Balancing Accuracy and Precision: Optimal b-Values for Diffusion Tensor Imaging along the Perivascular Space.

BACKGROUND AND PURPOSE: The glymphatic system facilitates perivascular clearance, and its dysfunction has been implicated in neurodegenerative diseases. Diffusion tensor imaging along the perivascular space (DTI-ALPS) has been proposed as an indirect approach to assess glymphatic function, but its reliability is debated. The choice of b-value is an aspect of possible improvement. While a b-value of 1000 seconds/mm2 is commonly used, the optimal b-value for DTI-ALPS remains unknown. This study aims to determine the optimal b-value for DTI-ALPS.

MATERIALS AND

METHODS

Simulations were conducted to examine how the choice of maximum b-value influences bias, precision, and effect size of the ALPS index. DTI-ALPS was applied in a cohort of 194 participants divided into 4 groups: healthy controls (HCs) (n=42), patients with Parkinson disease (n=119), patients with Parkinson disease dementia (PDD) (n=16), and patients with progressive supranuclear palsy (PSP) (n=17). ALPS indices were calculated by manually placing regions of interest on projection and association fibers in each hemisphere. Group differences in ALPS indices across b-values were analyzed using mixed models.

RESULTS

In vivo, ALPS indices were higher at a b-value of 500 and 250 seconds/mm2 compared with a b-value of 1000 seconds/mm2 in both hemispheres. Simulations indicated a bias-variance trade-off: very low b-values reduced sensitivity and compromised precision, while high b-values improved precision but reduced accuracy. The simulated effect size of the ALPS index peaked at intermediate b-values (≈700 seconds/mm2). In vivo, ALPS indices were lower in patients with PDD and PSP compared with HCs, though differences varied across b-values.

CONCLUSIONS

Both simulations and in vivo results suggest that the commonly used b-value of 1000 seconds/mm2 is not optimal for assessing diffusion in the perivascular spaces. Intermediate b-values at approximately 700 seconds/mm2 appear more suitable. However, further optimization of acquisition parameters is needed.