Dedicated breast CT: effect of adaptive filtration on dose distribution
  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. 
Polad M. Shikhaliev
physics.med-ph