Towards the molecular origin of glutamate CEST (GluCEST) imaging in rat brain

Purpose Chemical exchange saturation transfer (CEST) signals at 3 ppm acquired with high irradiation powers, termed GluCEST, have been suggested as an indicator of glutamate in neural systems. However, amines in proteins may also contribute to the GluCEST signal, but they have been ignored in previous studies. This study aims to investigate the molecular origins and specificity of GluCEST signal in rat brain. Methods Dialysis was used to selectively remove small molecules, such as glutamate and other metabolites, from tissue homogenates prepared from rat brain. This approach allowed the specific influence of proteins on GluCEST to be measured. Clean CEST effects were also quantified by a 2-step analysis, which first used a fitting approach to quantify the asymmetric magnetization transfer effect and an inverse subtraction to remove it, and then combined an apparent exchange-dependent relaxation method with asymmetric analysis to remove the direct water saturation and T-1 weighting. Results The removal of glutamate and other small metabolites by dialysis decreases the GluCEST signal by approximately 24%, indicating that a major contributor to GluCEST in the brain is amines in proteins under our experimental conditions. Conclusion Data from in vivo GluCEST measurements should be interpreted with caution, especially in disorders in which changes in the content or conformation of proteins are expected.