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CT

Physics and Instrumentation

TermDefinition
the measurement of attenuation that occurs along each path through the patient from xray tube to detector data acquisition
the computerized processing of the transmission measurements into the CT image data reconstruction
display of the reconstructed gray-scale image in two-dimensional (2D) and/or three-dimension (3D) format a representation of the attenuation that occurred across the scanned volume of tissue multidimensional image display
display as well as storage, both short-term and long-term (archival), of images on computer workstations image archival and communication
another name for a scout image scanogram
the angle of the tube and detectors in relationship to the patient position during scout acquisition azimuth
eliminates the need for cables by utilizing a system of contact brushes that supply electricity to power the system and enable the passage of transmission data to the computer system slip-ring technology
involves the electromagnetic steering of the electron beam emitted from the cathode beam is directed toward two separate locations on the rotating anode, resulting in two sources of x-radiation flying focal spot technology
adjusts the mA throughout an acquisition to reduce patient radiation dose to a minimum automatically alters the applied mA according to a predetermined noise index that is acceptable for appropriate image quality automatic tube current modulation (ATCM)
quantity of x-radiation photon fluence
the rate at which the photon fluence passes through a unit area over unit time photon flux
calculated mAs value per acquired slice formula... effective mAs effective mAs=mAs/pitch
filter that is thicker at the ends than in the middle help shape the beam to reduce pt radiation exposure bow-tie filters
two components for MSCT collimation beam collimation and detector (section) collimation
amount of tissue in the longitudinal or z-axis that is represented by the 2D CT image controlled directly by beam collimation during SSCT controlled by beam collimation and detector collimation settings during MSCT section width (slice thickness)
the process of determining section width in the MSCT system detector collimation
the spacing between two adjacent CT images measured as the distance between the center of one section and the center of the next adjacent section section interval
acquired with equal section thickness and interval contiguous images
acquired with an interval greater than the section width noncontiguous images
reconstructed with a section interval that is less than the section width used predominantly to improve the quality of multiplanar reformation (MPR) and 3D reconstructions overlapping images
relationship between collimation and table movement during scanning pitch
reduction of intensity of a radiation beam as it passes through a substance attenuation
ability to quickly measure xrays and then recover before the next measurement response time
the tendency of scintilation crystal detectors to continue to glow in response to xrays once the exposure source has been terminated afterglow
small high-pressure chambers of xenon gas interaction with xrays causes a proportional amount of ionization of the xenon gas,resulting in the discharge of a small electrical signal gas ionization detectors
consists primarily of scintillating crystal material emits a quantity of light energy proportional to the xray incident upon an individual detector solid state detector
consisted primarily of two detectors (paired adjacently along the z-axis) exposed to a tightly collimated pencil beam of xrays first generation
utilizes a traslate-rotate geometry but with an increase in the number of detectors allowing for greater increments of rotation between translations and a reduction in overall scan time fan beam second generation
expanded curvilinear array of detectors rotates with an xray tube around the pt third generation
gantry houses a stationary,circular ring of detectors and the xray tube rotates around the pt fourth generation
utilizes a unique technical configuration devoid of moving parts stationary-stationary geometry, with both the xray source and detectors fixed during scanning electron beam CT (EBCT)
controls section width for traditional SSCT beam collimation
controls section width for MSCT detector collimation
the number, length and organization of the individual detector elements in an MSCT system detector configuration
format of MSCT detector configuration that utilizes multiple detectors in the longitudinal direction, each the same length uniform matrix-array
format of MSCT detector configuration where detectors are configured with the thinnest widths at the center, surrounded by detectors of incrementally increasing widths along the z-axis adaptive-array
format of MSCT detector configuration that consists of two detector sizes narrower detectors are positioned midline, then flanked by the wider detectors hybrid-array
the electronic combination of signal from adjacent detectors to form a reconstructed slice that is thicker than the individual detector width binning
involves multiple axial scans obtained at a single anatomic level over a predetermined period cine CT acquisition
consists of electronic components responsible for measuring the transmitted x-radiation absorbed by the detectors data acquisition system (DAS)
component of the DAS responsible for converting the electronic signal into digital form digital data then transmitted to the system computer for image reconstruction analog-to-digital converter (ADC)
responsible for controlling the signal emitted from the activated detectors exposed to the collimated xray beam during MSCT DAS
capability of performing multiple task simultaneously Central Processing Unit (CPU) is capable of this parallel processing
additional feature of the CT computers systems that further improves the speed and efficiency with which it can perform multiple simultaneous functions, including detector signal pre-processing, convolution, post-processing, and image manipulation pipelining
common choice for mass storage of CT computer system data rewrittable, nonremovable storage system that must be capable of storing a tremendous amount of data hard disk drive
main software of the CT computer controls the utilization of the hardware resources, such as the available memory, CPU time, and disk space operating system (OS)
the portion of the xray beam transmitted through the patient and incident upon a single detector ray
measurement of transmitted radiation made by an individual detector equals the total attenuation occurring along a straight-line path from tube detector ray sum (transmission measurement)
each data sample made by the DAS contains transmission measurements made along multiple rays, each corresponding to a particular detector (ray sums) view
determines the quantity of samples, and therefore transmission data, acquired during each gantry rotation views per rotation (VPR)
ability of an object to attenuate the xray beam is assigned a value linear attenuation coefficient
illustrates the relationship between radiation intensity and object density, and allows for the calculation of an objects linear attenuation coefficient only valid for homogeneous pencil-beam radiation sources lambert-beer equation l=(lo)e^(-ux) e=Euler's constant (2.718) and x=object thickness
the mathematical process primarily responsible for CT image reconstruction the ray sum data acquired from each projection (data sample) is projected back onto a matrix back-projection
2D grid of numbers arranged in rows and columns matrix
applied to reduce image unsharpness an algorithm or convolution kernal acts as a mathematical filter, modifying the ray sum data and removing the unwanted blurring effect of the back-projection convolution
back-projection of the ray sum data onto a matrix combined with mathematical filtration constitutes the current method for image recontruction in the modern MSCT system the filtered back projection
the mathematical process whereby data from tube rotations just above and just below a given slice position are used for image reconstruction allows the reconstruction of a thin, motion-free image from a volumetric data set acquired from a moving patient interpolation
technique that uses two sets of projection data acquired 360 degrees apart to form an image at a precise z-axis location 360 degrees linear interpolation (360LI)
technique that interpolates data acquired at a distance only 180 degrees away from the reconstructed slice location more commonly used for current MSCT image reconstruction 180 degree linear interpolation (180LI)
allows for thin sections to be reconstructed at any along the acquired z-axis volume utilizes multiple complimentary rays beyond those immediately above and below the particular slice plane x-filtering
an image of an object that is numerically based digital image
the individual boxes arranged in the matrix used to display the CT image abbreviation of picture element pixels
the volume of tissue represented by a pixel in the matrix used to display the CT image abbreviation of volume element voxel
a relative value that standardizes the attenuation value of a tissue by comparing it with the attenuation of water for aparticular CT system formula... CT number CT number=[(Ut-Uw)/Uw]X k
occurs when multiple types of tissues are represented by a single voxel attenuation coefficients for each tissue type are averaged, yielding a single pixel value whose HU attempts to represent the entire contents of the voxel partial volume averaging
the process of grayscale mapping of the CT image windowing
number of information bits contained within each pixel controls the max number of CT Housfield values that may be assigned to any pixel bit depth
middle pixel value the window width is centered on window level
provides a quantitive analysis of the Hounsfield values of a specific anatomic area region of interest (ROI)
graphic used for precise size determination, distance for intervential procedures, and so on linear distance measurement
allows the user to review large quantities of CT images at a high rate images viewed in a motion loop, the speed of which is controlled by the user cine viewing mode
voxels with equal dimensions along the x-, y-, and z-axes isotropic
the process of displaying CT images in a different orientation from the one used in the original reconstruction process multiplanar reformation (MPR)
planes that are at right angles to each other orthogonal planes
any plane that is tilted from the coronal, sagittal or axial plane oblique
plane that is a graphically designed reformation that samples an anatomic structure along points placed by the user curved
display only the maximum pixel value along a ray traced through the object to the viewer's assumed perspective in front of the viewing monitor maximum intensity projection (MIP)
the process of displaying volumetrically acquired CT data with a perception of 3D depth on a computer monitor or recording film Three-dimensional (3D) CT
uses 3D algorithms to provide a specific perspective to the construction of the 3D model rendering
allows the user to select the range of pixel values rendered in the 3D model threshold
a technique that uses the concept of thresholding to limit the displayed volumetric data surface rendering or shaded-surface display (SSD)
adjusts the opacity of voxels included in the 3D model according to their tissue characteristics does not exclude voxels, but instead alters their appearance so that the 3D model contains the entire volume data set volume rendering (VR)
the method of externally viewing a 3D reconstructed object orthographic volume rendering
provides a viewpoint of being within the lumen of the object, similar to an endoscopic view commonly utilized during CT colonography and CT bronchography perspective volume rendering or immersive rendering
the unwanted appearance of individual sections on a multiplanar or 3D reformation image resulting in a loss of sharpness and detail step artifact
a complex computer technique that reduces the size of digital CT image data image compression
image is digitally compressed without loss of data and is identical to the original lossless compression (reversible)
data are lost during the compression process and the image does not exactly match the original capable of much higher compression rates than lossless compression lossy compression (irreversible)
developed in 1985 by the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) confirms the process of recording, storing, printing, and transmitting medical image data Digital Imaging and Communications in Medicine (DICOM) Standard
storage of CT image data on digital image film hard copy
CT image data are stored in digital file format soft copy
type of optical discs that contain image data for review only the data cannot by modified or added to read-only discs
type of optical discs which allow the initial storage of large quantities of digital image data and subsequent unlimited viewing, without the ability to reuse the discs write-only, read-many times (WORM) discs
type of optical disc that allow data to be recorded multiple times rewritable discs
computerized networks charged with the responsibility of storing, receiving, distributing, and displaying CT and other digital medical images Picture Archival and Communications System (PACS)
the ability of a PACS to transmit image data across a local area network (LAN) or wide area network (WAN) from the imaging facility to an off-site location teleradiology
ability of a CT imaging system to display fine details separately spatial resolution or high-contrast spatial resolution
component of spatial resolution for helical CT system function of the actual object, and the accuracy or fidelity with which the object is represented on the image in-plane spatial resolution
a graphical representation of a CT system's response to a spatial frequency that serves as a measurement of the system's inplane spatial resolution modulation transfer function (MTF)
a quality control device typically composed of a radiolucent plastic material containing specialized inserts that is used to measure specific image quality criteria phantom
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 point spread function (PSF)
shapes the spatial frequencies used during the image reconstruction process reconstruction algorithm (convolution kernal)
the number of views obtained by the CT system during the data acquisition of an image sampling frequency
the sampling rate of transmission measurements acquired during each gantry rotation determines the quantity of samples, and therefore transmission data acquired during each gantry rotation views per rotation (VPR)
dictate 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 Nyquist Theorem
way of describing how patient movement during volumeteric acquisition can affect image detail longitudinal spatial resolution
the amount of broadening that occurs along the z-axis during volumetric data acquisition slice sensitivity profile (SSP)
the effective section width of the SSP measured by examining the SSP at half of its maximum height full width at half maximum (FWHM)
an inherent component of the reconstruction process that is not influenced by the operator applied to the image reconstruction of helically acquired images to reduce SSP-broadening effects spiral interpolation algorithm
ability of the CT sytem to detect an object with a small difference in linear attenuation coefficient as compared to the surrounding tissue CT system's ability to differentiate between two adjacent tissues of similar attenuation values contrast resolution (low-contrast detectability or sensitivity)
the thickness and atomic density of an object largely determine its image contrast as compared with surrounding tissue inherent subject contrast
stop-motion capability of a CT system temporal resolution
digital imaging system is any portion of the signal that contains no useful information noise
another term for random stochastic
the result of an insufficient xray photon flux per voxel inversely related to the amount of radiation exposed to each voxel quantum noise
signal may be lost and noise introduced to the reconstruction process by the various electronic components of the CT system electronic system noise
artifacts viewed as a type of noise portion of the signal that contains no useful information and may obscure the information present in the image artifactual noise
the amount of noise in a displayed CT image signal-to-noise ratio (SNR)
equals the rate at which xray photons pass through a given unit of tissue over a given unit of time determined by the operator-controlled settings of mA and time xray photon flux
ability of a CT system to maintain relatively consistent CT values across the entire image of an object of equal density spatial uniformity
the relative accuracy between calculated CT numbers and their respective linear attenuation coefficients linearity
CT number for air -1000 HU
CT number for water 0 HU
monitors the overall technical performance of the CT system by measuring specific aspects of its function and comparing the measurements with a set standard quality control (QC) program
measurement of the scanner's performance through quality testing procedures and evaluation of the test results quality assurance
implementation of corrective actions to improve any identified performance inadequacies of the CT system quality control
a form of noise on the CT image resulting from errors during the measurement of transmitted radiation by the detectors contains no useful information and can severely disrupt the quality of the image artifact
specific type of streaking beam-hardening artifact that occurs in the posterior fossa of the brain Hounsfield bar
an 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 appears as a generalized unsharpness or haziness of the border of objects partial volume artifact
inaccuracy in pixel values that occurs when the associated voxels contain attenuation coefficients for multiple tissue types values are averaged together to yield a single pixel value that attempts to represent an assortment of different materials partial volume averaging
occurs when a portion of the patient has been positioned outside of the scan field of view out-of-field artifact
the unwanted appearance of individual sections on a multiplanar of 3D reformation image resulting in a loss of sharpness and detail step artifact
occurs when the section width equals the pixel dimension and the voxel is cube-shaped isotropic acquisitions
list of equipment-induced artifacts rings, streaks, aliasing, tube arcing, cone beam, and windmill
equipment-induced artifact caused by faulty detectors an error in the detector results in the back-projection of an incorrect ring of density on the reconstructed CT image rings
equipment-induced artifact caused by detector malfunction and/or misalignment of the tube and detectors streaks
equipment-induced artifact caused by insufficent number of views (data samples) obtained during data acquisition aliasing
equipment-induced artifact caused by short-circuiting within the xray tube during data acquisition that results in severe streak artifacts in the CT image tube arcing
equipment-induced artifact that worsen as the number of detector rows increases unique to the cone beam geometry of MSCT systems that appear similar to partial volume artifacts cone beaming
equipment-induced artifact that may occur from the use of increased pitch during a volumetric acquisition has a spiraling appearance of shading artifact that appears as a large number of volumetrically acquired images are paged through for review windmill or z-spacing or interpolation artifact
thickness of the slice that is actually represented on the CT image effective slice thickness
Created by: mkontio