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The sympathetic nervous system of the heart is critically involved in the maintenance of cardiovascular homoeostasis by regulation of cardiac contractility, conduction, heart frequency and peripheral vasoconstriction.1 It has been shown that a dysfunctional cardiac sympathetic nervous system exerts detrimental effects on the structural and functional integrity of the myocardium, leading to a marked increase in the morbidity and mortality rates of patients with cardiac disease.2 3 For this reason, the cardiac sympathetic nervous system has been used for prognostication of patients with cardiovascular disease.2–6
Evaluation of the cardiac sympathetic nervous system can be performed with radionuclide imaging of [123I]m-iodobenzylguanidine ([123I]MIBG). [123I]MIBG is a norepinephrine analogue which uses similar uptake and storage mechanisms within the sympathetic neuron as norepinephrine. Cardiac uptake of [123I]MIBG is predominantly determined by two pathways: it can be cleared from the synaptic cleft by (1) the norepinephrine transporter protein located at the dilated endings of the sympathetic neuron or (2) the non-neuronal transport mechanism located at the post-synaptic site of the cleft. Importantly, once taken up by the sympathetic neuron, [123I]MIBG is not metabolised by proteolytic enzymes. As a consequence, [123I]MIBG is preserved with its original molecular structure, which makes [123I]MIBG suitable for visualisation of cardiac sympathetic innervation and activation.
A complete [123I]MIBG imaging protocol consists of planar and single photon emission computed tomography (SPECT) imaging performed at 10–20 min (early imaging) and 3–4 h (delayed imaging) after administration of the tracer. Planar imaging provides information on global sympathetic innervation or activation, whereas SPECT imaging can be used for assessment of regional cardiac sympathetic innervation or activation (figure 1). On planar imaging, the heart-to-mediastinum (H/M) ratio is determined by dividing the mean counts per pixel within the cardiac region …