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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.

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Obsolete imaging techniques

It is almost axiomatic that as time passes, the imaging techniques by which patients are evaluated, eventually become obsolete, due to continued scientific and technological innovations. Although the reality of new-fangled technology coming along and supplanting established methodology is of c...

Virtual grid

Virtual grid softwares are a relatively new innovation utilizing no physical grid at all. Instead, the original purpose of a grid is replicated by an algorithm 1 based on fundamental mathematics (i.e. Laplace transformation, wavelet transformation and Gaussian decomposition) which iteratively re...

O-arm

The O-arm is a movable CT imaging structure developed for intraoperative 3D fluoroscopic imaging. It is utilized during surgery for the identification of bony details in complex procedures such as spinal fixation or microdiscectomy. See also C-arm

Dark-field computed tomography

Dark-field computed tomography is an emerging medical imaging technology. While conventional CT measures differential attenuation properties of the various tissues, dark-field CT utilizes their small-angle scattering (dark field) characteristics. Physics Instead of attenuation of x-ray photon...

Timed barium esophagogram

The timed barium esophagogram (TBO) is a simple physiologic assessment and objective method for assessing the esophageal emptying used in patients with suspected achalasia and to evaluate and follow up patients who have been treated with myotomy or pneumatic dilatation1,3. Technique Several te...

Salt and pepper noise (MRI artifact)

Salt and pepper noise, also known as impulse noise, has been used to describe the characteristic appearance of a certain artifact seen on MRI. The artifact looks like innumerable black and white pixels throughout the image. Smoothing filters are algorithms designed to diminish the noise whilst ...

Echocardiography

Echocardiography refers broadly to the use of diagnostic ultrasound as it pertains to the heart and cardiovascular system. The features of the imaging equipment used, as well as the principles underlying image generation, are analogous to other sonographic applications. It is primarily used to n...

Arterial input function

Arterial input function (AIF) is commonly defined as the concentration of the contrast medium in an artery measured over time by placing a region of interest. Use in MRI and CT It is important to be precise that on MRI the estimation of the concentration is obtained indirectly from the induced...

LumiFlow

LumiFlow is a postprocessing technique for color or power Doppler ultrasound, which produces a relief-like visualization of the depicted vasculature. Physics Lumiflow can be applied to both color and power Doppler imaging. It applies a shading with an artifical light source to create a three-...

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. ...

Taurine

Taurine is one of the compounds examined in MR spectroscopy. It resonates at 3.4 ppm chemical shift. It is elevated in medulloblastomas.

CT chest abdomen-pelvis (protocol)

The CT chest-abdomen-pelvis protocol serves as an outline for an examination of the trunk covering the chest, abdomen and pelvis. It is one of the most common CT examinations conducted in routine and emergencies. It can be combined with a CT angiogram. Note: This article aims to frame a genera...

CT abdomen-pelvis (protocol)

The CT abdomen-pelvis protocol serves as an outline for an examination of the whole abdomen including the pelvis. It is one of the most common CT protocols for any clinical questions related to the abdomen and/or in routine and emergencies. It forms also an integral part of trauma and oncologic ...

Phase-sensitive inversion recovery

Phase-sensitive inversion recovery (PSIR), also known as phase-corrected inversion recovery (PCIR), refers to an inversion recovery MRI pulse sequence that accounts for the positive and negative polarities and preserves the information of tissue magnetization during the recovery from the initial...

CT pancreas (protocol)

The CT pancreas protocol serves as an outline for a dedicated examination of the pancreas. As a separate examination, it is usually conducted as a biphasic contrast study and might be conducted as a part of other scans such as CT abdomen-pelvis, CT chest-abdomen-pelvis. Note: This article aims...

CT elbow (protocol)

The CT elbow protocol serves as an examination for the bony assessment of the elbow and is usually performed as a non-contrast study. It can be also combined with a CT arthrogram for the evaluation of chondral and osteochondral injuries. Note: This article aims to frame a general concept of a ...

CT hand and wrist (protocol)

The CT hand and wrist protocol serves as an examination for the bony assessment of the wrist and is often performed as a non-contrast study and less often as a contrast-enhanced study. A CT wrist can be also conducted as a CT arthrogram for the evaluation of ligamentous injuries and the triangul...

CT pelvis (protocol)

The CT pelvis protocol serves as an outline for the acquisition of a pelvic CT. As a separate examination, it might be performed as a non-contrast or contrast study or might be combined with a CT hip or rarely with a CT cystogram. A pelvic CT might be also conducted as a part of other scans such...

CT neck (protocol)

The CT neck protocol serves as a radiological examination of the head and neck. This protocol is usually performed as a contrast study and might be acquired separately or combined with a CT chest or CT chest-abdomen-pelvis. On rare occasions, it will be performed as a non-contrast study. Dependi...

CT thoracic spine (protocol)

The CT thoracic spine or T-spine protocol serves as an examination for the assessment of the thoracic spine. As a separate examination, it is often performed as a non-contrast study. It might be combined or simultaneously acquired with a CT chest or CT chest-abdomen-pelvis as part of a trauma or...

CT lumbar spine (protocol)

The CT lumbar spine or L-spine protocol serves as an examination for the assessment of the lumbar spine. As a separate examination, it is most often performed as a non-contrast study. It might be combined or simultaneously acquired with a CT abdomen. It also forms a part of a polytrauma CT or mi...

CT cervical spine (protocol)

The CT cervical spine or C-spine protocol serves as an examination for the assessment of the cervical spine. It is usually performed as a non-contrast study. In certain situations, it might be combined or simultaneously acquired with a CT angiography of the cerebral arteries or a CT of the neck....

CT shoulder (protocol)

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...

CT foot (protocol)

The CT foot protocol serves as an examination for the bony assessment of the fore and mid-foot and is almost always performed as a non-contrast study. It can also be combined with a CT ankle protocol or can be acquired as dual-energy CT depending on the clinical question. Note: This article aim...

Zero echo time imaging

Zero echo time (ZTE) imaging is a relatively recent development in MR technology, with the aim to better visualize tissues such as bone with the shortest T2 values. Physics In ZTE, the signal is acquired immediately after applying the radiofrequency pulse resulting in near-zero echo times. Af...

Archives of the Roentgen Ray

The Archives of the Roentgen Ray was a general radiology journal published from 1897 to 1915. In 1915, it was renamed the Archives of Radiology and Electrotherapy. The Archives was a forerunner publication of the British Journal of Radiology (BJR). History In 1897, after a single year in print...

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

Mache

The mache (ME) is an obsolete unit of concentration of radioactivity which was originally expressed by a concentration of elemental radon per liter of water. One mache unit is equivalent to 3.7 x 10-7 curie or 13,760 becquerel per milliliter. History and etymology This eponymous unit was named...

Laser interstitial thermal therapy

Laser interstitial thermal therapy (LITT) or focal laser ablation is a surgical technique for selective ablation of a lesion or tissue using laser-generated heat. Compared to other minimally invasive techniques such as radiofrequency, microwave, or cryoablation, lasers are able to create a more ...

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...

Photoacoustic imaging

Photoacoustic or optoacoustic imaging (PAI) is an emerging imaging modality that utilizes a hybrid approach by using optical illumination of endogenous materials or administered fluorescent tracers, and consequent detection of the ultrasound waves released due to thermoelastic expansion. Physi...

MR cerebral venography

MR cerebral venography (MRV) is an MRI examination of the head with either contrast-enhanced or non-contrast sequences to assess patency of the dural venous sinuses and cerebral veins. NB: This article is intended to outline some general principles of protocol design. The specifics will vary d...

Cardiac ischemia protocol (MRI)

The cardiac MRI ischemia or stress protocol encompasses a set of different MRI sequences for the assessment of myocardial ischemia. Note: This article aims to frame a general concept of a cardiac MRI protocol in the setting of vasodilator stress perfusion testing. Protocol specifics will vary ...

Dalton (unit)

The dalton (symbol: Da), also known as an atomic mass unit, is a unit of mass that is equal to one twelfth of the mass of a free carbon-12 atom at rest. Its value is approximately equal to 1.660 x 10−27 kg. The molar mass of an entity, when measured in daltons, is approximately equal to the sum ...

Coherent system of units

A coherent system of units consists of a set of base units (typically time, length, mass, electric current and temperature) and a set of derived units. The derived units are formed from the product of base units raised to specific powers with a constant factor of one. Some derived units have spe...

Pierre Curie

Pierre Curie (1859–1906) was a French physicist who was co-awarded the Nobel Prize for Physics in 1903, with his wife Marie Curie, and Henri Becquerel, for their pioneering work on radioactivity, which included the joint discovery with his wife of radium and polonium. He also discovered piezoele...

Chronic appendicitis

Chronic appendicitis (rare plural: appendicitides) is defined by inflammation of the appendix over time with symptoms lasting for more than three weeks duration (cf. acute appendicitis) 1. The condition should be differentiated from recurrent appendicitis, in which one or more episodes of flares...

Ultrasound probe defect

Ultrasound probe defects are hardware failures of the ultrasound transducer manifesting as various abnormalities of the scan image, in severe cases even causing complete signal loss. These are a form of ultrasound artifact. Therefore, awareness of the various signs of equipment damage is crucia...

James Ambrose

James "Jamie" Ambrose (1923-2006) was a neuroradiologist and co-developer of the first CT scanner with physicist Godfrey Hounsfield. Together they performed the first ever CT scan on a living patient in 1971 1. Early life James Abraham Edward Ambrose was born on 5 April 1923 in Pretoria, South...

MR tagging

Cardiac MR tagging or myocardial tagging refers to a MRI based acquisition method designed for myocardial deformation analysis. Methodology The method exploits tissue magnetization as a tissue property. A local magnetic saturation grid of dark lined tissue markers known as tags are induced ont...

Joule

The joule (J) is the derived SI unit of energy quantity. One joule represents the work exerted by a force of one newton acting over a distance of one meter in the direction of that force. Terminology As for all other eponymous units when the name is written out in full it is not capitalized, b...

Tungsten

Tungsten (chemical symbol, W) is a hard refractory metallic element with remarkable resilience which forms the basis for its industrial uses. It is the metal of choice in the filaments and targets of x-ray tubes. There is no evidence that tungsten is required by the human body, although some mic...

William D Coolidge

William D Coolidge (1873-1975) was an American physicist who revolutionised radiology with his groundbreaking x-ray tube, the underlying technology of which remains at the core of every machine more than a century later. Early life William David Coolidge was born on 23 October 1873 on a small ...

MR feature tracking

MR feature tracking refers to an MRI based post-processing method, used on normal cine SFFP sequences for the analysis of myocardial deformation and the determination of myocardial strain parameters. Methodology MRI feature tracking is a two-dimensional software algorithm applied on standard c...

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...

Dexel

Dexels, a portmanteau of detector elements, analogous to pixels, refers to the individual radiation-sensitive elements of the detector component of a scanner, e.g. computed tomography. It is important to appreciate that there is not necessarily a one-to-one correspondence between dexels and the ...

Velocity encoding

Velocity encoding or Venc is referred to as an operator-controlled parameter for the determination of the maximum velocity within a velocity encoded phase contrast imaging study. Usage Velocity-encoding (Venc) gradients are used to generate a phase shift in magnetic resonance phase contrast im...

Chemical exchange saturation transfer

Chemical exchange saturation transfer (CEST) imaging is a novel molecular MR technique that enables imaging certain compounds at concentrations that are too low to impact the contrast of standard MR imaging and too low to directly be detected in MRS at typical water imaging resolution 1. Amide ...

Parathyroid 4D CT

Parathyroid four-dimensional (4D) CT refers to multiphase computed tomography of the neck used to localize abnormal parathyroid glands (i.e., involved with adenoma, hyperplasia, or, rarely, carcinoma). The "4D" indicates that imaging is performed in multiple phases of contrast, with time being t...

Low-field magnetic resonance imaging

Low-field magnetic resonance imaging is an emerging approach to MRI imaging, which aims to provide diagnostic image quality using devices with several magnitude lower field strength (typically well under 0.1T) than most stationary units. The reduced field strength of these devices allows improv...

Electromagnetic radiation

Electromagnetic radiation refers to the waves or quanta of the electromagnetic field as they propagate through space. The speed of electromagnetic waves is invariant in a vacuum, being ~3x108 m/s and represented by the symbol, c, otherwise known as the speed of light. The types of electromagneti...

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 ...

Noise power spectrum

The noise power spectrum (NPS), also known as the power spectral density, of a signal, is the Fourier transform of the noise autocorrelation. It gives the intensity of noise as a function of spatial frequency. It is used in all the main radiological modalities, most commonly x-ray-based, i.e. ra...

Transmutation

Nuclear transmutation occurs when a decay process alters the number of protons in a nucleus to form a new element. Processes which cause nuclear transmutation include beta particle emission, alpha particle emission, and electron capture.

Advanced multiple beam equalization radiography

The advanced multiple beam equalization radiography (AMBER) system is used to control the distribution of local exposure to the film. An array of independently functioning detectors is used to send feedback signals to the modulators kept in front of the x-ray tube to modulate the exposure levels.

B-Flow

B-Flow is a type of ultrasound imaging that allows visualization of blood flow by selectively enhancing the signal from moving blood cells while simultaneously suppressing tissue signal 1. Unlike color Doppler, it does not show flow direction or amplitude. B-flow is used clinically to image the...

Nyquist limit

The Nyquist limit represents the maximum Doppler shift frequency that can be correctly measured without resulting in aliasing in color or pulsed wave ultrasound. Physics The Nyquist limit always equals Pulse Repetition Frequency (PRF)/2. The US machine can display the Nyquist limit either as ...

Electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPR) and spectroscopy is a preclinical imaging modality with potential to be translated into a clinical imaging technique in the future. In brief, electron paramagnetic resonance imaging allows detection and quantification of free radical molecules with u...

Magnetomotive ultrasound

Magnetomotive ultrasound (MMUS) is an emerging medical imaging modality, which utilizes the discrete tissue vibration caused by superparamagnetic iron oxide (SPIO) nanoparticles under an external magnetic field. Physics If an external time-varying magnetic field is applied (e.g. using an elec...

Cardiac gating (MRI)

Cardiac gating or cardiac triggering refers to the gain of information about specific time points and their use for image acquisition during the cardiac cycle. Technique Cardiac synchronization can be achieved by the ECG signal or with a peripheral pulse transducer. The following two types of ...

Cine imaging (MRI)

Cine imaging, a.k.a. cine sequences or cine MRI, are a type of MRI sequence acquired to capture motion. Imaging technique Cine images are obtained by repeatedly imaging the area of interest for a certain time typically within a single slice, although 3D solutions already exist 3. For the hear...

MRI protocol article structure

Articles describing specific MRI protocols require a different set of subheadings as the usual epidemiology, clinical presentation, pathology, etc. are not relevant. Example article: ankle protocol (MRI) ======================================================================= An introductory s...

Elbow protocol (MRI)

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

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...

Ankle protocol (MRI)

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

Intermediate weighted images

An intermediate weighted image is acquired by a sequence with a proton-density like long repetition time and a prolonged echo time usually 35-60 ms 1,2. It combines the ability to depict the detailed anatomy of a proton density-weighted image with the fluid sensitivity of a T2-weighted sequence,...

Hip protocol (MRI)

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

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...

Blur

Blurring, or unsharpness, refers to the distortion of the definition of objects in an image, resulting in poor spatial resolution. Types of blur geometric blur limited by focal spot size, distance from the source to the patient, distance from patient to receptor detector blur limited by...

Binning

Binning is a technique by which signals arriving from adjacent physical elements of an electronic detector are combined to produce a larger pixel/voxel. This increases the signal to noise ratio to provide better contrast resolution, with the trade-off being reduced spatial resolution.

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 ...

Dose area product

The dose area product (DAP) or kerma area product (KAP) is a method of radiation dose monitoring used in radiographic and fluoroscopic studies. It provides an indication of the radiation dose received by a patient. It is calculated as the product of dose and beam area (Gy.cm2), and is measured ...

Diagnostic reference level

A diagnostic reference level (DRL) is a specified radiation dose for a given imaging study that is not expected to be exceeded. If a radiation dose does exceed the diagnostic reference level for a particular study, this should prompt an investigation into radiographic technique or equipment per...

Compression paddle

A compression paddle is a device found in mammographic units which is used to compress the breast. It consists of a flat radiolucent plate positioned parallel to the support table, and attached to either a mechanical or pneumatic assembly. It is controlled by the operator by a foot pedal. Full...

Compression in mammography

In mammography, compression of the breast is performed to reduce its thickness. By doing so, the following benefits are achieved: improved subject contrast (by reducing scattered radiation) improved density uniformity improved visualization of breast tissue near chest wall (by spreading out s...

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...

Doubling dose

The doubling dose refers to the radiation dose required to double the number of spontaneous genetic mutations in a given population of cells. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) have traditionally estimated the doubling dose in humans at 1.0 Gy. ...

5 gauss line

The 5 gauss line is the safety line drawn around the perimeter of the main magnet of the MRI scanner, specifying the distance at which the stray magnetic field is equivalent to 5 gauss (0.5 mT). Five gauss and below are considered 'safe' levels of static magnetic field exposure for the general ...

Contrast improvement factor

The contrast improvement factor of an anti-scatter grid is the ratio of contrast obtained with the use of the grid to the contrast without the use of the grid. Hence, the contrast improvement factor reflects the increased image quality obtained from grid use. Most grids will have a contrast im...

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...

Bucky factor

The Bucky factor is the ratio of radiation on the anti-scatter grid to the transmitted radiation. Hence, the Bucky factor reflects the increased radiation dose required from anti-scatter grid use, as any increase in mAs proportionally increases dose. The Bucky factor changes with: change in k...

Automatic brightness control (ABC)

Automatic brightness control (ABC) is a device incorporated into the fluoroscopic unit. Its function is to maintain a consistent overall appearance of the image by automatically adjusting the kVp and/or mAs. In this regard, ABC systems are analogous to the AEC systems utilized in radiography. ...

Automatic exposure control

Automatic exposure control (AEC) is a device incorporated into radiographic and mammographic imaging systems. Its function is to automatically terminate exposure when a preset amount of radiation has been detected. Automatic exposure control systems help to provide a consistent optical density/...

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...

Nuclide

A nuclide is a nuclear-centric term describing an atomic species by its nuclear composition and nuclear energy state. A nuclide has a specific number of protons and neutrons and will additionally have a specific energy state of its nucleus. Radionuclides are unstable nuclides that undergo radi...

Metastable state

A metastable state of an isomer is defined as an excited state that exists for greater than 10-6 seconds. In chemical notation, metastable species are identified by the letter 'm'. Typically, excited nuclei will instantaneously decay to a more stable energy state (within 10-15 seconds), emittin...

Isomer

In nuclear physics, isomers are atomic species that are identical in nuclear composition, sharing the same mass and atomic numbers, but differ in their relative energy states, and will therefore differ in their manner of radioactive decay. This term should not be confused with the similarly sou...

Nuclear shell model

The nuclear shell model is a theoretical construct in nuclear physics which describes that the nucleus of an atom can exist in discrete energy states. This model is partially analogous to the electron orbitals of the atomic shell model. The ground state is the lowest energy state for a specific...

Isotone

Isotones are atomic species that share the same number of neutrons and differ in the number of protons. Examples of isotones include carbon-12, nitrogen-13 and oxygen-14. These atoms all have six neutrons and six, seven and eight protons respectively. A mnemonic that can be used to differentia...

Isobar

Isobars are atomic species that have the same mass number (A), but a different atomic number (Z). Isobars should not be confused with isotopes, which share the same atomic number, and therefore belong to the same chemical element, but have varying mass numbers. Examples of isobars include 14,6...

Electronvolt (unit)

An electronvolt (eV) is defined as the energy required to accelerate a single electron at rest through an electron potential difference of one volt in a vacuum. 1 eV ≈ 1.603 x 10-19 J Electronvolts are a more mathematically convenient method to describe the miniscule quantity of energy associa...

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%. ...

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...

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