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 continuously improved upon by countless contributing members. Our dedicated editors oversee each edit for accuracy and style. Find out more about articles.
717 results found
Article
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 several 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
Terminology
It...
Article
BOLD imaging
Blood oxygenation level dependent (BOLD) imaging is the standard technique used to generate images in functional MRI (fMRI) studies, and relies on regional differences in cerebral blood flow to delineate regional activity.
Blood flow in the brain is highly locally controlled in response to oxy...
Article
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 magnetic resonance spectroscopy (MRS). This artifact occurs in the frequency-encoding direction and is due to spatial misregistration of fat and water molecules.
Chemi...
Article
Fetal dosimetry
NB: Please consult original article(s) and discuss with your local radiology department/radiation physicist before making any clinical decision.
Although radiation exposure to the gravid uterus is to be avoided whenever possible, and only deliberately performed after careful weighing up of the ...
Article
MR spectroscopy
The technique of magnetic resonance spectroscopy (usually shortened to MR spectroscopy or 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 (disease...
Article
Magic angle effect (MRI artifact)
The magic angle is an MRI artifact that occurs in sequences with a short TE (less than 32 ms) - T1 weighted, proton density weighted, and gradient echo sequences.
It is confined to regions of tightly bound collagen at 54.74° from the main magnetic field (B0), and appears hyperintense, thus pote...
Article
Mammography
Mammography is a dedicated radiographic technique for imaging the breast, and the resultant images are known as mammograms.
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 ...
Article
Nephrogenic systemic fibrosis
Nephrogenic systemic fibrosis (NSF), also known as nephrogenic fibrosing dermopathy, occurs almost exclusively in patients with renal impairment and is associated with the administration of gadolinium-based contrast agents (GBCAs) used in MRI.
The American College of Radiology (ACR) has divide...
Article
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...
Article
Ring artifact
Ring artifacts are a CT phenomenon that occurs due to the miscalibration or failure of one or more detector elements in a CT scanner. Less often, it can be caused by insufficient radiation dose or contrast material contamination of the detector cover 2.
Features
This artifact usually occurs i...
Article
Tube arcing
Tube arcing occurs when there is a short-circuit within the tube, typically from the cathode to the tube envelope. The result is a temporary loss of x-ray output and a localized artifact.
A number of causes of tube arcing are recognized 1:
insulator surface flashover
insulator breakdown
va...
Article
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 a medium, and so has the unit joules (J) per kilogram (kg), with the adopted name of gray (Gy) where 1 Gy = 1 J.kg-1.
The absorbed dose is not a good indica...
Article
Equivalent dose
Equivalent dose (symbol HT) is a measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effects of different types of ionizing radiation. In quantitative terms, equivalent dose is less fundamental than absorbed dose, but it is more b...
Article
Stochastic effects
Stochastic effects occur by chance and can be compared to deterministic effects which result in a direct effect. Cancer induction and radiation induced hereditary effects are the two main examples of stochastic effects.
Models
Cancer induction as a result of exposure to radiation is thought by...
Article
Deterministic effects
Deterministic effects describe a cause and effect relationship between ionizing radiation and certain side-effects. They are also known as non-stochastic effects to contrast them with chance-like stochastic effects (e.g. cancer induction).
These effects depend on dose, dose rate, dose fractiona...
Article
Kerma
Kerma is a measure of energy transferred from radiation to matter and is an acronym for kinetic energy released to matter. It is related to, but not the same as absorbed dose. Kerma is measured by the SI unit, the gray (joules per kilogram) 1,3.
Kerma measures the amount of energy that is trans...
Article
Structure of the atom
The structure of the atom is key to the majority of the techniques used in radiology, and a general understanding of atomic structure is worthwhile. An atom is the smallest constituent of a chemical element that still maintains the chemical properties of that element.
The following is a simplif...
Article
Valence shell
The valence shell of an atom is the outermost shell of the electron cloud. It plays a large part in determining the chemical, thermal, optical and
electrical properties of the element. This occurs because it often not
full and movement of electrons may occur between it and a) electrons
from othe...
Article
Electron
The electron is a subatomic particle that has negligible mass (9.109 x 10-31 kg) and is negatively charged (-1.6 x 10-19 Coulombs). The properties of x-rays and their interaction with matter concern the orbiting electrons within the atom.
In classical physics the electrons orbit the central pos...
Article
T1 weighted image
T1 weighted image (also referred to as T1WI or the "spin-lattice" relaxation time) is one of the basic pulse sequences in MRI and demonstrates differences in the T1 relaxation times of tissues.
A T1WI relies upon the longitudinal relaxation of a tissue's net magnetization vector (NMV). Basicall...
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