Abstract
AIM: Assessment of amino acid transport using PET tracers such as [18F]-fluor-ethyl-thyrosine (FET) is a valuable supplement to routine contrast enhanced structural MRI in the initial and follow up evaluations of patients with brain tumors. MRI blood volume (BV) imaging using dynamic susceptibility contrast (DSC) has also been shown to give important information about tumor angiogenesis. The aim of the study was to investigate the feasibility of simultaneous imaging of structural MRI, angiogenic activity (BV DSC), and metabolic activity (FET) in a short single session using an integrated PET/MRI scanner. METHODS AND MATERIALS: Twenty-five scans were obtained in 22 referred brain tumor patients. A 20 min static simultaneous PET/MRI (Siemens mMR) acquisition 20 min. p.i. of 200 MBq FET was performed. The MRI protocol included axial FLAIR and post-contrast T1 MPRAGE. For BV imaging the contrast agent (0.1-0.2 ml/kg, Gadovist 1 mmol/ml) was administered using a power injector and BV maps were calculated using dedicated software. Metabolically active tumor volume (FET-vol) was defined in tissue as activity > 1.6 x background activity (B), and metabolic activity, Tmax/B, as maximal tumor activity/B. A low-dose CT scan was obtained for attenuation correction. RESULTS: The PET/MRI scans were well-accepted by patients. Fourteen had high grade glioma (glioblastoma n = 9, oligodendroglioma III n = 4, astrocytoma III n = 1), 7 patients had low grade glioma (ganglioglioma =1, brain stem glioma n = 1, astrocytoma II n = 1, pontine glioma, oligodendroglioma n = 2, unspecified n = 1) and one patients has brain metastasis (malignant melanoma). Five patients had been exposed to anti-angiogenetic chemotherapy (Bevacizumab + irinotecan). Median tumor volume was 1.6 ml (range 0-138 ml). Areas with visually increased BV were observed in 15 of 18 scans with metabolically active tumor tissue. Scans with increased BV had significantly larger median FET-vol (4.1 ml vs 0 ml, p < 0.0005) and median Tmax/B (2.5 vs 1.4, p < 0.005) compared to those with without increased BV. Excluding scans without metabolically active tumor tissue, neither FET-vol nor Tmax/B was different. There was a high variability in the regional signals observed using FLAIR, FET PET, BV, and post-contrast T1, and poor spatial congruence between FET and BV. The fraction of scans with and without increased BV was not related to tumor grade or exposure to Bevacizumab. CONCLUSION: In the clinical management of brain tumors a simultaneous and comprehensive evaluation of structure, metabolism and blood volume is feasible in a short single session FET PET/MRI DSC scan yielding complementary high quality information. This modality may provide valuable insight into tumor biology and increase the overall diagnostic quality, decrease clinical decision time, and increase acceptance of PET imaging with patients and treating clinicians in clinical management and in trials.
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