PET/MR: Finding its Niche

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 - PET/MR Whole Body Scan
Whole-body PET/MR scan
Source: Siemens Healthcare

The final diagnostic form the evolving hybrid PET/MR tree will take is uncertain, but it has already begun to blossom. In fact, low-hanging clinical fruit has lured a number of major medical centers to early adoption.

The attraction is combined PET and MRI images, with the potential of opening new diagnostic opportunities. Fused PET and MRI data, for instance, allow temporal registration and visualization of diseased tissue as it proliferates over time at the molecular level. This is expected to help identify various disease states sooner.      

Despite the potential clinical roles, the high cost of PET/MR units, as well as the increased size of the machine, could inhibit many hospitals from embracing the technology as soon as they may wish.  

Frost & Sullivan estimates the cost of a PET/MR hybrid to be between $2.5 and $3 million, slightly higher than the $1.9 to $2.4 million price tag for a PET/CT scanner.

Best of both worlds

PET/MR appeared on the diagnostic scene nearly a decade after PET/CT. Design changes in PET and MRI scanners were necessary to overcome a number of obstacles; for instance, MRI magnets distorted images from nearby PET scanners, and there wasn't space enough inside an MRI system to house a PET scanner.

Among the design changes, LCD workstations prevent image distortion and higher-performing gradients free up enough space to accommodate a PET inside the MR gantry, while at the same time widening the bore. FDA approval in June paved the way for clinical use in the U.S.

The fusion of PET and MRI images and data will capitalize on the strengths of each, providing a hybrid technology that is greatly superior to the sum of its parts, says Habib Zaidi, PhD, head of the PET instrumentation and neuroimaging laboratory at Geneva University Hospital.

With PET/MR, MRI images provide anatomy and tissue characterization combined with metabolic imaging obtained from PET, making it possible to see tissue function and metabolism.

"More importantly, using simultaneous rather than sequential scanning will enable the resolution of many of the impediments to precise co-registration of anatomo-molecular information," Zaidi says.

The immediate impact of PET/MR will likely be most significant in oncology, where the combination of PET and MR in one imaging device offers advantages over conventional imaging modalities. Oncologic application of PET/MR allows imaging of the four main processes in cancer formation: apoptosis resistance, angiogenesis, proliferation and metastasis.

"PET/MR will take functional characterization of tissue to another level, and allow one to marry the beautiful soft-tissue contrast of MRI with the tissue characterization of PET," says Vikas Gulani, MD, PhD, director of MRI at Case Western Reserve University in Cleveland.

Moreover, Gulani says, study contrasts will not necessarily be limited to customary FDG. The goal is to produce contrast agents that will light up in particular disease or physiological states.

"While molecular imaging with MR is of major interest worldwide, particularly because of the high tissue contrast of MR, it is made difficult by the inherently low sensitivity of MR," Gulani says. It is anticipated that PET/MR will allow radiologists to combine the best of both worlds in molecular imaging, beyond FDG.

In the long run, various molecular imaging possibilities could open up, Gulani says.

"The limited role of PET/CT in certain clinical indications, such as central nervous system disorders, orthopedic infections, inflammatory disorders and in the evaluation and follow-up of metastatic disease, is well-established," Zaidi says. "The better soft-tissue contrast observed on MRI is also well-established and emphasizes the ineffectiveness of PET/CT for this indication, as well as the potential role of PET/MR."

Constellation of configurations

Breast cancer. Source: Geneva Hospital/Philips Healthcare

PET/MR has been rolled out in three different design configurations.

In one design, the PET and MR systems are operated separately, but are positioned in the same room (Ingenuity TF PET/MR, Philips Healthcare), about three meters apart at opposite ends of a patient turntable. The patient, fitted with MR coils, is first scanned by the 3T system, then withdrawn, rotated 180° and slid into the PET bore. This allows for sequential acquisition and later fusion of data.

"The advantage is the patient will be in the same reference frame when imaged in PET and in MRI, so you