Does the packed impermeable sphere model explain time-dependent diffusion of brain tumors measured with clinical scanners?

Motivation: While biophysical models typically represent tumor as packed impermeable spheres, it is nontrivial whether such picture is appropriate within the range of diffusion time available with clinical scanner. Goal(s): We aimed to identify the power-law scaling that describes the time-dependent diffusion of brain tumors measured with a clinical scanner. Approach: We measured the diffusion coefficient of seventy-six brain tumors at diffusion times between 35-300 ms using a stimulated-echo based sequence. Results: The diffusion coefficient of meningiomas and metastatic tumors decreased with diffusion time slower than the prediction from the packed impermeable sphere model. Impact: The tissue features dominating diffusion time dependence of tumors may vary depending on histology and may possibly deviate from the packed impermeable sphere model.