Estimating B1+ of the breast at 7T using a generic distribution
Michael J van Rijssel1, Josien P W Pluim1, Peter R Luijten1, Alexander J Raaijmakers1, and Dennis W J Klomp1

1Center for Image Sciences, UMC Utrecht, Utrecht, Netherlands

Synopsis

Quantitative DCE-MRI requires reliable B1+ information. This study presents a simulation-based fast B1+ estimation method for DCE breast imaging at 7T. Numerical FDTD simulations were conducted to assess the inter-subject differences in B1+ for four volunteers using segmented breast images for the simulation model. Inter-subject differences are shown to be comparable to the accuracy of popular B1+ mapping methods, justifying the use of one generic B1+ distribution for B1+ estimation (coil template). This template was created by averaging the simulated B1+ distributions over the four volunteers. We demonstrate the feasibility of this method in three in-vivo cases.

Purpose

Quantitative DCE-MRI or fast T1 mapping using variable flip-angle approaches requires reliable information on the B1+ field, especially at inhomogeneous fields. B1+ mapping methods are generally limited in terms of SNR or time efficiency. The present work explores the feasibility of using one generic coil-specific B1+ distribution (coil template) to estimate B1+ fields for breast imaging at 7T, while circumventing B1+ mapping.

Methods

FDTD simulations for five healthy female volunteers (1-5) representing a wide range in breast anatomies presented in previous work were used to assess theoretical inter-subject differences in B1+.1 Simulated B1+ fields were aligned on coil position using multi-resolution intensity-based rigid image registration using the mutual information similarity metric and B-spline interpolation in elastix.2 One volunteer’s simulation was excluded, since data was not available for full inclusion of the coil. To rule out differences in absolute RF power settings in constructing the template, all simulations were multiplied by a scaling factor relative to volunteer 5 (arbitrarily chosen): $$$Scale(i)=median_{all\ r}(\frac{Simulation_5(r)}{Simulation_i(r)})$$$, where i is the volunteer number and r the position in the simulation. The average of the remaining four scaled simulations was computed and served as a coil-specific B1+ template.

Using the same coil used in the simulations,3 a 3D B1 map (DREAM4), 3D T1-weighted gradient echo images (FFE) at four flip-angles (2°, 3°, 11° and 17°) and a 2D single-slice Look-Locker measurement using a water-selective adiabatic inversion pulse were obtained unilaterally from three healthy female volunteers (6-8) aged 23 - 30.5 To assess the proposed method’s feasibility in vivo, rigid registration was applied between the measured B1+ map of the subject and the coil-specific B1+ template, using masks to exclude regions where the mapping failed and where the simulated B1+ < 20% or > 100% of the nominal angle. The registered template was scaled using: $$$Scale=median_{r\in M}(\frac{Map(r)}{Template(r)})$$$, where M represents Map>70%.

As a preliminary analysis, T1 maps were calculated for these volunteers using the DESPOT1 method, without B1+ information or incorporating B1+ information from either the measured map or the registered template.6, 7 An independent reference T1 map was calculated from the Look-Locker real channel data by a three-parameter non-linear least-squares fit.

Results

Figure 1 shows the range in breast anatomies included in the simulations by means of their fat-suppressed T1w scans.

Figure 2 shows a comparison between the designed coil-specific template and subject-specific simulations. Volunteer 2 showed least agreement; the mean difference was 0.87% of the nominal flip angle with a standard deviation of 3.92%.

Figure 3 shows a comparison between the measured B1+ map and the scaled and registered template. Volunteer 7 showed least agreement; the mean difference was 2.87% of the nominal flip angle with a standard deviation of 11.94%.

As a visual illustration, Figure 4 shows the T1 maps generated for volunteer 8 with the DESPOT1 method using no B1+ information, the measured B1+ map and the registered template respectively, alongside the Look-Locker based reference.

Discussion

The agreement between B1+ simulations of different volunteers is better than the accuracy of popular B1+ mapping methods,8 which indicates that B1+ inhomogeneity in the breast at 7T when using a quadrature setup is mostly determined by the transmit coil, and that variations between subjects contribute to this only slightly. This finding can most likely be explained by the dielectric properties of fat, which have a modest effect on the RF wavelength and, by extent, cause smooth B1+ fields in fat-dominated areas such as the breast. This opens up the possibility to estimate B1+ using a coil-specific template, provided this template can be reliably positioned and scaled.

When the position and scaling of the template are obtained directly from a B1+ measurement (by rigid registration and scaling as described above), the accuracy of the registered and scaled template is excellent, although the standard deviation of the difference with the measurement is higher mainly due to the noisy nature of the B1+ map.

Preliminary results of T1 mapping with DESPOT1 show that both T1 maps that incorporate B1+ information, either from the DREAM map or the registered template, show fair agreement with the Look-Locker based reference.

Conclusion

Simulations show that inter-subject differences in B1+ fields of the breast at 7T are comparable to the accuracy of popular B1+ mapping methods reported in literature when using a quadrature setup. This opens up the possibility of using one generic B1+ map, instead of subject-specific mapping.

Acknowledgements

No acknowledgement found.

References

1. van der Velden T A, Italiaander M, van der Kemp W J, et al. Radiofrequency configuration to facilitate bilateral breast (31)P MR spectroscopic imaging and high-resolution MRI at 7 Tesla. Magn Reson Med. 2014.

2. Klein S, Staring M, Murphy K, et al. elastix: A Toolbox for Intensity-Based Medical Image Registration. IEEE Trans Med Imaging. 2010;29(1):196-205.

3. Klomp D W J, van de Bank B L, Raaijmakers A, et al. (31)P MRSI and (1)H MRS at 7T: initial results in human breast cancer. NMR Biomed. 2011;24(10):1337-1342.

4. Nehrke K, and Börnert P. DREAM - a novel approach for robust, ultrafast, multislice B-1 mapping. Magn Reson Med. 2012;68(5):1517-1526.

5. Look D C, and Locker D R. Nuclear Spin-Lattice Relaxation Measurements by Tone-Burst Modulation. Phys Rev Lett. 1968;20(18):987-989.

6. Homer J, and Beevers M S. Driven-Equilibrium Single-Pulse Observation of T1 Relaxation - a Reevaluation of a Rapid New Method for Determining NMR Spin-Lattice Relaxation-Times. J Magn Reson. 1985;63(2):287-297.

7. Deoni S C L, Rutt B K, and Peters T M. Rapid combined T-1 and T-2 mapping using gradient recalled acquisition in the steady state. Magn Reson Med. 2003;49(3):515-526.

8. Nehrke K, Sprinkart A M, and Börnert P. An in vivo comparison of the DREAM sequence with current RF shim technology. Magn Reson Mater Phy. 2015;28(2):185-194.

Figures

Fat-suppressed T1w scans of the four included simulation volunteers (sagittal view). Bright dots mark the location of vitamin tablets used to determine the position of the coil in the images.

A-D: absolute difference between template and individual simulations. E-F: template on different color scales. G: line profiles corresponding to same-colored lines in A-F. H: histogram of difference between template and simulation 2 (which showed least agreement).

For all volunteers: A: B1+ map measured with DREAM technique. B: B1+ template registered and scaled to measured map, red line indicates breast outline as in A. C: difference between template and map. D: histogram of difference between template and map.

A: T1 map using DESPOT1 and no B1+ correction. B: T1 map using DESPOT1 and B1+ map measured with DREAM. C: T1 map using DESPOT1 and B1+ template registered and scaled to measured map. D: T1 map using Look-Locker (reference).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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