Innovative component allows development of PET/MR hybrid

The fact that metal and MRI do not mix has, until recently, thwarted attempts to develop hybrid PET/MR scanners. Now engineers are maneuvering around the problem by eliminating the metal from PET scanners.

The main stumbling block has been the photomultiplier tube, an essential component of a PET system. PMTs contain enough metal to disturb the magnetic field of an MRI system. Because field distortions translate into image artifacts, attempts to combine PET and MRI have not succeeded.

Avalanche photodiode chips are helping overcome the problem, however. APDs can play the same role as PMTs in measuring the timing and location of scintillation events in an MR scanner's crystal detectors and translating that information into electrical impulses.

APDs consist mainly of silicon, which poses little threat to MR field homogeneity, and they are extremely small, which is a plus for a PET detector system that must slide compactly into an MRI magnet bore.

Siemens Health Solutions engineers based in Erlangen, Germany, working with colleagues at the former CTI PET Systems in Knoxville, are assembling a prototype PET/MR scanner that overcomes the technical barriers to clinical use of such a hybrid system. The prototype will be equipped with a tubular PET insert, featuring APD detectors in lieu of photomultiplier tubes, inserted into the bore of a commercially available 3T MR scanner.

MR and PET images will provide anatomic and physiological data, similar to fusion imaging with PET/CT, said Dr. Arne Hengerer, director of Siemens' MR molecular imaging program.

Siemens' prototype and the first clinical scanner based on the platform will address neurologic applications, according to lead project engineer Ralf Ladebeck. That approach was necessary because the current PET/MRI insert provides only a limited field-of-view. The prototype will be completed in 2006.

Many of the same goals that led to development of PET/CT have spurred the quest for a workable PET/MRI system, said Bernd Pichler, Ph.D., a medical physicist at the University of Tubingen in Germany. Both hybrid approaches fuse physiological data acquired with FDG-PET and anatomic information captured by either CT or MRI to identify the origins of metabolic activity.

But because MRI does not involve ionizing radiation, PET/MRI may be preferable to PET/CT for pediatric applications and for patients requiring repeat imaging. MR's high-contrast resolution could make its pairing with PET the first choice for brain studies, stem cell trafficking, and other applications in which the clinician must be confident of the modality's soft-tissue characterization, Pichler said.

Radiologists would also like to explore the diagnostic potential of combined PET and functional MRI studies. The ability to simultaneously acquire FDG-PET images with MR spectroscopy, blood oxygen level-dependent, or diffusion tensor imaging could improve the characterization of multiple sclerosis, Parkinson's disease, Alzheimer's disease, and other brain disorders.

The PET insert pairs APDs in a block configuration with lutetium oxyorthosilicate crystal scintillators to maximize their energy resolution within the confined space, said Matthias J. Schmand, Ph.D., director of detector R&D at Siemens. LSO provides a better light yield than the alternative bismuth germanate detectors. The diameter of the insert is about 5 cm overall, reducing the width of the MRI bore by 10 cm.

Phantom testing of an experimental configuration of the scanner produced a 3.2 cm2 field-of-view. Structures as small as 2 mm could be resolved with the PET detector, and objects as small as 1 mm could be seen with a host MR scanner. The signal-to-noise ratio of the MRI equipped with the insert was about 15% lower than after the component was removed. The SNR loss for the PET component was 20%, Hengerer said.

Siemens engineers are aiming at 2.5-mm isotropic resolution with the prototype system, Schmand said.

"The goal is to develop a system so we have no compromises," Ladebeck said.

Mr. Brice is senior editor of Diagnostic Imaging.