Dedicated Solid State Cardiac Systems for Myocardial Perfusion Imaging

Multiple Detector Systems

Myocardial perfusion imaging (MPI) has evolved from planar scintigraphy to single detector single-photon emission computed tomography (SPECT) imaging, to dual detector SPECT imaging to our present state of multiple (more than two) detector SPECT imaging. In recent years manufacturers have begun to break away from the conventional SPECT imaging approach to create innovative designs of dedicated cardiac imagers. These imagers' designs have in common that all available detectors are constrained to simultaneously imaging just the cardiac field of view. These new designs vary in the number and type of scanning or stationary detectors, and whether crystals or solid state detectors are used.^1,2,3

Three commercially available cardiac-centric SPECT systems exist which use multiple solid state detectors and have been validated in multicenter clinical trials. These are:

1. The Digirad^® Cardius, which uses three parallel hole collimator cameras consisting of cesium-iodide (CsI) crystals/diode module detectors³
2. The Spectrum Dynamics D-SPECT^TM, which uses nine cadmium-zinc-telluride (CZT) fanning detectors with tungsten parallel large hole collimators⁴
3. The General Electric Discovery NM 530c, which uses 19 CZT static detectors with pinhole collimators (tungsten insert)⁵

In the CZT-based systems the conventional photomultiplier cameras with sodium iodide (NaI) crystals have been replaced with CZT solid state detectors which have a superior energy and spatial resolution. These improvements come about because the energy of the photon being imaged is converted directly to an electrical pulse within a 2.46 mm² pixel. This direct conversion preserves the energy information much better than the conventional cameras that use the Anger camera electronics and is resolved in a single small pixel.

These systems have in common the potential for a five- to ten-fold increase in effective count sensitivity at no loss or even a gain in energy, spatial and contrast resolution, resulting in the potential for acquiring a stress myocardial perfusion scan injected with a standard dose in two minutes or less or in trading off some of this added count sensitivity for a reduction in the injected dose at comparable image quality. Studies have shown that images from these solid state imagers using conventional myocardial perfusion imaging (MPI) doses of 10 mCi for rest and 30 mCi from stress acquired at four minutes for rest and two minutes for stress are of higher quality and at least of equal diagnostic value to those acquired on conventional two-detector SPECT cameras using 14 minutes for rest and 12 minutes for stress acquisition.^4,5 Other studies have shown that with these multiple detector imagers the injected dose may be reduced by half (5 mCi rest/15 mCi stress) and the rest images acquired in eight minutes and the stress in five minuets and still yield images of superior quality over conventional acquisition times with conventional SPECT systems.⁶

Reduced Dose vs. Increased Efficiency

The increased count sensitivity of these new systems may be traded off for a reduced injected dose and thus a reduced total effective dose to the patient. It is clear that these more efficient imaging systems also allow for high-quality images that are obtained using a lower injected radiopharmaceutical dose and thus a decrease in the radiation dose that is absorbed by the patient and staff. This reduction in dose comes at an increase in acquisition time, even if the total time is still less than what has been traditionally used in conventional systems. Recently, the American Society of Nuclear Cardiology published an information statement⁷ recommending that laboratories use imaging protocols that achieve on average a radiation exposure of less than or equal to 9 mSv in 50% of the studies by 2014. Although there are many different protocols that may be implemented to accomplish this exposure goal, use of the more efficient imagers described above would greatly facilitate this goal and allow for increases in efficiency over the imaging protocols used today.

References

1. Garcia EV, Faber TL, Esteves FP. Cardiac Dedicated Ultrafast SPECT Cameras: New Designs and Clinical Implications. J Nucl Med, 2011; 52:210-217. Available at http://jnm.snmjournals.org/content/52/2/210.full.pdf
2. Sharir T, Slomka PJ, Berman DS. Solid-state SPECT technology: fast and furious. J Nucl Cardiol, 2010;17:890-6. Available at http://www.ncbi.nlm.nih.gov/pubmed/20740337
3. Maddahi J, Mendez R, Mahmarian J, Thomas G, Babla H, Bai C, Arram S, Maffetone P, Conwell R: Prospective multi-center evaluation of rapid gated SPECT myocardial perfusion upright imaging. J Nucl Cardiol, 2009;16:351-7. Available at http://www.ncbi.nlm.nih.gov/pubmed/19247734
4. Sharir T, Slomka PJ, Hayes SW, DiCarli MF, Ziffer JA, Martin WH, Dickman D, Ben-Haim S, Berman DS: Multicenter trial of high-speed versus conventional single-photon emission computed tomography imaging: quantitative results of myocardial perfusion and left ventricular function. J Am Coll Cardiol, 2010:55:1965-74. Available at http://www.ncbi.nlm.nih.gov/pubmed/20430269
5. Esteves FP, Raggi P, Folks RD, Keidar Z, Askew JW, Rispler S, O’Connor MK, Verdes L, Garcia EV. Novel solid-state-detector dedicated cardiac camera for fast myocardial perfusion imaging: multicenter comparison with standard dual detector cameras. J Nucl Cardiol 2009, 16:927-24. Available at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2776146/
6. Duvall WL, Croft LB, Ginsberg ES et al. Reduced isotope dose and imaging time with a high-efficiency CZT SPECT camera. J Nucl Cardiol, 2011;18:847-57. Available at http://www.ncbi.nlm.nih.gov/pubmed/21528422
7. Cerqueira MD, Allman KC, Ficaro EP, et al. Recommendations for reducing radiation exposure in myocardial perfusion imaging. J Nucl Cardiol, 2010. 17: 709-18.