Pulse sequences for diffusion-weighted MRI

Magnetic resonance imaging (MRI) data collection is described in terms of pulse sequences, i.e. prescribed temporal applications of radio-frequency and gradient magnetic fields, which are in turn used for signal creation, spatial encoding, and control of image contrast. The pulse sequence determines what the image will look like, including the type of image contrast, and its resolution and quality. Understanding pulse sequences used for diffusion-weighted (DW) MRI requires some level of understanding of the underlying physics of MRI. Magnetic resonance images primarily reflect water and fat concentration; specifically, they reflect the signal from hydrogen nuclei. Additional gradient magnetic fields can be used in a pulse sequence to make the MR image contrast sensitive to thermal motion of water, but the resulting sensitization to cardiac and patient motion must be accounted for in order to obtain high quality images with meaningful information. Almost any MRI pulse sequence can be modified to become sensitive to diffusion. Very large diffusion-weighting gradients are required to produce incoherent phase shifts sufficient to produce signal decay in the presence of microscopic thermal motion. This feature of diffusion-weighted MRI is also its single greatest challenge. Given the challenges of multi-shot DWI, the most common pulse sequence for DW MRI is currently single-shot Echo Planar Imaging (SSEPI). The single-shot nature of this method, beyond removing disparities in the motion-induced phase of the data, also makes it a very fast imaging method. © 2009