MRI with Zero Echo Time

Abstract

MRI with zero echo time is a robust, fast, and silent approach for three-dimensional (3-D) imaging of samples with short transverse relaxation times. In this article, the principles of acquiring and reconstructing data with zero echo time are described and complemented by discussions of implementation details, potential limitations, and selected applications, as well as advanced and related techniques. MR imaging of samples with very short T2s requires spatial encoding and data acquisition to be started and completed rapidly after signal creation. One powerful way of meeting these requirements is with 3-D radial center-out k-space encoding. In particular, when RF excitation is performed after switching on the projection gradient, such schemes achieve an echo time of zero and immediately have available maximum k-space speed. MRI with zero echo time poses two major challenges. First, RF pulses need to be of high bandwidth to uniformly excite the full range of Larmor frequencies induced by the projection gradient. Second, transmit–receive switching results in an initial period of dead time, because of which a certain range of data in the center of k-space is not acquired. This can be handled by acquisition oversampling in conjunction with algebraic image reconstruction. Keywords: short T2; radial MRI; high bandwidth; dead time; transmit–receive switching; silent imaging; algebraic reconstruction; ZTE imaging; lung; tendon; teeth