PET/MRI Enters the U.S. Market

The new imaging modality of positron emission tomography (PET)/magnetic resonance imaging (MRI) was introduced in the U.S. market in June. The Siemens Biograph mMR (molecular MR) gained U.S. Food and Drug Administration (FDA) 510(k) clearance as the first dedicated PET/MRI system. It is also the only system to combine both modalities into one machine, allowing simultaneous imaging of location, function and metabolic activity of organs in a single image.

"The Siemens PET/MRI system allows two tests to run simultaneously without having to move the patient to a different scanning system," said Alberto Gutierrez, Ph.D., director of the Office of In Vitro Diagnostic Device Evaluation and Safety in the FDA's Center for Devices and Radiological Health. "Minimizing changes in a patient's position between tests allows physicians to compare images more easily and helps them get the most accurate information possible."

The Biograph mMR uses MRI rather than computed tomography (CT) to produce detailed images of the internal structures of the body. Because MRI makes images based mainly on the concentration of water in the body, it can produce greater detail of nearly all the internal structures of the body as compared with CT X-ray imaging. This may provide the physician with additional information about a patient's condition.

Additionally, the Biograph mMR system allows physicians to acquire images at a significantly lower radiation dose compared to a PET/CT system. Although the radiation dose from the PET exam remains unchanged, MRI does not use ionizing radiation, so the entire ionizing radiation dose from the CT scan is eliminated.

In addition to Siemens, Philips Healthcare and GE Healthcare are also developing PET/MRI systems. All three vendors took different approaches in how their systems operate and in overcoming the technical issues of combining PET in or near intense MRI magnetic fields.  

Promising Future for New Modality
“PET/MR is likely to have a major impact in the future, as PET/CT did when it was introduced,” said Michael Graham, M.D., Ph.D., director of nuclear medicine, University of Iowa, during a press conference on key molecular imaging research at the 2011 Society of Nuclear Medicine (SNM) meeting in June. “I think we will see this technology proliferate over the next few years.”

He said it might be used to image complex soft tissue, such as the brain, head, neck and the pelvis. It will also be very appealing for pediatric imaging to help reduce radiation dose from CT scans.

While MRI provides exquisite morphological and functional details in soft tissue, PET goes further to investigate the human body at the level of cellular activity and metabolism.

PET/MR is a tailored imaging modality for women and children, said Bruce Rosen, M.D., Ph.D., professor of radiology, Harvard Medical School, director of the Martinos Center of Biomedical Engineering at Massachusetts General Hospital. He said it eliminates CT radiation dose and MRI is better at imaging soft tissue anatomy, such as female pelvic organs. It is also ideal for oncology; Rosen said MRI is good at imaging tumor angiogenesis, monitoring hypoxia to see if treatment is working, and tractography to show the tracts of neuron activity to determine what is tumor and what is healthy brain tissue. MRI can also measure blood-brain barrier permeability.

“Combining MRI technology with PET in a single integrated system adds the advantages of the extremely broad spectrum of diagnostic MRI procedures to the arsenal of available PET procedures,” said Alexander Drzezga, M.D., TU Muenchen, Munich, Germany, lead author of a study of the Siemens system, which he presented at SNM. “This could potentially result in the development of new imaging agents that bring together specific diagnostic strengths of PET and MRI. It offers exciting scientific options to image physiologic and pathophysiologic processes at the same time and to improve our understanding of both. This and further studies could potentially open a whole new hybrid imaging discipline within the field of nuclear medicine.”

Siemens’ Approach
The Siemens Biography mMR (molecular MR) received European CE mark in the first week of June and FDA clearance the second week of June 2011. Four machines are already installed at Massachusetts General Hospital in Boston under an FDA investigational device exemption (IDE).

Graham said the Siemens PET/MRI system is a major departure from previous PET systems. Traditional photomultiplier tubes could not be used because of the immediate proximity of the MRI magnets. Instead, Siemens used avalanche photo diodes (APDs), new types of detectors that use ceramic instead of metal components.

Another development that helped facilitate a combined PET/MRI scanner was larger bore sizes. The introduction of 70 cm-bore MRI systems in recent years allows for additional space in which to install the PET detectors, which reduces the bore size back down to about 60 cm.

The Siemens system allows for completely simultaneous imaging of both anatomy and function, which cannot be done in either the Philips or GE solutions. This exact coordination in images may allow new avenues of clinical research, such as mapping brain response to stimuli.

Siemens representatives are quick to point out there is no time savings or benefit in using two separate devices and to create two separate imaging exams and then merging them. They say that type of exam can take 60-90 minutes, whereas the Siemens mMR can accomplish the same imaging in about 30 minutes. They say it is similar to an MRI exam, except with the addition of an FDG injection for PET.

New types of artifacts will be encountered with PET/MRI systems due to the MRI coils. Siemens says using the coils built into the mMR system table is not an issue, because the system’s software has already mapped these coils and will correct their image artifacts. But the use of moveable coils may present artifacts that need to be corrected. This artifact issue is overcome in the GE and Philips systems by separating the imaging systems.

Earlier this year, Siemens Healthcare received the 2011 North American Frost & Sullivan Award for new product innovation for the Biograph mMR.

Philips Approach
The Philips Ingenuity TF PET/MR gained European CE mark in January 2011 and is pending FDA 510(k) clearance, with a system being used for investigational purposes at Mt. Sinai Hospital. The company hopes the clearance will come sometime in 2011.

Philips Healthcare created a system that uses a separate PET and MRI scanner located in the same room. The innovation is the use of an automated table gantry. The system images the patient on the MRI and then rotates 180 degrees to image the patient in the PET scanner. The mobile table allows imaging of patients without them moving from one table and scanner to another, which leads to misalignments on the merged hybrid images.

The system requires more space than a single MRI or PET scanning room, instead taking the space of between one-and-a-half to two rooms.

GE Healthcare’s Approach
Instead of developing a new, dedicated PET/MRI system, GE developed a single mobile patient table that is transferred between its PET/CT and MRI scanners for registered, tri-modality imaging. It maintains patient positioning between the two scanners for better registration by keeping the patient in the same position. The table loads into each scanner using a set of identical rails. GE highlighted its FDA-cleared PET/MRI system during the SNM 2011 meeting in June.

Vivek Bhatt, general manager, PET/CT, GE Healthcare, said GE took the approach of examining the clinical value of PET/MRI first, without spending a lot of time and money to develop a standalone system before a definite need has been established. He said the GE system offers a cost-effective solution that uses hospitals’ existing GE scanning equipment and allows clinicians to research the modality.

Gustav von Schulthess, M.D., Ph.D., department of medical radiology, University Hospital Zurich in Switzerland, has been using the GE PET/CT+MR solution to study its clinical value.

All three vendors are expected to show their new PET/MRI systems in November at the Radiological Society of North America (RSNA) 2011 meeting in Chicago.