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Improving breast imaging B1+ homogeneity with B1 shimming at 1.5T: a modeling study
Xin Chen1, Xin Li2, Danielle Kara3, Mingdong Fan3, Joseph Rispoli2, Michael Steckner1, and Robert Brown3

1Toshiba Medical Research Institute USA, Mayfield Village, OH, United States, 2Purdue University, West Lafayette, IN, United States, 3Case Western Reserve University, Cleveland, OH, United States

Synopsis

B1+ inhomogeneity is a challenge for breast imaging and B1 shimming is standard for all 3T clinical MRI applications. While B1+ homogeneity is typically better at 1.5T than 3T due to reduced wavelength effects, breast image quality is still challenged by B1+ inhomogeneity at 1.5T. This work shows B1 shimming can significantly improve B1+ homogeneity for breast imaging at 1.5T as compared to conventional quadrature drive.

Introduction

B1+ homogeneity is an important image quality factor in breast imaging, both to ensure optimal image quality and to facilitate comparisons. The number of breast MRI procedures is growing rapidly and it is important to investigate mechanisms that improves image quality. B1+ inhomogeneity in breast imaging has been evaluated on quadrature driven 1.5T and 3T clinical MR systems, and showed similar inhomogeneity patterns. Quantitative measurements at both field strengths suggest inhomogeneity at 1.5T only slightly less severe than 3T.1 While B1 shimming has been widely applied at 3T and has provided improved B1+ homogeneity for breast imaging,1,2 application to 1.5T has not received as much consideration because, in general, B1+ homogeneity is considered sufficient at 1.5T. This work shows B1 shimming can improve B1+ homogeneity for breast imaging at 1.5T as compared to conventional quadrature drive (QD).

Methods

Numerical FDTD EM modeling was performed with a generic whole body Tx coil based on ISO/TS109743 (16-rung high-pass birdcage, diameter 750mm, length 650mm. RF shield diameter 790mm, length 850mm4,5) with software package Sim4Life (v3.4, ZMT, Zurich, Switzerland).The coil was tuned to 64MHz, and driven with two voltage sources located on one end ring. Female human models Hanako6 (1.60m, 53kg) and Ella7 (1.63m, 58.7kg) were studied respectively. The human models were positioned prone, feet first in the Tx coil (Figure 1). Anatomically accurate breast enhancements8 were added to the models’ chest to simulate a realistic prone breast imaging scenario. The human models were elevated by a typical breast coil height so that the breasts were at isocenter. Ella was also posed to represent the abdominal curvature over a breast coil. Simulations were performed by varying the phase and amplitude of one voltage source to achieve B1 shimming. Transmit B1 field (B1+) was analyzed in the central axial slice (Z=0) across the breasts.

Results

The coefficient of variation (CV) of the modeled B1+ was calculated for each B1 shim setting over each individual breast as well as the entire breasts. Additionally, average B1+ was calculated over left and right breast respectively, and left/right ratio was calculated to quantitatively measure B1+ asymmetry between two breasts. The results are shown in Tables 1 and 2 for Ella and Hanako, respectively. QD is used as reference for comparison (B1 shim setting #1 in both tables). All power gain settings in Tables 1 and 2 are only 0dB (which means input power are equal between two feeding sources) because no significant improvements were otherwise realized. Figures 2 and 3 show B1+ maps in the central axial slice (Z = 0) with B1 shim settings corresponding to entries in Tables 1 and 2 for Ella and Hanako respectively.

Discussion

Despite of differences of body habitus and posture between Ella and Hanako (in particular the differences of breasts), modeling results showed that increasing the phase difference between two feeding sources can 1) improve B1+ homogeneity in each individual breast, 2) reduce B1+ difference between two breasts, and 3) improve overall breast B1+ homogeneity. Furthermore, the phase difference between two feeding sources had a larger impact on B1+ than amplitude difference, as for a given phase varying the amplitude of one source did not show significant impact on B1+.

Conclusion

Modeling with anatomically accurate and detailed female human and breast models showed that B1+ field inhomogeneity and asymmetry presently challenging 1.5T breast imaging can be significantly reduced by B1 shimming using two feeding sources.

Acknowledgements

We would like to acknowledge Dr. Susan Hagness’s group at University of Wisconsin-Madison for the breast models.

References

  1. Trop I, Gilbert G, Ivancevic M, et al. Breast MR Imaging at 3 T with Dual-Source Radiofrequency Transmission Offers Superior B1 Homogeneity: An Intraindividual Comparison with Breast MR Imaging at 1.5 T. Radiology. 2013; 267(2):602‐608.
  2. Winker S, Rutt B. Practical methods for improved B1+ homogeneity in 3T breast imaging. Proc. Intl. Soc. Mag. Reson. Med. 21. 2013: 397.
  3. ISO/TS 10974:2012 Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device. Geneva, Switzerland: International Organization for Standardization, 2012.
  4. Murbach M, Neufeld E, Kainz W, et al. Whole-body and local RF absorption in human models as a function of anatomy and position within 1.5T MR body coil. Magn. Reson. Med. 2014; 71(2):839-845.
  5. Murbach M, Cabot E, Neufeld E, et al. Local SAR enhancements in anatomically correct children and adult models as a function of position within 1.5 T MR body coil. Prog Biophys Mol Biol. 2011; 107(3):428-433.
  6. Nagaoka T, Watanabe S, Sakurai K, et al. Development of Realistic High-Resolution Whole-Body Voxel Models of Japanese Adult Male and Female of Average Height and Weight, and Application of Models to Radio-Frequency Electromagnetic-Field Dosimetry. Physics in Medicine and Biology. 2004; Vol.49: 1-15.
  7. Gosselin MC, Neufeld E, Moser H, et al. Development of a new generation of high-resolution anatomical models for medical device evaluation: the Virtual Population 3.0. Physics in Medicine and Biology. 2014; 59(18): 5287-5303.
  8. Li X, Rispoli J. Thermal and Specific Absorption Rate Simulation of Heterogeneous Breast Models in The Prone Position Fused with Two Human Whole-Body Models. ISMRM Workshop on Ensuring RF Safety in MRI, McLean, VA, 2017.

Figures

Figure 1. Ella (left) and Hanako (right) with realistic breast enhancements simulating realistic prone breast imaging. Ella is posed to reflect the abdominal curvature over a breast coil.

Figure 2. Normalized B1+ (B1+/average B1+ over the entire breasts) in central axial slice for Ella with four B1 shim settings corresponding to Table 1. All figures are drawn on the same color scale shown on the right. The number under each figure is the phase difference (degree) between two feeding sources. The input power of two sources are equal for all cases.

Figure 3. Normalized B1+ (B1+/average B1+ over the entire breasts) in central axial slice for Hanako with four B1 shim settings corresponding to Table 2. All figures are drawn on the same color scale shown on the right. The number under each figure is the phase difference (degree) between two feeding sources. The input power of two sources are equal for all cases.

Table 1. B1+ homogeneity CV evaluation within breasts with different B1 shim settings for Ella model. Amplitude variations between two feeding sources did not produce useful improvements.

Table 2. B1+ homogeneity CV evaluation within breasts with different B1 shim settings for Hanako model. Amplitude variations between two feeding sources did not produce useful improvements.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
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