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
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
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 darker and vice versa, creating a fals...
Article
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 for detection and quantificat...
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
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...
Article
Mu-metal
Mu-metal is an alloy of primarily nickel and iron, with high magnetic permeability 1. It provides a low resistance pathway for external magnetic fields to flow, thereby 'shielding' the area housed within 1. It is therefore a useful component of the housing for medical imaging devices which requi...
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
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
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
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
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, decibel is a unit of "loudness", which...
Article
Image intensifier
Image intensifiers are used to convert low-energy x-rays into visible light images. Image intensifiers are several thousand times more sensitive compared to standard 400-speed screen-film combinations, and in practice can produce images using several thousand times less radiation 3,4.
The bigge...
Article
Grids
Grids are placed between the patient and the x-ray film to reduce the scattered radiation reaching the detector (produced mainly by the Compton effect) and thus improve image contrast.
They are made of parallel strips of high attenuating material such as lead with an interspace filled with low ...
Article
Transformer
A transformer is a passive electrical device used to transfer electrical energy from one circuit to another, via the phenomenon of electromagnetic induction.
It is fundamental in the modulation of voltage and current in the x-ray generator.
Components
primary coil
a set of insulated wires at...
Article
Air gap technique
The air gap technique is a radiographic technique that improves image contrast resolution through reducing the amount of scattered radiation that reaches the image detector. In select situations, this technique can be used instead of an antiscatter grid as the primary scatter reduction method in...
Article
Scatter to primary ratio
The scatter to primary ratio is a ratio of the scattered radiation to the primary unscattered radiation transmitted by the object being imaged. Hence, the scatter to primary ratio provides an indication of the degree of unwanted scattered radiation arising from a particular imaging study.
The s...
Article
Transient arterial phase respiratory motion-related artifact
Transient arterial phase respiratory motion-related artifact refers to common self-limited dyspnea observed immediately after administering gadoxetate disodium during liver MRI studies. Awareness about this potential artefact when choosing this hepatospecific contrast agent (Primovist/Eovist) is...
Article
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 movemen...
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