The Evolution of PET/MR Is Hindered by Our Field’s Reluctance to Provide Critical Evaluation

Abstract

Over the last 10 years, our field has been able to review and develop opportunities for integrating commercially available PET/ MR into clinical practice. We have done so with a coalition of industry, academia, and health-care providers. Although the intention to deliver better technology to patients through dissemination of PET/MR is laudable, the delineation between academic and clinical utility is often omitted from the literature; this impedes the field in finding the proper context for the technology. Under this circumstance, I am writing to follow up on a recent article published in The Journal of Nuclear Medicine: ‘‘PET/MRI Versus PET/CT for Whole-Body Staging: Results from a SingleCenter Observational Study on 1,003 Sequential Examinations’’ by Martin et al. (1). The work that is described in this publication portrays a benchmark comparison of new and classic technologies relative to the classic domain of oncology in PET, but there are notable biases in the study design worthy of comment. Furthermore, the publication presents an opportunity to remark on many of the practical differences between PET/MR and PET/CT, which when presented alongside the authors’ analysis, could portray a case for arriving at an opposite conclusion. Ultimately, 10 years into the commercial PET/MR era, the field has yet to contextualize the technology in academic and clinical-value equations, which promotes exclusivity and hinders opportunities to identify and address the barriers that encumber clinical impact. The work performed by Martin et al. compares diagnostic capacities between an implementation of a PET/CT system and an implementation of a PET/MR system in a given population. However, study design differences across cameras were numerous and uniformly biased in favor of the perceived superior performance of the PET/MR system. The largest bias stems from the differences in PET hardware in the systems being compared, that is, a Siemens Biograph mCT PET/CT and a Siemens mMR PET/ MR. The specifications and a performance comparison of these same, or similar, model systems have recently been reported (2). The 2 scanners have differences in bore size, transaxial coverage, crystal-to-detector encoding ratio, and photon amplifier technologies (avalanche photo diode vs. photomultiplier tubes). All these design differences favor the PET scanner associated with the mMR model (better sensitivity, better resolution, better noise discrimination) and are unrelated to the MR component of the instrument. It is therefore no surprise that the authors see improved diagnostic image quality on the mMR instrument. With respect to the study design: although not all details have been provided, we can deduce that PET image quality was biased toward PET/MR because the acquisition time per bed position was twice as long as that for the PET/CT device and the acquisition took place less than one isotope half-life later; that is, the PET images in the PET/MR acquisitions were generated with more count statistics and fewer random-coincidence detections. This difference is compounded by the fact that different postreconstruction filters were used between the scanners, and physiologic differences favor increased sensitivity in the later PET/MR images (as noted by the authors). It is also worth remark that the mCT system performance appears dumbed down by not using the timeof-flight reconstruction capacity of the system, which is readily available to users and has been shown to offer notable imagequality enhancement on the PET/CT device (2). Ultimately, it is not clear if the improved lesion detectability reported by Martin et al. stemmed from having readily accessible simultaneously acquired MR images available to clinicians or from having better PET data from the incrementally superior PET hardware in the mMR machine. The distinction is quite important to many readers because it directly impacts the answer to a principal question: is PET/MR worth the additional costs and complexity of operation when compared alongside PET/CT? An objective comparison of the 2 technologies with different overheads and workflows is challenging. It is a reasonable contention that such comparison research is laying the groundwork of benchmarking the technology to support further developments, and such benchmarking fits within an academic context aligned with a vision of continued expansion. This sentiment is portrayed by the authors’ concluding that their work will ‘‘. . .hopefully further pave the way toward a widespread introduction of PET/MRI into clinical patient care.’’ However, this analysis is at odds with the clinical-value context. Clinically speaking, consideration of the large gap of approximately 5% of PET/CT patients for whom the authors were not able to obtain PET/MR images because they were subject to ‘‘abortion by patient’’ or negated by ‘‘technical problems of PET/ MRI’’ is perplexingly omitted from the analysis and at odds with the authors’ conclusions on the equivalence of the devices. Furthermore, from an operational perspective, comparable performance, relative to complexities and the cost of PET/MR technology, portray a case for its limited capacity and removal from the market as a PET/CT alternative, that is, the antithesis of the authors’ concluding statement. These discrepancies reveal a discordance in PET/MR academic and clinical paradigms that is prevalent in the field. Received Nov. 25, 2020; revision accepted Jan. 3, 2021. For correspondence or reprints contact: Adam L. Kesner, Memorial Sloan Kettering Cancer Center, 1250 First Ave., Room S-1119E (Box 84), New York, NY 10065. E-mail: [email protected] Published online Jan. 15, 2021. COPYRIGHT© 2021 by the Society of Nuclear Medicine and Molecular Imaging. DOI: 10.2967/jnumed.120.260950