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Initial Clinical Experience with Microwave Breast Imaging in Women with Normal Mammography

Posted Jan 31, 2007 Updated Apr 7, 2023
By Paul M. Meaney , Margaret W. Fanning , Timothy Raynolds , Colleen J. Fox , Qianqian Fang , Christine A. Kogel , Steven P. Poplack , Keith D. Paulsen
10 min read

Rationale and Objectives

We have developed a microwave tomography system for experimental breast imaging.

Materials and Methods

In this article, we illustrate a strategy for optimizing the coupling liquid for the antenna array based on in vivo measurement data. We present representative phantom experiments to illustrate the imaging system’s ability to recover accurate property distributions over the range of dielectric properties expected to be encountered clinically. To demonstrate clinical feasibility and assess the microwave properties of the normal breast in vivo, we summarize our initial experience with microwave breast exams of 43 women with negative mammography according to the Breast Imaging Reporting and Data System (BI-RADS 1).

Results

The clinical results show a high degree of bilateral symmetry in the whole breast average microwave properties. Focal assessments of microwave properties are associated with breast tissue composition evaluated through radiographic density categorization verified through magnetic resonance image correlation in selected cases. Specifically, both whole-breast average and local microwave properties increase with increasing radiographic density, in which the latter exhibits a more substantial rise.

Conclusion

These findings support our hypothesis that water content variations in the breast play an influential role in dictating the overall dielectric property distributions and indicate that the microwave properties in the breast are more heterogeneous than previously believed based on ex vivo property measurements reported in the literature.

In 2004 alone, more than 200,000 new cases of breast cancer were diagnosed in the United States, with roughly 40,000 women dying from the disease, making breast cancer the second largest cause of female cancer deaths in the United States ( ). There have been numerous reports demonstrating that early detection is the single most significant predictor of long-term survival ( ); therefore, improvements in detection may help to reduce the high mortality rates that exist. Mammography is the front-line screening modality, but its weaknesses in terms of sensitivity and specificity are well documented ( ). Although magnetic resonance (MR) and ultrasound are used primarily in a diagnostic setting, there is room for improvement in differential diagnosis as well.

Over the past decade, there has been a steady increase in the interest in using microwave imaging as a way of detecting breast cancers. Ex vivo studies have indicated that there is significant contrast in the dielectric properties of normal and malignant breast tissue ( ). However, it is well known that the breast is a heterogeneous organ whose composition varies significantly depending on a variety of factors ( ). For example, Woodard and White ( ) have shown that the mammary glands typically contain significantly more water than adipose tissue. Thus fibroglandular breast tissue would be expected to have higher dielectric properties than fat, but with properties that are still lower than high water-content tissues. Further, studies have shown that the amount of fibroglandular tissue can vary widely from fattier breasts (with very little) to dense breast (with substantially larger proportions), suggesting that the baseline electrical properties of the normal breast may be highly variable ( ).

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Methods

Imaging Procedure

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Table 1

Summary of the Age and Breast Density Distributions for the 43 Women Recruited for this Imaging Study

Age Density Total Fatty Scattered Heterogeneously Dense Extremely Dense 40–49 0 7 3 1 11 50–59 2 4 6 0 12 60–69 1 8 3 0 12 7079 2 4 2 0 8 Total 5 23 14 1 43

Table 2

Summary of Microwave Tomography System Performance Specifications

Performance Specifications Exam geometry Breast pendant in liquid bath Imaging planes 7 Coupling bath 80:20 and 86:14 glycerin:water mixtures Total number of images 49 (7 frequencies × 7 planes) Exam time 7 min Frequencies 500–1,700 MHz in 200-MHz increments Power transmitted <1 mW Measurement sensitivity −130 dBm Antenna array 16 monopole antennas on a 15.2-cm diameter circle, noncontacting Measurements/image set 240 (16 transmitters × 15 receivers per transmitter) Image properties Electrical permittivity (ε r ) and conductivity (σ) Reconstruction time <5 min/image

Figure 1, (a) Photograph of the clinical microwave breast imaging prototype showing the illumination tank, exam platform, and electronics cart (underneath bed); (b) two-dimensional schematic diagram of the illumination and reception configuration; and representative (c) amplitude and (d) phase projections for a set of measured clinical data for a single imaging plane at 1,300 MHz (data for only 4 of the 16 illuminations are shown).

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Phantom Experiments

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Figure 2, Typical phantom experiment with liquid containers suspended from above the tank and integrated with an alignment fixture for accurate positioning.

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In Vivo Tissue Property Study

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Figure 3, Flow graph of the process followed for patient recruiting, examination, and data processing.

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Water Content and MR Comparison

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Results

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Background Liquid Selection

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Figure 4, Maximum phase projection data from 900 MHz imaging experiments of 147 volunteers using several different coupling baths (0.9% saline, and 70:30, 80:20, and 87:13 glycerin:water coupling baths) for women with a range of breast densities.

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Phantom Results

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Figure 5, The 1,300 MHz permittivity (top) and conductivity (bottom) images for a (a) 10-cm phantom with a 2-cm inclusion, and a (b) 7.5-cm phantom with a 1-cm inclusion, respectively. Phantom designations (left to right): fatty(FT) , scattered(SC) , heterogeneously dense(HD) , and extremely dense (ED) have microwave properties that mimic those determined from in vivo images (see text for details). The imaging field of view is 13.5 cm in diameter in all cases and properties are reported on the common scales shown (left most image pair). Spatial dimensions are also the same in each case and a representative scale (in meters) is shown on the bottom right most image for the larger phantom case.

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In Vivo Normal Subject Study

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Figure 6, Scatter plots of the 1,300 MHz average (a) permittivity and (b) conductivity for the right versus left breasts of the 43 normal subjects.

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Figure 9, Microwave (top row, permittivity; middle row, conductivity at 1,100 MHz) and coregistered magnetic resonance (bottom row) images in the same anatomically coronal view for the right breast of a woman with heterogeneously dense tissue. The labels (P1–P7) above each image correspond to the plane of acquisition relative to the chest wall. The property and spatial scales are the same for all images. The microwave image field of view is fixed at 13.5 cm in diameter.

Figure 7, Bar graphs of the 1,300 MHz overall average and fibroglandular average (a) permittivity and (b) conductivity values as a function of breast density for the 43 normal subjects.

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Figure 8, Scatter plots of the 1,300 MHz fibroglandular average (a) permittivity and (b) conductivity values grouped by radiographic density and graphed as a function of patient age. P values for the trend lines are shown for each density category.

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MR Comparison Results

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Figure 10, Microwave (top row, permittivity; middle row, conductivity at 1,100 MHz) and coregistered magnetic resonance (bottom row) images in the same anatomically coronal view for the left breast of a woman with fatty to scattered radiographic density. P1 though P7 (labels above each image column) indicate microwave tomograms spaced 1 cm apart beginning near the chest wall.

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Discussion

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Academic Radiology, Volume 14, Issue 2
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