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Amide proton transfer imaging with improved robustness to magnetic field inhomogeneity and magnetization transfer asymmetry using saturation with frequency alternating RF irradiation
Author(s) -
Scheidegger Rachel,
Vinogradov Elena,
Alsop David C.
Publication year - 2011
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.22912
Subject(s) - magnetization transfer , asymmetry , nuclear magnetic resonance , saturation (graph theory) , homogeneity (statistics) , proton , magnetization , materials science , irradiation , robustness (evolution) , magnetic field , analytical chemistry (journal) , chemistry , molecular physics , physics , magnetic resonance imaging , computer science , nuclear physics , mathematics , chromatography , medicine , biochemistry , quantum mechanics , combinatorics , machine learning , gene , radiology
Amide proton transfer (APT) imaging has shown promise as an indicator of tissue pH and as a marker for brain tumors. Sources of error in APT measurements include direct water saturation, and magnetization transfer (MT) from membranes and macromolecules. These are typically suppressed by postprocessing asymmetry analysis. However, this approach is strongly dependent on B 0 homogeneity and can introduce additional errors due to intrinsic MT asymmetry, aliphatic proton features opposite the amide peak and radiation damping‐induced asymmetry. Although several methods exist to correct for B 0 inhomogeneity, they tremendously increase scan times and do not address errors induced by asymmetry of the z‐spectrum. In this article, a novel saturation scheme—saturation with frequency alternating RF irradiation (SAFARI)—is proposed in combination with a new magnetization transfer ratio (MTR) parameter designed to generate APT images insensitive to direct water saturation and MT, even in the presence of B 0 inhomogeneity. The feasibility of the SAFARI technique is demonstrated in phantoms and in the human brain. Experimental results show that SAFARI successfully removes direct water saturation and MT contamination from APT images. It is insensitive to B 0 offsets up to 180 Hz without using additional B 0 correction, thereby dramatically reducing scanning time. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.