PET/MRI prototype adds new facet to hybrid scanning

Early research illustrates promise and problems

The popularity of PET/CT has led some visionaries to wonder about the viability of other such hybrids, particularly PET and MRI. Simon Cherry is one of them.

Cherry, a professor of biomedical engineering at the University of California, Davis, has developed a second-generation prototype PET/MRI system. The compact system, designed for imaging animals, is built around an MR-compatible PET detector ring consisting of small lutetium oxyorthosilicate (LSO) scintillator elements. These tiny detectors are positioned inside the magnet and read using optical fibers that transfer the light resulting from scintillation to multichannel photomultiplier tubes. The tubes and their associated electronics reside in a radio-frequency-shielded enclosure several feet from the PET detector ring and the MR magnet whose field surrounds it.

This configuration eliminates the need to place photon detectors and their electronics directly inside the MR magnet, which would be problematic because the magnetic field causes distortions. RF shielding and the separation of electronics eliminates most of the interference.

The system also incorporates a fiducial ring filled with MR contrast agent that provides the basis for registering MR and PET data.

Cherry describes his project as "end of the middle stage." He has been investigating PET/MRI since 1996 and began with a simple system he built from the ground up. Cherry moved onto the current-generation technology about two years ago.

Cherry's approach works well, at least when imaging animals. Problems would arise, however, if the system were sized to image people.

"When you go to human-sized scanners, the volume of fiber optics that you need becomes pretty unmanageable," Cherry said. "I suspect that to go into humans, we're going to need a more elegant solution."

Cherry hopes to further develop PET/MRI technology in collaboration with MRI researchers at the California Institute of Technology. Their grant application to the National Institute of Biomedical Imaging and Bioengineering proposes the development of improved PET/MRI technology, as well as novel dual-modality imaging tracers. The research, if funded, could move PET/MRI closer to clinical application, but human studies are at least several years away.

"Bringing these two modalities together is tough," Cherry said.

It may not even be necessary. The sale of PET/CT scanners has reawakened interest in the development and application of software that registers data sets obtained separately using different modalities. This may be all physicians need, according to Cherry.

"There's some question as to whether we can show applications with a combined PET/MRI that significantly improve the information you get versus doing two scans separately and using software techniques to register the data together," he said.

Animal studies now under way are aimed at resolving that question. Meanwhile, developmental work on a hybrid PET/MRI is progressing, although on a limited scale. For the most part, vendors of imaging equipment have shown little more than curiosity in Cherry's project. One reason is the difficulty in building such a scanner.

"There's a lot of value added with MR over CT, but it's also much harder to integrate the technologies," he said.

Size is the biggest problem, at least for the current concept of using fiber optics. This might be mitigated, if early human testing were constrained to brain studies. A dedicated brain scanner would be far more compact than one suitable for general-purpose imaging, Cherry said. Consequently, its construction would pose fewer problems.

But even if the technology does not pan out for clinical applications, PET/MR scanners might be commercialized. Hybrid systems might ultimately be developed and marketed for animal imaging alone, as these products would be far smaller and less expensive than scanners required for humans, Cherry said.