A step forward in the use of SPECT imaging with I-123 MIBG

With the publication of the AdreView Myocardial Imaging for Risk Evaluation in Heart Failure Study (ADMIRE-HF), it has become clear that a simple ratio of heart-to-mediastinum (H/M) activity of I-123 metaiodobenzylguanidine (MIBG), on an anterior planar image is a strong predictor of cardiac prognosis in heart failure patients with a reduced left ventricular ejection fraction and New York Heart Association class II or III symptoms. The planar I-123 MIBG H/M ratio provides predictive information for outcome in heart failure patients beyond that available from clinical variables, left ventricular ejection fraction, and b-type natriuretic peptide. It remains unclear, however, whether further improvement in the prognostic information available from the planar I-123 MIBG H/M ratio can be provided through the use of single-photon emission computed tomography (SPECT). Previously, Chen et al. compared the accuracy of planar and SPECT imagings for noninvasive quantitation of I-123 MIBG activity in a cardiac phantom model. They noted that SPECT imaging with iterative reconstruction produced more accurate heart-to-calibration ratios than planar imaging. Cardiac SPECT imaging is usually performed with a low-energy, high-resolution (LEHR) collimator. In addition to its predominant 159 keV energy, I-123 has a low abundance, high-energy photon that penetrates the septa of a LEHR collimator, blurring the MIBG images, and impairing the accuracy of quantitative estimates. Those authors had previously reported that deconvolution of septal penetration (DSP) by those high-energy photons combined with iterative reconstruction yielded quantitative estimates of I-123 MIBG activity that were similar to measurements without septal penetration using a mediumenergy all-purpose collimator. This suggests that the combination of iterative reconstruction and DSP might substantially improve quantitation of I-123 MIBG activity in the heart in patient studies. These previous investigations notwithstanding, the I-123 MIBG H/M ratio from SPECT imaging has not entered into common use due to a lack of a standardized and practical method for calculation of the H/M ratio, as well as the technical challenges of calculating the H/M ratio in heart failure patients with very low uptake of the tracer into the myocardium. In their study, Chen et al. describe a new method for calculating the I-123 MIBG H/M ratio from SPECT images. The method was initially developed by comparing its ability to separate abnormal I-123 MIBG H/M ratios in 53 patients with coronary artery disease, prior myocardial infarction, and left ventricular ejection fraction B40%, from 14 normal volunteers with no history of heart disease. Three versions of the SPECT method (filtered back-projection, iterative reconstruction, and iterative reconstruction with DSP) plus a standard planar H/M method were then tested for their ability to separate 957 heart failure patients from 94 controls from the ADMIRE-HF study. Using receiver operator characteristics (ROC) analysis, the SPECT method with iterative reconstruction and the planar H/M ratio method provided comparable separation of heart failure patients from controls. A finding that was likely unexpected is that iterative reconstruction with DSP did not separate patients from controls as effectively as iterative reconstruction alone. Improved separation of I-123 MIBG activity in the heart from background and scattered activity results in larger values of the H/M ratio with the SPECT compared to planar methods. Merlet et al. provided an early report documenting the high risk for adverse cardiac events in patients with a planar H/M ratio less than 1.2. In the ADMIRE-HF study, a planar value of 1.6 was prospectively tested to separate high-risk from low-risk patients. The value of 1.6 represents two standard deviations below the mean planar H/M ratio in normal subjects as reported in the literature. In the present study, the optimal cut-off value for the planar H/M ratio for separating heart failure patients from controls in the validation cohort was 1.58, and the optimal H/M ratio using SPECT imaging with iterative reconstruction was 2.61. This difference likely represents less contamination of the heart region of interest from background From the Division of Cardiovascular Diseases, Department of Internal Medicine, University of Cincinnati College of Medicine, Cincinnati, OH. Reprint requests: Myron C. Gerson, MD, Division of Cardiovascular Diseases, Department of Internal Medicine, University of Cincinnati College of Medicine, P.O. Box 670542, Cincinnati, OH 45267-0542; myron.gerson@uc.edu. J Nucl Cardiol 2012;19:16–8. 1071-3581/$34.00 Copyright 2011 American Society of Nuclear Cardiology. doi:10.1007/s12350-011-9495-4