A novel planar type antenna printed on a high permittivity Rogers’ substrate is proposed for early stage microwave breast cancer detection. The design is based on a p-shaped wide-slot structure with
Since the Federal Communication Commission (FCC) has allocated the frequency band 3.1–10.6 GHz for commercial application [
Since UWB transmits narrow short pulses instead of continuous wave, therefore, a reasonably wideband antenna is needed in order to capture the important information of the scatterers in the time domain. Conventional UWB antennas have been introduced for this application such as dipole [
For breast imaging, a planar-type design is preferred as the antenna can be placed directly on the target. In addition to wideband characteristic, this antenna should also preferably be small in size, lightweight, and compact. One new emerging design which fulfills these requirements is the microstrip printed slot antennas. Due to its favorable impedance characteristics, this type of antenna, particularly the printed wide-slot antenna, has been designed and evaluated in the laboratory for medical imaging. Slot antennas provide large magnetic fields which are less prone to near-field coupling with nearby objects [
The geometry and configuration of the proposed antenna are shown in Figure
Optimized design parameters.
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Unit (mm) | 6 | 6.5 | 7.2 | 7.5 | 1.2 | 16 | 16 | 1.27 |
The proposed p-shaped wide-slot antenna. (a) The schematic and (b) the actual designs.
In this paper, we present a radar-based breast cancer detection system. It uses a circular antenna array and a breast phantom model with dielectric properties similar to real breast tissues. Our experimental system was built in such way that it can be used directly with real breast cancer patients in future work. The array is formed by 16 proposed antennas, placed equidistantly around the breast phantom, making an angle of 22.5° from the origin. The breast phantom consists of a 2 mm thick layer of skin and 88 mm diameter of the breast. In this geometry the longest and shortest sensing distances correspond to two opposite and adjacent antennas, respectively. The distance between any two opposing antennas is 90 mm whereas the distance of any two adjacent antennas is approximately 20 mm. Figure
Circular antenna array for full view geometry.
The data acquisition system used is based on the 16-curved circular array, surrounding the target. All the 16 antennas are driven via a
Actual design of the antenna holder.
Experiment setup of the imaging system.
To experiment the methods for breast cancer detection, the heterogeneous glandular model was constructed using chemical materials which are formulated based on the relative permittivity and conductivity of realistic human tissues over the frequency of interest given in Table
Relative permittivity and conductivity of breast tissue at frequency range from 200 MHz to 5 GHz [
Tissue | Relative permittivity | Conductivity |
---|---|---|
Fat | 15 | 0.15 |
Skin | 38 | 1.9 |
Glandular | 32 | 1.5 |
Tumour | 54 | 2.5 |
The realistic chemical-based heterogeneous breast phantom. (a) Top view. (b) Side view.
UWB radar imaging aims to reconstruct the presence and location of the target arising from the dielectric contrast between healthy and malignant tissues. The qualitative image is displayed as a variance of energy intensity. Figure
Sensing geometry of multistatic aperture radar with 16 antennas.
The image reconstruction algorithm investigated in this application is similar to delay and sum (DAS) algorithm which is among the earliest qualitative radar techniques developed for breast cancer detection [
Following time alignment, a focusing-quality procedure utilizing the coherence factor (CF) is implemented. Traditionally, the CF is used for anomaly correction and side lobe suppression in ultrasound imaging applications [
The focusing quality
The high-coherence signals (high coherence factor) represent the main lobe part, which creates an image with good focusing quality, and their amplitude should be maintained. On the other hand, the incoherent signals (low coherence factor) represent the side lobe part, which creates an image with poor focusing quality, and their amplitude should be suppressed. Consequently, the amplitude of each image pixel is adaptively weighted by the corresponding CF such that the unwanted side lobes are effectively reduced.
The enhancement in tumor detection capability of EDAS algorithm is based on the supplementary weighting process involving pairing multiplication of the backscattered signals. In this case, each signal pair measured from the same transmitter is multiplied with one another and their products are summed. The results from these procedures are squared and then integrated over a window length,
In order to investigate the performance and establish some salient characteristics of the proposed design, series of simulated experiments were firstly performed and, secondly, followed by laboratory experiments. Image reconstruction experiment was also performed in order to assess the feasibility of applying the overall system for breast cancer detection.
A few sensitive design parameters were investigated numerically in order to study antenna characteristics and behavioral performance across the entire band. Wide-slot antenna is well known for its wide impedance bandwidth; however, it is restricted by the degradation of the radiation patterns at higher frequencies. In this particular design the bandwidth of the antenna is determined by the shapes and sizes of the slot.
Figure
Simulated reflection coefficient for different slot shapes.
Another factor which has increased the impedance bandwidth of the proposed design is due to the simple and flat structure of the radiating microstrip line. Compared to other designs the radiating element is formed by etching the microstrip line directly onto the PCB substrate above the ground plane. In this case the size of the ground plane is determined by the frequency as follows [
Simulated reflection coefficient for various substrate sizes.
Figure
Simulated and measured radiation patterns at
The simulated reflection coefficient and
Reflection coefficients of the proposed p-shaped wide-slot antenna, comparing simulated and measured values.
The transmission coefficient
Transmission coefficients comparing simulated and measured values.
Another important characteristic of the antenna is the shape and size of pulse since this information tells general dielectric properties of the medium, particularly the scatterers. Generally, microwave detection of breast cancer relies on changes in the dielectric properties of malignant tissues as compared to healthy ones. Theoretically, the pulse shape of the signals is affected as it travels from the transmitter to target and back to receiver. For this reason the shape of the incident pulses is geometrically different from the shape of the arriving pulses. Therefore, it is important for the antenna to transmit or receive a signal with minimal distortion so that the image of the scatterer can be accurately reconstructed. In most of UWB designs, the antennas emit high and low frequency components from different portions of the geometry, so the radiated waveforms are highly distorted. This undesirable characteristic is not suitable for impulse applications including the breast cancer detection. It is pertinently important for the design to fulfill the requirement of having phase center that does not change with frequency so that the radiated emissions are minimally distorted. In order to assess the performance of the proposed antenna in transmission and reception of narrow pulses in distortionless condition, the time domain impulse response of the antenna was measured. The measurement was performed in nearly lossless media using a vegetable oil as the homogeneous background. In this way the background interferences especially the air-skin interface are minimized. Furthermore, the oil-based medium provides the best performance in terms of matching media and coupling material [
Measured response of the proposed antenna.
Commonly in radar-based imaging, the ideal pulse produced by the antenna must be as short as possible with minimal late-time ringing [
Another factor used to evaluate the antenna is the fidelity factor [
Fidelity factor as a function of distance between two antennas.
In order to demonstrate the feasibility of this proposed antenna in detecting breast cancer at the early stage of the disease, an experiment was performed using the heterogeneously dense breast phantoms as shown in Figure
Image reconstruction of the heterogeneously dense breast phantom. (a) Actual image. (b) Image reconstructed using EDAS.
This paper presents a novel compact, wideband p-shaped wide-slot antenna for mapping cancerous cells at their early stage of development inside the heterogeneously dense breast tissues. A p-shaped wide-slot antenna is implemented to obtain a good reflection coefficient that can cover a wider impedance bandwidth, which is an important requirement for this type of application. The proposed antenna exhibits a wide impedance bandwidth averaging at 71% in the frequency range from 4.5 to 10.9 GHz. The antenna is printed on a square shaped Rogers’ substrate RT6010 of size
The authors declare that there is no conflict of interests regarding the publication of this paper.
This project was supported by USM Fundamental Research Grant Scheme 6071222.