Table 1.

The comparison of the novel tracer-based MRI and classical approaches.

	Tracer-based MRI	Real-time ion introduction	Integrative optical imaging
Tracer	Gd-DTPA	Positive ion (TMA+)	Fluorescent probe (Dextron)
Signal	Radio wave	Electric potential	Fluorescence
Detection Capability	Global	Distance < 200 μm	Depth < 200 μm
Imaging	3D	No	2D
Parameters	D, λ, t1/2, k	D, λ, α	D, λ
Influenced by brain activity	No	Yes	No
Mathematical model	3D anisotropic diffusion equation	1D Isotropic diffusion equation	2D Isotropic diffusion equation

TMA⁺: cation tetramethylammonium.

D: diffusion coefficient.

λ tortuosity.

α volume fraction.

t_1/2 half-life.

k clearance rate constant.