ages.Scanning pulse sequences for DWIThe DWI protocol can be used with a variety of MRI pulse sequences, including fast spin echo single shot, high speed STEAM (stimulated echo acquisition mode) imaging, incoherent gradient echo, magnetization prepared rapid gradient echo, echo-planar imaging and navigator echo. However, the pulse sequences must generate signals fairly quickly to detect areas of early ischemia. For this reason, echo-planar imaging is the most frequently used sequence. It requires a higher gradient strength specification. A contrast agent usually is not used with the scanning pulse sequences because water self-diffusion serves as a source of contrast on the MR images. Additional techniques include modified fast spin echo (high speed STEAM) and gradient echo sequence (MP-RAGE, or magnetization prepared rapid gradient echo). The navigator echo can be added to any pulse sequence to decrease motion artifact. The MRI pulse sequences most commonly used during DWI are listed below.Fast Spin Echo Single Shot (FSE)Standard fast spin echo produces images similar in appearance to conventional spin echo sequences. However, standard fast spin echo includes extra phase encoding gradients that follow each 180 degree pulse. This type of imaging generates MR signals and measures all of the raw image data (k-space) in a single acquisition. As a result, this sequence has an inherent decrease in the signal-to-noise-ratio (SNR) compared with conventional spin echo.High-speed STEAMThe high-speed STEAM technique is a modified fast spin echo sequence consisting of transverse magnetization, a vector component in the xy plane, that refocuses after the initial RF pulse similar to conventional spin echo. Unlike conventional spin echo, high speed STEAM temporarily stores transverse magnetization in the z plane. A stimulated echo is produced when the transverse magnetization is restored and refocused. When using this sequence as a fast s...
