On the interference from agar in chemical exchange saturation transfer MRI parameter optimization in model solutions

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Chemical Physics

Background:

• Chemical exchange saturation transfer (CEST) MRI is emerging in clinical practice for detecting low-concentration molecules.
• Glucose, glutamate, and dextran derivatives are promising CEST contrast agents.
• Model solutions, often using agar, are crucial for optimizing CEST MRI parameters, assuming agar itself has negligible CEST effects.

Purpose of the Study:

• To investigate the assumption that agar does not contribute significant CEST effects in model solutions.
• To characterize the CEST effects of agar across varying pH, concentration, and temperature.
• To develop methods for mitigating agar's confounding influence in CEST MRI.

Main Methods:

• Agar solutions at various pH, concentrations, and temperatures were analyzed using a 9.4 T MRI scanner.
• High power presaturation (4 μT) was applied, optimized for intermediate to fast exchanging groups.
• Postprocessing involved spatiotemporal corrections for B0 and spatial corrections for B1+.

Main Results:

• Significant CEST effects, up to 3% of the bulk water signal, were observed in agar solutions.
• The observed CEST effects demonstrated dependency on pH, concentration, and temperature, confirming agar's contribution.
• Methods for correcting or removing agar's contribution from CEST MRI data were demonstrated.

Conclusions:

• Agar exhibits a notable intrinsic CEST effect that must be accounted for during CEST MRI parameter optimization.
• To ensure accurate CEST MRI results, agar's contribution can be managed by using sufficiently high CEST agent concentrations or matched control samples.
• Pure agarose showed reduced but still present CEST effects, indicating it is not a neutral baseline.