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

NMR Biomed. 2021 Jan;34(1):e4403. doi: 10.1002/nbm.4403. Epub 2020 Sep 15.

Abstract

Chemical exchange saturation transfer (CEST) MRI is currently set to become part of clinical routine as it enables indirect detection of low concentrated molecules and proteins. Recently, intermediate to fast exchanging functional groups of glucose and its derivatives, glutamate and dextran, have gained attention as promising CEST contrast agents. To increase the specificity of CEST MRI for certain functional groups, the presaturation module is commonly optimized. At an early stage, this is performed in well-defined model solutions, in which, for instance, the relaxation times are adjusted to mimic in vivo conditions. This often involves agar, assuming the substance would not yield significant CEST effects by itself, which the current study proves to be an invalid assumption. Model solutions at different pH values and concentrations of agar were investigated at different temperatures at a 9.4 T human whole body MR scanner. High power presaturation of around 4 μT, optimal for investigating intermediate to fast exchanging groups, was applied. Postprocessing included spatiotemporal corrections for B0 and spatial corrections for B1+ . CEST effects of up to 3 % of the bulk water signal were observed. From pH, concentration and temperature dependency, it was concluded that the observed behavior reflects a CEST effect of agar. It was also shown how to remove this undesirable contribution from CEST MRI data. It was concluded that if agar is involved in the CEST MRI parameter optimization process, its contribution to the observed effects has to be taken into account. CEST agent concentration must be sufficiently high to be able to neglect the contribution of agar, or a control sample at matched pH is necessary for correction. Experiments on pure agarose showed reduced CEST effects compared with agar but did not provide a neutral baseline either.

Keywords: CEST effect agar, CEST MRI parameter optimization, gluCEST MRI optimization, glutamate CEST.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Agar / chemistry*
  • Glutamic Acid / analysis
  • Hydrogen-Ion Concentration
  • Magnetic Resonance Imaging*
  • Models, Theoretical*
  • Solutions

Substances

  • Solutions
  • Glutamic Acid
  • Agar