Techniques of motion suppression during free breathing include the use of respiratory triggering, respiratory monitoring with navigator pulse, and rotatory k-space sampling. Most T2-weighted SSFSE and 3D MRCP sequences are too long to be performed within a breath-hold and require respiratory triggering. Traditionally, this has been performed using pneumatic bellows placed around the lower chest to detect respiratory motion. The use of 2D navigator pulses is a more recent development. The most commonly used navigator technique is 2D PACE (prospective acquisition correction encoding) .
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Rotatory filling of k-space allows inherent motion correction capabilities. With these techniques, named PROPELLER (periodically rotated overlapping parallel lines with enhancement reconstruction) or BLADE (Siemens Healthcare), the entire k-space is covered by multiple rectangular regions shaped like blades rotated around the center . Each blade consists of a small number of phase-encoding lines that can be filled with a multiple echo acquisition after a single excitation. Any in-plane motion that occurs between the acquisitions of the two blades can be determined by comparing the k-space data in the overlapping part of two blades and may be corrected. After repeating the process for all the blades, the full k-space can be created from motion-corrected blades to reconstruct an image with reduced motion artifacts . Because of the redundancy in k-space data, these techniques require longer scanning times than conventional rectilinear data acquisition. However, the oversampling in the center of the k-space also improves the SNR. Because blade motion correction is based on the assumption of rigid body motion, it is not as effective in correcting elastic motion for organs, such as liver and pancreas, and also does not correct for through-plane motion. With rotatory k-space filling techniques, image quality is best with wider blades, longer echo train lengths, and oversampling of k-space. Inadequate k-space sampling may result in streak artifacts . Increasing echo-train length also improves flow suppression. Studies have shown the improved diagnostic quality of upper abdominal organs with rotatory k-space filling technique sequences compared with conventional breath-hold and navigator-corrected T2-weighted SSFSE sequences
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