Effective Modeling of Continuous Wave Z -Spectra for Quantitative CEST and CESL MRI Across Exchange Regimes

Area of Science:

• Magnetic Resonance Imaging
• Biophysics
• Spectroscopy

Background:

• Chemical exchange saturation transfer (CEST) and chemical exchange-sensitive spin-lock (CESL) MRI offer high sensitivity for spectroscopic imaging.
• Conventional analysis methods struggle with fast-exchange limits, impacting quantification accuracy.

Purpose of the Study:

• Introduce the AB-MT+n model for robust and reproducible quantification of continuous-wave Z-spectra.
• Enable accurate analysis across various chemical exchange regimes in MRI.

Main Methods:

• The AB-MT+n model uses a five-parameter basis for water saturation, semi-solid magnetization transfer, and fast exchange.
• The model can be extended with off-resonant components for separable slow- and intermediate-exchanging pools.
• Performance was validated through simulations, phantom studies, and in vivo glucose-enhanced CESL (glucoCESL) in mouse brains.

Main Results:

• Simulations demonstrated AB-MT+n recovers maximal information from Z-spectra with minimal parameterization.
• Phantom studies showed reliable quantification of glutamate but limitations for fast-exchanging glucose with conventional methods.
• The pool-unspecific asymmetry parameter (A) showed strong linearity with glucose concentration (R² > 80%).
• Reduced model orders proved sufficient for complex simulations and in vivo glucoCESL data.

Conclusions:

• AB-MT+n provides an accurate and acquisition-independent description of continuous-wave Z-spectra.
• The modular nature of the model enhances quantification, particularly in the challenging fast-exchange regime.
• This advancement improves the utility of CEST and CESL MRI for sensitive spectroscopic imaging.