Comparison of ¹⁸F-FET PET and Perfusion-Weighted MR Imaging: A PET/MR Imaging Hybrid Study in Patients with Brain Tumors

Abstract

PET using O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) provides important diagnostic information in addition to that from conventional MR imaging on tumor extent and activity of cerebral gliomas. Recent studies suggest that perfusion-weighted MR imaging (PWI), especially maps of regional cerebral blood volume (rCBV), may provide similar diagnostic information. In this study, we directly compared ¹⁸F-FET PET and PWI in patients with brain tumors. Methods: Fifty-six patients with gliomas were investigated using static ¹⁸F-FET PET and PWI. For comparison, 8 patients with meningiomas were included. We generated a set of tumor and reference volumes of interest (VOIs) based on morphologic MR imaging and transferred these VOIs to the corresponding ¹⁸F-FET PET scans and PWI maps. From these VOIs, tumor-to-brain ratios (TBR) were calculated, and normalized histograms were generated for ¹⁸F-FET PET and rCBV maps. Furthermore, in rCBV maps and in ¹⁸F-FET PET scans, tumor volumes, their spatial congruence, and the distance between the local hot spots were assessed. Results: For patients with glioma, TBR was significantly higher in ¹⁸F-FET PET than in rCBV maps (TBR, 2.28 ± 0.99 vs. 1.62 ± 1.13; P < 0.001). Histogram analysis of the VOIs revealed that ¹⁸F-FET scans could clearly separate tumor from background. In contrast, deriving this information from rCBV maps was difficult. Tumor volumes were significantly larger in ¹⁸F-FET PET than in rCBV maps (tumor volume, 24.3 ± 26.5 cm³ vs. 8.9 ± 13.9 cm³; P < 0.001). Accordingly, spatial overlap of both imaging parameters was poor (congruence, 11.0%), and mean distance between the local hot spots was 25.4 ± 16.1 mm. In meningioma patients, TBR was higher in rCBV maps than in ¹⁸F-FET PET (TBR_, 5.33 ± 2.63 vs. 2.37 ± 0.32; P < 0.001) whereas tumor volumes were comparable. Conclusion: In patients with cerebral glioma, tumor imaging with ¹⁸F-FET PET and rCBV yields different information. ¹⁸F-FET PET shows considerably higher TBRs and larger tumor volumes than rCBV maps. The spatial congruence of both parameters is poor. The locations of the local hot spots differ considerably. Taken together, our data show that metabolically active tumor tissue of gliomas as depicted by amino acid PET is not reflected by rCBV as measured with PWI.