Acquisition-weighted stack of spirals for fast high-resolution three-dimensional ultra-short echo time MR imaging

Ultra-short echo time (UTE) MRI requires both short excitation (similar to 0.5 ms) and short acquisition delay (< 0.2 ms) to minimize T-2-induced signal decay. These requirements currently lead to low acquisition efficiency when high resolution (< 1 mm) is pursued. A novel pulse sequence, acquisition-weighted stack of spirals (AWSOS), is proposed here to acquire high-resolution three-dimensional (3D) UTE images with short scan time (similar to 72 s). The AWSOS sequence uses variable-duration slice encoding to minimize T-2 decay, separates slice thickness from in-plane resolution to reduce the number of slice encodings, and uses spiral trajectories to accelerate in-plane data collections. T-2- and off-resonance induced slice widening and image blurring were calculated from 1.5 to 7 Tesla M through point spread function. Computer simulations were performed to optimize spiral interleaves and readout times. Phantom scans and in vivo experiments on human heads were implemented on a clinical 1.5T scanner (G(max) = 40 mT/m, S-max = 150 T/m/s). Accounting for the limits on B-1 maximum, specific absorption rate (SAR), and the lowered amplitude of slab-select gradient, a sinc radio-frequency (RF) pulse of 0.8ms duration and 1.5 cycles was found to produce a flat slab profile. High in-plane resolution (0.86 mm) images were obtained for the human head using echo time (TE) = 0.608 ms and total shots = 720 (30 slice-encodings x 24 spirals). Compared with long-TE (10 ms) images, the ultrashort-TE AWSOS images provided clear visualization of short-T-2 tissues such as the nose cartilage, the eye optic nerve, and the brain meninges and parenchyma.