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
Dexel
Dexels, a portmanteau of detector elements, analogous to pixels, refers to the individual radiation-sensitive elements of the detector component of a scanner, e.g. computed tomography. It is important to appreciate that there is not necessarily a one-to-one correspondence between dexels and the ...
Article
Signal-to-noise ratio (MRI)
Signal-to-noise ratio (SNR) is a generic term which, in radiology, is a measure of true signal (i.e. reflecting actual anatomy) to noise (e.g. random quantum mottle). On MRI the signal-to-noise ratio is measured frequently by calculating the difference in signal intensity between the area of int...
Article
T1 values (1.5 T)
T1 values are a few hundred milliseconds (ms) for most tissues examined.
The following are approximate T1 values (ms) of several tissues for B0 = 1.5 T
fat = 260
liver = 500
muscle = 870
brain white matter = 780
brain grey matter = 920
CSF = 2500
Tissues that will have high signal on T1-...
Article
X-ray interaction with matter
X-rays in the diagnostic range interact with matter primarily via two major processes, which are fundamental in understanding how an image is formed in a radiographic exam. These processes are the:
photoelectric effect
Compton scatter
Article
Voxel size
Voxel size is an important component of image quality. Voxel is the 3-D analog of a pixel. Voxel size is related to both the pixel size and slice thickness. Pixel size is dependent on both the field of view and the image matrix. The pixel size is equal to the field of view divided by the matrix...
Article
K-trans (MR perfusion)
Ktrans 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 var...
Article
Pierre Curie
Pierre Curie (1859–1906) was a French physicist who was co-awarded the Nobel Prize for Physics in 1903, with his wife Marie Curie, and Henri Becquerel, for their pioneering work on radioactivity, which included the joint discovery with his wife of radium and polonium. He also discovered piezoele...
Article
Ring down artifact
Ring down artifact is a special type of resonance artifact. Its appearance is similar to the ladder-like reverberation of comet-tail artifact, but it is produced by a completely different mechanism.
The artifact is only associated with gas bubbles, and occurs when an ultrasound pulse encounters...
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
Focusing cup
A focusing cup is a negatively charged, shallow depression on the surface of the cathode of an x-ray tube, which concentrates the electron beam towards the focal spot of the anode. It is typically composed of nickel.
The negative charge of the focusing cup helps to accelerate the electrons towa...
Article
Helical CT image acquisition
Helical (a.k.a. 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 he...
Article
Free induction decay
Free induction decay (FID) refers to a short-lived sinusoidal electromagnetic signal which appears immediately following the 90° pulse. It is induced in the receiver coil by the rotating component of the magnetization vector in the x-y plane which crosses the coil loops perpendicularly. It does ...
Article
RANZCR applied imaging technology examination
The RANZCR applied imaging technology (AIT) examination is one of two examinations comprising the RANZCR Phase 1 examinations, with the other examination being the anatomy examination.
These examinations can be sat together or independent of each other, however trainees have a maximum of four c...
Article
Reticuloendothelial MRI contrast agents
Reticuloendothelial MRI contrast agents can best be discussed in terms of those used for liver and spleen imaging and those for lymph node imaging.
Liver and spleen
The use of Gd-DTPA with routine imaging sequences of the liver is unsatisfactory. Particulate contrast agents targeted to the ret...
Article
RF overflow artifact
RF overflow artifact causes a nonuniform, washed-out appearance to an image. This artifact occurs when the signal received by the scanner from the patient is too intense to be accurately digitized by the analog-to-digital converter. Autoprescanning usually adjusts the receiver gain to prevent th...
Article
Intravenous MRI contrast agents
Intravenous MRI contrast agents include chelates of paramagnetic ions, both ionic and non-ionic. Gadolinium-based contrast agents (GBCAs) are the most common type. The particulates, sequestered in the liver, spleen, and lymph nodes, the intravascular agents, confined to the blood pool, and tumor...
Article
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...
Article
Zero fill artifact
Zero fill artifact is one of many MRI artifacts and is due to data in the K-space array missing or set to zero during scanning. The abrupt change from signal to no signal results in artifacts in the images showing alternating bands of shading and darkness, often in an oblique direction.
A spike...
Article
Relaxation
Relaxation is the process in which spins release the energy received from a radiofrequency pulse. MRI signal is influenced, among other factors, by different types of relaxation:
T1 relaxation (spin-lattice or longitudinal relaxation)
T2 relaxation (spin-spin or transverse relaxation)
T2* rel...
Article
Superparamagnetism
Superparamagnetic materials consist of individual domains of elements that have ferromagnetic properties in bulk. Their magnetic susceptibility is between that of ferromagnetic and paramagnetic materials.
The figure illustrates the effect of a superparamagnetic material (grey circle) on the ma...
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