Off-center object-of-interest (OoI) imaging in filtered region of interest rotational angiography (FROI-RA)

Rotational angiography (RA) is widely used clinically to obtain 3D data. In many procedures, e.g., neurovascular interventions, the imaged field of view (FOV) is much larger than the region of interest (ROI), thereby subjecting the patient to unnecessary x-ray dose. To reduce the dose in these procedures, we have proposed placing an x-ray attenuating filter with an open aperture (ROI) in the x-ray beam (called filtered region of interest (FROI) RA. We have shown that this approach yields high quality data for centered objects of interest (OoIs). In this study, we investigate the noise behavior of the FROI approach for off-center OoIs. Using filter-specific attenuation and noise characteristics, simulated FROI projection images were generated. The intensities in the peripheral region were equalized, and the 3D data reconstructed. For each reconstructed voxel, the intersections with the full intensity beam (ROI) were determined for each projection, and noise properties were evaluated. Off-center OoIs intersect the high intensity beam in more than 60% of the projections (ROI having 40% FOV area), with intersection frequency increasing with increasing ROI area and OoI proximity to the central region. The noise increases with distance from the central region up to a factor of two. Integral dose reductions range between 40% and 85%, depending on ROI area and filter thickness. Substantial dose reductions (40-85%) are achieved with less than a factor of two increase in noise for OoIs peripheral to the central region, indicating the FROI approach might be an alternative for reducing dose during standard procedures.