Water selective excitation (WSE) is an efficient means for fat suppressed dynamic contrast enhanced (DCE) breast MRI; particularly at 7 T^1-4. However, despite the availability of 3^rd order shim and an increased chemical shift between water and fat, residual fat signal often remains visible². The availability of local transmit arrays for the breasts, opens up capabilities for B₁⁺ shimming that go beyond phase and amplitude shimming. In this study, we suggest to employ frequency B₁⁺ shimming, where the resonance frequency of the RF pulse can be different between both breasts, improving the homogeneity of WSE.

Experiments were performed on a 7 T whole body MR system (Philips Healthcare, Cleveland, OH) equipped with a 4-element breast transceiver (MR Coils BV, Drunen, NL). The elements of each breast were driven in quadrature and connected to a separate 4 kW RF amplifiers. B₀ shimming was performed and optimized for either 16 or 17 terms (1^st, 2^nd and 3^rd order and one or two 0th order terms, separated for left and right breast). The difference between the two 0th order terms was incorporated as a frequency offset in the RF pulse of one of the breasts.

Simulations on B₀ maps of eight healthy volunteers were performed to investigate the potential of the proposed method. To simulate the work flow of the scanner, the resonance frequency was based on a single breast. Performance of both methods were quantified by assessing the percentage of voxels in the breast, where fat would experience an excitation of 15% or more of the requested flip angle.

In vivo performance was assessed in healthy female volunteers. The protocol consisted of two 3D acquisitions; one with regular WSE (using 16 B₀ shim terms), and one with the proposed method (using 17 B₀ shim terms). Both acquisitions had a 1-3-3-1 binomial pulse as WSE pulse. See the table in figure 1 for more parameters.

The B₀ maps calculated after B₀ and frequency shimming clearly shows a more uniform frequency distribution when compared to B₀ shimming alone (fig 2a). For all subjects, the addition of frequency shimming substantially reduced the excitation of lipids (fig 2b).

Figure 3 shows two slices of the same subject, using the regular and the frequency shimmed fat suppression. Both slices demonstrate the improvement in fat suppression when incorporating frequency shimming.

Simulations and experiments show the potential of using different excitation frequencies for each breast in fat suppressed MRI. In contrast to multi-dimensional RF pulses that incorporate B₀ information, this technique of frequency shimming can be used without switched B₀ gradients. Consequently, for the short RF pulses used for spectral-spatial excitation (i.e. < 500$\mu$s for spatial selection), frequency shimming can be applied without effecting the pulse durations.In bilateral breast MRI where the spins in each breast can be excited with a separate transmit coil, frequency shimming can be applied that can improve chemical shift selective excitation for both breasts. Consequently very efficient fat suppression can be obtained for bilateral breast MRI at 7T.