Spin-lock imaging of early tissue pH changes in ischemic rat brain

Area of Science:

• Magnetic Resonance Imaging
• Neuroscience
• Biochemistry

Background:

• Spin-lattice relaxation rates (R_1ρ) in the rotating frame are influenced by chemical exchange, particularly in brain tissue.
• Ischemic stroke alters brain tissue pH, affecting proton exchange rates and potentially impacting MRI signal.
• Amide proton transfer (APT) detects pH changes but suffers from reduced specificity due to confounding magnetization transfer effects.

Purpose of the Study:

• To evaluate the specificity of ΔR_1ρ, a modified R_1ρ technique, for detecting pH variations associated with ischemic stroke.
• To compare the pH sensitivity of ΔR_1ρ with conventional R_1ρ and APT in animal models of stroke.

Main Methods:

• Applied a spin-locking MRI sequence to acquire R_1ρ data at two different spin-locking amplitudes.
• Calculated ΔR_1ρ as the difference between R_1ρ values to isolate chemical exchange contributions.
• Performed numerical simulations and analyzed tissue homogenates to validate ΔR_1ρ specificity for acidosis.
• Acquired ΔR_1ρ and APT data in rat brains post-stroke and analyzed correlations.

Main Results:

• ΔR_1ρ demonstrated significant correlation with APT metrics, indicating sensitivity to pH changes.
• R_1ρ acquired with a single spin-locking amplitude did not show this correlation.
• Numerical simulations and tissue homogenate measurements confirmed ΔR_1ρ's specificity for detecting tissue acidosis.

Conclusions:

• ΔR_1ρ is a more specific MRI method than conventional R_1ρ or APT for detecting pH variations in ischemic stroke.
• This technique holds promise for non-invasive assessment of tissue acidosis in neurological conditions.