Simultaneous PET-MRI in oncology: a solution looking for a problem?

Abstract

With the recent development of integrated positron emission tomography-magnetic resonance imaging (PET-MRI) scanners, new possibilities for quantitative molecular imaging of cancer are realized. However, the practical advantages and potential clinical benefits of the ability to record PET and MRI data simultaneously must be balanced against the substantial costs and other requirements of such devices. In this review, we highlight several of the key areas where integrated PET-MRI measurements, obtained simultaneously, are anticipated to have a significant impact on clinical and/or research studies. These areas include the use of MR-based motion corrections and/or a priori anatomical information for improved reconstruction of PET data, improved arterial input function characterization for PET kinetic modeling, the use of dual-modality contrast agents, and patient comfort and practical convenience. For widespread acceptance, a compelling case could be made if the combination of quantitative MRI and specific PET biomarkers significantly improves our ability to assess tumor status and response to therapy, and some likely candidates are now emerging. We consider the relative advantages and disadvantages afforded by PET-MRI and summarize current opinions and evidence as to the likely value of PET-MRI in the management of cancer.

Fig. 1

PET/CT (top row) vs. PET/MR (bottom row) of a patient with an inflamed plaque. Higher soft-tissue contrast of the MR image (bottom left) compared to the CT image (top left) allows better delineation of the luminal area and therefore improves quantification of the PET uptake signal.

Fig. 2

Use of MR time-of-flight (ToF) sequence to highlight the arterial blood pool. (A) 3D-ToF on neck region and 3D image reconstruction after arterial segmentation (B). (C) MR-angiography of the brain using time-of-flight (ToF) MR sequence.

Fig. 3

A retrospectively registered PET-MRI approach to monitoring neoadjuvant chemotherapy (NAC) in an invasive ductal carcinoma. Specifically, quantitative DCE- and DW-MRI parameters have been registered to an FDG-PET scan at three time points during NAC: 1) pre-therapy (column 1), 2) after one cycle of therapy (column 2), and 3) at the conclusion of NAC but prior to surgery (column 3). The first three rows present data available from the MRI study: K^trans, v_e, v_p, and ADC, respectively. The final row presents the FDG-PET map at each time point. The ability to simultaneously acquire such rich data provides the opportunity for many studies described in the text.