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

Our plain-English summary of PET/CT versus body coil PET/MRI: how low can you go?, by P. Appenzeller, C. Mader, M.W. Huellner, et al (Insights Into Imaging, 2013).

The study

• Authors: P. Appenzeller, C. Mader, M.W. Huellner, D. Schmidt, D. Schmid, A. Boss, G. von Schulthess, P. Veit-Haibach
• Journal: Insights Into Imaging
• Published: 2013;4(4):481-90
• Cited by: 33 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: 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.

Methods And Materials: 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.

Results: 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.

Conclusion: 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.

Main Messages: • 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.

In plain English

This prospective study from University Hospital Zurich compared two cancer scanning methods. It did not study asbestos exposure. The 63 adult patients had various cancers, including 3 with pleural mesothelioma; a table lists their PET-positive findings, but the methods were not compared separately for them. Each patient had a standard low-dose PET/CT scan and then a quick MRI using one sequence and only the body coil. Of the 63 patients, 46 had at least one lesion; 126 lesions were evaluated. Across all lesions, how clearly lesion borders could be seen did not differ significantly between the methods (p=0.095; mean score 2.93 for PET/CT, 2.75 for PET/MRI). Seventeen lesions were visible only on PET/CT and six only on PET/MRI. PET/CT showed lung lesions (p=0.016) and lymph nodes (p=0.033) significantly more clearly. Bone lesions scored higher on PET/CT, but that difference was not statistically significant. The authors concluded that this basic PET/MRI might only serve as a back-up in very few patients. They noted that not all lesions were confirmed by tissue examination and that the patient group was mixed.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Prospective comparison of two imaging methods in the same patients: standard low-dose PET/CT versus PET/MRI using one MRI sequence with the body coil only.
• Who was studied: 63 adult patients (mean age 58 years, range 19 to 86; 23 women, 40 men) at University Hospital Zurich, referred for staging or restaging/follow-up of various cancers. These included 3 patients with pleural mesothelioma. The study dates are not given.
• Exposure: No asbestos or other environmental exposure was studied. The comparison was between imaging methods: 18F-FDG PET with unenhanced low-dose CT, and PET with a whole-body MRI using a single gradient echo sequence, no contrast medium and no dedicated surface coils (body coil only).
• Main results:
  • In the 46 of 63 patients who had at least one lesion, 126 PET-positive lesions were evaluated.
  • The patient table lists each of the three patients with pleural mesothelioma as having 3 PET-positive findings. One had pleura (3). One had lymph node, bone and pleura. One had lymph node, mesenterium and abdominal wall. No separate PET/CT-versus-PET/MRI comparison is given for these patients.
  • Across all lesions, conspicuity (how clearly lesion borders could be seen) did not differ significantly between the methods: p=0.095, mean score 2.93 for PET/CT and 2.75 for PET/MRI.
  • 101 lesions were visible on both methods. Seventeen (16.8 %) were visible only on PET/CT, six (6 %) only on PET/MRI, and two (1.9 %) on neither, being seen only on the PET images.
  • Among the 101 lesions visible on both, lesion size did not differ significantly (p=0.251). Mean diameter was 20.21 mm on CT and 19.94 mm on MRI.
  • PET/CT showed pulmonary (lung) lesions significantly more clearly than PET/MRI (p=0.016).
  • PET/CT showed lymph nodes significantly more clearly than PET/MRI (p=0.033).
  • Bone lesions had a higher mean conspicuity score on PET/CT than on PET/MRI, but the difference was not statistically significant.
• The authors' conclusion: The authors concluded that basic PET/MRI with body coil acquisition does not entirely match the diagnostic accuracy of standard low-dose PET/CT, and might only serve as a back-up solution in very few patients.
• Limitations the authors state:
  • Tissue (histological) confirmation, the reference standard, was not available for all lesions evaluated.
  • The slice thickness differed between PET/MRI and PET/CT.
  • The patient population was mixed (inhomogeneous), which limits conclusions about specific tumour types.
  • The numbers of patients and lesions were somewhat low.

Topics

Medical terms in this study, explained in our Lexicon: Staging, Lung Cancer, Pleural Mesothelioma, Esophageal Cancer, Colorectal Cancer, Stomach Cancer, Discovery, Statistical Significance.

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