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27 Cards in this Set
- Front
- Back
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Spatial resolution |
The ability of a CT imaging system to display fine details separately. Given in units of line pairs per centimeter (lp/cm) |
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In-plane spatial resolution |
Can be demonstrated as a function of the actual object, and accuracy of fidelity with which the object is represented kn the image |
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Modulation transfer function (MTF) |
Graphical representation of a CT systems response to a spatial frequency. Ability of a scanner to record a spatial frequency and thereby resolve an object can be shown graphically using an MTF curve |
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Point spread function |
A measure of a ct systems in plane spatial resolution that evaluate the amount of spread inherent in an orthogonal image of a thin wire |
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Longitudinal spatial resolution |
Way to describe how patient movement during volumetric acquisition can affect image detail. Qualified by the degree of broadening that occurs to the slice sensitivity profile during volumetric scanning |
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Contrast resolution |
Ability of the CT system to detect an object with a small difference in linear attenuation coefficients as compared to the surrounding tissue. Aka low contrast detectability or sensitivity of the ct system |
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Noise |
Any portion if the signal that contains no useful information. Random statistical variation in the signal. Appears as overall graininess on image |
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Quantum noise |
Result of insufficient xray photon flux per voxel. Inversely related to the amount of radiation exposed to each voxel. Changing voxel dimension or xray flux affects noise |
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Electronic system noise |
Signal may be lost and noise introduced by various components of the CT system |
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Artifactual noise |
Artifacts can represent a portion of the signal that contains no useful information and may obscure information in image. |
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Signal to noise ratio (SNR) |
Quantification of the amount of noise in a displayed CT image. SNR is calculated as the standard deviation. In the ROI measurement of a water phantom image |
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Temporal resolution |
Ability of a CT system to freeze motion and provide an image free from blurring |
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Nyquist theorem |
A sampling law dictating that the data sampling frequency must be at least twice the objects spatial frequency in order for the object to be resolved by a CT system |
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Uniformity (spatial) |
Describes a systems ability to maintain relatively consistent CT values across the entire image of an object of equal density. |
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Linearity |
Relative accuracy between calculated CT numbers and their respective linear attenuation coefficients. Checked with daily calibration checks with water phantom |
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Quality assurance |
Measurement of the scanners performance through quality testing procedures and evaluation of test results |
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Quality control |
Implementation of corrective actions to improve any identified performance inadequacies of the ct system |
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Artifact |
Contains no useful information and van severely disrupt the quality of an image by interfering with anatomic detail contained on the ct image |
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Beam hardening |
Phenomenon whereby lowenergy photons are absorbed as the xray beam passes through an object resulting in an increase in average photon energy of the beam |
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Edge gradient artifact |
Caused by the systems inability to process the high spatial frequency signal that represents the interface between two substances with widely different attenuation values |
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Ring artifact |
Caused by faulty detectors. An error in a detector results in the back projection of an incorrect ring if density in the reconstructed image. |
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Streak artifact |
Maybe be causes by detector malfunction or misalignment of tube and detectors |
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Aliasing artifact |
Another form of streak artifact caused by an insufficient number if views (data samples) obtained during data aquisition. |
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Tube arching |
Short circuiting within the xray tube during data aquisition that results in severe streak artifacts |
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Cone beam artifacts |
Worsens as detector rows increase. Similar to partial volume artifacts. |
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Windmill artifacts |
Aka z spacing or interpolation artifact. May occur from increased pitch during a volumetric aquisition. Results in a spiraling appearance of shading artifact |
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Misregistration |
An artifact that occurs when a patient suspends respiration at different depths during scans. Results in loss of info |
