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D-SPECT —
Looking for Cardiac Nuclear Medicine’s Next Generation Dramatically decreasing image acquisition time could someday allow for calculating absolute measurements of blood flow simply not possible with current perfusion studies. By producing higher-resolution SPECT images in much less time, Spectrum Dynamics Ltd. hopes to introduce the modality’s most significant advancement in more than 40 years. The company was founded to develop new functional molecular imaging technology, accessories, and applications. The Haifa, Israel-headquartered firm’s new acquisition technique, called dynamic SPECT (D-SPECT), is an attempt to take SPECT beyond the Anger camera—the pioneering device invented by the late Hal O. Anger, which is considered the most enduring piece of nuclear medicine technology developed in the past 50 years. In developing its D-SPECT system—currently an investigational device in the United States—the company replaced the Anger-based gamma camera with a compact camera comprised of tungsten collimators and 10 small, solid-state cadmium zinc telluride (CZT) digital detectors. The resulting detector/collimator columns sweep back and forth like search lights, collecting photons from hundreds to thousands of different angles. The new system is designed for cardiac imaging and could significantly change nuclear cardiology. Investigational work with the D-SPECT system has shown improvements in sensitivity up to a factor of 10 compared with a conventional Anger camera. This sensitivity translates to shorter acquisition times and, potentially, reduced radiopharmaceutical doses to patients without sacrificing image quality. Dramatically decreasing image acquisition time could someday allow for absolute quantitation, which could lead to calculating absolute measurements of blood flow that is simply not possible with current perfusion studies. Saving Time The company also points out that with a conventional SPECT camera, attempts to increase the acceptance angle of the collimator automatically reduce image quality. However, its research suggests that with D-SPECT, the reconstructed image exhibits improve sensitivity and better spatial resolution. “Those are the two main advantages that I see over currently available systems, as the D-SPECT system is optimized for collecting data from the heart,” says James A. Patton, PhD, professor of radiology and physics with the department of radiology and radiological sciences at Vanderbilt University Medical Center in Nashville, Tenn., where D-SPECT will begin its beta testing in December. “Because of the system’s geometry, users can position detectors closer to the heart, which provides those improvements in sensitivity and resolution.” The system is capable of independently controlled scanning from different directions, which enables differential scanning of the volume of interest, further allowing a localized increase of resolution and sensitivity. This opens the door to kinetic perfusion tracer models for a variety of applications, including absolute myocardial perfusion and coronary flow reserve measurements. Sensitivity & Resolution “I believe D-SPECT’s incredible sensitivity will open up everyone’s minds in terms of what drugs they might conceive of developing,” says Jack A. Ziffer, PhD, MD, FACC, chairman of radiology at Baptist Hospital of Miami and a consultant for Spectrum Dynamics. “For instance, it is possible to develop agents that bind to different types of plaque. In cardiology, it would be very important to identify vulnerable plaques. I don’t think people thought they could ever do that with SPECT, because of the former issues with patient dosimetry and time of acquisition. The markedly improved sensitivity and energy resolution of D-SPECT will stimulate development of new agents.” Increasing Throughput Because of the rapid growth in the number of nuclear cardiology procedures, Spectrum Dynamics turned its eyes to cardiac nuclear imaging. “In the U.S. nuclear medicine SPECT market today, cardiology is the only segment that has experienced significant growth in the last few years,” says Josh Gurewitz, market consultant with Spectrum Dynamics. Cardiac nuclear imaging has become increasingly important for diagnosis and management of heart disease, resulting in rapid growth in the number of nuclear cardiology procedures. This led to more cardiologists buying their own gamma cameras and providing studies outside the hospital setting. Cardiac nuclear imaging is time-consuming, taking anywhere from 12 to 20 minutes for each gated SPECT study. The time limits patient throughput and can lead to patient backlogs for the office-based practitioners and outpatient imaging centers. Spectrum Dynamics wanted to speed things up without sacrificing image quality. “Today, an average site can do about six or seven studies a day with a conventional system,” Gurewitz says. “But D-SPECT offers a potentially large improvement in throughput, allowing sites to add additional patients per day without increasing the number of cameras or personnel costs, so there is a lot of excitement about that.” Spectrum Dynamics’ D-SPECT Cardiac Scanner, a rapid functional imaging camera dedicated to myocardial perfusion imaging, can shorten procedure time to two minutes. In the future, this may enable performance of serial scans during a single study, resulting in dynamic imaging and absolute myocardial perfusion and coronary flow reserve measurements. “Outpatient cardiac imaging should provide the
biggest market for this system,” says Patton. “I think the
significant throughput improvement is the biggest advantage that D-SPECT
provides. I’ve seen very high-quality images acquired in just
two minutes.” Validation The RSNA introduction included a presentation, “D-SPECT: A Novel Camera for High Speed Quantitative Molecular Imaging: Initial Description and Validation,” delivered by Daniel S. Berman, MD, FACC, professor of medicine at the University of California, Los Angeles School of Medicine and a consultant for Spectrum Dynamics. Berman and colleagues sought to demonstrate how D-SPECT resolves the conflict between sensitivity and resolution inherent in standard SPECT. In their research, they performed phantom studies to assess the sensitivity and resolution of D-SPECT compared with conventional SPECT with an Anger camera. In all studies, the D-SPECT system demonstrated sensitivity that was more than 10 times greater than conventional SPECT. In addition, D-SPECT spatial resolution was two times higher, despite less imaging time than conventionally (one tenth). Berman also reported that researchers obtained excellent images in human volunteers in two minutes with D-SPECT, compared with 17 minutes with conventional SPECT. Berman, lead author of the study, concluded that “D-SPECT is an entirely new technology with marked increase in sensitivity and resolution compared to [traditional] SPECT. The resultant shorter acquisition times allow rapid, sequential SPECT acquisitions [dynamic SPECT]. The improved energy resolution of CZT could allow D-SPECT of multiple tracers simultaneously, creating new opportunities for quantitative molecular imaging.” As Berman indicated, the capability of D-SPECT to perform dual isotope imaging, facilitated by the CZT detectors, could be one of the technology’s most significant advantages. “Because of the CZT detectors, D-SPECT has exceptional energy resolution and an excellent ability to discriminate between different energy peaks,” says Berman. “From an application standpoint, D-SPECT will be able to perform dual isotope stress/rest imaging simultaneously.” This could have significant impact for nuclear cardiologists, as far as the time factor. “Today, when dual isotope studies are performed, they have to be done sequentially,” says Gurewitz. “Current technologies can’t discriminate between those two energies simultaneously, so these studies have a detrimental effect on patient throughput.” Beta Testing “The first kind of testing we’ll do is myocardial perfusion imaging, with the goal of validating the approaches that lets us speed up imaging dramatically without any loss of diagnostic content,” Ziffer says. Hybridizing D-SPECT “Current hybrid systems are integrated; that is, you can’t separate them,” Gurewitz says. “Users will be able to connect our product to a legacy CT when they need D-SPECT/CT studies. When they don’t, they can undock the product and have a separate gamma camera and CT scanner.” Fusion software is commercially available to create hybrid images taken from separate modalities, but many people involved in imaging believe images taken in the same exam provide much better registration of one image on the other. More Patients Essentially, in a SPECT/CT hybrid, the CT always waits for the SPECT system to finish its work. Only then can the next study be performed. “But D-SPECT acquires the data so rapidly, we are beginning to approach CT throughput with a SPECT system,” comments Gurewitz. “This will make hybrid technology economically feasible,” says Ziffer. “More places will be able to afford it. Also, users won’t be tying up a high-end CT scanner for an hour with every nuclear study.” Ziffer believes it makes perfect sense to couple a D-SPECT to a CT scanner because the hybrid also allows users to view objects that previously couldn’t be localized. He offers an example involving vulnerable plaque. “Let’s say I wanted to look at a vulnerable plaque in a patient who needed a drug-eluting stent or some other form of treatment in that plaque,” Ziffer says. “If there were such an agent to identify the vulnerable plaque, we could give it to the patient, but we’d have no idea where it was localized. With a D-SPECT scanner coupled to a CT device, we’d not only be able to identify uptake in the vulnerable plaque; as importantly, we’d be able to tell where that plaque is located, and that would ultimately influence management. We may be able to do all of this because you can couple D-SPECT to a CT scanner. It makes it economically and technologically feasible.” Ziffer believes the initial D-SPECT application could be just the tip of the molecular imaging iceberg. “Over the next five years, when this technology has become available, and as we started using it more and more, we’re going to get a lot of new ideas about how to use it that we haven’t even dreamt of yet,” he says. One of the most important impacts he anticipates is that D-SPECT will bring molecular imaging into the mainstream. “One day, we are going to look all around us, at everything available, and we’re going to say this is what D-SPECT has enabled. Then we’ll look back at this time and think, ‘That’s where it all started.’” — Dan Harvey is a freelance writer based in
Wilmington, Del., and a frequent contributor to Radiology Today.
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July
31, 2006
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