• Jian Hou, Vincent Wai-Sun Wong, Baiyan Jiang, Yi-Xiang Wang, Grace Lai-Hung Wong, Anthony Wing-Hung Chan, Winnie Chiu-Wing Chu, and Weitian Chen.
• Department of Imaging and Interventional Radiology, the Chinese University of Hong Kong, Hong Kong SAR, China.
• Magn Reson Med. 2020 Dec 1; 84 (6): 3157-3171.

PurposeIn MRI, the macromolecular proton fraction (MPF) is a key parameter of magnetization transfer (MT). It represents the relative amount of immobile protons associated with semi-solid macromolecules involved in MT with free water protons. We aim to quantify MPF based on spin-lock MRI and explore its advantages over the existing MPF-mapping methods.MethodsIn the proposed method, termed MPF quantification based on spin-lock (MPF-SL), off-resonance spin-lock is used to sensitively measure the MT effect. MPF-SL is designed to measure a relaxation rate (Rmpfsl ) that is specific to the MT effect by removing the R1ρ relaxation due to the mobile water and chemical exchange pools. A theory is derived to quantify MPF from the measured Rmpfsl . No prior knowledge of tissue relaxation parameters, including T1 or T2 , is needed to quantify MPF using MPF-SL. The proposed approach is validated with Bloch-McConnell simulations, phantom, and in vivo liver studies at 3.0T.ResultsBoth Bloch-McConnell simulations and phantom experiments show that MPF-SL is insensitive to variations of the mobile water pool and the chemical exchange pool. MPF-SL is specific to the MT effect and can measure MPF reliably. In vivo liver studies show that MPF-SL can be used to detect collagen deposition in patients with liver fibrosis.ConclusionA novel MPF imaging method based on spin-lock MRI is proposed. The confounding factors are removed, and the measurement is specific to the MT effect. It holds promise for MPF-sensitive diagnostic imaging in clinical settings.© 2020 International Society for Magnetic Resonance in Medicine.

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