Metabolic Imaging of Cerebral Gliomas: Spatial Correlation of Changes in O-(2-¹⁸F-Fluoroethyl)-L-Tyrosine PET and Proton Magnetic Resonance Spectroscopic Imaging

¹ Department of Neurosurgery, University of Erlangen-Nuremberg, Erlangen, Germany; MR Physics Group, Department of Radiology, Landesklinikum St. Poelten, St. Poelten, Austria
² Clinic of Nuclear Medicine, University of Erlangen-Nuremberg, Erlangen, Germany
³ Department of Neurosurgery, University of Erlangen-Nuremberg, Erlangen, Germany
⁴ MR Physics Group, Department of Radiology, Landesklinikum St. Poelten, St. Poelten, Austria

^* To whom correspondence should be addressed. E-mail: andi{at}nmr.at.

	Abstract

The aim of this study was to determine the spatial correlation of O-(2-¹⁸F-fluoroethyl)-L-tyrosine (¹⁸F-FET) uptake and the concentrations of choline (Cho), creatine (Cr), and total N-acetylaspartate (tNAA) determined with proton magnetic resonance spectroscopic imaging (¹H MRSI) in cerebral gliomas for the multimodal evaluation of metabolic changes. Methods: ¹⁸F-FET PET and 2-dimensional ¹H MRSI were performed in 15 patients with cerebral gliomas of World Health Organization (WHO) grades II–IV. PET and ¹H MRSI datasets were coregistered by use of mutual information. On the basis of their levels of ¹⁸F-FET uptake, 4 different areas in a tumor (maximum, strong, moderate, and low ¹⁸F-FET uptake) were defined on PET slices as being congruent with the volume of interest in the ¹H MRSI experiment. ¹⁸F-FET uptake in lesions was evaluated as tumor-to-brain ratios. Metabolite concentrations for Cho, Cr, and tNAA and Cho/tNAA ratios were computed for these 4 areas in the tumor and for the contralateral normal brain. Results: In the area with maximum ¹⁸F-FET uptake, the concentration of tNAA (R = -0.588) and the Cho/tNAA ratio (R = 0.945) correlated significantly with ¹⁸F-FET uptake. In the areas with strong and moderate ¹⁸F-FET uptake, only the Cho/tNAA ratios (R = 0.811 and R = 0.531, respectively) were significantly associated with amino acid transport. At low ¹⁸F-FET uptake, analysis of the correlations of amino acid uptake and metabolite concentrations yielded a significant result only for the concentration of Cr (R = 0.626). No correlation was found for metabolite concentrations determined with ¹H MRSI and ¹⁸F-FET uptake in normal brain tissue. Maximum ¹⁸F-FET uptake and the tNAA concentration were significantly different between gliomas of WHO grades II and IV, with P values of 0.032 and 0.016, respectively. Conclusion: High ¹⁸F-FET uptake, which is indicative of tumor cell infiltration, associates with neuronal cell loss (tNAA) and changes in ratios between parameters representing membrane proliferation and those of neuronal loss (Cho/tNAA ratio), which can be measured by ¹H MRSI. The significant correlation coefficients detected for Cr in regions with low ¹⁸F-FET uptake suggests an association between the mechanism governing amino acid transport and energy metabolism in areas that are infiltrated by tumor cells to a lesser extent. These findings motivate further research directed at investigating the potential of ¹H MRSI to define tumor boundaries in a manner analogous to that of amino acid PET.

Key Words: metabolic imaging, brain tumor, magnetic resonance spectroscopy, PET, tyrosine

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