Usefulness of ¹³N-NH₃ PET in the evaluation of brain lesions that are hypometabolic on ¹⁸F-FDG PET

We investigated the usefulness of ¹³N-NH₃ PET in characterizing brain lesions which show hypometabolism on ¹⁸F-FDG PET. ¹³N-NH₃ PET was performed in 18 patients with brain lesions (in 14 for initial diagnosis and in 4 for detection of astrocytoma recurrence) that showed hypometabolism compared with normal brain tissue on ¹⁸F-FDG PET. The diagnoses were ten gliomas, one metastatic tumor, one dysembryoplastic neuroepithelial tumor (DNT), and six non-neoplastic lesions (including three cases of radiation necrosis, two cases of encephalitic foci, and one ischemic lesion). Diagnosis was verified by histopathological examination in 13 patients or was established by clinical follow-up and additional investigations in the remainder. Seven of 12 brain tumors (58%, sensitivity) showed increased ¹³N-NH₃ uptake despite hypometabolism on ¹⁸F-FDG PET. The three low-grade gliomas, one metastatic tumor, and one DNT showed decreased ¹³N-NH₃ uptake. The mean (±SD) uptake ratio of ¹³N-NH₃ was significantly higher than that of ¹⁸F-FDG (1.24 ± 0.57 vs. 0.67 ± 0.21, P < 0.01) in the tumors. By contrast, all six non-neoplastic lesions showed decreased ¹³N-NH₃ uptake (100% specificity). The mean (±SD) uptake ratio of ¹⁸F-FDG and ¹³N-NH₃ in the non-neoplastic lesions was 0.68 ± 0.15 and 0.70 ± 0.19, respectively, and there was no significant difference between them (P > 0.05). The mean (±SD) uptake ratio of ¹³N-NH₃ in the tumors was significantly higher than that in the non-neoplastic lesions (1.24 ± 0.53 vs. 0.70 ± 0.19, P < 0.05). The preliminary results of this study suggest that ¹³N-NH₃ PET may be helpful to detect and differentiate brain tumors which show hypometabolism on ¹⁸F-FDG PET from non-neoplastic lesions with high specificity, especially for cerebral astrocytomas, but the sensitivity is relatively limited.