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.

606 results found
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Tissue Doppler imaging (echocardiography)

A variant of pulsed wave Doppler, tissue Doppler imaging (TDI) is a modality in echocardiography that allows measurement of the velocity of myocardial movement throughout the cardiac cycle. Physics Color flow, pulsed wave, and continuous wave Doppler are designed to analyze the Doppler shifts ...
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Tissue to background ratio

The tissue to background ratio is a measure of the specificity of radiopharmaceutical uptake within the target organ in a nuclear medicine study. It is also referred to as tumor to background ratio in the literature. The presence of radiopharmaceutical in the background (tissues external to the...
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Tissue weighting factor

The tissue weighting factor (WT) is a relative measure of the risk of stochastic effects that might result from irradiation of that specific tissue. It accounts for the variable radiosensitivities of organs and tissues in the body to ionizing radiation. To calculate the effective dose, the indi...
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Training, testing and validation datasets

The division of the input data into training, testing and validation sets is crucial in the creation of robust machine learning algorithms. Firstly, machine learning algorithms require a training set to be trained on. Each iteration, it calculates the difference between the predicted and actual ...
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Transducer-related artifact

Artifacts in ultrasound imaging may result from faults in the transducer itself or in the transducer cable. 
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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...
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Transient elastography

Transient elastography most often refers to a type of elastography which relies on a mechanical pulse generated by an external probe. The principle is similar to shear wave elastography, in that the elastic modulus is generated from shear wave velocity, but the application of the pulse from an ...
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Transient interruption of contrast

Transient interruption of contrast (TIC) is a common flow artifact seen in CT pulmonary angiography (CTPA) studies. The contrast opacificiation of the pulmonary arteries is suboptimal due to an increase in the flow of unopacified blood from the inferior vena cava (IVC) to the right side of the h...
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Truncation artifact (CT)

Truncation artifact in CT is an apparently increased curvilinear band of attenuation along the edge of the image. This artifact is encountered when parts of the imaged body part remain outside the field of view (e.g. due to patient body habitus), which results in inaccurate measurement of atten...
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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 artefact.  A number of causes of tube arcing are recognized 1:  insulator surface flashover insulator breakdown va...
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Tube current modulation

Tube current modulation is a feature of modern CT scanners to alter the strength of the beam by modulating the current of the x-ray tube depending on the signal to noise ratio. The purpose of tube current modulation is to maintain a near constant signal to noise ratio in the image across differe...
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Tube rating

Tube ratings are the defined input parameters (kVp, mA, exposure) that can be safely used during its operation without causing damage to the x-ray tube itself and unique to each individual x-ray tube model. An x-ray tube rating is the maximum allowable kilowatts (kW) in 0.1 second 2.  When the ...
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Tube shielding

Tube shielding refers to the use of a material within the X-ray tube housing to limit leakage of scattered radiation, to protect both patients and staff from unnecessary exposure. Lead (Pb) is an ideal material for this purpose due to its high atomic density (Z = 82). Due to these properties, i...
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Tumor specific MRI contrast agents

Tumor specific MRI contrast agents are pharmaceuticals that are targeted to tumors, either specifically or nonspecifically. Monoclonal antibodies are targeted to specific tumors such as adenocarcinoma of the colon. Metalloporphyrins exhibit affinity for many tumor types including carcinoma, sarc...
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Turbo inversion recovery magnitude

Turbo inversion recovery magnitude (TIRM) is an inversion recovery MRI pulse sequence that displays the magnitude of a turbo/fast spin echo, without regard for the phase/polarity of the longitudinal magnetization. This method of image reconstruction is the dominant form of inversion recovery use...
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Twinkling artifact

Twinkling artifact is seen with color flow Doppler ultrasound 1. It occurs as a focus of alternating colors on Doppler signal behind a reflective object (such as a calculus), which gives the appearance of turbulent blood flow 2. It appears with or without an associated color comet-tail artifact ...
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Ultrahigh field MRI

Ultrahigh field (UHF) magnetic resonance imaging refers to imaging done on any MRI scanner with a main magnetic field (B0) strength of 7 tesla or greater. Until recently purely a research tool, following the introduction of the first 7 T clinical scanner in 2017, there are now a slowly increasin...
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Ultrasound artifacts

Ultrasound artifacts are commonly encountered and familiarity is necessary to avoid false diagnoses. Artifacts acoustic enhancement acoustic shadowing aliasing artifact anisotropy beam width artifact blooming artifact color bruit artifact color flash artifact comet tail artifact color...
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Ultrasound elastography

Ultrasound elastography, also called as sono-elastography, is a modern evolutionary method of sonographic imaging. Techniques include shear wave elastography (also known as transient elastography) and strain elastography (also known as static or compression elastography). These techniques assess...
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Ultrasound frequencies

Ultrasound frequencies in diagnostic radiology range from 2 MHz to approximately 15 MHz.  It is important to remember that higher frequencies of ultrasound have shorter wavelengths and are absorbed/attenuated more easily. Therefore, higher frequencies are not as penetrating. This explains why h...
Article

Ultrasound (introduction)

Ultrasound (US) is an imaging technology that uses high-frequency sound waves to characterize tissue. It is a useful and flexible modality in medical imaging, and often provides an additional or unique characterization of tissues, compared with other modalities such as conventional radiography o...
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Ultrasound transducer

An ultrasound transducer converts electrical energy into mechanical (sound) energy and back again, based on the piezoelectric effect. It is the hand-held part of the ultrasound machine that is responsible for the production and detection of ultrasound waves. It consists of five main components:...
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Units of measurement

For units of measurement the use of SI units (both base and derived units) in articles and cases on Radiopaedia.org is preferred. This is in line with best scientific practice and helps maintain consistency across the site. Terminology By scientific convention: for eponymous units, the full ...
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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 o...
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Vicarious contrast media excretion

Vicarious contrast media excretion (VCME) is defined as excretion of intravascularly-administered water-soluble contrast media in a way other than via normal renal excretion. The most common vicarious excretion of water-soluble contrast material is via the liver, resulting in increased bile dens...
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Virtual reality

Advancements of technology have enabled various simulated reality devices, including virtual reality (VR) and augmented reality (AR). Both technologies provide stereoscopic and three-dimensional (3D) immersion of a simulated object. VR simulates a virtual environment while AR overlays simulated ...
Article

Volume coils

Volume coils are the transmit and receive radiofrequency coils which are used to both transmit and receive the radiofrequency signal in MRI. Most MRI scanners have what is called a body coil – which is a volume coil built into the bore of the magnet which transmits the radiofrequency for most ex...
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Volume rendering

Volume rendering is a type of data visualization technique which creates a three dimensional representation of data. CT and MRI data are frequently visualized with volume rendering in addition to other reconstructions and slices.  Volume rendering techniques can also be applied to tomosynthesis ...
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Voxel

Voxel is a portmanteau of contractions of the two words 'volume' and 'element' and was coined as a 3-D equivalent of a pixel. It is an individual point in space on a 3-dimensional, regular matrix. The location of each voxel is encoded by its relative relationship to other voxels. A tensor may b...
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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...
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Wall filter

The wall filter in ultrasound is a way of filtering out low or high frequency Doppler signals. In clinical ultrasound, it is usually used to filter out very low frequencies that may add noise to a spectral Doppler waveform. A typical use is removing the low frequency reverberation of an arteria...
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Weber (SI unit)

The weber (symbol: Wb) is the SI derived unit of magnetic flux, and superseded the maxwell, the CGS unit for magnetic flux.  Terminology As per all other eponymous SI units when the unit is written out in full it is not capitalized, but when shortened to its symbol it is capitalized. History ...
Article

Wilhelm Roentgen

Wilhelm C Roentgen (1845-1923) was a German physicist who is celebrated globally for his discovery of x-rays on 8 November 1895. Early life Wilhelm Conrad Roentgen (Röntgen in German) was born on 27 March 1845 in Lennep, Germany. He attended the primary and secondary school run by Martinus Her...
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Windmill artifact

In CT imaging, the windmill artifact is an image distortion in the axial plane, encountered during helical multidetector acquisitions. The telltale appearance is characterized by equally distanced bright streaks diverging from a focal high-density structure. The streaks seemingly rotate while sc...
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Wrist protocol (MRI)

The MRI wrist protocol encompasses a set of MRI sequences for the routine assessment of the wrist joint. Note: This article aims to frame a general concept of an MRI protocol for the assessment of the wrist. Protocol specifics will vary depending on MRI scanner type, specific hardware and softw...
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Xenon-127

Xenon-127 is a radiopharmaceutical principally used when a performing VQ scan. It is not a widely used alternative to xenon-133 with the main advantage being a higher proton energy allowing for post perfusion scanning.  photon energy: 203 KeV physical half life: 36.3 days
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X-ray artifacts

X-ray artifacts can present in a variety of ways including abnormal shadows noted on a radiograph or degraded image quality, and have been produced by artificial means from hardware failure, operator error and software (post-processing) artifacts.  There are common and distinct artifacts for fi...
Article

X-ray film

X-ray film displays the radiographic image and consists of emulsion (single or double) of silver halide (silver bromide (AgBr) is most common) which when exposed to light, produces a silver ion (Ag+) and an electron. The electrons get attached to the sensitivity specks and attract the silver ion...
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 
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X-ray production

X-rays are produced due to sudden deceleration of fast-moving electrons when they collide and interact with the target anode. In this process of deceleration, more than 99% of the electron energy is converted into heat and less than 1% of energy is converted into x-rays. Definitions Generator ...
Article

X-ray quantity and quality

X-ray photon quantity refers to the number of photons produced during an exposure. Factors influencing x-ray quantity includes: peak voltage (kVp): beam quantity is approximately proportional to the square of the tube potential generator type/voltage waveform: reducing ripple increases beam q...
Article

X-rays

X-rays (or much more rarely, and usually historically, x-radiation or Roentgen rays) represent a form of ionizing electromagnetic radiation. They are produced by an x-ray tube, using a high voltage to accelerate the electrons produced by its cathode. The produced electrons interact with the anod...
Article

X-ray tube

An x-ray tube functions as a specific energy converter, receiving the electrical energy and converting it into two other forms of energy: x-radiation and heat. Heat is considered the undesirable product of this conversion process; therefore x-radiation is created by taking the energy from the el...
Article

Young's modulus

Young's modulus is a relationship between elasticity, strain, and stress: elasticity x (change in length / original length) = (force / area) put another way, this is elasticity x (strain) = stress or elasticity = stress / strain Elasticity is measured in kilopascals (kPa). This relationsh...
Article

Zebra stripes

Zebra stripes/artifacts appear as alternating bright and dark bands in a MRI image. The term has been used to describe several different kind of artifacts causing some confusion. Artifacts that have been described as a zebra artifact include the following: Moire fringes 1,2 Zero-fill artifact...
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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...
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Zero filling interpolation

Zero filling interpolation (ZIP) is the substitution of zeroes for unmeasured data points in order to increase the matrix size of the new data prior to Fourier transformation of MR data. This results in pixels smaller than the actual resolution of the image. The zero filling occurs in the periph...
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Zipper artifact

In MR imaging, zipper artifact refers to a type of MRI artefact where one or more spurious bands of electronic noise extend perpendicular to the frequency encode direction and is present in all images of a series.         There are various causes for zipper artifacts in images. Most of them are...

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