Simultaneous MR-microwave breast imaging: Initial phantom experiments

MRI is widely used for breast cancer screening for women at a high risk including those with dense breasts. Gadolinium-based contrast agents provide excellent sensitivity for tumor detection; however, some reports suggest that significant health risks may be associated with gadolinium use. Alternative techniques to improve breast MRI specificity that do not reply on gadolinium injection are needed.

Microwave dielectric properties provide excellent endogenous contrast between malignant and normal breast tissue. While standalone microwave imaging (MI) methods are available, they typically suffer from poor spatial resolution. Integrating concurrent MI with MRI would allow high resolution from MRI to be combined with high specificity from MI. We have developed a MI system that operates inside the bore of an MR scanner without disruption to image acquisition from either modality. Results are presented from the first set of experiments in anthropomorphic breast phantoms conducted with the integrated imaging configuration.

MRI and microwave data were acquired simultaneously with a system which met size, materials, and operational constraints imposed by the MRI scanner bore and associated electromagnetic environment. Functionality included coaxial antenna feedlines mounted to a plate underneath an imaging tank with integrated breast imaging coils, which was moved vertically to provide multiples planar views of the imaging region of interest. The MI technique imported MR spatial information directly into a soft prior-based algorithm for 3D image reconstruction. The method did not utilize any prior information other than the MRI structural data.

MI recovered spherical fibroglandular and tumor equivalent tissue inclusions within a predominant adipose tissue. Data were presented at three frequencies-900, 1100, and 1300 MHz, respectively-with comparable results. Microwave property distributions were relatively homogeneous across each tissue zone with steep gradients at their associated interfaces. Permittivity images recovered properties well for all three tissue types, while conductivity images maintained fidelity for adipose and tumor tissues but became inaccurate in the fibroglandular tissue equivalent zones.

To the best of our knowledge, these microwave images are the first to be reconstructed from 3D data acquired inside a MRI scanner bore where MRI data were incorporated as spatial priors within the resultant microwave images. MR-MI system logistical challenges were overcome sufficiently to recover accurate images of phantoms making the multi-modality approach ready for actual patient examinations in the future.