Yoshihiko Kuwahara

Additional information is available at the end of the chapter Yoshihiko Kuwahara

http://dx.doi.org/10.5772/intechopen.69562 Additional information is available at the end of the chapter

#### Abstract

We overview the research trend on microwave imaging for early breast cancer detection. The technologies have two categories: ultra-wide band (UWB) radar that reconstructs the scattering power distribution in the breast and inverse scattering problem that reconstructs the dielectric properties distribution. We have developed a clinical equipment using UWB radar and carried out clinical test 4 years ago. Through the experiments, we concluded that the UWB radar was insufficient for the clinical equipment, because the UWB radar cannot discriminate cancerous tumor and other lesions. Therefore, we have been studying inverse scattering. It is a challenging task to develop an equipment using inverse scattering technologies. We have proposed a microwave mammography that has four features: (1) sensor with breast fixing by absorption, (2) small sensor with multipolarization, (3) image reconstruction program linking the commercial EM simulator, and (4) hybrid imaging method using UWB radar and inverse scattering.

Keywords: early breast cancer detection, microwave imaging, radar, inverse scattering, hybrid imaging

### 1. Introduction

Early detection and treatment of breast cancer, which has the highest rate of incidence in women, are important. Although X-ray mammography is widely used, it has the disadvantages of X-ray exposure, detection failure owing to low contrast, and pain during inspection. An echograph is a well-established alternative to X-ray mammography. However, the inspection quality of this device depends on the skill of the inspector, and the reproducibility of results is poor [1]. Recently, microwave imaging for breast cancer detection has attracted attention [2]. However, the low contrast between fibroglandular tissue and malignant tissue in the microwave frequency range poses a challenge to many researchers [3].

Breast cancer detection through microwave imaging is broadly grouped into two categories: tomography [4] and ultra-wide bandwidth (UWB) radar [5]. Tomography can reconstruct such

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

organization structures as fatty, fibroglandular, and malignant tissues. However, the electromagnetic analysis including the antennas, supporting structure, and environment must be accurately carried out because measuring error, noise, and modeling error significantly affect the image reconstruction. Moreover, since the calculation load is enormous, it is unsuitable for mass examination. UWB radar cannot reconstruct the organization structure accurately [6]. However, since it is tolerant to measuring error and noise in comparison with tomography, it is easy to manufacture. Furthermore, since the calculation load is small, it is suitable for mass examination.

We have developed multistatic UWB radar for early breast cancer detection. Our equipment features multistatic microwave imaging via space time (MS-MIST) algorithm, which extends the MIST algorithm to multistatic UWB radar [5] and a conformal array, which fixes the breast to the inner shape of a sensor via suction [7]. Through numerical simulations and experiments with phantoms, MS-MIST was confirmed to have high resolution with low artifacts. In addition, our sensor requires neither placement of the breast in a tank filled with a coupling liquid nor measurement of the breast shape. The proposed system has low failure rate of inspection in comparison with the already developed UWB radar [6] because the sensor with suction and fixation restrains the patient from moving and breathing during the scan. Moreover, the inspection time is short because the number of antennas is reduced by MS-MIST with high resolution and low artifact. Hence, it results in small size and low cost. First, we describe the clinical equipment developed and demonstrate the imaging results, including numerical and clinical experiments.

The clinical test results demonstrate that the system can detect cancer that has a clear boundary and is isolated from the fibroglandular tissue. However, if the boundary is irregular or if the tumor is buried under the fibroglandular tissue, the system is unable to correctly reconstruct the shape of the tumor [6]. Therefore, we are currently working on the development of microwave tomography [8–11].

In order to achieve accurate image reconstruction, it is necessary to obtain diverse observation data. Several methods can be employed to obtain diverse observation data. More observation data can be obtained by increasing the number of antennas; however, the scale of the apparatus increases and the computational cost becomes substantial. Furthermore, the signal-to-noise ratio (SNR) is degraded by increasing the size, which degrades the image reconstruction. A method using multiple frequencies has been proposed [12]. In general, biological tissue is a medium with frequency dependence, and its behavior is modeled using the Debye approximation with several parameters. Consequently, the number of unknown parameters increases with the number of frequencies; thus, the reconstruction becomes difficult.

The multiple-polarization method has been examined as a means to obtain a variety of observation data. The impact of polarization on image reconstruction was evaluated in Ref. [13], and it was concluded that the effectiveness was limited. However, the physical considerations related to antenna arrangement have not yet been investigated. Second, we review a compact-sized imaging sensor using multipolarization. We use the distorted Born iterative method (DBIM) described in Ref. [12] to solve the inverse scattering problem.

Microwave tomography can reconstruct complex structures if the measurement system is modeled completely and there is no measurement error. In order to reduce the modeling error, an image-reconstructing program that solves the forward problem using a commercial electromagnetic simulator has been developed [9]. Considering the actual device, this program includes an algorithm that applies the scattering parameters provided by the vector network analyzer (VNA) to the inverse scattering equation [10]. Third, we present microwave mammography with these technologies.

We could successfully reconstruct the complex numerical breast phantom using the proposed microwave mammography. Subsequently, we developed simple microwave tomography and carried out experiments. However, we could not reconstruct a sufficiently high-quality image owing to the deviations between the calculated and measured backscattered signals. It is well known that the settings of the initial complex permittivity distribution are important. Previously, initial permittivity in the imaging area was set to be uniform. Finally, we propose a method in which the backscattered power distribution is reconstructed by the radar, and the distribution is subsequently used as the prior knowledge in the inverse scattering problem. The effectiveness of the proposed method is confirmed by experiments.
