The Feasibility of Novel 124I-MIBG Tracer in Evaluation of Myocardial Sympathetic Denervation and Assessment of Neuroendocrine Tumors. Comparison With 123I-MIBG.

The autonomic nervous system consists of sympathetic and parasympathetic innervation.

The neurotransmitter of the sympathetic system is norepinephrine (NE) which in response to a stimulus, is released to the synaptic space Guanethidine is a false neurotransmitter analog of NE that is not catabolized . When labeled with radioactive iodine the radiotracer meta-iodobenzylguanidine (as I-MIBG) may be used for localization of different clinical and pathological conditions. There are two main indications for scintigrpahy with labeled MIBG 1. Whole body imaging of neuroendocrine tumors such as phechromocytoma and neuroblastoma. Scintigraphy allows staging of the disease as well as follow up in patients with MIBG-avid tumors. Moreover, when labeled with high doses of 131I it can be used for treatment in patients with metastatic neuroendocrine tumors. 2. the other indication of labeled MIBG scintigraphy is for assessment of myocardial sympathetic nerve endings Autonomic disturbances are of the common symptoms in Parkinson disease (PD). They may appear early in the disease occasionally prior to the motor symptoms, thus raising a potential use of autonomic system assessment as a biological marker for prediction of future development of motor PD.

Labeled MIBG imaging is a potential test for determining of the integrity of the individual autonomic function. Reduced MIBG uptake indicates postganglionic sympathetic dysfunction, which has been observed during the early stages of PD. MIBG scintigraphy is thought to be a sensitive modality for the diagnosis of PD, assisting in the differential diagnosis of other parkinsonian diseases, PD and other movement disorders. This method has also been reported to be of a high specificity.

The labeled MIBG used routinely is the tracer labeled with 123I which can be used with gamma camera for performance of planar imaging or 3D SPECT images. 123I emits predominantly γ photons with energies of 159 keV and has a half-life of 13.2 hour.

Quantitation of 123I MIBG myocardial uptake by the Heart to-Mediastinum ratio (H/M) measured from planar imaging has been found to be of predictive value for assessment of prognosis for cardiac events. H/M obtained from 123I MIBG SPECT was reported to improve the differentiation between subjects with normal and those with abnormal MIBG uptake.