Nuclear Cardiology Boards Practice Test

In I-123 MIBG cardiac imaging, the heart-to-mediastinal (H/M) ratio measures how much tracer the heart takes up relative to background mediastinal activity, reflecting cardiac sympathetic nerve function. In congestive heart failure, sympathetic denervation and impaired norepinephrine handling reduce myocardial MIBG uptake, lowering the H/M ratio. A lower H/M ratio on this imaging is consistently associated with worse outcomes, including higher mortality and more heart-failure–related events, because it indicates more advanced sympathetic dysfunction. A higher ratio suggests relatively preserved innervation and a better prognosis. While increased mediastinal uptake or no heart uptake represent abnormal patterns, the prognostic value most commonly comes from a decreased heart-to-mediastinal ratio.