Multimodality cardiac imaging

Abstract

For the last three decades myocardial perfusion imaging (MPI) with single photon emission computed tomography (SPECT) has been an excellent tool for guiding clinicians in the management of patients with known or suspected coronary artery disease (CAD). However, despite its documented high diagnostic accuracy myocardial perfusion SPECT may fail to detect the true extent of coronary atherosclerosis and thus underestimate the coronary risk in individual patients. In fact, large observational studies have shown that 43% of patients suffering a myocardial infarction and 31% of patients dying from a cardiac cause had a normal or only mildly abnormal prior perfusion scan. A possible explanation for these shortcomings is inherent in the nature of the technique. MPI evaluates the hemodynamic relevance of coronary stenoses and therefore can only detect obstructive coronary lesions. However, approximately half of patients with normal SPECT perfusion have subclinical CAD on coronary CT angiography (CTA). Histopathological correlation studies demonstrate that almost 70% of coronary occlusions (leading to myocardial infarction) result from thrombosis of lesions with a stenosis of less than 50% before infarction. This explains to some extent why— despite evidence of normal myocardial perfusion— patients with subclinical CAD are at higher risk of coronary events, and underlines the importance of assessing the full (i.e., subclinical) extent of CAD to guide therapeutic decisions. Recent years have witnessed tremendous technological advances in coronary multislice CTA paralleled by an increased use of this technology in clinical practice. Given its high spatial resolution, its noninvasive nature, its relative ease of use, and its complementary value to MPI, CTA has been used as an ideal partner modality for hybrid (or multimodality) imaging with nuclear techniques. Additionally, the increased availability of dedicated fusion software packages for three-dimensional coregistration of CTA and MPI have facilitated the use of hybrid imaging in clinical practice. The feasibility and clinical robustness of noninvasive hybrid imaging was first documented by Namdar and coworkers in a clinical study involving fusion of N-ammonia positron emission tomography (PET) with 4-slice CTA in 25 patients with CAD. The hybrid PET/CTA images allowed to identify flow-limiting coronary lesions which required a revascularization procedure (as defined by invasive coronary angiography and PET) with a sensitivity, specificity, positive, and negative predictive value of 90%, 98%, 82%, and 99%, respectively. These encouraging results were confirmed by a similar study with SPECT/CTA showing that the hybrid approach resulted in a significant improvement in specificity (from 63% to 95%) and positive predictive value (from 31% to 77%) compared to CTA alone for detecting flow-limiting coronary stenoses. Santana and colleagues showed significantly higher diagnostic performance for fused SPECT/CT imaging compared to SPECT alone (P 0.001) and to the side-by-side analysis of SPECT and CT (P = 0.007) for the diagnosis of obstructive CAD on invasive coronary angiography. Interestingly, this improved diagnostic performance was mainly a result of a higher sensitivity in patients with multivessel disease. A recent study implementing motion-frozen SPECT data and CTA-guided SPECT contour and territory adjustments, found that the improved diagnostic value of hybrid imaging was mainly driven by higher diagnostic indices in the left circumflex (LCX) and right coronary artery (RCA) territories. Traditionally, LCX and RCA are subject to more variable patterns of coronary artery anatomy than the left anterior descending artery, which explains the particular value of hybrid imaging in these territories. The study presented in this issue of the Journal of Nuclear Cardiology by Sato and coworkers adds importantly to previously published studies and further underscores the complementary and incremental From the MRC Clinical Sciences Centre and National Heart and Lung Institute, Hammersmith Hospital, Imperial College, London, United Kingdom; Cardiac Imaging, University Hospital Zurich, Switzerland; Zurich Center for Integrative Human Physiology (ZIHP), University of Zurich, Switzerland. Reprint requests: Oliver Gaemperli, MRC Clinical Sciences Centre and National Heart and Lung Institute, Hammersmith Hospital, Imperial College London, Hammersmith Campus, Du Cane Road, London W12 0NN, United Kingdom; oliver.gaemperli@csc.mrc.ac.uk. J Nucl Cardiol 2010;17:4–7. 1071-3581/$34.00 Copyright 2009 by the American Society of Nuclear Cardiology. doi:10.1007/s12350-009-9161-2