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Articles are a collaborative effort to provide a single canonical page on all topics relevant to the practice of radiology. As such, articles are written and edited by countless contributing members over a period of time. A global group of dedicated editors oversee accuracy, consulting with expert advisers, and constantly reviewing additions.

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1.5 T vs 3.0 T

Comparing 1.5 T vs 3.0 T (1.5 tesla vs 3.0 tesla) MRI systems identifies several differences; a 3 T system has increased signal-to-noise ratio (SNR) increased spatial resolution increased temporal resolution increased specific absorption rate (SAR) increased acoustic noise Terminology It...

3D fast spin echo (MRI sequence)

3D fast spin-echo sequences are relatively recent MRI pulse sequences that are able to rapidly image relatively large volumes of tissue with high resolution whilst retaining many of the advantages of fast spin-echo sequences. They are able to create the same weightings as traditional 2D sequen...

5 gauss line

The 5 gauss line is the safety line drawn around the perimeter of the main magnet of the MRI scanner, specifying the distance at which the stray magnetic field is equivalent to 5 gauss (0.5 mT). Five gauss and below are considered 'safe' levels of static magnetic field exposure for the general ...

Absorbed dose

Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation. It is equal to the energy deposited per unit mass of a medium, and so has the unit joules (J) per kilogram (kg), with the adopted name of gray (Gy) where 1 Gy = 1 J.kg-1. The absorbed dose is not a good indica...

Absorption (ultrasound)

In ultrasound, absorption is the reduction in intensity of the sound waves as it passes through tissue. Most of the energy lost is in the form of heat.

Acoustic enhancement

Acoustic enhancement also called posterior enhancement or enhanced through transmission, refers to the increased echoes deep to structures that transmit sound exceptionally well. This is characteristic of fluid-filled structures such as cysts, the urinary bladder and the gallbladder. The fluid ...

Acoustic impedance

Acoustic impedance (Z) is a physical property of tissue. It describes how much resistance an ultrasound beam encounters as it passes through a tissue. Acoustic impedance depends on: the density of the tissue (d, in kg/m3) the speed of the sound wave (c, in m/s) and they are related by: Z = ...

Acoustic shadowing

The phenomenon of acoustic shadowing (sometimes, somewhat tautologically, called posterior acoustic shadowing) on an ultrasound image is characterized by a signal void behind structures that strongly absorb or reflect ultrasonic waves. It is a form of imaging artifact. This happens most frequent...

Acquisition time

The time of acquisition for a conventional spin echo or gradient echo sequence is the product of the repetition time, phase encoding steps, and number of averages (TR x phase steps x NEX). For example, with a one second TR, 128 phase steps, and two averages we would get an acquisition time of ab...

Additive manufacturing

Additive manufacturing is a process, such as stereolithography, in which objects are created by adding layer after layer from the ground up. This process can be contrasted with subtractive manufacturing, a process in which unneeded material is removed to create the desired object in the same way...

Advanced multiple beam equalization radiography

The advanced multiple beam equalization radiography (AMBER) system is used to control the distribution of local exposure to the film. An array of independently functioning detectors is used to send feedback signals to the modulators kept in front of the x-ray tube to modulate the exposure levels.

Air bubble artifact

The air bubble artifact on CT is due to the presence of abnormal gas in the oil coolant which surrounds the x-ray tube. The artifact manifests as subtle low density, which has only been described on brain scans. Cause The x-ray tube in a CT scanner is prevented from overheating by a heat excha...

Air gap technique

The air gap technique is a radiographic technique that improves image contrast resolution through reducing the amount of scattered radiation that reaches the image detector. In select situations, this technique can be used instead of an antiscatter grid as the primary scatter reduction method in...

Air gap technique (general radiography)

The utilization of the air gap technique in general radiography is limited due to the need for equipment facilitation to create the air gap when it is not inherent in the standard technique. Horizontal-beam lateral hip There are many different methods of performing the horizontal beam lateral ...

Air gap technique (mammography)

The air gap technique is utilized for the magnification mammography view. Magnification mammography is a high dose imaging technique which is generally utilized as a follow-up to a standard mammogram image series when a focal area needs to be more clearly examined 1. The air gap technique is ut...

Aliasing artifact (CT)

Aliasing artifact, otherwise known as undersampling, in CT refers to an error in the accuracy proponent of analog to digital converter (ADC) during image digitization. Image digitization has three distinct steps: scanning, sampling, and quantization. When sampling, the brightness of each pix...

Aliasing in MRI

Aliasing in MRI, also known as wrap-around, is a frequently encountered MRI artifact that occurs when the field of view (FOV) is smaller than the body part being imaged. The part of the body that lies beyond the edge of the FOV is projected onto the other side of the image. This can be correcte...

Aliasing phenomenon (ultrasound)

Aliasing is a phenomenon inherent to Doppler modalities which utilize intermittent sampling in which an insufficient sampling rate results in an inability to record direction and velocity accurately. Physics Unlike continuous wave Doppler, pulsed wave and color flow Doppler modalities alterna...

Allan Macleod Cormack

Allan M Cormack (1924-1998) was a South African-American physicist who was instrumental in the development of CT 1,3. Early life Allan Macleod Cormack was born on 23 February 1924 in Johannesburg, South Africa. His parents - teacher Amelia MacLeod and engineer George Cormack - had both emigrat...

Alpha decay

Alpha decay is the process in which an alpha particle (containing two neutrons and two protons) is ejected from the nucleus. An alpha particle is identical to the nucleus of a helium atom. All nuclei with the atomic number (Z) greater than 82, are considered unstable. These are considered “neutr...

Ammonia (N-13)

13NH3 is a PET tracer used for studies of myocardial perfusion imaging. It is produced in a cyclotron by proton irradiation of the enriched water of the oxygen-16. Ammonia (N-13) is administered intravenously, at a dose of 10-20 mCi (370-740 Mbq) in adults; its physical half-life is 10 minutes. ...

Anisotropy

Anisotropy is an artefact encountered in ultrasound, notably in muscles and tendons during a musculoskeletal ultrasound. In musculoskeletal applications, the artefact may prompt an incorrect diagnosis of tendinosis or tendon tear. When the ultrasound beam is incident on a fibrillar structure as...

Ankle protocol (MRI)

The MRI ankle protocol encompasses a set of MRI sequences for the routine assessment of the ankle joint. Note: This article aims to frame a general concept of an MRI protocol for the assessment of the ankle. Protocol specifics will vary depending on MRI scanner type, specific hardware and softw...

Anode angle

The anode angle refers to the angle the target surface of the anode sits at in relation to the vertical. Most x-ray tubes have an anode angle of 12-15 degrees but greater or lesser angles can also be used depending on the application. The degree of angulation of the anode affects the effective...

Anode heel effect

Anode heel effect refers to the lower field intensity towards the anode in comparison to the cathode due to lower x-ray emissions from the target material at angles perpendicular to the electron beam. Basic concept The conversion of the electron beam into x-rays doesn’t simply occur at the sur...

Anode (x-ray tube)

The anode (or anticathode) is the component of the x-ray tube where x-rays are produced. It is a piece of metal, shaped in the form of a bevelled disk with a diameter between 55 and 100 mm, and thickness of 7 mm, connected to the positive side of the electrical circuit. The anode converts the en...

Antoine Henri Becquerel

Antoine Henri Becquerel (1852-1908) was a French scientist renowned for his work and subsequent discovery of radioactivity for which he was awarded the Nobel Prize for physics in 1903. Early life Antoine Henri Becquerel was born on 15 December 1852 in Paris, France to a family of nobility and ...

Apparent diffusion coefficient

Apparent diffusion coefficient (ADC) is a measure of the magnitude of diffusion (of water molecules) within tissue, and is commonly clinically calculated using MRI with diffusion-weighted imaging (DWI) 1. Basics Diffusion-weighted imaging (DWI) is widely appreciated as an indispensable tool i...

Arterial spin labeling MR perfusion

Arterial spin labeling (ASL) MR perfusion is an MR perfusion technique which does not require intravenous administration of contrast (unlike DSC perfusion and DCE perfusion). Instead it exploits the ability of MRI to magnetically label arterial blood below the imaging slab. The parameter most c...

As low as reasonably achievable (ALARA)

As low as reasonably achievable (ALARA) is a principle of radioprotection stating that whenever ionizing radiation has to be applied to humans, animals or materials exposure should be as low as reasonably achievable. It is fundamental to the principles of radiation protection.

Attenuation coefficient

The attenuation coefficient is a measure of how easily a material can be penetrated by an incident energy beam (e.g. ultrasound or x-rays). It quantifies how much the beam is weakened by the material it is passing through. See also attenuation (ultrasound) linear attenuation coefficient mass...

Attenuation (ultrasound)

The amplitude and intensity of ultrasound waves decrease as they travel through tissue, a phenomenon known as attenuation. Given a fixed propagation distance, attenuation affects high frequency ultrasound waves to a greater degree than lower frequency waves. This dictates the use of lower freque...

Automated full-field volumetric ultrasound

An automatic full-field volumetric breast ultrasound scanner (AFFBUS) is a developing technology which was initiated to overcome the drawback of dense breast and to get a three-dimensional view of the breast. Components scan station view station Scan station Automatic ultrasound imaging ac...

Automatic brightness control (ABC)

Automatic brightness control (ABC) is a device incorporated into the fluoroscopic unit. Its function is to maintain a consistent overall appearance of the image by automatically adjusting the kVp and/or mAs. In this regard, ABC systems are analogous to the AEC systems utilized in radiography. ...

Automatic exposure control (AEC)

Automatic exposure control is a device incorporated into radiographic and mammographic imaging systems. Its function is to automatically terminate exposure when a preset amount of radiation has been detected. Automatic exposure control systems help to provide a consistent optical density/signal...

Axial plane for imaging of the brain

A consistent axial plane for imaging of the brain needs to be chosen to allow for reproducible image acquisition and comparison. Unlike the sagittal plane, which is intrinsically defined by our inherent left-right plane of symmetry, axial and coronal planes need to be agreed upon and over the ye...

Axial resolution (ultrasound)

Axial resolution in ultrasound refers to the ability to discern two separate objects that are longitudinally adjacent to each other in the ultrasound image. Axial resolution is generally around four times better than lateral resolution. Axial resolution is defined by the equation: axial resolut...

B0

The B0 in MRI refers to the main static magnetic field and is measured in teslas (T). The majority of MRI systems in clinical use are 1.5 T, with increasing numbers of 3 T systems being installed. Since 2017, 7 T clinical scanners have been available, see ultrahigh field MRI. Altering the field...

Background radiation

Background radiation refers to exposure to ionizing radiation in day-to-day life, excluding occupational exposures. It is measured in millisieverts (mSv). Ionizing radiation occurs naturally in the environment 1,2: radioactive gas (e.g. radon, thoron): 0.2-2.2 mSv/year external terrestrial (e....

Background radiation equivalent time

Exposing a patient to radiation is a measured, justified means aiding patient care. Each medical imaging examination utilizing ionizing radiation adheres to the fundamental principles of radiation protection. The general public's understanding of ionizing radiation is limited 1; this article pr...

Beam collimators

Beam collimators are 'beam direction' devices used in the x-ray tube housing, along with an arrangement of mirrors and lights, in such a way that the light and x-ray fields match each other. They are made of lead shutters which completely absorb the photons, and thus reduce the patient dose as w...

Beam focusing

Beam focusing refers to creating a narrow point in the cross-section of the ultrasound beam called the focal point. It is at the focal point where the lateral resolution of the beam is the greatest also. Before the focal point is the near field or Fresnel zone, where beams converge. Distal to th...

Beam steering

Beam steering refers to altering the angle of the ultrasound beam with respect to the transducer without moving the probe. Beam steering allows a point on an image to be insonated from multiple angles from a single probe and a single position of the probe. Beam steering is accomplished by adding...

Beam width artifact

Ultrasound beam width artifact occurs when a reflective object located beyond the widened ultrasound beam, after the focal zone, creates false detectable echoes that are displayed as overlapping the structure of interest. To understand this artifact, it is important to remember that the ultraso...

Becquerel (SI unit)

The becquerel (symbol: Bq) is the SI unit of radioactivity and is defined as one nuclear disintegration per second 1; it officially replaced the curie (Ci), the unit in the superseded cgs system, in 1975. On Radiopaedia.org we primarily express all radioactive doses in becquerels. However as th...

Beta decay

Beta particles occur with either negative or positive charge (β- or β+) and are known to be either electrons or positrons, respectively, therefore beta decay represents radioactive decay, in which a beta particle is emitted. The kinetic energy of beta particles has a continuous spectrum. Beta m...

B-Flow

B-Flow is a type of ultrasound imaging that allows visualization of blood flow by selectively enhancing the signal from moving blood cells while simultaneously suppressing tissue signal 1. Unlike color Doppler, it does not show flow direction or amplitude. B-flow is used clinically to image the...

Binning

Binning is a technique by which signals arriving from adjacent physical elements of an electronic detector are combined to produce a larger pixel/voxel. This increases the signal to noise ratio to provide better contrast resolution, with the trade-off being reduced spatial resolution.

Biological effects of ultrasound

The biological effects of ultrasound refer to the potential adverse effects the imaging modality has on human tissue. These are primarily via two main mechanisms: thermal and mechanical. Despite this, ultrasound has a remarkable record for patient safety with no significant adverse bioeffects re...

Biomolecular radiation damage

Biomolecular radiation damage may result from exposure of biological tissues to ionizing radiation from direct exposure, or via Compton scattering. Mechanism of tissue radiation damage Direct effect if a biological macromolecule (e.g. DNA, RNA, protein) becomes ionized or excited by an ionizi...

Black boundary artifact

Black boundary artifact, also known as India ink artifact or type 2 chemical shift artifact, is an artificially-created black line located at fat-water interfaces such as those between muscle and fat. This results in a sharp delineation of the muscle-fat boundary lending the image an appearance ...

Blooming artifact (CT)

Blooming artifact in CT is caused by small, high density structures such as artery calcifications and metallic objects, which appear larger than their true size. The cause of the artifact is that the high Hounsfield value of such objects coupled with the use of smoothing filter kernels results...

Blooming artifact (MRI)

Blooming artifact is a susceptibility artifact encountered on some MRI sequences in the presence of paramagnetic substances that affect the local magnetic milieux. Although it is an artifact, it may be deliberately exploited to improve detection of certain small lesions, much as the T1 shortenin...

Blooming artifact (ultrasound)

Blooming or color bleed artifact occurs when the color signal indicating blood flow extends beyond its true boundaries, spreading into adjacent regions with no actual flow. This artifact mainly affects the portion of the image distal to the vessel and the transducers. It is somewhat similar to ...

Blur

Blurring, or unsharpness, refers to the distortion of the definition of objects in an image, resulting in poor spatial resolution. Types of blur geometric blur limited by focal spot size, distance from the source to the patient, distance from patient to receptor detector blur limited by...

BOLD imaging

Blood oxygenation level dependent (BOLD) imaging is the standard technique used to generate images in functional MRI (fMRI) studies, and relies on regional differences in cerebral blood flow to delineate regional activity. Blood flow in the brain is highly locally controlled in response to oxy...

Brain morphometry

Brain morphometry is the act of measuring various dimensions (typically volume) of parts of the brain. Historically, this was only performed post-mortem. In modern practice, this is performed in vivo using MRI. A volumetric scan of the brain (typically T1 weighted) is obtained and segmented i...

Bremsstrahlung radiation

X-rays are produced by high-energy electrons bombarding a target, especially targets that have a high proton number (Z). When bombarding electrons penetrate into the target, some electrons travel close to the nucleus due to the attraction of its positive charge and are subsequently influenced by...

Bucky factor

The Bucky factor is the ratio of radiation on the anti-scatter grid to the transmitted radiation. Hence, the Bucky factor reflects the increased radiation dose required from anti-scatter grid use, as any increase in mAs proportionally increases dose. The Bucky factor changes with: change in k...

b values

b value measures the degree of diffusion weighting applied, thereby indicating the amplitude (G), time of applied gradients (δ) and duration between the paired gradients (Δ) and is calculated as: b = γ² G² δ² (Δ−δ/3) Therefore, a larger b value is achieved by increasing the gradient amplitude ...

Calcium mass score

The calcium mass score was introduced to determine the absolute mass of coronary artery calcium with the help of a cardiac calibration phantom and the use of correction factors 1,4. The method itself comprises the integration of signal above a given threshold 3. Even though, higher sensitivity ...

Cardiac gating (MRI)

Cardiac gating or cardiac triggering refers to the gain of information about specific time points and its use for image acquisition during the cardiac cycle. Technique Cardiac synchronization can be achieved by the ECG signal or alternatively with a peripheral pulse transducer. The following t...

Cardiac ischemia protocol (MRI)

The cardiac MRI ischemia or stress protocol encompasses a set of different MRI sequences for the assessment of myocardial ischemia. Note: This article aims to frame a general concept of a cardiac MRI protocol in the setting of vasodilator stress perfusion testing. Protocol specifics will vary ...

Cardiac MRI

Cardiac MRI consists of using MRI to study heart anatomy, physiology, and pathology. Advantages In comparison to other techniques, cardiac MRI offers: improved soft tissue definition protocol can be tailored to likely differential diagnoses a large number of sequences are available dynamic...

Cassette

Cassettes are rigid holders used in conventional and computed radiography (CR) for the screen film system and imaging plate respectively. The back side of the cassette has rubber or felt for adequate contact between the screen film system or with the imaging plate. The front of the cassette is...

Cathode (x-ray tube)

The cathode is part of an x-ray tube and serves to expel the electrons from the circuit and focus them in a beam on the focal spot of the anode. It is a controlled source of electrons for the generation of x-ray beams. The electrons are produced by heating the filament (Joule heating effect) i.e...

Central point artifact

The central point artifact is a focal dot of increased signal in the center of an image. It is caused by a constant offset of the DC voltage in the receiver. After Fourier transformation, this constant offset gives the bright dot in the center of the image as shown in the diagram. The axial MRI...

Central volume principle

The central volume principle is used generating perfusion maps. It relates three perfusion parameters to each other as follows: CBF = CBV/MTT This means that provided you can measure two of these parameters you can calculate the third.

Cerebral blood flow (CBF)

Cerebral blood flow (CBF) is one of the parameters generated by perfusion techniques (CT perfusion and MR perfusion). CBF is defined as the volume of blood passing through a given amount of brain tissue per unit of time, most commonly milliliters of blood per minute per 100 g of brain tissue 1. ...

CGS system

The CGS (or cgs) system (or centimeter-gram-second) of units predated the current International System (also known as SI units), which is the current iteration of the metric system. Although many fields, including most of the healthcare sciences have abandoned CGS for everyday work, there are s...

Characteristic radiation

Characteristic radiation is a type of energy emission relevant for X-ray production. This energy emission happens when a fast-moving electron collides with a K-shell electron, the electron in the K-shell is ejected (provided the energy of the incident electron is greater than the binding energy ...

Charge-coupled device detector

Charge-coupled device (CCD) detectors are used in digital radiography for the indirect conversion of x-ray photons into an electric charge (indirect because the x-ray photons are first converted into light via a scintillating screen). Structure A charge-coupled device can either be an area arr...

Charles Thurstan Holland

Charles Thurstan Holland (1863-1941) was a pioneering radiologist who played a pivotal role in establishing radiology as a specialty in its own right. Early life Charles Thurstan Holland was born in Bridgwater, Somerset on 7 March 1863. He studied medicine at University College Hospital in...

Chemical exchange saturation transfer

Chemical exchange saturation transfer (CEST) imaging is a novel molecular MR technique that enables imaging certain compounds at concentrations that are too low to impact the contrast of standard MR imaging and too low to directly be detected in MRS at typical water imaging resolution 1. Amide ...

Chemical shift

The chemical shift is the local change in resonant frequency due to different chemical environments. The external magnetic field causes the electron cloud surrounding the nucleus to induce an electron current, which in turn produces a local magnetic field at the nucleus opposed in direction to t...

Chemical shift artifact

Chemical shift artifact or misregistration is a type of MRI artifact. It is a common finding on some MRI sequences and used in MRS. This artifact occurs in the frequency-encoding direction and is due to spatial misregistration of fat and water molecules. Chemical shift is due to the difference...

Choline C-11

Carbon-11-choline (choline C-11 or 11C-choline) is the most studied isotope carbon-11 PET radiopharmaceutical. The molecule is used for oncologic Imaging 1-4. Choline is one of the components of phosphatidylcholine, a fundamental element of cell membrane phospholipids 5. Cancer cells tend to ha...

Chromium-labeled red blood cells

Chromium-labeled red blood cells is an intravascular MRI contrast agent. The use of 51Cr-labeled RBCs in nuclear medicine suggested the use of paramagnetic Cr(III)-labeled RBCs as an intravascular contrast agent for MRI. In dogs, significant enhancement of the liver and spleen is noted with mini...

Chronic appendicitis

Chronic appendicitis is defined by inflammation of the appendix over time with symptoms lasting for more than three weeks duration 1. The condition should be differentiated from recurrent appendicitis, in which one or more episodes of flares of symptoms last 24 to 48 hours and subside on their o...

Cinematic rendering

Cinematic rendering is a novel 3D rendering algorithm that produces a more photorealistic representation of 3D images with enhanced depth and shape perception than achieved with standard volume rendering (VR) 1. It was developed by Siemens Healthineer 2. It is not currently approved for clinical...

Citrate peak

Citrate is a compound examined in MR spectroscopy in the setting of possible prostate carcinoma. Citrate resonates at 2.6 ppm and is decreased in prostate cancer. For more information go to: MR spectroscopy in prostate cancer

CLEAR

CLEAR, an abbreviation of constant level appearance, is an MR technique to achieve homogeneity correction by using coil sensitivity maps acquired in a reference scan. CLEAR is a term utilized by Philips and is comparable to PURE in Siemens MR scanners.

Cobalt

Cobalt (chemical symbol Co) is an essential trace element. Its most important function in humans is as the active metal atomic center of vitamin B12. Chemistry Basic chemistry Cobalt is a shiny grey-silvery transition metal with an atomic number 27 and atomic weight 58.93. Its oxidation state...

Coherent scattering

Coherent scattering (also known as unmodified, classical or elastic scattering) is one of three forms of photon interaction which occurs when the energy of the x-ray or gamma photon is small in relation to the ionization energy of the atom. It, therefore, occurs with low energy radiation. Upon ...

Coherent system of units

A coherent system of units consists of a set of base units (typically time, length, mass, electric current and temperature) and a set of derived units. The derived units are formed from the product of base units raised to specific powers with a constant factor of one. Some derived units have spe...

Collimator (Gamma camera)

The collimator of a Gamma camera used in nuclear medicine differs in structure and function to the beam collimators used in general radiography. They typically consist of a lead disc drilled with tens of thousands of closely packed holes, separated from each other by septa. Each hole only acce...

Color flash artifact

The color flash artifact is a commonly encountered artifact on color Doppler ultrasound, representing spurious flow signal arising due to tissue/transducer motion. Physics The flash artifact is caused by movement of reflective tissues (e.g. due to respiration), or the transducer, which genera...

Color flow Doppler (ultrasound)

The use of color flow Doppler (CFD) or color Doppler imaging (CDI) (or simply color Doppler) sonography allows the visualization of flow direction and velocity within a user defined area. A region of interest is defined by the sonographer, and the Doppler shifts of returning ultrasound waves wit...

Color-write priority

Color-write priority is an adjustable setting of color Doppler duplex ultrasound and determines whether a particular pixel on the image displays color or grayscale B-mode information at the moment. Color-write priority is rarely changed directly during routine ultrasound imaging, even though it...

Color comet-tail artifact

The color comet-tail artifact is an ultrasonographic sign seen in a number of situations when color Doppler scanning is performed. Typically the artifact, which resembles the grey-scale comet-tail artifact, is seen in a situation when a small highly reflective (usually calcific) object is inter...

Comet-tail artifact

The comet-tail artifact is a grey-scale ultrasound finding seen when small calcific / crystalline / highly reflective objects are interrogated and is believed to be a special form of reverberation artifact. It is similar to the color comet-tail artifact and is seen in similar situations, althou...

Compression in mammography

In mammography, compression of the breast is performed to reduce its thickness. By doing so, the following benefits are achieved: improved subject contrast (by reducing scattered radiation) improved density uniformity improved visualization of breast tissue near chest wall (by spreading out s...

Compression paddle

A compression paddle is a device found in mammographic units which is used to compress the breast. It consists of a flat radiolucent plate positioned parallel to the support table, and attached to either a mechanical or pneumatic assembly. It is controlled by the operator by a foot pedal. Full...

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