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.

359 results found
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Repetition time

The repetition time (TR) is the time from the application of an excitation pulse to the application of the next pulse. It determines how much longitudinal magnetization recovers between each pulse. It is measured in milliseconds.
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Resonance and radiofrequency

Protons in a magnetic field have a microscopic magnetisation and act like tiny toy tops that wobble as they spin.The rate of the wobbling or precession is the resonance or Larmor frequency. In the magnetic field of an MRI scanner at room temperature, there is approximately the same number of pro...
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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. NOTE: This article has been transferred from mritutor.org and was last updated in March 5, 1996. Review and edit pending.  Liver and spleen The u...
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Reverberation artifact

Reverberation artifact occurs when an ultrasound beam encounters two strong parallel reflectors. When the ultrasound beam reflects back and forth between the reflectors ("revereberates"), the ultrasound transducer interprets the sound waves returning from the reverbration as deeper structures s...
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RF overflow artifact

RF overflow artefact cause 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 thi...
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Ring artifact

Ring artifacts are a CT phenomenon that occur due to miscalibration or failure of one or more detector elements in a CT scanner. They occur close to the isocentre of the scan and are usually visible on multiple slices at the same location. They are a common problem in cranial CT. The remedy is ...
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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...
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Roentgen

Roentgen is the unit used to measure radiation exposure. It is defined as the quantity of c-rays that produces 2.580 × 10-4 coulombs of charge collected per unit mass (kilograms) of air at standard temperature and pressure (STP): 1 R = 0.000258 C/kg air. History and etymology Named after Wilhe...
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Saturation recovery sequences

Saturation recovery (SR) sequences are rarely used for imaging now. Their primary use at this time is as a technique to measure T1 times more quickly than an inversion recovery pulse sequence. Saturation recovery sequences consist of multiple 90 degree RF pulses at relatively short repetition ti...
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Scattering (ultrasound)

Scattering occurs when a sound wave strikes a structure with both a different acoustic impedance to the surrounding tissue and a wavelength less than that of the incident sound wave. Such structures are known as “diffuse reflectors,” with examples being red blood cells and non-smooth surfaces of...
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SeHCAT

SeHCAT (23-seleno-25-homo-tauro-cholic acid) is a radiopharmaceutical used in the investigation of bile salt malabsorption, which is a cause of chronic diarrhoea.  Characteristics physical half-life: 118 days Uses, dosage and timings A capsule containing SeHCAT is ingested with water. The pa...
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Sensitivity

Sensitivity of a test/investigation is defined as the ability of the test to identify true positive cases of the disease under question. Calculation Sensitivity = true positives detected by test / total cases of the disease where, total cases with the disease = true positive + false negative
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Sensitivity and specificity

Sensitivity and specificity are fundamental characteristics of diagnostic imaging tests. The two characteristics derive from a 2x2 box of basic, mutually exclusive outcomes from a diagnostic test (Figure 1): true positive (TP): an imaging test is positive and the patient has the disease/condit...
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Sensitivity and specificity of multiple tests

Sensitivity and specificity of multiple tests is a common statistical problem in radiology because frequently two tests (A and B) with different sensitivities and specificities are combined to diagnose a particular disease or condition. These two tests can be interpreted in an "and" or an "or" ...
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Shading artifact

Shading artifact in MRI refers to loss of signal intensity in one part of the image, leading to dark shading in this portion of the image. Causes uneven excitation of nuclei within the field; due to RF pulses applied at flip angles other than 90 and 180 degrees abnormal loading of coil or cou...
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Shear wave elastography

Shear wave elastography is a developing variation of ultrasound imaging. The concept is similar to strain elastography, but instead of using transducer pressure to compare a shift in an ultrasound A-line (thereby measuring changes in strain), a higher intensity pulse is transmitted to produce s...
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Short tau inversion recovery

Short tau inversion recovery (STIR), also known as short T1 inversion recovery, is a fat suppression technique with an inversion time TI = ln(2)·T1fat,where the signal of fat is zero. This equates to approximately 140 ms at 1.5 T. To distinguish two tissue components with this technique, their ...
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Side lobe artifact

Side lobe artifacts occur where side lobes reflect sound from strong reflector that is outside of the central beam, and where the echoes are displayed as if they originated from within the central beam. Ultrasound transducer crystals expand and contract to produce primary ultrasound beams in th...
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Signal to noise ratio

Signal to noise ratio (SNR) is a generic term which, in radiology, is a measure of true signal (e.g. reflecting actual anatomy) to noise (e.g. random quantum mottle). A lower signal to noise ratio generally results in a grainy appearance to images.  Each modality has its own source(s) of noise ...
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Space charge and space charge effect

Space charge refers to the collection of electrons which are emitted from the metal surface, after the application of tube current, at a short distance away from the metal surface. These electrons collect and form a cloud of charge around the metal surface. This space charge limits the further ...
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Spatial resolution

Spatial resolution in radiology refers to the ability of the imaging modality to differentiate two objects. Low spatial resolution techniques will be unable to differentiate between two objects that are relatively close together. See also spatial resolution (CT) spatial resolution (MRI) cont...
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Spatial resolution MRI

Spatial resolution determines how "sharp" the image looks. Low resolution will give either fuzzy edges, or a pixelly appearance to the image. In MRI, spatial resolution is defined by the size of the imaging voxels. Since voxels are three dimensional rectangular solids, the resolution is frequen...
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Specificity

Specificity of a test/investigation is the ability of a test to be correctly negative (true negative) in persons without the disease in question. Calculation Specificity = true negatives detected by test / total cases without the disease where, total cases without the disease = true negative ...
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Speckle artifact

Speckle artifact may be encountered in ultrasound. It is caused by the scattering of waves from the surface of small structures within a certain tissue. The artifact produces a textured appearance. See also ultrasound artifacts
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Speckle tracking echocardiography

Speckle tracking echocardiography is a new player in the cardiac arena which is rapidly making its mark in cardiac and radiology circles alike. It is an objective, robust, semiautomatic and reproducible technique with a short post processing time. Traditionally, left ventricular function is mea...
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Speed displacement artifact

Speed displacement artifact is a gray scale ultrasound finding that can be identified as an area of focal discontinuity and displacement of an echo deeper than that its actual position in an imaged structure.  The artifact is due to the image processor's assumption that the velocity of the ultra...
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Spin echo sequences

Spin-echo pulse sequences are one of the earliest developed and still widely used (in the form of fast spin-echo) of all MRI pulse sequences. The pulse sequence timing can be adjusted to give T1-weighted, proton density, and T2-weighted images. Dual echo and multiecho sequences can be used to ob...
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Spiral pulse sequences

Spiral scanning in MRI is unlike spiral scanning in CT where the x-ray tube is continuously rotating and data is continuously being acquired. In MRI the word "spiral" refers to the pattern of sampling k-space. In conventional imaging sequences including spin echo and gradient echo and in fast im...
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Spoiled gradient echo MRI

Spoiled gradient echo MRI is an MRI technique which destroys residual transverse magnetization at the end of each excitation cycle. 
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Standard uptake value

The standard uptake value (SUV)  is a simple way of determining activity in PET imaging, most commonly used in Flourodeoxyglucose (FDG) imaging. It is also known as the dose uptake ratio (DUR). As the name suggests it is a mathematically derived ratio of tissue radioactivity concentration at a p...
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Steady-state free precession MRI

Steady-state free precession MRI (SSFP) is a type of gradient echo MRI pulse sequence in which a steady, residual transverse magnetisation (Mxy) is maintained between successive cycles. The sequence is noted for its superiority in dynamic/cine assessment of cardiac function. Discussion To unde...
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Step-and-shoot tomosynthesis (breast)

Step-and-shoot is a technology of image acquisition in digital breast tomosynthesis characterised by stop scanning at every single angle during images acquisition. Step-and-shoot technology allows advantages in microcalcifications conspicuity, spatial resolution, signal-to-noise Ratio improveme...
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Strain elastography

Strain elastography (also known as tissue strain elastography/static elastography/compression elastography) is a developing form of ultrasound that assesses tissues' macro structure through the strain modulus. This is different from normal B-mode grayscale ultrasound which characterizes a tissue...
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Structure of the atom

The structure of the atom is key to the majority of the techniques used in radiology, and a general understanding of atomic structure is worthwhile. The following is an uncomplicated overview of the structure of the atom. Nucleus The composition of an atom is principally 'empty space'. Its ma...
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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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Susceptibility weighted imaging

Susceptibility weighted imaging (SWI) is an MRI sequence which is particularly sensitive to compounds which distort the local magnetic field and as such make it useful in detecting blood products, calcium, etc. Physics SWI is a 3D high-spatial resolution fully velocity corrected gradient echo ...
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T1 rho

T1 rho, also referred to as T1ρ or "spin lock", is an MRI sequence that is being developed for use in musculoskeletal imaging. At the moment it is mostly investigational and does not yet have widespread clinical use. The "rho" in the sequence name refers to a "ro"tating frame and the sequence h...
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T2 relaxation

T2 relaxation refers to the progressive dephasing of spinning dipoles following the 90° pulse as seen in a spin-echo sequence due to tissue-particular characteristics, primarily those that affect the rate of movement of protons, most of which are found in water molecules. This is alternatively k...
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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 ...
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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...
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T2* relaxation

T2* decay refers to an exponential decrease in Mxy  (i.e. signal strength) following the initial excitation pulse as a function of time constant T2*. A picture of the signal or free induction decay (FID) is shown on the right, occurring immediately after a 90o RF excitation pulse in a liquid pha...
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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 pha...
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Tc-99m labeled RBC

Tc-99m labelled RBC is one of the technetium radiopharmaceuticals used in renal imaging. Characteristics photon energy: 140 KeV physical half life: 6 hours biological half life:  normal distribution: heart, vessels, spleen miscellaneous facts: threshold for detection is 0.05-0.1 m...
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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...
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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: ...
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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...
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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...
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Thallium 201 chloride

Thallium 201 Chloride is a radiopharmaceutical used in cardiac imaging. Characteristics photon energy: 80 keV physical half life: 55 hours biological half life rest: 3 minutes exercise: 30 seconds normal distribution: myocardium, skeletal muscle, GI tract, liver, kidneys excretion: renal...
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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...
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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...
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Thermionic emission

Thermionic emission is the emission of electrons from a heated metal (cathode). This principle was first used in the Coolidge's 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 circui...
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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 aluminium card with TLD discs the discs are made of a thermoluminescent material, commonly calcium sulphate dope...
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Thyroid scan (Tc-99m)

Tc-99m [pertechnetate] thyroid scan is a functional nuclear medicine study used to assess the thyroid gland. patient preparation fast for 4 hours prior to exam radiopharmaceutical Tc-99m pertechnetate dose and route of administration 3-5 mCi IV time of imaging 20 minutes after Tc-99m per...
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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...
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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...
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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 ionising radiation. To calculate the effective dose, the indi...
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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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Tube arcing

Tube arcing occurs when there is a short-circuit within the tube, typically from the cathode to tube envelope. This causes a momentary loss of x-ray output. New tubes are more prone to this problem due to residual gas within them.
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Tumour 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 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 tumours 2.
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Twinkle artifact

Twinkle artifact is the result of intrinsic machine noise seen with colour Doppler ultrasound 1. It occurs as a focus of alternating colours on Doppler signal behind a reflective object (such as calculi), which gives the appearance of turbulent blood flow 2. And it appears with or without an ass...
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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 artifacts

Ultrasound artifacts are commonly encountered and familiarity is necessary to avoid false diagnosis. Artifacts acoustic shadowing acoustic enhancement beam width artifact comet tail artifact colour comet tail artifact mirror image artifact reverberation artifact ring down artifact refr...
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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...
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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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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 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...
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Voxel

Voxel is a contraction of the 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 is a voxel that contains ...
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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...
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X-ray film

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

Xenon-127

Xenon-127 is a radiopharmaceutical principally used when a performing VQ scan. It is an not 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

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 The units of elasticity are in kPa. This relationship is fun...
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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...
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...

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