Quantitative Macromolecular Proton Fraction Imaging using Pulsed Spin-Lock

TL;DR

This paper introduces MPF-PSL, a pulsed spin-lock MRI technique that enables robust, high-SNR macromolecular proton fraction imaging within hardware limits, demonstrated through simulations, phantom, and in vivo studies for clinical use.

Contribution

The paper presents a novel pulsed spin-lock method for MPF imaging that overcomes hardware constraints and enhances clinical applicability.

Findings

MPF-PSL is insensitive to water pool parameters.

It enables high-SNR MPF quantification.

Validated in vivo for liver fibrosis detection.

Abstract

Purpose: Recent studies have shown that spin-lock MRI can simplify quantitative magnetization transfer (MT) by eliminating its dependency on water pool parameters, removing the need for a T1 map in macromolecular proton fraction (MPF) quantification. However, its application is often limited by the requirement for long radiofrequency (RF) pulse durations, which are constrained by RF hardware capabilities despite remaining within specific absorption rate (SAR) safety limits. Methods: To address this challenge, we propose a novel method, MPF mapping using pulsed spin-lock (MPF-PSL). MPF-PSL employs a pulsed spin-lock train with intermittent free precession periods, enabling extended total spin-lock durations without exceeding hardware and specific absorption rate limits. A comprehensive analytical framework was developed to model the magnetization dynamics of the two-pool MT system under pulsed spin-lock, demonstrating that MPF-PSL achieves MT-specific quantification while minimizing confounding effects from the water pool. The proposed method is validated with Bloch-McConnell simulations, phantoms, and in vivo studies at 3T. Results: Both Bloch-McConnell simulations and phantom validation demonstrated that MPF-PSL exhibits robust insensitivity to water pool parameters while enabling high-SNR MPF quantification. In vivo validation studies confirmed the method's clinical utility in detecting collagen deposition in patients with liver fibrosis. Conclusion: MPF-PSL presents a practical solution for quantitative MT imaging, with strong potential for clinical applications.