Rapid magnetization transfer magnetic resonance imaging

Magnetization transfer (MT) imaging provides a contrast reflecting the properties of hydrogen atoms bound to macromolecules in the tissue. These components, which are short-lived in nature, cannot be captured by conventional MRI methods. However, by saturating the magnetization of macromolecules using an off-resonance radio-frequency pulse, a signal intensity drop is induced, enabling the generation of MT contrast. In principle, MT is expressed in two different ways: (a) Magnetization transfer ratio (MTR), which is a semi-quantitative measure, and (b) quantitative magnetization transfer (qMT) which represent quantitative parameters underlying the MT effect. Both methods have demonstrated their usefulness in the diagnosis and prognosis of various pathologies, such as multiple sclerosis (MS). However, the integration of MT imaging into daily clinical practice remains a challenge due to the general long acquisition time of qMT imaging, the transmit field nonuniformities at high field, and the limited signal-to-noise at low fields.
This thesis aims to address these issues by developing fast and robust methodologies for MT imaging at both low (0.55 T) and high (3 T) field strengths. To this end, a fast spiral multi-slice spoiled gradient echo (SPGR) sequence, combined with a low-resolution B1-mapping for accurate MTR imaging in less than one minute was implemented. The same method was further developed to obtain accurate and easy-to-calculate whole-brain qMT maps within 5 minutes. Moreover, the feasibility of MT imaging at low field was investigated with an MT-sensitized balanced steady-state free precession (bSSFP) sequence, which slightly outperformed the product SPGR in terms of signal-to-noise ratio (SNR). We further demonstrated that an extremely efficient bSSFP-based sequence, termed bSTAR can be extended to MT imaging at low-field with a submillimeter isotropic resolution within the clinically acceptable scan time.
The techniques presented in this thesis facilitate a wider use of MT imaging in clinics for the diagnosis and prognosis of various diseases.