Dedicated breast CT: effect of adaptive filtration on dose distribution

Purpose: The purpose of the current work was experimental investigations of the mean breast dose and volume dose distributions with and without adaptive filtration. Adaptive filtration of the x-ray beam is used in dedicated breast CT to reduce dynamic range requirements on the detector, and to improve image quality. The adaptive filter has a predetermined shape, it is placed after the x-ray source, and it decreases x-ray exposure to the periphery of the breast such that 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, which was investigated in this work. Methods: The breast dose was measured using a 14 cm diameter cylindrical breast phantom made from tissue equivalent material (acrylic). An acrylic adaptive filter was fabricated to match the 14 cm diameter of the breast phantom. The breast dose was measured using a CT ion chamber inserted into a series of axial holes distributed along the radius of the phantom from the center to its 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 at a fixed air kerma of 6.6 mGy for all tube voltages. Results: The adaptive filter decreased mean breast dose by a factor of 1.6 – 2.2 depending on the tube voltage. To maintain CT image quality, the x-ray tube output was increased by these factors when adaptive filtration 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, with reversed magnitude of non-uniformity as compared to the case without adaptive filtration. Conclusion: While adaptive filtration provides important advantages on detector dynamic range, CT number uniformity, and CT noise uniformity, the volume uniformity of the dose with adaptive filter depended 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 of this work allowed also for determining necessary dose increase factor to compensate for x-ray absorption in the adaptive filter. Corresponding author: [email protected]