Prog Med Phys.  2018 Mar;29(1):29-41. 10.14316/pmp.2018.29.1.29.

Preliminary Phantom Experiments to Map Amino Acids and Neurotransmitters Using MRI

Affiliations
  • 1Department of Biomedical Engineering, Graduate School, Kyung Hee University, Yongin, Korea.
  • 2Biomedical Research Center, Asan Institute for Life Sciences, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.
  • 3Department of Radiology, Kyung Hee University Hospital at Gangdong, College of Medicine, Kyung Hee University, Seoul, Korea. ghjahng@gmail.com
  • 4Department of Biomedical Engineering, College of Electronics and Information, Kyung Hee University, Yongin, Korea.

Abstract

The objective of this study was to evaluate the chemical exchange saturation transfer (CEST) effect of amino acids and neurotransmitters, which exist in the human brain, depending on the concentration, pH, and amplitude of the saturation radiofrequency field. Phantoms were developed with asparagine (Asn), γ-aminobutyric acid (GABA), glutamate (Glu), glycine (Gly), and myoinositol (MI). Each chemical had three different concentrations of 10, 30, and 50 mM and three different pH values of 5.6, 6.2, and 7.4. Full Z-spectrum CEST images for each phantom were acquired with a continuous-wave radiofrequency (RF) saturation pulse with two different B₁ amplitudes of 2 µT and 4 µT using an animal 9.4T MRI system. A voxel-based CEST asymmetry was mapped to evaluate exchangeable protons based on amide (-NH), amine (-NHâ‚‚), and hydroxyl (-OH) groups for the five target molecules. For all target molecules, the CEST effect was increased with increasing concentration and B1 amplitude; however, the CEST effect with varying pH displayed a different trend depending on the characteristics of the molecule. On CEST asymmetric maps, Glu and MI were well visualized around 3.0 and 0.9 ppm, respectively, and were well separated macroscopically at a pH of 7.4. The exchange rates of Asn, Glu, BABA, and Gly usually decreased with increasing pH. The CEST effect was dependent on the concentration, acidity of the target molecules, and B1 amplitude of the saturation RF pulse. The CEST effect for Asn can be observed in a 9.4T MRI system. The results of this study are based on applying the CEST technique in patients with neurodegenerative diseases when proteins in the brain are increased with disease progression.

Keyword

9.4T MRI; Chemical exchange saturation transfer; Amino acid; Neurotransmitter; Exchange rate
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