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Ch15Beam Restriction

Principles of Radiographic Imaging - Carlton

QuestionAnswer
Negative effects of scatter radiation Not part of useful beam, unwanted density on the radiograph, overall graying effect (fog), contributes to patient and operator dose
Factors contributing to Compton scatter High kVp values, tissue volume, field size, and patient thickness
Increase kVp Decreases patient dose (less photoelectric interactions, more Comptons), longer scale of contrast, gray haze of entire image (scatter_
Decrease in kVp Decreased transmission through patient, increased photoelectric absorption, decreased Compton scatter
Decrease in kVp Increases patient dose, increased photoelectric absorption, to maintain image density/exposure a decrease in kVp is accompanied by an increase in mAs
Decrease in kVp Shorter scale of contrast, fewer grays
Field Size = Collimation increased field size increases the volume of tissue irradiated, resulting in increased scatter, longer scale of contrast in image
Field Size Use the smallest field size possible, decreases scatter production
The Trade Off By decreasing the field size fewer photons reach the IR so the exposure is decreased, you must increase mAs to maintain exposure
Patient Thickness Thicker body parts produce more scatter, more atoms, more photon interactions, resulting in an increase scatter production
Decreasing Part Thickness Compression devices can be used to improve spatial resolution and contrast. Results in lower patient dose, brings tissue closer to field, mammography primarily
Beam Restricting Devices 1. Reduce patient dose 2. Limits scatter produced 3. Enhances contrast 4. Visibility of detail
Variable Aperture Collimator Function-reduces field size and controls shape of field (automatically)
Upper Stage (1st) reduces off focus radiation
Lower Stage (2nd) decreases penumbra
Positive Beam Limitating Device PBL- Automatically collimates when a cassette is placed in the bucky
Beam Filtration Removes low energy radiation, placed between tube part and collimator, above 70 kVp--- 2.5mm Al required (federal regulation)
Created by: lorperkins
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