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Investigating the origin of pH‐sensitive magnetization transfer ratio asymmetry MRI contrast during the acute stroke: Correction of T 1 change reveals the dominant amide proton transfer MRI signal
Author(s) -
Wu Limin,
Jiang Liang,
Sun Phillip Zhe
Publication year - 2020
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.28313
Subject(s) - magnetization transfer , nuclear magnetic resonance , chemistry , relaxation (psychology) , nuclear medicine , contrast (vision) , magnetic resonance imaging , stroke (engine) , diffusion mri , medicine , radiology , physics , optics , thermodynamics
Purpose Amide proton transfer (APT) MRI is promising to serve as a surrogate metabolic imaging biomarker of acute stroke. Although the magnetization transfer ratio asymmetry (MTR asym ) has been used commonly, the origin of pH‐weighted MRI effect remains an area of investigation, including contributions from APT, semisolid MT contrast asymmetry, and nuclear Overhauser enhancement effects. Our study aimed to determine the origin of pH‐weighted MTR asym contrast following acute stroke. Methods Multiparametric MRI, including T 1 , T 2 , diffusion and Z‐spectrum, were performed in rats after middle cerebral artery occlusion. We analyzed the conventional Z‐spectrumI Δ ωI 0and the apparent exchange spectrumR exΔ ω, being the difference between the relaxation‐scaled inverse Z‐spectrum and the intrinsic spinlock relaxation rateR 1·cos 2 θ ·I 0 I Δ ω- R 1 ρΔ ω. The ischemia‐induced change was calculated as the spectral difference between the diffusion lesion and the contralateral normal area. Results The conventional Z‐spectrum signal change at −3.5 ppm dominates that at +3.5 ppm (−1.16 ± 0.39% vs. 0.76 ± 0.26%, P < .01) following acute stroke. In comparison, the magnitude of ΔR ex change at 3.5 ppm becomes significantly larger than that at −3.5 ppm (−2.80 ± 0.40% vs. −0.94 ± 0.80%, P < .001), with their SNR being 7.0 and 1.2, respectively. We extended the magnetization transfer and relaxation normalized APT concept to the apparent exchange–dependent relaxation image, documenting an enhanced pH contrast between the ischemic lesion and the intact tissue, over that of MTR asym . Conclusion Our study shows that after the relaxation‐effect correction, the APT effect is the dominant contributing factor to pH‐weighted MTR asym following acute stroke.