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 (∼0.5 ms) and short acquisition delay (<0.2 ms) to minimize T2‐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 (∼72 s). The AWSOS sequence uses variable‐duration slice encoding to minimize T2 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. T2‐ and off‐resonance induced slice widening and image blurring were calculated from 1.5 to 7 Tesla (T) 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 (Gmax = 40 mT/m, Smax = 150 T/m/s). Accounting for the limits on B1 maximum, specific absorption rate (SAR), and the lowered amplitude of slab‐select gradient, a sinc radiofrequency (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 × 24 spirals). Compared with long‐TE (10 ms) images, the ultrashort‐TE AWSOS images provided clear visualization of short‐T2 tissues such as the nose cartilage, the eye optic nerve, and the brain meninges and parenchyma. Magn Reson Med 60:135–145, 2008. © 2008 Wiley‐Liss, Inc.