JNM: Combo nuc imaging could better identify ICD candidates

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Combined imaging techniques of using a neurotransmitter analog to detect impairment of cardiac sympathetic innervation and myocardial perfusion imaging (MPI) to detect impaired myocardium can provide crucial information on the selection of patients at greater risk for sudden death and who would most likely benefit from prophylactic implantable cardioverter-defibrillator (ICD) use, according to a study published in this month's Journal of Nuclear Medicine.

The study included 60 patients who underwent both cardiac sympathetic nerve imaging using 123I-metaiodobenzylguanidine (MIBG) and MPI at rest using Tc-99m tetrofosmin within two weeks of ICD implantation in the study.

Kimio Nishisato, MD, from the cardiology division of Muroram City General Hospital in Muroram, Japan, and colleagues obtained cardiac planar and tomographic MIBG images at fasting and resting conditions at 15 to 30 minutes (early) and four hours (late) after an intravenous tracer injection, using a gamma-camera equipped with a low-energy general-purpose collimator.

MPI was performed at rest at 60 minutes after an intravenous tracer injection. Electrocardiogram-gated SPECT data were obtained for 60 seconds per projection from 60 projections during a 360 degree rotation, with a frame rate of eight, using a three-head gamma-camera with a low-energy, high-resolution parallel-hole collimator and a Butterworth filter.

Cardiac MIBG activity was quantified by the researchers as a heart-to-mediastinum ratio (HMR), and impaired Tc-99m uptake was graded as a summed score (SS) using a computerized technique with a percentage of tracer uptake by Nishisato and colleagues.

Patients with ICD shocks had a significantly smaller HMR and a greater SS than did those without, according to Nishisato and colleagues. During a mean 29-month interval, ICD shock was documented in 30 patients (50 percent) and three cardiac deaths were also observed in this group of patients.

“Kaplan–Meier analysis showed that in patients who had both an HMR of 1.90 or less and an SS of 12 or greater had a significantly greater ICD discharge rate than did those who had both an HMR greater than 1.90 and an SS less than 12 (94 percent vs. 18 percent),” wrote Nishisato and colleagues.

"While implantable cardioverter-defibrillator therapy is an effective option over anti-arrhythmic medications to prevent sudden cardiac death, the balance of clinical benefits, efficacy and risks is still a matter of discussion," wrote Ichiro Matsunari, MD, PhD, director of the clinical research department at the Medical & Pharmacological Research Center Foundation in Hakui, Japan, in an invited perspective on the Nishisato et al study.

Matsunari and colleagues added that there are several issues to be addressed before this new imaging method for planning ICD placement can come into widespread clinical use. First, the cost required for 123I-MIBG and myocardial perfusion SPECT should be justified in light of the benefits of this method. Second, although an appropriate ICD shock is considered a surrogate marker for SCD, it does not necessarily represent actual SCD. Third, regional heterogeneity in sympathetic innervation was not assessed in the study because of low cardiac 123I MIBG uptake in some patients, in whom high-quality SPECT images are difficult to obtain.

“Despite these unresolved questions, the study provides a rationale for further studies investigating the role of nuclear imaging for better selection of ICD candidates beyond left ventricular ejection measurements,” added Matsunari and colleagues.