Fast PET crystals gain popularity as customers seek high performance

Philips, Siemens, CTI leverage proprietary technologies

PET technology is changing, metamorphosing at a molecular level. The performance of dedicated PET and PET/CT scanners depends on the type of scintillation crystal used in the detector, and oxyorthosilicates appear to be winning.

Siemens Medical Solutions reports that more than half of its orders last year were for scanners with the LSO (lutetium oxyorthosilicate) crystal, compared with about 30% of orders the previous year.

"LSO has become the crystal of choice, and we only see its popularity growing," said Jonathan Frey, Siemens' worldwide manager for PET marketing.

Philips Medical Systems is experiencing a similar increase in demand for PET scanners with high-performance crystal detectors. Like LSO, Philips' proprietary GSO (gadolinium oxyorthosilicate) promotes improved throughput and image quality by allowing more counts than BGO (bismuth germanate) and sodium iodide, the older, established crystal technologies.

"Fast scintillators allow 3D imaging, which provides the most counts you can get from a PET scanner," said Deepak Malhotra, Philips' senior director of PET marketing.

The use of 3D in PET scanning refers to data acquisition. Photons can be recorded in all three planes using LSO and GSO, due to those crystals' unique physical properties.

Philips and Siemens offer PET scanners with BGO scintillators as lower cost alternatives to their premium systems. But Philips promotes its high-performance Allegro and Siemens its Ecat Accel, both built from fast scintillators, as the wave of the future.

These vendors, along with CTI Molecular Imaging, sense an opportunity to gain not only revenue but a marketing edge over rival GE Medical Systems. That edge can be achieved for both dedicated PET and PET/CT scanners, according to company executives, because GE's PET systems are equipped with only BGO.

"When providers can do scans on one of our LSO-based PET/CT scanners in as little as seven minutes, why would they choose a competitor's product that takes much longer?" said Terry Douglass, CTI chairman and CEO.

The joint venture between CTI and Siemens, called CTI PET Systems (CPS), supplies both companies with LSO-based detectors. CTI reported booming sales last quarter, mostly attributed to the growing popularity of LSO. Douglass predicts that LSO-based products will soon account for all CTI sales of PET scanners (DI SCAN 2/19/03).

Siemens' sales far surpass those of CTI, in large part because of Siemens' name recognition and its well-established distribution channels in radiology. The company also expects an ongoing shift toward LSO, but Frey believes there will continue to be a market for the less expensive crystals.

Speed accounts for the rising popularity of LSO and GSO. After emitting a flash of light in response to being struck by a high-energy photon, both crystals rapidly return to a resting or passive state, ready to emit another flash in about 60 nanoseconds. BGO requires a period five times as long. The time needed to acquire the number of photon counts needed for a diagnostic quality image is thus much shorter for GSO- and LSO-based systems than those built around BGO. Shorter scans translate into increased productivity and improved patient comfort.

Scans with a dedicated PET scanner using sodium iodide or BGO take 40 to 60 minutes. GSO and LSO can cut that in half. PET/CTs with LSO or GSO detectors are even faster. Attenuation correction is completed using the CT rather than a standard, slower emission source, which reduces scan times to about 15 minutes, Frey said. CTI claims that one site can complete diagnostic scans in less than 10 minutes.

"One reason our providers want LSO-based scanners is that they hope and expect that demand will build to eight, 10, 12, or even 16 studies a day," Douglass said. "To be able to do that number of studies, you need an LSO scanner. This all fits together; it's what you need to build procedure volume and improve business."

The oxyorthosilicates are especially suited to clinical studies requiring high count rates. Frey believes there is a great untapped opportunity in rubidium scanning for cardiac perfusion that might be achieved using systems with fast scintillators. Research applications requiring high counts may also make the transition to the clinical mainstream in the future. The adoption of high-performance systems, therefore, could lay the groundwork for the later expansion of PET clinical practice.

In addition to faster scintillation times, the oxyorthosilicates allow 3D scanning, which means photons can be recorded in any of three planes, leading to more counts. BGO and sodium iodide are not suited to 3D imaging. They rely on septa, which collimate photons into two dimensions, the x and y planes. The ability to set a tight energy window using LSO and GSO gives these scintillators the advantage, as the window helps filter out image-degrading scatter and random photons from the PET radionuclide.

But LSO and GSO are not exactly alike. Experience in the user base indicates that LSO does not completely exclude scatter and random counts, although these counts do not appear to have a substantial impact on the images' diagnostic quality. The same may be true for GSO, as no method is completely effective at negating extraneous counts. GSO, however, has an advantage over LSO in this respect, according to Johann Fernando, Ph.D., Philips' director of PET science.

The crystal is more resistant to temperature changes than LSO, he said. As a result, the energy window can be set to a narrower range.

"This cuts down on the number of scatter and random counts," Fernando said.

One thing is certain in the world of PET: Customers are buying into the idea of high-performance crystals, despite their higher price. PET scanners outfitted with fast scintillators typically cost 30% more than the other products. Customers that buy the more expensive systems are planning for the future, Frey said, and companies that make them are recouping past investments.

"It's taken eight years to get where we are today with LSO," he said. "As for all new technologies, development costs have to be recovered."