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Half-value layer and intensity variations as a function of position in the radiation field for film-screen
J A Terry1, R G Waggener, M A Miller Blough
1Department of Radiology, Tulane University Medical School, New Orleans, Louisiana 70112-2699, USA.
Abstract:
Differences in half-value layer (HVL) and radiation intensity are investigated as a function of position in the mammographic radiation field. Sources of systematic variation include the heel effect, the inverse square law, and differential photon path lengths through thicknesses of inherent and added filtration. The combination of these effects can increase the HVL by as much as 9% and reduce intensity by as much as 40% along the cathode-anode axis. To the left and right of the x-ray field central axis, reductions in radiation intensity of up to 9% and minor increases in HVL are noted as well. Optical density variations as a function of position in the field correlate well with the measured radiation intensity changes.