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Imaging and radio acquisition (propeller)

Sampling data using the conventional Cartesian technique, with line-by-line sampling of k-space, results in artifacts associated with phase and frequency encoding. Chemical shift results in artifact in the frequency-encoded direction, and motion results in artifact in the phase-encoded direction. Other methods that sample the central and peripheral regions of k-space simultaneously, rather than sequentially in a grid, radio acquisition  can reduce these artifacts. Other patterns or trajectories of k-space sampling include radial, spiral, and PROPELLER imaging. PROPELLER fast spin-echo  is a radial k-space filling technique. A range of ultra-low-resolution images are acquired, one per repetition time, with as many phase-encoding steps as echoes in the echo train length (ETL). Each of these scans corresponds to a different rotational direction within the scan plane. Since the data are collected in a series of rectangular strips, serially rotating around central k-space, central k-space is repeatedly sampled. The data set looks like a freeze-frame photograph of a rotating propeller, with the hub located in the center of k-space. Data reconstruction then combines these different data collections in the image domain and corrects or even rejects (based on relative phase information) data acquisitions that are degraded by motion. The resulting final image is relatively free of motion artifact imaging and radio acquisition. Also, of particular benefit to musculoskeletal imaging, radial scanning produces fully isotropic in-plane resolution with no frequency and phase direction. Consequently, chemical shift radio acquisition and pulsatile flow artifacts are diffused across the whole image plane, so these artifacts are far less discernible than with Cartesian imaging