An improved method for estimating the heart-to-mediastinum ratio from cardiac sympathetic nerve imaging with low-energy high-resolution collimators.

BACKGROUND: Septal penetration causes underestimation of the heart-to-mediastinum (H/M) ratio in cardiac (123)I-metaiodobenzylguanidine (MIBG) imaging with a low-energy high-resolution (LEHR) collimator. We aimed to improve the method of estimating the H/M ratios using the LEHR collimator.

METHODS AND RESULTS: 4 hours after (123)I-MIBG injection, 40 patients were imaged successively with the medium-energy (ME) and LEHR collimators using gamma cameras having 3/8-inch crystals. Severe underestimation of the H/M ratios was observed with the LEHR collimator when compared to the ME collimator. Narrowing the energy window width did not reduce the underestimation. Application of (123)I-dual-window (IDW) correction using a narrow or wide subwindow reduced the underestimation substantially but not entirely. The H/M ratios estimated from the LEHR images with or without IDW correction were corrected based on their correlations with the ratios estimated from the ME images. This empiric correction removed systematic underestimation, and residual errors were reduced when the H/M ratios after IDW correction were converted using the empiric equation. The conversion equation was successfully applied to the correction of the H/M ratios determined in another 40 patients using a 5/8-inch crystal.

CONCLUSIONS: In estimating the H/M ratios using an LEHR collimator, empiric correction combined with IDW correction improves concordance with ME-based values in comparison with empiric correction alone.