Metabolic activity diffusion imaging (MADI): I. Metabolic, cytometric modeling and simulations.

Area of Science:

• Biophysics
• Biomedical Imaging
• Cell Physiology

Background:

• Cellular water efflux rate constant (k_io) reflects metabolic rate of Na+, K+-ATPase (NKA).
• Current methods require contrast agents (CAs), limiting in vivo application due to distribution issues.
• Noninvasive methods are needed to determine k_io in living tissues.

Purpose of the Study:

• To develop a noninvasive, CA-free method for determining the cellular water efflux rate constant (k_io) in vivo.
• To explore the use of diffusion-weighted MRI (DWI) for assessing cellular metabolic activity.
• To establish metabolic activity diffusion imaging (MADI) as a novel approach.

Main Methods:

• Utilized Monte Carlo simulations of water diffusion in virtual cellular ensembles.
• Employed a primitive model incorporating cell number density (ρ) and mean cell volume (V).
• Analyzed diffusion tensor magnitude across the entire b-space in DWI.

Main Results:

• The MADI model demonstrates that diffusion in semipermeable, compartmented spaces is not Gaussian and is diffusion time-dependent.
• Steady-state diffusion is primarily limited by membrane permeation (k_io) and cell encounter probabilities (ρ, V), not nanoscopic diffusion (D_n).
• The model accurately matches existing experimental data.

Conclusions:

• Metabolic Activity Diffusion Imaging (MADI) provides a noninvasive method to determine k_io in vivo.
• MADI leverages DWI to assess cellular metabolic rates, independent of contrast agents.
• This approach has significant implications for interpreting DWI data and understanding tissue physiology.