Off-resonance-robust balanced SSFP cardiac cine MRI
Keywords: Electrical Engineering.

Cardiac cine MRI is used to assess ventricular function and wall motion, and balanced steady-state free precession (bSSFP) is an attractive sequence for cine imaging due to its speed and desirable blood-myocardium contrast. However, the sequence suffers from dark band artifacts -- regions of signal dropout in the image -- if the magnetic field strength, and thus the precession frequency of proton spins, cannot be made sufficiently uniform over the field of view. In the presence of non-uniformity in the frequency of spins, known as off-resonance, blood flow results in additional artifacts. Balanced SSFP's sensitivity to off-resonance hampers its use at high field, e.g., 3 T. We propose a set of methods that can be integrated to form a bSSFP cardiac cine sequence that is much more robust to off-resonance. Multiple-acquisition bSSFP, in which a set of phase-cycled component images are acquired and then combined, is normally used to eliminate the signal nulls in the bSSFP spectral profile that result in dark band artifacts. During the acquisition of each phase cycle, the RF phase is incremented by a different amount to shift the spectral profile; the images thus have offset bands and are then combined into a banding-free image. To be able to use multiple-acquisition bSSFP in the heart, however, the most severe of near-band flow artifacts first have to be mitigated so that the artifacts originating from the bands in the component images do not corrupt the final combined image. Simulations, phantom images, and in vivo images show that partial dephasing, which slightly unbalances the gradients on the slice-select axis, substantially reduces the transient-related artifacts and hyperintensity that can result from through-plane flow near a bSSFP steady-state stopband while minimally affecting the desired, on-resonant signal. Multiple-acquisition bSSFP increases the scan time by a factor of the number of phase cycles acquired. A combined acquisition and reconstruction strategy is proposed to counter this scan time increase and acquire three phase cycles within a breath-hold. In frequency-modulated SSFP, the RF phase cycling is incremented by a small amount each TR to slowly shift the bSSFP spectral profile without spoiling the signal. A frequency modulation scheme is designed for cardiac applications to obviate the need for signal stabilization periods between phase cycles and facilitate acquisition of interleaved phase cycles. A highly accelerated acquisition is enabled by a reconstruction that enforces consistency between phase cycles and over time. In vivo results at 1.5 and 3 T show that the proposed methods enable banding-free bSSFP cine within a short breath-hold, although some residual flow artifacts remain. Finally, we present a non-Cartesian accelerated frequency-modulated bSSFP cine sequence. A projection-reconstruction (PR) trajectory is used in this proof-of-concept, which illustrates that three phase cycles and a B0 field map can be reconstructed from data acquired within a breath-hold. A bSSFP sequence has rewinders at the end of the TR to null the gradient areas, bringing the trajectory back to the center of k-space. Acquiring data during the rewind enables generation of a field map using BMART -- B0 mapping using rewinding trajectories, where the rewind data forms the second TE image for calculating the field map. This lengthens the TR by only 5% but facilitates inclusion of only passband signal in the final combined image and exclusion of stopband and near-band flow signal based on the static field strength at each position. The PR sequence thus facilitates field map combination of the phase cycle images, which results in more homogeneous blood pool signal and reduced signal contributions from out-of-slice spins than root-sum-of-squares. Therefore, we propose partial dephasing to mitigate flow artifacts in component images, a highly undersampled frequency-modulated sequence to enable acquisition of three phase cycles within a breath-hold, and a PR trajectory to facilitate reconstruction of a field map from the acquired cine data, enabling exclusion of residual flow artifacts during phase cycle combination. Together, these methods result in an off-resonance-robust balanced SSFP cardiac cine sequence