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.

369 results found
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Units of measurement

For units of measurement the use of SI units in articles and cases on Radiopaedia.org is preferred. This is in line with best scientific practice and helps maintain consistency across the site. Terminology By scientific convention, if the unit is named after an individual, then when writing t...
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MRI pulse sequence abbreviations

This article contains a list of commonly and less commonly used MRI pulse sequence abbreviations and their meaning. If available, an explanation is included in a separate article. spin echo sequences (SE) T1: T1 weighted IR: inversion recovery T2 : T2 weighted RARE: rapid acquisition with r...
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Spatial resolution

Spatial resolution in radiology refers to the ability of the imaging modality to differentiate two objects. Low spatial resolution techniques will be unable to differentiate between two objects that are relatively close together. See also spatial resolution (CT) spatial resolution (MRI) cont...
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Spatial resolution (CT)

Spatial resolution in CT is the ability to distinguish between object or structures that differ in density. A high spatial resolution is important for one to discriminate between structures that are located within a small proximity to each other.  Factors affecting CT spatial resolution field...
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Noise

Noise is present in all electronic systems, and originates from a number of sources including electronic interference. It appears as irregular granular pattern in all images and degrades image information. It may be inapparent or render images non-diagnostic, depending on the severity. Radiogra...
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Blooming artifact

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...
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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- ...
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Ionising radiation

Ionising radiation is the term given to forms of radiation that are energetic enough to displace orbiting electrons from the atoms they interact with, thus forming ions.  Forms of ionising radiation Indirect X-rays and gamma radiation are the commonest forms of ionising radiation. Occasionall...
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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, gradient e...
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Magnets (types)

Magnets used for MRI are of three types: permanent, resistive and superconductive. Permanent MRI magnets use permanently magnetised iron like a large bar magnet that has been twisted into a C-shape where the two poles are close together and parallel. In the space between the poles the magnetic ...
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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,...
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International System of Units

The International System of Units, or the SI units (shortened from the French Système international d'unités) is the globally-adopted system of units of measurement. It is the modern iteration of the metric system. It superseded all prior systems including CGS and MKS, although in certain fields...
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Sievert (SI unit)

The sievert (symbol Sv) is the SI unit of dose equivalent and is dimensionally-equivalent to one joule per kilogram. The sievert represents the stochastic effects of ionising radiation as adjusted by a radiation weighting factor to account for differing responses of different human tissues to io...
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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 ionising 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...
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Rutherford (unit)

The rutherford (symbol Rd) is an obsolete unit of radioactivity which was superseded by the introduction of the becquerel in 1975. One rutherford was equivalent to 1,000,000 nuclear disintegrations per second, or alternatively one becquerel equated to one microrutherford (μRd).  Terminology As...
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Roentgen (unit)

The roentgen (symbol R) or röntgen (in German) is a legacy unit to measure radiation exposure. It was defined as the quantity of x-rays that produces 2.580 × 10-4 coulombs of charge collected per unit mass (kilograms) of air at standard temperature and pressure (STP): 1 R = 0.000258 coulombs per...
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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...
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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...
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Henry (SI unit)

The henry (symbol: H) is the SI derived unit of electrical inductance.  Terminology As per all other eponymous SI units when the unit is written out in full it is not capitalised, but when shortened to its symbol it is capitalised. History and etymology The henry is named in honour of Joseph...
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Weber (SI unit)

The weber (symbol: Wb) is the SI derived unit of magnetic flux, and superseded the maxwell, the CGS unit for magnetic flux.  Terminology As per all other eponymous SI units when the unit is written out in full it is not capitalised, but when shortened to its symbol it is capitalised. History ...
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Maxwell (CGS unit)

The maxwell (symbol: Mx) is the CGS unit of magnetic flux and was superseded by the weber, the unit in the SI system.  Terminology As per all other eponymous measurement units when the unit is written out in full it is not capitalised, but when shortened to its symbol it is capitalised. Histo...
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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...
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Tesla (SI unit)

The tesla (symbol T) is the derived SI unit of magnetic flux density, which represents the strength of a magnetic field. One tesla represents one weber per square metre. The equivalent, and superseded, cgs unit is the gauss (G); one tesla equals 10,000 gauss.  Terminology As for all eponymous ...
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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...
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MR enterography

MR enterography is a non-invasive technique for diagnosis of small bowel disorders. Indications The most common indication is to evaluate patients with Crohn disease (CD). Other less common indications would include coeliac disease, postoperative of adhesions, radiation enteritis, scleroderma,...
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Gallium 67 scintigraphy

Gallium 67 is a photon-emitting radiotracer 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 transferrin), and has a pred...
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Meglumine iotroxate (Biliscopin)

Meglumine iotroxate (BiliscopinTM) is an iodinated, intravenous contrast agent that is preferentially excreted into the biliary tree and is used in CT intravenous cholangiography. The typical dose is 100 mL Biliscopin (105 mg meglumine iotroxate/mL; 5.0 g iodine), which is administered via slow...
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Thallium-201 chloride

Thallium-201 chloride is a radiopharmaceutical used primarily in cardiac imaging. Characteristics photon energy: 80 keV physical half life: 73 hours biological half life rest: 3 minutes exercise: 30 seconds normal distribution: myocardium, skeletal muscle, GI tract, liver, kidneys excret...
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MRI sequences (overview)

An MRI sequence is a number of radiofrequency pulses and gradients that result in a set of images with particular appearance. This article presents a simplified approach to recognising and thinking about common MRI sequences, but does not concern itself with the particulars of each sequences. F...
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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...
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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 arterial waveforms are 1-2: triphasic: forward flow in systole reverse flow in late systole / early d...
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Wall filter

The wall filter in ultrasound is a way of filtering out low or high frequency Doppler signals. In clinical ultrasound, it is usually used to filter out very low frequencies that may add noise to a spectral Doppler waveform. A typical use is removing the low frequency reverberation of an arteria...
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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...
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Power Doppler

Power Doppler is a technique that uses the amplitude of Doppler signal to detect moving matter. Power Doppler: is independent of velocity and direction of flow, so there is no possibility of signal aliasing is independent of angle, allowing detection of smaller velocities than colour Doppler, ...
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Piezoelectric effect

The Piezoelectric effect converts kinetic or mechanical energy, due to crystal deformation, into electrical energy. This is how ultrasound transducers receive the sound waves. The same effect can be used in reverse – inverse piezoelectric effect – whereby the application of an electric field to...
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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...
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MRI artifacts

MRI artifacts are numerous and give an insight into the physics behind each sequence. Some artifacts affect the quality of the MRI exam while others do not affect the diagnostic quality but may be confused with pathology. When encountering an unfamiliar artifact, it is useful to systematically ...
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Radiation-induced carcinogenesis

Radiation-induced carcinogenesis is widely but not universally believed to occur at exposures from ionising radiation used in medical imaging. It is thought to be a stochastic effect of ionising radiation, with the linear no-threshold theory (LNT) proposing no "safe" level of radiation exposure,...
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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....
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Background radiation equivalent time

Exposing a patient to radiation is a measured, justified means aiding patient care. Each medical imaging examination utilising ionising radiation adheres to the fundamental principles of radiation protection. The general public's understanding of ionising radiation is limited 1; this article pr...
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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...
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Radiation damage (biomolecular)

Biomolecular radiation damage can result when biological tissues are exposed to ionising radiation from direct exposure or via Compton scattering. Ionisation is known to lead to the production of free radicals; free radicals are uncharged molecules that possess an unpaired valence electron. Con...
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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...
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Voxel

Voxel is a portmanteau of contractions of the two words 'volume' and 'element' and was coined as a 3-D equivalent of a pixel. It is an individual point in space on a 3-dimensional, regular matrix. The location of each voxel is encoded by its relative relationship to other voxels. A tensor may b...
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Diffusion tensor imaging

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

A pixel (or pel or picture element) may refer to either the smallest discrete element of the physical display or to the smallest element of the image. Voxel is its 3-dimensional equivalent, as employed in CT and other cross-sectional imaging modalities. History and etymology The history of the...
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Positron emission tomography

Positron emission tomography (PET) is a modern non-invasive imaging technique for quantification of radioactivity in vivo. It involves the intravenous injection of a positron-emitting radiopharmaceutical, waiting to allow for systemic distribution, and then scanning to detect and quantification ...
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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...
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Young's modulus

Young's modulus is a relationship between elasticity, strain, and stress: elasticity x (change in length / original length) = (force / area) put another way, this is elasticity x (strain) = stress or elasticity = stress / strain Elasticity is measured in kilopascals (kPa). This relationsh...
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Strain elastography

Strain elastography (also known as tissue strain elastography/static elastography/compression elastography) is a developing form of ultrasound that assesses tissues' macroscopic structure through the strain modulus. This is different from normal B-mode grayscale ultrasound which characterizes a ...
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Shear wave elastography

Shear wave elastography is a developing variation of ultrasound imaging. The concept is similar to strain elastography, but instead of using transducer pressure to compare a shift in an ultrasound A-line (thereby measuring changes in strain), a higher intensity pulse is transmitted to produce s...
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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...
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MRI safety

MRI scanners, although free from potentially cancer-inducing ionising radiation found in plain radiography and CT, have a host of safety issues which must be taken very seriously. MRI safety can be divided into: main magnetic field varying magnetic (gradient) fields radiofrequency NOTE: This...
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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...
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Metal artifact reduction sequence

A metal artifact reduction sequence (MARS) is intended to reduce the size and intensity of susceptibility artifacts resulting from magnetic field distortion. A variety of techniques are used for reducing metal artifacts at MRI, both for addressing artifacts due to the presence of metal in the i...
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MRI contrast agents

MRI contrast agents have become an indispensable part of modern magnetic resonance imaging. Although MRI was initially hoped to provide a means of making definitive diagnoses non-invasively, it has been found that the addition of contrast agents in many cases improves sensitivity and/or specific...
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T2 relaxation

T2 relaxation refers to the progressive dephasing of spinning dipoles following the 90° pulse as seen in a spin-echo sequence due to tissue-particular characteristics, primarily those that affect the rate of movement of protons, most of which are found in water molecules. This is alternatively k...
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Black boundary artifact

Black boundary artifact (also known as India ink artifact, chemical shift artifact of the 2nd kind (or type 2), phase cancellation artifact, or black line artifact) is an artificially-created black line located at fat-water interfaces such as those between muscle and fat. This results in a sharp...
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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...
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Myoinositol peak

Myoinositol is one of the compounds images with MR spectroscopy (MRS) at both 1.5 T and 3 T and is seen to resonate at 3.5 ppm chemical shift (right of the choline peak).  Myoinositol is a precursor of both phosphatidylinositol (the major inositol-containing phospholipid) and of phosphatidylino...
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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. Chemical shift is due to the differences between resonance frequencies of fat and water. It occurs in the frequency-encode direction where a shift in the detected...
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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...
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MRI contrast agent safety

Though considered safer than the frequently used iodinated contrast agents used in x-ray and CT studies, there are safety issues with MRI contrast agents as well. Paramagnetic metal ions suitable as MRI contrast agents are all potentially toxic when injected IV at or near doses needed for clinic...
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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...
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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...
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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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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...
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Quenching

Quenching refers to rapid expulsion of the liquid cryogen used to maintain the MRI magnet in a superconducting state. Discussion Modern MRI scanners contain superconducting magnets which have very low energy consumption, made possible by maintaining internal subzero temperatures by way of a 'c...
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Magnetism

Magnetism is a property of matter that is a result of the orbiting electrons in atoms. The orbiting electrons cause the atoms to have a magnetic moment associated with an intrinsic angular momentum called spin.  Magnetic field strengths are measured in tesla (T), a derived SI unit. The equivale...
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Magnetic susceptibility artifact

Magnetic susceptibility artifacts (or just susceptibility artifact) refer to a variety of MRI artifacts that share distortions or local signal change due to local magnetic field inhomogeneities from a variety of compounds.  They are especially encountered while imaging near metallic orthopedic ...
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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...
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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...
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Godfrey Hounsfield

Sir Godfrey N Hounsfield (1919-2004) pioneered the CT scanner making him one of the greats in the history of radiology. For his work, he received the Nobel Prize in Physiology or Medicine in 1979. This was remarkable because he had had no previous experience of working in the medical field but w...
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Physics and imaging technology: x-ray

Knowledge of the physics and imaging technology involved in the production of X-rays is vitally important for medical imaging specialists. See also physics and imaging technology: X-ray physics and imaging technology: Ultrasound physics and imaging technology: CT physics and imaging technol...
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Physics and imaging technology: CT

Knowledge of the physics and imaging technology involved in the production of CT scans is vitally important for medical imaging specialists. See also physics and imaging technology: x-ray physics and imaging technology: ultrasound physics and imaging technology: CT physics and imaging techn...
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Physics and imaging technology: MRI

Knowledge of the physics and imaging technology involved in the production of MRI scans is vitally important for medical imaging specialists. See also physics and imaging technology: x-ray physics and imaging technology: ultrasound physics and imaging technology: CT physics and imaging tech...
Article

Physics and imaging technology: ultrasound

Knowledge of the physics and imaging technology involved in the production of ultrasound (US) scans is vitally important for medical imaging specialists. See also physics and imaging technology: x-ray physics and imaging technology: ultrasound physics and imaging technology: CT physics and ...
Article

Physics and imaging technology: nuclear medicine

Knowledge of the physics and imaging technology involved in the production of nuclear medicine scans is vitally important for medical imaging specialists. See also physics and imaging technology: x-ray physics and imaging technology: ultrasound physics and imaging technology: CT physics and...
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Physics curriculum

The physics curriculum is one of our curriculum articles and aims to be a collection of articles that represent core physics and imaging technology knowledge: physics and imaging technology: X-ray physics and imaging technology: Ultrasound physics and imaging technology: CT physics and imagi...
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Physical principles of ultrasound

Medical ultrasound is based on the use of high-frequency sound to aid in the diagnosis and treatment of patients. Ultrasound frequencies range from 2 to approximately 15 MHz, although even higher frequencies may be used in some situations. The ultrasound beam originates from mechanical oscillat...
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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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CGS system

The CGS (or cgs) system (or centimetre-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...
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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 ...
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Absorbed dose

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

The rem (an acronym for roentgen equivalent man) was the cgs unit of effective dose and was officially replaced by the sievert many years ago.  One rem was a large quantity of radiation, and therefore for practical day to day use the millirem (mrem), representing one-thousandth of a rem, was us...
Article

Rad (unit)

The rad (symbol rad) is a legacy unit in the cgs system for the absorbed dose of ionising radiation, although it remains in widespread use in the United States.  The rad is defined as the dose represented by 100 ergs of energy being absorbed by one gramme of matter. The erg is the unit of energ...
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 ionising radiation. In quantitative terms, equivalent dose is less fundamental than absorbed dose, but it is more b...
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

Tissue weighting factor

The tissue weighting factor (WT) is a relative measure of the risk of stochastic effects that might result from irradiation of that specific tissue. It accounts for the variable radiosensitivities of organs and tissues in the body to ionising radiation. To calculate the effective dose, the indi...
Article

Pair production

Pair production (PP), like the photoelectric effect, results in the complete attenuation of the incident photon. Pair production can only occur if the incident photon energy is at least 1.022 MeV. As the photon interacts with the strong electric field around the nucleus it undergoes a change of ...
Article

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...
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Photoelectric effect

Photoelectric effect, or photoelectric absorption (PEA) is a form of interaction of X-ray or gamma photon with the matter. A low energy photon interacts with the electron in the atom and removes it from its shell. The probability of this effect is maximum when the energy of the incident photon...
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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)...

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