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

Objective: To investigate the usefulness of ¹³N-NH₃ PET in detecting brain lesions which show hypometabolism on ¹⁸F-FDG PET. Methods: ¹³N-NH₃ PET imaging was performed for a prospective study in 18 patients with brain lesions that showed hypometabolism compared with normal brain tissue on ¹⁸F-FDG PET scans. Fourteen patients underwent ¹⁸F-FDG PET imaging for initial diagnosis and 4 patients for detection of astrocytoma recurrence (13 males, 5 females, age 20-68 (42.4 ± 12.6) years). Ten gliomas, 1 metastatic tumor, 1 dysembryoplastic neuroepithelial tumor (DNT) and 6 non-neoplastic lesions (including 3 cases of radiation necrosis, 2 cases of encephalitic foci, and 1 case of ischemic lesion) were verified by histopathological examination (n=13) or clinical follow-up (n=5). The tumor-to-contralateral brain tissue ratios (T/C) were calculated by the ROI method. The diagnostic efficacy of ¹³N-NH₃ PET was evaluated. Paired t test and two-sample t test were performed to analyze the differences of T/C between different groups. Results: Seven (5 astrocytomas and 2 glioblastomas) of 12 brain tumors (sensitivity: 58%, 7/12) showed increased ¹³N-NH₃ uptake (higher uptake than the contralateral brain tissue), while 3 low-grade gliomas, 1 metastatic tumor, and 1 DNT showed decreased ¹³N-NH₃ uptake (no uptake or lower uptake than the contralateral brain tissue). The uptake ratio of ¹³N-NH₃ was significantly higher than that of ¹⁸F-FDG (1.24 ± 0.66 vs 0.67 ± 0.24, t=-3.740, P<0.05) in the tumors. All six non-neoplastic lesions showed decreased ¹³N-NH₃ uptake (specificity: 6/6). The T/C ratios of ¹⁸F-FDG and ¹³N-NH₃ in the non-neoplastic lesions were 0.68 ±0.15 and 0.70 ±0.19, respectively, and there was no significant difference between them (t=-0.246, P>0.05). The T/C 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, t=2.624, P<0.05). Conclusion: ¹³N-NH₃ PET imaging may be helpful to detect and differentiate brain tumors with hypometabolism as detected by ¹⁸F-FDG PET imaging from non-neoplastic lesions with high specificity, especially for cerebral astrocytomas, but the sensitivity is relatively limited. (authors)