Rapid 3D Whole Brain Molecular MRI at 7T

Abstract

Magnetic resonance imaging (MRI) has emerged as a leading, non-ionizing diagnostic tool in medical practice over the last decades. Beyond, MRI has evolved into an expansive field of research, yielding a multitude of novel advancements in technical innovations and a variety of new contrasts. The MR signal provides information not only regarding the tissue composition, but also concerning the magnetic, chemical, and physical interactions of the spin system. Here, chemical exchange saturation transfer (CEST) imaging utilizes the chemical exchange of protons and its associated magnetization transfer to provide insights into biochemistry with unprecedented resolution. The CEST technique represents a promising in-vivo approach to studying mobile proteins non-invasively. It has the potential to facilitate new insights into the pathogenesis of neurodegenerative diseases (NDs), given that many forms are associated with the accumulation of misfolded proteins. <br /> Aim of this thesis was the development of a robust and rapid technique for whole-brain CEST imaging at 7T, as the method benefits significantly from ultra-high field (UHF) strength in particular from the increased spectral resolution. Hence, a saturation scheme containing a train of Gaussian-shaped RF pulses was designed with the objective of maximizing the amide contrast resulting from the peptide bond in the backbone of mobile proteins while minimizing the required time. By employing a snapshot approach with centric-reordered 3D-EPI readout, a whole-brain field of view (FOV) is acquired after a single saturation block. One challenge associated with whole-brain imaging at UHF is the wide range of B1 variations, which significantly affects the CEST signal. To that end, it was necessary to implement an effective B1 correction method. <br /> The proposed approach enables the robust acquisition of artifact-free whole-brain CEST maps at 7T within a total scan time of less than 15min. In the snapshot 3D-EPI CEST sequence, the saturation block lasts for 3.6s, while the readout time ranges from 734ms to 1.1s, resulting in an isotropic resolution of 2mm and 1.61mm respectively. Consequently, the total duration of the sequence is less than 4min including 43 frequency offsets and two unsaturated images. The acquisition of two supplementary repetitions with varying B1 allows for the correction of the deviations in B1 within the cerebrum. The transfer of the sequence to a parallel transmission excitation (pTx) allows for the reduction of required repetitions. Furthermore, the so-called PUSHUP approach enables the quantification of CEST effects within the cerebellum. <br /> The snapshot 3D-EPI CEST approach is well-suited for large-scale and multi-modality studies given its manageable time effort and high degree of flexibility.