Improved fat suppression of the breast using discretized frequency shimming
The current workhorse in the assessment of breast tumors with MR, dynamic contrast‐enhanced (DCE) MRI, has a sensitivity of over 90% and a specificity of 70 to 90% in the detection of breast tumors 3. Morphological and kinetic information from this type of acquisition can be used to differentiate between malignant and benign tumors. It was recently found that the first few minutes of the DCE MRI can be sufficient in detecting breast cancer 5.
The signal‐to‐noise ratio increase obtained by moving to higher field strengths ( ≥ 7 Tesla (T)) can be used to obtain high spatial resolutions at these high temporal constraints 6. Moreover, a high density of receiver arrays can be used to accelerate MRI with sensitivity encoding, enabling a 1.5‐mm isotropic spatial resolution within a temporal resolution of 6.7 s.
However, with higher resolutions, slight movement between acquisitions might result in errors in the calculated enhancement curves. This can lead to subtraction errors that appear as rim‐enhanced lesions, which may be diagnosed incorrectly as an aggressive tumor with poor outcome 8. To reduce these errors, the high signal of fat surrounding the lesions must be suppressed 9. This way, even with subtle motion during image acquisition, the subtraction artifact can be negligible.
Multiple fat suppression techniques are available for DCE MRI, such as water‐selective excitation (WSE) 10, Dixon 11, or spectral presaturation inversion recovery 12. In previous work, we have shown that one of those techniques, WSE, is the preferred method in the T1‐weighted sequence for breast DCE MRI at 7 T when scanning at submilliliter resolutions 13. However, residual fat signal may remain visible, especially close to the axillary regions.
The cause of residual fat signal can be found in an inhomogeneous B0 field. Water‐selective excitation is a binomial radiofrequency (RF) pulse that has a selectivity in the spatial as well as the spectral domain. In a uniform B0 field, the RF pulse excites water (at 4.7 ppm) only, while excluding excitation of the lipids (at 1.3 ppm). With a strong, nonuniform B0 field, which is not uncommon with breast MRI 14, the lipid spins may get excited instead of water spins.
Optimizing the B0 field in the breasts requires careful positioning of the breasts in the coil and higher‐order B0 shimming. However, magnetic field distortions in both breasts cannot be completely removed, even with the availability of third‐order B0 shimming, available on most 7T systems 16. Fortunately, the development of hardware for breast MRI at 7 T is an active field of research 17. Although not widely available, recently developed 7T breast coils can make use of the possibilities that parallel transmit systems provide (ie, the transmitter for each breast is connected to a separate RF amplifier) 18. These types of setups have the option to independently excite spins in each breast, and are used mostly to perform JOURNAL/mrim/04.02/01445475-201801000-00060/math_60MM1/v/2017-12-21T175206Z/r/image-png phase and amplitude shimming for each breast, such that both breasts experience a similar JOURNAL/mrim/04.02/01445475-201801000-00060/math_60MM2/v/2017-12-21T175206Z/r/image-png .
In this study, we propose a method to improve the quality of the images obtained with WSE by exploiting the capabilities of the parallel transmit system even further. In addition to phase and amplitude shimming for JOURNAL/mrim/04.02/01445475-201801000-00060/math_60MM3/v/2017-12-21T175206Z/r/image-png , we will demonstrate the use of different resonant frequencies for the RF pulses to mitigate off‐resonance effects caused by B0 distortions in both breasts.