Dissertation / PhD Thesis

DZNE-2026-00931

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Rapid, High-resolution Quantitative MRI in Human Brain at 3T and 7T

Wang, D.DZNE*

2025

Abstract: Magnetic Resonance Imaging (MRI) is an essential tool in clinical diagnostics and research, offering non-invasive visualization of internal structures with exceptional soft-tissue contrast. Traditional MRI focuses on generating qualitative images optimized for contrast in regions of interest, aiming for tissue differentiation. In contrast, quantitative MRI (qMRI) produces parametric maps that provide measurable tissue-specific properties, such as longitudinal relaxation time (T1), transverse relaxation times (T2 and T∗ 2), proton density (PD), and magnetization transfer saturation (MTsat). These quantitative parameters enhance our understanding of tissue microstructure and pathology, improving diagnostic accuracy and reproducibility.Multi-Parameter Mapping (MPM) and Phase-based T2 mapping are two important qMRI techniques that enable comprehensive tissue characterization with high precision and consistency. MPM utilizes multiple echoes with varying magnitude image contrasts to acquire 𝑇1, 𝑇∗ 2 , 𝑃𝐷, and 𝑀𝑇sat parameters simultaneously, while Phase-𝑇2 is extracted from phase images. Both methods were developed based on traditional gradient echo (GRE) sequences, which is a well-established sequence but can be timeconsuming. 3D Echo Planar Imaging (3D-EPI) is a highly efficient sequence offering improved signal-to-noise ratio efficiency. In this thesis, a skipped-CAIPI 3D-EPI (SC-EPI) sequence is utilized for the MPM approach and for a novel Magnitude-Phase-based Relaxometry (MPR) method introduced here to improve the Phase-𝑇2 approach. The SC-EPI acquisition is explored in this work to achieve rapid, high-resolution quantitative imaging with improved efficiency, accuracy, and reproducibility. These advancements are intended to support both clinical and research applications.The first significant contribution of this thesis is the adaptation and validation of an MPM framework using SC-EPI. This work focuses on acquisition efficiency while maintaining the accuracy and reproducibility of quantitative parameter maps. The MPM approach, implemented within the hMRI toolbox, facilitates the generation of 𝑇1, 𝑇∗ 2 , 𝑃𝐷, and 𝑀𝑇sat maps using three multi-echo gradient echo images. The protocol of the SC-EPI sequence was optimized for MPM at 3T and 7T. Quantitative parameter maps were validated with phantom and in-vivo measurements. The reproducibility of these parameter maps was evaluated through scan-rescan experiments, using Coefficient of Variation analysis to validate the reliability of this method within a short scan time. Furthermore, this thesis includes first results of ultra-high resolution MPM at 7T, where parallel transmission (pTx) techniques were employed to address B1 inhomogeneity at ultra-high fields.The second major contribution focuses on the development and validation of MPR for improved T2 mapping. Building on previously proposed Phase-based T2 mapping, magnitude data was incorporated to modify the method and improve its accuracy. Monte Carlo simulations were conducted to optimize the MPR method, and the accuracy and precision of 𝑇2 estimates were validated by comparison to the Phase-based T2 through phantom and in-vivo experiments performed at 3T and 7T. To further mitigate residual biases caused by RF field inhomogeneities at 7T, pTx pulses were utilized by the MPR framework, demonstrating promising results.This thesis contributes to the development and assessment of rapid and reliable qMRI techniques, increasing their clinical feasibility, highlighting their potential for high-resolution imaging and addressing critical challenges associated with ultra-high-field MRI.

Note: Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn, 2025

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Contributing Institute(s):

1. MR Physics (AG Stöcker)

Research Program(s):

1. 354 - Disease Prevention and Healthy Aging (POF4-354) (POF4-354)

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