Cardiac Sympathetic Innervation (I-123-MIBG)

Quick Answer

I-123-MIBG images the heart's sympathetic (adrenergic) innervation by acting as a norepinephrine analog taken up by cardiac nerve terminals. Reduced uptake — a low heart-to-mediastinum (H/M) ratio and increased washout rate — reflects sympathetic denervation/dysfunction and independently predicts worse outcomes in heart failure (arrhythmic events, cardiac death; ADMIRE-HF), refining risk beyond ejection fraction. The same reduced cardiac uptake distinguishes Parkinson disease / dementia with Lewy bodies (postganglionic denervation) from multiple system atrophy. Critically, the absolute H/M threshold is collimator-dependent.

Background & Physiology

MIBG (meta-iodobenzylguanidine) is taken up by the norepinephrine transporter into presynaptic sympathetic nerve terminals and stored in vesicles, imaging the integrity and function of cardiac adrenergic innervation. In heart failure, chronic sympathetic overactivity and denervation reduce and destabilize neuronal uptake — a process that precedes and predicts adverse events.

Quantification

• Heart-to-mediastinum (H/M) ratio: a planar anterior-chest ROI over the heart divided by a mediastinal ROI, at early (~15 min) and delayed (~3–4 h) imaging. A low delayed H/M (commonly below ~1.6, threshold camera/collimator-dependent) indicates impaired innervation and higher risk.
• Washout rate: the fall in cardiac uptake from early to delayed imaging; increased washout reflects heightened sympathetic drive and adverse prognosis.
• Collimator dependence: low-energy vs medium-energy collimation strongly changes the absolute H/M (septal-penetration effects) — standardize and use local/normalized thresholds (phantom cross-calibration allows conversion to a standardized medium-energy value).

Clinical Roles

• Heart-failure risk stratification: ADMIRE-HF established the prognostic value of a low H/M ratio for cardiac events and mortality; MIBG refines risk beyond LVEF and may inform device/therapy decisions (e.g., ICD consideration in intermediate-risk patients).
• Parkinsonian syndromes: reduced cardiac MIBG supports Parkinson disease / DLB (postganglionic denervation) versus preserved uptake in MSA — complementary to DAT (ioflupane) imaging, which shows presynaptic nigrostriatal loss in both.

Interpretation & Decision

A low delayed H/M + high washout marks a higher-risk heart-failure phenotype independent of ejection fraction. For parkinsonism, reduced cardiac uptake favors PD/DLB and argues against MSA. Because absolute values are collimator-dependent, serial and cross-site comparisons require standardization.

Differential Diagnosis / Confounders

Interfering medications (as with any MIBG study), physiologic lung/mediastinal background, and technical (collimator/ROI) variability. In parkinsonism: PD/DLB (low cardiac uptake) vs MSA/essential tremor (preserved).

Reporting Checklist

• Report early and delayed H/M and washout rate, with the collimator and any standardization applied.
• State the prognostic implication (heart-failure risk) or the PD/DLB vs MSA distinction.
• Note interfering-drug status.

Common Pitfalls

• Comparing H/M across sites/collimators without standardization.
• Interfering medications (sympathomimetics, tricyclics, labetalol) reducing uptake.
• Physiologic lung/mediastinal background degrading the ratio.

Board Pearls

Board numbers at a glance:

I-123-MIBG images the heart's sympathetic innervation (a norepinephrine analog taken up by cardiac nerve terminals). A low delayed heart-to-mediastinum (H/M) ratio and increased washout reflect denervation and independently predict worse heart-failure outcomes — arrhythmic events and cardiac death (ADMIRE-HF), refining risk beyond ejection fraction.

The absolute H/M threshold is collimator-dependent (low- vs medium-energy septal penetration) — standardize and use local/normalized values, especially for serial or cross-site comparison.

The same reduced cardiac MIBG uptake distinguishes Parkinson disease / DLB (postganglionic sympathetic denervation) from MSA (preserved cardiac uptake) — complementary to DAT imaging, which is abnormal in both. Interfering drugs and physiologic mediastinal background are the recurring technical confounders.

Related Pages

• Tracer: I-123-MIBG; related: nuclear cardiology, DAT (ioflupane) SPECT.

Figure / Diagram Suggestions

• An H/M ROI planar diagram (heart vs mediastinum, early/delayed).
• A washout-rate time-course schematic.
• A PD/DLB (low) vs MSA (preserved) cardiac-uptake comparison.

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