Interleaved balanced SSFP imaging: artifact reduction using gradient waveform grouping.

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. Copyright (c) 2009 Wiley-Liss, Inc.