Variation of T 2 relaxation times in pediatric brain tumors and their effect on metabolite quantification

Background Metabolite concentrations are fundamental biomarkers of disease and prognosis. Magnetic resonance spectroscopy (MRS) is a noninvasive method for measuring metabolite concentrations; however, quantitation is affected by T 2 relaxation. Purpose To estimate T 2 relaxation times in pediatric brain tumors and assess how variation in T 2 relaxation affects metabolite quantification. Study Type Retrospective. Population Twenty‐seven pediatric brain tumor patients ( n  = 17 pilocytic astrocytoma and n  = 10 medulloblastoma) and 24 age‐matched normal controls. Field Strength/Sequence Short‐ (30 msec) and long‐echo (135 msec) single‐voxel MRS acquired at 1.5T. Assessment T 2 relaxation times were estimated by fitting signal amplitudes at two echo times to a monoexponential decay function and were used to correct metabolite concentration estimates for relaxation effects. Statistical Tests One‐way analysis of variance (ANOVA) on ranks were used to analyze the mean T 2 relaxation times and metabolite concentrations for each tissue group and paired Mann–Whitney U ‐tests were performed. Results The mean T 2 relaxation of water was measured as 181 msec, 123 msec, 90 msec, and 86 msec in pilocytic astrocytomas, medulloblastomas, basal ganglia, and white matter, respectively. The T 2 of water was significantly longer in both tumor groups than normal brain ( P  < 0.001) and in pilocytic astrocytomas compared with medulloblastomas ( P  < 0.01). The choline T 2 relaxation time was significantly longer in medulloblastomas compared with pilocytic astrocytomas ( P  < 0.05), while the T 2 relaxation time of NAA was significantly shorter in pilocytic astrocytomas compared with normal brain ( P  < 0.001). Overall, the metabolite concentrations were underestimated by ∼22% when default T 2 values were used compared with case‐specific T 2 values at short echo time. The difference was reduced to 4% when individually measured water T 2 s were used. Data Conclusion Differences exist in water and metabolite T 2 relaxation times for pediatric brain tumors, which lead to significant underestimation of metabolite concentrations when using default water T 2 relaxation times. Level of Evidence: 3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:195–203.