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Raditude20Ch14

Radiation Dosimetry in CT

QuestionAnswer
The areas of scatter into adjacent tissue are sometimes referred to as? Tails
What are the factors that contribute to patient dose when considering scatter radiation? Patient size, physical makeup, and the kVp used.
In general, how much dose will the tails contribute to the entire study? 25%-40%
What constitutes total dose? Central slice radiation dose, plus the scatter overlap (or tails)
What is the term for total dose, when the central slice radiation dose plus the scatter overlap (or tails)? Multiple scan average dose (MSAD)
Will the MSAD increase or decrease if slices overlap? Increase
Will the MSAD increase or decrease if there are gaps between the slices? Decrease
Single slice dose+amount scattered= total exposure
Dose calculated from multiple scans MSAD
What is the dose reported to the FDA CTDI
What is another type of radiation dose measurement in CT? Computed tomography dose index (CTDI)
What happens if there is slice overlap or gaps in the CTDI? The CTDI is multiplied by the ratio of slice thickness to slice increment.
What is the device physicists use to measure CTDI? Pencil ionization chamber
What is the preferred expression of radiation dose in CT dosimetry? CTDIvol
What is the price paid for excellent low-contrast resolution seen on CT images? relatively high radiation dose
What percentage can CT typically resolve and display visual differences between small objects that have only a minimal difference in density? 0.1%-0.5%
How much higher is average dose of CT than general radiography? 100 times greater
How much higher is the skin dose for CT than general radiography? 10 times higher
How does filtration affect the radiation dose? Removes the soft/low energy x-rays
How does detector absorption efficiency affect radiation dose to patients? The less efficient detector requires higher radiation exposure to produce an adequate image, therefore increasing patient dose.
What does the term "overbeaming" mean? To have the same x-ray intensity reach all of the detectors in an MDCT system, the collimators must be opened slightly more, allowing the x-ray penumbra to fall outside the active detectors.
What is pitch? The spacing of CT slices obtained with a spiral (or helical) scan process.
Define pitch. The table distance traveled in one 360 degree rotation divided by the collimated width of the x-ray beam.
How does pitch directly influence patient radiation dose? As pitch increases, the time that any one point in space spends in the x-ray beam is decreased.
What is the difference in how much beam is attenuated between smaller patients and larger patients? The smaller patient always absorbs the higher dose and that difference is at least a factor of 2.
How does higher mAs and kVp affect the patient? Higher dose to patient.
What is the term for the localization scan performed before scanning? Scout image
What are the 2 concerns with new CT technology? 1. Expanded technology resulting in more CT studies being performed. 2. Higher radiation doses associated with the newer scaners.
How much greater is the dose when the radiation doses used in adult protocols are used in neonates or young children? 50%
What is the term for images obtained beyond the desired area of interest? They contribute to patient dose without adding useful diagnostic information. Extra images
At what time is the fetus most radiosensitive? 0-3 months
What are some ways to reduce CT radiation dose? Limit region covered, adjust mAs based on pt. size, region, and clinical indication, consider an increase in pitch, limit the use of thin slices, consider pt. shielding.
Created by: amy wiseman