Effects of echo time and modeling of compartmental T2 on VERDICT parameter estimation in brain tumors

VERDICT MRI leverages the diffusion time-dependence of the diffusion MRI signal to estimate microstructural parameters in tumors. However, estimated parameters may be biased due to differences in T2 relaxation times between microstructural compartments, which are often neglected in conventional modelling. This study investigates how choice of echo time affects the estimated parameters in brain tumors and evaluates the use of relaxation-VERDICT, which incorporates T2 relaxation into the model to potentially reduce estimation biases and improve tumor microstructure estimation.

Ten adult patients with intracranial tumors (mean age 57 years, range 32-78 years, 5 males) underwent diffusion MRI using a dual-echo acquisition with varying echo times, b-values, and diffusion times. VERDICT model fitting was performed using datasets with either a fixed echo time or a variable minimum echo time, and with models that either included or excluded compartment-specific T2 relaxation.

Our results show that VERDICT parameter estimates vary significantly with the inclusion of T2 relaxation in the model, e.g. leading to a substantial increase of intracellular volume fraction. Estimated T2 relaxation was significantly shorter for the intracellular space (median 65 ms, IQR 60-70 ms) than for the extracellular space (median 246 ms, IQR 218-252 ms).

Results indicate that accounting for T2-related echo time effects is important for proper biophysical interpretation of estimated VERDICT parameters and may thereby improve diagnostic reliability for brain tumors.