Readout-segmented DWIWhy does readout-segmented DWI produce better looking images?
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Conventional diffusion-weighted imaging (DWI) is typically performed using a single-shot echo-planar (ss-EPI) sequence. This has the advantage of speed but suffers from susceptibility artifacts/distortions, low signal-to-noise, and spatial blurring. To improve image quality (at the expense of increased imaging time) readout-segmented echo-planar diffusion methods (rs-DWI) have been developed. Siemens currently markets one such sequence under the trade name RESOLVE (REadout Segmentation Of Long Variable Echo trains).
In rs-DWI k-space sampling occurs in a small number (3-6) of "shots". Each shot has limited transversal of k-space in the readout direction, but full resolution along the phase-encode axis. Because motion artifacts between segments can create substantial artifacts, 2D navigator readout segments are acquired during the second and subsequent echoes. At k-space center, the navigator is repeatedly acquired to account for shot-dependent nonlinear phase differences that arise from non-rigid motion of the head or other imaged body part. As shown in the example below, improved signal-to-noise, spatial resolution, and decreased susceptibility artifacts are noted using this strategy
K-space sampling strategy for readout-segmented DWI |
Comparison of single-shot echo-planar DWI with multi-shot DWI. Note decreased susceptibility distortion at the skull base on multi-shot image. Better signal-to-noise and spatial resolution is also observed on the multi-shot image, a result of longer imaging time.
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PROPELLER/BLADE diffusion methods may also be used to reduce susceptibility artifacts and improve spatial resolution compared to ss-EPI methods. These employ fast (turbo) spin echoes and sample k-space in a radial fashion. Image quality is significantly improved and motion artifacts reduced. These sequences may be superior to rs-DWI-EPI methods where susceptibility artifacts are most problematic, such as in evaluating temporal bone cholesteatomas. Notwithstanding these advantages, image acquisition time may be comparatively longer compared to rs-DWI and imaging artifacts may limit acquisition to primarily the axial plane. Additionally, the use of turbo spin echoes increases specific absorption rate (SAR) and produces more tissue heating compared to gradient echo EPI methods.
References
Morelli JN, Saettele MR, Rangaswamy RA, et al. Echo planar diffusion-weighted imaging: possibilities and considerations with 12- and 32-channel head coils. J Clin Imaging Sci 2012;2:31.
Pipe JG, Farthing VG, Forbes KP. Multishot diffusion-weighted FSE using PROPELLER MRI. Magn Reson Med 2002; 47:42-52.
Porter DA, Heidemann RM. High resolution diffusion‑weighted imaging using readout‑segmented echo‑planar imaging, parallel imaging and a two‑dimensional navigator‑based reacquisition. Magn Reson Med 2009;62:468‑75.
Morelli JN, Saettele MR, Rangaswamy RA, et al. Echo planar diffusion-weighted imaging: possibilities and considerations with 12- and 32-channel head coils. J Clin Imaging Sci 2012;2:31.
Pipe JG, Farthing VG, Forbes KP. Multishot diffusion-weighted FSE using PROPELLER MRI. Magn Reson Med 2002; 47:42-52.
Porter DA, Heidemann RM. High resolution diffusion‑weighted imaging using readout‑segmented echo‑planar imaging, parallel imaging and a two‑dimensional navigator‑based reacquisition. Magn Reson Med 2009;62:468‑75.
Related Questions
What is echo-planar imaging (EPI)? Is this the same as Fast Spin Echo (FSE)?
You alluded to different trajectories for filling k-space. What are they?
How does PROPELLER reduce motion artifacts?
What is echo-planar imaging (EPI)? Is this the same as Fast Spin Echo (FSE)?
You alluded to different trajectories for filling k-space. What are they?
How does PROPELLER reduce motion artifacts?