Quantification of T₁ and T₂ of subarachnoid CSF: Implications for water exchange between CSF and brain tissues

Abstract Purpose To quantify the T 1 and T 2 values of CSF in the subarachnoid space (SAS) at 3 T and interpret them in the context of water exchange between CSF and brain tissues. Methods CSF T 1 was measured using inversion recovery, and CSF T 2 was assessed using T 2 ‐preparation. T 1 and T 2 values in the SAS were compared with those in the frontal horns of lateral ventricles, which have less brain‐CSF exchange. Phantom experiments were performed to examine whether there were spatial variations in T 1 and T 2 that were unrelated to brain‐CSF exchange. Simulations were conducted to investigate the relationship between the brain‐CSF exchange rate and the apparent T 1 and T 2 values of SAS CSF. Results The CSF T 1 and T 2 values were 4308.7 ± 146.9 ms and 1885.5 ± 67.9 ms, respectively, in the SAS and were 4454.0 ± 187.9 ms and 2372.9 ± 72.0 ms in the frontal horns. The SAS CSF had shorter T 1 ( p = 0.006) and T 2 ( p < 0.0001) than CSF in the frontal horns. Phantom experiments showed negligible (< 6 ms for T 1 ; < 1 ms for T 2 ) spatial variations in T 1 and T 2 , suggesting that the T 1 and T 2 differences between SAS and frontal horns were largely attributed to physiological reasons. Simulations revealed that faster brain‐CSF exchange rates lead to shorter apparent T 1 and T 2 of SAS CSF. However, the experimentally observed T 2 difference between SAS and frontal horns was greater than that attributable to typical exchange effect, suggesting that the T 2 shortening in SAS may reflect a combined effect of exchange and deoxyhemoglobin susceptibility. Conclusion Quantification of SAS CSF relaxation times may be useful to assess the brain‐CSF exchange.