Macromolecular proton fraction mapping based on spin‐lock magnetic resonance imaging
Jian Hou, Vincent Wai‐Sun Wong, Baiyan Jiang, Yi-Xiang Wang, Grace Lai‐Hung Wong, Anthony W.H. Chan, Chiu‐Wing Winnie Chu, Weitian Chen
Abstract
Purpose In 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. Methods In 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 (R mpfsl ) that is specific to the MT effect by removing the R 1ρ relaxation due to the mobile water and chemical exchange pools. A theory is derived to quantify MPF from the measured R mpfsl . No prior knowledge of tissue relaxation parameters, including T 1 or T 2 , 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. Results Both 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. Conclusion A 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.