PET/CT versus body coil PET/MRI: how low can you go?

The purpose of this study was to evaluate if positron emission tomography (PET)/magnetic resonance imaging (MRI) with just one gradient echo sequence using the body coil is diagnostically sufficient compared with a standard, low-dose non-contrast-enhanced PET/computed tomography (CT) concerning overall diagnostic accuracy, lesion detectability, size and conspicuity evaluation.

Sixty-three patients (mean age 58 years, range 19-86 years; 23 women, 40 men) referred for either staging or restaging/follow-up of various malignant tumours (malignant melanoma, lung cancer, breast cancer, Hodgkin's lymphoma, non-Hodgkin's lymphoma, CUP, gynaecology tumours, pleural mesothelioma, oesophageal cancer, colorectal cancer, stomach cancer) were prospectively included. Imaging was conducted using a tri-modality PET/CT-MR set-up (full ring, time-of-flight Discovery PET/CT 690, 3 T Discovery MR 750, both GE Healthcare, Waukesha, WI). All patients were positioned on a dedicated PET/CT- and MR-compatible examination table, allowing for patient transport from the MR system to the PET/CT without patient movement. In accordance with RECIST 1.1 criteria, measurements of the maximum lesion diameters on CT and MR images were obtained. In lymph nodes, the short axis was measured. A four-point scale was used for assessment of lesion conspicuity: 1 (>25 % of lesion borders definable), 2 (25-50 %), 3 (50-75 %) and 4 (>75 %). For each lesion the corresponding anatomical structure was noted based on anatomical information of the spatially co-registered PET/CT and PET/MRI image sections. Additionally, lesions were divided into three categories: "tumour mass", "lymph nodes" and "lesions". Differences in overall lesion detectability and conspicuity in PET/CT and PET/MRI, as well as differences in detectability based on the localisation and lesion type, were analysed by Wilcoxon signed rank test.

A total of 126 PET-positive lesions were evaluated. Overall, no statistically significant superiority of PET/CT over PET/MRI or vice versa in terms of lesion conspicuity was found (p = 0.095; mean score CT 2.93, mean score MRI 2.75). A statistically significant superiority concerning conspicuity of PET/CT over PET/MRI was found in pulmonary lesions (p = 0.016). Additionally, a statistically significant superiority of PET/CT over PET/MRI in "lymph nodes" regarding lesion conspicuity was also found (p = 0.033). A higher mean score concerning bone lesions were found for PET/CT compared with PET/MRI; however, these differences did not achieve statistical significance.

Overall, PET/MRI with body coil acquisition does not match entirely the diagnostic accuracy of standard low-dose PET/CT. Thus, it might only serve as a back-up solution in very few patients. Overall, more time needs to be invested on the MR imaging part (higher matrix, more breath-holds, additional surface coil acquired sequences) to match up with the standard low-dose PET/CT.

• Evaluation of whether PET/MRI with one sequence using body coil is diagnostically sufficient compared with PET/CT • PET/MRI with body coil does not match entirely the diagnostic accuracy of standard low-dose PET/CT • PET/MRI might only serve as a backup solution in patients.