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  • Title: Interleaved balanced SSFP imaging: artifact reduction using gradient waveform grouping.
    Author: Nielsen JF, Nayak KS.
    Journal: J Magn Reson Imaging; 2009 Mar; 29(3):745-50. PubMed ID: 19243071.
    Abstract:
    PURPOSE: To analyze steady-state signal distortions in interleaved balanced steady-state free precession (bSSFP) caused by slightly unbalanced eddy-current fields and develop a general strategy for mitigating these artifacts. MATERIALS AND METHODS: We considered bSSFP sequences in which two gradient waveforms are interleaved in a "groupwise" fashion, ie, each waveform is executed consecutively two or more times before switching to the other waveform (we let "N" count the number of times each waveform is executed consecutively). The steady-state signal profile over the bSSFP passband was calculated using numerical Bloch simulations and measured experimentally in a uniform phantom. The proposed "grouped" interleaved bSSFP strategy was applied to cardiac velocity mapping using interleaved phase-contrast imaging with N=2 and N=6 in one healthy volunteer. RESULTS: Simulation and phantom measurements show that signal distortions are systematically reduced with increasing grouping number N. For most tissues, significant suppression was achieved with N=4, and increasing N beyond this value produced only marginal gains. However, signal distortions for blood remain relatively high even for N>4. In vivo cardiac velocity mapping using interleaved phase-contrast imaging with N=6 demonstrated reduced image artifact levels compared to the N=2 acquisition. CONCLUSION: Gradient waveform "grouping" offers a simple and general strategy for mitigating steady-state eddy-current distortions in bSSFP sequences that interleave two different gradients. Blood exhibits significant distortion even with "grouping," which is a major obstacle for cardiovascular bSSFP approaches that interleave multiple gradient waveforms. The grouping concept may also benefit applications that acquire images during the transient approach to steady state.
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