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An ultrasonic examination

Ultrasound is sound with frequencies greater than 20 kilohertz.[1] This frequency is the approximate upper audible limit of human hearing in healthy young adults. The physical principles of acoustic waves apply to any frequency range, including ultrasound. Ultrasonic devices operate with frequencies from 20 kHz up to several gigahertz.

Ultrasound is used in many different fields. Ultrasonic devices are used to detect objects and measure distances. Ultrasound imaging or sonography is often used in medicine. In the nondestructive testing of products and structures, ultrasound is used to detect invisible flaws. Industrially, ultrasound is used for cleaning, mixing, and accelerating chemical processes. Animals such as bats and porpoises use ultrasound for locating prey and obstacles.[2]

History

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Galton whistle, one of the first devices to produce ultrasound

Acoustics, the science of sound, starts as far back as Pythagoras in the 6th century BC, who wrote on the mathematical properties of stringed instruments. Echolocation in bats was discovered by Lazzaro Spallanzani in 1794, when he demonstrated that bats hunted and navigated by inaudible sound, not vision. Francis Galton in 1893 invented the Galton whistle, an adjustable whistle that produced ultrasound, which he used to measure the hearing range of humans and other animals, demonstrating that many animals could hear sounds above the hearing range of humans.

The first article on the history of ultrasound was written in 1948.[3] According to its author, during the First World War, a Russian engineer named Chilowski submitted an idea for submarine detection to the French Government. The latter invited Paul Langevin, then Director of the School of Physics and Chemistry in Paris, to evaluate it. Chilowski's proposal was to excite a cylindrical, mica condenser by a high-frequency Poulsen arc at approximately 100 kHz and thus to generate an ultrasound beam for detecting submerged objects. The idea of locating underwater obstacles had been suggested prior by L. F. Richardson, following the Titanic disaster. Richardson had proposed to position a high-frequency hydraulic whistle at the focus of a mirror and use the beam for locating submerged navigational hazards. A prototype was built by Sir Charles Parsons, the inventor of the vapour turbine, but the device was found not to be suitable for this purpose. Langevin's device made use of the piezoelectric effect, which he had been acquainted with whilst a student at the laboratory of Jacques and Pierre Curie.[4] Langevin calculated and built an ultrasound transducer comprising a thin sheet of quartz sandwiched between two steel plates. Langevin was the first to report cavitation-related bioeffects from ultrasound.[5]

Definition

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Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications

Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz". In air at atmospheric pressure, ultrasonic waves have wavelengths of 1.9 cm or less.

Ultrasound can be generated at very high frequencies; ultrasound is used for sonochemistry at frequencies up to multiple hundreds of kilohertz.[6][7][8] Medical imaging equipment uses frequencies in the MHz range.[9] UHF ultrasound waves have been generated as high as the gigahertz range.[10][11][12][13]

Characterizing extremely high-frequency ultrasound poses challenges, as such rapid movement causes waveforms to steepen and form shock waves.[14]

Perception

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Humans

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The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the middle ear. Auditory sensation can occur if high‐intensity ultrasound is fed directly into the human skull and reaches the cochlea through bone conduction, without passing through the middle ear.[15]

Children can hear some high-pitched sounds that older adults cannot hear, because in humans the upper limit pitch of hearing tends to decrease with age.[16] An American cell phone company has used this to create ring signals that supposedly are only audible to younger humans,[17] but many older people can hear the signals, which may be because of the considerable variation of age-related deterioration in the upper hearing threshold.

Animals

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Bats use ultrasounds to navigate in the darkness.
A dog whistle, which emits sound in the ultrasonic range, used to train dogs and other animals

Bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey. They can detect frequencies beyond 100 kHz, possibly up to 200 kHz.[18]

Many insects have good ultrasonic hearing, and most of these are nocturnal insects listening for echolocating bats. These include many groups of moths, beetles, praying mantises and lacewings. Upon hearing a bat, some insects will make evasive manoeuvres to escape being caught.[19] Ultrasonic frequencies trigger a reflex action in the noctuid moth that causes it to drop slightly in its flight to evade attack.[20] Tiger moths also emit clicks which may disturb bats' echolocation,[21][22] and in other cases may advertise the fact that they are poisonous by emitting sound.[23][24]

Dogs and cats' hearing range extends into the ultrasound; the top end of a dog's hearing range is about 45 kHz, while a cat's is 64 kHz.[25][26] The wild ancestors of cats and dogs evolved this higher hearing range to hear high-frequency sounds made by their preferred prey, small rodents.[25] A dog whistle is a whistle that emits ultrasound, used for training and calling dogs. The frequency of most dog whistles is within the range of 23 to 54 kHz.[27]

Toothed whales, including dolphins, can hear ultrasound and use such sounds in their navigational system (biosonar) to orient and to capture prey.[28] Porpoises have the highest known upper hearing limit at around 160 kHz.[29] Several types of fish can detect ultrasound. In the order Clupeiformes, members of the subfamily Alosinae (shad) have been shown to be able to detect sounds up to 180 kHz, while the other subfamilies (e.g. herrings) can hear only up to 4 kHz.[30]

No bird species have been reported to be sensitive to ultrasound.[31]

Commercial ultrasonic systems have been sold for supposed indoors electronic pest control and outdoors ultrasonic algae control. However, no scientific evidence exists on the success of such devices for these purposes.[32][33][34]

Detection and ranging

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Non-contact sensor

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An ultrasonic level or sensing system requires no contact with the target. For many processes in the medical, pharmaceutical, military and general industries this is an advantage over inline sensors that may contaminate the liquids inside a vessel or tube or that may be clogged by the product.

Both continuous wave and pulsed systems are used. The principle behind a pulsed-ultrasonic technology is that the transmit signal consists of short bursts of ultrasonic energy. After each burst, the electronics looks for a return signal within a small window of time corresponding to the time it takes for the energy to pass through the vessel. Only a signal received during this window will qualify for additional signal processing.

A popular consumer application of ultrasonic ranging was the Polaroid SX-70 camera, which included a lightweight transducer system to focus the camera automatically. Polaroid later licensed this ultrasound technology and it became the basis of a variety of ultrasonic products.

Motion sensors and flow measurement

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A common ultrasound application is an automatic door opener, where an ultrasonic sensor detects a person's approach and opens the door. Ultrasonic sensors are also used to detect intruders; the ultrasound can cover a wide area from a single point. The flow in pipes or open channels can be measured by ultrasonic flowmeters, which measure the average velocity of flowing liquid. In rheology, an acoustic rheometer relies on the principle of ultrasound. In fluid mechanics, fluid flow can be measured using an ultrasonic flow meter.

Nondestructive testing

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See also: Macrosonics and Ultrasonic testing
Principle of flaw detection with ultrasound. A void in the solid material reflects some energy back to the transducer, which is detected and displayed.

Ultrasonic testing is a type of nondestructive testing commonly used to find flaws in materials and to measure the thickness of objects. Frequencies of 2 to 10 MHz are common, but for special purposes other frequencies are used. Inspection may be manual or automated and is an essential part of modern manufacturing processes. Most metals can be inspected as well as plastics and aerospace composites. Lower frequency ultrasound (50–500 kHz) can also be used to inspect less dense materials such as wood, concrete and cement.

Ultrasound inspection of welded joints has been an alternative to radiography for nondestructive testing since the 1960s. Ultrasonic inspection eliminates the use of ionizing radiation, with safety and cost benefits. Ultrasound can also provide additional information such as the depth of flaws in a welded joint. Ultrasonic inspection has progressed from manual methods to computerized systems that automate much of the process. An ultrasonic test of a joint can identify the existence of flaws, measure their size, and identify their location. Not all welded materials are equally amenable to ultrasonic inspection; some materials have a large grain size that produces a high level of background noise in measurements.[35]

Non-destructive testing of a swing shaft showing spline cracking

Ultrasonic thickness measurement is one technique used to monitor quality of welds.

Ultrasonic range finding

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Principle of an active sonar
Main article: Sonar

A common use of ultrasound is in underwater range finding; this use is also called sonar. An ultrasonic pulse is generated in a particular direction. If there is an object in the path of this pulse, part or all of the pulse will be reflected back to the transmitter as an echo and can be detected through the receiver path. By measuring the difference in time between the pulse being transmitted and the echo being received, it is possible to determine the distance.

The measured travel time of Sonar pulses in water is strongly dependent on the temperature and the salinity of the water. Ultrasonic ranging is also applied for measurement in air and for short distances. For example, hand-held ultrasonic measuring tools can rapidly measure the layout of rooms.

Although range finding underwater is performed at both sub-audible and audible frequencies for great distances (1 to several kilometers), ultrasonic range finding is used when distances are shorter and the accuracy of the distance measurement is desired to be finer. Ultrasonic measurements may be limited through barrier layers with large salinity, temperature or vortex differentials. Ranging in water varies from about hundreds to thousands of meters, but can be performed with centimeters to meters accuracy

Ultrasound Identification (USID)

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Ultrasound Identification (USID) is a Real-Time Locating System (RTLS) or Indoor Positioning System (IPS) technology used to automatically track and identify the location of objects in real time using simple, inexpensive nodes (badges/tags) attached to or embedded in objects and devices, which then transmit an ultrasound signal to communicate their location to microphone sensors.

Imaging

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Sonogram of a fetus at 14 weeks (profile)
An explanatory video about medical ultrasound technology by the U.S. National Institute of Biomedical Imaging and Bioengineering.
Head of a fetus, aged 29 weeks, in a "3D ultrasound"

The potential for ultrasonic imaging of objects, in which a 3 GHz sound wave could produce resolution comparable to an optical image, was recognized by Sergei Sokolov in 1939. Such frequencies were not possible at the time, and what technology did exist produced relatively low-contrast images with poor sensitivity.[36] Ultrasonic imaging uses frequencies of 2 megahertz and higher; the shorter wavelength allows resolution of small internal details in structures and tissues. The power density is generally less than 1 watt per square centimetre to avoid heating and cavitation effects in the object under examination.[37] Ultrasonic imaging applications include industrial nondestructive testing, quality control and medical uses.[36]

Acoustic microscopy

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Acoustic microscopy is the technique of using sound waves to visualize structures too small to be resolved by the human eye. High and ultra high frequencies up to several gigahertz are used in acoustic microscopes. The reflection and diffraction of sound waves from microscopic structures can yield information not available with light.

Human medicine

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Medical ultrasound is an ultrasound-based diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs to capture their size, structure and any pathological lesions with real time tomographic images. Ultrasound has been used by radiologists and sonographers to image the human body for at least 50 years and has become a widely used diagnostic tool.[38] The technology is relatively inexpensive and portable, especially when compared with other techniques, such as magnetic resonance imaging (MRI) and computed tomography (CT). Ultrasound is also used to visualize fetuses during routine and emergency prenatal care. Such diagnostic applications used during pregnancy are referred to as obstetric sonography. As currently applied in the medical field, properly performed ultrasound poses no known risks to the patient.[39] Sonography does not use ionizing radiation, and the power levels used for imaging are too low to cause adverse heating or pressure effects in tissue.[40][41] Although the long-term effects due to ultrasound exposure at diagnostic intensity are still unknown,[42] currently most doctors feel that the benefits to patients outweigh the risks.[43] The ALARA (As Low As Reasonably Achievable) principle has been advocated for an ultrasound examination – that is, keeping the scanning time and power settings as low as possible but consistent with diagnostic imaging – and that by that principle nonmedical uses, which by definition are not necessary, are actively discouraged.[44]

Ultrasound is also increasingly being used in trauma and first aid cases, with emergency ultrasound being used by some EMT response teams. Furthermore, ultrasound is used in remote diagnosis cases where teleconsultation is required, such as scientific experiments in space or mobile sports team diagnosis.[45]

According to RadiologyInfo,[46] ultrasounds are useful in the detection of pelvic abnormalities and can involve techniques known as abdominal (transabdominal) ultrasound, vaginal (transvaginal or endovaginal) ultrasound in women, and also rectal (transrectal) ultrasound in men.

Veterinary medicine

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See also: Preclinical imaging

Diagnostic ultrasound is used externally in horses for evaluation of soft tissue and tendon injuries, and internally in particular for reproductive work – evaluation of the reproductive tract of the mare and pregnancy detection.[47] It may also be used in an external manner in stallions for evaluation of testicular condition and diameter as well as internally for reproductive evaluation (deferent duct etc.).[48]

By 2005, ultrasound technology began to be used by the beef cattle industry to improve animal health and the yield of cattle operations.[49] Ultrasound is used to evaluate fat thickness, rib eye area, and intramuscular fat in living animals.[50] It is also used to evaluate the health and characteristics of unborn calves.

Ultrasound technology provides a means for cattle producers to obtain information that can be used to improve the breeding and husbandry of cattle. The technology can be expensive, and it requires a substantial time commitment for continuous data collection and operator training.[50] Nevertheless, this technology has proven useful in managing and running a cattle breeding operation.[49]

Processing and power

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High-power applications of ultrasound often use frequencies between 20 kHz and a few hundred kHz. Intensities can be very high; above 10 watts per square centimeter, cavitation can be inducted in liquid media, and some applications use up to 1000 watts per square centimeter. Such high intensities can induce chemical changes or produce significant effects by direct mechanical action, and can inactivate harmful microorganisms.[37]

Physical therapy

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Main article: Therapeutic ultrasound

Ultrasound has been used since the 1940s by physical and occupational therapists for treating connective tissue: ligaments, tendons, and fascia (and also scar tissue).[51] Conditions for which ultrasound may be used for treatment include the follow examples: ligament sprains, muscle strains, tendonitis, joint inflammation, plantar fasciitis, metatarsalgia, facet irritation, impingement syndrome, bursitis, rheumatoid arthritis, osteoarthritis, and scar tissue adhesion.

Relatively high power ultrasound can break up stony deposits or tissue, increase skin permeability, accelerate the effect of drugs in a targeted area, assist in the measurement of the elastic properties of tissue, and can be used to sort cells or small particles for research.[52]

Ultrasonic impact treatment

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Ultrasonic impact treatment (UIT) uses ultrasound to enhance the mechanical and physical properties of metals.[53] It is a metallurgical processing technique in which ultrasonic energy is applied to a metal object. Ultrasonic treatment can result in controlled residual compressive stress, grain refinement and grain size reduction. Low and high cycle fatigue are enhanced and have been documented to provide increases up to ten times greater than non-UIT specimens. Additionally, UIT has proven effective in addressing stress corrosion cracking, corrosion fatigue and related issues.

When the UIT tool, made up of the ultrasonic transducer, pins and other components, comes into contact with the work piece it acoustically couples with the work piece, creating harmonic resonance.[54] This harmonic resonance is performed at a carefully calibrated frequency, to which metals respond very favorably.

Depending on the desired effects of treatment a combination of different frequencies and displacement amplitude is applied. These frequencies range between 25 and 55 kHz,[55] with the displacement amplitude of the resonant body of between 22 and 50 μm (0.00087 and 0.0020 in).

UIT devices rely on magnetostrictive transducers.

Processing

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Main article: Sonication

Ultrasonication offers great potential in the processing of liquids and slurries, by improving the mixing and chemical reactions in various applications and industries. Ultrasonication generates alternating low-pressure and high-pressure waves in liquids, leading to the formation and violent collapse of small vacuum bubbles. This phenomenon is termed cavitation and causes high speed impinging liquid jets and strong hydrodynamic shear-forces. These effects are used for the deagglomeration and milling of micrometre and nanometre-size materials as well as for the disintegration of cells or the mixing of reactants. In this aspect, ultrasonication is an alternative to high-speed mixers and agitator bead mills. Ultrasonic foils under the moving wire in a paper machine will use the shock waves from the imploding bubbles to distribute the cellulose fibres more uniformly in the produced paper web, which will make a stronger paper with more even surfaces. Furthermore, chemical reactions benefit from the free radicals created by the cavitation as well as from the energy input and the material transfer through boundary layers. For many processes, this sonochemical (see sonochemistry) effect leads to a substantial reduction in the reaction time, like in the transesterification of oil into biodiesel.[citation needed]

Schematic of bench and industrial-scale ultrasonic liquid processors

Substantial ultrasonic intensity and high ultrasonic vibration amplitudes are required for many processing applications, such as nano-crystallization, nano-emulsification,[56] deagglomeration, extraction, cell disruption, as well as many others. Commonly, a process is first tested on a laboratory scale to prove feasibility and establish some of the required ultrasonic exposure parameters. After this phase is complete, the process is transferred to a pilot (bench) scale for flow-through pre-production optimization and then to an industrial scale for continuous production. During these scale-up steps, it is essential to make sure that all local exposure conditions (ultrasonic amplitude, cavitation intensity, time spent in the active cavitation zone, etc.) stay the same. If this condition is met, the quality of the final product remains at the optimized level, while the productivity is increased by a predictable "scale-up factor". The productivity increase results from the fact that laboratory, bench and industrial-scale ultrasonic processor systems incorporate progressively larger ultrasonic horns, able to generate progressively larger high-intensity cavitation zones and, therefore, to process more material per unit of time. This is called "direct scalability". It is important to point out that increasing the power of the ultrasonic processor alone does not result in direct scalability, since it may be (and frequently is) accompanied by a reduction in the ultrasonic amplitude and cavitation intensity. During direct scale-up, all processing conditions must be maintained, while the power rating of the equipment is increased in order to enable the operation of a larger ultrasonic horn.[57][58][59]

Ultrasonic manipulation and characterization of particles

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A researcher at the Industrial Materials Research Institute, Alessandro Malutta, devised an experiment that demonstrated the trapping action of ultrasonic standing waves on wood pulp fibers diluted in water and their parallel orienting into the equidistant pressure planes.[60] The time to orient the fibers in equidistant planes is measured with a laser and an electro-optical sensor. This could provide the paper industry a quick on-line fiber size measurement system. A somewhat different implementation was demonstrated at Pennsylvania State University using a microchip which generated a pair of perpendicular standing surface acoustic waves allowing to position particles equidistant to each other on a grid. This experiment, called acoustic tweezers, can be used for applications in material sciences, biology, physics, chemistry and nanotechnology.

Ultrasonic cleaning

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Main article: Ultrasonic cleaning

Ultrasonic cleaners, sometimes mistakenly called supersonic cleaners, are used at frequencies from 20 to 40 kHz for jewellery, lenses and other optical parts, watches, dental instruments, surgical instruments, diving regulators and industrial parts. An ultrasonic cleaner works mostly by energy released from the collapse of millions of microscopic cavitation bubbles near the dirty surface. The collapsing bubbles form tiny shockwaves that break up and disperse contaminants on the object's surface.

Ultrasonic disintegration

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Similar to ultrasonic cleaning, biological cells including bacteria can be disintegrated. High power ultrasound produces cavitation that facilitates particle disintegration or reactions. This has uses in biological science for analytical or chemical purposes (sonication and sonoporation) and in killing bacteria in sewage. High power ultrasound can disintegrate corn slurry and enhance liquefaction and saccharification for higher ethanol yield in dry corn milling plants.[61][62]

Ultrasonic humidifier

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The ultrasonic humidifier, one type of nebulizer (a device that creates a very fine spray), is a popular type of humidifier. It works by vibrating a metal plate at ultrasonic frequencies to nebulize (sometimes incorrectly called "atomize") the water. Because the water is not heated for evaporation, it produces a cool mist. The ultrasonic pressure waves nebulize not only the water but also materials in the water including calcium, other minerals, viruses, fungi, bacteria,[63] and other impurities. Illness caused by impurities that reside in a humidifier's reservoir fall under the heading of "Humidifier Fever".

Ultrasonic humidifiers are frequently used in aeroponics, where they are generally referred to as foggers.

Ultrasonic welding

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In ultrasonic welding of plastics, high frequency (15 kHz to 40 kHz) low amplitude vibration is used to create heat by way of friction between the materials to be joined. The interface of the two parts is specially designed to concentrate the energy for maximum weld strength.

Sonochemistry

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Main article: Sonochemistry

Power ultrasound in the 20–100 kHz range is used in chemistry. The ultrasound does not interact directly with molecules to induce the chemical change, as its typical wavelength (in the millimeter range) is too long compared to the molecules. Instead, the energy causes cavitation which generates extremes of temperature and pressure in the liquid where the reaction happens. Ultrasound also breaks up solids and removes passivating layers of inert material to give a larger surface area for the reaction to occur over. Both of these effects make the reaction faster. In 2008, Atul Kumar reported synthesis of Hantzsch esters and polyhydroquinoline derivatives via multi-component reaction protocol in aqueous micelles using ultrasound.[64]

Ultrasound is used in extraction, using different frequencies.

Other uses

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When applied in specific configurations, ultrasound can produce short bursts of light in a phenomenon known as sonoluminescence.

Ultrasound is used when characterizing particulates through the technique of ultrasound attenuation spectroscopy or by observing electroacoustic phenomena or by transcranial pulsed ultrasound.

Wireless communication

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Audio can be propagated by modulated ultrasound.

A formerly popular consumer application of ultrasound was in television remote controls for adjusting volume and changing channels. Introduced by Zenith in the late 1950s, the system used a hand-held remote control containing short rod resonators struck by small hammers, and a microphone on the set. Filters and detectors discriminated between the various operations. The principal advantages were that no battery was needed in the hand-held control box and, unlike radio waves, the ultrasound was unlikely to affect neighboring sets. Ultrasound remained in use until displaced by infrared systems starting in the late 1980s.[65]

In July 2015, The Economist reported that researchers at the University of California, Berkeley have conducted ultrasound studies using graphene diaphragms. The thinness and low weight of graphene combined with its strength make it an effective material to use in ultrasound communications. One suggested application of the technology would be underwater communications, where radio waves typically do not travel well.[66]

Ultrasonic signals have been used in "audio beacons" for cross-device tracking of Internet users.[67][68]

Safety

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Occupational exposure to ultrasound in excess of 120 dB may lead to hearing loss. Exposure in excess of 155 dB may produce heating effects that are harmful to the human body, and it has been calculated that exposures above 180 dB may lead to death.[69] The UK's independent Advisory Group on Non-ionising Radiation (AGNIR) produced a report in 2010, which was published by the UK Health Protection Agency (HPA). This report recommended an exposure limit for the general public to airborne ultrasound sound pressure levels (SPL) of 70 dB (at 20 kHz), and 100 dB (at 25 kHz and above).[70]

In medical ultrasound, guidelines exist to prevent inertial cavitation from happening. The risk of inertial cavitation damage is expressed by the mechanical index.

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Ultrasound

View on Grokipedia
from Grokipedia
Ultrasound refers to with frequencies greater than 20 kHz, beyond the upper limit of hearing. These waves have diverse applications across , industry, , and , including non-invasive and therapeutics, industrial and , animal perception such as echolocation in bats and dolphins, and underwater communication. In clinical practice, diagnostic ultrasound employs that generate high-frequency pulses, typically in the range of 1 to 20 MHz, which propagate through body tissues at approximately 1540 m/s in , reflecting off interfaces between structures of differing to produce real-time images of internal organs, blood flow, and fetal development. These reflections, known as echoes, are detected by the same , converted into electrical signals, and processed to form two-dimensional, three-dimensional, or even four-dimensional visualizations, enabling the assessment of tissue where denser structures like appear bright and fluids appear dark. Key physical principles governing ultrasound include , where wave energy diminishes due to absorption, , and reflection—often at a rate of about 0.5 dB per MHz per cm of tissue depth—and the trade-off between frequency, resolution, and penetration, with higher frequencies providing finer detail but shallower imaging depth. relies on piezoelectric crystals in transducers to both emit and receive pulses, with modes such as B-mode for brightness-based imaging, Doppler for velocity detection in vascular flow, and for evaluating tissue stiffness, all contributing to its versatility in point-of-care diagnostics. Therapeutically, high-intensity focused ultrasound (HIFU) concentrates energy to ablate abnormal tissues, such as tumors in the or , without incisions, and has received FDA approval for specific applications like treating and bone metastases. Ultrasound's safety profile is a major advantage, as it involves no —unlike X-rays or CT scans—reducing risks during monitoring or pediatric exams, though prudent use is advised to minimize potential effects from prolonged exposure. Common applications span for fetal anomaly detection, via , abdominal evaluations for organ , and procedural guidance for biopsies, making it an essential, cost-effective tool in modern healthcare with over 140 years of development since the discovery of in 1880 and initial medical adoption in the .

Fundamentals

Definition and Principles

Ultrasound consists of mechanical waves, specifically longitudinal compression waves, with frequencies greater than 20 kHz, exceeding the upper limit of human hearing and distinguishing it from audible . These waves propagate through elastic media as pressure disturbances, enabling a wide range of applications due to their ability to interact with materials at scales invisible to the human ear. The fundamental behavior of ultrasound propagation is described by the acoustic wave equation: ∂2p∂t2=c2∇2p\frac{\partial^2 p}{\partial t^2} = c^2 \nabla^2 p where pp represents the acoustic pressure variation, cc is the in the medium, and ∇2\nabla^2 denotes the Laplacian operator. This linear models how small-amplitude pressure waves travel without dissipation in homogeneous, isotropic media, assuming the disturbances are much smaller than the ambient . In practice, ultrasound waves are generated primarily through piezoelectric transducers, which exploit the inverse piezoelectric effect: an applied alternating causes certain crystalline materials, such as or (PZT), to deform and produce mechanical vibrations at the desired frequency. Ultrasound waves exhibit several key properties that govern their interaction with media, including attenuation—the progressive decrease in due to absorption, , and ; reflection and at boundaries between media with differing properties; and , defined as Z=ρcZ = \rho c, where ρ\rho is the medium's . Mismatches in between adjacent media cause partial reflection of the incident wave, quantified by the , while matched impedances minimize such losses to enhance transmission. The cc varies by medium due to differences in and elasticity; representative values include approximately 1540 m/s in soft biological tissue and 1480 m/s in at standard conditions. Frequency ranges for ultrasound are tailored to specific uses, balancing and resolution: diagnostic applications commonly operate in the 1–20 MHz range to image internal structures with high detail, therapeutic uses employ frequencies above 1 MHz (typically 1–3 MHz) to deliver for heating or , and industrial processes often utilize lower frequencies of 20–100 kHz for tasks like or material processing, where deeper penetration is prioritized over fine resolution.

Historical Development

The earliest observations linking sound waves to navigation came from Italian biologist Lazzaro Spallanzani's 1793 experiments on bats, where he demonstrated that bats rely on echolocation rather than sight to navigate in darkness, laying foundational insights into acoustic sensing that later informed ultrasound principles. In the late 19th century, French physicists and Jacques Curie discovered the piezoelectric effect in 1880, observing that certain crystals generate an electric charge under mechanical stress, which became essential for developing ultrasound transducers capable of converting electrical energy into sound waves and vice versa. During , French physicist advanced practical applications by inventing an ultrasonic device between 1915 and 1917 to detect submarines, using piezoelectric crystals to generate and receive high-frequency sound pulses underwater, marking the first large-scale use of ultrasound technology. The inception of occurred in 1942 when Austrian neurologist Karl Theo Dussik pioneered hyperphonography, attempting the first human imaging by measuring ultrasound transmission through the to detect brain tumors, though the technique was limited by poor resolution. Post-World War II, ultrasound gained momentum in medicine, with Scottish physician Ian Donald introducing its use in in 1958 through collaborative work with engineer Tom Brown, adapting industrial scanners to visualize fetal development safely and non-invasively, which revolutionized . Key inventions in the 1940s included the pulse-echo technique developed by American physician George Ludwig around 1949, who used it to detect gallstones in animal models by sending short ultrasound pulses and analyzing echoes, establishing a core method for diagnostic . The 1960s saw the introduction of phased-array transducers, first conceptualized in 1967 by researchers at the , enabling electronic for more precise and dynamic ultrasound scanning without mechanical movement. Modern milestones emerged in the 1970s with the development of real-time two-dimensional using linear transducers, pioneered by companies like Technicare, allowing continuous visualization of moving structures such as heart valves. The 1980s brought widespread adoption of Doppler ultrasound, building on earlier work by Japanese researcher Shigeo Satomura in the late 1950s but commercialized through color flow mapping systems like those from ATL in 1982, which measured flow velocity and direction to assess vascular conditions. In the 1990s, three-dimensional and four-dimensional ultrasound techniques advanced, with the first commercial 3D systems introduced by Kretztechnik in 1996, using volume acquisition to reconstruct spatial images, enhancing diagnostic accuracy for complex anatomies like fetal anomalies. The 2010s and 2020s integrated into ultrasound image analysis, with seminal contributions like a 2017 framework from automating fetal biometry measurements, improving efficiency and reducing operator variability in diagnostics. As of 2025, recent developments emphasize portable ultrasound devices, such as the FDA-cleared Butterfly iQ+ in 2021 with ongoing AI enhancements, and advanced contrast-enhanced agents like Lumason, enabling point-of-care diagnostics in remote settings for faster in emergencies.

Perception

In Humans

Humans perceive ultrasound primarily through their auditory system, with the upper limit of audible frequencies generally reaching approximately 20 kHz in young adults, though sensitivity declines markedly above 10 kHz due to reduced responsiveness of the to high frequencies. This threshold varies individually, and sounds exceeding 20 kHz are classified as ultrasound, typically inaudible via air conduction. However, can facilitate the perception of ultrasonic frequencies in some cases, where vibrations transmitted through the skull bones stimulate the directly, enabling "hearing" of sounds up to 100 kHz or higher under specific conditions. Beyond auditory perception, ultrasound can induce non-auditory effects such as thermal sensations arising from tissue absorption, where intensities greater than 1 W/cm² lead to localized heating that is perceptible as warmth on the skin or within tissues. Occupational exposure to ultrasonic environments, including those involving ultrasonic cleaners used in industrial or laboratory settings, carries risks of for workers due to potential audible harmonics or prolonged high-frequency noise that may cause temporary or permanent threshold shifts, particularly if equipment lacks proper enclosure. Ultrasound also elicits vestibular and tactile responses in humans; for instance, low-frequency ultrasound near the audible range can provoke through stimulation of the , manifesting as involuntary eye movements, while higher frequencies produce tactile vibrations via acoustic on the skin. Age-related changes exacerbate these perceptual limits, as progressively impairs high-frequency hearing, often beginning with losses above 8 kHz in and worsening to affect frequencies as low as 2-4 kHz by late adulthood, due to degeneration of cochlear hair cells.

In Animals

Animals employ ultrasound for echolocation, communication, and predator avoidance, leveraging frequencies beyond human auditory limits. In bats, echolocation involves the emission of short ultrasonic pulses ranging from 20 to 200 kHz, which reflect off objects to provide spatial information. These pulses allow bats to detect prey and navigate in complete darkness. To discern target velocity, bats utilize the Doppler shift in returning echoes, governed by the formula Δf/f=2v/c\Delta f / f = 2v / c, where Δf\Delta f is the frequency shift, ff is the emitted frequency, vv is the relative speed of the target, and cc is the speed of sound. This mechanism enables precise adjustments during flight, such as compensating for their own motion to maintain echo frequencies within optimal hearing ranges. Marine toothed whales, such as dolphins, also rely on biosonar systems operating in the 10-150 kHz range. Dolphins produce high-frequency clicks that propagate through specialized nasal structures, with the —a fatty organ in the —facilitating to focus the sound into a narrow directional beam for enhanced target resolution. This adaptation improves the detection of prey in turbid waters or at depth, where visual cues are limited. Some toothed whales exhibit similar capabilities, though their lower-frequency variants suit longer-range communication and in oceanic environments. Insects such as moths have evolved ultrasonic hearing sensitive to 20-100 kHz to detect approaching bats and initiate evasive maneuvers. This triggers rapid flight alterations, like erratic turns or dives, increasing survival rates against echolocating predators. Beyond defense, ultrasound serves communicative roles in terrestrial animals; for instance, produce ultrasonic vocalizations in the 20-100 kHz spectrum for social signaling, including mating calls and territorial warnings. Evolutionary adaptations underpin these abilities, particularly in auditory structures. Bats possess specialized cochleae with elongated basilar membranes and densely packed cells tuned for high-frequency processing, enabling the resolution of fine temporal and details in echoes. These modifications, driven by the , represent in echolocating mammals, enhancing sensitivity to ultrasonic cues essential for survival.

Sensing and Measurement

Ranging and Detection

Ultrasound ranging and detection rely on the time-of-flight (ToF) principle, where an ultrasonic pulse is emitted from a transducer, reflects off an object, and returns as an echo; the round-trip time tt is measured to calculate distance dd using the formula d=c×t2d = \frac{c \times t}{2}, with cc being the speed of sound in the medium. This method enables non-contact measurement over distances typically from centimeters to several meters, depending on the sensor design and environmental conditions. Ultrasonic sensors for ranging commonly employ piezoelectric transducers, which convert electrical signals into mechanical vibrations at ultrasonic frequencies (above 20 kHz) for transmission and vice versa for reception upon echo arrival. A widely used example in is the HC-SR04 module, operating at 40 kHz, which features integrated piezoelectric elements to emit short bursts of eight pulses and detect echoes with a resolution of about 3 mm over a 2–400 cm range. These sensors are compact, low-cost, and interface easily with microcontrollers for real-time processing in mobile platforms. In practical applications, ultrasonic ranging supports vehicle parking sensors, where arrays of transducers mounted on bumpers detect nearby by measuring times to assist low-speed maneuvering and prevent collisions. Similarly, in drones, ultrasonic sensors enable avoidance by providing proximity data for autonomous navigation, particularly in indoor or low-altitude environments where they complement other sensing modalities. The resolution of ultrasonic detection is fundamentally limited by the wavelength λ=cf\lambda = \frac{c}{f}, where ff is the ; objects smaller than approximately half a wavelength may not be reliably detected, as the echo becomes indistinguishable from noise or effects. For a 40 kHz in air, with c≈343c \approx 343 m/s at , λ\lambda is about 8.6 mm, setting a practical minimum detectable size around 4 mm. Environmental factors, such as temperature, influence accuracy since cc in dry air approximates 331+0.6T331 + 0.6T m/s, where TT is in degrees Celsius; a 10°C rise increases cc by about 6 m/s, potentially introducing up to 1% error in distance calculations if uncompensated./Book%3A_University_Physics_I_-Mechanics_Sound_Oscillations_and_Waves(OpenStax)/17%3A_Sound/17.03%3A_Speed_of_Sound) Calibration or temperature sensors are often integrated to mitigate this dependence in precision applications.

Flow and Motion Measurement

Ultrasound techniques for flow and motion measurement primarily rely on the Doppler effect and time-of-flight principles to detect velocity in fluids and moving objects without invasive probes. Doppler ultrasound measures the frequency shift in reflected waves from moving scatterers, such as blood cells or particles, enabling non-contact assessment of motion dynamics. The core principle of Doppler ultrasound is the frequency shift Δf\Delta f given by the equation Δf=2vf0cos⁡θc\Delta f = \frac{2 v f_0 \cos \theta}{c}, where vv is the velocity of the scatterer along the beam axis, f0f_0 is the transmitted frequency, θ\theta is the angle between the ultrasound beam and the direction of motion, and cc is the speed of sound in the medium. This double-Doppler shift arises because the ultrasound wave travels to the moving target and back, amplifying the effect. The cosine term accounts for the directional sensitivity, with maximum shift occurring when the beam is aligned parallel to the flow (θ=0∘\theta = 0^\circ). Two main Doppler modes exist: continuous-wave (CW) and pulsed-wave (PW). CW Doppler transmits and receives signals continuously, providing high sensitivity to a wide range of velocities without aliasing but lacking spatial resolution as it cannot distinguish depth. In contrast, PW Doppler uses short pulses to gate echoes from specific depths, offering excellent spatial localization for targeted measurements, though it is limited by aliasing at high velocities due to the Nyquist limit (maximum detectable velocity approximately vmax⁡=c4f0Tv_{\max} = \frac{c}{4 f_0 T}, where TT is the pulse repetition period). The trade-off favors CW for high-speed flows like valvular jets and PW for precise, range-resolved assessments such as arterial blood velocity profiles. Transit-time flowmeters, an alternative to Doppler methods, measure flow by detecting the difference in propagation times of ultrasound pulses sent upstream and downstream across a pipe or vessel. The time differential Δt\Delta t is proportional to the vv via Δt=2Lvcos⁡ϕc2−v2cos⁡2ϕ\Delta t = \frac{2 L v \cos \phi}{c^2 - v^2 \cos^2 \phi}, where LL is the path length between transducers and ϕ\phi is the beam angle; for low velocities, this approximates to Δt≈2Lvc2\Delta t \approx \frac{2 L v}{c^2}. This bidirectional approach excels in clean, low-turbulence fluids, providing accurate volumetric flow rates independent of fluid composition. In medical applications, PW Doppler is widely used to quantify blood flow in arteries, such as peak systolic velocity in carotid or femoral vessels, aiding in the diagnosis of by comparing flow speeds to normal ranges (e.g., >125 cm/s indicating significant narrowing). Industrially, transit-time flowmeters monitor pipe flows in water distribution or chemical processing, offering non-intrusive installation on existing lines with accuracies up to ±1% for velocities from 0.01 to 10 m/s. For comprehensive flow mapping, ultrasound particle image velocimetry (uPIV) adapts optical PIV principles to acoustic , tracking speckle patterns or contrast agents in B-mode images to generate vector fields of . By cross-correlating sequential ultrasound frames, uPIV achieves non-invasive, high-spatiotemporal resolution mapping of complex flows, such as microvascular or turbulent jets, with sub-millimeter accuracy over depths up to several centimeters. This technique has been validated in phantom and studies, outperforming traditional Doppler in resolving multi-directional flows without angle dependency.

Nondestructive Testing

Ultrasonic (NDT) employs high-frequency sound waves to evaluate the integrity of materials and components without causing damage, primarily by detecting internal flaws through the analysis of wave reflections at material interfaces. This technique relies on the principle that ultrasonic waves reflect when encountering boundaries between materials of differing acoustic impedances, allowing inspectors to identify discontinuities such as cracks or voids. The pulse-echo technique is a fundamental method in ultrasonic NDT, where a generates short bursts of ultrasonic pulses that propagate through the material and reflect back from flaws or boundaries to the same , which then converts the echoes into electrical signals for analysis. The resulting data is typically displayed as an A-scan, a one-dimensional showing echo on the vertical axis versus time or equivalent depth on the horizontal axis, enabling the localization and assessment of defects based on echo timing and strength. Ultrasonic testing can be performed using contact or immersion methods, each requiring appropriate acoustic to transmit waves efficiently between the and test piece. In contact testing, a couplant such as or paste is applied directly to the surface to eliminate air gaps and ensure wave transmission, making it suitable for on-site inspections of rough or irregular surfaces. Immersion testing, in contrast, submerges both the and the component in a couplant like , which provides uniform and is ideal for automated scanning of complex geometries or delicate parts, though it may introduce corrections due to the medium. Common applications of ultrasonic NDT include weld inspection in components, where it detects fusion defects and ensures structural safety in high-stakes environments, and thickness gauging in pipelines to monitor and wall thinning for preventive maintenance. These methods help identify critical defect types such as cracks, voids, and inclusions, which can compromise material strength; defect sizing is often achieved using the Krautkramer method, also known as the distance-gain-size (DGS) technique, which correlates echo amplitude with flaw size by comparing signals to reference reflectors while accounting for distance and beam spread. Phased-array ultrasonic testing (PAUT), introduced in industrial applications during the , advances traditional methods by using an of elements with electronic time delays to steer, focus, and shape the ultrasonic beam electronically, allowing rapid scanning from multiple angles without physical probe movement. By the 2020s, PAUT has integrated for automated defect classification, where algorithms analyze A-scan or imaging data to distinguish and categorize flaws with reduced human interpretation error, enhancing efficiency in complex inspections.

Imaging

Acoustic Microscopy

Acoustic microscopy employs high-frequency ultrasound waves to achieve micron-scale resolution imaging of structures, distinguishing it from conventional ultrasound techniques by its focus on microscopic scales. Scanning acoustic microscopy (SAM), the primary modality, utilizes focused ultrasonic transducers operating at frequencies typically between 10 and 100 MHz to probe samples immersed in a coupling medium such as , enabling non-destructive visualization of internal features. At these frequencies, the technique balances resolution and , with higher frequencies yielding finer details but shallower imaging depths in opaque . The core principle of SAM involves a piezoelectric transducer that generates short acoustic pulses, which are focused onto the sample via a lens and scanned mechanically across the surface. Reflected or transmitted echoes are detected to form images, where contrast arises primarily from differences in acoustic impedance between materials or interfaces. Acoustic impedance ZZ, defined as Z=ρcZ = \rho c with ρ\rho as density and cc as the speed of sound, governs the reflection coefficient at boundaries, highlighting defects like voids, delaminations, or cracks. This impedance-based contrast provides insights into mechanical properties, such as elasticity and hardness, without requiring optical transparency. Resolution in SAM is fundamentally limited by acoustic diffraction and approximated by λ2\frac{\lambda}{2}, where λ=cf\lambda = \frac{c}{f} is the wavelength, cc the speed of sound in the medium, and ff the frequency; focused transducers achieve lateral resolution of approximately 7.4 μm at 100 MHz in water (c≈1480c \approx 1480 m/s), corresponding to a wavelength of about 14.8 μm. Axial resolution, determined similarly by pulse duration, further refines depth discrimination. In applications, SAM excels in inspection, where it detects subsurface voids, cracks, or delaminations in integrated circuits and without disassembly, supporting and in manufacturing. For biological samples, it enables 3D of cell morphology, revealing internal structures like nuclei or cytoskeletal elements in tissues such as or , offering label-free mechanical property mapping at cellular scales. These uses leverage SAM's ability to image non-destructively through opaque media, providing complementary data to optical methods. Recent advances in the have integrated photoacoustic principles into acoustic microscopy, combining optical excitation with ultrasonic detection to generate contrast from light-absorbing structures, enhancing multimodal imaging for biological and material studies. These photoacoustic variants, such as wide-field scanning systems, achieve higher speeds and resolutions by leveraging laser-induced thermoelastic expansion, as demonstrated in high-speed prototypes resolving features below 1 μm. Such developments, building on foundational SAM designs from Quate and Lemons in 1974, expand applications in nanoscale biomechanics and hybrid material characterization.

Medical Imaging

Medical ultrasound imaging employs high-frequency sound waves to generate real-time images of internal organs and tissues, serving as a primary diagnostic tool in clinical practice due to its non-invasive nature and absence of ionizing radiation. Introduced to medicine in the 1950s for applications like echocardiography, it has evolved into a versatile modality for visualizing soft tissues and fluid-filled structures. The foundational technique is B-mode (brightness mode) imaging, which produces two-dimensional grayscale images by processing the amplitude envelopes of reflected ultrasound echoes to represent tissue interfaces based on acoustic impedance variations. This method displays brighter pixels for stronger echoes, enabling visualization of anatomical details such as organ boundaries and cysts. However, B-mode images often exhibit speckle noise—a multiplicative interference pattern arising from coherent scattering within tissues—which can obscure fine details; speckle reduction algorithms, including spatial compounding (acquiring images from multiple angles) and frequency compounding (using varied transmit frequencies), mitigate this by averaging speckled patterns while preserving resolution. Doppler ultrasound modes extend B-mode by incorporating the to evaluate blood flow dynamics. Color flow Doppler overlays color-encoded maps on B-mode images, with hues indicating direction (typically red for flow toward the and blue for away) and brightness denoting speed, facilitating vascular mapping in arteries and veins. Doppler, in contrast, provides a graphical of over time along a sample volume, allowing quantification of severity through metrics like peak systolic and end-diastolic ratios, which correlate with pressure gradients across narrowed vessels. Clinical applications span multiple specialties. In , ultrasound performs fetal biometry by measuring parameters such as biparietal diameter, head circumference, abdominal circumference, and femur length to estimate , monitor growth, and detect anomalies like . In , assesses heart function, including chamber sizes, wall motion, , and valvular integrity, aiding in the diagnosis of conditions such as and congenital defects. Abdominal ultrasound evaluates organs like the liver for tumors, distinguishing benign hemangiomas from malignant based on echotexture, vascular patterns, and enhancement characteristics. To enhance diagnostic accuracy, ultrasound contrast agents—microbubbles filled with inert gases and stabilized by lipid or protein shells—improve visualization of tissue and lesion vascularity by oscillating in response to ultrasound waves, generating strong echoes. The first microbubble agent, Optison (albumin-shelled with perfluoropropane gas), received FDA approval in 1998 for left ventricular opacification in , with subsequent agents like Definity (lipid-shelled) approved in 2001 and expanded indications in the 2020s for liver and other organ . The advent of portable and handheld ultrasound devices has driven the expansion of point-of-care ultrasound (POCUS) since the 2010s, allowing rapid bedside assessments in emergency departments, intensive care units, and outpatient settings to guide procedures like vascular access and evaluate conditions such as pleural effusions or . Despite its advantages, imaging faces limitations, including poor acoustic penetration through and air-filled structures like lungs or bowel gas, which cause shadowing and obscure deeper tissues. Additionally, it is highly operator-dependent, with image quality and diagnostic reliability varying based on the practitioner's skill in probe manipulation, machine settings, and interpretation.

Veterinary Imaging

Veterinary ultrasound imaging adapts probe designs to accommodate diverse animal anatomies and sizes, enabling precise diagnostics across species. Linear probes, operating at higher frequencies such as 7-18 MHz, are commonly used for small animals like or pets, providing high-resolution images for superficial structures such as hearts or canine vascular assessments. In contrast, curvilinear (convex) probes, typically in the 2-5 MHz range, are suited for larger animals, offering deeper penetration for examining equine tendons or bovine reproductive organs. These adaptations ensure optimal image quality tailored to anatomical variations, with linear probes producing rectangular fields of view for detailed near-field and curvilinear probes generating sector-shaped views for broader abdominal surveys. Key applications of veterinary ultrasound include reproductive monitoring in , where it facilitates early detection in like and sheep by visualizing as early as 25-30 days post-breeding, reducing economic losses in farming operations. In companion animals, cardiac ultrasound () assesses heart function in pets such as dogs and cats, identifying conditions like through measurements of chamber size and valve motion. For performance animals, musculoskeletal evaluates and integrity in racehorses, aiding in and rehabilitation by detecting subtle tears or . These uses leverage real-time B-mode for non-invasive, repeatable evaluations, often integrated with Doppler for blood flow analysis. Challenges in veterinary ultrasound arise from anatomical barriers like or hide, which cause signal and require clipping or gel application to improve acoustic , particularly in long-haired breeds. Sedation is often necessary for uncooperative or anxious patients to minimize motion artifacts, with protocols like recommended for short procedures, though many scans in calm animals proceed without it. Portable ultrasound units address field-based needs in husbandry, allowing on-site in remote settings without full facilities, though they may compromise on image depth compared to stationary systems. Emerging techniques, such as (CEUS), enhance tumor evaluation in veterinary by quantifying microvascular and , as demonstrated in 2020s trials on canine mammary tumors where CEUS parameters correlated with expression and microvessel density. Unlike human , which typically employs 2-15 MHz frequencies optimized for adult body sizes, veterinary applications span a broader 2-18 MHz range to image everything from tiny avian structures to large equine abdomens, accommodating size disparities across species.

Therapeutic Applications

Physical Therapy

Therapeutic ultrasound is a non-invasive modality employed in to facilitate tissue healing, reduce , and improve function in musculoskeletal conditions, primarily through low-intensity applications that generate controlled or non-thermal effects. Typically delivered via a handheld applied to the skin with a coupling , it penetrates tissues to depths of 2-5 cm depending on frequency, promoting physiological responses that aid rehabilitation without surgical intervention. Common therapeutic frequencies range from 1 to 3 MHz, with 1 MHz selected for deeper structures like joints and 3 MHz for superficial tissues such as tendons; intensities are generally set between 0.5 and 2 /cm² to achieve effects that elevate tissue temperature by 4-5°C, enhancing metabolic activity and extensibility. The primary mechanisms involve non-thermal effects like —where microscopic gas bubbles oscillate or collapse under acoustic pressure—and acoustic streaming, which induces fluid currents that stimulate cellular activity and promote , thereby increasing blood flow and nutrient delivery to injured areas. These processes collectively support repair in soft tissues by reducing and accelerating synthesis. In practice, addresses injuries such as sprains, strains, and tendinopathies by improving circulation and breaking down adhesions, while phonophoresis enhances topical —often anti-inflammatories like —through the skin via acoustic streaming and cavitation-induced membrane permeability. Protocols distinguish between continuous mode, which delivers uninterrupted waves for benefits (e.g., 1-1.5 W/cm² for 5-10 minutes), and pulsed mode, typically at a 20% (e.g., 1 ms on, 4 ms off) to minimize heating while emphasizing non-thermal effects like streaming for acute injuries. Treatment areas are calculated as 2-3 times the effective radiating area (ERA) of the to ensure even coverage. Evidence supports its efficacy particularly for of the shoulder, where pulsed ultrasound at 0.89 MHz and 2.5 W/cm² over 24 sessions reduced size by up to 50% and improved pain scores compared to , as shown in a seminal randomized . A 2023 randomized further demonstrated that combined with exercises significantly reduced size and improved function in chronic calcific shoulder tendinitis compared to exercises alone.

Targeted Drug Delivery

Targeted drug delivery using ultrasound leverages acoustic to enhance the transport and release of therapeutic agents at specific sites, minimizing systemic exposure and side effects. This approach primarily relies on microbubbles as contrast agents that oscillate or collapse under ultrasound exposure, generating mechanical forces to permeabilize cell membranes and vascular barriers. Sonoporation, a key mechanism, involves the transient formation of pores in cell membranes due to microbubble oscillations, enabling efficient uptake of drugs and nucleic acids while preserving cell viability. Focused ultrasound combined with intravenously administered microbubbles has emerged as a non-invasive method to temporarily disrupt the blood-brain barrier (BBB), facilitating drug penetration into the . Clinical s initiated in the 2010s and continuing through 2025 have validated its safety for applications in neurodegenerative diseases, including Alzheimer's, where repeated BBB opening in targeted brain regions improved amyloid plaque clearance and showed potential cognitive benefits without significant adverse effects; a 2022-2023 published in 2025 reported amyloid reduction in 4 of 6 participants and neuropsychiatric symptom improvement in 5 of 6. In , ultrasound-enhanced delivery augments efficacy by promoting and tumor penetration of agents like , as demonstrated in phase I trials for pancreatic and cancers, where microbubble increased drug accumulation and therapeutic response rates. For , sonoporation supports non-viral vector delivery, such as plasmids, by enhancing cellular efficiency and reducing compared to viral methods, with preclinical studies showing sustained in targeted tissues. Optimal ultrasound parameters for cavitation-mediated delivery typically include frequencies of 0.5–2 MHz and a exceeding 0.3, which initiates stable microbubble oscillations leading to bioeffects like pore formation without excessive tissue damage. Recent 2020s research has advanced hybrid systems, such as porphyrin-based agents responsive to both optical and ultrasound stimuli, allowing spatiotemporal control over drug release through combined photoacoustic and mechanisms for precise theranostics.

High-Intensity Focused Ultrasound (HIFU)

High-intensity focused ultrasound (HIFU) represents a non-invasive therapeutic modality that concentrates acoustic at a precise focal point within the body to achieve tissue ablation, offering an alternative to traditional surgical interventions for various conditions. The technique relies on focusing mechanisms such as geometric focusing with single-element concave transducers, which direct ultrasound waves to a fixed , or electronic focusing using multi-element phased arrays that enable dynamic and shaping for targeting irregular volumes. These methods achieve intensities greater than 1000 W/cm² at the focal point—typically 1–3 mm in width and up to 10 mm in length—while maintaining low intensities in the propagation path to protect intervening tissues. The core therapeutic effect of HIFU stems from thermal , wherein the focal energy absorption rapidly heats the target tissue to 60–100°C for 1–5 seconds, leading to irreversible through protein , cell fusion, and vascular stasis. This process confines the ablation to a discrete ellipsoidal , with exposure durations calibrated to ensure complete without excessive mechanical disruption. Frequencies between 0.8–3.5 MHz are commonly used, balancing and focal precision for deep-seated targets. Clinically, HIFU has gained regulatory approval for ablating prostate tissue in treatment, with the FDA clearing devices like the Sonablate 450 in 2015 and the Exablate Prostate in 2021; for uterine fibroids, the Exablate 2000 received approval in 2004, enabling symptom relief in patients with leiomyomas up to 13 cm; for , the Exablate Neuro was approved in 2016, with expansion to staged bilateral treatments in December 2022; as of November 2025, additional approvals include staged bilateral treatment for in July 2025 and noninvasive treatment of liver tumors with the HistoSonics Edison system in October 2023. These approvals highlight HIFU's role in outpatient procedures, often reducing recovery time compared to invasive options. Precision in HIFU delivery is enhanced by real-time (MRI) guidance, which provides high-resolution 3D anatomical localization for treatment planning and proton shift-based thermometry to monitor focal maps dynamically. This feedback allows operators to adjust sonication parameters—such as power and duration—to sustain 60–65°C in the target while preventing or overheating in adjacent areas, achieving non-perfused volumes exceeding 60% for effective . Despite its advantages, HIFU carries risks including skin burns from prolonged energy coupling at the entry site and unintended effects, where inertial bubbles can cause mechanical damage or hemorrhage beyond the focus. To mitigate these, models simulate acoustic propagation, heat deposition, and to predict size and optimize exposure protocols, ensuring safe energy delivery tailored to patient anatomy.

Industrial and Processing Uses

Cleaning and Disintegration

Ultrasound facilitates cleaning by inducing acoustic in liquids, where microscopic bubbles form, grow, and collapse violently, generating localized extreme conditions that dislodge contaminants from surfaces without mechanical contact. The collapse of these cavitation bubbles produces transient hot spots with temperatures reaching approximately 5000 K and pressures up to 1000 , creating microjets and shock waves that effectively remove dirt, oils, and residues. This process is particularly advantageous for delicate items, as it minimizes physical abrasion while penetrating complex geometries. For aggressive cleaning tasks, low-frequency ultrasound in the 20-40 kHz range is commonly employed, as it promotes larger bubbles and stronger implosions suitable for robust contaminant removal. Applications include jewelry, optical lenses, watches, and medical instruments, where this frequency range ensures thorough while preserving material integrity. In disintegration processes, ultrasound disrupts biological materials through cavitation-induced shear forces, enabling in applications such as from microbial or plant cells. Frequencies between 20 and 100 kHz are typically used, as they generate sufficient to rupture cell walls and membranes, releasing intracellular contents efficiently without excessive heat buildup. Ultrasonic humidifiers operate on a different , utilizing higher around 1.7 MHz to generate fine mists through the excitation of capillary waves on liquid surfaces, which break into droplets without relying on . This method produces aerosolized water for air humidification, with the determining droplet size for optimal dispersion. Efficiency in ultrasonic cleaning and disintegration depends on factors such as , typically ranging from 1 to 10 W/L to balance energy input with intensity, and the need for the cleaning solution to remove dissolved gases that could otherwise inhibit bubble formation. , often achieved by initial low-power or , ensures consistent performance and prevents uneven cleaning.

Welding and Manipulation

Ultrasonic welding is a solid-state joining process that utilizes high-frequency acoustic vibrations, typically at 20 kHz, to generate frictional heat at the interface of materials such as thermoplastics and metals, enabling fusion without the need for fillers, fluxes, or external heating. The vibrations cause localized molecular agitation, the contact surfaces under applied pressure to form strong bonds upon cooling, with the process occurring in seconds due to the rapid energy input. This method is particularly effective for dissimilar materials, as it minimizes distortion and preserves material properties. In industrial applications, is widely used for assembling automotive components, such as wiring harnesses and housings, and for packaging , including battery tabs and circuit board enclosures, where cycle times often fall below 1 second to support high-volume production. These short cycles enhance efficiency in automated lines, reducing energy consumption and enabling precise, clean joints that meet stringent quality standards in sectors demanding reliability and speed. Beyond joining, ultrasound facilitates particle manipulation through , where standing waves created by ultrasonic transducers suspend and position small particles or objects in a contactless manner, preventing in sensitive processes. This technique leverages from interfering sound waves to counteract gravity, allowing precise handling of droplets, powders, or delicate components in air. For characterization, ultrasonic spectra measure how sound waves are absorbed or scattered by suspensions, enabling non-invasive determination of distributions based on frequency-dependent patterns. Ultrasonic impact treatment, often termed , bombards metal surfaces with high-frequency vibrations from a sonotrode-mounted tool, inducing compressive residual stresses that enhance resistance by up to several fold in components like welds and parts. This surface modification refines microstructures and closes microcracks, improving durability under cyclic loading without altering bulk properties. Applications include treating and structures, where the process extends service life in high-stress environments.

Sonochemistry

Sonochemistry refers to the promotion or initiation of chemical reactions through the application of ultrasound in liquids, primarily driven by acoustic cavitation, where the formation, growth, and implosive collapse of microbubbles generate extreme localized conditions. These conditions include temperatures up to 5000 K and pressures exceeding 1000 atm within the bubbles, facilitating reactions that would otherwise require harsh conditions. The primary mechanisms in sonochemistry involve within the hot spots created during bubble collapse and interfacial reactions at the bubble-liquid boundary. occurs in the gas phase inside the collapsing bubbles, where volatile molecules decompose due to the intense heat, while interfacial reactions take place in the thin shell surrounding the bubbles, where radicals and solutes interact under elevated temperatures and pressures. In aqueous systems, sonolysis of exemplifies radical formation: the extreme conditions lead to the homolytic cleavage of molecules, producing radicals (H•) and hydroxyl radicals (OH•) as primary reactive . These radicals can then propagate chain reactions or directly oxidize substrates. Optimal ultrasonic frequencies for sonochemical typically range from 20 kHz to 500 kHz, as lower frequencies (around 20-100 kHz) promote violent bubble collapses for intense hot spots, while higher frequencies (up to 500 kHz) increase the number of events for broader reaction enhancement. Ultrasonic power influences yield by affecting intensity; higher power levels generally increase radical production and reaction rates, though excessive power can lead to diminished efficiency due to bubble coalescence. In , sonochemistry accelerates reactions such as the Diels-Alder cycloaddition, where ultrasound reduces reaction times from hours to minutes and improves yields by enhancing molecular mixing and activating dienes through cavitation-induced energy input. For , sonochemical processes degrade persistent organic pollutants, such as pharmaceuticals and dyes, via OH• radical attack, achieving up to 100% removal in some cases within 60 minutes by mineralizing contaminants into CO₂, H₂O, and inorganic ions. Sonochemistry aligns with principles by enabling solvent-free or reduced-solvent reactions, minimizing waste, and operating under milder conditions than traditional heating, thus lowering energy consumption. Industrial adoption has grown since the 2010s, with scaled ultrasonic reactors implemented for processes like synthesis and effluent remediation, as evidenced by commercial systems achieving higher throughput and metrics by 2025.

Communication and Identification

Underwater and Wireless Communication

Ultrasound plays a critical role in communication, where in the ultrasonic range (typically above 20 kHz) enable data transmission over distances unattainable by radio frequencies due to water's high of electromagnetic signals. In , low frequencies between 20 and 100 kHz are employed for long-range communication spanning kilometers, as higher frequencies suffer greater absorption and , limiting their effective . This range balances bandwidth needs with minimal , allowing signals to travel from surface vessels to submerged assets like autonomous vehicles (AUVs). Modulation techniques such as (FSK) and (PSK) are commonly used in ultrasonic modems for AUVs and other subsea devices, enabling robust links in noisy environments. FSK, often non-coherent, supports rates around 5 kbps in the 20-30 kHz band, while coherent PSK schemes like 4-PSK can achieve up to 7-10 kbps over 1-2 km under favorable conditions. These methods encode digital information onto carrier waves, with experimental systems demonstrating reliable transmission for in AUV operations. Key applications include monitoring, where ultrasonic modems relay from seafloor instruments to research vessels, and diver communication systems that transmit voice or text alerts over hundreds of meters using wrist-worn transceivers at 25-33 kHz. However, underwater ultrasonic communication faces significant limitations, including multipath interference from surface and bottom reflections, which causes (ISI) with delays up to 10 ms, and frequency-dependent absorption in , approximated by α ≈ 0.1 dB/km per kHz, leading to rapid signal loss at higher frequencies. Absorption arises primarily from molecular relaxation processes involving , , and water viscosity, exacerbating attenuation in warm, saline conditions. In contrast, ultrasonic wireless communication in air operates over much shorter ranges due to rapid atmospheric attenuation, typically using 40 kHz carriers for applications in Internet of Things (IoT) sensor networks. At this frequency, signals propagate effectively up to 10-20 meters in indoor environments, supporting low-power, directional links for device pairing or asset tracking without interfering with audible sound or radio spectra. Data rates remain modest, around 100 bps with frequency-shift keying (FSK) modulation, suitable for transmitting sensor readings like proximity or environmental data in smart homes or medical monitoring. Limitations include severe signal decay beyond a few meters from air's viscosity and humidity effects, as well as multipath from room reflections, necessitating line-of-sight setups for reliable performance.

Ultrasound Identification Systems

Ultrasound identification systems (USID) utilize passive ultrasonic tags to enable non-contact tracking and of objects in various environments. These tags operate by reflecting or resonating with incoming ultrasonic waves, allowing interrogators to detect unique signatures without requiring onboard power sources. Unlike traditional (RFID) systems, USID leverages mechanical sound waves, which propagate effectively through challenging media such as liquids and metals. Passive tags in USID typically consist of resonators tuned to ultrasonic frequencies or acoustic reflectors. Resonators can be implemented as micromachined structures, such as membranes over cavities, operating in the 200–400 kHz range, or simple acoustic reflectors like holes of varying depths in a tag body, which produce distinct patterns based on their . When illuminated by the interrogator's signal, the tag backscatters modulated ultrasonic waves encoding identification data through frequency-specific resonances or reflections, enabling remote detection up to several meters in air or . This passive design ensures low cost and durability, with tags capable of supporting thousands of unique identifiers via combinations of resonant frequencies. Applications of USID include tracking in retail settings, where tags affixed to products allow rapid scanning of stock without line-of-sight requirements, and anti-counterfeiting measures, such as embedding tags in for high-value like wine bottles to verify authenticity through liquid-penetrating scans. For instance, resonators can confirm product integrity by detecting fill levels or unique material signatures non-invasively. These systems enhance security by providing tamper-evident identification that resists replication. USID offers distinct advantages over RFID, including greater resistance to cloning due to the physical precision required for resonant structures, which are difficult to duplicate without specialized fabrication, and superior performance through liquids, where RF signals attenuate rapidly while ultrasonic waves maintain . This makes USID particularly suitable for moist or metallic environments common in retail and . Additionally, tags are less susceptible to , ensuring reliable operation in industrial settings. USID readers incorporate techniques to resolve multiple tags simultaneously, directing focused ultrasonic beams toward specific locations to minimize and improve . These systems use array transducers to steer and concentrate energy, allowing interrogation of distributed tags with centimeter-level accuracy, even in dense deployments. Such capabilities support scalable inventory audits by isolating individual responses from groups of items. Recent developments in the have explored ultrasonic technologies for proximity detection and communication using consumer smartphones, leveraging built-in microphones and speakers, though dedicated hardware remains primary for passive USID tag reading.

Safety and Biological Effects

Mechanisms of Interaction

Ultrasound interacts with biological tissues primarily through and mechanical mechanisms, which can lead to bioeffects depending on exposure parameters. effects arise from the absorption of acoustic , converting it into . The absorption (α) in soft tissues is typically around 0.5 dB/cm/MHz, resulting in that increases with and distance traveled. This absorption generates via the (SAR), defined as SAR = \frac{2 \alpha I}{\rho}, where I is the acoustic intensity (W/cm²), α is the absorption (in Nepers/cm, convertible from dB), and ρ is the tissue density (approximately 1 g/cm³). Prolonged exposure can elevate tissue temperature, potentially causing cellular damage if thresholds are exceeded. Mechanical effects stem from the pressure oscillations of the ultrasound wave, inducing —the formation, growth, and of gas bubbles in tissue fluids. Cavitation manifests in two main types: stable cavitation, characterized by sustained bubble oscillations that produce microstreaming (fluid flows at speeds up to several m/s around the bubble), and inertial (or transient) cavitation, involving rapid bubble that generates localized shock waves, high shear stresses, and temperatures exceeding 5000 momentarily. These processes can disrupt cell membranes or enhance permeability without significant heating. Non-thermal effects include acoustic , a steady force exerted by the propagating wave on particles or interfaces, leading to displacement of cells, contrast agents, or tissue components. This , proportional to the intensity gradient, enables targeted manipulation, such as pushing microbubbles toward vessel walls, without relying on thermal mechanisms. To mitigate bioeffects in diagnostic applications, exposure is limited by indices like the (MI), which predicts inertial risk and is kept below 1.9, and the thermal index (TI), indicating potential temperature rise and maintained under 1 to avoid significant heating. These thresholds ensure safe operation while allowing imaging. Absorption varies across tissue types, with higher coefficients in muscle (around 0.6–1.0 dB/cm/MHz for non-fatty tissues) compared to (about 0.4 dB/cm/MHz), influencing local heating patterns and wave propagation.

Guidelines and Regulations

Guidelines and regulations for ultrasound safety aim to minimize potential biological effects by limiting acoustic exposure while ensuring diagnostic efficacy. The primary principle guiding these is ALARA (As Low As Reasonably Achievable), which emphasizes using the lowest output settings necessary for image quality. This approach is endorsed by major organizations including the American Institute of Ultrasound in Medicine (AIUM), the Federation for Ultrasound in Medicine and Biology (WFUMB), and the (FDA). Regulatory frameworks are established by the FDA in the United States and the (IEC) internationally. The FDA regulates ultrasound devices as Class II medical devices requiring 510(k) premarket clearance, with acoustic output data documented in the to ensure compliance with exposure limits. The IEC standard 60601-2-37 specifies basic safety and essential performance requirements for ultrasonic diagnostic equipment, including measurement of acoustic fields and limits on pressure and intensity to prevent and mechanical hazards. Manufacturers must display real-time and mechanical indices on devices to inform users of potential risks. Safety indices include the (MI), which estimates the potential for (non-thermal effects), and the Thermal Index (TI), which predicts temperature rise based on tissue type ( [TIS], bone [TIB], or fluid [TIC]). For general diagnostic use, the FDA sets derated spatial-peak temporal-average intensity (I_SPTA.3) at ≤720 mW/cm² and MI ≤1.9 under Track 3 voluntary standards, with stricter limits for ophthalmic applications (MI ≤0.23, I_SPTA.3 ≤50 mW/cm²). WFUMB recommends keeping TI below 1.0 for most scans, up to 3.0 for peripheral vascular, and ≤0.7 as a default for obstetric imaging to protect the . Exposure durations should be minimized; for example, live B-mode scanning limited to 30 minutes, and pulsed Doppler to 5-10 minutes, especially in early . AIUM guidelines stress prudent use exclusively for medical benefit, prohibiting non-diagnostic applications like or keepsake imaging. In pregnancy, routine Doppler is discouraged in the first trimester unless clinically indicated, with TI ≤1.0 and exposure kept brief. For educational and research settings, exposures must adhere to conservative limits, with and no repetitive scanning on vulnerable subjects like pregnant participants. Neonatal and ophthalmic scans require extra caution due to higher sensitivity, with MI ≤0.3 recommended for ultrasound in infants.
ApplicationRecommended MI LimitRecommended TI LimitMax Exposure Time
General Diagnostic≤1.9<1.0 (soft tissue)30 minutes (B-mode)
Obstetric (default)≤1.9≤0.7Minimal, with pauses
Ophthalmic≤0.23≤1.0As short as possible
Fetal Doppler (11-14 weeks)≤1.9≤1.05-10 minutes
Neonatal Lung≤0.3<1.0Brief scans only
These limits establish scale for risk mitigation, derived from epidemiological and in vitro studies showing no confirmed adverse effects at compliant levels but potential for heating or cavitation above thresholds. Compliance is enforced through device labeling, user training, and periodic maintenance to verify output accuracy.

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