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CT registry Prep
Dose Measurements
Term | Definition |
---|---|
Dose Profile | area of tissue exp to radiation per slice. |
Exposure | ability of xrays to Ionize a volume of air. measured in Roentgens (R) |
Absorbed Dose | amt of xray energy absorbed in a unit of mass. measured in Grays (Gy) |
Kerma | describes amnt of radiation in a unit of air. |
Effective Dose (efd) | accounts for different types of tissue and their assigned weighted factors. Weighted factors are determined by radiosensitivity. approximates amt of relative risk measured in Sieverts (sv) |
Ct Dose index | CTDI approx. measure of dose received in a single section/slice. Calculated for he central slice in a series |
CTDI100 | fixed measurement taken with a 100mm long pencil ionization chamber. |
CTDIw | internationally accepted weighted dose index. calculated by adding 2/3 of the exposure at the periphery of the field plus 1/3 of the centrally located dose. More accurate than dose approximation. Only for Step and Shoot Axial measured in milligray ( |
CTDIvol | approx. dose for each section during a helical scan. CTDIvol=CTDIw/pitch as pitch increases, dose per slice decreases and CTDIvol decreases. measured in milligray (mGy) |
MSAD | Multiple scan average dose older term for CTDIvol. Avg cumulative dose to each slice in the center of the scan with multiple slices. MSAD=slice thickeness/increments(spacing) |
MSAD for Axial | when slice thickness is equal to bed index(pitch), msad is equal to the CTDI |
MSAD for Helical/Spiral | MSAD=CTDI/Pitch where pitch=amnt of table travel per tube rotation divided by collimation. |
Dose Length Product | (DLP) measure of CT dose. DLP=MSAD x slice thickness x number of slices in scan volume. or DLP=CTDIvol x scan length |