Computational electromagnetics models of microwave interactions with the human breast serve as an invaluable tool for exploring the feasibility of new technologies and improving design concepts related to microwave breast cancer detection and treatment. In this paper we report the development of a collection of anatomically realistic 3D numerical breast phantoms of varying shape, size, and radiographic density which can be readily used in FDTD computational electromagnetics models. The phantoms are derived from T1-weighted magnetic resonance images (MRIs) of prone patients. Each MRI is transformed into a uniform grid of dielectric properties using several steps. First, the structure of each phantom is identified by applying image processing techniques to the MRI. Next, the voxel intensities of the MRI are converted to frequency-dependent and tissue-dependent dielectric properties of normal breast tissues via a piecewise-linear map. The dielectric properties of normal breast tissue are taken from the recently completed large-scale experimental study of normal breast tissue dielectric properties conducted by the Universities of Wisconsin and Calgary. The comprehensive collection of numerical phantoms is made available to the scientific community through an online repository.

Non-invasive microwave hyperthermia treatment of breast cancer is investigated using threedimensional (3-D) numerical breast phantoms with anatomical and dielectric-properties realism. 3-D electromagnetic and thermal finite-difference time-domain simulations are used to evaluate the focusing and selective heating efficacy in four numerical breast phantoms with different breast tissue densities. Beamforming is used to design and focus the signals transmitted by an antenna array into the breast. We investigate the use of propagation models of varying fidelity and complexity in the design of the transmitted signals. An ideal propagation model that is exactly matched to the actual patient's breast is used to establish a best-performance baseline. Simpler patient-specific propagation models based on a homogeneous breast interior are also explored to evaluate the robustness of beamforming in practical clinical settings in which an ideal propagation model is not available. We also investigate the performance of the beamformer as a function of operating frequency and compare single-frequency and multiple-frequency focusing strategies. Our study suggests that beamforming is a robust method of non-invasively focusing microwave energy at a tumor site in breasts of varying volumes and breast tissue density.

Purpose: To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two-port body coil on B 1 þ uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response. Methods: RF shimming alters induced current distribution, which may result in large changes in the level and location of absorbed RF energy. We investigated this effect with six anatomical human models from the Virtual Population in 10 imaging landmarks and four RF coils. Three thermoregulation models were applied to estimate potential local temperature increases, including a newly proposed model for impaired thermoregulation. Results: Two-port RF shimming, compared to circular polarization mode, can increase the B 1 þ uniformity on average by þ32%. Worst-case SAR excitations increase the local RF power deposition on average by þ39%. In the first level controlled operating mode, induced peak temperatures reach 42.5 C and 45.6 C in patients with normal and impaired thermoregulation, respectively. Conclusion: Image quality with 3T body coils can be significantly increased by RF shimming. Exposure in realistic scan scenarios within guideline limits can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode. Magn Reson Med 76:986-997, 2016. V C 2015 Wiley Periodicals, Inc.