Multidetector systems prepare to test concept for new PET scanner platform

Siemens, CTI bank on LSO as detector crystal for the futurea

A novel pair of dedicated PET scanners that use detectors rather than rings have received FDA clearance--but neither will be commercially available any time soon, according to their developer.

The systems, the Vision 3000 and 5000, are being developed by CTI PET Systems (CPS) Innovations, a joint venture of CTI Molecular Imaging and Siemens Medical Solutions. CPS received 510(k) clearance for the systems in August (SCAN 10/25/02). Many of the systems that gain FDA clearance from August to November are showcased at the RSNA meeting later that year, but this will not happen for the Vision series.

"We are going to place them in select research institutions strictly as part of a research platform," said Brad Herrington, senior director of product marketing at CPS. "The FDA clearance allows us to get clinical data for validation without having to get IDE investigational device exemption or institutional review board approval."

Herrington would not speculate on when or whether a product based on the concept might appear.

"The promise is there, but we have a lot of work to do," he said.

The real news at the upcoming RSNA meeting, according to Herrington, is the company's switch to LSO (lutetium oxyorthosilicate) crystal technology and the integration of a multislice scanner on its PET/CT hybrid. This system is marketed by CTI as the ECAT Reveal and by Siemens as the biograph. The decision to switch from the widely used BGO (bismuth germanate) crystal underscores the clinical advantages of LSO technology, which promises substantially reduced scan times.

The first PET/CT systems incorporating LSO were installed this summer at the University of California, Los Angeles, and the University of Southern California, where exams now typically take between five and 10 minutes, Herrington said. The speed is largely due to the use of LSO. The crystal features better efficiency or stopping power than BGO, faster scintillation, and five times the luminescence, according to proponents, giving it a distinct advantage in speed, image quality, and even patient comfort.

Usually, patients undergoing PET/CT are scanned with their arms at their sides, because they cannot comfortably keep their arms motionless over their heads for long periods of time. This position may cause beam-hardening artifacts on CT, however.

"Now that PET scans are down to a reasonable time, patients can be allowed to put their arms over their heads, and, utilizing the CT scan as attenuation correction, we come up with a low-noise data set," he said.

CPS hopes to drive home the advantages of LSO during the RSNA meeting--not the advantages of the still-experimental Vision series of scanners, Herrington said. The new series is more a validation of the concept that detector heads rather than rings can be used effectively to gather counts during positron imaging.

The Vision systems combine rotating flat-panel detector heads and coincidence point source transmission scan capability. This design makes the Vision 3000, which is configured with three detector heads, and the Vision 5000 with five heads fundamentally different from other PET scanners developed by CPS and marketed by CTI and Siemens. Both the Accel and Emerge, for example, depend on ring detectors. The Accel uses a full ring and the Emerge employs a rotating partial ring.

The Vision systems are not a complete break with the past, however. LSO-based detector heads and coincidence point source transmission scanning first appeared on the Siemens e.cam LSO 3111, which allowed PET/SPECT imaging. The e.cam system was never commercialized due to "marketability issues," said Vilim Simcic, vice president and general manager of PET for Siemens.

Simcic did not elaborate on what those issues were, but cost was likely one of them. LSO is inherently more expensive to produce than other crystals. At the time the e.cam LSO 3111 was being developed, coincidence cameras were viewed as a cost-effective offering between conventional gamma cameras and PET scanners.

"The cost of producing LSO is coming down (with increasing volume)," Herrington said. "Not as rapidly as we would like, but customers are seeing the value of the extra cost of LSO--and they are paying for it."

Unlike the e.cam LSO 3111, the Vision series is being developed as a dedicated PET scanner. Any commercial successors to the Vision prototypes, therefore, would be less affected by cost concerns than if the system were primarily a SPECT system.

The ability to configure a PET system with multiple detector heads could, in fact, become a selling point. Simcic said this concept could provide the basis for a family of PET scanners whose members could offer different levels of performance at different price points. Better performance might be achieved by going from three to five heads, for example. For this, customers might be willing to pay more, just as they pay a premium for gamma cameras configured with detectors and CT scanners with detectors capable of multiple slices. This approach differs markedly from the current state of PET productization.

"If I wanted to change the performance/price points for the Ecat Accel or any PET product that has a fixed detector ring, it would be difficult, if not impossible, without reengineering the product," Simcic said. "With panel detectors, the customer could decide at the point of ordering whether a lower cost, lower performance system or a higher cost, higher performance system is desired."

Multiple detector heads offer other advantages than scalability, including potentially higher performance, higher value for the customer, and simplified inventory management, he said. The Vision systems are expected to be priced comparably to ring PET systems, but if productivity improvements were demonstrated with the Vision series, CTI and Siemens might even be able to charge a premium.

Although the price range and marketing strategy for the Vision series have yet to be determined, Simcic said Siemens has the right to market every system manufactured by CPS. When or even whether that right will be exercised, however, is still up in the air.

"It's very difficult to see how we'll position this system relative to all the other systems," he said. "The data just aren't there yet."