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Improved method for post‐processing correction of B 1 inhomogeneity in glutamate‐weighted CEST images of the human brain
Author(s) -
Cember Abigail T. J.,
Hariharan Hari,
Kumar Dushyant,
Nanga Ravi P. R.,
Reddy Ravinder
Publication year - 2021
Publication title -
nmr in biomedicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.278
H-Index - 114
eISSN - 1099-1492
pISSN - 0952-3480
DOI - 10.1002/nbm.4503
Subject(s) - human brain , nuclear magnetic resonance , glutamate receptor , image processing , chemistry , physics , computer science , artificial intelligence , neuroscience , image (mathematics) , psychology , biochemistry , receptor
Glutamate‐weighted CEST (gluCEST) imaging is nearly unique in its ability to provide non‐invasive, spatially resolved measurements of glutamate in vivo. In this article, we present an improved correction for B 1 inhomogeneity of gluCEST images of the human brain. Images were obtained on a Siemens 7.0 T Terra outfitted with a single‐volume transmit/32‐channel receive phased array head coil. Numerical Bloch‐McConnell simulations, fitting and data processing were performed using in‐house code written in MATLAB and MEX (MATLAB executable). “Calibration” gluCEST data was acquired and fit with a phenomenological functional form first described here. The resulting surfaces were used to correct experimental data in accordance with a newly developed method. Healthy volunteers of varying ages were used for both fitted “calibration” data and corrected “experimental” data. Simulations allowed us to describe the dependence of CEST at 3.0 ppm (gluCEST) on saturation B 1 using a new functional form, whose validity was confirmed by successful fitting to real human data. This functional form was used to parameterize surfaces over the space ( B 1 , T 1 ), which could then be used to correct the signal from each pixel. The resulting images show less signal loss in areas of low B 1 and greater contrast than those generated using the previously published method. We demonstrate that, using this method with appropriate nominal saturation B 1 , the major limitation of correcting for B 1 inhomogeneity becomes the effective flip angle of the acquisition module, rather than inability to correct for inhomogeneous saturation. The lower limit of our correction ability with respect to both saturation and acquisition B 1 is about 40% of the nominal value. In summary, we demonstrate a more rigorous and successful approach to correcting gluCEST images for B 1 inhomogeneity. Limitations of the method and further improvements to enable correction in regions with severe pathology are discussed.