letter from the editor: combining pet with mri
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Author's Accepted Manuscript
Letter from the Editor: Combining PET with MRI
Jannette Collins MD, MEd, FCCP, FACR
PII: S0037-198X(14)00007-8DOI: http://dx.doi.org/10.1053/j.ro.2014.01.006Reference: YSROE50461
To appear in: Seminar in Roentgenology
Cite this article as: Jannette Collins MD, MEd, FCCP, FACR, Letter from the Editor:Combining PET with MRI, Seminar in Roentgenology, http://dx.doi.org/10.1053/j.ro.2014.01.006
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Seminars in Roentgenology
July 2014
Letter from the Editor: Combining PET with MRI
PET/MRI is a hybrid imaging technology that incorporates MRI soft tissue morphological imaging and
PET functional imaging. MRI yields superb soft‐tissue contrast and functional information on perfusion,
diffusion, or metabolism. PET enables physicians to visualize molecular tracers with picomolar
sensitivity, providing information about cell metabolism and receptor status (1). Initial experience with
PET/MRI has shown promise for oncologic applications, while cardiac imaging and neuroimaging are also
being explored.
Three companies offer PET/MRI imaging devices, which come in two varieties: integrated or
adjacent. The integrated design is a PET detector inserted in the bore of an MRI system (2). This design
allows simultaneous acquisition of PET and MRI data, whereas the adjacent or tandem approach
requires sequential data acquisition with the patient moving between the PET and MRI scanners while
remaining in the exact same position on the scanner table (2). Simultaneous PET and MRI scans
eliminate the need to move patients from one imaging unit to another. In the tandem approach, MRI is
performed either before or after PET data acquisition, resulting in longer examination times. A major
drawback to this system is a lack of temporal matching between 2 functional datasets, such as for MRI
perfusion or spectroscopy and PET tracer uptake. Both scanner designs incorporate 3.0 Tesla MRI and
time‐of‐flight (TOF) PET technology.
Creation of the PET/MRI combination required three technologic steps (1). First, the
photomultiplier technology had to be replaced with magnetic field‐insensitive avalanche photodiodes.
Second, compact PET detectors had to be constructed to be invisible to the MRI and to not interfere
with the field gradients or MRI radiofrequency. Third, the MRI scanner had to be adapted to
accommodate the PET detectors and to allow simultaneous data acquisition without mutual
interference. The radiofrequency coils of the MRI scanner had to be integrated into the PET system. In
addition, dedicated coils and new strategies for PET attenuation correction, based on MRI information,
had to be developed.
Initial studies have shown that therapeutically relevant information is frequently obtained by
PET or MRI but not necessarily CT. For example, MRI is more sensitive than PET/CT in the detection of
brain, bone, and liver metastases, whereas PET/CT is more accurate in the detection of lymph node
metastases, characterization of soft‐tissue masses, and therapy monitoring (1).
There is not yet an established indication for when PET/CT can be replaced by PET/MRI, which
would significantly reduce radiation exposure (especially given the number of repeat studies during
treatment), decrease the number of sedations required for pediatric patients, decrease cost, and add
clinical value (3). Defined PET/MRI protocols need to be developed and validated. PET/MRI won’t
become a routine clinical examination until its impact on diagnostic accuracy has been proven, and its
effect on therapy management and cost‐efficiency has been considered and validated. More in‐depth
research is needed on potential uses of the technology, especially in areas such as lung cancer, where
the benefit of using PET/MRI isn’t yet clear.
I’m very fortunate to have Dr. Ros guest edit this and the next issue of Seminars in
Roentgenology on PET/MRI. Dr. Ros is the Theodore J. Castele Professor and Chairman of Radiology at
Case Western Reserve University in Cleveland, associated with one of the first cancer center hospitals in
the world to house PET/MRI technology in a purely clinical setting. From 2011, when the machine was
received, to September 2013, the center had examined 145 cancer patients with the double‐scanning
protocol of PET/CT followed by PET/MRI. Much of the work described in the articles in this and the next
issue was done at this institution (University Hospitals Seidman Cancer Center) by the article authors.
They report, based on their experience, that PET/MRI not only provided a more accurate detection of
disease and areas where it had spread, but in some cancer types it distinguished more effectively
malignant tumors from benign ones (3).
Jannette Collins, MD, MEd, FCCP, FACR
Editor‐in‐Chief
References
1. Pichler BJ, Kolb A, Nagele T, Schlemmer H‐P. PET/MRI: paving the way for the next generation of
clinical multimodality imaging applications. J Nucl Med 2010; 51:333‐336.
2. Herrmann KA, Kohan AA, Gaeta MC, Rubbert C, Vercher‐Conejero JL, Paspulati RM, et al.
PET/MRI: applications in clinical imaging. Curr Radiol Rep 2013; 1:161‐176.
3. Kaplan DA. PET/MRI: reflections two years after FDA approval. Diagnostic Imaging, June 12,
2013. Available at http://www.diagnosticimaging.com/pet‐ct/petmri‐reflections‐two‐years‐
after‐fda‐approval. Accessed 1‐17‐14.