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.

359 results found
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. Doppler equation  F = 2fo(v/c)cos(Q) where: F is doppler frequency shift fo is transmitted frequen...
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Doppler waveforms

Doppler waveforms are often misinterpreted and/or overlooked. They can provide great deal of information if carefully understood. Radiographic features Ultrasound Doppler The three basic waveforms are 1-2: triphasic: triphasic waveform forward flow in systole reverse flow in late systole ...
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Dose limits

Dose limits are recommended by the International Commission on Radiological Protection (ICRP), they are in place to ensure that the individuals are not exposed to an unnecessarily high amount of ionising radiation. Dose limits are a fundamental component of radiation protection, and breaching th...
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Double inversion recovery sequence

Double inversion recovery (DIR) is a MRI pulse sequence which suppresses signal from the CSF as well as from the white matter and hence enhances any inflammatory lesion. To obtain such sequence in 3T MRI scanner, two inversion times are required. TI1 which is used for suppression of CSF and usu...
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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)....
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Dual energy CT

Dual energy CT utilises two separate energy sets to examine the differing attenuation properties of matter, having a significant advantage over traditional single energy CT. Independent attenuation values at two energy sets can create virtual non-contrast images from contrast enhanced imaging as...
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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 ...
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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...
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Dynamic nuclear polarisation

Dynamic nuclear polarisation (DNP) is a phenomenon by which polarisation is transferred from a polarising agent to a biological tracer, enhancing the nuclear energy difference and thereby increasing the signal-to-noise ratio (SNR) dramatically.
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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...
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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 180o RF pulses following the 90°/180° in a spin-echo sequence. This is accomplished by rapidly reversing the readout or frequency- ...
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

In accordance with Faraday's Law of Induction, rapidly changing gradient magnetic fields can induce stray currents, known as eddy currents, in the surrounding conducting materials. Eddy currents are unwanted as they generate their own magnetic fields, which oppose the original magnetic field vi...
Article

Eklund technique

Eklund modified compression technique  is a technique which can be used for patients with augmented or reconstructed breasts post mastectomy.  Technique It consists of postero-superior displacement of the implants simultaneously to an anterior traction of the breast, pushing the implants towar...
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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  strain elastography (also known as static o...
Article

Electron-positron annihilation

Electron-positron annihilation is the process in which a positron (from B+ decay) collides with an electron resulting in their annihilation. Being of opposite charges and same mass they act as a collision of subatomic particle and anti-particle. According to the law of conservation of energy, t...
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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 magnetisation 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 is a component of the image intensifier in fluoroscopic systems which converts the energy from x-rays into light photons. It is composed of a fluorescent material such as caesium iodide activated with sodium (CsI:Na) and coats the entrance surface of the image intensifier. ...
Article

Entrance skin dose

The entrance skin dose 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 skin dose is often a benchmark measurement used to ...
Article

Entry slice phenomenon

Entry slice phenomenon occurs when unsaturated spins in blood first enter into a slice or slices. It is characterized by 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 mecha...
Article

Ernst angle

The Ernst angle is the flip angle that maximises signal in T1-weighted sequences that have a short repetition time (TR). When the TR is very short, the best flip angle to maximise 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-rays to i...
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

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 you local radiology department/radiation physicist before making any clinical decision. Although exposure to the gravid uterus is to be avoided when ever possible, and only deliberately performed after careful weighing up of the pros and ...
Article

Filament circuit

The tungsten cathode needs to be heated for thermionic emission to take place. Thus a 10 V potential difference and a 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 cathode...
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 crossectional image reconstruction.  It utilises simultaneou...
Article

Filters

Filters are metallic sheets that are designed to mainly absorb the low-energy x-ray photons from the spectrum. If unfiltered these low-energy x-ray photons are absorbed in the patient, thus not contributing to image formation and just increase dose. Therefore using a filter reduces the patient d...
Article

Fixing solution

Fixing solution is used in dark room 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 hardner: potassium alum, to harden the gelatin emulsion in film acidifi...
Article

Flat panel detectors

Flat panel detectors (FPD) are used in direct digital radiography (DDR) for conversion of X-rays to light (indirect conversion) or charge (direct conversion) which is read out using thin film transistors (TFT array). Types They are of two types: indirect conversion FPDs outermost layer is sc...
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 T1 and T2-weighted images) 2. Physi...
Article

Fluid attenuation inversion recovery

Fluid attenuation inversion recovery (FLAIR) is a special inversion recovery sequence with a long inversion time (TI) which results in removing signal from the cerebrospinal fluid (CSF) from the resulting images.1  To null the signal from fluid, the inversion time (TI) of the FLAIR pulse sequen...
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. 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 electron beam is determined by: dimens...
Article

Focussed Assessment with Sonography for Trauma (FAST) scan

Focussed Assessment with Sonography for Trauma (FAST) scan is a point-of-care ultrasound examination performed at the time of presentation of a trauma patient.  It is invariably performed by a clinician, who should be formally trained, and is considered as an 'extension' of the trauma clinical ...
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

Free induction decay

Free induction decay (FID) refers 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 not...
Article

Functional MRI

Functional magnetic resonance imaging (fMRI) is a technique used to obtain functional information by visualising 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 pre-surgical patients...
Article

Gadobenate dimeglumine

Gadobenate dimeglumine (also known as MultiHanceTM) is an extracellular intravenous contrast agent used in magnetic resonance imaging. linear, ionic molecule  95-96% renal excretion, 4-5% hepatic excretion T1 relaxivity @ 1.5 T: 6.0-6.6 concentration: 0.5 mmol/ml recommended dosage: 0.1 mmo...
Article

Gadobutrol

Gadobutrol (also known as Gadovist/GadavistTM) is an extracellular intravenous contrast agent used in magnetic resonance imaging. cyclic, nonionic molecule  100% renally excreted T1 relaxivity @ 1.5 T: 4.9-5.5 (slightly higher than other extracellular contrast agents) concentration: 1.0 mmol...
Article

Gadodiamide

Gadodiamide (also known as OmniscanTM) is an extracellular intravenous contrast agent used in magnetic resonance imaging. linear, nonionic molecule  100% renally excreted T1 relaxivity @ 1.5 T: 4.0-4.6 concentration: 0.5 mmol/ml recommended dosage: 0.1 mmol/kg Indications As an extracellu...
Article

Gadofosveset trisodium

Gadofosveset trisodium (also known as AblavarTM or VasovistTM) is an intravenous blood pool contrast agent used in magnetic resonance imaging. It was designed as an agent for contrast-enhanced MR angiography since it exhibits strong binding to plasma proteins, thus remaining in the blood stream...
Article

Gadolinium

Gadolinium (Gd) is a metallic element (atomic number 64) that can be chelated into paramagnetic agents that are injected intravenously during MR imaging. The gadolinium ion is useful as an MRI agent because it has seven unpaired electrons, which is the greatest number of unpaired electron spins...
Article

Gadopentetate dimeglumine

Gadopentetate dimeglumine (also known as MagnevistTM) is an extracellular intravenous contrast agent used in magnetic resonance imaging. linear, ionic molecule  100% renally excreted T1 relaxivity @ 1.5 T: 3.9-4.3 concentration: 0.5 mmol/ml recommended dosage: 0.1 mmol/kg Indications As a...
Article

Gadoterate meglumine

Gadoterate meglumine (also known by the tradename DotaremTM) is an intravenous extracellular MRI contrast agent. cyclic, ionic molecule  100% renally excreted T1 relaxivity @ 1.5 T: 3.4-3.8 (slightly lower than other extracellular contrast agents) concentration: 0.5 mmol/ml recommended dosa...
Article

Gadoteridol

Gadoteridol (also known as ProHanceTM) is an extracellular intravenous contrast agent used in magnetic resonance imaging. cyclic, nonionic molecule  100% renally excreted T1 relaxivity @ 1.5 T: 3.9-4.3 concentration: 0.5 mmol/ml recommended dosage: 0.1 mmol/kg Indications As an extracellu...
Article

Gadoversetamide

Gadoversetamide (also known as OptiMARKTM) is an extracellular intravenous contrast agent used in magnetic resonance imaging. linear, nonionic molecule  100% renally excreted T1 relaxivity @ 1.5 T: 4.4-5.0 concentration: 0.5 mmol/ml recommended dosage: 0.1 mmol/kg Indications As an extrac...
Article

Gadoxetate disodium

Gadoxetate disodium (also known by the tradenames PrimovistTM and EovistTM) is a hepatospecific paramagnetic gadolinium-based contrast agent, used exclusively in MRI liver imaging. Its chief use is in hepatic lesion characterisation, i.e. assessing focal liver lesions identified on other imaging...
Article

Gallium 67 scintigraphy

Gallium 67 is a radiotracer which is used in the form of various salts like citrate and nitrate. Gamma camera imaging includes planar (2 dimensional) , SPECT and SPECT/CT scans. It has a predilection to sites of inflammation. It binds to inflammatory proteins and thus it pools up at the sites of...
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

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

Gd-DTPA labeled albumin

Gd-DTPA labeled albumin is an intravascular MRI contrast agent. Gd-DTPA is covalently bonded to albumin in ratios from 16:1 to 31:1 providing excellent enhancement of liver, spleen, myocardium, brain, and slow moving blood of rats and rabbits. The albumin has a molecular weight of about 92,000 a...
Article

Gd-DTPA labeled dextran

Gd-DTPA labeled dextran is an intravascular MRI contrast agent. Dextran is a polysaccharide consisting of a polymer of glucose molecules with a molecular weight between 75,000-100,000. Dextran has a high level of safety and is broken down more rapidly than albumin. Approximately 15 Gd-DTPA molec...
Article

Gibbs and truncation artifacts

Gibbs 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 reconstructed from k-space which is a finite sampling of the...
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 Hounsfield (b. 28 Aug 1919, d. 12 Aug 2004) pioneered the CT scanner making him one of the greats in the history of radiology. For his work, he received the Nobel Prize in Stockholm in 1979. This was remarkable because Godfrey had no previous experience of working in the medical fiel...
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 magnetisation flip angles of less than 90° Compared to the spin echo and inversion recovery sequences, GRE sequen...
Article

Grid cut off

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...
Article

Grids

Grids are placed between the patient and the X-ray film to reduce the scattered radiation (produced mainly by compton effect) and thus improve image contrast. They are made of parallel strips of lead with an interspace having an aluminum or organic spacer. The strips can be oriented either line...
Article

Gyromagnetic ratio

The gyromagnetic ratio, often denoted by the symbol γ (gamma) is the ratio of the magnetic momentum in a particle to its angular momentum. The SI unit is the radian per second per Tesla (rad⋅s−1⋅T−1).  The gyromagnetic ratio of the proton is 2.675 221 900(18) x 108 s-1 T-1. Since a proton wil...
Article

Half life time

Physical half life time (Tp) The time interval required for an amount of certain radioactive nuclei to decay to its half of original value. Biological half life time (Tb)  The time interval required for the body to eliminate 50% of any substance by normal routes of elimination: metabolic turn...
Article

Half value layer

Half value layer (HVL) is the width of a material required to reduce the air kerma of a x-ray or gamma-ray to half its original value. This applies for narrow beam geometry only as broad-beam geometry will experience a large degree of scatter, which will underestimate the degree of attenuation. ...
Article

Harmonic imaging

Harmonic imaging is a technique in ultrasonography that provides images of better quality as compared with conventional ultrasound technique. Physics Harmonic imaging exploits non-linear propagation of ultrasound through the body tissues. The high pressure portion of the wave travels faster th...
Article

Helical CT image acquisition

Helical ("spiral") CT image acquisition was a major advance on the earlier stepwise ("stop and shoot") method. With helical CT, the patient is moved through a rotating x-ray beam and detector set. From the perspective of the patient, the x-ray beam from the CT traces a helical path. The helical...
Article

Hepatobiliary MRI contrast agents

Hepatobiliary MRI contrast agents are specialised agents used to aid diagnosis in MRI. They are separated into three categories: gadolinium-based agents, manganese-based agents and superparamagnetic iron oxide particles.  Gadolinium based agents Gadolinium (Gd) based contrast agents are class...
Article

Herringbone artifact

Herringbone artifact, also called as crisscross artifact or corduroy artifact, is an MRI artifact, it appears as a fabric of herring bone. The artefact is scattered all over the image in a single slice or multiple slices. Causes electromagnetic spikes by gradient coils fluctuating power suppl...
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High resolution CT

High resolution CT is a scanning protocol in which thin sections (usually 0.625 to 1.25 mm) are acquired and reconstructed using a sharp algorithm (e.g. bone algorithm). It has been used for: lung imaging temporal bone imaging Lung imaging Two techniques have been used: spaced axial (non-h...
Article

High voltage generator

X-ray units require a high voltage generator to achieve the necessary power required of an x-ray tube. AC power will supply x-ray units with sinusoidal currents, resulting in 'peaks and troughs', limiting an x-ray tube to produce x-rays only half of the 1/60th of s second cycle.  A single-phase...
Article

High-intensity focused ultrasound

High-intensity focused ultrasound (HIFU) is a promising non-invasive thermal ablation technique. Unlike diagnostic ultrasound, HIFU focuses the ultrasound waves precisely upon a target. In a similar way to how light can be focused to a burning point by a magnifying glass, ultrasound passes thro...
Article

Historical aspects of ultrasound in medicine

The first written document dealing with the use of waves in spatial orientation dates back to 1794, when Lazaro Spallanzani (“Opus coli di fisica”), analyzed the basic mechanisms of spatial orientation of the bats, proposing other mechanisms of spatial orientation than the visual – ophthalmic sy...
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Hormesis

Hormesis is a controversial concept in toxicology. The proposed concept is that very tiny amounts of a toxin may potentially have beneficial biological effects, rather than deleterious effects. In the context of radiology it refers to a proposed modification to the linear no-threshold theory to...
Article

Hounsfield unit

The Hounsfield unit (HU) is a quantity commonly used in computed tomography (CT) scanning to express CT numbers in a standardised and convenient form. Hounsfield units, created by and named after Sir Godfrey Hounsfield, are obtained from a linear transformation of the measured attenuation coeffi...
Article

Hunter's angle

Hunter's angle (HA) is a term coined from a neurosurgeon, Hunter Sheldon, at Huntington Medical Research Institutes. He placed his comb on the spectrum at approximately a 45° angle and connected several of the peaks. If the angle and peaks roughly corresponded to the 45° angle, the curve was con...
Article

Hyperintense on T1-weighted images (mnemonic)

Mnemonics for bright or hyperintense T1-weighted lesions include: My Best Friend is Pretty Cool 3 Fs and 4 Ms Mnemonic My Best Friend is Pretty Cool: M: melanin B: blood (i.e. methaemoglobin in subacute haemorrhage) F: fat and slow flow P: protein; paramagnetic substances (e.g. manganese...
Article

I-123 Ioflupane

I123 Ioflupane is a radiopharmaceutical for the diagnosis of Parkinson disease and its differential diagnoses. Characteristics iodine-123 is a cyclotron product physical half-life is 13.22 hours predominant energy of its gamma spectrum is 159 keV Ioflupane is the international nonproprietar...
Article

I-131

I-131​ (or 131I) is a radioisotope used in the diagnosis and treatment of thyroid lesions. It is one of the oldest radiotracers used in nuclear medicine, in use for over 50 years. It is predominately used in thyroid ablation therapy, for patients post thyroidectomy, and for metastatic thyroid ca...
Article

Image intensifier

Image intensifiers are utilised to convert low energy radiation into visible light images. Frequently the detector portion of an x-ray c-arm use in operating theatres, the image intensifier has a low scatter input portion comprised of low absorption substances such as titanium or aluminium 1,2. ...
Article

Image plate artifact

Image plate artifact is caused by backscatter radiation. Backscatter radiation is transmitted through the back of the cassette to the cassette hinge where the lead coating gets weakened or cracked.  To reduce backscatter, the radiographer should collimate where possible.
Article

Image reconstruction (CT)

Image reconstruction in computed tomography is a rapidly evolving industry, the race to produce an efficient yet accurate image reconstruction method while keeping scan dose to a minimum has defined improvements in CT over the past decade. The mathematical problem that CT image reconstruction i...
Article

In-phase and out-of-phase sequences

In-phase (IP) and out-of-phase (OOP) sequences correspond to paired MRI gradient echo (GRE) sequences obtained with the same repetition time (TR) but with two different echo time (TE) values.  Applications The main application of the IP-OOP sequences is to identify pathological (microsopic) fa...
Article

India ink artifact

The so-called india ink artifact also known as the black boundary artifact is an MRI feature seen on out-of-phase imaging and represents signal drop out in voxels that contain both fat and non-fat components. As such it is seen surrounding interfaces, lending the image an appearance as if someon...
Article

Indium-111 OncoScint

Indium-111 OncoScint is a radiopharmaceutical used in SPECT imaging. It is a labelled monoclonal antibody that is directed against TAG-72, which is a tumour-associated antigen associated with ~95% of colorectal carcinomas and 100% of ovarian carcinomas 1,2. Background hepatic uptake limits sensi...
Article

Indium-111 oxine labelled white blood cell scan

Indium-111 oxine labelled white blood cell scan is a nuclear medicine test which attempts to localise infection and/or inflammation by injecting the patient's previously extracted and radioactively labelled white blood cells.  Procedure The patient's blood is withdrawn and white blood cells ex...
Article

Intensifying screen

Intensifying screen is used in the cassette to intensify the effect of the X-ray photon by producing a larger number of light photons. It decreases the mAs required to produce a particular density and hence decreases the patient dose significantly. In cassettes, which use double emulsion films, ...
Article

Intensity modulated radiation therapy (IMRT)

Intensity Modulated Radiation Therapy (IMRT) represents the radiotherapeutic modality where the intensity of the radiation delivered, could be modulated during the treatment to focus on the tumour tissue and spare the adjacent anatomical structures/tissue(s). Therefore the increased dose of radi...
Article

Intravascular (blood pool) MRI contrast agents

Intravascular MRI contrast agents normally remain confined to the intravascular space, compared to Gd-DTPA which distributes throughout the extracellular fluid space. This is a result of intravascular agents having a molecular weight of approximately 70,000 and above, compared to a molecular wei...
Article

Intravenous MRI contrast agents

Intravenous MRI contrast agents include chelates of paramagnetic ions, both ionic and nonionic. The particulates, sequestered in the liver, spleen, and lymph nodes, the intravascular agents, confined to the blood pool, and tumour specific agents are discusses separately (see bottom).  NOTE: Thi...
Article

Inverse square law

The inverse square law describes the principle of dose reduction as the distance from the source increases. This assumes a point source. If radiation spreads over a spherical area, as the radius increases, the area over which the dose is distributed increases according to A=4πr^2 where A is the...
Article

Inversion recovery sequences

Inversion recovery pulse sequences are a type of MRI sequence used to selectively null the signal for certain tissues (e.g. fat or fluid). Inversion recovery can also generate heavily T1-weighted images and was originally developed for this purpose. Physics Basically, an inversion recovery (I...
Article

Iodinated contrast media

Iodinated contrast media are contrast agents frequently used via intravenous administration in computed tomography, although they are also used in fluoroscopy, angiography and venography, and even occasionally, plain radiography. Although the intravenous route is common, they are also administer...
Article

Iodinated contrast-induced thyrotoxicosis

Iodinated contrast-induced thyrotoxicosis is rare and may occur in patients with pre-existing thyroid disease and through thyrotoxicosis may be fatal (e.g. cardiac arrhythmia). Patients with a normal thyroid gland are unaffected.  Patients with existing thyrotoxicosis should not receive iodinat...

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