PET/CT scanners proliferate and add new capabilities

PET/CT has come a long way. Improved PET detectors with better optical response, faster electronics, and software that rapidly matches CT and PET scans have dramatically advanced the hybrids, promising whole-body exams in 15 minutes or less. Yet nothing says progress like number of slices. The CT component has marched forward from quadslice to eight-slice, then from eight to 16. Now 64-slice PET/CT is at hand.

The big three vendors in PET/CT will be showing their superpremium wares at the RSNA meeting. Siemens Medical Systems and GE Healthcare talked up 64-slice versions of their PET/CTs at the 2004 meeting, and Philips countered with talk of a 64-slice hybrid at the June 2005 Society of Nuclear Medicine meeting. While these hybrids may be on the exhibit floor in Chicago, however, none is expected to become widely available until next year at the earliest.

The supporting casts for these marquee stars will garner most of the commercial attention on the exhibit floor. Slices will range from as few as two to as many as 16. This comes as no surprise to anyone familiar with CT, where vendors have populated virtually every niche imaginable with scanners of varying capabilities. The proliferation of PET/CTs, however, prompts questioning as to how many slices are enough, too few, or even too many.

The University of Rochester chose 16 slices for the PET/CT installed in midsummer at its University Imaging at Science Park center. The freestanding imaging center affiliated with Strong Memorial Hospital, which is part of the University of Rochester, fills unbooked time on its PET/CT doing diagnostic CT scans.

Vendors claimed, when they first introduced PET/CT four years ago, that the CT component could be used separately while referrals for PET built up. Now that argument has been turned to influence the choice of CT. University Imaging expects eventually to fill all its available time on the PET/CT doing positron-related studies, according to codirector Dr. John G. Strang. In the meantime, however, the 16-slice CT can cover any radiological need that comes along.

St. Luke's Hospital in Milwaukee uses an eight-slice PET/CT. Eight is enough, said Dr. David L. Yuille, a St. Luke's staff physician board-certified in both radiology and nuclear medicine. The medical center, known throughout Wisconsin for cardiac care, has instead dedicated its PET/CT to oncology.

"There has been very little use of the scanner for myocardial viability," Yuille said. "We looked at it and said forget it."

The cost of a rubidium generator and the relatively low reimbursement for cardiac scans were deciding factors, he said.

Vendors will urge RSNA meeting visitors to consider technologies that make the most of their slices. GE's Discovery ST will be shown in four-, eight-, 16-, and 64-slice versions. Philips' Gemini portfolio will include two-, six-, 10-, 16-, and 64-slice configurations. The biograph line will offer many of the same slice choices as Gemini, but without the 10-slice midstep. Hitachi Medical will join them with its Sceptre P3, a rotational PET scanner outfitted with a quadslice CT.

Among the distinctions will be each company's crystals. GE's Discovery ST uses bismuth germinate. Siemens' biographs and Hitachi's Sceptre P3 (with PET components supplied by Siemens) use lutetium oxyorthosilicate. Philips' Gemini scanners are built around gadolinium oxyorthosilicate.

The general designs linking a PET and CT end to end are roughly the same, although Philips has engineered its hybrid with an opening between the two scanners. Its OpenView gantry, a staple of the Gemini line, provides a less confining environment for patients, according to the company.

While choices will abound, much of the emphasis will be on 16-slice configurations. Philips has been shipping its Gemini GXL 16 Power configuration for about a year. In March, the company upgraded the system with enhanced postprocessing and connectivity to PACS, HIS/RIS, and radiation therapy planning systems, as well as cardiac acquisition, review, and analysis packages. Philips is planning to migrate by year's end at least some of these capabilities to lesser slice scanners, including its two-slice version.

With its billion-dollar acquisition of CTI Molecular Imaging completed in midyear, Siemens now has all the pieces to its PET/CT puzzle. The company, which had been in partnership for decades with CTI for the supply of PET components, now can leverage both sides of the technological equation: advanced PET detectors and electronics, and the z-Sharp and Straton x-ray tube technologies, which allow the acquisition of two CT slices almost simultaneously. The distribution of PET radiopharmaceuticals through PETNET, obtained through the CTI acquisition, will buoy the use of these systems.

GE will highlight its Discovery STE, the "elite" version of its ST product. Discovery STE was first shown at the SNM meeting five months ago. GE will frame the system at the RSNA meeting as the means for satisfying current oncologic applications as well as emerging demands in neurology and cardiology. The cornerstone of the enhanced product is its Dimension Console, which streamlines clinical protocols such as IV contrast administration to allow diagnostic CT scans, tailors x-ray dose to minimize radiation exposure, generates static and 4D motion studies in a single acquisition, and allows advanced 3D and 4D postprocessing.

Workflow tools may be critically important. A quarter or more of the CT acquisitions done as part of PET/CT exams at St. Luke's are diagnostic, according to Yuille. These are typically accompanied by a low-dose study for attenuation correction. Both are interpreted.

"We can evaluate the bones better on the nondiagnostic CT than on the diagnostic CT because it is a thinner slice," he said. "I know you can reformat all this stuff, but then you are going back and telling the techs to reformat (the diagnostic CT) on thinner slices to look at bone. You are not going to do that when you're busy."

The need for more slices enters the picture with cardiac applications. Sixteen-slice CTs are required for the coronary CT angiography work being explored at Duke University Medical Center. But 16-slice PET/CT may be necessary even for sites not interested in such studies of the heart, said Dr. R. Edward Coleman, chief of nuclear medicine and vice chair of radiology at Duke.

"You need to meet whatever is considered the standard for diagnostic CT in your practice," he said. "Patients getting PET scans frequently need diagnostic CTs, and if they were not done on our scanner, they would go elsewhere in our department."

Coleman notes, however, that an eight-slice scanner may meet the standard of care. It does at St. Luke's, according to Yuille, who considers investing in this level of technology wise over both the short and long term. GE has assured the medical center that its eight-slice Discovery scanner can be upgraded to 16 slices in the future.

"As long as you have companies willing to upgrade your CT without upgrading the PET, you are probably safe with an eight," he said.

The picture for PET/CT may be muddied even more in the future, if SPECT/CT catches on. Philips, Siemens, and GE each offer at least one such hybrid. For the time being, GE is sticking with its single-slice Infinia Hawkeye, although it is looking into multislice versions. The Infinia Hawkeye 4, shown as a work-in-progress at the SNM meeting in June, reflects the company's cautious approach to a historically cost-sensitive marketplace. By linking a SPECT camera to a CT built only for attenuation correction and localization, GE might keep its overall costs below a half-million dollars and within reach of most nuclear medicine departments, according to the company.

Competitors have not exercised such caution. Philips offers six- and 16-slice Precedence SPECT/CTs and has announced plans to bring a 64-slice SPECT/CT to market possibly by the end of 2006. Siemens offers SPECT/CT in single-, dual-, and six-slice configurations as part of its Symbia TruePoint portfolio.

Vendors hope SPECT/CT will do for gamma cameras what PET/CT did for positron imaging. The great unknown is what SPECT/CT may do to PET/CT. Yuille sees little future for 64-slice PET/CT. But a superpremium CT linked to a SPECT camera is another story.

"The key is attenuation correction," he said. "If you have an attenuation-corrected MIBI image, it would probably be nearly as accurate as a rubidium image and cost a lot less."