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CT Physics Terms III

Image Quality & Quality Assurance

Factors that can be controlled by the operator & affect the quality of the image produced. These factors include mA, scan time, slice thickness, FOV, reconstruction algorithm, & kVp. When using helical scan methods, operator also has a choice of pitch scan parameters
Time the x-ray beam is on for the collection of data for each slice. Most often it is the time required for the gantry to make a 360 degree rotation, although with overscanning and partial scanning options there may be some mild variation scan time
One a single-detector row system this is controlled by the width of the collimator opening. On a multidetector row system it is controlled by a combination of collimation and detector configuration slice thickness
The number of line pairs visible per unit length spatial frequency
Indicates the amount of CT number variance within the ROI standard deviation
The number of x-ray photons detected per pixel in CT signal-to-noise ratio (SNR)
Relates to the inherent properties of the object scanned. For ex., the lung possesses high ___ b/c it's primarily air-filled. The low attenuation lungs provide a background that makes nearly any other object visible b/c of its difference in density subject contrast
How rapidly data are acquired. It is controlled by gantry speed, the number of detector channels in the system, and the speed with which the system can record changing signals temporal resolution
Theorem that states because an object may not lie entirely within a pixel, the pixel dimension should be half the size of the object to increase the likelihood of that object being resolved. sampling theorem
Result of measuring and charting the relationship between object size and visibility contrast-detail curve
The relationship between object size and visibility contrast-detail response
The displayed contrast of an image is dependent on the window settings used for its display display contrast
Image accuracy image fidelity
Equal in all directions; a voxel that is cube-shaped isotropic
Resolution in the xy direction in-plane resolution
Most commonly used method of describing spatial resolution ability. It is often used to graphically represent a system’s capability of passing information to the observer modulation transfer function (MTF)
Charts that depict spatial frequency (object size) on the x axis and the MTF along the y axis MTF graph
A phantom used to measure spatial resolution. This type of phantom is made of acrylic and has closely spaced metal strips embedded in it line pairs phantom
Ability of a system to resolve, as separate forms, small objects that are very close together. Also called spatial resolution or detail resolution high-contrast resolution
Defines the quality (average energy) of the x-ray beam kVp
The subjectivity inherent in the method of evaluating contrast resolution that requires an observer to detect objects as destine. Result can vary because different observers will often look at the same image and evaluate it differently receiver operator characteristics
Measure of the tube current used in the production of x-ray energy. In conjunction with the scan time, it is the quantitative measure of the x-ray beam milliamperes
The product of mA setting and scan time mAs
Determines how the data are filtered in the reconstruction process. The appropriate reconstruction algorithm selection depends on which parts of the data should be enhanced or suppressed to optimize the image for diagnosis reconstruction algorithm
With digital technology, the image is not as directly linked to the dose, so even an mA or kVp setting that is too high is used, a good image results. This effect can make it difficult to identify when a dose that is higher than necessary is used uncoupling effect
Ability of the system to differentiate between objects with similar densities. Also called low-contrast resolution contrast detectability
The spatial frequency possible on a given CT system at an MTF equal to 0.1 limiting resolution
Resolution in the z direction longitudinal resolution
Because an object may not lie entirely within a pixel, the pixel dimension should be half the size of the object to increase the likelihood of that object being resolved Nyquist sampling theorem
Occurs when there are an insufficient number of photons detected. It is inversely related to the number of photons used to form the image. Hence, as the number of x-ray photons used to create an image decreases, noise increases. quantum mottle; AKA quantum noise
A solid phantom that contains 4 modules & is made primarily from a water equivalent material. Each is 4cm deep & 20cm in diameter, w/ external align't markings to center phantom in the x, y, & z axes, & is used to measure diff. aspects of image quality ACR CT accreditation phantom
Artifacts that result from insufficient projection data; cause fine stripes that appear to be radiation from a dense structure aliasing
Artifacts that result from lower-energy photons being preferentially absorbed, leaving higher-intensity photons to strike the detector array beam-hardening artifacts
Artifacts that relate to the cone-shaped beam required for MDCT helical scans. These artifacts are more pronounced for the outer detector rows. The larger the cone beam (i.e., more detector channels), the more pronounced the effect cone-beam artifacts
Phantoms used to measure the radiation dose delivered for various CT examinations. CTDI phantoms
Streak artifact or shading (both light and dark) arising from irregularly shaped objects that have a pronounced difference in density from surrounding structures edge gradient
Result in subtle inaccuracies in CT numbers and can be easily misinterpreted as disease. These artifacts can best be avoided by using a low pitch whenever possible helical interpolation artifacts
Anything appearing on the image that is not present in the object scanned image artifacts
Determination of the accuracy of the alignment of the laser light used for patient positioning laser light accuracy
The relationship between CT numbers and the linear attenuation values of the scanned object at a designated kVp value linearity
Inaccuracies in the image caused when parts of the patient are located outside the scan field of view. These artifacts occur because the anatomy outside the SFOV attenuates and hardens the x-ray beam, but is ignored in the image reconstruction process out-of-field artifacts
Artifact that can result when an object does not appear on all views. Inconsistencies between views cause shading artifacts on the image partial volume artifacts
A special cylindrical dosimeter used in conjunction with a CTDI phantom to assess the radiation dose in CT pencil ionization chamber
Values published by the ACR regarding the radiation dose that is acceptable for a variety of CT scans reference dose values
Determination of the accuracy of the slice thickness selected by the operator versus the width of the collimator opening slice thickness accuracy
Undesired surge of electrical current within the x-ray tube. A common cause of equipment-induced artifact tube arcing
Insufficient projection data that cause inaccuracies related to reproducing sharp edges and small objects and result in an artifact known as aliasing undersampling
The ability of the scanner to yield the same CT number regardless of the location of an ROI within a homogeneous object uniformity
Appear only on MDCT helical systems and relate to the cone-shaped beam required. They appear as either streaks or as bright and dark shading near areas of large density differences (e.g., bone and muscle) windmill artifacts
Created by: sspatel