Spectral optimization for dedicated breast CT

Abstract

The authors investigated the choice of spectra for the optimization of the dose-weighted contrast-to-noise ratio (CNRD) for a dedicated breast CT scanner. The objective is to provide the desired image quality level at minimal dose values. The CNRD was investigated as a function of energy or tube voltage and filtrations for various breast sizes and contrasts. The authors performed simulations of the pendant female breast as cylinders consisting of a homogeneous mixture of adipose and glandular tissue with diameters from 6 to 18 cm. The contrasts of adipose tissue, calcium hydroxyapatite, and iodine contrast agent relative to glandular tissue were analyzed using inserts of 9 mm in diameter. Simulations were conducted for monochromatic and polychromatic radiation with a 3 mm Al or a 0.3 mm Cu filter. Simulations and measurements on an experimental micro-CT scanner were performed for validation purposes with a 6 cm water-equivalent cylinder, with inserts representing a pure density difference of 10%, calcium hydroxyapatite, and iodine contrast agent. A breast tissue sample embedded in paraffin was investigated to confirm the simulation results. Optimal tube voltages were found to be in the range of 30-55 kV for breast CT imaging. For example, with 3 mm Al or 0.3 mm Cu filtration, optimal tube voltages were about 53 and 48 kV, respectively, for the contrast iodine/glandular tissue for all diameters. With 3 mm Al filtration, optimal tube voltages increased from 30 to 37 kV and from 30 to 47 kV for the contrast calcium hydroxyapatite/glandular tissue and adipose/glandular tissue, respectively. Tube power requirements were estimated and the change in tube current relative to 80 kV was found to be between 4 and 14 and between 11 and 214 with 3 mm Al filtration or 0.3 mm Cu filtration, respectively. These numbers show that realizing the low optimal tube voltages may not be feasible in most cases due to power requirements. Depending on the filtration, the authors assume that a compromise solution has to be found between the highest potential dose reduction and a solution working with available x-ray sources. In view of the tube power constraints, the authors recommend aiming for tube voltages in the range of 50 kV and higher.