Quantitative pulsed CEST-MRI using Ω-plots

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Metabolomics

Background:

• Chemical Exchange Saturation Transfer (CEST) enables indirect detection of low-concentration metabolites in vivo using MRI.
• Current clinical CEST applications are limited by the use of pulsed radiofrequency (RF) saturation, which is theoretically challenging for quantification due to specific absorption rate (SAR) regulations.
• Accurate quantification of pulsed CEST is difficult, hindering its full potential in clinical settings.

Purpose of the Study:

• To develop a novel analytical framework for pulsed CEST MRI.
• To enable accurate quantification of pulsed CEST experiments.
• To provide deeper theoretical insight into pulsed CEST mechanisms.

Main Methods:

• An extension of the interleaved saturation-relaxation approach was used for theoretical analysis.
• Analytical integration of the continuous wave (cw) eigenvalue as a function of RF pulse shape was performed.
• The method was validated using creatine phantoms on a 7T MRI scanner and further assessed through simulations.

Main Results:

• A new analytical formula for pulsed CEST was derived, mirroring the structure of cw CEST but including pulse-shape-dependent form factors.
• The approach allows for analytical Z-spectrum calculations, offering enhanced understanding of pulsed CEST.
• The extended method successfully adapted Dixon's Ω-plot for pulsed saturation, enabling independent determination of exchange rate and relative proton concentration.

Conclusions:

• The developed analytical treatment provides a robust method for quantitative pulsed CEST MRI.
• The derived form factors allow direct comparison between pulsed and continuous wave (cw) saturation effects.
• This advancement facilitates more accurate and insightful application of CEST MRI in clinical research and diagnostics.