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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 a number of 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 Signal-to-n...

Absorbed dose

Absorbed dose is a measure of the energy deposited in a medium by ionising 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

Acoustic shadowing on an ultrasound image is characterized by a signal void behind structures that strongly absorb or reflect ultrasonic waves. This happens most frequently with solid structures, as sound conducts most rapidly in areas where molecules are closely packed, such as in bone or stone...

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...

Air bubble artifact (CT)

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 (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...

Air gap technique (plain 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 ...

Aliasing artefact (ultrasound)

Aliasing is a commonly encountered artefact in color and spectral Doppler ultrasound, which is caused by blood flow velocity exceeding the upper limits of the scale set by the user 1. The maximum velocity that can be displayed is limited by the pulse repetition frequency (PRF). If the PRF is too...

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 digitisation. Image digitisation 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...

Alpha decay

All nuclei with the atomic number Z>82, are considered unstable. These are considered “neutron-rich” and undergo the decay process by emitting a particle containing two neutrons and two protons. Alpha decay is the process in which an alpha particle (containing two neutrons and two protons) is e...

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...

Anode

The anode 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 the diameter between 55 and 100 mm, and thickness of 7 mm, connected to the positive side of the electrical circuit. The anode converts the energy of the ele...

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 effects 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...

Antoine Henri Becquerel

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

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 labelling MR perfusion

Arterial spin labelling (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...

Artifact

Artifacts in radiology refer to something seen on an image that is not present in reality but appears due to a quirk of the modality itself. The commonest artifact seen in radiology is image noise, which is inherent to every modality and technique, and can be mitigated but never eliminated. As...

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

Attenuation is the reduction in power and intensity of sound waves as they travel through tissue.

Attenuation coefficient

The attenuation coefficient is quantified as the measure of how easily a material can be penetrated by an x-ray beam. It quantifies how much the beam is "attenuated" (i.e. weakened) by the material it is passing through.

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...

Background radiation

Background radiation refers to exposure to ionising radiation in day-to-day life, excluding occupational exposures. It is measured in millisieverts (mSv). Ionising 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 ionising radiation adheres to the fundamental principles of radiation protection. The general public's understanding of ionising 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 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, the unit in the superseded cgs system, in 1975. Terminology One becquerel is a very small unit and is invariably used with a prefix, e.g. mega...

Beta decay

Beta particles occurs 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. Kinetic energy of beta particles has a continuous spectrum. Beta minu...

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...

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 artefact (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 ...

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...

Bremsstrahlung

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...

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 ...

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 a rubber or felt for adequate contact between screen film system or with the imaging plate. The front is made of low atomi...

Cathode

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 i.e. a coil of wire made f...

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...

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

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 of K-shell electron) leaving behind a 'hole'. An outer shell electron fills this hole (from the L-shell, M-shell, ...

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 differenc...

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...

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.

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 ionisation energy of the atom. It therefore occurs with low energy radiation. Upon i...

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...

Compton effect

Compton effect or Compton scatter is one of three principle forms of photon interaction. It is the main cause of scattered radiation in a material. It occurs due to the interaction of the x-ray or gamma photon with free electrons (unattached to atoms) or loosely bound valence shell (outer shell)...

Computed tomographic (CT) colonography

Computed tomographic (CT) colonography, also called CTC, virtual colonoscopy (VC) or CT pneumocolon, is a powerful minimally invasive technique for colorectal cancer screening. Indications screening test for colorectal carcinoma colon evaluation after incomplete or unsuccessful conventional c...

Computed tomography

Computed tomography (CT) scanning, also known as, especially in the older literature and textbooks, computerized axial tomography (CAT) scanning, is a diagnostic imaging procedure that uses x-rays to build cross-sectional images ("slices") of the body. Cross-sections are reconstructed from measu...

Computed tomography texture analysis

Computed tomography texture analysis (or CTTA) is a method to obtain new useful biomarkers that provide objective and quantitative assessment of tumor heterogeneity by analyzing the differences and patterns within the pixel values of an image. CTs can be worked with as a matrix of numbers, corre...

Cone beam CT

Cone beam CT (CBCT) is a variant type of computed tomography (CT), and is used particularly in dental and extremity imaging but has recently found new application in dedicated breast imaging 4,5. It differs from conventional CT in that it uses cone-shaped x-ray beam and two dimensional detectors...

Contrast enhanced mammography

Contrast enhanced mammography (CEM) is a complementary breast imaging modality. A finite number of sequential images are obtained with X-ray beam produced at a high energy, above the K-edge of Iodine, and with an intravenous non-ionic Iodine contrast agent injected between pre and post contrast...

Contrast enhanced MR angiography

Contrast enhanced MR angiography (MRA) is a technique involving 3D spoiled gradient-echo (GE) sequences, with administration of gadolinium-based contrast agents (GBCA). It can be used to assess vascular structures of almost any part of the body. Its key features are: T1 weighted spoiled gradien...

Contrast-enhanced ultrasound

Contrast-enhanced ultrasound (CEUS) involves the administration of intravenous contrast agents containing microbubbles of perfluorocarbon or nitrogen gas. The bubbles greatly affect ultrasound backscatter and increase vascular contrast in a similar manner to intravenous contrast agents used in C...

Contrast enhancement

Contrast enhancement is a ubiquitous term in radiology and can be used in two ways. Firstly, it may refer to methods of exaggerating the visible difference between adjacent structures on scans by administering contrast agents. This includes differentiating between normal structures. There are ...

Contrast resolution

Contrast resolution in radiology refers to the ability of the imaging modality to distinguish between differences in image intensity. The inherent contrast resolution of a digital image is given by the number of possible pixel values, and is defined as the number of bits per pixel value. See al...

Convex array

Convex (sequential) arrays, also known as curvilinear or curved linear arrays, are similar to linear arrays but with piezoelectric elements arranged along a curved transducer head. Ultrasound beams are emitted at 90 degrees to the transducer head. This arrangement results in a trapezoidal field ...

Coronary MR angiography

Coronary MR angiography (coronary MRA) is a developing approach to imaging the coronary arteries. Advantages of coronary MRA include avoidance of the intravenous iodinated contrast and ionizing radiation used in coronary CT angiography and conventional angiography. A disadvantage of coronary M...

Coulomb per kilogram

The SI unit for exposure to ionising radiation is coulomb per kilogram (Ckg-1) and curiously unlikely other SI radiation units, a specific name has not been adopted for this unit. This unit officially replaced the old unit, the roentgen in 1975, with an official transition period lasting at leas...

Creatine peak

Creatine is one of the compounds examined in MR spectroscopy. It resonates at 3.0 ppm chemical shift and is found in metabolically active tissues (brain, muscle, heart) where it is important in storage and transfer of energy. It tends to be maintained at a relatively constant level, and is predo...

Cross-excitation artifact (MRI)

Cross-excitation artifact is a type of MRI artifact and refers to the loss of signal within a slice due to pre-excitation from RF pulse meant for an adjacent slice. The frequency profile of the RF pulse is imperfect; this means that during slice selection there is some degree of excitation of t...

CSF flow studies

CSF flow studies are performed using a variety of MRI techniques and are able to qualitatively assess and quantify pulsatile CSF flow. The most common technique used is time-resolved 2D phase contrast MRI with velocity encoding. Note, when referring to CSF flow in the context of imaging we are...

CT artifacts

CT artifacts are common and can occur for various reasons. Knowledge of these artifacts is important because they can mimic pathology (e.g. partial volume artefact) or can degrade image quality to non-diagnostic levels. CT artifacts can be classified according to the underlying cause of the ar...

CT cholangiography

CT cholangiography is a technique of imaging the biliary tree with the usage of hepatobiliary excreted contrast. It is useful in delineating biliary anatomy, identifying a bile leak or looking for retained gallstones within the biliary system. Indications Second-line test (after ultrasound) wh...

CT dose index

CT dose index (CTDI) is a standardized measure of radiation dose output of a CT scanner which allows the user to compare radiation output of different CT scanners. In the past CTDI100 (measured over a 100 mm long ionization chamber) and CTDIw (weighted average of dose across a single slice) were...

CT enteroclysis

Computed tomographic (CT) enteroclysis refers to a hybrid technique that combines the methods of fluoroscopic intubation-infusion small bowel examinations with that of abdominal CT 1. Indications CT enteroclysis is complementary to capsule endoscopy in the elective investigation of small-bowel...

CT fluoroscopy

Computed tomography (CT) fluoroscopy combines the conventional advantages of both CT and fluoroscopy and has an important role in image-guided interventions where real-time imaging is required. Historically, fluoroscopy was the main image guidance tool for interventional radiology procedures. T...

CT scanner (evolution)

CT scanners were first introduced in 1971 with a single detector for brain study under the leadership of Godfrey Hounsfield, an electrical engineer at EMI (Electric and Musical Industries Ltd). Thereafter, it has undergone several changes with an increase in the number of detectors and decrease ...

CT stair-step artifact

The CT stair-step artifact is found in straight structures which are oriented obliquely with respect to movement of the table and appear around the edges of sagittal and coronal reformatted images when wide collimations and non-overlapping reconstruction intervals are used. It is also seen in c...

Curie (unit)

The curie (symbol Ci) was the unit for radioactive decay in the cgs system. One curie was defined as the radioactivity of one gram of pure radium-226; this is equivalent to 3.7 x 1010 decays per second. It was officially replaced by the becquerel in 1975. Terminology One curie was too large t...

Decibel

The decibel (dB) is a unit that measures the relative difference between two sound intensities. The relationship is logarithmic: dB = 10 log (I2 / I1) dB = relative intensity of the sounds I1 = intensity of sound 1 I2 = intensity of sound 2 Informally, we use decibel as a unit of "loudness,...

Dependence of magnetisation (proton density, field strength and temperature)

The dependence of magnetism is based on proton density (PD), field strength and temperature. There is a frictional interchange of energy between the protons and the lattice (spin-lattice interaction), such that a balanced exchange occurs between the two energy states and the thermal equilibrium ...

Detective quantum efficiency

Detective quantum efficiency (DQE) is one of the fundamental physical variables related to image quality in radiography and refers to the efficiency of a detector in converting incident x-ray energy into an image signal. The words "quantum efficiency" have a precise meaning, because the DQE me...

Diamagnetism

Diamagnetism is the property of materials that have no intrinsic atomic magnetic moment, but when placed in a magnetic field weakly repel the field, resulting in a small negative magnetic susceptibility. Materials like water, copper, nitrogen, barium sulfate, and most tissues are diamagnetic. T...

Diastolic pseudogating

Diastolic pseudogating appears as periodic bright and dark signal in arteries such as the aorta as one progresses through a series of images. Synchronization of the cardiac cycle and the pulse sequence results in high signal in the artery during diastole when blood is relatively stationary and l...

Dielectric effect artifact

Dielectric effect artifact is an MRI artifact encountered most often on body MRI with 3 T units. Artifact At 3 T, the radiofrequency (RF) wavelength measures 234 cm in air, and the speed and wavelength of the RF field is shortened to ~26 cm within the body as a result of dielectric effects. Ho...

Diffusion kurtosis imaging

Diffusion kurtosis imaging (DKI) is an advanced neuroimaging modality which is an extension of diffusion tensor imaging by estimating the kurtosis (skewed distribution) of water diffusion based on a probability distribution function. It provides a high order diffusion of water distribution and a...

Diffusion tensor imaging

Diffusion tensor imaging (DTI) is an extension of diffusion weighted imaging that allows data profiling based upon white matter tract orientation. Diffusion weighted imaging is based on the measurement of Brownian motion of water molecules. This motion is restricted by membranous boundaries. In...

Diffusion-tensor MRI imaging and fiber tractography

Diffusion-tensor imaging (DTI) is an MRI technique that uses anisotropic diffusion to estimate the axonal (white matter) organization of the brain. Fiber tractography (FT) is a 3D reconstruction technique to access neural tracts using data collected by DTI. Within cerebral white matter, water m...

Diffusion weighted imaging

Diffusion-weighted imaging (DWI) is a form of MR imaging based upon measuring the random Brownian motion of water molecules within a voxel of tissue. In general simplified terms, highly cellular tissues or those with cellular swelling exhibit lower diffusion coefficients. Diffusion is particular...

Diffusion weighted MRI in acute stroke

Diffusion weighted imaging (DWI) is a commonly performed MRI sequence for evaluation of acute ischemic stroke, and is sensitive in the detection of small and early infarcts. Conventional MRI sequences (T1WI, T2WI) may not demonstrate an infarct for 6 hours, and small infarcts may be hard to appr...

Digital radiography

Digital radiography is based on the use of discrete values in comparison to conventional radiography which uses analog/continuous values. It removes the requirement of dark room procedures. Types computed radiography (CR) uses photostimulable phosphor plates (PSP) in cassettes direct digital...

Direct digital radiography

Direct digital radiography (DDR) refers to direct digital registration of the image at the detector with no intermediate processing step required to obtain the digital signals as in computed radiography (CR). There are two primary methods of conversion are indirect and direct: Indirect convers...

Disintegrations per second

Disintegrations per second (dps), also known as decays per second, represents the number of atoms of a radioactive isotope that decay per second. One becquerel is equivalent to one disintegration per second. Counts per second (cps) is the number of disintegrations per second, as measured by a ...

Distance measurement

Ultrasound machines perform distance measurement to synthesise images from returning echoes. To generate images for an ultrasound scan, machines need to determine the distance of reflective interfaces from the transducer. Simply, the formula used is: distance = (speed x time)/2 Where: distanc...

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