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.
716 results found
Article
b values
In MRI with diffusion-weighted imaging (DWI), the b value measures the degree of diffusion weighting applied, thereby indicating the amplitude (G), time of applied gradients (δ) and duration between the paired gradients (Δ).
It is calculated as:
b = γ² G² δ² (Δ−δ/3)
Therefore, a larger b valu...
Article
Acquisition time
The time of acquisition for a conventional spin echo or gradient echo sequence is the product of the repetition time, phase encoding steps, and number of averages (TR x phase steps x NEX).
For example, with a one-second TR, 128 phase steps, and two averages, we would get an acquisition time of...
Article
Pitch (CT)
Pitch (P) is a term used in helical CT with two terminologies depending on whether single slice or multislice CT scanners are used 1-3.
Single slice CT (SSCT)
The term detector pitch is the table distance traveled in one 360° gantry rotation divided by beam collimation 2.
For example, if the ...
Article
Continuous X-ray spectrum
The continuous X-ray spectrum refers the range of photon energies produced in an X-ray tube due to the properties of Bremsstrahlung radiation.
The energy of X-ray photons can take a value from zero to the maximum kinetic energy of the incident electrons.
Both the continuous X-ray spectrum and ...
Article
Blur
Blurring, or unsharpness, refers to the distortion of the definition of objects in an image, resulting in poor spatial resolution.
Types of blur
geometric blur
in terms of X-ray based imaging, reducing focal spot size, reducing the distance from patient to receptor (termed object to image d...
Article
Photomultiplier tube (Gamma camera)
The photomultiplier tube array of a Gamma camera detects the visible light produced by the scintillator and converts it to a measurable electronic signal 1.
A series of photomultiplier tubes are mounted behind the scintillation crystal 1. Each photomultiplier tube is composed of a tightly seal...
Article
Scintillator (gamma camera)
The scintillator is the component of a gamma camera which receives the gamma rays emitted from a radionuclide in a nuclear medicine scan and converts it to visible light photons. It is located just behind the collimator device.
A scintillator crystal may have properties of phosphorescence, wher...
Article
Reflection
Reflection of a sound wave occurs when the wave passes between two tissues of different acoustic impedances and a fraction of the wave 'bounces' back. This forms one of the major principles of ultrasound imaging as the ultrasound probe detects these reflected waves to form the desired image.
An...
Article
MR fingerprinting
MR fingerprinting (MRF) describes an approach to the acquisition and evaluation of MRI data that is 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 evoluti...
Article
3D fast spin echo (MRI sequence)
3D fast spin-echo sequences are MRI pulse sequences that are able to rapidly image relatively large volumes of tissue with high resolution whilst retaining many of the advantages of fast spin-echo sequences.
They are able to create the same weightings as traditional 2D sequences (i.e., T1, T2,...
Article
Dose length product
Dose length product (DLP) is a measure of CT tube radiation output/exposure (measured in mGy.cm). It is related to volume CT dose index (CTDIvol), but CTDIvol represents the dose through a slice of an appropriate phantom. The dose length product accounts for the length of radiation output along ...
Article
Slip-ring (CT)
Slip-ring functions to allow the transfer of electrical information and power between a rotating device and external components. They are used in helical CT and MRI scanners among other applications; in this setting, they allow image acquisition without progressive twisting of cables as the scan...
Article
Charge-coupled device detector
Charge-coupled device (CCD) detectors are used in digital radiography for the indirect conversion of x-ray photons into an electric charge (indirect because the x-ray photons are first converted into light via a scintillating screen).
Structure
A charge-coupled device can either be an area arr...
Article
Line focus principle
The line focus principle in radiography explains the relationship between the actual focal spot on 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...
Article
Mean glandular dose
The mean glandular dose (MGD) is an estimate of the average absorbed dose to the glandular tissues of a breast during mammography. It is measured in Gray (Gy).
The most commonly accepted method of calculating the mean glandular dose is described by Dance et al (2000):
...
Article
Anode heel effect
Anode heel effect refers to the lower field intensity towards the anode in comparison to the cathode due to lower x-ray emissions from the target material at angles perpendicular to the electron beam.
Basic concept
The conversion of the electron beam into x-rays doesn’t simply occur at the sur...
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