Quantification of scattered radiation in projection mammography: four practical methods compared.

PURPOSE

Four different practical methodologies of quantifying scattered radiation for two different digital mammographic systems are compared. The study considered both grid in and grid out geometries for two different antiscatter grid types, a typical linear grid and a cellular grid design. The aim was to find quick and reproducible methods that could be used in place of the beam stop technique.

METHODS

The scatter to primary ratio (SPR) and the scatter fraction (SF) were used to quantify scattered radiation as a function of poly(methyl methacrylate) (PMMA) thickness, grid position, and beam quality. The four scatter estimation methods applied were (1) the beam stop method, (2) a hybrid method that combined measured detector (scatter-free) modulation transfer function (MTF) data and a Monte Carlo simulation of the scatter point spread function, (3) from the low frequency drop data taken from the system MTF, and (4) from the edge spread function (ESF) measured in the presence of PMMA. Repeatability error was assessed for all methods.

RESULTS

SPR results acquired with the beam stop method ranged from 0.052 to 0.187 for the system with linear grid and from 0.012 to 0.064 for the cellular grid system, as PMMA thickness was increased from 20 to 80 mm. With the grid removed, beam stop SPR was similar for both systems, ranging between 0.268 and 1.124, for corresponding MTF thicknesses. The direct MTF method had a maximum difference of 24% from the beam stop SPR and SF data for all conditions except the cellular grid in geometry, where maximum difference in SPR was 0.044 (164%). The ESF technique gave large differences from the beam stops for both grid geometries but agreement was within 21% for the grid out geometry. Repeatability error with beam stops was between 1% and 5% for the grid out geometries, while for the grid in cases it was 13% and 87% for the linear and cellular grids, respectively. Repeatability error for the direct MTF method applied to both systems and grid geometries ranged between 3% and 12%.

CONCLUSIONS

All three alternative methods to the beam stop technique gave reasonable estimates of SPR without grid, with a maximum difference of 24% (mean difference 8%). For the grid in geometry, the direct MTF method gave a maximum difference of 24% for the linear grid system, while maximum percentage difference was 119% (absolute difference of 0.042) for the system with the cellular grid, where SPR values were low. Except for cases where the SPR is very low, the direct MTF method offers a quick and reproducible alternative to the beam stop technique.