SPECT breast imaging combining horizontal and vertical axes of rotation

Vertical-axis-of-rotation (VAOR) geometries, in which a gamma camera orbits a pendulous breast of a prone patient, have been proposed for SPECT breast imaging. Dedicated VAOR systems would be advantageous since they allow for minimal radius of rotation around the breast, which improves resolution-sensitivity tradeoffs, and there is less attenuation and scatter between the breast and the camera. However, the torso may not be viewed at enough angles to estimate contamination from torso activity. Also, VAOR geometries may involve camera tilt to scan for tumors near the chest wall. These tilted VAOR projections may be insufficient to establish even breast activity alone. Thus, we investigated the utility of designing VAOR systems to function inside some general-purpose horizontal-axis-of-rotation (HAOR) scanners, with the HAOR scanner orbiting a 192° angular range posteriorly, alleviating the insufficient-data problems. We simulated the expected projection data from six various-sized lesions with different activity concentrations in the breast and axilla, additionally considering the contribution from background activity from the torso. The data were then statistically reconstructed considering 360° HAOR, VAOR plus small additional arcs and combined HAOR-VAOR orbits, and the results were compared with the known activity concentration distribution. Substantial improvement in the noise versus bias was observed in the breast and axilla, an important and otherwise difficult region to image