Proceedings of the 2008 IEEE, CIBEC'08 978-1-4244-2695-9/08/$25.00 ©2008 IEEE

COMPACT ANTENNA FOR MICROWAVE IMAGING SYSTEMS

A. M. Abbosh

School of ITEE, The University of Queensland, St. Lucia, Qld 4072, Australia E-mail: abbosh@itee.uq.edu.au

Abstract − A planar antenna with ultra wideband directive performance is presented. It is aimed to be part of a microwave imaging system for breast cancer detection. The antenna is designed to operate efficiently across the ultra wideband frequency (3.1 GHz to 10.6 GHz). It has a very compact size with overall dimensions of 0.9 cm×1 cm. The antenna is assumed to be immersed in a liquid of a high dielectric constant to improve the matching with the breast tissues, and thus to increase the dynamic range of the system. The time domain performance of the antenna shows negligible distortion, which makes it suitable for the medical imaging systems utilizing a short pulse for its operation. The effect of the multilayer breast tissues on performance of the antenna is also investigated by calculating the fidelity factor across all layers of the breast. It is shown that due to multiple reflection/ scattering, the fidelity factor decreases as the signal propagates inside the breast. However, the fidelity factor is always larger than 80% revealing the possibility of using the proposed antenna in the microwave imaging systems. Keywords - Imaging system, breast cancer, microwave, ultra wideband

I. INTRODUCTION

Ultra wideband (UWB) microwave imaging is a promising method for some medical applications such as breast cancer detection. This originated from the fact that the UWB signal has good penetration and resolution characteristics. The possibility of using UWB technology for cancer detection originates from the significant contrast in dielectric properties between normal and cancerous tissue [1-3].

In the UWB imaging systems, a very narrow pulse is transmitted from an antenna to penetrate the part of the human body under investigation, which is the breast in this paper. The scattered signal due to different layers of the breast tissues is collected by array of antennas surrounding the breast. Signal processing algorithms can then be used to investigate the existence of any cancerous tissues. For an accurate imaging system with high resolution and dynamic range, the transmitting/receiving UWB antenna should be planar, compact in size and directive with distortionless performance.

Many types of UWB antennas were suggested to be part of the microwave imaging systems [4-7]. The tapered slot antennas [4-6] satisfy the requirements for imaging systems in terms of bandwidth, gain and impulse response. However, the achieved performance is at the expense of a relatively large size. Other types of UWB antennas [8] have a compact

size and low distortion; however, they have an omnidirectional radiation, which reduces the dynamic range of the system.

In the presented work, a planar UWB antenna is proposed. It has a very compact size with dimensions of (0.9 cm×1cm). The simulated performance of the proposed antenna in an environment, which is similar to the real situation in the breast imaging case, shows an ultra wideband behavior with a distortionless pulse operation.

II. ANTENNA DESIGN

The antenna presented in this paper is to be used in a microwave imaging system, which is designed for breast cancer detection. The imaging system includes a circular array of the proposed antenna, which encircles the breast. The antennas are assumed to be immersed in a liquid with a high dielectric constant to achieve the best possible matching with the breast tissues [6]. This reduces the reflected/ scattered signals at the skin layer interface, and thus increases the dynamic range of the imaging system. One of the antennas is used to transmit a UWB pulse, while rest of the antennas in the array receive the scattered pulses. The measured data is collected and then the procedure is repeated with the second antenna transmitting the signal, while the remaining are used for receiving the scattered signal. This process is repeated until all antennas in the array perform the transmitting role. The antenna array can be moved up and down automatically via a computer-controlled high-precision linear actuator. This facilitates the collection of the required data to form a three dimensional image.

Configuration of the proposed antenna is shown in Fig. 1. It uses Rogers RT6010 (εr = 10.2, thickness= 0.64mm) as a substrate. The antenna is immersed in a liquid with a dielectric constant, which is taken in this paper to be equal to that of the substrate for a good matching with the gap between the antenna and the breast and eventually with the breast tissues. The overall dimension of the antenna (the length L and the width W in Fig. 1) is chosen initially to be equal to half of the effective wavelength calculated at the centre frequency of operation (6.85 GHz). The radiator, which is located at the top layer of the substrate, and the ground plane at the bottom layer are in the form of quarter an ellipse with a major diameter equal to W. There is a slot (S) between the radiator and the ground plane. The initial value for this slot is chosen to be equal to the substrate’s thickness. The secondary diameter of the quarter ellipses representing

Proceedings of the 2008 IEEE, CIBEC'08 978-1-4244-2695-9/08/$25.00 ©2008 IEEE

the radiator and the ground plane is initially chosen to be equal to the major radius minus the slot value S. With this choice for the antenna dimensions, the overall size is very compact; however, it will have an inferior performance at the low frequency band (around 3 GHz). To improve the performance at that band, pairs of symmetrical slots are cut from the radiator and the ground plane in the manner shown in Fig. 1. Those slots increase path of the current near the end of the structure. This makes the path length of the current effectively larger than the physical length. This eventually improves the performance at the lower end of the band without a significant effect on the rest of the covered band.

The antenna’s structure is then optimized using the software CST Microwave Studio. The final dimensions are: L=10.5 mm, W=9 mm, Ws= 0.4 mm, Ls=2 mm, ds=3 mm, S= 0.48 mm. The antenna is fed using a microstrip line with width equal to Wf= 0.45 mm.

Fig. 1 Configuration of the proposed antenna.

III. RESULTS AND DISCUSSIONS

Performance of the proposed antenna when used in the UWB imaging system was verified using the commercial software package, CST Microwave Studio.

Fig. 2 shows the simulated return loss of the antenna. As can be seen from this figure, the 10 dB return loss bandwidth extends from 2.1 GHz to more than 11 GHz covering the required UWB band (3.1 GHz – 10.6 GHz).

The far-field radiation pattern of the antenna was calculated using the software Microwave Studio and it is depicted in Fig. 3 at 6 GHz. The antenna shows directive properties with an average front-to-back ratio which is greater than 10 dB making it a good candidate for microwave imaging applications.

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0

Frequency(GHz)

Ret

urn

loss

(dB

)

Fig. 2 Variation of the return loss with frequency.

Fig. 3 The three dimensional radiation pattern at 6 GHz.

The time domain performance of the proposed antenna was also calculated. A narrow pulse was assumed to be transmitted from one antenna and the received pulse was calculated at a co-polarized receiving antenna which is at a distance of 30 cm from the transmitter. The pulse shape was chosen such that it contains the UWB frequency spectrum of 3.1 to 10.6 GHz. Shapes of the transmitted and received pulses are shown in Fig. 4. Note that the excited pulse and the received pulse are normalized with respect to their peak values. The figure reveals that the pulse distortion occurs below the 0.2 level with respect to the peak level of 1, and thus it is almost negligible. The observed result indicates that the designed antenna supports distortionless narrow pulse which makes it an excellent choice for the purpose of a microwave imaging with high resolution.

Proceedings of the 2008 IEEE, CIBEC'08 978-1-4244-2695-9/08/$25.00 ©2008 IEEE

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

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1

Normalized time

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mal

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am

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de

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Fig. 4 The impulse response of the antenna.

In the imaging system for breast cancer detection, the antenna is to be at a close distance from the breast. Therefore, effect of the breast tissues on the antenna’s performance is investigated. The electromagnetic model used to simulate the breast contains two layers: The first layer is the skin layer with thickness= 2 mm, dielectric constant=36 and conductivity=4 S/m. The second layer is the breast tissue, which extends to a width of 10 cm, with a dielectric constant= 9 and conductivity=0.4 S/m [9]. Results of the simulation are shown in Fig. 5 for different distances between the antenna and the breast. Fig. 5 indicates clearly that the antenna maintains its ultra wideband performance despite being very close to the breast tissues.

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0

Frequency(GHz)

Re

turn

loss

(dB

)

10mm

15mm20mm

Fig. 5 Variation of the return loss with frequency at different distances from

the breast.

The imaging system in which the antenna is to be used contains an array of antennas surrounding the breast. Thus, it is important to investigate value of the mutual coupling between those antennas. The mutual coupling between two identical antennas at different frequencies was calculated

assuming two values for the distance between them. The two mutually coupled antennas were assumed to be parallel to each other in the manner shown in Fig. 6. Results of the calculation are shown in Fig. 6. It shows that the coupling is less than -25 dB across the whole ultra wideband when the distance between the two antennas is 20 mm, which is around half of the effective wavelength at the lowest frequency of operation. Fig. 6 also shows that the coupling decreases as the distance between the two antennas increases.

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B)

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Fig. 6 Variation of the mutual coupling between two identical antennas with

frequency at different distances.

It is also important to study the distortion when the radiated pulse propagates through the breast tissues. The antenna fidelity is used as an indication for that distortion. The fidelity factor is the maximum magnitude of the cross correlation between the observed pulse at a certain distance and the excited pulse [10]. The finite difference time domain method was used for this purpose [11]. In order to reduce the computation domain, a perfectly matched layer was applied as an absorbing boundary condition [12]. To include the frequency dependence of the dielectric constant εi and the conductivity σi of the breast tissues over the UWB, the first order Debye dispersion model was used [9]:

)2/(212 o

o

ii fj

fjfj επσ

τπεεε

επσε Δ

∞Δ∞ −

+−

+=− (1)

where τ is the relaxation time, εΔ and ε∞ and σΔ are the Debye model parameters which were selected according to the published data for the breast tissues [9]: Normal tissue: εΔ = 10, ε∞=7, τ =7 ps, σΔ =0.15 S/m, tumor: εΔ = 54, ε∞=4, τ =7 ps, σΔ =0.4 S/m. For the skin: ε =36, and σ =4 S/m.

The result, which is depicted in Fig. 7, indicates that as the signal propagates through the breast, the fidelity factor decreases due to an increasing pulse distortion inside the breast tissues. For the antenna presented in this paper, the fidelity factor is within reasonable values (more than 80%) even inside the breast tissues.

Proceedings of the 2008 IEEE, CIBEC'08 978-1-4244-2695-9/08/$25.00 ©2008 IEEE

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82

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96

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elity

(%)

Coupling medium Skin Breast tissue

Fig. 7 The calculated fidelity factor with distance from the antenna in the

presence of breast tissues.

IV. CONCLUSIONS

The design of a planar ultra wideband antenna for use in a microwave imaging system has been presented. The antenna has a very compact size of 0.9 cm×1 cm. To improve the matching between the antenna and the breast tissues, the antenna is assumed to be immersed in a liquid of a high dielectric constant. The simulated characteristics of the antenna have shown that it covers the band from 2.1 GHz to more than 11 GHz even when it operates at a very close distance from the breast tissues.

The time domain performance of the antenna has also been studies. It has been shown that the proposed antenna has the ability to send and receive very short pulses in a distortionless manner. It has been shown that the designed antenna has more than 80% fidelity factor even when the pulse is calculated inside the breast tissues.

Because the designed antenna is to be used within array of antennas surrounding the breast, the mutual coupling between two identical antennas has been simulated. It has been shown that the mutual coupling is very low (less than -25 dB) when the distance between the neighboring antennas is more than half of the effective wavelength calculated at the lowest frequency of operation.

REFERENCES

[1] S. K. Davis, B. D. Van Veen, S. C. Hagness, and F. Kelcz, "Breast tumor characterization based on ultrawideband microwave backscatter," IEEE Transactions on Biomedical Engineering, vol. 55, no. 1, pp. 237-246, January 2008 [2] E. Fear, X. Li, S. C. Hagness, and M. Stuchly, "Confocal microwave imaging for breast cancer detection: Localization of tumors in three dimensions," IEEE Trans. Biomed. Eng., vol. 49, no. 8, pp. 812-822, August 2002 [3] W. Khor, M. Bialkowski, A. Abbosh, N. Seman, and S. Crozier, “An ultra wideband microwave imaging system for breast cancer detection,” IEICE Trans. Comm., vol. E-90B, no. 9, pp. 2376-2381, 2007. [4] M. Chiappe and G. Gragnani, “Vivaldi antennas for microwave imaging: theoretical analysis and design considerations,” IEEE Transactions on Instrumentation and Measurement, vol. 55, no. 6, pp. 1885-1891, 2006. [5] A. Abbosh, H. Kan, M. Bialkowski, “Compact ultra-wideband planar tapered slot antenna for use in a microwave imaging system,” Microwave and Optical Technology Letters, vol.48, no.11, pp. 2212-2216, 2006. [6] A. Abbosh, and M. Bialkowski, “A UWB directional antenna for microwave imaging applications,” IEEE Antennas and propagation Symposium, APS2007, USA, 2007. [7] A. Abbosh and M. Bialkowski, “Design of ultra wideband planar monopole antennas of circular and elliptical shape,” IEEE Trans. Antennas and Propagation, vol. 56, no.1, pp.17-23, 2008. [8] A. Abbosh, M. Bialkowski, and S. Crozier, “Investigations into optimum characteristics for the coupling medium in UWB breast cancer imaging systems”, IEEE Antennas and Propagation Symposium, San Diego, USA, 2008. [9] S. Davis, H. Tandradinata, S. Hagness, and B. Van Veen, “Ultra wideband microwave breast cancer detection: A detection-theoretic approach using the generalized likelihood ratio test,” IEEE Trans. Biomed. Eng., vol. 52, pp. 1237–1250, 2005. [10] D. Lamensdorf and L. Susman, “baseband-pulse antenna techniques,” IEEE Antennas and Propagation Magazine, vol. 36, pp. 20-30, 1994. [11] D. Sullivan, Electromagnetic simulation using the FDTD method, John Wiley & Sons Inc., 2000. [12] J. Berenger, "A perfectly matched layer for the absorption of electromagnetic waves,” Journal of Computational Physics, vol. 114, pp. 185-200, 1994.