Articles

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.

502 results found
Article

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,...
Article

Dependence of magnetization (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 ...
Article

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...
Article

dGEMRIC (delayed gadolinium-enhanced MRI of cartilage)

dGEMRIC, or delayed gadolinium-enhanced MRI of cartilage is a technique that can be used quantitatively to assess glycosaminoglycan content of cartilage or qualitatively to asses the overall structures of cartilage. When the technique is adapted and applied to menisci it is called dGEMRIM. At a...
Article

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...
Article

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...
Article

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...
Article

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...
Article

Diffusion tensor 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 assess neural tracts using data collected by diffusion tensor imaging. Diffusion-weigh...
Article

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...
Article

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...
Article

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...
Article

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, either indirect or direct: Indirect conv...
Article

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 ...
Article

Distance measurement

Ultrasound machines perform distance measurement to synthesize 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...
Article

Dixon method

The Dixon method is an MRI sequence based on chemical shift and designed to achieve uniform fat suppression. It has been gaining popularity as it has some advantages over other fat suppression techniques, namely:  suppression of fat signal is more uniform and less affected by artifacts than man...
Article

Doppler angle

Doppler angle corrects for the usual clinical situation when an ultrasound beam is not parallel to the Doppler signal. For instance, if one wants to evaluate an artery, the best angle for evaluation would be at zero degrees (parallel to the vessel). The strongest signal and best waveforms would...
Article

Doppler shift

Doppler shift or Doppler effect is defined as the change in frequency of sound wave due to a reflector moving towards or away from an object, which in the case of ultrasound is the transducer. Terminology When sound of a given frequency is discharged and subsequently reflected from a source th...
Article

Doppler waveforms

Doppler waveforms refer to the morphology of pulsatile blood flow velocity tracings on spectral Doppler ultrasound. Waveforms differ by the vascular bed (peripheral, cerebrovascular, and visceral circulations) and the presence of disease. Radiographic features Ultrasound Doppler Most authori...
Article

Dose length product

Dose length product (DLP) measured in mGy*cm is a measure of CT tube radiation output/exposure. It is related to CTDIvol, but CTDIvol represents the dose through a slice of an appropriate phantom. DLP accounts for the length of radiation output along the z axis (the long axis of the patient). D...
Article

Dose limits

Dose limits are recommended by the International Commission on Radiological Protection (ICRP). They are in place to ensure that individuals are not exposed to an unnecessarily high amount of ionizing radiation. Dose limits are a fundamental component of radiation protection, and breaching these ...
Article

Dosimeters

Dosimeters are a form of radiation monitoring devices. They are used regularly for a number of key roles including both patient and personal dosimetry, environmental monitoring, spectroscopy, radiopharmaceutical and equipment checking Function  Energy enters the dosimeter which converts this e...
Article

Double inversion recovery sequence

Double inversion recovery (DIR) is an inversion recovery MRI pulse sequence that uses two different inversion pulses. The technique can be used to suppress signal from two different tissues or to suppress signal that moved between the two pulses. In the first instance, used in neuroimaging, two...
Article

Double oblique multiplanar reconstruction

Double oblique is a type of multiplanar reconstruction used in cardiac cross-sectional imaging. It is useful for an accurate assessment of the ascending aorta and aortic annulus, and is particularly useful for pre- and post-procedure evaluation of a transcatheter aortic valve replacement (TAVR)....
Article

D-SPECT

D-SPECT represents the next step in the evolution of SPECT technology and is based on a unique acquisition geometry. It has nine arrays of cadmium zinc telluride (CZT) detectors, each of which rotates around its central axis with programmable angular rotation 1. The detectors are very compact al...
Article

Dual energy CT

Dual energy CT, also known as spectral CT, is a computed tomography technique that uses two separate x-ray photon energy spectra, allowing the interrogation of materials that have different attenuation properties at different energies. Whereas conventional single energy CT produces a single imag...
Article

Dual-energy mammography

Dual-energy digital mammography is a complementary breast imaging modality. The technique consists of high-energy and low-energy digital mammograms after administration of iodinated contrast agent. Breast is exposed to low- and high-energy x-ray beams during a single breast compression in MLO ...
Article

Dynamic contrast enhanced (DCE) MR perfusion

Dynamic contrast-enhanced (DCE) MR perfusion, sometimes also referred to as permeability MRI, is one of the main MRI perfusion techniques which calculates perfusion parameters by evaluating T1 shortening induced by a gadolinium-based contrast bolus passing through tissue. The most commonly calcu...
Article

Dynamic nuclear polarization

Dynamic nuclear polarization (DNP) is a phenomenon by which polarization is transferred from a polarizing agent to a biological tracer, enhancing the nuclear energy difference and thereby increasing the signal-to-noise ratio (SNR) dramatically.
Article

Dynamic susceptibility contrast (DSC) MR perfusion

Dynamic susceptibility contrast (DSC) MR perfusion is one of the most frequently used techniques for MRI perfusion, and relies on the susceptibility induced signal loss on T2* weighted sequences which results from a bolus of gadolinium-based contrast passing through a capillary bed.  The most co...
Article

Echo planar imaging

Echo planar imaging is performed using a pulse sequence in which multiple echoes of different phase steps are acquired using rephasing gradients instead of repeated 180 degree radio frequency pulses following the 90°/180° in a spin echo sequence. This is accomplished by rapidly reversing the rea...
Article

Echo time

The echo time (TE) refers to the time between the application of the radiofrequency excitation pulse and the peak of the signal induced in the coil. It is measured in milliseconds. The amount of T2 relaxation is controlled by the TE.
Article

Eddy currents

Eddy currents (also known as Foucault currents) are the result of rapidly changing gradient magnetic fields that in turn induce stray currents in the surrounding conducting materials. They form in accordance to Faraday's Law of Induction. Eddy currents are unwanted as they generate their own ma...
Article

Effect of gradient strength and bandwidth on slice thickness

Slice thickness is dependent on gradient strength and bandwidth, to attain a slice thickness, a range of frequencies must be transmitted to produce resonance across the whole slice. This range of frequencies is the transmitter RF (radiofrequency) bandwidth (tBW). Slice thickness is determined by...
Article

Elastography

Elastography is a newer technique that exploits the fact that a pathological process alters the elastic properties of the involved tissue. This change in elasticity is detected and imaged using elastography. Radiographic technique Sono-elastography  Sono-elastography is the term used when ult...
Article

Electrical interference artifact (ultrasound)

Electrical interference artifact is an ultrasound artifact usually caused by the ultrasound machine being too close to the unshielded electrical equipment. The disturbance appears as arc-like moving bands in the ultrasound image.  While the presence of electrical equipment  (e.g unshielded vent...
Article

Electromagnetic induction

Electromagnetic induction is the induction of electric current via changing magnetic fields. Magnetic fields are generated by moving charges (equivalent to electrical current). Ampere’s law or Fleming’s right hand rule determines the magnitude and direction (i.e. clockwise or anti-clockwise) of ...
Article

Electron

The electron is a subatomic particle that has negligible mass and is negatively charged. The properties of x-rays and their interaction with matter concern the orbiting electrons within the atom. In classical physics the electrons orbit the central positively charged nucleus in shells, the oute...
Article

Electron binding energy

The electron binding energy describes the work that is required to remove an electron from an atom, as electrons are held in place by the electrostatic pull of the positively charged nucleus. The electron binding energy is measured in electron volt (eV), where 1 eV = 1.6 x 10-19 J.  The magnitu...
Article

Electron-positron annihilation

Electron-positron annihilation is the process in which a positron collides with an electron resulting in the annihilation of both particles.  Electrons (or β- particles) and positrons (or β+ particles) are of equal mass but opposite charge. Positrons are the antimatter equivalent of an electron,...
Article

Energy difference between spin up and spin down states

The energy difference between spin up and spin down states of hydrogen are important in understanding net magnetization vector of tissue for magnetic resonance imaging. Each hydrogen atom is formed by one proton and one orbiting electron. Because the atomic number is 1, it has a spin quantum nu...
Article

Entrance phosphor

The entrance phosphor (or input phosphor) is a component of the image intensifier in fluoroscopic systems that converts the energy from x-rays into light photons. It is composed of a fluorescent material such as cesium iodide activated with sodium (CsI:Na) and coats the entrance surface of the i...
Article

Entrance skin dose

The entrance skin dose (or entrance surface dose), abbreviated as ESD, is the measure of the radiation dose that is absorbed (mGy) by the skin as it reaches the patient. Entrance skin dose is a directly measurable quantity, often, measured using thermoluminescent dosimeters (TLD) 1. Entrance ski...
Article

Entry slice phenomenon

Entry slice phenomenon occurs when unsaturated spins in blood first enter into a slice or slices. It is characterized by the bright signal in a blood vessel (artery or vein) at the first slice that the vessel enters. Usually, the signal is seen on more than one slice, fading with distance. This ...
Article

Equivalent dose

Equivalent dose (symbol HT) is a measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effects of different types of ionizing radiation. In quantitative terms, equivalent dose is less fundamental than absorbed dose, but it is more b...
Article

Erg (unit)

The erg is the unit of energy in the cgs system, where one erg = 1 x 10-7 joules. The rad, a legacy unit of absorbed radiation dose, was defined as the absorption of 100 ergs per gramme of irradiated material.
Article

Ernst angle

The Ernst angle is the flip angle that maximizes signal in T1-weighted sequences that have a short repetition time (TR). When the TR is very short, the best flip angle to maximize signal can be quite small. Choosing the Ernst angle in this setting can increase signal by several fold. However, i...
Article

Exposure

The term exposure refers to the concentration, in air, of radiation at a specific point and is the ionization produced in a specific volume of air: E = Q / m where E is exposure, Q is the quantity of charge on the ions and m is the unit mass of air. Exposure describes the ability of X-ray photo...
Article

F-18 fluorodeoxyglucose

F-18 fluorodeoxyglucose (FDG) is the most common PET radiotracer. Structure The radiopharmaceutical consists of the fluorine-18 radionuclide substituting the hydroxyl group at the C-2 position of glucose. The IUPAC chemical name is 2-deoxy-2-[F-18]fluoroglucose. Production F-18 fluoride ion ...
Article

Factors affecting T1

Factors affecting T1 and T2 relaxation times of different tissues are generally based on molecular motion, size and interactions. The protons giving rise to an NMR signal are mainly those in cell water and lipids (i.e. protons that are free to move), while those in protein and solids usually do...
Article

Faraday cage

A Faraday cage is a conductive enclosure used to shield the inner space from electromagnetic interference. In MR imaging, this type of structure provides radiofrequency shielding to the scanning room to minimize occurrence of interference-associated imaging artifact. Operation The cage is usua...
Article

Fast spin echo

Fast or turbo spin echo (FSE/TSE) is an adaptation of conventional spin-echo (SE) acquisition technique designed to reduce imaging time. It has largely supplanted the original spin-echo technique due to vastly improved imaging speed. Basic spin echo sequence In a basic SE sequence, a single ec...
Article

Fat suppressed imaging

Fat suppression is commonly used in magnetic resonance (MR) imaging to suppress the signal from adipose tissue or detect adipose tissue 1. It can be applied to both T1 and T2 weighted sequences.  Due to short relaxation times, fat has a high signal on magnetic resonance images (MRI). This high ...
Article

Fat-water swapping artifact

Fat-water swapping artifact is seen in a significant proportion of fat/water suppressed sequences using the Dixon method. The artifact follows a computational error in areas of field inhomogeneity resulting in incorrectly determining whether a voxel contains water or fat. The images have geogra...
Article

Ferromagnetism

Ferromagnetic materials generally contain iron, nickel, or cobalt. These materials include magnets, and various objects that might be found in a patient, such as aneurysm clips, parts of pacemakers, shrapnel, etc.  These materials have a large positive magnetic susceptibility, i.e. when placed ...
Article

Fetal dosimetry

NB - Please consult original article(s) and discuss with your local radiology department/radiation physicist before making any clinical decision. Although radiation exposure to the gravid uterus is to be avoided whenever possible, and only deliberately performed after careful weighing up of the...
Article

Fiber assignment by continuous tracking algorithm (FACT)

Fiber assignment by continuous tracking (FACT) algorithms are a commonly used deterministic post-processing algorithm for magnetic resonance tractography studies. In these algorithms, axonal fiber bundles are reconstructed - voxel by voxel - following the direction of the main eigenvector. The ...
Article

Filament circuit

The tungsten cathode needs to be heated for thermionic emission to take place. Thus a 10 voltage potential difference and 3-6 amperes of filament current is supplied, which forms the filament circuit.  This should not be confused with tube current which determines the flow of electrons from the...
Article

Film dosimeter

Film dosimeters are used to measure radiation exposure to workers to monitor radiation safety and ensuring that they receive doses below the appropriate limit.  Film badges are the cheapest and most common monitoring device. They consist of a small case with a piece of film situated between fil...
Article

Filtered back projection

Filtered back projection is an analytic reconstruction algorithm designed to overcome the limitations of conventional back-projection; it applies a convolution filter to remove blurring. It was, up until recently the primary method in cross sectional image reconstruction.  It utilizes simultane...
Article

Filters

Filters are metal sheets placed in the x-ray beam between the window and the patient that are used to attenuate the low-energy (soft) x-ray photons from the spectrum. Filtering is the removal of these low energy x-rays from the beam spectrum which would otherwise not contribute to image quality ...
Article

Fixing solution

Fixing solution is used in the darkroom to fix (i.e. remove unexposed silver) the developed and rinsed x-ray film in conventional radiography. Components fixing agent: sodium/ammonium thiosulfate, dissolves unexposed silver hardener: potassium alum, to harden the gelatin emulsion in film aci...
Article

FLAIR vascular hyperintensities

FLAIR vascular hyperintensities are hyperintensities encountered on FLAIR sequences within subarachnoid arteries related to impaired vascular hemodynamics 1,2. They are usually seen in the setting of acute ischemic stroke and represent slow retrograde flow through collaterals (and not thrombus) ...
Article

Flash mode (CEUS)

Flash mode is a technique specific to CEUS-capable ultrasound devices, and in brief is a short ultrasound pulse with a very high mechanical index, resulting in almost complete destruction of the contrast agent microbubbles in the imaging plane.  Physics The microbubbles utilized as contrast ag...
Article

Flat panel detector

Flat panel detectors (FPD) are used in direct digital radiography (DDR) for the conversion of x-rays to light (indirect conversion) or charge (direct conversion) which is read out using a thin film transistor (TFT) array. Types indirect conversion FPDs outermost layer is scintillator: phospho...
Article

Flip angle

The flip angle is an MRI phenomenon by which the axis of the hydrogen proton shifts from its longitudinal plane (static magnetic field B0) Z axis to its transverse plane XY axis by excitation with the help of radiofrequency (RF) pulses. A RF pulse is sent in at the precise Larmor frequency in re...
Article

Flow void

Flow voids refer to a signal loss occurring with blood and other fluids, like CSF or urine, moving at sufficient velocity relative to the MRI apparatus. It is a combination of time-of-flight and spin-phase effects usually seen in spin-echo techniques (such as T2-weighted images) 2. Physics Dur...
Article

Fluid attenuation inversion recovery

Fluid attenuation inversion recovery (FLAIR) is a special inversion recovery sequence with a long inversion time. This removes signal from the cerebrospinal fluid in the resulting images 1. Brain tissue on FLAIR images appears similar to T2 weighted images with grey matter brighter than white ma...
Article

Fluorography

Fluorography is the use of relatively intense (50-1000mA), pulsed x-ray exposures (pulses are of short duration and applied at 1-12 pulses/second) to form an x-ray image.  The resultant images have a relatively high signal to noise ratio (SNR), i.e the images are of better quality than those ac...
Article

Fluoroscopy

Fluoroscopy is an imaging procedure that allows real-time x-ray viewing of the patient with high temporal resolution. It is based on an x-ray image intensifier (II) coupled to a photographic and/or video camera. In recent years flat panel detectors, (which are similar to the digital radiography ...
Article

Fluoroscopy vs fluorography

Fluoroscopy and fluorography are very similar imaging techniques and, in many instances, can be performed on the same equipment. Fluoroscopy vs fluorography Fluoroscopy low current (0.5-5 mA), continuous or near-continuous x-ray exposures relatively low signal to noise ratio (SNR) prioritis...
Article

Flying focus tomosynthesis

Flying focus is a technology of image acquisition in digital breast tomosynthesis characterized by a continuous sweep during shooting. Sharpness in digital systems is determined by the modulation transfer function (MTF), which determine contrast transfer as a function of spatial frequency. Sin...
Article

Focal spot

Focal spot is the area of the anode surface which receives the beam of electrons from the cathode. It is the apparent source of x-rays.  Basic concept Size and shape of the focal spot is determined by the size and shape of the electron beam when it strikes the anode 1. Size and shape of the e...
Article

Fourier transform

Fourier transform is a mathematical operation which converts a time domain signal into a frequency domain signal. Discussion Fourier transform is integral to all modern imaging, and is particularly important in MRI. The signal received at the detector (receiver coils in MRI, piezoelectric disc...
Article

Fourier transformation

The Fourier transform is a mathematical formula that converts a signal in time (or spatial) domain t to one in the frequency domain ω. A frequency domain signal F(ω) is obtained by modulating the time domain signal f(t) to a special sinusoidal wave e-jωt across all time (from negative infinity ...
Article

Fractionation (radiation therapy)

Radiation therapy is usually divided or “fractionated” over a treatment course lasting multiple weeks. Fractionation in the context of radiotherapy is the process of dividing a dose of radiation into multiple “fractions”. This practice seeks to maximize the destruction of malignant cells while m...
Article

Free induction decay

Free induction decay (FID) refers to a short-lived sinusoidal electromagnetic signal which appears immediately following the 90° pulse. It is induced in the receiver coil by the rotating component of the magnetization vector in the x-y plane which crosses the coil loops perpendicularly. It does ...
Article

Fringe field (MRI)

The fringe field is the peripheral magnetic field outside of the magnet core. It is important because it can cause interference with nearby electronic devices, such as pacemakers. Although the strength of the magnetic fields decreases with distance from the core of the magnet, the effect of the ...
Article

Functional MRI

Functional magnetic resonance imaging (fMRI) is a technique used to obtain functional information by visualizing cortical activity. fMRI detects subtle alteration in blood flow in response to stimuli or actions. It is used in two broad ways: clinical practice typically in presurgical patients ...
Article

Gadolinium contrast agents

Molecular complexes containing the rare earth metal gadolinium, chelated to a carrier ligand, form the gadolinium contrast agents (a type of paramagnetic contrast agent) which are the primary class of MRI contrast media. The intravenous route of administration is the commonest. Chemistry The g...
Article

Gallium-67 scintigraphy

Gallium-67 is a photon-emitting radiotracer used for scintigraphy which is used in the form of various salts like citrate and nitrate. Once administered, imaging may consist of planar (2 dimensional) , SPECT, and SPECT-CT acquisitions. Once injected it binds to plasma proteins (especially transf...
Article

Gamma-aminobutyric acid (GABA) peak

Gamma-aminobutyric acid (GABA) is the principle inhibitory neurotransmitter in the central nervous system 1 and as such, is one of the compounds examined in MR spectroscopy.  It is present in the human brain at a concentration of about 1 mM, a whole order of magnitude lower than some of the more...
Article

Gamma camera

Gamma cameras (also called scintillation cameras or Anger cameras) are the predominant nuclear medicine imaging machine currently in use. They permit the acquisition of planar images. Design From the exterior to interior a camera is comprised of 1: head cover: usually made from glass collima...
Article

Gastrointestinal MRI contrast agents

Gastrointestinal MRI contrast agents are varied and can be either positive or negative agents. Acceptance of the use of MRI in abdominal imaging has been limited in part by difficulty in distinguishing bowel from intra-abdominal masses and normal organs. The use of enteric contrast agents can ai...
Article

Gauss (unit)

The gauss (symbol: G or Gs) is a legacy CGS unit of magnetic flux density, which was superseded by the tesla. One gauss is defined as one maxwell per cm2 (Mx/cm2), which equates to 10-4 tesla, and is therefore a small unit. This is one of the reasons for its stubborn persistence in some scientif...
Article

Gibbs and truncation artifacts

Gibbs artifact, also known as truncation artifact or ringing artifact, is a type of MRI artifact. It refers to a series of lines in the MR image parallel to abrupt and intense changes in the object at this location, such as the CSF-spinal cord and the skull-brain interface. The MR image is reco...
Article

Glutamine-Glutamate peak

Glutamate-Glutamine (Glx) peak is one of the regions assessed on MR spectroscopy, and resonates between 2.2 and 2.4 ppm chemical shift. It overlaps with the GABA peak and cannot be routinely separated from each other.
Article

Godfrey Hounsfield

Sir Godfrey N Hounsfield (1919-2004) pioneered the CT scanner making him one of the greats in the history of radiology and medicine in general. For his work, he received the Nobel Prize in Physiology or Medicine in 1979. This was remarkable because he had had no previous experience of working in...
Article

Gradient coils

Gradient coils are used to produce deliberate variations in the main magnetic field (B0). There are three sets of gradient coils, one for each direction. The variation in the magnetic field permits localization of image slices as well as phase encoding and frequency encoding. The set of gradient...
Article

Gradient echo sequences

Gradient echo sequences (GRE) are an alternative technique to spin echo sequences, differing from it in two principal points: utilization of gradient fields to generate transverse magnetization flip angles of less than 90° Compared to the spin echo and inversion recovery sequences, gradient e...
Article

Gray (SI unit)

The gray (symbol Gy) is the SI unit of absorbed dose and is defined as the absorption of one joule of energy, in the form of ionizing radiation, per kilogram of matter, i.e. one gray = 1 J/kg 2. Terminology One gray is a large unit and is usually used with a prefix, e.g. milligray (mGy), micro...
Article

Grey scale imaging (ultrasound)

Commonly referred to as B (brightness) mode, the use of grey scale imaging in ultrasound renders a two-dimensional image in which the organs and tissues of interest are depicted as points of variable brightness. The formation of a B-mode image relies on the pulse-echo principle; assuming the spe...
Article

Grid cutoff

Grid cutoff is an unwanted absorption of x-rays via an x-ray grid, observed when a grid is employed incorrectly, most often seen with parallel grids. The term cutoff stems from the phenomenon in which the primary x-ray beam is 'cut off' by grid lines, leading to an overall decrease in optical de...

Updating… Please wait.

 Unable to process the form. Check for errors and try again.

 Thank you for updating your details.