Mri Sequences

An MRI sequence is an ordered combination of RF and gradient pulses designed to acquire the data to form the image. In this chapter I will describe the basic gradient echo, spin echo and inversion recovery sequences used in MRI. The data to create an MR image is obtained in a series of steps. First the tissue magnetisation is excited using an RF pulse in the presence of a slice select gradient. The other two essential elements of the sequence are phase encoding and frequency encoding/read out, which are required to spatially localise the protons in the other two dimensions. Finally, after the data has been collected, the process is repeated for a series of phase encoding steps. The MRI sequence parameters are chosen to best suit the particular clinical application. The parameters affecting soft tissue contrast are described, and advanced sequences such as STIR, FLAIR, FISP, and FLASH are briefly introduced at the end of the chapter.