Validity of the mediastinum as a reference region to evaluate cardiac accumulation of iodine-123 metaiodobenzylguanidine.

OBJECTIVE

The heart-to-mediastinum (H/M) ratio, the heart count normalized for the mediastinum count, is commonly used in cardiac (123)I-metaiodobenzylguanidine (MIBG) imaging. However, there are reports describing age-dependent increases in the mediastinum count with or without correction for the injected dose (ID) and a poor correlation between the H/M ratio and heart count normalized for ID. We evaluated the validity of the mediastinum count as a reference in comparison with the ID.

METHODS

Results of cardiac (123)I-MIBG imaging in 200 patients were analyzed. The mean counts for the heart and mediastinum were estimated to calculate the H/M ratio. Additionally, the heart and mediastinum counts were normalized for ID measured with a dose calibrator. ID was corrected for body size represented by body weight, body surface area, or lean body mass.

RESULTS

The coefficient of variance of the ID-normalized mediastinum count was reduced by correcting ID for body size. The indicators of body size showed significant negative correlations with age. Although a positive correlation was found between age and the ID-normalized mediastinum counts, the age-dependence was reduced by body size correction. There was a close correlation between the H/M ratio and ID-normalized heart counts, and body size correction improved the correlation.

CONCLUSION

The results of this study indicate the validity of the mediastinum as a reference region and support the use of the H/M ratio as an index of cardiac accumulation of (123)I-MIBG.