New PET crystal promises faster scans, better images

The worth of crystals used in SPECT and PET imaging is determined by how quickly they scintillate following the impact of a high-energy photon - and how long it takes the flash to extinguish. The faster this cycle is completed, the more photons can be counted by the detector, leading to better image resolution and faster scan times.

Siemens Medical Solutions and CTI Molecular Imaging have been talking up the high performance capabilities of their LSO (lutetium oxyorthosilicate) crystal since 1999. Executives at Saint Gobain Crystals, the world's largest producer of detectors for gamma cameras, say their new crystal is better.

The crystal, called BrilLanCe 380, is at least twice as fast as LSO - so fast that it will allow time-of-flight imaging for PET. Time-of-flight provides information about where the event (the annihilation of an electron by a positron, spawning a photon) took place inside the patient. In PET imaging, this would translate into substantially improved sensitivity.

"Currently, with coincidence detection, you know that the annihilation occurred somewhere on a line between the two events you detect," said Dominique Rothan, marketing manager for Saint Gobain Crystals. "By detecting at which moment the photons arrive on the detectors, you are able to determine the position on that line of the annihilation and thereby better determine the location of the radionuclide."

BrilLanCe 380 is fast enough to do that, he said. It also is very sensitive, which further leads to a very low energy resolution and higher signal-to-noise ratio.

The crystal is named for its components: bromide (Br) and lanthanum (La) doped with cerium (Ce). Its development began some five years ago at the University of Delft in the Netherlands. Saint Gobain acquired exclusive license to the crystal and has since refined the process for its manufacture.

The market for PET technology has cooled this year, largely due to the expanding installed base of PET/CTs and relatively low throughput at clinical sites, some of which average only two or three patients per day. The weakening market for PET, however, does not diminish the upside potential of BrilLanCe, according to Rothan.

"The opportunity comes from having a better system that makes better images, that does time-of-flight," he said. "It improves the way you detect cancer. This is the main driver, particularly in heavy patients, who are difficult to image."

Several manufacturers of PET equipment - large OEMs noted for the development of clinical scanners, as well as small companies more focused on animal PET - are looking into the new crystal. Much of the work has been to study its properties. Although the crystal is available for prototyping, no one has yet taken this step.

Sure to appeal to OEMs is the likelihood that this new crystal will be not only faster but also less costly than the current gold standard LSO. The raw material for LSO is expensive and melts into a usable crystalline form only at very high temperatures - on the order of 2000º C, which adds to the cost. In contrast, the raw material for BrilLanCe is relatively inexpensive and melts at about 1000° C.

The actual cost of BrilLanCe for OEMs has yet to be determined, however, partly because a few bugs in its manufacture still have to be worked out. The biggest is a tendency for the crystal to break. Rothan is confident, however, that a process for overcoming this problem will be developed.

Another issue is that BrilLanCe must be kept dry, as moisture causes the crystal's performance to degrade. It will, therefore, have to be encapsulated. This should not pose a major challenge, however, as the same issue affects sodium iodide, which is routinely encapsulated as part of its integration into SPECT gamma cameras, he said.

Advanced electronics will have to be developed to take advantage of BrilLanCe's improved performance, particularly its time-of-flight capabilities. There is, however, no reason why the electronics couldn't follow the introduction of the crystal into PET scanners. Siemens and CTI did exactly that with their LSO-based scanners, which were commercially released some three years before a data pipeline was available to capitalize on their faster count rates (SCAN 1/8/04).

Another speed bump on the road to market is the need to install equipment that can produce the crystal in quantities large enough to satisfy demand coming from OEMs. This could take up to two years, Rothan said, from the time Saint Gobain signs its first contract with an OEM - and there is no telling when that will happen.

Some OEMs are deeply committed to their own crystals, having invested heavily in proprietary R&D programs. Siemens' partner, CTI Molecular Imaging, first announced its intent to commercialize LSO in 1999. Siemens showed the first LSO-based PET scanner two years later at the RSNA meeting (SCAN 12/26/01).

Rothan recognizes that Siemens and CTI will be hesitant to give up on their investments in favor of a new crystal. Other companies might be more interested, if they can be convinced that BrilLanCe 380 offers an edge over LSO. For the imaging community to reap the benefits of BrilLanCe, Saint Gobain will have to win these companies over, one at a time.