A Rapid T 1 ‐Independent Sequence for Cerebrospinal Fluid Water Standard Correction: Application to Improved Proton Density Mapping in Brain

ABSTRACT Purpose To improve brain water content measures relative to cerebrospinal fluid (CSF) by including a rapid T 1 ‐independent sequence to correct for CSF T 1 errors. Methods Proton density (PD) was estimated from a 3D multiple echo gradient echo sequence by extrapolating to TE = 0 and then correcting for T 1 losses using a T 1 map from 3D MP2RAGE. To overcome errors in CSF T 1 , a T 1 ‐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 T 1 . Results Phantom experiments confirmed the validity of the approach, reducing T 1 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 T 1 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 T 1 ‐independent sequence enabled correction of CSF‐referenced PD maps, avoiding the difficulty of direct CSF T 1 measurement.