Influence of myocardial region of interest definition on quantitative analysis of planar 123I-mIBG images

Purpose

In planar ¹²³I-mIBG myocardial imaging, definition of the heart region of interest (ROI) is a critical step in quantifying uptake. The present study evaluated the impact of changes in heart ROI size on quantitative results in subjects with good and poor uptake.

Methods

Reference irregular whole-heart and square upper mediastinum ROIs were defined visually on 531 planar ¹²³I-mIBG images. Based on the reference heart ROI, an automated program created two other ROIs: one larger (+1 pixel) and one smaller (?1 pixel), the stated numbers representing the spacing intervals between each epicardial boundary pixel. Two additional smaller ROIs (?2 and ?3 pixels) were drawn for the 100 images (19 %) with a heart/mediastinum (H/M) ratio ≤1.30. The number of pixels, the counts per pixel, and the H/M ratio for each heart ROI were calculated and compared to that in the reference ROI. Washout rate and changes as a function of ROI size were also calculated for 110 subjects who had both early and late images.

Results

The mean changes in heart ROI size ranged from +19.0 % for the +1 pixel to ?44.4 % for the ?3 pixels ROI. For the +1 and ?1 pixel ROIs, mean relative counts per pixel changes were ?1.2 % and +0.7 %, respectively, with corresponding ranges of change in the H/M ratio of ?0.12 to +0.05 and ?0.05 to +0.11. For scans with H/M ratio ≤1.30, mean relative counts per pixel changes were 0, 0, ?0.7 %, and ?1.4 % for the four ROIs, with range of change in the H/M ratio from ?0.13 to +0.05. Mean washout rates were almost identical for the reference ROIs (45.0 %) and the +1 pixel and ?1 pixel heart ROIs (44.9 % and 45.1 %, respectively).

Conclusion

Significant changes in the size of the best visually defined heart ROI produce minimal, clinically inconsequential changes in the H/M ratio and washout rate, even in subjects with significantly reduced myocardial uptake of ¹²³I-mIBG.