Water diffusion and exchange as they influence contrast enhancement

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biophysics
• Physiology

Background:

• Contrast-enhanced MRI signal intensity is influenced by contrast agent effects on tissue water magnetization, not directly by concentration.
• Accurate interpretation of MRI studies necessitates understanding water movement's impact on the magnetic resonance signal.

Purpose of the Study:

• To review how water diffusion and exchange between biological compartments affect MR signal enhancement.
• To discuss the mechanisms by which contrast agents influence water magnetization: direct relaxivity and indirect susceptibility effects.
• To explore the implications of these effects for extracting physiologic information from MRI.

Main Methods:

• Theoretical presentation of relaxation enhancement for T1-dominant contrast agents.
• Review of relevant physiologic time constants for water movement impacting relaxation enhancement.
• Discussion of experimental challenges in measuring relaxation enhancement.
• Analysis of susceptibility effects, considering contrast agent and compartment properties, and water movement dynamics.

Main Results:

• Water diffusion and exchange significantly alter MR signal enhancement.
• Relaxivity agents' effects are explained by relaxation enhancement theory, influenced by water movement time constants.
• Susceptibility effects are complex, dependent on agent/compartment geometry and water flow, but potentially powerful.

Conclusions:

• Understanding water dynamics is critical for accurate contrast-enhanced MRI interpretation.
• Both relaxivity and susceptibility effects of contrast agents are modulated by water movement.
• Susceptibility effects, despite modeling complexity, offer potential utility in MRI even without full quantification.