New scintillators promise faster, more accurate PET images

The development of new crystal technology is crucial to the advancement of PET imaging. Executives at ADAC Laboratories are betting on GSO (gadolinium oxyorthosilicate) for their flagship PET system, Allegro.

Crystals convert high-energy photons into flashes of light that are recorded instantly, transmitted, and assembled into images. To a large extent, they will determine the efficiency and success of future PET systems. GSO enables more “counts”-recorded flashes of light-to be collected more rapidly, according to Josh Gurewitz, director of nuclear medicine and PET marketing at ADAC, a Philips Medical Systems company. The material is four times faster than the most widely used scintillators, bismuth germanate (BGO) and sodium iodide (NaI), promising higher quality images and faster throughput, he said.

”GSO is a fast scintillator; it has very high stopping power; and when you compare it to other scintillators, the energy resolution is among the very best,” Gurewitz said.

The immediate effect of these advantages according to Steve Atkinson, ADAC’s director of PET tactical marketing, is high sensitivity, high count rate capability, a low number of erroneous counts, and reduced scatter.

”These qualities are essential for the widespread adoption of PET, which demands 3-D data acquisition as a means to increasing throughput,” Atkinson said.

NaI and BGO limit PET throughput by requiring whole-body scan times of 40 to 60 minutes. GSO promises to trim that to 25 minutes or less, according to Gurewitz. Traditionally PET data from scanners using BGO-based detectors have been acquired in the two transaxial planes: the x- and y-axes across the patient. Data from the BGO scanners have generally not been acquired in the head-to-toe plane, because there was nothing to control the scatter. The use of a high-performance scintillator with good energy resolution allows the GSO systems to reduce scatter through electronic means. This enables more efficient 3-D imaging.

”This means you are acquiring a richer data set, because you’re collecting counts from an increased number of angles and this leads to more true counts,” Atkinson said.

GSO was so important to the development of ADAC’s Allegro that engineers designed the scanner’s detectors to optimize the properties of the crystal, creating a continuous light guide for uniform energy distribution.

The roots of GSO development go back three years to when ADAC Laboratories, working in concert with the University of Pennsylvania, began the R&D process that might lead to the next-generation scintillator. University researchers led by Gerd Muehllehner, Ph.D., an adjunct professor of radiologic physics and Philips’ director of PET technology, were looking for a material exhibiting the best qualities of the current generation of materials: the superior energy resolution of sodium iodide and the excellent stopping power of BGO. At the time, the popularity of PET was growing on the heels of expanding reimbursement and expanding recognition of clinical utility, and ADAC wanted to make the most of it.

”PET has been a clinical modality for over a decade and a research modality for nearly 30 years, but only in the last four or five years has the return been there for manufacturers to invest the R&D effort to take the technology to a new level,” Atkinson said.

The discovery and development of GSO involved scientific analysis and a bit of serendipity, he said. The research team focused on oxyorthosilicates, because these glass-like substances have attractive scintillation properties. They efficiently convert photons into visible light, then quickly return to a passive state-the perfect combination to boost throughput while producing higher quality images.

Muehllehner headed ADAC’s research team, which focused on several members of the oxyorthosilicates family, including GSO. They examined pure samples as well as combinations of materials. In the end ADAC chose pure GSO. One reason was its ability to stop and convert particles to light, then revert quickly to a passive state. (The shorter the scintillation event the better; more events can be counted.) Another is the material’s uniform crystalline structure, which provides good energy resolution. GSO is highly stable, regardless of temperature changes. It also has the advantage of not emitting background radiation, which occurs naturally with some other types of oxyorthosilicates.

The search continues for materials that perform even better. This search ultimately will push the performance of PET scanners to new levels of image quality and throughput, Gurewitz said.