Is T2*Enough to Assess Oxygenation? Quantitative Blood Oxygen Level-Dependent Analysis in Brain Tumor

Purpose: To analyze the contribution of the transverse relaxation parameter (T2), macroscopic field inhomogeneities (Delta B-0), and blood volume fraction (BVf) to blood oxygen level-dependent (BOLD)-based magnetic resonance (MR) measurements of blood oxygen saturation (SO2) obtained in a brain tumor model. Materials and Methods: This study was approved by the local committee for animal care and use. Experiments were performed in accordance with permit 380820 from the French Ministry of Agriculture. The 9L gliosarcoma cells were implanted in the brain of eight rats. Fifteen days later, 4.7-T MR examinations were performed to estimate T2*, T2, BVf, and T2*Delta B-0corrected in the tumor and contralateral regions. MR estimates of SO2 were derived by combining T2, BVf, and T2*Delta B-0corrected according to a recently described quantitative BOLD approach. Scatterplots and linear regression analysis were used to identify correlation between parameters. Paired Student t tests were used to compare the tumor region with the contralateral region. Results: No significant correlations were found between T2* and any parameter in either tumor tissue or healthy tissue. T2* in the tumor and T2* in the uninvolved contralateral brain were the same (36 msec +/- 4 [standard deviation] vs 36 msec +/- 5, respectively), which might suggest similar oxygenation. Adding T2 information (98 msec +/- 7 vs 68 msec +/- 2, respectively) alone yields results that suggest apparent hypo-oxygenation of the tumor, while incorporating BVf (5.3% +/- 0.6 vs 2.6% +/- 0.3, respectively) alone yields results that suggest apparent hyperoxygenation. MR estimates of SO2 obtained with a complete quantitative BOLD analysis, although not correlated with T2* values, suggest normal oxygenation (68% +/- 3 vs 65% +/- 4, respectively). MR estimates of SO2 obtained in the contralateral tissue agree with previously reported values. Conclusion: Additional measurements, such as BVf, T2, and Delta B-0, are needed to obtain reliable information on oxygenation with BOLD MR imaging. The proposed quantitative BOLD approach, which includes these measurements, appears to be a promising tool with which to map tumor oxygenation.