Improved fat suppression of the breast using discretized frequency shimming

Purpose: Robust fat suppression is essential in bilateral breast MRI at 7 Tesla. The lack of good fat suppression can result in errors when calculating the enhancement curve from dynamic contrast-enhanced acquisitions. In this work we propose discretized frequency shimming to improve the quality of fat suppression by exploiting the capabilities of a parallel transmit setup. Methods: With discretized frequency shimming, different resonance frequencies are used for the different transmitters of the radiofrequency coil. This can be particularly useful for frequency-selective radiofrequency pulses, such as water excitation. The potential of this method has been investigated by simulations on seven acquired B ₀ maps. Prospective experiments on phantoms and four healthy volunteers were performed to illustrate the performance of discretized frequency shimming. Results: Simulations show a decrease in off-resonance excitation for all subjects when discretized frequency shimming is used. This decrease is highly subject-dependent, but can result in up to 30% better fat suppression, as observed in phantom and volunteer experiments. Conclusions: Discretized frequency shimming improves fat suppression in bilateral breast MRI at 7 Tesla. Magn Reson Med 79:593–599, 2018.