DTI-ALPS is affected by fiber orientation differences

What the study found: DTI-ALPS (diffusion tensor image analysis along the perivascular space) values were influenced by differences in the orientation of projection and association fibers, and by off-axis fiber orientation. The study found that the DTI-ALPS index can be confounded by non-glymphatic factors.
Why the authors say this matters: The authors conclude that DTI-ALPS measures based on diffusion along Cartesian axes are affected by off-axis orientation of ALPS-associated fibers. They indicate that this should be considered when interpreting the DTI-ALPS index.
What the researchers tested: The researchers prospectively studied 44 healthy adults using 3 T diffusion-weighted spin-echo echo-planar imaging. They compared three ways of calculating the DTI-ALPS index, using delineations from four observers, and assessed whether deviation from expected anatomical orientation biased the measure.
What worked and what didn't: Inter-observer agreement was generally good to excellent. ALPS values differed significantly across methods, with higher values for methods that reduced bias from tract orientation deviation; left-sided values were higher than right-sided values for all three methods. Deviation from expected tract orientation significantly affected ALPS values, especially for the left projection pathway.
What to keep in mind: The study was done in 44 healthy adults, so the available summary does not describe results in other groups. The abstract does not provide additional limitations beyond the note that technical efficacy was Stage 1.

Key points

• DTI-ALPS values were influenced by off-axis orientation of associated fibers.
• The authors say DTI-ALPS can be confounded by non-glymphatic factors.
• Three calculation methods gave significantly different ALPS values.
• Methods that reduced orientation bias produced higher ALPS values.
• Left DTI-ALPS values were higher than right-sided values for all three methods.