Multiband phase‐constrained parallel MRI

PURPOSE: Parallel MRI methods are typically associated with a degradation of the signal-to-noise ratio (SNR). High scan time reduction factors are therefore restricted to applications with high intrinsic SNR. One possibility to increase the intrinsic SNR is to simultaneously excite several slices by means of multiband radio-frequency (RF) pulses and subsequently separate the slices by parallel MRI reconstruction algorithms. However, the separation of closely spaced slices may suffer from severe noise amplification when there is insufficient coil sensitivity variation along the slice direction. The purpose of this work is to apply a phase-constrained reconstruction for multiband experiments to minimize the noise amplification. METHODS: Pre-defined phase differences between neighboring slices are induced and slice separation is performed by a phase-constrained parallel MRI reconstruction. Phase differences between neighboring slices are tailored to achieve optimal slice separation with minimized noise amplification. The potential of the method is demonstrated through multiband in-vivo experiments. RESULTS: Noise amplification in multiband phase-constrained reconstructions is significantly reduced in comparison to standard multiband reconstruction when the phase difference between neighboring slices (distance = 12 mm) is 90°. CONCLUSIONS: Multiband phase-constrained parallel MRI has the potential for accelerated multi-slice imaging with an improved SNR performance.