Detector designed for orbit promises gains for digital gamma cameras

Arizona researchers investigate brain imager

The search for black holes and new stars has led NASA engineers to develop a gamma ray detector that could vastly improve the resolution of nuclear medicine cameras. Engineers at NASA's Goddard Space Flight Center in Greenbelt, MD, are developing astronomy instruments using the new technology, which is based on cadmium zinc telluride (CZT) detector material. But they are also collaborating with researchers at the University of Arizona to adapt the technology for nuclear medicine uses, such as within a solid-state digital gamma camera.

The NASA solid-state detector is roughly similar in concept to a detector incorporated into a solid-state digital gamma camera being commercialized by Digirad, which received clearance for its Digirad TC 2020 Imager in May (SCAN 6/11/97). In fact, the San Diego company and another company called eV Products of Saxonburg, PA, have been providing CZT detector material to Goddard engineers. CZT chips are assembled into flat detectors at the Goddard center, much as they are by Digirad. But that is where the similarity ends.

The team at Goddard has devised a novel way to record the electrical signals that result when gamma rays strike the material. NASA has criss-crossed extremely fine strip electrodes on the front and back of the detectors. The strips, placed about 100 micrometers from each other, are packed in a checkerboard manner. Algorithms that describe the way electrical charges flow through CZT allow the localization of gamma ray impacts to an accuracy of about 30 microns, a level of precision unprecedented in nuclear medicine.

"The astronomy application of CZT detectors calls for really fine spatial resolution," said Dr. Neil Gehrels, head of the gamma ray and cosmic ray astrophysics branch at Goddard. "We've been pushing in that direction probably more than most companies have."

The strip detectors developed so far at Goddard have about half a million pixels spread over 60 square cm. The detector, about 7 square inches, comprises 36 individual CZT chips, each bonded together and then connected to electronics that capture the signals generated by the impact of gamma rays.

"We had quite a challenge coming up with a contact technology that, first of all, stuck really well to the CZT so it wouldn't pull up when we bonded the chips, and at the same time gave good electrical conductivity to the material," Gehrels said.

Commercial companies have not yet come up with this high-tech "glue," according to Gehrels, because they have not been working with electronic arrays as small as the ones at Goddard.

The most immediate application of the new detector will be to locate gamma ray bursts from deep space. Gehrels believes that the high precision of the detector will allow assembly of a relatively small-and cheap-satellite. The technology could have uses in other fields in which gamma ray detection is involved, however, such as nuclear medicine. To this end, Goddard is collaborating with researchers at the University of Arizona in Tucson to develop a CZT-based gamma camera for brain imaging.

Nuclear medicine applications. The same precision that astronomers need in their field promises to markedly advance the capability of gamma cameras used in nuclear medicine, said Dr. Harrison Barrett, regents professor of radiology and optical sciences at the university.

"The greatest advantage of these solid-state detectors is their spatial resolution," Barrett said. "You can get more pixels in a given area by having smaller pixels."

The resolution possible with detectors being developed at the University of Arizona outshines what is possible with detectors produced by Digirad, Barrett said. The research team has built a detector capable of resolving 0.4 mm.

"That is almost a factor of ten smaller than Digirad's detectors," Barrett said.

While the strip-electrode fabrication developed at Goddard is responsible for the high resolution achieved by the Tucson group, CZT itself offers several advantages. CZT is ideally suited for nuclear medicine, not only because of its ability to stop gamma rays and convert the photons to electricity, but because of its energy resolution. For example, the ability to distinguish photons at less than optimal energies can allow for correction of Compton scatter at the detector level.

"When a photon undergoes Compton scatter, it changes direction and loses almost all of its information about its point of origin. It also loses some energy," Barrett said. "So you would be able to reject any photon that comes out with less than 140 keV that you should get, for example, during a technetium study."

Studies at the University of Arizona indicate that energy changes as low as 6% can be detected using CZT. And that may be just the beginning. CZT has the potential to afford substantially better energy resolution, Barrett said.

CZT can also record the impact of x-rays, but the cost of the material is likely to limit applications in digital radiography. Whereas brain and cardiac applications in nuclear medicine require relatively small detectors, radiography tends to require the coverage of much larger areas, Barrett said.

The Arizona researchers hope to develop a gamma ray detector that might be used in a dedicated brain imager. But to succeed, they must first overcome certain cost issues.

"The cost of the basic crystal is a major factor," Barrett said. "We have bought crystals from Digirad and from eV and both charge about $3500 for a 1-inch-square detector."

Barrett is hoping that overseas suppliers might be found. One such source that promises to be an order of magnitude less expensive may exist in the former Soviet Union.

"That's one of the things we'll be investigating," he said.