Echo Planar Imaging For Diffusion / Perfusion

Conventional MRI sequences acquiring only one line of spatial frequency data per RF excitation or repetition interval (TR) can be additionally fitted with diffusionlperfusion sensitizing gradient pulses which alter pixel intensities. Ideally, pixel values from images created at different gradient levels can be used to calculate local diffusionlperfusion maps of tissue. However, conventional imaging methods, with relatively long scan times, often allow bulk motion, flow and system instability artifacts (at or below levels tolerable for normal imaging) to corrupt the diffusionlperfusion sensitized data. This imposes constraints on patient motion which are often uncomfortable, difficult or impossible to achieve. Methods such as Echo Planar Imaging (EPI) that acquire reconstructable spatial frequency data within one or a few RF excitations are becoming methods of choice for motion artifact reduced imaging. An overview of the differences between conventional imaging and EPI will be discussed, emphasizing areas relating to pulse sequences, magnetlgradient hardware requirements and reconstruction corrections.