13N-NH3: a selective contrast-enhancing tracer for brain tumor

The purpose of this study is to determine whether there is a qualitative and quantitative relationship between the breakdown of the blood-brain barrier, defined radiologically by the contrast enhancement of MRI, and the uptake of N-NH3 in brain tumors.

The results of N-NH3 PET in 42 patients suspected of having brain tumors were compared with the findings of contrast-enhanced MRI. A histopathological diagnosis was carried out in 33 patients, and a clinical diagnosis was performed in the remaining patients. PET and MRI images were visually inspected, and the contrast index (CI) of MRI and the uptake index (UI) of N-NH3 were measured.

Contrast enhancement of MRIs was seen in 20 of 29 brain tumors (69%). Increased uptake of N-NH3 was seen in 24 of 29 brain tumors (83%). Nineteen of 20 contrast-enhancing brain tumors exhibited the increased uptake of N-NH3 (95%). Areas of the increased N-NH3 uptake corresponded to areas enhanced on MR images in the majority of cases. Five out of nine nonenhancing tumors exhibited increased uptake of N-NH3 (56%). The contrast enhancement of MRIs was seen in nine of 13 nonneoplastic lesions (69%). None of the nonneoplastic lesions showed increased uptake of N-NH3, yielding a specificity of 100% for brain tumors (0 of 13). CI for tumor tissue was 1.46+/-0.64. UI of N-NH3 for tumor tissue was 1.64+/-0.71. CI and UI for tumor tissue were significantly correlated (r=0.86, P<0.01). A statistically significant difference in uptake levels of N-NH3 between contrast-enhancing tumors and nonenhancing tumors (1.88+/-0.66, n=20 vs. 1.11+/-0.52, n=9, P<0.01) was observed. UI was higher in brain tumors compared with the nonneoplastic lesions (1.64+/-0.71 vs. 0.71+/-0.19, P<0.01).

N-NH3 is a potential selective contrast-enhanced tracer for brain tumor, and may prove especially useful for evaluating the contrast-enhancing lesions on MRI to distinguish brain tumors from nonneoplastic lesions.