Using myocardium-to-background ratio to get the optimal starting angle for a non-360-degree upright cardiac SPECT acquisition

Background: Myocardium counts in the projection data and myocardium wall ratios in the reconstructed images have been used to find the optimal angular sampling for 180° supine SPECT imaging. However, supine and upright images demonstrate different soft tissue attenuation patterns. In this work, we propose the use of myocardium-to-background ratio (MBR) in the projection data as a figure-of-merit to determine the optimal starting angle for upright cardiac SPECT imaging. We hypothesize that higher MBR in the projection data means better signal-to-noise ratio and leads to better quality reconstructed images. Methods: We performed MBR analysis for 76 patient images (38 stress-rest studies) acquired with upright SPECT camera. Out of the 76, 22 were acquired with starting angle of right anterior oblique (RAO) 45° in 180°, 26 with RAO 30° in 180° and 22 with RAO 38° in 202°. We separated the male (34) and female (42) data and compared the MBR at each projection angle. For the MBR analysis, the myocardium wall and the background in each projection were identified in four steps: (1) a volume image was reconstructed using FBP algorithm, (2) left ventricle was segmented from the volume image, (3) the segmented image was reprojected to obtain the region of interest (ROI) to calculate the average counts in the myocardium, and (4) the ROI in step (3) was blurred then subtracted by the ROI in step (3) to derive the ROI to calculate background counts. Results: MBR reached a local peak at RAO 30° for all the data. For males, the MBR at RAO 45° and RAO 38° was lower than RAO 30° but higher than left posterior oblique (LPO) 60° (towards the end of the acquisition). For females, the MBR at RAO 38° and RAO 45° was lower than RAO 30° and LPO 60°. Conclusions: Using MBR as the figure of merit, we conclude that for males, starting angle of RAO 38° or RAO 45° is preferred over RAO 30° for upright cardiac SPECT imaging. For females, however, starting angle of RAO 30° is preferred.