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.

423 results found
Article

T2 shine through

T2 shine-through refers to high signal on DWI images that is not due to restricted diffusion, but rather to high T2 signal which 'shines through' to the DWI image. T2 shine through occurs because of long T2 decay time in some normal tissue. This is most often seen with subacute infarctions due ...
Article

T2 washout

T2 washout is a phenomenon encountered on diffusion weighted imaging (DWI) which results in DWI images (e.g. B = 1000) appearing normal despite abnormal ADC maps.  For the phenomenon to occur a particular combination of ADC and T2 signal intensity is required.  increased T2 signal facilitated...
Article

Tc-99m HMPAO labelled WBC

Tc-99m HMPAO (hexamethylpropyleneamine oxime) labelled WBC is one of the technetium radiopharmaceuticals used in WBC imaging. Characteristics photon energy: 140 KeV physical half life: 6 hours biological half life: 4 hours normal distribution: spleen, liver, kidney, bone marrow pharmacokin...
Article

Tc-99m labeled RBC

Tc-99m labelled RBC is one of the technetium radiopharmaceuticals used in the assessment of GI bleeding. Characteristics photon energy: 140 KeV physical half life: 6 hours biological half life:  normal distribution: heart, vessels, spleen miscellaneous facts: threshold for detection is 0....
Article

Tc-99m MAG3

Tc-99m MAG3 (mercaptoacetyltriglycine) is one of the technetium radiopharmaceuticals used in renal imaging. Characteristics photon energy: 140 KeV physical half life: 6 hours biological half life: 4 hours normal distribution: kidneys (100%) pharmacokinetics: uptake by tubular secretion (9...
Article

Tc-99m pertechnetate

Tc-99m pertechnetate is one of the technetium radiopharmaceuticals used in imaging of thyroid, colon, bladder and stomach. Characteristics photon energy: 140 keV physical half-life biological half-life: 6 hours normal distribution: stomach, thyroid, salivary glands, (testicles) excretion: ...
Article

Tc-99m sulfur colloid

Technetium-99m sulfur colloid is one of the technetium radiopharmaceuticals. Characteristics photon energy: 140 keV physical half-life: 6 hours biological half-life normal distribution: liver: 85% spleen: 10% bone marrow: 5% excretion: hepatic target organ: liver, spleen pharmacokinet...
Article

Technetium 99m-methyl diphosphonate

Technetium 99m-methyl diphosphonate (99mTc MDP) is a radiotracer used in nuclear medicine especially for bone scans. Any disease process which results in extracellular fluid expansion will lead to accumulation of this tracer. Radionuclide profile photon energy: 140 keV physical half life: 6 h...
Article

Temporal resolution

Temporal resolution relates to the duration of time for acquisition of a single frame of a dynamic process, i.e., cine imaging. Discussion The concept of temporal resolution is fundamental to cardiac CT and MRI, in which a rapidly beating heart is imaged over the order of milliseconds into mul...
Article

Tesla (SI unit)

The tesla (symbol T) is the derived SI unit of magnetic flux density, which represents the strength of a magnetic field. One tesla represents one weber per square meter. The equivalent, and superseded, cgs unit is the gauss (G); one tesla equals 10,000 gauss. Most current medical magnetic reson...
Article

Thallium-201 chloride

Thallium-201 chloride is a radiopharmaceutical used primarily in cardiac imaging. Characteristics photon energy: 80 keV physical half life: 73 hours biological half life rest: 3 minutes exercise: 30 seconds normal distribution: myocardium, skeletal muscle, GI tract, liver, kidneys excret...
Article

The 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 anti-scatter grid as the primary scatter reduction method i...
Article

Thermal index

The thermal index (TI) is intended as a measure of an ultrasound beam's thermal bioeffects. It is often displayed on ultrasound screens (along with the mechanical index). Absorption of sound waves may cause heating in tissue. The thermal index depends on: a measure of time-averaged acoustic po...
Article

Thermionic emission

Thermionic emission is the emission of electrons from a heated metal (cathode). This principle was first used in the Coolidge tube and then later in the modern day x-ray tubes. Before the discovery of the principle, gas tubes were used for x-ray production. The cathode has its filament circuit ...
Article

Thermoluminescent dosimeter

Thermoluminescent dosimeter (TLD) is a passive radiation detection device that is used for personal dose monitoring or to measure patient dose.  Parts plastic holder nickel-coated aluminum card with TLD discs the discs are made of a thermoluminescent material, commonly calcium sulphate doped...
Article

Thomas Edison

Thomas Edison (1847-1931) was a prolific inventor, considered by many to be the greatest inventor in American history. Edison holds around 1093 US patents; however, this article will focus on his work on fluoroscopy.  Early Life and career Thomas Alva Edison was born on the 11th February 1897 ...
Article

Thorotrast

Thorotrast is a suspension of radioactive thorium dioxide first produced in Germany in 1928 and used as a contrast agent until the 1950s. Its principal use was for cerebral angiography: 90% of the estimated 50,000-100,000 patients treated received it for this purpose. Basic principles Thorotra...
Article

Thyroid scan (Tc-99m)

Tc-99m pertechnetate thyroid scan is a functional nuclear medicine study used to assess the thyroid gland. Technique patient preparation fast for 4 hours prior to exam radiopharmaceutical Tc-99m pertechnetate dose and route of administration 111-185 MBq (3-5 mCi) IV  time of imaging 20 ...
Article

Time gain compensation

A way to overcome ultrasound attenuation is time gain compensation (TGC), in which signal gain is increased as time passes from the emitted wave pulse. This correction makes equally echogenic tissues look the same even if they are located in different depths. The basis of this is that of return...
Article

Time of flight angiography

Time of flight angiography (TOF) is an MRI technique to visualize flow within vessels, without the need to administer contrast. It is based on the phenomenon of flow-related enhancement of spins entering into an imaging slice. As a result of being unsaturated, these spins give more signal that s...
Article

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 ionising radiation. To calculate the effective dose, the indi...
Article

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 ...
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 artefact.  A number of causes of tube arcing are recognized 1:  insulator surface flashover insulator breakdown va...
Article

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. When the electron beam strikes the target material in the anode only 1% of the kinetic e...
Article

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...
Article

Turbo inversion recovery magnitude

Turbo inversion recovery magnitude (TIRM) is a type of inversion recovery MRI pulse sequence. It has been shown to be superior in the assessment of osteomyelitis in bone 1 and in the assessment of head and neck tumors 2.
Article

Twinkling artifact

Twinkling artifact is the result of intrinsic machine noise seen with color Doppler ultrasound 1. It occurs as a focus of alternating colors on Doppler signal behind a reflective object (such as calculi), which gives the appearance of turbulent blood flow 2. It appears with or without an associa...
Article

Ultrasound artifacts

Ultrasound artifacts are commonly encountered and familiarity is necessary to avoid false diagnosis. Artifacts acoustic enhancement acoustic shadowing aliasing artifact beam width artifact blooming artifact comet tail artifact color comet tail artifact mirror image artifact reverberati...
Article

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...
Article

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:...
Article

Units of measurement

For units of measurement the use of SI 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 name is not capitalized, but t...
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 o...
Article

Vicarious contrast material excretion

Vicarious contrast material excretion (VCME) defines excretion of water-soluble contrast material in a way other than via normal renal secretion.  The most common vicarious excretion of water-soluble contrast material is via the liver, resulting in increased bile density seen in the gallbladder...
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...
Article

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...
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

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...
Article

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 capitalised, but when shortened to its symbol it is capitalised. History ...
Article

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
Article

X-ray artifacts

Artifacts can present in a variety of ways including abnormal shadow 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 film, comp...
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 processes, which are fundamental in understanding how an image is formed in a radiographic exam. These process are the photoelectric effect  Compton scatter 
Article

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 Cathode T...
Article

X-rays

X-rays represent a form of electromagnetic radiation. They are produced by the x-ray tube, using the high voltage to accelerate the electrons produced by the cathode. The produced electrons interact with the anode, thus producing x-rays. The x-rays produced include Bremsstrahlung and the charact...
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...
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

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...
Article

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...

Updating… Please wait.

 Unable to process the form. Check for errors and try again.

 Thank you for updating your details.