A Rapid T1-Independent Sequence for Cerebrospinal Fluid Water Standard Correction: Application to Improved Proton Density Mapping in Brain.

PURPOSE: To improve brain water content measures relative to cerebrospinal fluid (CSF) by including a rapid T1-independent sequence to correct for CSF T1 errors. METHODS: Proton density (PD) was estimated from a 3D multiple echo gradient echo sequence by extrapolating to TE = 0 and then correcting for T1 losses using a T1 map from 3D MP2RAGE. To overcome errors in CSF T1, a T1-independent single-shot echo planar imaging (EPI) sequence was introduced to determine the CSF-to-tissue signal ratio and then rescale the resulting relative PD map into a quantitative measurement. The technique is demonstrated in phantom and five healthy subjects. In addition to producing PD scaled to CSF water content, the method enables calculation of CSF T1. RESULTS: Phantom experiments confirmed the validity of the approach, reducing T1 error after EPI correction from 54.2% to 1.0% in comparison with an inversion recovery (IR) gold standard. In five healthy subjects, the average CSF reference correction factor was 1.48, and the mean CSF T1 error was reduced from (45.8% ± 0.8%) down to (1.1% ± 0.5%) compared to the IR gold standard. Regional PD measurements within the human brain were in agreement with previous literature. CONCLUSION: Introduction of a rapid T1-independent sequence enabled correction of CSF-referenced PD maps, avoiding the difficulty of direct CSF T1 measurement.