Dose assessment and reconstruction algorithm optimization in simultaneous breast and lung CT imaging.

Cancer is the second leading cause of death in the world, and therefore, there is an undeniable need to ensure early screening and detection systems worldwide. The aim of this project was to study the feasibility of a Cone Beam Computed Tomography (CBCT) scanner for simultaneous breast and lung lesion imaging. Additionally, the development of reconstruction algorithms and the study of their impact to the image quality was considered. Monte Carlo (MC) simulations were performed using the PENELOPE code system. A geometry model of a CBCT scanner was implemented for energies of 30 keV and 80 keV for hypothetical scanning protocols. Microcalcifications were inserted into the breast and lung of the computational phantom (ICRP Adult Female Reference), used in the simulations for dose assessment and projection acquisition. Dosimetric and imaging performances were evaluated through Computed Tomography Dose Index and Catphan phantoms, respectively. Reconstructed images were analyzed in terms of Contrast-to-Noise Ratio (CNR) and dose calculations were performed for two protocols, using a normalization factor of 2 mGy in the breast and another with 5 mGy in the lungs. MC geometry model and reconstruction algorithm were validated by means of on-field measurements and data acquisition in a clinical center. Results indicate that for both protocols, the absorbed dose in both organs is the same, allowing the optimization of protocol strategies regarding radiosensitive organs. The best implementation of the reconstruction algorithm was achieved with 80 keV, using linear interpolation and hanning filter. More specifically for a spherical lung lesion (r = 7 mm) a 30% CNR gain was found when the number of projections varied from 12 to 36 (corresponding to a dose increase of a factor of 3). This study suggests the applicability of a CBCT modulated beam scanner for imaging simultaneously breast and lung lesions while ensuring dose reduction without compromising image quality as a possibility.