In vivo pH mapping with omega plot-based quantitative chemical exchange saturation transfer MRI

Area of Science:

• Biomedical Imaging
• Magnetic Resonance Imaging
• Metabolite Quantification

Background:

• Chemical exchange saturation transfer (CEST) MRI is valuable for detecting dilute metabolites and microenvironment properties.
• CEST MRI is increasingly used for imaging disorders like stroke and cancer.
• In vivo CEST MRI quantification is challenging due to combined effects of labile proton concentration and exchange rate.

Purpose of the Study:

• To independently quantify amide proton concentration and exchange rate in vivo.
• To investigate these parameters in a cardiac arrest-induced global ischemia rat model.

Main Methods:

• Utilized amide proton CEST (APT) effect analysis.
• Employed the image downsampling expedited adaptive least-squares (IDEAL) fitting algorithm on Z-spectra.
• Applied modified omega plot analysis to decouple and determine amide proton concentration and exchange rate simultaneously.

Main Results:

• Global ischemia significantly decreased the amide proton exchange rate (from 29.6 ± 5.6 to 12.1 ± 1.3 s⁻¹).
• Amide proton concentration showed minimal change (0.241 ± 0.035% vs. 0.202 ± 0.034%).
• The findings indicate that pH effects predominantly influence APT contrast during tissue ischemia.

Conclusions:

• Successfully determined labile proton concentration and exchange rate underlying in vivo APT MRI.
• The significant change in exchange rate, not concentration, highlights the dominant role of pH in APT contrast during ischemia.
• This method offers improved quantification for CEST MRI applications in disease imaging.