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Focal Changes in Diffusivity on Apparent Diffusion Coefficient MR Imaging and Amino Acid Uptake on PET Do Not Colocalize in Nonenhancing Low-Grade Gliomas

Low-grade gliomas (LGGs) may harbor malignant foci, which are characterized by increased tumor cellularity and angiogenesis. We used diffusion-weighted MR imaging (apparent diffusion coefficient [ADC]) and PET with the amino acid O-(2-18F-fluorethyl)-l-tyrosine (18F-FET) to search for focal changes of diffusion (ADC) and amino acid uptake and to investigate whether focal changes in these parameters colocalize within LGGs. Methods: We retrospectively selected 18 patients with nonenhancing LGG. All patients had undergone 18F-FET PET and MR imaging for preoperative evaluation or for therapy monitoring in recurrent or progressive LGG. Region-of-interest analysis was performed to compare 18F-FET uptake and ADC values in areas with focal intratumoral maximum metabolic activity and diffusion restriction and between tumor and normal brain. 18F-FET uptake was normalized to the mean cerebellar uptake (ratio). ADC values were also compared with the 18F-FET uptake on a voxel-by-voxel basis across the whole tumor. Results: The mean focal maximum (mean ± SD, 1.69 ± 0.85) and global 18F-FET uptake in tumors (1.14 ± 0.41) exceeded that of normal cortex (0.85 ± 0.09) and cerebrospinal fluid (0.82 ± 0.20). ADC values in the area with most restricted diffusion (1.07 ± 0.22 × 10−3 mm2/s) and in the whole tumor (1.38 ± 0.27 × 10−3 mm2/s) were in the range between normal cortex (0.73 ± 0.06 × 10−3 mm2/s) and cerebrospinal fluid (2.84 ± 0.09 × 10−3 mm2/s). 18F-FET uptake did not correlate with corresponding (colocalizing) ADC values, either in the area with focal maximum metabolic activity or in the area with most restricted diffusion or in the whole tumor. Conclusion: There is no congruency between 18F-FET uptake and diffusivity in nonenhancing LGG. Diffusion restriction in these tumors most likely represents changes in brain and tumor cell densities as well as alteration of water distribution and is probably not directly correlated with the density of tumor cells.