A CSF Background Suppression Scheme in Arterial Spin Labeling MRI

Arterial spin labeling (ASL) MRI suffers from low signal-to-noise ratio. Current background suppression (BS) methods focus on suppressing tissue signal. The present work aims to test the hypothesis that BS schemes to suppress CSF signal can produce a greater benefit, given the recent observations that water in CSF has more pulsation as part of the neurofluid circulation. We developed a CSF BS scheme that maximally suppresses residual CSF signal using two inversion pulses. Its performance was compared with regular BS and enhanced BS, both of which primarily suppress gray and white matter signals. All schemes were evaluated in single-delay and multi-delay pseudo-continuous ASL (pCASL) MRI. The single-delay scans assessed cerebral blood flow (CBF), while the multi-delay scans measured both CBF and arterial transit time (ATT). Reproducibility was assessed using voxel-wise coefficient of variation (CoV) and spatial Spearman correlation coefficients (Rs). When using the CSF BS scheme, CSF signals were suppressed to less than 1% of the equilibrium magnetization, with gray and white matter signals around 5% in opposite magnetization signs. Complex control/label subtraction effectively accounted for magnetization signs and obtained the correct difference signal. In single-delay pCASL, CSF BS reduced visually apparent hyper- and hypo-intensity signals in CBF maps, which were present in the tissue-focused regular and enhanced BS schemes. These artifactual signal fluctuations were particularly pronounced in the brain-stem regions where CSF pulsation was the most severe, but also spread along the z-encoding direction. Quantitatively, CSF BS resulted in a lower voxel-wise CoV (8.8%) and higher R_s (0.89) in the CBF maps, compared with tissue-focused BS (CoV 18.8%, R_s 0.64). In multi-delay pCASL, CSF BS yielded a CoV of 5.9% and 11.3% for CBF and ATT maps, respectively, outperforming both enhanced and regular BS. These results demonstrate that CSF BS can reduce spurious signals and improve image quality in ASL perfusion MRI.