Improved chest wall imaging through combined complex trajectories in dedicated dual modality SPECT-CT breast molecular imaging

In the hybrid SPECT-CT breast imaging system currently in development in our lab, patient positioning is a practical compromise between comfort and a need to maximize the imaged volume of breast and chest wall. The integrated imaging system rotates under the patient, with the current CT system restricted to purely azimuthal trajectories at a fixed height, while the flexible SPECT system is capable of fully 3D positioning around the pendant breast. The current patient bed, designed with the aforementioned compromises in mind, separates the top of the CT cone beam from the chest wall, thus limiting the system’s ability to image this important area. This study examines combined complex trajectories, including limited angle tomography for both modalities and raising the entire imaging system during the scan, to more effectively image lesions in or near the chest wall. While emphasizing new CT system trajectories, SPECT trajectories are also investigated to maximize the imaged volume while avoiding contact with the bed or patient. Various sized lesions filled with low and medium concentrations of ⁹⁹mTc activity (10:1 to 3:1) and CT contrast are imaged using different trajectories. Dual modality projections are post-processed to mimic limited angle trajectories or trajectories that raise the CT system for a portion of the scan. Reconstructed images from data sets with trajectories that removed 60° of SPECT and CT azimuthal data and trajectories combining limited angle acquisition with vertical system shift show a significant increase in observed breast volume while maintaining lesion visibility. Two task-based observer studies are used to further evaluate the visibility of small low-contrast lesions reconstructed with decreasing angular acquisitions and system shifting. Observer study results further indicate that limited angle trajectories and system shifting in mid-scan appear to improve chest wall imaging for this dual modality system.