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Medical imaging
One frame of a CT scan of the chest showing the heart and lungs
ICD-10-PCSB
ICD-987-88
MeSH003952 D 003952
OPS-301 code3
MedlinePlus007451

Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to reveal internal structures hidden by the skin and bones, as well as to diagnose and treat disease. Medical imaging also establishes a database of normal anatomy and physiology to make it possible to identify abnormalities. Although imaging of removed organs and tissues can be performed for medical reasons, such procedures are usually considered part of pathology instead of medical imaging.[citation needed]

Measurement and recording techniques that are not primarily designed to produce images, such as electroencephalography (EEG), magnetoencephalography (MEG), electrocardiography (ECG), and others, represent other technologies that produce data susceptible to representation as a parameter graph versus time or maps that contain data about the measurement locations. In a limited comparison, these technologies can be considered forms of medical imaging in another discipline of medical instrumentation.

As of 2010, 5 billion medical imaging studies had been conducted worldwide.[1] Radiation exposure from medical imaging in 2006 made up about 50% of total ionizing radiation exposure in the United States.[2] Medical imaging equipment is manufactured using technology from the semiconductor industry, including CMOS integrated circuit chips, power semiconductor devices, sensors such as image sensors (particularly CMOS sensors) and biosensors, and processors such as microcontrollers, microprocessors, digital signal processors, media processors and system-on-chip devices. As of 2015, annual shipments of medical imaging chips amount to 46 million units and $1.1 billion.[3]

The term "noninvasive" is used to denote a procedure where no instrument is introduced into a patient's body, which is the case for most imaging techniques used.

History

[edit]
Further information: Image processing § History

In 1972, engineer Godfrey Hounsfield from the British company EMI invented the X-ray computed tomography device for head diagnosis, which is commonly referred to as computed tomography (CT). The CT nucleus method is based on the projecting X-rays through a section of the human head, which are then processed by computer to reconstruct the cross-sectional image, known as image reconstruction. In 1975, EMI successfully developed a CT device for the entire body, enabling the clear acquisition of tomographic images of various parts of the human body. This revolutionary diagnostic technique earned Hounsfield and physicist Allan Cormack the Nobel Prize in Physiology or Medicine in 1979.[4] Digital image processing technology for medical applications was inducted into the Space Foundation's Space Technology Hall of Fame in 1994.[5]

By 2010, over 5 billion medical imaging studies had been conducted worldwide.[6][7] Radiation exposure from medical imaging in 2006 accounted for about 50% of total ionizing radiation exposure in the United States.[8] Medical imaging equipment is manufactured using technology from the semiconductor industry, including CMOS integrated circuit chips, power semiconductor devices, sensors such as image sensors (particularly CMOS sensors) and biosensors, as well as processors like microcontrollers, microprocessors, digital signal processors, media processors and system-on-chip devices. As of 2015, annual shipments of medical imaging chips reached 46 million units, generating a market value of $1.1 billion.[9][10]

Types

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Plain X-ray of the wrist and hand

In the clinical context, "invisible light" medical imaging is generally equated to radiology or "clinical imaging". "Visible light" medical imaging involves digital video or still pictures that can be seen without special equipment. Dermatology and wound care are two modalities that use visible light imagery. Interpretation of medical images is generally undertaken by a physician specialising in radiology known as a radiologist; however, this may be undertaken by any healthcare professional who is trained and certified in radiological clinical evaluation. Increasingly interpretation is being undertaken by non-physicians, for example radiographers frequently train in interpretation as part of expanded practice. Diagnostic radiography designates the technical aspects of medical imaging and in particular the acquisition of medical images. The radiographer (also known as a radiologic technologist) is usually responsible for acquiring medical images of diagnostic quality; although other professionals may train in this area, notably some radiological interventions performed by radiologists are done so without a radiographer.[citation needed]

As a field of scientific investigation, medical imaging constitutes a sub-discipline of biomedical engineering, medical physics or medicine depending on the context: Research and development in the area of instrumentation, image acquisition (e.g., radiography), modeling and quantification are usually the preserve of biomedical engineering, medical physics, and computer science; Research into the application and interpretation of medical images is usually the preserve of radiology and the medical sub-discipline relevant to medical condition or area of medical science (neuroscience, cardiology, psychiatry, psychology, etc.) under investigation. Many of the techniques developed for medical imaging also have scientific and industrial applications.[11]

Radiography

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Main article: Radiography

Two forms of radiographic images are in use in medical imaging. Projection radiography and fluoroscopy, with the latter being useful for catheter guidance. These 2D techniques are still in wide use despite the advance of 3D tomography due to the low cost, high resolution, and depending on the application, lower radiation dosages with 2D technique. This imaging modality uses a wide beam of X-rays for image acquisition and is the first imaging technique available in modern medicine.

  • Fluoroscopy produces real-time images of internal structures of the body in a similar fashion to radiography, but employs a constant input of X-rays, at a lower dose rate. Contrast media, such as barium, iodine, and air are used to visualize internal organs as they work. Fluoroscopy is also used in image-guided procedures when constant feedback during a procedure is required. An image receptor is required to convert the radiation into an image after it has passed through the area of interest. Early on, this was a fluorescing screen, which gave way to an Image Amplifier (IA) which was a large vacuum tube that had the receiving end coated with cesium iodide, and a mirror at the opposite end. Eventually the mirror was replaced with a TV camera.[citation needed]
  • Projectional radiographs, more commonly known as X-rays, are often used to determine the type and extent of a fracture as well as for detecting pathological changes in the lungs. With the use of radio-opaque contrast media, such as barium, they can also be used to visualize the structure of the stomach and intestines – this can help diagnose ulcers or certain types of colon cancer.[citation needed]

Magnetic resonance imaging

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Main article: Magnetic resonance imaging
One frame of an MRI scan of the head showing the eyes and brain

A magnetic resonance imaging instrument (MRI scanner), or "nuclear magnetic resonance (NMR) imaging" scanner as it was originally known, uses powerful magnets to polarize and excite hydrogen nuclei (i.e., single protons) of water molecules in human tissue, producing a detectable signal which is spatially encoded, resulting in images of the body.[12] The MRI machine emits a radio frequency (RF) pulse at the resonant frequency of the hydrogen atoms on water molecules. Radio frequency antennas ("RF coils") send the pulse to the area of the body to be examined. The RF pulse is absorbed by protons, causing their direction with respect to the primary magnetic field to change. When the RF pulse is turned off, the protons "relax" back to alignment with the primary magnet and emit radio-waves in the process. This radio-frequency emission from the hydrogen-atoms on water is what is detected and reconstructed into an image. The resonant frequency of a spinning magnetic dipole (of which protons are one example) is called the Larmor frequency and is determined by the strength of the main magnetic field and the chemical environment of the nuclei of interest. MRI uses three electromagnetic fields: a very strong (typically 1.5 to 3 teslas) static magnetic field to polarize the hydrogen nuclei, called the primary field; gradient fields that can be modified to vary in space and time (on the order of 1 kHz) for spatial encoding, often simply called gradients; and a spatially homogeneous radio-frequency (RF) field for manipulation of the hydrogen nuclei to produce measurable signals, collected through an RF antenna.[citation needed]

Like CT, MRI traditionally creates a two-dimensional image of a thin "slice" of the body and is therefore considered a tomographic imaging technique. Modern MRI instruments are capable of producing images in the form of 3D blocks, which may be considered a generalization of the single-slice, tomographic, concept. Unlike CT, MRI does not involve the use of ionizing radiation and is therefore not associated with the same health hazards. For example, because MRI has only been in use since the early 1980s, there are no known long-term effects of exposure to strong static fields (this is the subject of some debate; see 'Safety' in MRI) and therefore there is no limit to the number of scans to which an individual can be subjected, in contrast with X-ray and CT. However, there are well-identified health risks associated with tissue heating from exposure to the RF field and the presence of implanted devices in the body, such as pacemakers. These risks are strictly controlled as part of the design of the instrument and the scanning protocols used.[citation needed]

Because CT and MRI are sensitive to different tissue properties, the appearances of the images obtained with the two techniques differ markedly. In CT, X-rays must be blocked by some form of dense tissue to create an image, so the image quality when looking at soft tissues will be poor. In MRI, while any nucleus with a net nuclear spin can be used, the proton of the hydrogen atom remains the most widely used, especially in the clinical setting, because it is so ubiquitous and returns a large signal. This nucleus, present in water molecules, allows the excellent soft-tissue contrast achievable with MRI.[13][citation needed]

A number of different pulse sequences can be used for specific MRI diagnostic imaging (multiparametric MRI or mpMRI). It is possible to differentiate tissue characteristics by combining two or more of the following imaging sequences, depending on the information being sought: T1-weighted (T1-MRI), T2-weighted (T2-MRI), diffusion weighted imaging (DWI-MRI), dynamic contrast enhancement (DCE-MRI), and spectroscopy (MRI-S). For example, imaging of prostate tumors is better accomplished using T2-MRI and DWI-MRI than T2-weighted imaging alone.[14] The number of applications of mpMRI for detecting disease in various organs continues to expand, including liver studies, breast tumors, pancreatic tumors, and assessing the effects of vascular disruption agents on cancer tumors.[15][16][17]

Nuclear medicine

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Main article: Nuclear medicine

Nuclear medicine encompasses both diagnostic imaging and treatment of disease, and may also be referred to as molecular medicine or molecular imaging and therapeutics.[18] Nuclear medicine uses certain properties of isotopes and the energetic particles emitted from radioactive material to diagnose or treat various pathology. Different from the typical concept of anatomic radiology, nuclear medicine enables assessment of physiology. This function-based approach to medical evaluation has useful applications in most subspecialties, notably oncology, neurology, and cardiology. Gamma cameras and PET scanners are used in e.g. scintigraphy, SPECT and PET to detect regions of biologic activity that may be associated with a disease. Relatively short-lived isotope, such as 99mTc is administered to the patient. Isotopes are often preferentially absorbed by biologically active tissue in the body, and can be used to identify tumors or fracture points in bone. Images are acquired after collimated photons are detected by a crystal that gives off a light signal, which is in turn amplified and converted into count data.

  • Scintigraphy ("scint") is a form of diagnostic test wherein radioisotopes are taken internally, for example, intravenously or orally. Then, gamma cameras capture and form two-dimensional[19] images from the radiation emitted by the radiopharmaceuticals.
  • SPECT is a 3D tomographic technique that uses gamma camera data from many projections and can be reconstructed in different planes. A dual detector head gamma camera combined with a CT scanner, which provides localization of functional SPECT data, is termed a SPECT-CT camera, and has shown utility in advancing the field of molecular imaging. In most other medical imaging modalities, energy is passed through the body and the reaction or result is read by detectors. In SPECT imaging, the patient is injected with a radioisotope, most commonly Thallium 201TI, Technetium 99mTC, Iodine 123I, and Gallium 67Ga.[20] The radioactive gamma rays are emitted through the body as the natural decaying process of these isotopes takes place. The emissions of the gamma rays are captured by detectors that surround the body. This essentially means that the human is now the source of the radioactivity, rather than the medical imaging devices such as X-ray or CT.
  • Positron emission tomography (PET) uses coincidence detection to image functional processes. Short-lived positron emitting isotope, such as 18F, is incorporated with an organic substance such as glucose, creating F18-fluorodeoxyglucose, which can be used as a marker of metabolic utilization. Images of activity distribution throughout the body can show rapidly growing tissue, like tumor, metastasis, or infection. PET images can be viewed in comparison to computed tomography scans to determine an anatomic correlate. Modern scanners may integrate PET, allowing PET-CT, or PET-MRI to optimize the image reconstruction involved with positron imaging. This is performed on the same equipment without physically moving the patient off of the gantry. The resultant hybrid of functional and anatomic imaging information is a useful tool in non-invasive diagnosis and patient management.

Fiduciary markers are used in a wide range of medical imaging applications. Images of the same subject produced with two different imaging systems may be correlated (called image registration) by placing a fiduciary marker in the area imaged by both systems. In this case, a marker which is visible in the images produced by both imaging modalities must be used. By this method, functional information from SPECT or positron emission tomography can be related to anatomical information provided by magnetic resonance imaging (MRI).[21] Similarly, fiducial points established during MRI can be correlated with brain images generated by magnetoencephalography to localize the source of brain activity.

Ultrasound

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Main article: Medical ultrasound
Ultrasound image showing the liver, gallbladder and common bile duct.

Medical ultrasound uses high frequency broadband sound waves in the megahertz range that are reflected by tissue to varying degrees to produce (up to 3D) images. This is commonly associated with imaging the fetus in pregnant women. Uses of ultrasound are much broader, however. Other important uses include imaging the abdominal organs, heart, breast, muscles, tendons, arteries and veins. While it may provide less anatomical detail than techniques such as CT or MRI, it has several advantages which make it ideal in numerous situations, in particular that it studies the function of moving structures in real-time, emits no ionizing radiation, and contains speckle that can be used in elastography. Ultrasound is also used as a popular research tool for capturing raw data, that can be made available through an ultrasound research interface, for the purpose of tissue characterization and implementation of new image processing techniques. The concepts of ultrasound differ from other medical imaging modalities in the fact that it is operated by the transmission and receipt of sound waves. The high frequency sound waves are sent into the tissue and depending on the composition of the different tissues; the signal will be attenuated and returned at separate intervals. A path of reflected sound waves in a multilayered structure can be defined by an input acoustic impedance (ultrasound sound wave) and the Reflection and transmission coefficients of the relative structures.[20] It is very safe to use and does not appear to cause any adverse effects. It is also relatively inexpensive and quick to perform. Ultrasound scanners can be taken to critically ill patients in intensive care units, avoiding the danger caused while moving the patient to the radiology department. The real-time moving image obtained can be used to guide drainage and biopsy procedures. Doppler capabilities on modern scanners allow the blood flow in arteries and veins to be assessed.

Elastography

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Main article: Elastography

Elastography is a relatively new imaging modality that maps the elastic properties of soft tissue. This modality emerged in the last two decades. Elastography is useful in medical diagnoses, as elasticity can discern healthy from unhealthy tissue for specific organs/growths. For example, cancerous tumours will often be harder than the surrounding tissue, and diseased livers are stiffer than healthy ones.[22][23][24][25] There are several elastographic techniques based on the use of ultrasound, magnetic resonance imaging and tactile imaging. The wide clinical use of ultrasound elastography is a result of the implementation of technology in clinical ultrasound machines. Main branches of ultrasound elastography include Quasistatic Elastography/Strain Imaging, Shear Wave Elasticity Imaging (SWEI), Acoustic Radiation Force Impulse imaging (ARFI), Supersonic Shear Imaging (SSI), and Transient Elastography.[23] In the last decade, a steady increase of activities in the field of elastography is observed demonstrating successful application of the technology in various areas of medical diagnostics and treatment monitoring.

Photoacoustic imaging

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Main article: Photoacoustic imaging in biomedicine

Photoacoustic imaging is a recently developed hybrid biomedical imaging modality based on the photoacoustic effect. It combines the advantages of optical absorption contrast with an ultrasonic spatial resolution for deep imaging in (optical) diffusive or quasi-diffusive regime. Recent studies have shown that photoacoustic imaging can be used in vivo for tumor angiogenesis monitoring, blood oxygenation mapping, functional brain imaging, and skin melanoma detection, etc.

Tomography

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Basic principle of tomography: superposition free tomographic cross sections S1 and S2 compared with the (not tomographic) projected image P

Tomography is the imaging by sections or sectioning. The main such methods in medical imaging are:

  • X-ray computed tomography (CT), or Computed Axial Tomography (CAT) scan, is a helical tomography technique (latest generation), which traditionally produces a 2D image of the structures in a thin section of the body. In CT, a beam of X-rays spins around an object being examined and is picked up by sensitive radiation detectors after having penetrated the object from multiple angles. A computer then analyses the information received from the scanner's detectors and constructs a detailed image of the object and its contents using the mathematical principles laid out in the Radon transform. It has a greater ionizing radiation dose burden than projection radiography; repeated scans must be limited to avoid health effects. CT is based on the same principles as X-ray projections but in this case, the patient is enclosed in a surrounding ring of detectors assigned with 500–1000 scintillation detectors[20] (fourth-generation X-ray CT scanner geometry). Previously in older generation scanners, the X-ray beam was paired by a translating source and detector. Computed tomography has almost completely replaced focal plane tomography in X-ray tomography imaging.
  • Positron emission tomography (PET) also used in conjunction with computed tomography, PET-CT, and magnetic resonance imaging PET-MRI.
  • Magnetic resonance imaging (MRI) commonly produces tomographic images of cross-sections of the body. (See separate MRI section in this article.)

Echocardiography

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Main article: Echocardiography

When ultrasound is used to image the heart it is referred to as an echocardiogram. Echocardiography allows detailed structures of the heart, including chamber size, heart function, the valves of the heart, as well as the pericardium (the sac around the heart) to be seen. Echocardiography uses 2D, 3D, and Doppler imaging to create pictures of the heart and visualize the blood flowing through each of the four heart valves. Echocardiography is widely used in an array of patients ranging from those experiencing symptoms, such as shortness of breath or chest pain, to those undergoing cancer treatments. Transthoracic ultrasound has been proven to be safe for patients of all ages, from infants to the elderly, without risk of harmful side effects or radiation, differentiating it from other imaging modalities. Echocardiography is one of the most commonly used imaging modalities in the world due to its portability and use in a variety of applications. In emergency situations, echocardiography is quick, easily accessible, and able to be performed at the bedside, making it the modality of choice for many physicians.

Functional near-infrared spectroscopy

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Main article: Functional near-infrared spectroscopy

FNIR Is a relatively new non-invasive imaging technique. NIRS (near infrared spectroscopy) is used for the purpose of functional neuroimaging and has been widely accepted as a brain imaging technique.[26]

Magnetic particle imaging

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Main article: Magnetic particle imaging

Using superparamagnetic iron oxide nanoparticles, magnetic particle imaging (MPI) is a developing diagnostic imaging technique used for tracking superparamagnetic iron oxide nanoparticles. The primary advantage is the high sensitivity and specificity, along with the lack of signal decrease with tissue depth. MPI has been used in medical research to image cardiovascular performance, neuroperfusion, and cell tracking.

Industry

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Organizations in the medical imaging industry include manufacturers of imaging equipment, freestanding radiology facilities, and hospitals.

The global market for manufactured devices was estimated at $5 billion in 2018.[27][28] Notable manufacturers included Fujifilm, GE HealthCare, Siemens Healthineers, Philips, Shimadzu, Canon, Carestream Health, Hologic, and Esaote.[29] In 2016, the manufacturing industry was characterized as oligopolistic and mature; new entrants included in Samsung and Neusoft Medical.[30] In 2024, Fischer MVL in India began manufacturing MRI machines.[31]

In 2016, Toshiba exited the industry by selling its medical imaging division to Canon, which was ultimately renamed to Canon.[32] In 2019, Hitachi exited the industry by selling its business to Fujifilm for about $1.6 billion.[33] The simpler x-ray machines were being commoditized by 1998, when Kodak had about 30% market share globally;[34] Kodak later sold its medical imaging business in 2007[35] and the business was ultimately renamed to Carestream Health. In the 1970s, CT scanners were introduced, followed by MRI machines in the 1980s, with GE leading in both.[36]: 79  Digital radiography has replaced older computed radiography over time, which reduces radiation doses.[37]

In the United States, as estimate as of 2015 places the US market for imaging scans at about $100b, with 60% occurring in hospitals and 40% occurring in freestanding clinics, such as the RadNet chain.[38]

In pregnancy

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CT scanning (volume rendered in this case) confers a radiation dose to the developing fetus.
Main article: Medical imaging in pregnancy

Medical imaging may be indicated in pregnancy because of pregnancy complications, a pre-existing disease or an acquired disease in pregnancy, or routine prenatal care. Magnetic resonance imaging (MRI) without MRI contrast agents as well as obstetric ultrasonography are not associated with any risk for the mother or the fetus, and are the imaging techniques of choice for pregnant women.[39] Projectional radiography, CT scan and nuclear medicine imaging result some degree of ionizing radiation exposure, but have with a few exceptions much lower absorbed doses than what are associated with fetal harm.[39] At higher dosages, effects can include miscarriage, birth defects and intellectual disability.[39]

Maximizing imaging procedure use

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The amount of data obtained in a single MR or CT scan is very extensive. Some of the data that radiologists discard could save patients time and money, while reducing their exposure to radiation and risk of complications from invasive procedures.[40] Another approach for making the procedures more efficient is based on utilizing additional constraints, e.g., in some medical imaging modalities one can improve the efficiency of the data acquisition by taking into account the fact the reconstructed density is positive.[41][42]

Creation of three-dimensional images

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Volume rendering techniques have been developed to enable CT, MRI and ultrasound scanning software to produce 3D images for the physician.[43] Traditionally CT and MRI scans produced 2D static output on film. To produce 3D images, many scans are made and then combined by computers to produce a 3D model, which can then be manipulated by the physician. 3D ultrasounds are produced using a somewhat similar technique. In diagnosing disease of the viscera of the abdomen, ultrasound is particularly sensitive on imaging of biliary tract, urinary tract and female reproductive organs (ovary, fallopian tubes). As for example, diagnosis of gallstone by dilatation of common bile duct and stone in the common bile duct. With the ability to visualize important structures in great detail, 3D visualization methods are a valuable resource for the diagnosis and surgical treatment of many pathologies. It was a key resource for the famous, but ultimately unsuccessful attempt by Singaporean surgeons to separate Iranian twins Ladan and Laleh Bijani in 2003. The 3D equipment was used previously for similar operations with great success.

Other proposed or developed techniques include:

Some of these techniques[example needed] are still at a research stage and not yet used in clinical routines.

Non-diagnostic imaging

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Neuroimaging has also been used in experimental circumstances to allow people (especially disabled persons) to control outside devices, acting as a brain computer interface.

Many medical imaging software applications are used for non-diagnostic imaging, specifically because they do not have an FDA approval[44] and not allowed to use in clinical research for patient diagnosis.[45] Note that many clinical research studies are not designed for patient diagnosis anyway.[46]

Archiving and recording

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Further information: Picture archiving and communication system

Used primarily in ultrasound imaging, capturing the image produced by a medical imaging device is required for archiving and telemedicine applications. In most scenarios, a frame grabber is used in order to capture the video signal from the medical device and relay it to a computer for further processing and operations.[47]

DICOM

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The Digital Imaging and Communication in Medicine (DICOM) Standard is used globally to store, exchange, and transmit medical images. The DICOM Standard incorporates protocols for imaging techniques such as radiography, computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and radiation therapy.[48]

Compression of medical images

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Medical imaging techniques produce very large amounts of data, especially from CT, MRI and PET modalities. As a result, storage and communications of electronic image data are prohibitive without the use of compression.[49][50] JPEG 2000 image compression is used by the DICOM standard for storage and transmission of medical images. The cost and feasibility of accessing large image data sets over low or various bandwidths are further addressed by use of another DICOM standard, called JPIP, to enable efficient streaming of the JPEG 2000 compressed image data.

Medical imaging in the cloud

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There has been growing trend to migrate from on-premise PACS to a cloud-based PACS. A recent article by Applied Radiology said, "As the digital-imaging realm is embraced across the healthcare enterprise, the swift transition from terabytes to petabytes of data has put radiology on the brink of information overload. Cloud computing offers the imaging department of the future the tools to manage data much more intelligently."[51]

Use in pharmaceutical clinical trials

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Medical imaging has become a major tool in clinical trials since it enables rapid diagnosis with visualization and quantitative assessment.

A typical clinical trial goes through multiple phases and can take up to eight years. Clinical endpoints or outcomes are used to determine whether the therapy is safe and effective. Once a patient reaches the endpoint, he or she is generally excluded from further experimental interaction. Trials that rely solely on clinical endpoints are very costly as they have long durations and tend to need large numbers of patients.

In contrast to clinical endpoints, surrogate endpoints have been shown to cut down the time required to confirm whether a drug has clinical benefits. Imaging biomarkers (a characteristic that is objectively measured by an imaging technique, which is used as an indicator of pharmacological response to a therapy) and surrogate endpoints have shown to facilitate the use of small group sizes, obtaining quick results with good statistical power.[52]

Imaging is able to reveal subtle change that is indicative of the progression of therapy that may be missed out by more subjective, traditional approaches. Statistical bias is reduced as the findings are evaluated without any direct patient contact.

Imaging techniques such as positron emission tomography (PET) and magnetic resonance imaging (MRI) are routinely used in oncology and neuroscience areas.[53][54][55][56] For example, measurement of tumour shrinkage is a commonly used surrogate endpoint in solid tumour response evaluation. This allows for faster and more objective assessment of the effects of anticancer drugs. In Alzheimer's disease, MRI scans of the entire brain can accurately assess the rate of hippocampal atrophy,[57][58] while PET scans can measure the brain's metabolic activity by measuring regional glucose metabolism,[52] and beta-amyloid plaques using tracers such as Pittsburgh compound B (PiB). Historically less use has been made of quantitative medical imaging in other areas of drug development although interest is growing.[59]

An imaging-based trial will usually be made up of three components:

  1. A realistic imaging protocol. The protocol is an outline that standardizes (as far as practically possible) the way in which the images are acquired using the various modalities (PET, SPECT, CT, MRI). It covers the specifics in which images are to be stored, processed and evaluated.
  2. An imaging centre that is responsible for collecting the images, perform quality control and provide tools for data storage, distribution and analysis. It is important for images acquired at different time points are displayed in a standardised format to maintain the reliability of the evaluation. Certain specialised imaging contract research organizations provide end to end medical imaging services, from protocol design and site management through to data quality assurance and image analysis.
  3. Clinical sites that recruit patients to generate the images to send back to the imaging centre.

Risks and safety issues

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Medical imaging can lead to patient and healthcare provider harm through exposure to ionizing radiation, iodinated contrast, magnetic fields, and other hazards.[60]

Lead is the main material used for radiographic shielding against scattered X-rays.

In magnetic resonance imaging, there is MRI RF shielding as well as magnetic shielding to prevent external disturbance of image quality.[61]

Privacy protection

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Medical imaging are generally covered by laws of medical privacy. For example, in the United States the Health Insurance Portability and Accountability Act (HIPAA) sets restrictions for health care providers on utilizing protected health information, which is any individually identifiable information relating to the past, present, or future physical or mental health of any individual.[62] While there has not been any definitive legal decision in the matter, many studies have indicated that medical imaging contain biometric information that can uniquely identify a person, and as such qualify as PHI and/or special categories of personal data.[63][64][65][66][67][68]

The UK General Medical Council's ethical guidelines indicate that the Council does not require consent prior to making recordings of X-ray images.[69] However, the same guidance indicates that the images and recordings need to be anonimized, and acknowledges that in deciding whether a recording is anonymised, one should bear in mind that apparently insignificant details may still be capable of identifying a patient. As such, one should be particularly careful about the anonymity of a recordings of an X-ray image before using or publishing them without consent in journals and other learning materials, whether they are printed or in an electronic format.[70]

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United States

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As per chapter 300 of the Compendium of U.S. Copyright Office Practices, "the Office will not register works produced by a machine or mere mechanical process that operates randomly or automatically without any creative input or intervention from a human author" including "Medical imaging produced by X-rays, ultrasounds, magnetic resonance imaging, or other diagnostic equipment."[71] This position differs from the broad copyright protections afforded to photographs. While the Copyright Compendium is an agency statutory interpretation and not legally binding, courts are likely to give deference to it if they find it reasonable.[72] Yet, there is no U.S. federal case law directly addressing the issue of the copyrightability of X-ray images.

Derivatives

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In a derivative of a medical image created in the U.S., added annotations and explanations may be copyrightable, but the medical image itself remains public domain.

An extensive definition of the term derivative work is given by the United States Copyright Act in 17 U.S.C. § 101:

A "derivative work" is a work based upon one or more preexisting works, such as a translation...[note 1] art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a "derivative work".

17 U.S.C. § 103(b) provides:

The copyright in a compilation or derivative work extends only to the material contributed by the author of such work, as distinguished from the preexisting material employed in the work, and does not imply any exclusive right in the preexisting material. The copyright in such work is independent of, and does not affect or enlarge the scope, duration, ownership, or subsistence of, any copyright protection in the preexisting material.

Germany

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In Germany, X-ray images as well as MRI, medical ultrasound, PET and scintigraphy images are protected by (copyright-like) related rights or neighbouring rights.[73] This protection does not require creativity (as would be necessary for regular copyright protection) and lasts only for 50 years after image creation, if not published within 50 years, or for 50 years after the first legitimate publication.[74] The letter of the law grants this right to the "Lichtbildner",[75] i.e. the person who created the image. The literature seems to uniformly consider the medical doctor, dentist or veterinary physician as the rights holder, which may result from the circumstance that in Germany many X-rays are performed in ambulatory settings.

United Kingdom

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Medical images created in the United Kingdom will normally be protected by copyright due to "the high level of skill, labour and judgement required to produce a good quality X-ray, particularly to show contrast between bones and various soft tissues".[76] The Society of Radiographers believe this copyright is owned by employer (unless the radiographer is self-employed—though even then their contract might require them to transfer ownership to the hospital). This copyright owner can grant certain permissions to whoever they wish, without giving up their ownership of the copyright. So the hospital and its employees will be given permission to use such radiographic images for the various purposes that they require for medical care. Physicians employed at the hospital will, in their contracts, be given the right to publish patient information in journal papers or books they write (providing they are made anonymous). Patients may also be granted permission to "do what they like with" their own images.

Sweden

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The Cyber Law in Sweden states: "Pictures can be protected as photographic works or as photographic pictures. The former requires a higher level of originality; the latter protects all types of photographs, also the ones taken by amateurs, or within medicine or science. The protection requires some sort of photographic technique being used, which includes digital cameras as well as holograms created by laser technique. The difference between the two types of work is the term of protection, which amounts to seventy years after the death of the author of a photographic work as opposed to fifty years, from the year in which the photographic picture was taken."[77]

Medical imaging may possibly be included in the scope of "photography", similarly to a U.S. statement that "MRI images, CT scans, and the like are analogous to photography."[78]

See also

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Explanatory notes

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References

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Further reading

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Medical imaging

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Medical imaging encompasses a range of technologies that create visual representations of the interior structures and functions of the to facilitate clinical analysis, , monitoring, and treatment of medical conditions. It allows healthcare providers to visualize tissues, organs, and physiological processes non-invasively or minimally invasively, revealing abnormalities such as tumors, fractures, or infections that may not be detectable through alone. The origins of medical imaging trace back to 1895, when Wilhelm Conrad Röntgen discovered X-rays, enabling the first radiographic images of the human body and laying the foundation for diagnostic imaging technology. Over the subsequent decades, the field evolved rapidly: was explored for medical use in the , computed (CT) emerged in the 1970s for cross-sectional imaging, (MRI) was developed in the 1980s utilizing magnetic fields and radio waves, and techniques advanced in the late . This progression from analog to digital methods has transformed medical imaging into a cornerstone of modern healthcare, integrating with electronic health records and for enhanced precision. Central to these modalities is the use of waves—acoustic for ultrasound and electromagnetic for X-ray, CT, and MRI—enabling non-invasive visualization of internal structures. Key modalities in medical imaging include X-ray radiography, which uses to produce two-dimensional images excelling in the visualization of bones (e.g., fractures), chest structures (lungs, heart), abdomen (for issues like bowel obstruction), calcifications, and foreign bodies; it is quick, inexpensive, and widely available but provides poorer contrast for soft tissues and involves exposure to ionizing radiation; CT scanning, which combines X-rays with computer processing for detailed three-dimensional views; MRI, which employs strong magnetic fields and radiofrequency pulses to image soft tissues without radiation; ultrasound, relying on high-frequency sound waves for real-time imaging of soft tissues, organs, blood vessels, muscles, tendons, joints, thyroid, breast, and for monitoring pregnancy, offering the advantages of no ionizing radiation (making it safe for pregnant women and children), capability for guiding procedures, and visualization of moving structures or blood flow, though it is limited by interference from bone and air as well as operator dependence; and nuclear medicine scans like positron emission tomography (PET), which detect radioactive tracers to assess metabolic activity. Additional techniques, such as for dynamic imaging and for breast screening, expand its applications across specialties like , , and . Medical imaging plays a pivotal role in the healthcare continuum, from preventive screening and early disease detection to guiding interventions and evaluating treatment efficacy. It supports non-invasive diagnostics for conditions ranging from cardiovascular diseases to cancers, improving patient outcomes by enabling precise, timely decisions. However, modalities involving , such as X-rays and CT, carry risks of exposure, prompting regulatory oversight to ensure benefits outweigh potential harms through optimized protocols and dose reduction strategies.

History

Early developments

The discovery of X-rays is credited to German physicist Wilhelm Conrad Röntgen, who on November 8, 1895, observed that cathode rays from a high-voltage caused a nearby screen coated with barium platinocyanide to fluoresce, even when shielded from light. Röntgen conducted further experiments over the following weeks, determining that these unknown rays could penetrate materials opaque to light and produce images on photographic plates. He announced his findings in a preliminary report to the Physico-Medical on December 28, 1895, describing the rays' ability to pass through soft tissues while being absorbed by denser structures like bones. Röntgen's first medical application came shortly after, when he produced an image of his wife Anna Bertha's hand on December 22, 1895, revealing the bones and her ; this image, along with his detailed paper published in 1896, sparked immediate global interest in the potential for non-invasive internal visualization. Early photographic techniques for capturing images relied on existing silver halide emulsion plates, similar to those used in conventional , but exposed directly to the rays in a darkened room to avoid light interference; these plates required long exposure times of several minutes due to the low intensity of early X-ray sources. By around 1900, X-rays had become a standard tool for diagnosing fractures and locating foreign bodies, such as bullets or glass fragments embedded in tissues, as physicians worldwide reported their utility in confirming injuries that were difficult to assess through alone. These initial applications were particularly valuable in trauma cases, where images provided clear evidence of bone discontinuities or opaque intruders, reducing reliance on invasive surgical exploration. In 1896, American inventor Thomas Edison developed the first practical fluoroscope, a device using a calcium tungstate-coated screen that fluoresced under X-ray exposure, enabling real-time dynamic viewing of internal structures without the need for photographic development. Edison's innovation, tested extensively in his laboratory, allowed physicians to observe moving organs or guide procedures interactively, marking a significant step toward live medical imaging despite early concerns over radiation exposure.

20th century advancements

The marked a transformative era in medical imaging, building on the foundational X-ray techniques of the late to introduce modalities that enabled cross-sectional and functional visualization of the body. A pivotal early advancement occurred in with the invention of the in 1958 by Hal O. Anger at the , which allowed for real-time imaging of gamma-ray emissions from radiotracers, significantly improving the detection of organ function and disease distribution compared to prior rectilinear scanners. This device, also known as the Anger camera, incorporated a scintillator crystal, photomultiplier tubes, and collimators to produce two-dimensional images, laying the groundwork for subsequent tomographic techniques in the field. In the 1950s, ultrasound emerged as a non-ionizing method, pioneered by Scottish obstetrician Ian Donald, who adapted industrial technology for medical use and demonstrated its potential in visualizing abdominal masses and fetal development through his seminal 1958 publication in . Donald's work shifted ultrasound from (A-mode) displays to (B-mode) , enabling clearer anatomical depictions and establishing it as a safe, real-time tool initially focused on and gynecology. The 1960s and 1970s witnessed the rise of nuclear medicine's tomographic capabilities, with (SPECT) explored extensively by David E. Kuhl and colleagues starting in the early 1960s, using rotating cameras to reconstruct three-dimensional images from gamma emissions. Concurrently, (PET) advanced in the 1970s through the development of cyclotron-produced tracers like , enabling high-resolution of metabolic processes, with early whole-body systems operational by the mid-1970s. Computed tomography (CT) represented a breakthrough in 1971 when engineer at Laboratories constructed the first prototype scanner, which used computer algorithms to generate cross-sectional images, with the inaugural clinical scan performed on October 1, 1971, at Atkinson Morley Hospital in . By 1973, refined CT systems entered widespread clinical use, initially for and later adapted for whole-body applications, dramatically enhancing diagnostic precision over conventional . Magnetic resonance imaging (MRI) was introduced in the 1970s by . Lauterbur, who in 1973 published the first method for spatial encoding using magnetic field gradients to produce two-dimensional images, and by , who developed techniques for rapid image acquisition, including echo-planar imaging in 1977. Their foundational contributions, recognized with the 2003 in Physiology or Medicine, enabled non-invasive, high-contrast soft-tissue imaging without , revolutionizing diagnostics by the late .

21st century innovations

The early marked a pivotal shift in medical imaging from analog film-based systems to fully digital workflows, particularly with the widespread adoption of and picture archiving and communication systems (PACS). By the early , hospitals increasingly transitioned to computed radiography and direct , enabling immediate image capture and processing without physical films, which reduced processing times from hours to minutes and improved accessibility across departments. PACS, initially developed in the for cross-sectional modalities like CT and MRI, became integral by 2000-2005, allowing centralized storage, retrieval, and distribution of images via networks, thereby eliminating film libraries and supporting for remote consultations. This , accelerated by falling hardware costs and standardization efforts like , laid the groundwork for integrated healthcare imaging ecosystems. Advancements in (MRI) during the focused on higher field strengths and accelerated acquisition techniques to enhance resolution and reduce scan times. The introduction of 7T MRI scanners, approved for clinical use in and the around 2017, provided unprecedented signal-to-noise ratios for detailed and musculoskeletal imaging, enabling visualization of microstructures like cortical layers in the . , a mathematical framework for reconstructing images from undersampled data, emerged in the late and gained clinical traction in the , allowing up to 5-10 fold faster scans while maintaining diagnostic quality, as demonstrated in cardiac and abdominal protocols. These innovations built on 20th-century MRI foundations to address longstanding challenges in patient comfort and throughput. Artificial intelligence (AI) integration into medical imaging began gaining momentum in the early , with convolutional neural networks applied to tasks like lesion detection and segmentation around 2012, following breakthroughs in . By 2018, the US Food and Drug Administration (FDA) had approved the first AI-based tools for , such as software for automated of head CT scans to prioritize intracranial hemorrhages, marking a surge in clearances that reached over 100 by 2023. These tools improved workflow efficiency, with studies showing AI-assisted interpretations reducing radiologist reading times by 20-30% without compromising accuracy in chest X-rays and mammograms. In the 2020s, ultrasound imaging saw the rise of portable, handheld devices that democratized point-of-care diagnostics, particularly in resource-limited settings. Devices like the Butterfly iQ and Vscan Air, introduced around 2018-2020 and refined through the decade, offer wireless connectivity to smartphones and tablets, enabling rapid bedside assessments for emergencies like trauma or obstetrics. Concurrently, high-frequency transducers exceeding 20 MHz, such as the 46 MHz UHF probe launched in 2025, improved superficial imaging resolution for vascular and dermatological applications, achieving sub-millimeter detail comparable to optical methods. Hybrid imaging systems advanced significantly post-2010, combining functional and anatomical modalities for comprehensive diagnostics. The first commercial PET-MRI system, ' Biograph mMR, was introduced in 2010, integrating tomography's metabolic insights with MRI's soft-tissue contrast, which proved valuable for staging and , reducing the need for separate scans. By 2024, entered clinical trials for detection and vascular mapping, leveraging laser-induced waves to provide data with millimeter resolution and no , as evaluated in multicenter studies. As of 2025, emerging trends emphasize AI-driven multimodal integration and radiation reduction strategies. Multimodal AI platforms fuse data from CT, MRI, and to enhance predictive diagnostics, such as in tumor characterization, with models demonstrating improved specificity compared to single-modality approaches. reconstruction techniques in CT, like iterative denoising algorithms, enable low-dose protocols that cut by up to 80% while preserving image quality, as seen in photon-counting CT systems approved in 2021 and widely adopted by 2025. Prototype wearable imaging devices, including flexible patches for continuous cardiac monitoring, are in early testing, promising ambulatory applications for chronic management.

Fundamentals

Physical principles

Waves play a crucial role in medicine, enabling non-invasive diagnostic imaging and therapeutic treatments. Sound waves (acoustic waves) are used in ultrasound imaging to visualize internal organs, monitor pregnancies, and guide procedures without ionizing radiation. They also power shock wave therapy for pain relief, tissue regeneration, and musculoskeletal conditions. Electromagnetic waves support X-ray and CT scans for bone and tissue imaging, MRI (using radio waves) for detailed soft tissue views, and therapies like pulsed electromagnetic fields (PEMF) for bone healing, pain management, and wound repair. Medical imaging relies on various physical principles to generate images of the body's internal structures, primarily through interactions of energy forms with biological tissues. These principles can be broadly categorized into those involving , which uses high-energy photons capable of ejecting electrons from atoms, and , which employs lower-energy waves that do not typically cause such . Ionizing techniques, such as X-ray-based , exploit the of as it passes through matter, while non-ionizing methods, like (MRI) and , depend on magnetic fields or acoustic waves to probe tissues without the risks associated with . In modalities, s interact with tissues primarily through the and . The occurs when an incident photon is completely absorbed by an inner-shell , ejecting it and leading to characteristic secondary radiation, with probability increasing steeply with (Z^3) and inversely with (E^3). involves the photon colliding with a loosely bound outer-shell , transferring partial and scattering at an angle, which predominates at diagnostic energies (30-150 keV) and contributes to by reducing beam intensity. These interactions result in exponential attenuation of the beam, described by the Beer-Lambert law: I=I0e−μxI = I_0 e^{-\mu x}, where II is the transmitted intensity, I0I_0 is the initial intensity, μ\mu is the linear (dependent on tissue density and atomic composition), and xx is the material thickness. This law quantifies how denser tissues, like , attenuate more than soft tissues, enabling contrast in radiographic images. Non-ionizing techniques operate across lower-energy portions of the spectrum. In MRI, the principle hinges on , where hydrogen nuclei (protons) possess intrinsic spin, behaving as tiny bar magnets that align with an external (B_0). This alignment causes the spins to precess around the field direction at the Larmor (ω=γB0\omega = \gamma B_0, where γ\gamma is the ), and radiofrequency pulses at this tip the spins, inducing a detectable signal upon relaxation. imaging, conversely, uses mechanical pressure waves (typically 1-20 MHz) that propagate through tissues at speeds around 1540 m/s in , with image formation arising from partial reflection at interfaces between media of differing acoustic properties. The depends on acoustic impedance mismatch, defined as Z=ρcZ = \rho c (product of tissue ρ\rho and sound speed cc), where larger mismatches (e.g., tissue-bone) produce stronger echoes for better boundary visualization. Medical imaging modalities span the , from radio waves (3 kHz to 300 GHz) in MRI for spin excitation to gamma rays (above 10^19 Hz) in , where radionuclides emit high-energy photons for . s (10^16 to 10^20 Hz) bridge these for structural anatomy, while visible and light find limited use in optical imaging. Across all methods, image quality is governed by (SNR), the ratio of desired signal intensity to background noise standard deviation, which fundamentally limits detectability; higher SNR enhances detail but is constrained by factors like radiation dose or . Contrast mechanisms, such as density differences in or T1/T2 relaxation in MRI, exploit tissue-specific interactions to differentiate structures, with SNR influencing the efficacy of these contrasts.

Image acquisition and reconstruction

Image acquisition in medical imaging involves capturing raw signals generated from interactions between energy sources and biological tissues, converting them into detectable forms for processing. Analog systems traditionally rely on continuous signal representation, such as exposed to s in , where crystals form a proportional to intensity. In contrast, digital acquisition employs discrete sampling to produce quantifiable data, enabling post-processing and storage; this shift improves and reduces chemical waste. Digital detectors include charge-coupled devices (CCDs), which convert patterns into electrical charges via a layer and array of photosensitive pixels, commonly used in for high spatial fidelity. Photomultiplier tubes (PMTs), sensitive to low-light scintillation events, amplify signals through cascades, essential in for detecting gamma rays in and . Image reconstruction transforms these acquired projections or signals into viewable 2D or 3D images, addressing the of inferring tissue properties from incomplete data. Filtered back-projection (FBP), a foundational analytical method introduced by Ramachandran and Lakshminarayanan in 1971, reconstructs images by convolving projections with a ramp filter to compensate for blurring artifacts inherent in simple back-projection. The core formula for FBP in parallel-beam geometry is: f(x,y)=∫0π[p(θ,t)∗h(t)] t=xcos⁡θ+ysin⁡θdθf(x,y) = \int_{0}^{\pi} \left[ p(\theta, t) \ast h(t) \right]_{\ t = x \cos \theta + y \sin \theta} d\theta where f(x,y)f(x,y) is the reconstructed image density at point (x,y)(x,y), p(θ,t)p(\theta, t) is the projection data at angle θ\theta and distance tt, and h(t)h(t) is the filter kernel (typically a ramp function h(t)=∫−∞∞∣ω∣ei2πωtdωh(t) = \int_{-\infty}^{\infty} |\omega| e^{i 2\pi \omega t} d\omega). This approach offers computational efficiency but can amplify noise in low-dose scenarios. Iterative methods, such as the algebraic reconstruction technique (ART) based on Kaczmarz's 1937 iterative linear solver, refine estimates by sequentially projecting data onto hyperplanes defined by ray integrals, converging to a solution that minimizes inconsistencies. ART is particularly advantageous for sparse or noisy data, iteratively updating pixel values via x(k+1)=x(k)+λ(bi−aiTx(k))ai∥ai∥2x^{(k+1)} = x^{(k)} + \lambda (b_i - a_i^T x^{(k)}) \frac{a_i}{\|a_i\|^2}, where λ\lambda is a relaxation parameter, enhancing robustness over direct methods. Artifacts arise during acquisition and reconstruction, degrading image fidelity; common types include beam hardening and . Beam hardening occurs in X-ray-based modalities when polychromatic beams preferentially attenuate low-energy photons, shifting the spectrum and causing cupping or streaking in dense regions like bone. Corrections involve physical pre-filtration to harden the beam upstream, dual-energy scanning to estimate effective attenuation, or software-based polynomial fitting of projection data to linearize paths. , stemming from relative to the , manifests as wrap-around or ghosting when signal frequencies exceed detector capabilities, often in Fourier-based reconstructions. Mitigation strategies include oversampling to increase the field of view, filters to suppress high frequencies pre-digitization, or unfolding algorithms during reconstruction. Resolution metrics quantify image quality, guiding clinical utility. Spatial resolution measures the smallest distinguishable detail, typically assessed via line-pair phantoms in line pairs per millimeter (lp/mm); higher values (e.g., 5–10 lp/mm in CT) enable fine structure visualization. Temporal resolution captures motion without blur, expressed in milliseconds per frame; in dynamic , values below 100 ms reduce cardiac motion artifacts. Contrast resolution detects subtle intensity differences, often quantified by the minimum detectable contrast percentage; it depends on and is enhanced by averaging multiple acquisitions. These metrics interrelate, with trade-offs in acquisition time or dose influencing overall performance.

Imaging Modalities

Radiography

Radiography is a cornerstone of medical imaging, employing beams to produce two-dimensional projection images of the body's internal by exploiting differences in tissue . This technique, discovered in 1895 by , forms the basis for visualizing bones, lungs, and soft tissues through their varying absorption of . Conventional film-screen radiography, the traditional method, uses a cassette containing placed between rare-earth intensifying screens that fluoresce upon exposure, amplifying the signal to reduce patient dose while capturing the for chemical development. This approach dominated clinical practice for decades but suffered from limitations such as narrow , requiring precise exposure control to avoid over- or underexposure, and the need for wet chemical processing, which generated environmental waste. The advent of digital systems revolutionized radiography starting in the 1980s. Computed radiography (CR), introduced commercially in 1983 by Fuji Medical Systems, replaces film with reusable photostimulable phosphor plates that store the X-ray energy as a latent image, which is then read out by laser scanning to generate a digital signal. CR provides a broader latitude for exposure errors, post-processing capabilities like contrast adjustment, and filmless workflow integration with picture archiving and communication systems (PACS). Direct radiography (DR), emerging in the mid-1990s with flat-panel detectors, captures images directly in digital form using amorphous silicon or selenium arrays, eliminating the scanning step in CR for faster acquisition and higher spatial resolution. Compared to film-screen and CR, DR systems deliver superior image quality at reduced radiation doses, often 20-50% lower, due to efficient detector quantum efficiency. Key exposure factors in radiography are kilovoltage peak (kVp), which governs the beam's penetrating power and average energy, and milliampere-seconds (mAs), the product of tube current and exposure time that determines the total number of photons produced. Optimal kVp selection balances penetration for thicker body parts—typically 50-80 kVp for extremities and 100-120 kVp for chests—with contrast; higher kVp enhances visibility of soft tissues but diminishes detail. mAs primarily controls image density, with adjustments made to compensate for size; for instance, a 15% kVp increase necessitates a 50% mAs reduction to maintain equivalent receptor exposure. Patient radiation dose in radiography follows basic principles derived from X-ray output characteristics, approximated by the formula D=k⋅mAsd2D = k \cdot \frac{\mathrm{mAs}}{d^2}, where DD is the dose, kk is a system-specific constant influenced by kVp and filtration, mAs is the exposure product, and dd is the source-to-image distance (SID). This inverse square law relationship highlights that dose decreases quadratically with increasing distance, guiding clinical practice to use standard SIDs (e.g., 100-180 cm) to minimize exposure while ensuring adequate image sharpness. Radiography is a first-line imaging modality due to its speed, low cost, and widespread availability, providing excellent detail for bony and calcified structures. It excels in imaging bones (e.g., detecting fractures, assessing joint alignment, and evaluating orthopedic hardware), chest (e.g., identifying pneumonia, pleural effusions, and cardiomegaly through visualization of lung fields and cardiac silhouette), abdomen (e.g., detecting bowel obstruction through abnormal gas patterns, calcifications such as renal calculi, and radiopaque foreign bodies), and in detecting calcifications or foreign bodies. However, it involves exposure to ionizing radiation and provides poorer soft tissue contrast than modalities like ultrasound or magnetic resonance imaging. A principal limitation arises from its projectional nature, where overlapping anatomical structures in the third dimension can obscure subtle lesions, such as small pulmonary nodules superimposed on . Fluoroscopy represents a dynamic extension of , enabling real-time visualization by delivering pulsed or continuous low-dose X-rays to an image receptor, often amplified for fluoroscopic viewing on a monitor. This technique facilitates interventional procedures, such as guiding catheters in or monitoring gastrointestinal motility during studies, with modern digital systems incorporating dose-reduction features like last-image-hold to limit cumulative exposure.

Computed tomography

Computed tomography (CT), also known as computed axial tomography (), is a medical imaging technique that uses to generate cross-sectional images of the body, allowing for detailed visualization of internal structures in three dimensions. The process involves rotating an around the patient while detectors capture attenuated X-ray beams from multiple angles, with the data reconstructed into tomographic slices using algorithms such as filtered back-projection. This modality excels in providing high-resolution images of bones, blood vessels, and soft tissues, making it essential for diagnosing conditions like tumors, fractures, and vascular diseases. Specialized CT techniques expand its diagnostic utility in targeted clinical scenarios. CT angiography (CTA) utilizes intravenous contrast to enable rapid, detailed imaging of blood vessels and is particularly valuable in emergencies for detecting pulmonary embolism, aortic dissection, and aneurysms. High-resolution CT (HRCT) employs thin slices and optimized protocols to produce detailed images of lung parenchyma, serving as a key tool for diagnosing interstitial lung diseases, pulmonary fibrosis, bronchiectasis, and diffuse parenchymal conditions. CT colonography, also known as virtual colonoscopy, provides a non-invasive screening method for colorectal polyps and cancer through detailed CT imaging of the colon. Cone-beam CT (CBCT) generates three-dimensional images using a cone-shaped X-ray beam and is primarily applied in dentistry for implant planning, orthodontics, oral surgery, and maxillofacial applications, as well as in some otolaryngologic evaluations. The evolution of CT technology in the 1990s marked a significant advancement with the introduction of helical (or spiral) scanning in 1990, which enabled continuous as the patient table moves through the gantry, reducing scan times and motion artifacts compared to earlier step-and-shoot methods. Building on this, multi-detector CT (MDCT) emerged in 1998, featuring multiple rows of detectors along the z-axis to acquire volumetric data in a single rotation, dramatically increasing speed and resolution for applications like . These developments allowed for thinner slices (as low as 0.5 mm) and faster coverage of large body regions, transforming CT into a routine diagnostic tool. CT images are quantified using the Hounsfield unit (HU) scale, a standardized measure of radiodensity relative to water, defined by the formula: HU=1000×μ−μwaterμwater−μairHU = 1000 \times \frac{\mu - \mu_{\text{water}}}{\mu_{\text{water}} - \mu_{\text{air}}} where μ\mu represents the linear attenuation coefficient of the tissue. Water is assigned 0 HU, air -1000 HU, and bone around +1000 HU, facilitating consistent interpretation across scanners. In 2006, dual-energy CT (DECT) was introduced commercially with the first dual-source system, enabling material decomposition by acquiring datasets at two different X-ray energies (typically 80 kVp and 140 kVp) to differentiate tissues based on their energy-dependent attenuation properties, such as distinguishing iodine from bone. Radiation exposure in CT is managed through metrics like the computed tomography dose index (CTDI), which quantifies the average dose in a standardized phantom, and the dose-length product (DLP), calculated as CTDIvol multiplied by the scan length (in mGy·cm), providing an estimate of total radiation output. The ALARA (as low as reasonably achievable) principle guides dose optimization by adjusting parameters like tube current (mA) and voltage (kVp) while maintaining image quality, with modern protocols reducing effective doses to 1-10 mSv for routine exams. techniques further support ALARA by allowing lower doses without compromising diagnostic utility.

Magnetic resonance imaging

Magnetic resonance imaging (MRI) is a non-invasive imaging modality that utilizes strong and radiofrequency pulses to generate detailed images of internal body structures, particularly excelling in soft-tissue contrast without . It relies on the alignment and excitation of atomic nuclei, primarily protons in and molecules, within the body's tissues. Following excitation, these nuclei relax back to equilibrium, producing signals that are spatially encoded to form images. This technique provides superior visualization of organs like the , muscles, and joints compared to other modalities. The core of MRI signal generation involves two primary relaxation processes: longitudinal (T1) relaxation, where the net vector recovers along the direction of the external , and transverse (T2) relaxation, where the decays in the plane perpendicular to the field due to spin-spin interactions. T1 relaxation times vary by tissue type, typically shorter in (around 200-400 ms) than in water-rich tissues like (over 2000 ms), enabling T1-weighted images that highlight anatomical differences based on proton density and relaxation rates. T2 relaxation, conversely, is faster in fluids (100-200 ms) than in solids, allowing T2-weighted sequences to detect or through increased signal intensity in affected areas. These relaxation times are influenced by molecular environment and , with higher fields generally prolonging both T1 and shortening T2*. To enhance contrast in specific regions, MRI often employs gadolinium-based contrast agents (GBCAs), which are paramagnetic chelates that shorten T1 relaxation times locally, producing bright signals on T1-weighted images for better delineation of lesions, tumors, or vascular structures. Gadolinium shortens T1 more effectively than T2 at clinical doses, making it ideal for vascular and perfusion imaging, though its use requires screening for renal impairment due to risks of nephrogenic systemic fibrosis in patients with low glomerular filtration rates. Macrocyclic GBCAs, such as gadoterate, exhibit higher stability and lower dissociation rates compared to linear agents, reducing the potential for free gadolinium release and associated toxicities. Regulatory bodies like the FDA mandate warnings on GBCA retention in tissues, even in patients with normal renal function, emphasizing the need for judicious use. Image formation in MRI depends on pulse sequences that control excitation and signal readout. Spin-echo sequences use a 90° radiofrequency followed by a 180° refocusing to correct for field inhomogeneities, producing T2-weighted images with reduced susceptibility artifacts, ideal for anatomical of the and spine. Gradient-echo sequences, in contrast, employ partial flip angles (e.g., 30°) and gradient reversals without refocusing pulses, enabling faster acquisition and T1* or T2*-weighted contrast sensitive to variations, such as in or functional studies. These sequences fill k-space—a Fourier domain representation of the image—through phase-encoding and frequency-encoding gradients, where the center of k-space captures low-frequency contrast information and the periphery encodes high-frequency details for edge sharpness. Efficient k-space trajectories, like Cartesian or radial sampling, balance speed and resolution, with techniques accelerating scans via parallel . MRI systems operate at various field strengths, with 1.5 T being the clinical standard for broad applications due to its balance of image quality and accessibility. Higher fields like 3 T offer improved (SNR) roughly doubling that of 1.5 T, enhancing resolution for detailed or musculoskeletal imaging, though they increase susceptibility artifacts and (SAR). At 7 T, SNR can quadruple compared to 1.5 T, enabling ultra-high-resolution imaging of microstructures like cortical layers, but challenges include intensified B1 inhomogeneities, higher SAR—limited by FDA guidelines to 4 W/kg whole-body averaged over 15 minutes—and prolonged T1 times requiring adjusted sequences. SAR, measuring radiofrequency energy deposition as heat, scales quadratically with , necessitating pulse optimization at ultra-high fields to stay within safety thresholds and prevent tissue heating. Specialized MRI techniques extend its utility to targeted vascular and gastrointestinal applications. Magnetic resonance angiography (MRA) is a non-invasive method for visualizing the arterial and venous systems without ionizing radiation. It employs non-contrast techniques such as time-of-flight (TOF) or phase-contrast, which rely on blood flow properties, or contrast-enhanced approaches using gadolinium for improved detail. MRA is particularly effective for detecting aneurysms, arterial stenosis, vascular abnormalities, and conditions such as arteriovenous malformations or dissections in the brain, neck, aorta, and peripheral vessels. MR enterography is an optimized MRI protocol for detailed evaluation of the small bowel, involving oral contrast agents to distend the intestine and frequently intravenous gadolinium contrast. It is widely used for inflammatory bowel diseases, especially Crohn's disease, enabling assessment of disease extent, bowel wall thickening, mural edema, inflammation, strictures (inflammatory vs. fibrotic), fistulas, abscesses, and extraintestinal manifestations such as mesenteric hypervascularity. Its lack of ionizing radiation makes it suitable for repeated imaging in chronic conditions. Functional MRI (fMRI) extends anatomical imaging by mapping activity through blood-oxygen-level-dependent (BOLD) contrast, which exploits the paramagnetic properties of to detect hemodynamic changes following neural activation. Upon neuronal firing, local blood flow increases to deliver oxygenated , reducing deoxyhemoglobin concentration and thereby lengthening T2* relaxation times, yielding a positive BOLD signal (typically 1-5% change at 3 T) on gradient-echo sequences. This indirect measure of function, first demonstrated in the early , supports presurgical planning and cognitive research, with optimal sensitivity at higher fields like 3 T or 7 T due to amplified T2* effects, though it remains limited by low (seconds) compared to direct .

Ultrasound

Ultrasound imaging, also known as sonography, utilizes high-frequency sound waves to visualize internal body structures in real time, offering a non-invasive, portable, and cost-effective modality compared to other techniques. It is particularly advantageous for imaging soft tissues, including organs, blood vessels, muscles, tendons, joints, thyroid, and breast, as well as for pregnancy monitoring. Ultrasound uses no ionizing radiation, making it safe for pregnant women and children, and provides real-time imaging that is ideal for visualizing moving structures and blood flow, as well as for guiding interventional procedures. However, it is limited by interference from bone and air-filled structures, which can obscure visualization or cause artifacts, and is highly operator-dependent, with results varying based on the skill and experience of the examiner. It operates by emitting acoustic pulses that reflect off tissues, with echoes detected to form images based on differences in . This method excels in dynamic assessments, such as organ motion or blood flow, and is widely used in , , and musculoskeletal evaluations due to its safety, lacking . Transducers are the core components of ultrasound systems, converting into and vice versa. Common types include linear transducers, which produce rectangular images ideal for superficial structures like the or vessels, and phased- transducers, which generate sector-shaped images suitable for deeper penetration in cardiac or abdominal scans. These transducers typically operate in ranges of 2-18 MHz, where lower frequencies (2-5 MHz) provide deeper penetration for abdominal imaging, while higher frequencies (10-18 MHz) offer superior resolution for superficial applications. Imaging modes in ultrasound include B-mode (brightness mode), which displays a two-dimensional grayscale image of tissue anatomy by mapping echo amplitude, and M-mode (motion mode), which provides a one-dimensional graph of tissue movement over time, useful for assessing cardiac valve kinetics or fetal heart rates. Artifacts can compromise image quality; reverberation artifacts arise from repeated reflections between highly reflective surfaces and the transducer, appearing as equally spaced bright lines, while shadowing occurs when sound waves are blocked by dense structures like bones or calculi, resulting in dark areas distal to the obstacle. Doppler ultrasound extends B-mode by assessing flow and direction through frequency shifts in reflected waves from moving red cells. Color Doppler mode overlays a color-coded map of flow and direction on the B-mode image, with red typically indicating flow toward the and blue away, while spectral Doppler provides a graph of over time along a sample line for quantitative analysis. The , derived from , relates flow rates across vessel segments as Q=V1A1=V2A2Q = V_1 A_1 = V_2 A_2, where QQ is volume flow rate, VV is , and AA is cross-sectional area, enabling estimation of peak velocities in stenotic regions to assess severity. As of 2025, advancements in include AI-guided probes that automate image acquisition and interpretation, enhancing diagnostic accuracy in point-of-care settings by real-time probe positioning and detection. High-frequency transducers exceeding 20 MHz have improved superficial resolution, particularly for dermatological and vascular applications, allowing visualization of fine structures like layers or small vessels with minimal .

Nuclear medicine

Nuclear medicine is a branch of medical imaging that utilizes radioactive tracers, known as radiotracers, to visualize and quantify physiological processes at the molecular level, primarily through (SPECT) and (PET). Unlike anatomical imaging modalities, nuclear medicine emphasizes functional and metabolic information, where the distribution of the radiotracer reflects biological activity such as blood flow, receptor binding, or metabolic rates. Radiotracers typically consist of a attached to a biologically active , allowing targeted accumulation in tissues of interest. Common radionuclides include for SPECT, with a of 6 hours, and for PET, with a half-life of approximately 110 minutes, enabling on-site production via cyclotrons and timely imaging. A prominent example is 18F-fluorodeoxyglucose (FDG), a glucose analog used in PET to assess glucose metabolism, which is elevated in many malignant cells due to the Warburg effect. FDG is taken up by cells via glucose transporters and phosphorylated by but not further metabolized, leading to intracellular trapping proportional to metabolic demand. This tracer's development and validation for human brain imaging established its foundational role in , , and , revolutionizing the assessment of tumor viability and neurological disorders. In PET, positron-emitting radionuclides like decay to produce positrons that annihilate with electrons, emitting two 511 keV gamma photons in nearly opposite directions; these are detected via coincidence circuitry, which registers events only when both photons arrive simultaneously within a narrow time window (typically 6-12 nanoseconds), defining a line of response without physical collimation and improving sensitivity over SPECT. For SPECT, the , invented by Hal O. Anger in , serves as the core detection system, employing a thallium-doped (NaI(Tl)) scintillation crystal to convert incident gamma rays into visible light, which photomultiplier tubes (PMTs) amplify and positionally localize to form projection images. Multiple projections acquired by rotating the camera around the patient enable using filtered back-projection or iterative algorithms. correction is essential in both SPECT and PET to compensate for photon absorption and scatter in tissue, which can distort quantitative accuracy; methods include transmission scanning with an external radionuclide source to generate an attenuation map, or emission-based approaches that estimate the map from the tracer distribution itself, with segmented models dividing the body into uniform density regions for simplified correction. In PET, coincidence detection inherently rejects some scattered events, but dedicated scatter correction via modeling or dual-energy window techniques further refines images. Patient safety in relies on to estimate from radiotracers, guided by the Medical Internal Radiation Dose (MIRD) formalism developed by the Society of Nuclear Medicine in the late 1960s. The MIRD schema calculates the mean to a target region as the product of the cumulated activity in source regions, S-values (absorbed dose per unit cumulated activity), and a factor accounting for target mass, enabling organ-level predictions using mathematical phantoms like the Cristy-Eckerman model. This approach, formalized in early pamphlets, underpins regulatory guidelines and personalized dosing, balancing diagnostic yield against risks such as cancer induction.

Emerging techniques

Elastography represents a class of emerging ultrasound-based techniques that assess tissue mechanical properties, particularly , to aid in . Shear wave elastography (SWE), a prominent variant, employs acoustic force to generate shear waves within tissue, whose speed is measured to quantify elasticity. The shear wave speed csc_s is given by cs=μρc_s = \sqrt{\frac{\mu}{\rho}}
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