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Simultaneous Acquisition
— A PET Detector Inside an MRI Bore Makes It Feasible Fusion technology has clearly demonstrated the value gained from the coregistration of anatomical and functional data. The most compelling example is the impact the PET/CT hybrid scanner has had on diagnostic imaging. Lately, a great deal of interest has focused on fusing MRI and PET images because of their potential advantages as hybrid imaging technology. Researchers anticipated that the MRI/PET fusion could provide higher resolution of soft tissue images, reduce radiation exposure for patients (compared with PET/CT), and offer the capability of real-time, simultaneous acquisition of MRI and PET data—which, among other benefits, could provide motion correction. As the MRI/PET technology has developed, those expectations are showing some signs of proving out. If it continues, the new hybrid could become a valuable imaging tool with potential applications in oncology, neurology, and cardiology. Its most immediate impact appears to be in neuroimaging. In recent years, the MRI/PET fusion has been successfully tested in the imaging of small animals, but industry observers didn’t expect to see a commercially available clinical system until the end of the decade. However, development has moved along at a relatively rapid clip. In 2006, Siemens Medical Solutions acquired the first images of human subjects. By the end of that year, at RSNA 2006, Siemens previewed its hybrid system (which it terms MR/PET) as an investigational device. The company expects to make the first version of this system commercially available in the United States this year. That first commercial system will be a head-only system designed for neurological applications, particularly brain tumors and neurodegenerative conditions such as Alzheimer’s disease, stroke, and epilepsy. “Those are the key applications we’re looking at with this initial version,” says Jeffrey Bundy, PhD, Siemens’ senior director of MR research and development in the United States. Complementary Modalities “Considered in conjunction, MRI and PET are very complementary to each other. Magnetic resonance provides high resolutions of soft tissue while PET yields important functional information.” Also driving the development, Bundy adds, was the development of new MR-compatible PET detectors that employ scintillation crystals inside the magnet in conjunction with external photo sensors. “That’s the advancement that really got MR/PET over the technical hump,” he says. The biggest advantage that this MR/PET system presents is its capacity for simultaneous acquisition, as opposed to sequential MRI/PET scanning. Previously, clinicians had to manually fuse MR and PET images on a workstation—a process that involved a great deal of time, or dedicated software. “Simultaneous acquisition is the most unique thing about MR/PET, and it was a major motivating factor in developing the hybrid,” Bundy says. “It’s different than PET/CT, which has side-by-side modalities. You perform the CT scanning first and then do the PET scanning.” Design Challenges In merging the modalities, the most substantial technical challenge involved the photomultiplier tubes (PMTs) used with PET to detect light. These tubes don’t operate well within a magnetic field, according to Bundy. Also, the limited space within a magnet made PET detector placement problematic. Thus, a standard PET detector would be unfeasible for use in the MR/PET system. Siemens circumvented the problems by using MR-compatible Avalanche photodiodes, rather than the conventional PMT. “Simultaneous imaging wasn’t the only target of development. We needed uncompromised MR and PET scans,” Bundy says. “MR and PET had to be the best they could be to provide the best diagnostic information. We didn’t want to compromise the quality of either modality just for the sake of putting them together. To do that, we needed the MR-compatible detector.” Those were the primary challenges in the hardware environment, according to Bundy. The challenge on the software side involved integration. “We had to provide a software environment that could handle the simultaneous acquisition,” he says. “We did that by streamlining the acquisition software so that [the] operator could handle both the MR and PET acquisitions as efficiently as possible. Of course, software integration will be ongoing as the system evolves.” System Design Essentially, when the PET cylinder is inserted into the MRI scanner, the result is a dedicated, head-only MR/PET system. Insertion accomplishes the simultaneous acquisition. In turn, simultaneous acquisition eliminates problems related to coregistration issues. “Any time you have to move a patient between different imaging studies, you will have registration issues,” Bundy says. “But if you can acquire the separate data at the same time, you then remove those issues. When you remove registration errors, you know for sure that what you see in the MR images correlates with the activity in the PET data, which is vitally important from a clinical standpoint.” Flexibility MR/PET also provides users with workflow advantages. It enables patients to undergo a session in one setting. They won’t have to be imaged at different times or on different days or in more than one location. “Everything can be taken care of in one session, and that presents a huge advantage, both for the clinicians and the patients,” Bundy says. “Obviously, patients don’t want to be in the hospital any longer than necessary. For clinicians, the immediate, practical benefit is that they can get the right answer faster and then move on to appropriate treatment.” Workflow Challenges Bundy adds that a facility may also have to consider how it handles radioisotopes’ space and shielding considerations. “A site that does FDG-PET would have different kinds of shielding and siting requirements than a site that will be using other types of isotopes,” he says. Also, staff will need to be cross-trained in MRI and PET, and to be made aware of all the complex safety issues. Currently, you won’t find many physicians and technologists well-versed in both MRI and PET. “People on the MR side will have to get familiar with PET acquisition, and vice versa,” says Bundy. “That’s something we are currently working on internally. We’re preparing our people to do this cross-training. When we reach the point where we’re rolling out MR/PET in large numbers, we’ll send our specialists into the field to do training at sites.” Enthusiastic Response “Interest in our first version has been across the board,” Bundy says. “It’s present in the smaller community hospitals all the way up to the large research institutions. We’ve been talking with all different types of customers, so I don’t expect that usage will be limited to the larger hospitals and academic centers.” Those customers, he says, appear enthusiastic about the technology’s potential applications, its impact on clinical workflow, and its utility in core PET and MRI research. “Many people would like to get their hands on this system,” he says. Their enthusiasm appears justified. MR/PET makes possible simultaneous acquisition of anatomical and functional information. It will not only advance neurological imaging; it has potential applications in other areas, such as cardiology. — Dan Harvey is a freelance writer based in
Wilmington, Del., and a frequent contributor to Radiology Today.
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