With a dedicated breast CT system using a quasi-monochromatic x-ray source and flat-panel digital detector, the 2D and
3D scatter to primary ratios (SPR) of various geometric phantoms having different densities were characterized in detail.
Projections were acquired using geometric and anthropomorphic breast phantoms. Each phantom was filled with 700ml
of 5 different water-methanol concentrations to simulate effective boundary densities of breast compositions from 100%
glandular (1.0g/cm3) to 100% fat (0.79g/cm3). Projections were acquired with and without a beam stop array. For each
projection, 2D scatter was determined by cubic spline interpolating the values behind the shadow of each beam stop
through the object. Scatter-corrected projections were obtained by subtracting the scatter, and the 2D SPRs were
obtained as a ratio of the scatter to scatter-corrected projections. Additionally the (un)corrected data were individually
iteratively reconstructed. The (un)corrected 3D volumes were subsequently subtracted, and the 3D SPRs obtained from
the ratio of the scatter volume-to-scatter-corrected (or primary) volume. Results show that the 2D SPR values peak in the
center of the volumes, and were overall highest for the simulated 100% glandular composition. Consequently, scatter
corrected reconstructions have visibly reduced cupping regardless of the phantom geometry, as well as more accurate
linear attenuation coefficients. The corresponding 3D SPRs have increased central density, which reduces radially. Not
surprisingly, for both 2D and 3D SPRs there was a dependency on both phantom geometry and object density on the
measured SPR values, with geometry dominating for 3D SPRs. Overall, these results indicate the need for scatter
correction given different geometries and breast densities that will be encountered with 3D cone beam breast CT.