Multichannel Technology and Parallel Imaging4 - Magnetic Resonance Imaging - Lecture Slides, Slides of Radiology

Download Multichannel Technology and Parallel Imaging4 - Magnetic Resonance Imaging - Lecture Slides and more Slides Radiology in PDF only on Docsity! Parallel Imaging and Noise 43 Noise in parallel images is 1) increased, and 2) non-uniform. As shown in this SENSE example, unfolding the alias multiplies the noise within particular regions (non-uniform). A similar effect appears in k-space based methods. Larkman DJ et al. Magn Reson Med 2006; 55:153-160 docsity.com Parallel Imaging (k-space Example) K-space oriented parallel acquisition techniques: • SMASH (Simultaneous Acquisition of Spatial Harmonics), AUTO-SMASH • PILS (Parallel Imaging with Localized Sensitivities) • GRAPPA (Generalized Autocalibrating Partially Parallel Acquisition). Figure 5 shows fast spin echo T2 weighted sagittal scans of the lumbar spine, without (A) and with (B) parallel imaging (the latter using GRAPPA). In (B), every second Fourier line has been skipped (an iPAT factor of 2, or acceleration factor). Scan time is thus reduced by a factor of two (comparing B to A). 44 docsity.com Figures 6A,B (Ref. #2) 47 docsity.com Parallel Imaging (Drawbacks) Image-based reconstruction: If an aliasing artifact would be present in the chosen FOV for a non-parallel image sequence, then this aliasing will cause reconstruction problems if parallel imaging is attempted. K-space based reconstruction: The ability to construct effective sensitivities from the spatial sensitivities for each coil element depends on the sensitivity profile. This, in turn, depends on the coil element design; therefore, coil design is more critical with this technique. 48 docsity.com 49 GRAPPA (k-space) SENSE (image) docsity.com Figures 4A,B (Ref #2) 52 docsity.com Use #2: Reduction of T2-blurring Parallel imaging reduces T2-blurring because the readout time is shorter resolution is better. 53 Augustin Me et al. Top Magn Reson Imag 2004; 15:207 docsity.com • Parallel imaging reduces number of phase-encoding steps required per imaging time Top – normal acquisition, Bottom – R=2 acceleration Use #3: Reduction of Susceptibility Artifact (EPI) docsity.com