Water excitation with LIBRE pulses in three-dimensional variable flip angle fast spin echo for fat-free and large field of view imaging at 3 tesla

Three-dimensional (3D) variable flip angle fast spin echo sequence (vf-FSE) enables 3D acquisition of high spatial resolution, high signal-to-noise ratio (SNR), and a clinically acceptable scan time [1]. The 3D isotropic imaging of vf-FSE allows multi-directional and multi-planar reconstruction for clearly visualizing anatomical structures and lesions. The vf-FSE sequence has been commonly used in blood vessel wall imaging [2,3], peripheral nerve imaging [4,5], 3D magnetic resonance cholangiopancreatography (MRCP) imaging [1,6,7], musculoskeletal imaging [[8], [9], [10]], and spinal cord imaging [11,12]. However, fat suppression of vf-FSE in the large field of view imaging remains a challenge in clinical practices.

Fat suppression technique is commonly used for maximizing the contrast between tissue and lesions, reducing fat-related motion artifacts [13] and chemical shift artifacts [14]. As a 3D sequence, vf-FSE usually covers a large volume which increases the inhomogeneity of the main magnetic field (B0), leading to inhomogeneous and insufficient fat suppression. For instance, the conventional fat suppression technique (FatSat) which is commonly used in vf-FSE is sensitive to B0 inhomogeneities and thus may fail for fat suppression in large volume imaging [15].

Recently, a new water excitation (WE) module named lipid insensitive binomial off-resonant RF excitation (LIBRE) pulses was developed for fat suppression by Bastiaansen and Stuber [18]. Compared with the conventional WE module, LIBRE uses two off-resonance excitation pulses that are almost identical and without interval between them. Previous studies demonstrated that LIBRE is robust to magnetic field inhomogeneities for simultaneously fat suppression and water excitation in gradient echo sequences (GRE) [[18], [19], [20]]. However, the previous studies are based on GRE sequence, in which a small excitation flip angle (~22°) is employed. The flip angles of the sub-pulses in LIBRE are also small and can be set as the desired excitation flip angle, regardless of the offset resonance frequency. As the excitation of spin echo is as large as 90°, it is unclear if LIBRE is still valid for fat suppression, especially in 3D vf-FSE sequence for the large field of view imaging.

To address this issue, we further investigate the fat suppression effects of LIBRE pulses, especially incorporating it into 3D vf-FSE for fat-free and large volume imaging at 3 T by numerical simulation, phantom, and healthy volunteer studies.

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