Variation of T2 relaxation times in pediatric brain tumors and their effect on metabolite quantification

Abstract

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 T₂ relaxation.

Purpose: To estimate T₂ relaxation times in pediatric brain tumors and assess how variation in T₂ 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: T₂ 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 T₂ relaxation times and metabolite concentrations for each tissue group and paired Mann-Whitney U-tests were performed.

Results: The mean T₂ 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 T₂ 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 T₂ relaxation time was significantly longer in medulloblastomas compared with pilocytic astrocytomas (P < 0.05), while the T₂ 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 T₂ values were used compared with case-specific T₂ values at short echo time. The difference was reduced to 4% when individually measured water T₂ s were used.

Data conclusion: Differences exist in water and metabolite T₂ relaxation times for pediatric brain tumors, which lead to significant underestimation of metabolite concentrations when using default water T₂ relaxation times.

Level of evidence: 3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:195-203.

Keywords: MRS; MRS quantification; T2 relaxation; pediatric brain tumors.