Quantification of water exchange across the blood-brain barrier using noncontrast MR fingerprinting

Abstract

Purpose: A method is proposed to quantify cerebral blood volume ( $v_b$ ) and intravascular water residence time ( ${\tau }_b$ ) using MR fingerprinting (MRF), applied using a spoiled gradient echo sequence without the need for contrast agent.

Methods: An in silico study optimized an acquisition protocol to maximize the sensitivity of the measurement to $v_b$ and ${\tau }_b$ changes. Its accuracy in the presence of variations in $T_{1,t}$ , $T_{1,b}$ , and $B_1$ was evaluated. The optimized protocol (scan time of 19 min) was then tested in a exploratory healthy volunteer study (10 volunteers, mean age 24 $\pm$ 3, six males) at 3 T with a repeat scan taken after repositioning to allow estimation of repeatability.

Results: Simulations show that assuming literature values for $T_{1,b}$ and $T_{1,t}$ , no variation in $B_1$ , while fitting only $v_b$ and ${\tau }_b$ , leads to large errors in quantification of $v_b$ and ${\tau }_b$ , regardless of noise levels. However, simulations also show that matching $T_{1,t}$ , $T_{1,b}$ , $B_1^{+}$ , $v_b$ and ${\tau }_b$ , simultaneously is feasible at clinically achievable noise levels. Across the healthy volunteers, all parameter quantifications fell within the expected literature range. In addition, the maps show good agreement between hemispheres suggesting physiologically relevant information is being extracted. Expected differences between white and gray matter $T_{1,t}$ (p < 0.0001) and $v_b$ (p < 0.0001) are observed, $T_{1,b}$ and ${\tau }_b$ show no significant differences, p = 0.4 and p = 0.6, respectively. Moderate to excellent repeatability was seen between repeat scans: mean intra-class correlation coefficient of $T_{1,t}:0.91$ , $T_{1,b}:0.58$ , $v_b:0.90$ , and ${\tau }_b:0.96$ .

Conclusion: We demonstrate that regional simultaneous quantification of $v_b$ , ${\tau }_b$ , $T_{1,b},T_{1,t}$ , and $B_1^{+}$ using MRF is feasible in vivo.

Keywords: MR fingerprinting; blood brain barrier; water exchange.