Pulse sequence design for fast and flexible sodium magnetic resonance imaging

Sodium (²³Na) magnetic resonance imaging can be used to obtain information that is complementary to that of routine clinical hydrogen (¹H) MRI. Imaging ²³Na is challenging due to its low in-vivo signal-to-noise ratio and rapid transverse relaxation. Three-dimensional (3D) ultrashort echo time (UTE) sequences, often used for ²³Na MRI, involve whole-volume acquisitions that require scan times of 10–34 min. Two-dimensional (2D) UTE sequences can be used to image a reduced number of slices, but scan times range 8–10 min per slice. This dissertation explores the design of two novel pulse sequences that combine simultaneous multi-slice and UTE techniques to reduce scan times for ²³Na MRI: (1) The power independent of number of slices presaturated UTE (PINS-UTE) sequence uses a prepulse to saturate wide regions of magnetization and a non-selective pulse to simultaneously excite three slices; (2) The differential multi-block presaturated UTE (δMB-UTE) uses a prepulse to saturate two blocks of magnetization and a non-selective pulse to excite the surrounding magnetization. By subtracting acquisitions with shifted saturation blocks, four simultaneous slices are imaged. Both sequences are first implemented for ¹H MRI, then modified for ²³Na MRI. The δMB-UTE sequence enabled the rapid imaging of a reduced through-plane field-of-view (FOV) with both ¹H and ²³Na MRI. Images obtained with the PINS-UTE sequence exhibited signal variation due to unwanted signal between the slices. When acquiring ²³Na images with the same FOV, resolution, and repetition time, the scan times required by the PINS-UTE and δMB-UTE sequences are expected to be reduced by a factor of 12 as compared to 2D UTE sequences. Compared to similarly matched 3D UTE sequences, the scan time of the PINS-UTE sequence is expected to be increased by a factor of 1.75, while the scan time of the δMB-UTE sequence is expected to be comparable. For both sequences, greater acceleration relative to 3D UTE sequences may be achieved when fewer slices are imaged or when high in-plane resolution is desired. The sequences introduced in this dissertation may enable the integration of ²³Na MRI into clinical protocols and support studies of diseases such as multiple sclerosis, stroke, and cancer.