Optimized design of single-sided quadratic phase outer volume suppression pulses for magnetic resonance imaging

Abstract

In Magnetic Resonance Imaging, outer volume suppression pulses only need to saturate the magnetization on one side of the region of interest (ROI), so there is no need for them to be symmetric. Single-sided quadratic phase outer volume suppression pulses (OVS) make use of this fact to reduce the peak RF excitation pulse power, while maintaining the high selectivity on one side of the passband. Therefore, single-sided pulses are appropriate for single-shot fast spin echo cardiac imaging, as well as for spectroscopic imaging studies of brain and prostate cancer. The low peak RF value is achieved by designing Shinnar-Le Roux (SLR) polynomials with quadratic phase, which is a FIR filter design problem. Due to the relaxation of the magnitude constraints on one side of the passband (the ‘do not care’ region) it is possible to reduce the peak RF power by extending the quadratic phase beyond the passband. The polynomials are designed using a weighted least squares algorithm in which significant weighting is given to the frequency bands over which the magnitude profile is fixed, and a lower weight is applied beyond the transition width on the ‘do not care’ side. We improve the design of single-sided quadratic phase OVS pulses by optimizing the range of frequencies over which the phase of the pulse is quadratic. This is achieved by iteratively increasing the extent of the ‘do not care’ region, using convex optimization methods at each iteration to find the optimal polynomial with the lowest peak coefficient. Applying the SLR transform to the polynomial with the minimum peak, we get the lowest peak RF pulse that produces the desired single-sided saturation profile. Using this procedure the peak RF power can be reduced by additional 5-10 percent, while the profile’s ripple is comparable with the ripple of double-sided pulses.