Target Audience

Researchers and clinicians who are interested in CEST MRI.

Objectives

Participants should be able to:
1) understand CEST mechanism, signal enhancement principle, sequences, and quantification methods.
2) understand the specificity of a commonly used CEST quantification metric, MTR_asym.
3) know the specificity of CEST imaging of small metabolites.

Overview of Presentation

Chemical exchange saturation transfer (CEST) MRI provides a unique mechanism for producing contrast and makes MRI sensitive to the presence of metabolites, mobile macromolecules, and exogenous agents with exchangeable protons, as well as tissue chemical environment through their effects on the water signal. Because of these interesting contrasts, CEST has been applied in the diagnosis of tumor, ischemic stroke, neurologic disease, etc,. However, quantification of clean chemical exchange effect is challenging due to its strong dependence on multiple non-specific contributions. In the past decade, many quantification methods have been developed to increase its specificity.
This lecture will provide an overview of the CEST mechanism, signal enhancement principle, and sequences. We will also introduce several quantification methods including MTR_asym, multiple-pool Lorentzian fit, chemical exchange rotation transfer (CERT), variable delay multi-pulse (VDMP), quantifying exchange using saturation power or time (QUESP/QUEST), omega plot, and ratiometric approach. Among them, MTR_asym is the most widely used quantification method and can be easily performed. However, its specificity in quantifying amide proton transfer (APT) is still under debate. Here, we will investigate the specificity of MTR_asym through theoretical analysis and numerical simulations. We will show that although MTR_asym is influenced by the upfield nuclear Overhauser enhancement (NOE) effect, MTR_asym acquired in steady state at low field may be roughly insensitive to T₁ by carefully adjusting the saturation power, and MTR_asym acquired in non-steady state (e.g. 500 ms saturation time) may be roughly insensitive to T₁ and have weak dependence on magnetization transfer (MT) effect with low saturation power (e.g. < 1μT). Proteins also have exchangeable amine protons at 2 ppm and 3 ppm. Their CEST signals may overlap with those from creatine at 2 ppm and glutamate at 3 ppm. We will introduce a method which uses dialyzed tissue homogenates to remove small metabolites to investigate the influence of proteins on CEST signals at 2 ppm and 3 ppm.We will show that 34% CEST signal at 2 ppm and 76% CEST signal at 3 ppm in brain may be from proteins. This suggests that the interpretation of CEST contrast in diseases where there are significant change in the content of proteins should be careful.

Acknowledgements

No acknowledgement found.

References

No reference found.