Phantom- and simulation-based validation of combined diffusion relaxometry in ex vivo ADRD white matter

In principle, combined T₂-Diffusion (D) MRI has the microstructural and chemical sensitivity to detect axonal and myelin water changes in Alzheimer's disease and related dementias (ADRD), but its practical implementation may be hindered by demanding hardware requirements. This work assesses the feasibility and accuracy of T₂-D for ex vivo analysis of WM lesions in ADRD tissue.

A thawed ex vivo brain sample from the Michigan Brain Bank and a T₂-D phantom were scanned at 7T using a combined diffusion relaxometry (CDR) sequence. A non-negative least squares (NNLS) conventional data processing pipeline was used to disentangle water pools with unique T₂-D signatures. Simulations examined the effects of minimum TE and SNR on recovery of myelin water (short T₂, slow diffusion).

Across tissue types, T₂-D data consistently resolved three spectral components. Phantom experiments showed detection of short T₂ and slow diffusion features similar to those observed in ADRD ex vivo tissue, and confirmed CDR's ability to accurately resolve multiple components. Simulations indicated reliable T₂-D recovery for myelin with SNR > 30 dB and minimum TE < 25 ms.

Strong T₂ and D weighting could be combined to capture the expected axonal, myelin, and extracellular (EC) regions in T₂-D space. The observed short-T₂, restricted-D components are therefore unlikely to be artifacts and instead support interpretations as physically meaningful myelin and axonal water signatures.