Inverse relationship between cardiac accumulation of meta-[131I]iodobenzylguanidine (I-131 MIBG) and circulating catecholamines in suspected pheochromocytoma.

Heart intensity (HI) in the 24- and 48-hr images of meta-[131I]iodobenzylguanidine (I-131 MIBG), a pheochromocytoma-seeking guanethidine analog, were compared with concentrations of plasma and urinary catecholamines and their metabolites in nonpheochromocytoma and pheochromocytoma patients. HI was inversely related to plasma concentrations and urinary excretion rates of the hormones. Plasma norepinephrine had the highest inverse correlation with HI (r = -0.73 at 24 hr, -0.63 at 48 hr), and urinary metanephrine the lowest (r = -0.23 at 24 hr, -0.28 at 48 hr). A similar relationship was observed in the intensity of salivary-gland visualization, but with less marked variations. HI was much higher in nonpheochromocytoma patients than in pheochromocytoma patients. HI in an I-131 MIBG image provides useful information in the diagnosis of pheochromocytoma, and may provide a tool for the study of the influence of catecholamines on the heart.