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.

365 results found
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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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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...
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Iodine

Iodine (chemical symbol, I) is a chemical element with the atomic number 53. It is a shiny purplish solid in the halogen group. Its name is derived from ιωδησ, Greek for violet-coloured.
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Kilovoltage peak

Kilovoltage peak (kVp) is the peak voltage applied to the X-ray tube. It determines the highest energy of X-ray photon. It is responsible for the acceleration of electrons from the cathode to the anode. It also determines tube current in space charge region.  Increase in kVp shifts the X-ray sp...
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K space

K space is an abstract concept and refers to a data matrix containing the raw MRI data. This data is subjected to mathematical function or formula called a transform to generate the final image. A discrete Fourier or fast Fourier transform 1-3 is generally used though other transforms such as th...
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K-trans (MR perfusion)

K trans is a measure of capillary permeability obtained using dynamic contrast-enhanced (DCE) MR perfusion. It is calculated by measuring the accumulation of gadolinium-based contrast agent in the extravascular-extracellular space.  Increased permeability of vessels in the brain is seen in a va...
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Lactate peak

Lactate is one of the more important compounds assessed on MR spectroscopy, and resonates at 1.3 ppm chemical shift, with a characteristic double peak at long TEs. It is however superimposed on the lipid band, and using an intermediate TE (e.g. 144ms) will invert only lactate allowing it to be d...
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LaPlace's law

LaPlace's law is useful in thinking about dilated tubular structures, such as the bowel or vessels (e.g. aortic aneurysms). The relationship between wall tension and radius shows why more dilated regions of a tube develop more wall stress and therefore are at higher risk for perforation: wall t...
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Lead apron

Lead aprons are one of the key parts of personal radiation protection equipment along with lead gloves, lead glasses, and thyroid shields. In medical imaging, there are two main types 1: 0.25 mm lead equivalence weighs 1-5 kg ("lightweight apron") attenuates 75% of the x-ray beam at 50 kVp ...
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Linear array

Ultrasound transducers that produce images via linear array typically contain 256-512 elements, making them the largest assembly. Each element produces a scan line that makes up the ultrasound image. Multiple adjacent elements combine to produce an ultrasound beam that is emitted at 90 degrees ...
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Linear attenuation coefficient

Linear attenuation coefficent (µ) is the fraction of attenuated incident photons per unit thickness of a material. It represents the fraction of photons removed from a monoenergetic beam per unit thickness of material. Its complement is the transmitted portion of the beam. It is expressed numeri...
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Line focus principle

Line focus principle explains the relationship between the anode surface and the effective focal spot size. Basic concept The focal spot is the area of the target  upon which the electron beam impinges. The energy of the electrons in the electron beam is mostly converted into heat (approximate...
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Lipids peak

Lipids are a collection of related compounds examined in MR spectroscopy. They resonates at 1.3 ppm chemical shift, and are markers of severe tissue damage with liberation of membrane lipids, as is seen in cerebral infarction or cerebral abscesses. It is also encountered in cerebral metastases a...
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Loopogram

A loopogram is a fluoroscopic study of an ileal conduit, which is a type of urinary diversion. Terminology This procedure is also known is an ileal conduitogram, ileal loopography or ileostoureterography.  Procedure It is a retrograde study in which contrast is injected via the anterior abdo...
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Mach bands

Mach bands or the Mach effect refers to an optical phenomenon from edge enhancement due to lateral inhibition of the retina 2. This is an inbuilt edge enhancement mechanism of the retina, where the edges of darker objects next to lighter objects will appear lighter and vice versa; creating a fal...
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Magnetic resonance neurography

Magnetic resonance neurography (MRN) is a relatively new non-invasive imaging technique for dedicated assessment of peripheral spinal nerves. It is used to assess peripheral nerve entrapments and impingements as well as localization and grading of nerve injuries and lesions. Dedicated high-res...
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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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Magnetisation transfer

Magnetisation transfer (MT) imaging is an MRI technique that can be used to exploit contrast between tissues where 1H protons are present in three states1: bound to macromolecules in free water as water in the hydration layer between the macromolecules and the free water To assess MT, an off...
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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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Magnets

Magnets used for MRI are of three types: permanent, resistive and superconductive. Permanent MRI magnets use permanently magnetized 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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Mammography

Mammography is a dedicated radiographic technique for imaging the breast. Types of mammography In general terms, there are two types of mammography: screening and diagnostic. Mammography differs significantly in many respects from the rest of diagnostic imaging. Screening mammography  In ge...
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Maximum intensity projection (MIP)

Maximum Intensity Projection (MIP) consists of projecting the voxel with the highest attenuation value on every view throughout the volume onto a 2D image 1. Such an algorithm is rather simple: for each XY coordinate, only the pixel with the highest Hounsfield number along the Z-axis is represe...
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Mean transit time (MTT)

Mean transit time (MTT) corresponds to the average time, in seconds, that red blood cells spend within a determinate volume of capillary circulation. It is assessed as part of the CT perfusion protocol, which is basically used in the context of ischaemic stroke diagnosis and management, and MR p...
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Mechanical index

The mechanical index (MI) is an attempt to measure part of an ultrasound beam's bioeffects. The MI is found on most ultrasound display screens, along with the thermal index. Mechanical index is proportional to an ultrasound beam's peak negative pressure (or peak rarefaction pressure). It is al...
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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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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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Microbubbles

Microbubbles are intravenous contrast agents used in contrast-enhanced ultrasound. Microbubble contrast agent is different than the agitated saline contrast agent often used in echocardiographic studies. Microbubbles consist of a gas surrounded by a lipid, lipopolymer, or polymer shell. They ra...
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Milliamperage-seconds (mAs)

Milliamperage-seconds more commonly known as mAs is a measure of radiation produced (milliamperage) over a set amount of time (seconds) via an x-ray tube. It directly influences the radiographic density, when all other factors are constant. An increase in current (mA) results in a higher produc...
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Minimum intensity projection (MinIP)

Minimum intensity projection (MinIP) is a data visualization method that enables detection of low-density structures in a given volume. The algorithm uses all the data in a volume of interest to generate a single bidimensional image, in other words, its consists of projecting the voxel with the ...
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Mirror image artifact

Mirror image artifact in sonography is seen when there is a highly reflective surface (e.g. diaphragm) in the path of the primary beam. The primary beam reflects from such a surface (e.g. diaphragm) but instead of directly being received by the transducer, it encounters another structure (e.g. ...
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Modulation transfer function

The modulation tranfer function (MTF) is the spatial frequency response of an imaging system or a component. It is the contrast at a given spatial frequency relative to low frequencies. On the radiogram, objects having different sizes and opacity are displayed with different gray-scale value...
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Moiré fringes

Moiré fringes are an interference pattern most commonly seen when acquiring gradient echo images using the body coil. Because of lack of perfect homogeneity of the main magnetic field from one side of the body to the other, aliasing of one side of the body to the other results in superimpositio...
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Motion artifact

Motion artifact is a patient-based artifact that occurs with voluntary or involuntary patient movement during image acquisition. Misregistration artifacts, which appear as blurring, streaking, or shading, are caused by patient movement during a CT scan.  Blurring also occurs with patient moveme...
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MR angiography

MR angiography (MRA) is an alternative to conventional angiography and CT angiography, eliminating the need for iodinated contrast media and ionising radiation. It has evolved into several techniques with different advantages and applications: contrast enhanced MR angiography non-contrast enha...
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MR enterography

MR enterography is a new non-invasive technique for diagnosis of small bowel disorders. Indications The most common indication is to evaluate patients with Crohn disease (CD). Technique Actual procedure will vary depending on institutional protocol/guidelines but below is a typical descripti...
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MR fingerprinting

MR fingerprinting (MRF) is a relatively recent approach to the acquisition and evaluation of MRI data aimed at generating quantitative multiparametric data from a single acquisition.  The underlying process is acquiring data in a pseudorandom manner resulting in a unique pattern of signal evolu...
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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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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 noninvasively, it has been found that the addition of contrast agents in many cases improves sensitivity and/or specifici...
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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 MR contrast agents are all potentially toxic when injected IV at or near doses needed for clinica...
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MRI electronics and data processing

As an introduction to the electronics and data processing of the MRI scanner, a schematic diagram has been provided (figure 1). Starting from the right hand side, we have the computer that directs all of the action in the MRI acquisition and acquires and processes the data. The computer tells t...
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MRI physics

The basic process The way MR images are generated is complicated and is much harder to understand than plain radiography, CT and ultrasound. It has strong underpinnings in physics which must be understood before any real sense of 'how it works' is gained.  What follows is a very abbreviated, '...
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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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MRI pulse sequences

An MRI pulse sequence is a programmed set of changing magnetic gradients. Each sequence will have a number of parameters, and multiple sequences are grouped together into an MRI protocol.  Parameters A pulse sequence is generally defined by multiple parameters, including: time to echo (TE) t...
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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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MRI sequence parameters

Listed in the table below are the most common acquisition parameters for commonly used MRI pulse sequences (in msec). For a general introduction to these sequences please refer to MRI sequences (basic). The specific parameters for any given study varies from one manufacturer to another, and fr...
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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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MR liver iron quantification

MR liver iron quantification is a non-invasive means of measuring liver iron concentration, a key indicator in the management of patients with haemochromatosis (primary or secondary). Advantages Apart from being non-invasive, sampling occurs in a large cross-section of the liver, as opposed to...
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MR perfusion weighted imaging

Perfusion weighted imaging is a term used to denote a variety of MRI techniques able to give insights into the perfusion of tissues by blood.  There are three techniques in wide use to derive one or more perfusion values:  ​techniques ​dynamic susceptibility contrast (DSC) MR perfusion dynam...
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MR spectroscopy

MR spectroscopy (MRS) allows tissue to be interrogated for the presence and concentration of various metabolites. Grossman and Yousem said "If you need this to help you, go back to page 1; everything except Canavan has low NAA, high Choline". This is perhaps a little harsh, however it is fair to...
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Myo-inositol peak

Myo-inositol is one of the compounds images with MR spectroscopy (MRS) at both 1.5T and 3T and is seen to resonate at 3.5 ppm chemical shift (right of the choline peak).  Myo-inositol is a precursor of both phosphatidylinositol (the major inositol-containing phospholipid) and of phosphatidylino...
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N-acetylaspartate (NAA) peak

N-acetylaspartate (NAA) is one of the more important compounds assessed on MR spectroscopy, and resonates at 2.0 ppm chemical shift.  NAA is the acetylated form of the amino acid, aspartate, which is found in high concentrations in neurons and is a marker of neuronal viability. It is therefore ...
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Negative enhancement integral

The negative enhancement integral in MR perfusion is used to calculate the relative cerebral blood volume (rCBV).  It represents the area described by the baseline and the signal loss due to passage of contrast bolus in tissue. 
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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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Non contrast enhanced MR angiography

Non contrast enhanced MR angiography is performed in several ways including: time of flight angiography phase contrast angiography three-dimensional (3D) electrocardiograph-triggered half-Fourier fast spin echo Generally, these techniques are time-consuming as compared with contrast enhanced...
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Nuclear magnetic resonance

Nuclear magentic resonance is a powerful technique which involves placing nuclei within an external magnetic field enabling thus them to undergo precession. The 'resonance' part of the names implies the fact that a second (usually perpendicular) radiofrequency pulse tuned to the precessional fre...
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Octreotide scintigraphy

Octreotide scintigraphy uses 111In-labelled octreotide which is a somatostatin analog; it is also known as an OctreoscanTM, a brand name for 111In-labelled pentetreotide; pentetreotide is a DTPA-conjugated form of octreotide, originally manufactured by Mallinckrodt Nuclear Medicine LLC, which no...
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Ontology

Ontology is the study of the nature of being, existence or reality as such, as well as the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology deals with questions concerning what entities exist or can be ...
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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 ...
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Paramagnetism

Paramagnetic materials include oxygen and ions of various metals like Fe (iron), Mg (magnesium), and Gd (gadolinium). These ions have unpaired electrons, resulting in a positive magnetic susceptibility. The magnitude of this susceptibility is less than 0.1% of that of ferromagnetic materials. T...
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Partial volume averaging (CT artifact)

Partial volume artifact occurs when tissues of widely different absorption are encompassed on the same CT voxel producing a beam attenuation proportional to the average value of these tissues.  The last generation of CT scanners and its reduction in the volume of a voxel has substantially reduc...
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Pelvic cervical carcinoma protocol (MRI)

A dedicated pelvic MRI protocol is very useful for imaging assessment of cervical carcinoma. Although the FIGO is a clinical staging, the 2009 revised FIGO staging encourages the use of MRI to complement clinical staging. Preparation Imaging is optimally performed after three hours of fasting...
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Phase contrast imaging

Phase contrast imaging is an MRI technique that can be used to visualise moving fluid. It is typically used for MR venography as a non-IV-contrast requiring technique.  Spins that are moving in the same direction as a magnetic field gradient develop a phase shift that is proportional to the vel...
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Phased array

A phased array ultrasound transducer is typically 2-3 cm long, consisting of 64-128 elements. It is a smaller assembly than a sequential array and can be either linear or curvilinear. A sector field of view is produced by all elements firing to create a single waveform. Small delays in element ...
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Phase-encoded motion artifact

Phase-encoded motion artifact is one of many MRI artifacts occurring as a result of tissue/fluid moving during the scan. It manifests as ghosting in the direction of phase-encoding, usually in the direction of the short axis of the image (i.e left to right on axial or coronal brains, and anterio...
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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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Photon

A photon is, in simple terms, an elementary particle. It has a zero mass (rest mass) and travels at the speed of light. It is defined as stable with no electric charge and exhibits both wavelike and particle-like properties. For the sake of this article, a photon refers to an uncharged particle ...
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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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Physics curriculum

The physics curriculum is one of our curriculum articles and aims to be a collection of articles that represent core physics knowledge, including all modalities including x-ray, CT, MRI, ultrasound, nuclear medicine and mammography.  We've just started working on this, so only MRI is present at...
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Picture archiving and communication system

Picture archiving and communication system (PACS) is a modality of imaging technology which helps in image transmission from the site of image acquisition to multiple physically-disparate locations. This technology not only is economical (film-less department), but also convenient to access mult...
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Piezoelectric effect

Piezoelectric effect refers to the application of an electric field to a crystal, which causes realignment of the internal dipole structure. This realignment results in crystal lengthening or contraction, converting electrical energy into kinetic or mechanical energy. This is how ultrasound tra...
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Pitch

Pitch (P) is a term used in helical CT. It has two terminologies depending on whether single slice or multislice CT scanners are used 1-3. Single slice CT (SSCT) The term detector pitch is used and is defined as table distance traveled in one 360° gantry rotation divided by beam collimation 2....
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Portosystemic shunt ratio

The portosystemic shunt ratio is a measure performed using ultrasound to quantify the abnormal flow of portal venous blood that is shunted away from the hepatic sinusoidal circulation in the context of a congenital portosystemic shunt 1. Ultrasound The ratio is determined using the following e...
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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 use of a radiopharmaceutical injected into the body and its accumulation in the body is detected, quantified and interpreted. Another similar imaging techni...
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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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Production of radioactive nuclei

Naturally occurring radioactive nuclei have a physical half life time of  about 108 to 1010 years which makes them unsuitable for use in medical imaging. Thus radioactive nuclei used in medical practice are artificially made. Production of these nuclei involve bombarding stable nuclei with prot...
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Propagation speed

The propagation speed of sound waves through tissue is an important element of ultrasound scans. Ultrasound machines assume sound waves travel at a speed of 1540 m/sec through tissue 1. In reality, the speed of sound is affected by the density and elasticity of the medium through which it is tr...
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Prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a type II transmembrane glycoprotein, that has become an increasingly prominent imaging biomarker 1. PSMA has emerged as a useful target in PET imaging of prostate cancer, especially in the evaluation of small volume lymph node and bone metastases 2. ...
Article

Pseudoenhancement

Pseudoenhancement is an artifact in postcontrast CT evaluation of renal cysts. The distinction between cystic and solid lesions visualised in CT is primarily made on the basis of whether the lesion enhances with contrast administration. A renal cystic lesion is considered "enhancing" when there...
Article

Quantitative computed tomography

Quantitative computed tomography (QCT) is a bone mineral density (BMD) measurement technique in which the CT scanner is calibrated using solid phantoms (made of calcium hydroxyapatite, representing various bone mineral densities) placed under the patient in a pad. With this calibration, the Houn...
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Quenching

Quenching refers to rapid expulsion of the liquid cryogen used to maintain the MR 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 'cr...
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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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Radiation damage (skin injury)

Radiation-induced skin injuries can occur in both radiotherapy and fluoroscopic procedures such as interventional radiology.  Acute radiation doses above 2 Gy are known to result in erythema, permanent epilation will occur at 7 Gy and delayed skin necrosis transpires above doses of 12 Gy.  The...
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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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Radiation-induced lung cancer

Radiation-induced lung cancers have been reported as a rare late complication of radiation therapy in both post-operative breast cancer and Hodgkin lymphoma patients. These occur 5-10 years (or later) after treatment 1-4. There appears to be increased risk of lung cancer in these patients if the...
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Radiation protection

Radiation protection is based on the three fundamental principles of justification of exposure, keeping doses as low as reasonably achievable (optimisation) and the application of dose limits. The International Commission on Radiological Protection (ICRP) is responsible for the development of th...
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Radiofrequency coils

Radiofrequency coils (RF coils) are the "antennae" of the MRI system, broadcasting the RF signal to the patient and/or receiving the return signal. RF coils can be receive-only, in which case the body coil is used as a transmitter; or transmit and receive (transceiver). Surface coils are the si...
Article

Radioisotope spills

Radioisotope spills are classified as major or minor Examples of major spills include quantities equal to or larger than: 1mCi I-131 100 mCi Tc-99m or Tl-201 10 mCi Ga-67, In-111, or I-123
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Radiopharmaceuticals

Radiopharmaceuticals are drugs that are bound to radioactive substances.  The pharmaceutically active portion determines the activity that will be measured and the radioactive portion emits radiation that can be measured by the scanner. A number of radiopharmaceuticals are used in human imaging...
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RadLex

RadLex is a lexicon of radiological information that has been produced by the RSNA.  It is an ontological system whose principle aim is to develop a useful vocabulary for radiologists.  In the words of the RSNA "As images, imaging reports, and medical records move online, radiologists need a u...
Article

RANZCR examination

The RANZCR examination is set by the Royal Australian and New Zealand College of Radiologists, and consists of two parts: RANZCR part 1 typically first attempted during the first year of training, and is a barrier to progression into year 3 consists of written examinations in anatomy and phys...
Article

RANZCR part 1 examination

The RANZCR part 1 examination is a series of four papers covering anatomy and physics and are a hurdle requirement to advance into third year of radiology training in the Australian and New Zealand radiology training program.  Since September 2013 the exams have been delivered electronically. T...
Article

Receiver operating characteristic curve

The receiver operating characteristic (ROC) curve is a statistical relationship used frequently in radiology, particularly with regards to limits of detection and screening. The curves on the graph demonstrate the inherent trade-off between sensitivity and specificity: y-axis: sensitivity x-a...
Article

Rectal cancer protocol (MRI)

Pelvic MRI protocol for rectal cancer includes: through tumour (always ensure planes are exactly perpendicular to the wall) three-plane T2 axial T1 narrow field of view images to be included overviews axial T2FS axial T1 See also MRI protocols

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