PET/MRI: Reflections Two Years After FDA Approval

Siemens Biograph mMR

Philips Ingenuity TF PET/MRI

This is the first in a two-part series investigating PET/MRI’s status and future two years after FDA approval. Part two examines implementation and reimbursement challenges.

When the FDA approved the Siemens Biograph mMR system two years ago, it became the first (and still only approved) machine to perform PET and MRI scans simultaneously. Philips’ hybrid system gained FDA approval soon after, allowing for similar results with sequential scanning. In those two years, the handful of research centers adopting the technology continue to struggle with learning how to integrate the system with their workflow, comparing the utility of PET/MRI versus PET/CT, and determining which organs and disease processes are best imaged with PET/MRI.

PET/MRI vs PET/CT

The big question for those using a PET/MRI system is how it compares to PET/CT. When introducing PET/MRI to a medical center, the physicians incorporate studies to compare the results. At Texas Children’s Hospital (TCH) in Houston, Victor Seghers, MD, PhD, head of the pediatric radiology nuclear medicine section, said he’s optimistic they’ll prove equivalence, though they need sufficient data. Their Philips machine was just installed in May and they’ve only completed five studies so far.

Seghers said their research protocol is to perform a clinical PET/CT, followed by a research PET/MRI. They perform them sequentially, as the PET/CT and PET/MRI machines are across the hall from each other. They also use the same PET tracer dose from the clinical PET/CT, so they don’t expose the patient to additional radiation.

David Gilmore, MS, CNMT, RT, program director of Regis College’s Medical Imaging Programs in Weston, Mass., noted some of the differences he’s seen in some facilities. “With PET/CT, we’ve been able to localize things. Now we can give more precise location with MRI than with CT. MRI has the capability of doing functional imaging, combined with physiology-based imaging of PET. We’ve doubled the amount of things we can see with this.”

[[{"type":"media","view_mode":"media_crop","fid":"13924","attributes":{"alt":"","class":"media-image media-image-right","id":"media_crop_6758298614100","media_crop_h":"0","media_crop_image_style":"-1","media_crop_instance":"654","media_crop_rotate":"0","media_crop_scale_h":"0","media_crop_scale_w":"0","media_crop_w":"0","media_crop_x":"0","media_crop_y":"0","style":"margin: 5px; float: right; height: 92px; width: 100px;","title":"Omer Aras, MD","typeof":"foaf:Image"}}]]Since MRI provides more biological and functional data than CT - without radiation -the system is a huge advantage for pediatric patients and those needing multiple scans, said Omer Aras, MD, an incoming assistant attending radiologist at Memorial Sloan Kettering  Cancer Center in New York. 

While CT is better at evaluating small lung metastases, “MRI provides higher spatial resolution definition of tumor volume and local disease extent than CT for staging tumors in the brain, head and neck, spinal cord, liver, pelvis, breast and musculoskeletal system,” said E. Edmund Kim, MD, MS, radiology professor at University of California at Irvine and professor of molecular medicine at Seoul National University in Korea.

Aras added that PET/MRI scans are also more accurate at detecting and staging prostate cancer and possibly pancreatic cancer as well.

Changing the Protocol

Yet, more than just proving equivalence between PET/CT and PET/MRI, Seghers said he’d rather see if PET/MRI can change existing imaging protocols. MRI inherently provides more tissue characterization, he said, so if it can replace CT for anatomical screening, it adds value as well as decreases radiation exposure. Segher’s goal is to show superior data from PET/MRI alone, not as an addition to PET/CT. He wants to avoid using the MR sequences only for attenuation correction of the PET images. He’d like to see if all imaging can be done in one study.

For example, lymphoma patients often get multiple diagnostic CTs of the neck, chest, abdomen and pelvis, in addition to multiple PET/CTs. Seghers would like to investigate whether radiologists could perform a PET/MRI to replace the diagnostic CT and PET/CT. This would not only significantly reduce radiation exposure (especially given the number of repeat studies during treatment), but decrease the number of sedations required for pediatric patients. This would decrease risk and cost, and demonstrate safety and clinical value.

Seghers said he is hoping that researchers can create new types of studies where MR images and PET data can answer additional medical questions for therapeutic use. MRI can address questions of blood perfusion. If a tumor is shown to be metabolically active via PET, but shows poor tissue perfusion, does this have management implications for chemotherapeutic regimens? Should a physician change the chemo treatment if the patient has a cancer without enough blood flow?

Another topic is lymphoma treatment. MRI is good at detecting bone marrow edema, which indicates that lymphoma is spreading to the bones, Seghers said. Patients with lymphoma often receive PET scans as well as bone marrow biopsies. Can a PET/MRI offering information about marrow edema replace a painful biopsy? 

Siemens v. Philips

Currently there are two FDA-approved hybrid PET/MRI systems. The Siemens Biograph mMR simultaneously scans the patient with both modalities. The Biograph mMR is installed in eight U.S. locations, with an additional 25 internationally, according to Robert C. McKinstry, III, MD, PhD, radiology professor at Washington University School of Medicine.

Philips Ingenuity TF PET/MRI offers sequential scanning, and is equipped with Time-of-Flight PET scanning technology that measures the time between photon events, to improve image resolution. The Ingenuity has a scanner at opposite ends of the table, rotating the patient between the two scanners.

Washington University installed the machine almost two years ago, acquiring it soon after FDA approval in June 2011. McKinstry called it an “amazing piece of engineering,” adding, “The diagnostic future is spectacular.”

What impressed McKinstry was the clinical data gleaned from the Philips machine, which he saw when a colleague presented during a panel discussion. “I think that it’s a good transitional solution for that vendor,” he said. “They have a pretty robust solution given the layout and complexity of doing scans serially. They’re getting good data out of it.”

However he doesn’t feel it’s sufficient for the research work they’re doing at Washington University. “They can’t do the kind of science we’re doing,” he said. He thinks the Philips machine works well for routine replacement of a PET/CT scanner with a PET/MRI system for oncologic or neurological use. “That solution will be very viable,” he said.

Texas Children’s researched the technology for two years, Seghers said, and installed the Philips Ingenuity in May. They considered many factors when deciding between the machines. “On the Siemens’ side, it’s exciting to have simultaneous acquisition of PET and MR data,” he said. “However there are no clinical applications for simultaneous acquisition and we felt that in real-time scanning, neither the first-generation Siemens nor Philips PET/MR scanners would offer a significant speed advantage. However, the future of PET/MR does lie in an integrated machine to further decrease length of the scans, so as to take advantage of PET and robust MR sequences as well as to improve patient throughput.”

Another factor was cost. Texas Children’s made several other large equipment purchases at the same time, so they enjoyed some cost savings, plus the Ingenuity TF has a lower price tag compared to the Siemens machine.

Research or Clinical Use

Washington University is using the Siemens machine for research purposes only. As a research facility, physicians are interested in more than just dose reduction, McKinstry said. They also want to advance medical science and neuroscience. “The machine is perfectly suited to understand the relationship between structure and function and between metabolism and cerebral hemodynamics,” he said, noting that they could simultaneously image blood vessel perfusion and metabolics. “It will allow you to answer questions you can’t do serially.”

The department scans 10 to 15 patients per week on PET/MRI, all of whom are on study protocols, McKinstry said, and the hospital plans to start some purely clinical cases in a month.

Memorial Sloan Kettering will get their Siemens machine in the next six to 12 months, said Aras. They’ll be using it for diagnostic clinical work, though initially they’ll scan a small group of patients to compare the data sets to understand the benefits of PET/MRI over other imaging modalities. Later, they’ll use the machine for broad spectrum applications. “Memorial Sloan Kettering Hospital has a lot of cancer patients and rare cases no one else sees, so it will be an interesting application to use it,” Aras said.

While most facilities are using it for research, said Seghers, TCH is eager to use it for clinical work. “We’re the first freestanding children’s hospital to have a PET/MR,” he said. “We feel a great pressure to get it up and running to get radiation dose savings for the children we’re imaging.”

Indications for PET/MRI

The two major foci for researchers and clinicians using PET/MRI are neuroscience and oncology. “For us, the big wins are going to be dementia evaluation and cancer patients,” said McKinstry. “The area that has exploded the most here is dementia imaging, with the approval of amyloid tracers and the burgeoning field of tau imaging tracers.” For Alzheimer’s disease, he’d use PET/MRI predominantly to exclude the diagnosis, or to follow the disease course, though it’s still more in the research realm.

In terms of other neurological use, Seghers said that TCH will be using PET/MRI for epilepsy patients, looking for seizure foci.

Oncology is the other current focus. “We’re excited about the simultaneous application with FDG and new tracers to look at various aspects of oncologic imaging,” separate from the tumor metabolism, said McKinstry. MRI is much better than CT at imaging the pelvis, a complex anatomical structure CT. The liver will also be better imaged with PET/MRI. “The combination will help us sort out things that at the moment is difficult with PET/ CT,” he said.

PET/MRI is also good for imaging brain tumors, lymphoma, sarcoma, and Langerhans cell histiocytosis, said Seghers.

There’s talk of breast cancer being another potential application. MRI for breast cancer is much more innovative than CT, said McKinstry, so [[{"type":"media","view_mode":"media_crop","fid":"13922","attributes":{"alt":"","class":"media-image media-image-right","id":"media_crop_207400066928","media_crop_h":"0","media_crop_image_style":"-1","media_crop_instance":"653","media_crop_rotate":"0","media_crop_scale_h":"0","media_crop_scale_w":"0","media_crop_w":"0","media_crop_x":"0","media_crop_y":"0","style":"float: right; margin: 5px; height: 150px; width: 100px;","title":"Robert C. McKinstry III, MD, PhD","typeof":"foaf:Image"}}]]combining the dynamic information gleaned from MRI with PET’s metabolic and tracer information will provide future growth. “We don’t think (PET/MRI) will replace PET/CT for older adults who have cancers easily worked up by PET/CT, but selected applications like pediatric malignancies, pelvic malignancies, head and neck cancer, those areas are going to be extremely exciting.”

Further down the line are cardiac exams. McKinstry wants to develop a heart exam to leverage knowledge of myocardial infusion scanning, taken to the next level. He anticipates that a comprehensive PET/MRI myocardial exam will be better than some other studies, though that remains to be proven.

A number of tracers are available for cardiac PET/MRI, the University of California’s Kim noted, including C-11 hydroxyephedrine which is useful for delineation of cardiac regional innervation and control mechanism in patients with dilated cardiomyopathy or after cardiac transplantation. Neoangiogenesis can be evaluated with PET using F-18 galacto-RDG and MRI in tumor as well as infarcted myocardium. To understand myocardial or tumor apoptosis, F-18 caspase-3 is useful.

Siemens and Philips have done a good job of marketing the technology, and showing the benefits primarily to the academic centers, said Gilmore. “It’s new and on the radar - hybrid technology is not going away.”

Part two of our series will focus on the difficult issues in integrating PET/MRI into the workflow, avoiding turf battles with reading the exams and running the machines, criteria to consider in purchasing a system, and the billing conundrum.