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CT Review 4
Technical Factors that influence CT image formation
Term | Definition |
---|---|
List the 9 commonly adjusted CT imaging parameters: | Kv, mA, pitch, recon interval, table increment, slice thickness, FOV, matrix, recon filter |
A quality image is determined be what 4 factors? | density, contrast, resolution, distortion |
What is spatial resolution? | a measure of the size of the smallest object that can be visualized, or resolved, in an image. |
There are 4 geometric factors that affect spatial resolution, what are they? | focal spot size, detector aperture size, focal spot to pt distance, pr to detector distance |
pre and post collimation reduces what? | scatter radiation and pt dose |
what is another name for pre collimation? | dose collimation / source |
detector collimation significantly ___?___ image resolution and __?__ image artifacts. | increases, minimizes |
What formal is used to determine pixel size? | pixel size = DFOV/matrix |
What is contrast resolution? | the ability to differentiate small differences with CT in density on the image |
Tissues with differences in density of less than ___% can be differentiated on a CT. | 5% |
What is quantum noise? | electronic interference that comes from the pt and components of the CT scanner |
What is dose? | the amount of ionizing radiation absorbed by the pt per unit mass |
what factors affect dose? | mAs, Kvp, collimation, scan time |
What is standard deviation? | amount of noise, or spread, can be calculated and expressed by the statistical parameter |
spatial resolution is often expressed in terms of what? | line pairs |
What factors affect spatial resolution? | slice thickness, DFOV, image matrix, recon filters |
Noise can be controlled by what parameters? | mAs, Kvp, kernel, slice thickness, pt size, collimation, image display |
Noise has a tendency to make a CT image __?__ and this can obscure fine anatomical detail. | grainy |
Beam collimation if equal to _?_ _?_ on a single slice helical ct. | slice thickness |
Some ct scanners allow adjustments of two types of FOV's, what are they? | DFOV, SFOV |
A cross section of a pt's anatomy should be __?__ contained within the SFOV. | completely |
Reducing voxel size __?__ noise because __?__ protons are absorbed in voxel. | increases, fewer |
When we select the shade of gray assigned to the ct number of the pixels in the image it is known as what? | wl and ww |
What are the parameters that control the contrast in a ct image? | focal spot size, pixel size, kernel, recon filter |
What is attenuation? | the degree to which an xray beam is reduced by an object |
What is CTDI? | CT dose index-an approx measure of the radiation dose received in a single CT section or slice. |
The effects of increasing image matrix: | 1.contrast: unchanged 2.resolution: increase 3.noise: increase 4.dose: unchanged |
the effects of using sharper recon filters: | 1.contrast: decrease 2.resolution: increase 3.noise: increase 4.dose: unchanged |
the effects of increasing mAs: | 1.contrast: unchanged 2.resolution: decrease 3.noise: increase 4.dose: increase |
the effects of increasing Kvp: | 1.contrast: up or down 2.resolution: unchanged 3.noise: decrease 4.dose: increase |
the effects of increasing slice thickness: | 1.contrast: unchanged 2.resolution: decrease 3.noise: decrease 4.dose: decrease |
the effects of increasing table increments: | 1.contrast: unchanged 2.resolution: decrease 3.noise: unchanged 4.dose: decrease |
the effects of increasing pitch | 1.contrast: unchanged 2.resolution: increase 3.noise: unchanged 4.dose: decrease |
the effects of increasing recon interval | 1.contrast: unchanged 2.resolution: decrease 3.noise: unchanged 4.dose: unchanged |
the effects of increasing recon FOV | 1.contrast: unchanged 2.resolution: decrease 3.noise: decrease 4.dose: unchanged |
what is a pixel? | the individual boxes arranged in the matrix used to display the ct image |
what is a ray? | the portion of the xray beam transmitted thru the pt and incident upon a single detector |
what is a kernel? | mathematical filter applied to raw data during CT image recon to remove the blurring artifact inherent to back projection |
what is a matrix? | two dimensional grid of numbers arranged in rows and columns. |