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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.

666 results found
Article

Portosystemic shunt ratio

The portosystemic shunt ratio is a measure performed using ultrasound to quantify the abnormal flow of portal venous blood that is shunted away from the hepatic sinusoidal circulation in the context of a congenital portosystemic shunt 1. Measurement The ratio is determined on ultrasound using ...
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

Power Doppler

Power Doppler is a technique that uses the amplitude of Doppler signal to detect moving matter. Power Doppler: is independent of velocity and direction of flow, so there is no possibility of signal aliasing is independent of angle, allowing detection of smaller velocities than color Doppler, f...
Article

Practical radiography: A Hand-Book of the Applications of the X-Rays

The book Practical radiography: A Hand-Book of the Applications of the X-Rays was the first ever textbook on x-rays anywhere in the world. It was written by H Snowden Ward and first published in May 1896 by Dawbarn & Ward. This is a mere six months after Wilhelm Roentgen's discovery of x-rays. ...
Article

Production of radioactive nuclei

Naturally occurring radioactive nuclei have a physical half life time of ~108 to 1010 years which makes them unsuitable for use in medical imaging. Thus radioactive nuclei used in medical practice are artificially synthesized. Production of these nuclei involve bombarding stable nuclei with pro...
Article

Propagation speed

The propagation speed of sound waves through tissue is an important element of ultrasound scans. Ultrasound machines assume sound waves travel at a speed of 1540 m/sec through tissue 1. In reality, the speed of sound is affected by the density and elasticity of the medium through which it is tr...
Article

Prostate MRI protocol

Prostate MRI has become an increasingly frequent examination faced in daily radiological practice and is usually acquired as either multiparametric or biparametric MRI of the prostate. This article aims to outline the concept of an MRI protocol for the assessment of the prostate. Recommendatio...
Article

Pseudoenhancement

Pseudoenhancement is an artifact encountered with contrast-enhanced CT, whereby the calculated density of a lesion is inaccurately increased. This phenomenon is most often problematic during evaluation of renal cysts by CT. On CT, it can be challenging to distinguish cystic versus solid renal l...
Article

Pulsatile portal venous flow

A pulsatile portal venous flow pattern can result from both physiological and pathological causes. In well subjects mild pulsatility, or in rare situations, even marked pulsatility has been described, particularly in thin subjects, with a venous pulsatility index of >0.5 with an inverse correla...
Article

Pulsatility index (ultrasound)

The pulsatility index (PI) (also known as the Gosling index) is a calculated flow parameter in ultrasound, derived from the maximum, minimum, and mean Doppler frequency shifts during a defined cardiac cycle. Along with the resistive index (RI), it is typically used to assess the resistance in a ...
Article

Pulse height analyzer

A pulse height analyzer is an instrument used to analyze the frequency distribution of the spectrum of photon energies that are captured by a gamma camera.  Energy windows are selected to only allow certain photon energies that fall within a preset range to contribute to the output pulse.
Article

Pulse repetition frequency

Pulse repetition frequency (PRF) indicates the number of ultrasound pulses emitted by the transducer over a designated period of time. It is typically measured as cycles per second or hertz (Hz). In medical ultrasound the typically used range of PRF varies between 1 and 10 kHz 1. A number of ar...
Article

Quantitative computed tomography (bone)

Quantitative computed tomography (QCT) in bone imaging is a bone mineral density (BMD) measurement technique in which the CT scanner is calibrated using solid phantoms (made of calcium hydroxyapatite, representing various bone mineral densities) placed under the patient in a pad. With this calib...
Article

Quantitative computed tomography (thoracic imaging)

Quantitative computed tomography (QCT) in thoracic imaging has multiple potential applications although often not adopted in standard use in many centers at time of initial writing (2019). These include quantitating lung intensity / density and airway geometry in the normal adult human lung as...
Article

Quantum noise

Quantum noise, also called quantum mottle, is the main and the most significant source of noise in plain radiography. It is a random process due to fluctuations in the number of photons reaching the detector from point to point. This means that exposing the detector in the absence of an object w...
Article

Quenching

Quenching refers to rapid expulsion of the liquid cryogen used to maintain the MRI magnet in a superconducting state. Discussion Modern MRI scanners contain superconducting magnets which have very low energy consumption, made possible by maintaining internal subzero temperatures by way of a 'c...
Article

Rad (CGS unit)

The rad (symbol rad) is a legacy unit in the cgs system for the absorbed dose of ionizing radiation, although it remains in widespread use in the United States.  The rad is defined as the dose represented by 100 ergs of energy being absorbed by one gramme of matter. The erg is the unit of energ...
Article

Radiation damage (skin injury)

Radiation-induced skin injuries can occur in both radiotherapy and fluoroscopic procedures such as interventional radiology.  Acute radiation doses above 2 Gy are known to result in erythema, permanent epilation will occur at 7 Gy and delayed skin necrosis transpires above doses of 12 Gy.  The...
Article

Radiation dose considerations in CT fluoroscopy

Consideration needs to be made to the radiation dose to both the patient and radiologist in CT fluoroscopy, given the relatively high and continuous radiation exposure involved. Radiation exposure to the patient The patient surface dosage may range between 2 and 10 mGy/sec, with exposure times...
Article

Radiation effects on embryonic and fetal development

Radiation effects on embryonic and fetal development are generally considered low risk compared to the normal risks of pregnancy. Most diagnostic x-ray and nuclear medicine examinations are <50 mSv and have not been demonstrated to produce any significant impact on fetal growth and development. ...
Article

Radiation-induced carcinogenesis

Radiation-induced carcinogenesis is widely but not universally believed to occur at exposures from ionizing radiation used in medical imaging. It is thought to be a stochastic effect of ionizing radiation, with the linear no-threshold theory (LNT) proposing no "safe" level of radiation exposure,...
Article

Radiation-induced lung cancer

Radiation-induced lung cancers are a potential long-term complication of radiotherapy to the chest.  Besides lung cancer, sarcomas (osteosarcomas are the most common arising from the irradiated bones, and malignant fibrous histiocytomas the most frequently arising from the soft tissues), breast...
Article

Radiation protection

Radiation protection is based on the three fundamental principles of justification of exposure, keeping doses (of ionizing radiation) as low as reasonably achievable (optimization) and the application of dose limits. The International Commission on Radiological Protection (ICRP) is responsible f...
Article

Radiation risk factor

The radiation risk factor is the total lifetime risk of radiation-induced fatal cancer for the general population. The International Commission on Radiological Protection (ICRP) quantifies the radiation risk factor as 5% (5 in 100) per Sv, or 0.05% (1 in 20,000) per mSv. These figures are base...
Article

Radiation weighting factor

The radiation weighting factor (WR) is a dimensionless constant that accounts for the relative biological effectiveness (RBE) of various types of ionizing radiation. The radiation weighting factor is used to calculate the equivalent dose (HT) by the following equation: Absorbed dose (DT) x rad...
Article

Radicals

Radicals (formerly called free radicals) are uncharged atoms or molecules in which an electron orbit has a single unpaired electron. Terminology Historically the International Union of Pure and Applied Chemistry (IUPAC) also used the term radical for any joined up group of atoms forming a side...
Article

Radioactivity

Radioactivity, also known as radioactive decay, describes the process of spontaneous breakdown of unstable (or radioactive) nuclides, with the formation of daughter nuclei and release of subatomic particles and/or gamma radiation. A single decay (a.k.a. disintegration) refers to the degradation ...
Article

Radioactivity units

The amount of radioactivity present in a sample of material is expressed using radioactivity units. The becquerel (Bq), is the SI unit of radioactivity. Superseded units curie: still commonly used in the United States rutherford: obsolete mache: obsolete
Article

Radiofrequency coils

Radiofrequency coils (RF coils) are the "antennae" of the MRI system, broadcasting the RF signal to the patient and/or receiving the return signal. RF coils can be receive-only, in which case the body coil is used as a transmitter; or transmit and receive (transceiver). Surface coils are the si...
Article

Radiofrequency receiver

Radiofrequency coils are the radiofrequency receivers (as well as sometimes the transmitters) of the electromagnetic signal used in MRI. Radiofrequency coils can be either receive-only, or both receivers and transmitters of the radiofrequency signal. The receiver coils detect the electromagnetic...
Article

Radiofrequency shielding

Radiofrequency or RF shielding is a technique for creating electromagnetic (EM) isolation, which is critical during the acquisition of MR images. RF shielding serves to both prevent external EM interference from contaminating the target tissue signal, and to ensure that signals generated by MR i...
Article

Radiofrequency transmitter

The radiofrequency (RF) transmitter is the generator of the radiofrequency current which is delivered to the transmitting coil. This creates a signal which is used to excite protons in the imaging field. Radiofrequency coils can be both transmitters and receivers of the radiofrequency signal or ...
Article

Radiograph

A radiograph (or plain radiograph although the word 'plain' is strictly superfluous) is the radiologist's preferred term for the static image generated following the passage of x-rays through the patient. Non-imaging clinicians and the lay population generally use the term "x-ray" to refer to a ...
Article

Radioisotope spills

Radioisotope spills are classified as major or minor. Examples of major spills include quantities equal to or larger than: 1mCi I-131 100 mCi Tc-99m or Tl-201 10 mCi Ga-67, In-111, or I-123
Article

Radiological image artifact

Most artifacts in radiology refer to something seen on an image that are not present in reality but appear due to a quirk of the modality itself. Artifact is also used to describe findings that are due to things outside the patient that may obscure or distort the image, e.g. clothing, external c...
Article

Radiologist

What is a radiologist? A radiologist is a specialist doctor who uses medical imaging such as x-ray, MRI, CT, ultrasound and angiography, to diagnose and treat human disease or injury. Radiologists undergo lengthy training and assessment in order to be accredited by relevant governing boards and...
Article

Radiopharmaceuticals

Radiopharmaceuticals are drugs that are bound to radioactive substances.  The pharmaceutically active portion determines the activity that will be measured and the radioactive portion emits radiation that can be measured by the scanner. A number of radiopharmaceuticals are used in human imaging...
Article

RadLex

RadLex is a lexicon of radiological information that has been produced by the Radiological Society of North America (RSNA).  It is an ontological system whose principle aim is to develop a useful vocabulary for radiologists.  In the words of the RSNA "As images, imaging reports, and medical re...
Article

Raymond V Damadian

Raymond V Damadian (1936-fl.2021) is a pioneer in the field of MRI and inventor of one of the first MRI scanners. Early life Raymond Vahan Damadian was born on 16 March 1936 in Forest Hills, New York City and was a child prodigy. He studied violin at the Juilliard School of Music for eight yea...
Article

Receiver operating characteristic curve

The receiver operating characteristic (ROC) curve is a statistical relationship used frequently in radiology, particularly with regards to limits of detection and screening. The curves on the graph demonstrate the inherent trade-off between sensitivity and specificity: y-axis: sensitivity x-a...
Article

Rectal cancer protocol (MRI)

MRI protocol for rectal cancer is a group of MRI sequences put together for imaging staging of primary tumors of the rectum and assessment of response following neoadjuvant therapy. Modified versions of the protocol may also be used for the assessment of local recurrence. Note: This article is ...
Article

Recursive filtration

Recursive filtration or averaging is a technique used to reduce excessive noise in fluoroscopy, where parts of the current frame and several preceding frames are combined to create an 'averaged' image. This helps to increase the signal to noise ratio in the final image without contributing to pa...
Article

Reflection

Reflection of a sound wave occurs when the wave passes between two tissues of different acoustic speeds 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. Angle ...
Article

Refraction

Refraction of a sound wave occurs if it travels between tissues with different propagation speeds. As the incident pulse or returning echo strikes an interface of different density or elasticity and therefore a different propagation speed, the direction of the wave changes according to Snell’s l...
Article

Refraction artifact

Refraction artifact can occur when a transmitted ultrasound pulse strikes an interface at a non-perpendicular angle. The difference in propagation speeds between the two tissues can cause refraction to occur. Should the refracted incident sound wave strike a reflector and cause an echo to return...
Article

Relaxation

Relaxation is the process in which spins release the energy received from a radiofrequency pulse. MRI signal is influenced, among other factors, by different types of relaxation: T1 relaxation (spin-lattice or longitudinal relaxation) T2 relaxation (spin-spin or transverse relaxation) T2* rel...
Article

Relaxometry

Relaxometry is measurement of relaxation times from MR images. T1, T2 and T2* can be estimated using the appropriate pulse sequence and parameters. T2 relaxometry has found useful in quantitating signal changes on T2-weighted images as in evaluating mesial temporal sclerosis. Details T2 relaxo...
Article

Rem (unit)

The rem (an acronym for roentgen equivalent man) was the cgs unit of effective dose and was officially replaced by the sievert many years ago.  One rem was a large quantity of radiation, and therefore for practical day to day use the millirem (mrem), representing one-thousandth of a rem, was us...
Article

Renal artery calcification

Renal artery calcifications, also known as renovascular calcifications, are deposits of calcium salts on the wall of a renal artery, found incidentally on imaging, usually CT 1. They are associated with extrarenal atherosclerosis and linked to hypertension 2. Terminology The term “renal artery...
Article

Renal intraparenchymal acceleration time

Renal intraparenchymal acceleration time is a parameter used in assessing renal arterial stenosis on Doppler ultrasound. It is the time taken from the start of systole to peak systole.  normal range: a value of usually <0.07 seconds (<70 ms) is taken as being within normal limits 1
Article

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.
Article

Resonance and radiofrequency

Protons in a magnetic field have a microscopic magnetization 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 pr...
Article

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. Liver and spleen The use of Gd-DTPA with routine imaging sequences of the liver is unsatisfactory. Particulate contrast agents targeted to the ret...
Article

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 ("reverberates"), the ultrasound transducer interprets the sound waves returning as deeper structures since it took longer for...
Article

Reynolds number

The Reynolds number (Re) is the primary parameter used to define the transition of fluid motion between laminar and turbulent flow patterns 1. The Reynolds number represents the ratio of inertia forces to viscous forces, and as such has no units (i.e. is a dimensionless quantity) 1.  Calculatio...
Article

RF overflow artifact

RF overflow artifact causes 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 th...
Article

Ring artifact

Ring artifacts are a CT phenomenon that occurs due to miscalibration or failure of one or more detector elements in a CT scanner. Less often, it can be caused by insufficient radiation dose or contrast material contamination of the detector cover 2. They occur close to the isocenter of the scan ...
Article

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...
Article

Ripple

Ripple, or voltage ripple, refers to the fluctuation in voltage output of some X-ray generators. It is given a percentage value, and calculated as 100 x (Vmax - Vmin )/Vmax (%). Single-phase and two-phase generators have 100% ripple. Three-phase generators have ripple values between 5 and 15%. ...
Article

Roentgen (unit)

The roentgen (symbol R) or röntgen (in German) is a legacy unit to measure radiation exposure. It was defined as the quantity of x-rays that produce 2.580 × 10-4 coulombs of charge collected per unit mass (kilograms) of air at standard temperature and pressure (STP): 1 R = 0.000258 coulombs per ...
Article

Rotating envelope x-ray tube

Rotating envelope x-ray tubes (RET), are a relatively novel type of high-performance x-ray tube developed in the early 2000s. Their two main features are the direct contact of the anode plate with the cooling oil, and rotation of the entire envelope around the axis of the anode.  In rotating en...
Article

Rutherford (unit)

The rutherford (symbol Rd) is an obsolete unit of radioactivity which was superseded by the introduction of the becquerel in 1975. One rutherford was equivalent to 1,000,000 nuclear disintegrations per second, or alternatively one becquerel equated to one microrutherford (μRd).  Terminology As...
Article

Samarium-153

Samarium-153 (Sm-153) is a radioisotope used in metabolic radiotherapy for the treatment of pain from bone metastases. It is produced in nuclear reactors, by neutron irradiation of samarium-152 (Sm-152 Sm2O3). Samarium-153 decays by emitting both beta minus particles and gamma photons with a ch...
Article

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 a different acoustic impedance to the surrounding tissue and which is smaller than the wavelength of the incident sound wave. Such structures are known as “diffuse reflectors,” with examples being red blood cells and non-smooth surface...
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 ...
Article

Schuller's view

Schuller's view is a oblique radiographic projection used to demonstrate the petrous temporal bone, internal auditory canal and bony labyrinth. It has an increasingly limited role in contemporary clinical practice because of the universal use of CT and MRI for imaging the temporal bone.  Patien...
Article

Scintigraphy performed with Tc-99m labeled RBCs

Tc-99m labeled RBCs - with radiolabelling technique in vivo or in vitro of red cells 3 - is one of the technetium radiopharmaceuticals used in the non-invasive assessment of gastrointestinal (GI) bleeding 2, characterized by high sensitivity (93%) and specificity (95%) 4. It is capable, in fact,...
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. The scintillator consists of a large, single photoluminescent cryst...
Article

SeHCAT

SeHCAT™ (23-seleno-25-homo-tauro-cholic acid or tauroselcholic acid) is a radiopharmaceutical used in the investigation of bile salt malabsorption, which is a cause of chronic diarrhea.  Characteristics physical half-life: 118 days Uses, dosage and timings A capsule containing SeHCAT is inge...
Article

Sensitivity

The sensitivity of a test/investigation is defined as the ability of the test to identify true positive cases of the disease under question. Sensitive tests are useful for ruling out disease.  Calculation Sensitivity = true positives detected by test / total cases of the disease where total ca...
Article

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: true positive (TP): an imaging test is positive and the patient has the disease/condition false ...
Article

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" ...
Article

Sequential CT image acquisition

Sequential CT scanning, also referred to as "scan-move-scan" or "step and shoot", was the conventional method of image acquisition in computed tomography before the advent of helical CT.  In sequential scanning, the patient is moved forward along the longitudinal axis of the CT scanner, pausing...
Article

Shading artifact

Shading artifact on 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...
Article

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...
Article

Short tau inversion recovery

Short tau inversion recovery (STIR), also known as short TI 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 ...
Article

Shoulder protocol (CT)

The CT shoulder protocol serves as an examination for the assessment of the shoulder joint. It is often performed as a non-contrast study. It can be combined with a CT arthrogram for the evaluation of labral injuries or the rotator cuff if MRI is contraindicated or in a postoperative setting whe...
Article

Shoulder protocol (MRI)

The MRI shoulder protocol encompasses a set of different MRI sequences for the routine assessment of the shoulder joint. Note: This article aims to frame a general concept of an MRI protocol for the assessment of the shoulder joint. Protocol specifics will vary depending on MRI scanner type, sp...
Article

Side lobe artifact

Side lobe artifacts occur where side lobes reflect sound from a 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 ...
Article

Sievert (SI unit)

The sievert (symbol Sv) is the SI unit of dose equivalent and is dimensionally-equivalent to one joule per kilogram. The sievert represents the stochastic effects of ionizing radiation as adjusted by a tissue weighting factor to account for differing responses of different human tissues to ioniz...
Article

Signal-to-noise ratio (MRI)

Signal-to-noise ratio (SNR) is a generic term which, in radiology, is a measure of true signal (i.e. reflecting actual anatomy) to noise (e.g. random quantum mottle). On MRI the signal-to-noise ratio is measured frequently by calculating the difference in signal intensity between the area of int...
Article

Silver recovery

Silver recovery is the process by which pure metallic silver can be recycled from old x-ray films. The modern process is extremely efficient with a recovery of greater than 99.9% silver.  Historically all radiographic film media employed silver salts as part of the image-producing process. Alth...
Article

Single photon emission computed tomography (SPECT)

Single photon emission computed tomography (SPECT) is a three-dimensional nuclear medicine imaging technique combining the information gained from scintigraphy with that of computed tomography. This allows the distribution of the radionuclide to be displayed in a three-dimensional manner offerin...
Article

Size specific dose estimate

Size specific dose estimate (SSDE) measured in mGy, is a method of estimating CT radiation dose that takes a patient's size into account.  CTDIvol and DLP are common methods to estimate a patient's radiation exposure from a CT procedure. The exposures are the same regardless of patient size, bu...
Article

Slew rate

Slew rate refers to the speed at which a gradient can be turned on and off, and is defined as the maximum gradient strength of the gradient divided by the rise time.  MR imaging is a product of magnetic field gradients which are created by magnetic gradient coils. The quality and performance of...
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

Space charge

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 negative charge around the metal surface. This space charge repels and...
Article

Spatial pulse length (ultrasound)

Spatial pulse length in ultrasound imaging describes the length of time that an ultrasound pulse occupies in space. Mathematically, it is the product of the number of cycles in a pulse and the wavelength.  A shorter spatial pulse length results in higher axial resolution. Spatial pulse length c...
Article

Spatial resolution

Spatial resolution refers to the ability of an imaging modality to differentiate two adjacent structures as being distinct from one another. Other related terms include definition or visibility of detail. Spatial resolution is expressed in line pairs per mm (lp mm). The absence of spatial reso...
Article

Spatial resolution (CT)

Spatial resolution in CT is the ability to distinguish between object or structures that differ in density. A high spatial resolution is important for one to discriminate between structures that are located within a small proximity to each other.  Factors affecting CT spatial resolution field...
Article

Spatial resolution (MRI)

In MRI, spatial resolution is defined by the size of the imaging voxels. Since voxels are three-dimensional rectangular solids, the resolution is frequently different in the three different directions.  The size of the voxel and therefore the resolution depends on matrix size, the field-of-view,...
Article

Specific absorption rate

Specific absorption rate (SAR) is the rate that electromagnetic energy in the radiofrequency is absorbed by tissues during MR image acquisition represented as watts per kilogram (W/kg).  Both the International Electrotechnical Commission and the USA's Food and Drug Administration limit the amoun...

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