Specific APT and NOE Imaging Using DSP-CEST in Humans at 3 T

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Spectroscopy

Background:

• Chemical Exchange Saturation Transfer (CEST) imaging is crucial for quantifying biological processes but faces challenges with specificity, particularly at 3T where signal pools overlap.
• Conventional methods like asymmetry analysis and Lorentzian fitting have limitations in distinguishing overlapping CEST effects, such as amide proton transfer (APT) and nuclear Overhauser enhancement (NOE).
• The existing double saturation power (DSP)-CEST method, initially developed for continuous-wave saturation, requires adaptation for pulsed sequences and human imaging.

Purpose of the Study:

• To adapt and validate the double saturation power (DSP)-CEST method for pulsed saturation sequences in human 3T MRI.
• To assess the specificity and robustness of DSP-CEST in differentiating and quantifying amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) effects.
• To compare DSP-CEST results with conventional asymmetry analysis and Lorentzian difference (LD) analysis in vivo.

Main Methods:

• Development of a pulsed DSP-CEST sequence for 3T MRI.
• Validation using simulations and phantom experiments to confirm specificity.
• In vivo application in six healthy human subjects to evaluate quantification of APT and NOE effects using Magnetization Transfer Ratio (MTR) and Apparent Exchange-dependent Relaxation (AREX) metrics.

Main Results:

• DSP-CEST successfully eliminated confounding signals, enabling specific quantification of APT and NOE effects.
• Significant differences in APT effects were observed between white matter (WM) and gray matter (GM), with lower MTR and higher AREX values in WM.
• AREX-quantified NOE effects were significantly higher in WM than GM, while MTR showed no significant difference, supporting distinct origins for APT and NOE.

Conclusions:

• The pulsed DSP-CEST method provides a robust and specific approach for quantifying APT and NOE effects in human brain imaging at 3T.
• DSP-CEST offers improved specificity over asymmetry analysis and robustness over model-dependent fitting methods.
• The observed differences in APT and NOE quantification between WM and GM using DSP-CEST provide valuable insights into tissue microenvironment characteristics.