Diffusion-weighted imaging (DWI) is a sensitive and specific indicator of stroke, but is hindered in the clinical setting by its sensitivity to patient motion. This has precipitated the move towards single shot Echo Planar techniques (EPI) and high speed gradient hardware. The purpose of this work was to develop a multishot diffusion-weighted Echo Planar sequence in order to reduce the susceptibility sensitivity of EPI. Since the mutishot approach reintroduces the sensitivity to patient motion, we incorporated a navigator echo into the WPI sequence to compensate for motion effects. Methods and Results The modified EPI pulse sequence consists of a pair of diffusion-sensitizing gradients about a 1800 refocusing pulse. An EPI readout of 8-32 echoes follows the second diffusion sensitizing gradient. The sequence is repeated as many times as needed to make up the full image data set. We performed all imaging with resolution: 128x128, FOV: 24 cm, b values: 0-509 s/mm2, and peripheral gating. IEPI was performed at two gradient configurations with differing parameters and compared to single shot EPI performed with the high speed gradients. A navigator echo was acquired immediately before the readout in the IEPI sequence. The motion induced phase changes from each shot were estimated from the phase of the navigator echo after lDFT. This phase was used to correct all echoes in the image data from that shot. The navigator motion-correction scheme dramatically improved the image quality in all multishot images. Images acquired with navigated IEPI had less susceptibility distortion than the single shot images around the sinuses and minimal motion artifacts. Discussion Multishot Echo Planar Imaging, in combination with cardiac gating and phase correction with navigator echoes, is a robust and efficient means of acquiring diffusion-weighted data on either conventional or high speed gradient hardware.
Showing the most recent 10 out of 446 publications
