Hybrid MR-PET of brain tumours using amino acid PET and chemical exchange saturation transfer MRI

PET using radiolabelled amino acids has become a promising tool in the diagnostics of gliomas and brain metastasis. Current research is focused on the evaluation of amide proton transfer (APT) chemical exchange saturation transfer (CEST) MR imaging for brain tumour imaging. In this hybrid MR-PET study, brain tumours were compared using 3D data derived from APT-CEST MRI and amino acid PET using O-(2-¹⁸F-fluoroethyl)-L-tyrosine (¹⁸F-FET). Eight patients with gliomas were investigated simultaneously with ¹⁸F-FET PET and APT-CEST MRI using a 3-T MR-BrainPET scanner. CEST imaging was based on a steady-state approach using a B₁ average power of 1μT. B₀ field inhomogeneities were corrected a Prametric images of magnetisation transfer ratio asymmetry (MTR_asym) and differences to the extrapolated semi-solid magnetisation transfer reference method, APT and nuclear Overhauser effect (NOE), were calculated. Statistical analysis of the tumour-to-brain ratio of the CEST data was performed against PET data using the non-parametric Wilcoxon test. A tumour-to-brain ratio derived from APT and ¹⁸F-FET presented no significant differences, and no correlation was found between APT and ¹⁸F-FET PET data. The distance between local hot spot APT and ¹⁸F-FET were different (average 20 ± 13 mm, range 4-45 mm). For the first time, CEST images were compared with ¹⁸F-FET in a simultaneous MR-PET measurement. Imaging findings derived from¹⁸F-FET PET and APT CEST MRI seem to provide different biological information. The validation of these imaging findings by histological confirmation is necessary, ideally using stereotactic biopsy. (orig.)