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

To quantify the T₁ and T₂ 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.

CSF T₁ was measured using inversion recovery, and CSF T₂ was assessed using T₂ -preparation. T₁ and T₂ 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 T₁ and T₂ that were unrelated to brain-CSF exchange. Simulations were conducted to investigate the relationship between the brain-CSF exchange rate and the apparent T₁ and T₂ values of SAS CSF.

The CSF T₁ and T₂ 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 T₁ (p = 0.006) and T₂ (p < 0.0001) than CSF in the frontal horns. Phantom experiments showed negligible (< 6 ms for T₁ ; < 1 ms for T₂ ) spatial variations in T₁ and T₂ , suggesting that the T₁ and T₂ differences between SAS and frontal horns were largely attributed to physiological reasons. Simulations revealed that faster brain-CSF exchange rates lead to shorter apparent T₁ and T₂ of SAS CSF. However, the experimentally observed T₂ difference between SAS and frontal horns was greater than that attributable to typical exchange effect, suggesting that the T₂ shortening in SAS may reflect a combined effect of exchange and deoxyhemoglobin susceptibility.

Quantification of SAS CSF relaxation times may be useful to assess the brain-CSF exchange.