Respiratory-gated PET acquisition for the detection of liver metastases in patients with pancreatic ductal adenocarcinoma undergoing staging PET/MR

1375 Background: In staging pancreatic ductal adenocarcinoma (PDAC), PET/CT and contrast-enhanced MR currently serve as problem-solving tools for detecting metastatic disease1, particularly small hepatic metastases that may not be accurately characterized on traditional multiphase contrast enhanced CT. Recently, PET/MR has performed similarly to PET/CT in detecting metastases2, however, is not widely available and therefore not routinely used in clinical practice. Since 2018, our institution has performed over 70 PET/MR studies in patients with PDAC using a protocol with both free-breathing PET acquisition from the skull to the thighs, as well as respiratory-gated (RG) PET acquisition of the abdomen obtained simultaneously with RG MR sequences. We hypothesize that the addition of RG PET acquisition (Q.Static, GE Healthcare) provides greater liver lesion conspicuity, decreased background liver heterogeneity, and improved detection of liver metastases without increasing scan time. Methods: Adult patients evaluated on a GE Signa PET/MR scanner for staging of pancreatic adenocarcinoma from 9/11/2018 to 9/17/2020 were screened, with 75 PET/MR studies performed on 72 patients. Seventeen studies interpreted as positive for liver metastases were included, as well as 10 negative studies as controls. Measurements of background liver SUV maximum (SUVmax), SUV average (SUVavg), and SUV standard deviation (SUVstd) were compared between respiratory-gated (RG) and free-breathing (FB) acquisitions, as was lesion SUVmax. Studies were randomized and evaluated by nuclear medicine trained radiologists (5 attending radiologists, 1 fellow), with only PET images visible. Readers were blinded to acquisition type and identified the number of “definite” and “possible” lesions. Results: Respiratory-gated PET demonstrated decreased background liver FDG uptake compared to FB PET, with a mean SUVmax of 3.24 versus 3.82 (P<.001), mean SUVavg of 2.59 versus 2.75 (P<.001), and mean SUVstd of 0.18 versus 0.27 (P<.001). No statistically significant difference was seen in lesion SUVmax between RG and FB acquisitions, with a mean SUVmax of 5.32 versus 5.02 (P=.05). There was a higher ratio of lesion SUVmax to background liver SUVmax on RG images, with a mean ratio of 1.69 versus 1.41 (P<.001). The ratio of lesion SUVmax to background liver SUVavg was also higher for RG, with a mean ratio of 2.17 versus 1.94 (P=.002). Readers identified more lesions on RG images, with a mean of 48 lesions across 27 studies, versus 30 on FB (P=.004). Lesions were identified with greater confidence on RG, with “definite” lesions comprising 64% of total lesions versus 49% (P=.015). Across the 17 positive studies, there were fewer false negative studies on RG, with an average of 0.5 versus 2.3 (P=.006). Across 10 negative studies, there was no difference in false positive lesions, with a mean of 3.3 and 3.2 on RG and FB, respectively (P=.915). Discussion: PET/MR is an emerging modality in the staging of pancreatic adenocarcinoma. Without increasing scan time, respiratory-gated PET acquisition decreases background noise in the liver, increases metastasis conspicuity, results in detection of more lesions with greater confidence with fewer false negatives. Our institution recommends inclusion of RG PET acquisition in PET/MR protocols for pancreatic adenocarcinoma staging. Figure 1. Respiratory-gated PET (right) demonstrating decreased background liver uptake and improved lesion conspicuity of a single liver metastasis when compared to free-breathing acquisition (left).