Lower dose and shorter acquisition: Pushing the boundaries of myocardial perfusion SPECT

Abstract

For the past three decades myocardial perfusion imaging SPECT (MPI-SPECT) has managed to prevail as one of the most important non-invasive imaging techniques in cardiology. The strengths of MPI-SPECT include the reliance on physiology rather than anatomical features of the coronary circulation, its standardized and efficient methodology, and a large body of evidence supporting its clinical value. However, while taking advantage of all these benefits inherent to the method, we have for long time neglected the need to push on technical developments and innovations and it may seem as if MPI-SPECT has missed the step into the twenty-first century. In fact, almost all around the globe, MPI-SPECT is still performed in a similar manner to 20 or 30 years ago using essentially the same radionuclides, protocols, and camera designs. In an era of emerging technologies and multimodality imaging, MP-SPECT has to compete with other techniques that are faster or avoid ionizing radiation such as CT coronary angiography or cardiac magnetic resonance. Thus, a technique which requires 20to 30-minute acquisition time and exposes the patient to 10-25 mSv of ionizing radiation appears to be an endangered species in the cardiac imaging field. However, recent years have witnessed important developments in device technology and a number of novel camera designs for dedicated cardiac MPI-SPECT have entered commercial production. Solid-state semiconductor detectors made of a cadmium-zinc-telluride (CZT) alloy provide direct conversion of absorbed gamma-rays into an electrical signal and avoid the need for the bulky photomultiplier tubes used in conventional SPECT cameras. In 2008, GE Healthcare has introduced a CZT-based dedicated cardiac scanner (Discovery NM 530c) to the market. This device consists of 19 fixed arrays of pixilated CZT detectors assembled into a unique geometry that allows obtaining simultaneous views of the heart from different angles. This design coupled with the use of a proprietary multi-pinhole collimator improves count sensitivity compared to conventional devices while the CZT detector technology provides higher energy and spatial resolution. Indeed phantom studies have demonstrated improvements of 57-fold in count sensitivity, 1.7-2.5-fold in spatial resolution, and 1.65-fold in energy resolution. Similar technology has been introduced by other vendors. In nuclear cardiology image quality is determined to a large extent by high count statistics, which are based on a trade-off between injected radiotracer activity and duration of image acquisition. Thus, it can be anticipated that protocols consisting of lower radionuclide doses and shorter acquisition times would push the boundaries of image quality in conventional scanners. With higher sensitivity SPECT devices such as the Discovery NM 530c, incentives to reduce acquisition times rather than radiation exposure to the patient were higher given the prospect for improved efficiency and patient throughput. Hence, a number of reports have been published demonstrating high image quality with CZT devices and comparable diagnostic performance with acquisition times as short as 2 minutes. However, the potential risks and hazards from ionizing radiation have obtained increasing public attention given to exponential growth of radiation exposure from medical imaging in the past years. And the recent devastating nuclear accidents with emission of unknown amounts of radionuclides into the environment have further sensitized the public to this issue. Therefore, nuclear cardiologists have been urged to reduce patients’ radiation exposure from MPISPECT, if ever possible without increasing acquisition times and thereby reducing patient comfort. Some diagnostic centers have implemented stress-only MPSPECT protocols, thereby avoiding unnecessary rest studies in the presence of normal stress studies, which may reduce radiation exposure in a subgroup of patients. However, this approach requires an optimal set-up with From the Cardiovascular Center, Department of Radiology, Cardiac Imaging, University Hospital Zurich, Zurich, Switzerland; and Zurich Center of Integrative Physiology (ZIHP), University of Zurich, Zurich, Switzerland. Reprint requests: Philipp A. Kaufmann, MD, Department of Radiology, Cardiac Imaging, University Hospital Zurich, NUK C 32, Raemistrasse 100, 8091 Zurich, Switzerland; pak@usz.ch. J Nucl Cardiol 2011;18:830–2. 1071-3581/$34.00 Copyright 2011 American Society of Nuclear Cardiology. doi:10.1007/s12350-011-9410-z