  Purpose: The purpose of the work was experimental investigations of the
breast dose distributions with adaptive filtration. Adaptive filtration reduces
detector dynamic range and improves image quality. The adaptive filter with
predetermined shape is placed at the x-ray beam such that the x-ray intensity
at the detector surface is flat. However, adaptive filter alters the mean dose
to the breast, as well as volume distribution of the dose. Methods: The dose
was measured using a 14 cm diameter cylindrical acrylic breast phantom. An
acrylic adaptive filter was fabricated to match the 14 cm diameter of the
phantom. The dose was measured using ion chamber inserted into holes
distributed along the radius of the phantom from the center to the edge. The
radial distribution of dose was measured and fitted by an analytical function
and the volume distribution and mean value of dose was calculated. The
measurements were performed at 40, 60, 90, and 120 kVp tube voltages and 6.6
mGy air kerma. Results: The adaptive filter decreased mean breast dose by a
factor of 1.6-2.2 depending on the tube voltage. To maintain image quality, the
tube output was increased by these factors when adaptive filter was used. The
adaptive filter provided uniform volume distribution of dose at 40 kVp and 60
kVp tube voltages, but the distribution was non-uniform at 90 kVp and 120 kVp
tube voltages. Conclusion: While adaptive filtration helps with detector
dynamic range, CT number uniformity, and CT noise uniformity, the volume
non-uniformity of the dose with adaptive filter depends on tube voltage. On the
other hand, at the same average breast dose, volume non-uniformity of the dose
with adaptive filter was not higher that that without adaptive filter. The
results allowed also to determine necessary dose increase factor to compensate
for x-ray absorption in the adaptive filter.
