Comparison of reduced angle and fully 3D acquisition sequencing and trajectories for dual-modality mammotomography

A dual-modality SPECT-CT system for dedicated 3D breast cancer imaging is under development. Independent dedicated SPECT and CT imaging systems have been integrated onto a single gantry for uncompressed breast imaging. This study examines challenges and tradeoffs involved in integrating the acquisition procedures of two independent imaging systems into a single imaging protocol. The physical limitation of the rotating CT tube beneath the custom patient bed currently provides only a 294 degree scan with the bed low enough for the breast to be in the cone-beam CT field-of-view. The directly coupled SPECT system is therefore also limited if the scans are to be taken simultaneously or in an interleaved fashion. Thus, geometric phantoms are imaged to characterize image degradations due to reduced projection angles for both modalities. Two different acquisitions were performed: one with the central ray of the CT cone-beam aligned with the system's center of rotation and one offset from the center of rotation by 5 cm. Various sized activity- filled lesions in an anthropomorphic breast phantom were imaged, first with uniform aqueous background activity and then with added acrylic pieces to simulate a non-uniform background. Interleaving the SPECT and CT acquisitions into a single scan was also investigated. Iterative reconstruction algorithms are used to reconstruct the data, and the SPECT and CT images are co-registered. Both the cold rod and breast data indicate that removing 75deg of SPECT azimuth al data does not significantly reduce image quality. CT images were also minimally affected if the cone-beam is centrally aligned with the center of rotation, but degraded with the laterally offset cone-beam setup. In the course of these experiments, the patient bed was reconfigured with a larger central hole covered with flexible neoprene, gaining the ability to rotate completely around the breast and dramatically improving CT projection views through the chest wall.