How MR Can Help to Avoid Pitfalls in Assessment of Gynecological Malignancies on ¹⁸F-FDG PET/MRI

Objectives: Hybrid ¹⁸F-FDG (FDG) PET/MR imaging combines the strengths of both modalities with excellent spatial and contrast resolution of MR and ability to detect hypermetabolic tumor with FDG PET. The use of PET/MR is especially beneficial in gynecologic cancer patients who require both MRI and FDG PET imaging for treatment planning and surveillance. Several studies demonstrated superiority of PET/MR to PET/CT for local staging in women with cervical and endometrial cancers, where PET/MR performed better in the identification of local recurrence. Furthermore, PET/MR has been demonstrated to be equal or better than PET/CT for lymph node detection.The protocol used at our institution for imaging of gynecologic malignancies includes acquisition of contrast enhanced pelvic MRI and dedicated pelvic PET images, followed by a whole body PET/MR acquisition with T1-weighted and T2-weighted images. The protocol provides a single complete exam in one session averaging 60 minutes, which is very convenient for patients. In the literature, ionizing radiation exposure for hybrid PET/MR imaging has been reported to be up to 80% less when compared with PET/CT, which is especially important in young women who require multiple exams. With FDG PET/MR becoming increasing available for clinical work, readers should be familiar with technical limitations and challenges of this hybrid imaging including specific MR sequences, potential problems with attenuation correction, and artifacts. In addition, the interpreting physician needs to be aware of possible pitfalls in both modalities, including non-malignant FDG-avid lesions and post treatment appearance. This presentation will discuss the FDG PET/MR protocol used at our institution with emphasis on usefulness of specific sequences (e.g., DWI, in-phase/out-of-phase fat/water series) in addressing potential false positive findings on PET. We will provide illustrative examples of post treatment findings as well as mimics of malignancy with emphasis on complimentary role of both modalities in establishing the diagnosis. Selected References: 1. Beiderwellen K, Grueneisen J, Ruhlmann V, et al. [(18)F]FDG PET/MRI vs. PET/CT for whole-body staging in patients with recurrent malignancies of the female pelvis: initial results. Eur J Nucl Med Mol Imaging 2015;42(1):56-65. 2. Kim S-K, Choi HJ, Park S-Y, et al. Additional value of MR/PET fusion compared with PET/CT in the detection of lymph node metastases in cervical cancer patients. Eur J Cancer 2009;45(12):2103-9. 3. Kitajima K, Suenaga Y, Ueno Y, et al. Value of fusion of PET and MRI for staging of endometrial cancer: Comparison with 18F-FDG contrast-enhanced PET/CT and dynamic contrast-enhanced pelvic MRI. Eur J Radiol 013;82(10):1672-6. 4. Ohliger MA, Hope TA, Chapman JS, Chen LM, Behr SC, Poder L. PET/MR Imaging in Gynecologic Oncology. Magn Reson Imaging Clin N Am. 2017 Aug;25(3):667-684.

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