Ultrafast imaging using gradient echoes.

Ultrafast magnetic resonance (MR) imaging techniques can reduce scan times to less than 1 s. The rapid acquisition minimizes motion artifacts that have plagued MR studies of the heart and abdomen, and facilitates dynamic studies to observe physiological function. We first discuss fast gradient-echo methods, including various spoiled and steady-state gradient-echo techniques. Ultrafast methods are then considered, with the focus on turbo-fast low-angle shot (FLASH) (also known as snapshot or subsecond FLASH) imaging. Although turbo-FLASH is a subset of gradient echo methods, there are several distinguishing features. For instance, with T1- or T2-weighted turbo-FLASH, the magnetization never reaches a steady state, so that the phase encode order becomes an important imaging parameter. Furthermore, image contrast is obtained by adjusting the magnetization preparation module, which is independent of the data acquisition module that follows. The signal behavior and strategies for contrast optimization are discussed. Potential clinical applications, including perfusion imaging, cardiac cine, breath-hold abdominal imaging, angiography, diffusion imaging, and three-dimensional studies, are explored.