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CT Vocab
Vocabulary located in Mosby's Exam Review for CT
| Question | Answer |
|---|---|
| 180* linear interpolation | Type of algorithm used for MSCT image reconstruction in which data acquired from a shorter distance (180*) away from the reconstructed slice location are interpolated. |
| 360* linear interpolation | Type of algorithm used for MSCT image reconstruction in which two sets of projection data acquired 360* apart are used to form an image at a precise z-axis location |
| absorbed dose | the amount of x-ray energy absorbed in a unit of mass. Measured in grays (Gy) |
| What is absorbed dose measure in? | Grays (Gy) |
| absorption effeciency | the ability of an individual CT detector to absorb and measure the transmitted x-ray intensity incident upon it. Primarily controlled by the characteristics and physical makeup of the detector material |
| adaptive array | the type of MSCT detector array configured with the thinnest detectors at the center, surrounded by detectors of incrementally increasing widths along the z-axis |
| advanced single slice rebinning (ASSR) algorithm | cone beam correction algorithm utilized to overcome potential attenuation calculation errors from the divergent bean of MSCT systems |
| afterglow | the tendency of a scintillation-type CT detector to continue to glow in response to x-radiation after the exposure source has been terminated |
| Agatston score | quantification of the volume and density of calcium within the coronary arteries. As calculates during a CT coronary artery calcification (CAC) examination, the Agatston score is used to indicate a patient's risk of suffering a cardiac event |
| algorithm AKA kernel | mathematical filter applied to raw data during CT image reconstruction to remove blurring artifact inherent to back-projection. |
| aliasing | a form of streak artifact caused by an insufficient number of views (data samples) obtained during data acquisition |
| analog-to-digital converter (ADC) | component of the data acquisition system (DAS) responsible for converting the electronic signal emitted by the CT detectors into digital form |
| array processor (AP) | component of the CT computer system responsible for receiving raw scan data, performing all of the major processing of the CT image, and returning the reconstructed image to the storage memory of the host computer |
| arterial phase | period of peak arterial enhancement following the bolus IV administration of iodinated contrast material. For example, the hepatic arterial phase occurs approximately 25-35 seconds after the initiation of contrast |
| artifact | form of noise on the CT image resulting from errors during the measurement of transmitted radiation by the detectors |
| attenuation | the reduction in intensity of a radiation beam as it passes through a substance |
| attenuation coefficient | the value assigned to an object quantifying its ability to attenuate an x-ray beam |
| automatic tube current modulation (ATCM) | CT system software used to adjust the mA throughout an acquisition to reduce patient radiation dose to a minimum |
| back projection | mathematical process of CT image reconstruction whereby ray sum data are projected back onto a matrix |
| beam hardening | phenomenon whereby low-energy photons are absorbed as the x-ray beam passes through an object, resulting in an increase in the average photon energy of the beam |
| beam pitch | ratio of table feed per gantry rotation to the total collimation used during acquisition with a MSCT system |
| beam width | dimension of the primary beam in the longitudinal or z-axis as controlled by the pre-patient collimator |
| binning | process of electronically combining signal from adjacent detector elements to produce a reconstructed Ct image that is thicker than the individual detector width |
| bit depth | number of information bits contained within each pixel |
| blood urea nitrogen (BUN) | measurement of renal function, determined as the amount of nitrogen in the blood in the form of the waste product urea. Normal range 7-25 mg/dl |
| What is the normal range of BUN in adults? | 7-25 mg/dl |
| cardiac cycle | series of blood flow related events that occur from the beginning of one heartbeat to that of the next |
| cerebral blood flow (CBF) | measurement during CT perfusion studies of the brain. CBF is the quantity of blood (mL) that MOVES THROUGH 100 g of brain tissue each minute. Normal range in gray matter is 50-60 mL/100 g/min |
| cerebral blood volume (CBV) | measurement during CT perfusion studies of the brain. CBV is the quantity of blood (mL) CONTAINED WITHIN a 100 g volume of brain tissue. Normal range is 4-5 mL/100 g |
| cerebral perfusion | level of blood flow throughout brain tissue |
| contiguous images | CT images acquired with equal section thickness and reconstruction intervals |
| contrast resolution AKA low-contrast detectability AKA sensitivity | ability of a CT system to detect an object with a small difference in linear attenuation coefficient from the surrounding tissue. |
| contrast induced nephrotoxicity (CIN) | substantial decline in renal function that can occur after a patient receives IV contrast. Usually signified by a marked increase in serum creatinine. |
| convolution | mathematical filtration used by the CT system to remove blurring artifact during the back-projection method of image recons. |
| corticomedullary phase | a late arterial phase of renal enhancement beginning 30-40 seconds after the initiation of contrast. Optimal enhancement of the renal cortex and renal veins occurs during this period |
| creatinine | waste product of metabolism found in the bloodstream and measured as an indicator or renal function. Normal range 0.5-1.5 mg/dL |
| CT dose index (CTDI) | approximate measure of the radiation dose received in a single CT section or slice |
| CT enterocylsis | evaluation of the small bowel whereby enteral contrast is administrated directly into the duodenum through a NG tube |
| CT enterography | evaluation of the small bowel after the oral administration of low-density barium (0.1%) |
| CT number | relative value assigned to each pixel to quantify the attenuation occurring in each voxel in comparison with the attenuation of water. Hounsfield Units (HU) |
| The calculated CT number for a given pixel is called? | Hounsfield units AKA Pixel value |
| CT runoff | CT angiographic assessment of the peripheral arterial tree from the renal arteries though the lower extremities. |
| CT simulation | process of obtaining anatomic info with CT imaging that is used to calculate the beam arrangement for radiotherapy |
| cupping artifact | error occurring in the superior portion of the skull, where dense bone averages with the low-attenuating brain tissue. This partial volume artifact may result in abnormally dense-appearing brain parenchyma |
| data acquisition system (DAS) | electronic components of a CT system responsible for measuring the transmitted x-radiation absorbed by the detectors |
| data channel | pathway of data transmission from the detectors to the computerized components of the system's data acquisition system. The number of data channels of a CT system determines the maximum number of sections that may be acquired with each gantry rotation. |
| data transfer rate | the speed at which a computerized storage drive is able to transfer data |
| deconvolution | mathematical image processing technique used to generate blood perfusion maps during CT perfusion studies of the brain |
| detector | device responsible for measuring transmitted radiation and converting it into a proportionate electronic signal to be used for image recon. |
| detector array | CT image receptor, consisting of a series of detectors arranged in varying configuration. |
| detector collimation | process of determining section width in the MSCT system as determined by the defined beam width and the number and thickness of detectors utilized for image recon. |
| detector configuration | number, length and organization of the individual detector elements in an MSCT system |
| detector pitch | ratio of table feed per gantry rotation to the acquired section width. This definition of pitch is used for helical single-slice CT (SSCT) |
| diastole | portion of the cardiac cycle when the heart muscle is relaxed. Reflects the force exhibited on the arterial walls during relaxation of the heart muscle. |
| Digital Imaging and Communications in Medicine (DICOM) | process of recording, storing, printing, and transmitting medical image data |
| display field of view (DFOV) AKA zoom factor AKA target view | diameter of the acquired attenuation data displayed across the image matrix |
| dose length product (DLP) | measurement of dose for an entire series of CT images. Equal to calculated does /section X length of a CT acquisition along the z-axis |
| dose profile | section of the patient exposed to radiation at the gantry isocenter |
| duel energy CT | simultaneous acquisition, by a MSCT system, of attenuation date for a single anatomic section with 2 beams, each having a different energy spectrum. Rapidly switches between KVP , allowing for improved contrast between substances of different densities |
| duel source CT | system consists of 2 separate x-ray tubes and detector arrays mounted 90* from each other within the gantry, allowing for duel energy CT acquisition. |
| edge gradient | streak artifacts that occurs at the interface between a high-density object and the lower attenuation material surrounding it. |
| effective dose | approximation of relative risk from exposure to ionizing radiation. Calculated by assigning weighting factors to different tissues on the basis of their individual radiosensitivities. Measure in sieverts (Sv) |
| What is effective dose measure in? | Sieverts (Sv) |
| effective section width | the slice sensitivity profile (SSP) of the reconstructed CT section. It is measured at the full width at half maximum (FWHM) of the SSP for a given CT acquisition |
| electron beam CT (EBCT) | specialized CT design devoid of moving parts. Utilizes a beam of electrons bombarding a tungsten target to produce x-radiation. Produces short exposures which is ideal for cardiac scoring. |
| equilibrium phase | contrast phase of the liver when hepatic parenchymal enhancement dissipates and there is minimal attenuation difference between the intravascular and extravascular spaces. Usually occurs t 2-3 minutes after initiation of contrast |
| excretory phase | delayed imaging renal enhancement phase that begins approximately 3 minutes after the initiation of contrast. |
| exposure | ability of x-rays to ionize a volume of air. measured in roentgens (R) |
| what is exposure measured in? | roentgens (R) |
| extravasation | escape of contrast outside the blood vessel into the surrounding soft tissue |
| fan agle | angle of coverage by the x-ray beam as it emerges from the tube housing and exposes the detector array within the scan plane (x-y axis) |
| feldkamp-davis-kress (FDK) algorithm | cone beam correction algorithm utilized to overcome potential attenuation calculation errors from the divergent beam of MSCT system |
| filtered back projection | mathematical process of CT image recons that involves convolution of the raw data prior to their projection back onto a matrix. |
| filtration | removal of the low energy x-ray photons emitted from the x-ray tube to improve beam quality and reduce patient radiation dose. |
| flat panel detector | large area detector consisting of a film of scintillating crystals bonded to a matrix of silicon photosensors. may be used in place of segmented detector rows found in MDCT systems. |
| flying focal spot | electronic switching tech. where the electron beam is electromag. steered toward 2 separate locations on the rotating anode. Results in the emission of 2 sources of x-radiation from tube and a doubling of the data samples acquired from each rotation |
| full width at half maximum (FWHM) | midpoint of a slice sensitivity profile (SSP), where the effective section width for a CT acquisition is determined. |
| gantry | assembly that houses he xray tube, detectors, and additional data acquisition components of a CT system. |
| geometric efficiency | ability of a detector array to absorb and measure the transmitted xray intensity incident upon it. Primarily controlled by the physical arrangement of detectors within the array and the amount of interspace material required between adjacent detectors. |
| glomerular filtration rate (GFR) | approximation of creatinine clearance or the rate by which creatinine is filtered from the bloodstream. Normal range Men: 70 +/- 14 mL/min/m2 Women: 60 +/- 10 mL/min/m2 |
| helical AKA spiral | type of CT acquisition whereby the xray tube and patient continuously move during scanning. yielding a data set in the form of a helix. |
| hounsfield bar | specific type of streaking beam-hardening artifact that occurs in the posterior fossa of the brain. |
| hounsfield unit | the unit of the CT number scale assigned to each pixel to quantify relative attenuation. |
| hybrid-array | the type of MSCT detector array with narrower detectors positioned midline, flanked by the wider detectors. |
| hyperdense | possessing CT attenuation values greater than the values of the surrounding tissue. |
| hypodense | possessing CT attenuation values less than the values of the surrounding tissue. |
| image compression | complex computer technique that reduces the size of digital CT image data |
| image data | reconstructed data that have been projected back onto a matrix after convolution by an algorithm and displayed on a monitor as a gray scale CT image |
| insufflation | introduction of air into an organ or cavity for distention and improved visualization. |
| interpolation | mathematical process used for helical CT image reconstruction whereby data from tube rotations just above and just below a given slice position are used for image reconstruction. |
| isocenter | center point of gantry rotation |
| isodense | possessing CT attenuation values equal to the values of the surrounding tissue. |
| isotropic | having equal dimensions along the x, y and z axes |
| kerma | quantity of energy deposited in a unit of mass. used to describe the absorbed dose of x-radiation |
| linear attenuation coefficient (u) | the values assigned to an object to quantify the extent to which it attenuates xray |
| linearity | relative accuracy between calculated CT numbers and their respective linear attenuation coefficients. |
| lossless compression | reversible process whereby the CT image is digitally compressed in size without loss of data and is identical to the original. |
| lossy compression | irreversible process whereby data are lost during the compression process and the CT image does not exactly match the original. |
| matrix | two dimensional grid of numbers arranged in a row |
| maximum intensity projection (MIP) | multiplane reformation technique that displays only the maximum pixel value along a ray traced through the object to the viewer's assumed perspective in front of the viewing monitor. |
| mean transit time (MTT) | refers to the average transit time, in seconds, needed for blood to pass through a given region of brain tissue. common measurement during CT perfusion studies of the brain. |
| minimum intensity projection (min-IP) | multiplanar reformation technique that displays the minimum pixel value along each ray to the viewer |
| misregistration | artifact that occurs when patient motion between consecutive acquisitions causes misalignment of data and the potential loss of anatomic info |
| modulation transfer function (MTF) | graphical representation of a CT system's response to a spatial frequency that serves as a measurement of the system's inplane spatial resolution. |
| monoenergetic | consisting of a uniform photon energy; use in CT to describe a beam of radiation |
| multi slice CT (MSCT) | CT system with a detector array capable of acquiring more than on section for each gantry rotation |
| multiphase | CT data acquisitions during multiple times phases of contrast enchancement |
| multiplanar reformation (MPR) | process of displaying CT images in a different orientation from the one used in the original reconstruction |
| multiple scan average dose (MSAD) | calculation of the average cumulative radiation dose to each anatomic slice within the center of a CT scan consisting of multiple slices |
| multisegment reconstruction | method of improving the temporal resolution of a CT system whereby the data acquisition process is subdivided into separate components of smaller rotation angles |
| nephrographic phase | renal enhancement phase occurring between 70-90 seconds after the start of injection of contrast. Enhancement between renal cortex and medulla reaches equilibrium providing optimal sensitivity for parenchymal lesions. |
| noise | grainy appearance of the CT image due primarily to an insufficient xray flux per voxel. May also be described as any portion of the signal that contains no useful info |
| nyquist theorem | sampling law dictating that the data sampling frequency must be at least twice the object's spatial frequency in order for the object to be resolved by the CT system |
| opacification | increase in CT density of a structure due to filling with positive contrast material |
| operating system (OS) | main software of the CT computer, controlling the utilization of the hardware resources including the available memory, CPU time. disk space and so on |
| othogonal | imaging planes that are perpendicular to each other |
| orthographic volume rendering | technique yielding a 3D model with the perspective of externally viewing the reconstructed object |
| osmolality | propensity of an iodinate contrast medium to cause fluid from outside the blood vessel to move into the bloodstream |
| out of field artifact | hyperdense streaking that occurs when a portion of the patient has been positioned outside the scan field of view (SFOV) |
| overbeaming | expansion of the primary bean in an MSCT system to ensure that all detectors of the array are exposed to xrays of equal intensity |
| overlapping images | CT images produced with a reconstruction interval that is less than the section width |
| overranging | process of applying radiation dose before and after the acquisition volume to ensure sufficient data collection for the interpolation algorithm of helical CT recon. |
| pancreatic phase | delayed arterial enhancement phase occurring approx 35-45 seconds after the start of contrast . If adequate volume has been injected at a rate of 3mL/sec or greater |
| parallel processing | ability of a computer to perform multiple functions simultaneaously |
| partial volume artifact | error that occurs when a structure is only partly positioned within a voxel and the attenuation for the object is not accurately represented by a pixel value |
| partial volume averaging | inaccuracy in pixel values that occurs when the associated voxels contain attenuation coefficients for multiple tissue types. Values are averaged to yield a single pixel value that attempts to represent an assortment of different materials |
| perspective volume rendering AKA Immersive Rendering | technique yielding a 3D model with the perspective of being within the lumen of the object, similar to an endoscopic view |
| phantom | quality control device typically composed of a radiolucent plastic material containing specialized inserts that is used to measure specific image quality criteria |
| photodiode | device used by a solid state CT detector to convert the light emitted by a scintillation crystal into a proportional electronic signal |
| photon fluence | quantity of xray photons passing through a specified area |
| photon flux | rate at which a quantity of xray photons passes through a unit area over a unit time |
| pipelining | form of parallel processing used by a computer to improve computation speed |
| pitch | relationship between collimation and table movement per gantry rotation |
| pixel (picture element) | refers to the individual boxes arranged in the matrix used to display the CT image |
| point spread function (PSF) | a measure of a CT system's in plane spatial resolution that evaluates the amount of spread inherent in an orthogonal image of a thin wire. |
| polyenergetic | consisting of a spectrum of differing energies; used to describe the beam of the x-radiation |
| portal venous phase | period of peak hepatic parenchymal enhancement when contrast redistributes from the blood into the extravascular spaces. Typically occurs 60-70 seconds after the initiation of contrast |
| prospective ECG gating | method of improving the temporal resolution of an MSCT system during examinations of the heart and mediastinum. Data is acquired in an axial step-and-shoot mode and only during the diastolic portion of the R-R interval |
| prospective reconstruction | the initial construction of the acquired raw data into CT image data with selected display field of view, algorithm, image center and so on |
| pseudoenhancement | minimal increase in attenuation demonstrated by a structure that typically does not enhance following intravenous contrast administration. Most common during MSCT evaluation of cysts |
| quality assurance | measurement of the scanner's performance through quality testing procedures and evaluation of the test results. |
| quality control | implementation of corrective actions to improve any identified performance inadequacies of CT system |
| raw data | transmission measurements obtained by the detectors used to mathematically reconstruct the CT image |
| ray | portion of the xray beam transmitted through the patient and incident upon a single detector |
| ray sum | measurement of transmitted radiation made by an individual detector used to determine the attenuation occurring along a ray |
| region of interest (ROI) | user defined graphic outline that calculates the average CT number of a given anatomic area |
| rendering | use of 3D algorithms to provide a specific perspective to the construction of a 3D model |
| response time | ability of a CT detector to quickly measure xray and then recover before the next measurement |
| ring artifact | an incorrect ring of density on the reconstructed CT image resulting from detector malfunction |
| scan delay | the time between the initiation of contrast administration and CT data acquisition. The chosen scan delay determines the phase of contrast enhancement for a given CT acquisition |
| scan field of view (SFOV) AKA Calibration Field | a parameter that controls the diameter of the circular data acquisition field within the CT gantry as determined by the number of activated detectors along the x-y axes. |
| scintillation | the production of light energy by a CT detector material in response to absorbed xray energy |
| section inerval | the distance between the center of one CT section and the center of the next adjacent section |
| section width AKA Slice thickness | the dimension of a reconstructed CT slice along the longitudinal direction of acquisition (z-axis) |
| signal | electronic current emitted by the CT detector in response to the absorption and measurement of transmitted radiation. |
| signal to noise ratio (SNR) | quantification of the amount of noise in a displayed CT image. Calculated as the standard deviation in the ROI measurement of a water phantom image |
| slice sensitivity profile (SSP) | graph demonstrating the broadening of the section width that is inherent to volumetric (helical) CT acquisition |
| slip ring | rotating assembly used to enable the passage of electrical signal during continuous rotation of the helical CT system |
| spatial frequency | the waveform of signal that represents the varied objects imaged by a CT system |
| spatial resolution | the ability of a CT imaging system to display fine details separately. Given in units of line pairs per centimeter (lp/cm) |
| step artifact | the unwanted appearance of individual sections on a multiplanar or 3D reformation image resulting in a loss of sharpness and detail |
| stranding | hazy increase in density of the fat surrounding an organ on CT exam that usually indicates an inflammatory process |
| surface rendering AKA Shaded surface display (SSD) | construction of a 3D model of a specific tissue type by limiting the displayed volumetric data on the basis of an attenuation threshold. |
| temporal resolution | the ability of a CT system to freeze motion and provide an image free of blurring |
| total collimation | the combined thickness of all of the sections that are simultaneously acquired with each gantry rotation during MSCT acquisition. |
| triple rule-out | comprehensive MSCT examination pf the chest for cardiac and noncardiac pain. Evaluates for coronary artery disease, aortic aneurysm and pulmonary embolism |
| tube arcing | short circuiting within the xray tube during data acquisition that results in severe streak artifacts in the CT image |
| uniform matrix array | type of MSCT detector array that utilizes multiple detectors in the longitudinal direction, each of the same length |
| uniformity | maintenance of relatively consistent CT values across the entire image of an object of equal density |
| view | each data sample made by the DAS during CT data acquisition |
| views per rotation (VPR) | the sampling rate of transmission measurements acquired during each gantry rotation |
| volume rendering | 3D modeling technique that utilizes the entire acquired data set but adjusts the opacity of voxels included in the 3D image according to their tissue characteristics |
| voxel (voxel elment) | refers to the volume of tissue represented by a pixel in the matrix used to display the CT image |
| window | the user defined range of pixel values that will be assigned a particular shade of gray |
| window level | the pixel value, given in Hounsfield units, at the center of the window width. Controls the brightness(density) of the CT image |
| window width | range of pixel values assigned a shade of gray in the displayed CT image. It controls the contrast of the CT image |
| windowing | the process of gray scale mapping of the CT image on the basis of the CT number assigned to each pixel |
| x-y axis | the plane perpendicular to the axis of the data acquisition (z-axis). It is parallel to the plane of the CT gantry |
| z-axis | longitudinal direction of the coordinate system used to spatially describe the location of acquired CT sections. It corresponds to the axis of data acquisition |
| z-filtering | mathematical process utilized by MSCT systems to reconstruct thin sections at any point along the acquired z-axis volume |
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