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Simultaneous experimental determination of labile proton fraction ratio and exchange rate with irradiation radio frequency power-dependent quantitative CEST MRI analysis

Authors

  • Phillip Zhe Sun,

    Corresponding author
    • Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, USA
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  • Yu Wang,

    1. Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, USA
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  • Gang Xiao,

    1. Department of Mathematics and Applied Mathematics, Hanshan Normal University, Chaozhou, Guangdong, China
    2. Department of Radiology, 2nd Affiliated Hospital of Shantou University Medical College, Shantou, Guangdong, China
    3. Provincial Key Laboratory of Medical Molecular Imaging, Shantou, Guangdong, China
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  • Renhua Wu

    Corresponding author
    1. Department of Radiology, 2nd Affiliated Hospital of Shantou University Medical College, Shantou, Guangdong, China
    2. Provincial Key Laboratory of Medical Molecular Imaging, Shantou, Guangdong, China
    • Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, USA
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Phillip Zhe Sun, Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA 02129, USA. Email: pzhesun@nmr.mgh.harvard.edu

Renhua Wu, Department of Radiology, 2nd Affiliated Hospital of Shantou University Medical College, Shantou, Guangdong, China. Email: rhwu@stu.edu.cn

Abstract

Chemical exchange saturation transfer (CEST) imaging is sensitive to dilute proteins/peptides and microenvironmental properties, and has been increasingly evaluated for molecular imaging and in vivo applications. However, the experimentally measured CEST effect depends on the CEST agent concentration, exchange rate and relaxation time. In addition, there may be non-negligible direct radio-frequency (RF) saturation effects, particularly severe for diamagnetic CEST (DIACEST) agents owing to their relatively small chemical shift difference from that of the bulk water resonance. As such, the commonly used asymmetry analysis only provides CEST-weighted information. Recently, it has been shown with numerical simulation that both labile proton concentration and exchange rate can be determined by evaluating the RF power dependence of DIACEST effect. To validate the simulation results, we prepared and imaged two CEST phantoms: a pH phantom of serially titrated pH at a fixed creatine concentration and a concentration phantom of serially varied creatine concentration titrated to the same pH, and solved the labile proton fraction ratio and exchange rate per-pixel. For the concentration phantom, we showed that the labile proton fraction ratio is proportional to the CEST agent concentration with negligible change in the exchange rate. Additionally, we found the exchange rate of the pH phantom is dominantly base-catalyzed with little difference in the labile proton fraction ratio. In summary, our study demonstrated quantitative DIACEST MRI, which remains promising to augment the conventional CEST-weighted MRI analysis. Copyright © 2013 John Wiley & Sons, Ltd.

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