Radical gamma camera design promises to shake up nuclear medicine

Israeli start-up Spectrum Dynamics has developed technology that can generate 10 times the sensitivity and double the spatial resolution of conventional Anger cameras, according to the company. Such dramatic increases raise the possibility of real-time imaging that shows the perfusion of one or more radiotracers through the myocardium introducing a radical change in the way cardiac patients are evaluated.

Preliminary results are based on phantom studies and early tests on human volunteers that have produced high-resolution cardiac images in two to three minutes compared with 15 to 20 minutes for a conventional exam.

Executives from Spectrum Dynamics described a prototype of the D-SPECT (dynamic SPECT) gamma camera at this year's RSNA meeting. The solid-state detectors, made from cadmium zinc telluride (CZT) crystals, differ markedly from traditional detectors, which are based on photomultiplier tubes or photodiodes. The detector moves inside casing, recording data as it moves across the patient. These differences do not, however, completely explain the improved performance.

"It is really putting together the whole concept - the sensitivity of the camera, the geometry (of the detector), the algorithms, the detector material - but also our (optimization of the) radiopharmaceuticals and the patient data," said Michael Nagler, Ph.D., executive vice president and general manager of Spectrum Dynamics.

Clinical tests will begin at a hospital in Israel early in 2006, with a multisite clinical trial set to begin in the U.S. by midyear. Spectrum Dynamics plans to begin shipping production units in the U.S. by the end of the year.

The systems will be sold either by a direct sales force or by sales people employed by distributors. The company is weighing options on how to approach the market.

"We want to proliferate the use of the camera as fast as we can, so we will choose the fastest possible way to do that," Nagler said. "We already have 300 names from people who came by the booth during the recent (RSNA) meeting who want to be contacted down the road when we come out with this."

Nagler emphasized that the interest has come from the description of mock-ups clearly described as investigational, pending FDA clearance. Pricing has not yet been determined. And only preliminary data were discussed with booth visitors.

Because the commercial launch of D-SPECT will not depend on the outcome of clinical trials (the FDA does not require proof of clinical efficacy to clear gamma cameras for marketing), production units will be available about the time the system is commercially launched.

"The clinical trials are mostly to establish market superiority," said Dalia Dickman, Ph.D., vice president of clinical and regulatory affairs and chief biologist for Spectrum Dynamics.

If D-SPECT lives up to expectations, the camera will resolve areas down to 6 mm or 7 mm across. While the data for these high-resolution images are being acquired, however, D-SPECT is expected to deliver a streaming video comprising tens of thousands of frames. The first of these frames will be snapped the moment the radioisotope is injected into the patient. Subsequent images will show blood flow that can be quantified in milliliters per minute, according to Nagler, as well as the uptake and redistribution of radioisotopes.

Conventional gamma cameras show the relative dispersal of a radioisotope over the 15 or 20 minutes they take to acquire data. As a result, cardiac assessments compare one part of the heart to others, with physicians flagging abnormalities if a part shows less perfusion than the rest of the heart.

"If all the arteries are stenosed, the whole heart will be hypoperfused, and there is no way of seeing that," Dickman said.

D-SPECT images, however, can serve as the basis for qualitative assessments, as well as absolute measurements for voxels throughout the heart muscle, Nagler said. The ability to document radiopharmaceutical kinetics may provide a better understanding of disease processes.

"The uptake slope of thallium is very much dependent on the physiological condition of the myocardium," he said.

Early users of D-SPECT will begin with images very much like the ones they see now, albeit with a much higher spatial resolution. This will provide a basis to interpret the information in other windows that show the dynamics of the radiopharmaceutical, the video, and the accumulation and redistribution of the tracer.

"That is why we are calling it D-SPECT," Nagler said.

Dr. Daniel Berman, a professor of medicine at the University of California, Los Angeles and a consultant for and shareholder in Spectrum Dynamics, suggests this new camera could allow faster and more accurate cardiac stress imaging and better identify patients at risk. Dr. Jack Ziffer, chair of the radiology department at Baptist Hospital of Miami and a member of the Spectrum Dynamics medical advisory board, speculates that the availability of this camera could spur the development of new tracers for cardiac and oncological applications.

Both base their opinions on preliminary experience gained in Spectrum Dynamics' engineering labs in Haifa. Berman and Ziffer and luminaries at Vanderbilt University and Brigham and Women's Hospital are scheduled to conduct clinical tests on the device in 2006.

They will examine static images for conventional interpretations along with video showing the uptake and redistribution of one or more radiotracers. Physicians might look at thallium, technetium, and iodine-123 simultaneously. Voxels indicating each radioisotope, which has specific energies, will be color-coded and displayed in context.

"We think this will bring a completely new dimension of analysis and appreciation of SPECT," Nagler said. "It will be the difference between black and white television and color television."

The primary application of D-SPECT will be as a stand-alone cardiac gamma camera, but the floor-mounted, lightweight, and highly portable unit will eventually be fitted for docking to a 64-slice CT scanner. The high speed of D-SPECT, and its ability to be undocked and moved out of the way, will help increase utilization of the scanner while providing SPECT data for fusion with CT as needed.

The company expects to release a version designed for SPECT/CT about a year after making the stand-alone SPECT camera available. The connection between the two will be a simple piece of software that tells the CT when the SPECT exam is done and that the CT acquisition should begin, according to Nagler. After that will come the postprocessing needed to fuse the data sets together, which, again, is fairly straightforward.

SPECT/CT, therefore, will be relatively easy to achieve, as the D-SPECT unit will be compatible with just about any of the tens of thousands of CT scanners installed worldwide, according to Nagler. The greatest bang for the buck will come, however, in hooking up with the latest generation of CTs optimized for imaging the heart, Dickman said.

"With this you will have a very efficient production system for doing functional imaging together with anatomic imaging in a few minutes without having to move the patient," she said. "We well understand the advantages of that."

But D-SPECT need not stop there. The CZT detector can record very high energy photons, meaning it might be tuned for PET.

These possibilities will be examined, according to Nagler and Dickman, but not right away. First, the company wants to establish the credentials of its new camera. Doing this will provide the imaging community enough to chew on for a while.