Spectral Half-Value Layer Determination of Mammographic Beam Qualities
Abstract
Purpose
Half-value layer (HVL) is a common beam quality parameter, but physical measurement is often time-consuming and difficult. This work presents a faster spectral method for determining HVL for four sets of polyenergetic mammography beam qualities using a solid-state spectrometer.
Methods
A CdTe detector (X-123CdTe, Amptek Inc.) was used to determine the spectrum of four sets of mammography beam qualities. Detector response modelling was performed in Geant4 using monoenergetic beams from 0-50 keV in 0.125 keV intervals. An unfolding algorithm utilized the detector response matrix to recover the intrinsic spectrum, from which air kerma was determined. The aluminum thickness required to reduce air kerma by one half was then calculated to obtain the spectral HVL, and physical measurements from a free-air chamber (FAC) provided the reference standard for comparison.
Results
Spectral HVLs showed good agreement with the reference standard across considered beam qualities. Overall, 86% of the spectral HVLs agreed with measured values within 0.10 mm Al, and all results were within 0.15 mm Al. The average and median differences between spectral and physical HVLs were 0.05 mm Al and 0.02 mm Al, respectively. The largest discrepancies occurred for the highest tube potential beam (50 kV).
Conclusion
HVLs determined from measured spectra have been implemented as a secondary means of beam quality characterization. Comparison with physically measured HVLs demonstrates good agreement, with the majority of HVLs agreeing within 0.1 mm Al. These results support the feasibility of using spectral HVLs for beam characterization; however, future work will focus on improving the underlying physical models, including detector response corrections, charge sharing, and incomplete charge collection.