Automated modification and fusion of voxel models to construct body phantoms with heterogeneous breast tissue: Application to MRI simulations

Abstract

Background Human voxel models incorporating detailed anatomical features are vital tools for the computational evaluation of electromagnetic (EM) fields within the body. Besides whole-body human voxel models, phantoms representing smaller heterogeneous anatomical features are often employed; for example, localized breast voxel models incorporating fatty and fibroglandular tissues have been developed for a variety of EM applications including mammography simulation and dosimetry, magnetic resonance imaging (MRI), and ultra-wideband microwave imaging. However, considering wavelength effects, electromagnetic modeling of the breast at sub-microwave frequencies necessitates detailed breast phantoms in conjunction with whole-body voxel models. Methods Heterogeneous breast phantoms are sized to fit within radiofrequency coil hardware, modified by voxel-wise extrusion, and fused to whole-body models using voxel-wise, tissue-dependent logical operators. To illustrate the utility of this method, finite-difference time-domain simulations are performed using a whole-body model integrated with a variety of available breast phantoms spanning the standard four tissue density classifications representing the majority of the population. Results The software library uses a combination of voxel operations to seamlessly size, modify, and fuse eleven breast phantoms to whole-body voxel models. The software is publicly available on GitHub and is linked to the file exchange at MATLAB® Central. Simulations confirm the proportions of fatty and fibroglandular tissues in breast phantoms have significant yet predictable implications on projected power deposition in tissue. Conclusions Breast phantoms may be modified and fused to whole-body voxel models using the software presented in this work; user considerations for the open-source software and resultant phantoms are discussed. Furthermore, results indicate simulating breast models as predominantly fatty tissue can considerably underestimate the potential for tissue heating in women with substantial fibroglandular tissue.

Homogeneous B1+ for bilateral breast imaging at 7 T using a five dipole transmit array merged with a high density receive loop array

“…Finite difference time domain simulations (Sim4Life, Zurich Med Tech, Zurich, Switzerland) were performed to evaluate transmit efficiency and RF safety limits of the setup assuming full decoupling to the receiver coils (see Supplementary File CoilModel.smash). These simulations were also performed on three adjusted human models of Ella of the virtual family, where the breasts were replaced by breast models to better correspond to the breast shape in prone position . The breast models were segmented from T 1 ‐weighted scans performed at 7 T with and without fat suppression from three volunteers with different breast sizes (small/medium/large) to account for differences in coil loading.…”

“…These simulations were also performed on three adjusted human models of Ella of the virtual family, 24 where the breasts were replaced by breast models to better correspond to the breast shape in prone position. 14,25 The breast models were segmented from T 1 -weighted scans performed at 7 T with and without fat suppression from three volunteers with different breast sizes (small/medium/large) to account for differences in coil loading. Scan parameters were and the corresponding dielectric permittivity and conductivity were assigned.…”

Open birdcage coil for head imaging at 7T

“…In these simulations, we used the time domain solver of CST Microwave Suite 2019 (Dassault Systèmes) with approximately 8 to 10 million of mesh cells. Instead of a spherical phantom, we used the multi‐tissue Ella voxel model 24‐26 . This model, cropped at the shoulder level, had an isotropic voxel of 2 mm 3 .…”

“…Instead of a spherical phantom, we used the multi-tissue Ella voxel model. [24][25][26] This model, cropped at the shoulder level, had an isotropic voxel of 2 mm 3 . In all simulation series, the coils were tuned and matched using an electrical simulator extension (CST Schematic by Dassault Systèmes).…”