Planar Ultra-Wideband Antenna for Microwave Imaging System for Medical Application - Case Study Example

Planar Ultra Wideband Antenna for Microwave Imaging System for Medical Application Submitted by November 2009 CWMA School of Information Technology and Electrical Sydney Introduction Worldwide, breast cancer has been classified as the second most common type of cancer after lung cancer. Presently the most common modalities used as diagnostic screening technologies for detecting and diagnosing breast cancer are X-ray mammography and breast magnetic resonance imaging (MRIs). Its been over 3 decades that microwaves have been used for treatment of cancer (microwave diathermy and microwave deep diathermy units). Recently, microwave imaging has received a considerable amount of interest with respect to the task of detection and location of malignant tissue in the woman’s breast. A microwave imaging system is considered as a viable alternative to X-ray mammography and breast MRIs due to its several advantages such as cost and insignificant side-effects. Microwave imaging involves the propagation of very low levels of microwave energy through the breast tissue. The basis for tumor detection and location is the difference in the electrical properties of normal and malignant breast tissue. Normal breast tissue is largely transparent to microwave radiation, while the malignant lump contains more water and blood, resulting in microwave signal back scattering. This scattered signal can be picked up by a microwave antenna (ultra-wideband antenna elements preferably in the planar format) and can be analyzed using a computer An alternative ultra-wideband UWB planar antenna element, which finds use in radar applications, is a uniplanar or antipodal Vivaldi antenna. The present designs of these antennas do not make them straight-forwardly applicable to microwave imaging applications. In the recently reported microwave imaging systems for breast cancer detection, resistively loaded monopoles [3, 5] and miniature pyramidal horn antennas [5, 6] have been used. The proposed antenna element is in the form of a planar tapered slot made of a high dielectric constant substrate material to achieve its compact size. It features a very low loss across the desired band, and its radiation efficiency exceeds 90% with a relatively high gain. The design of UWB antenna as proposed in this report is accomplished using simple design formulas. This is an advantage with the previously reported UWB antenna designs [8 –11] relying on a trial and error method and simulation tools and the design presented in this report offers a compact antenna size. 1.2 Motivation Although worldwide, breast cancer is the second most common type of cancer after lung cancer but in Australia breast cancer is the most common cause of cancer related death amongst women. Over a 25 year period (1982-2006), the number of new breast cancer cases in women has more than doubled. In 2005, over 12,000 Australians were diagnosed with breast cancer and there were over 2500 deaths. In other words, it implies that in Australia 42 women will be diagnosed with breast cancer every day in 2015 (Australian Institute of Health and Welfare & National Breast and Ovarian Cancer Centre 2009). Owing to high prevalence of breast cancer in Australian population and because of the high burden of disease related to it, a substantial component of budget is earmarked for the same. Around 14% of annual healthcare budget is set aside for the treatment of breast cancer. Hospital and research institutions in Australia are continuously seeking funding and support to enhance their infrastructural and human resources for combating with this killer disease. Keeping in mind the current scenario and the future projections it can be stated that there is a dire need to come up with cheaper and newer technologies to arrest the spread of breast cancer at early/treatable age. While addressing this need, the report presents a novel low cost technology that will assist in early detection and diagnosis of breast cancer. 2. Wider Professional Issues 2.1 Intellectual Property Centre for Wireless Monitoring and Applications (CWMA) at the University excels in developing technologies those are centered around the applications of electronics engineering, biomedical measurements, smart antenna technologies, wireless communications etc. The Centre has successfully developed technologies those have contributed to an enhanced human environment, more efficient and effective mining operation, improved athlete performance, and injury diagnosis and recovery monitoring. Planar Antenna design has been one of the fortes for CWMA, numerous publications have been published in leading international journals and CWMA holds patents of numerous similar engineered products. Antenna modules designed and developed as the outcome of this research will be the intellectual property of CWMA. All of the previously published research has been appropriately referenced in this project. Since the antenna has applications in medical domain, that is, a safety as well as security intense domain, it calls for compliance with biomedical as well as security standards. Accordingly, the project manager will make sure that the design not only complies with the biomedical and environmental regulations but will address various security concerns. As regards the risk and the manufacturing related issues, these will be handled jointly by the project manager and the industry partner. As per the understanding between the project manager and the industry partner, the former would hold all copyrights with respect to the intellectual property and the latter would hold the patent. Steps to enhance awareness about the product and the marketing will be taken care of by the project manager. 2.2 Design for Manufacture This project is concentrated on Ultra-Wideband (UWB) technology, although the technology has been around since 1980s and has primarily been used for radar based systems (Foerster et al., 2001). It is owing to high-speed switching requirements that the technology has founds its uses in designing low-cost systems. In this project UWB antenna has been designed and engineered for medical applications. The final goal is to put the antenna on the market for medical applications besides its possible use in UWB short range wireless systems. 3.0 Proposed Circular Cylindrical Antenna Array The UWB plannar antenna elements for a microwave imaging system (Figure 1) includes a circular array. One of the antennas is used as a transmitter of microwave signal, and the remaining antennas in the array act as receivers of the scattered signals. The process or measuring data continues in iterations and the data is collected, the measurement cycle repeats for the next antenna as it transmits the signal, in the meanwhile the remaining ones are used as receivers of scattered signal. This process continues until all antennas in the array have performed as transmitters. The high precision linear actuator (computer-controlled) moves the antenna array up and down automatically to facilitate the collection of data for creating a 3D object image. Figure 1 Configuration of a microwave imaging system including a circular array of wideband antenna elements. 4.0 Advantages and Disadvantages of Tapered Slot Antennas Significant progresses in the domain of broadband wireless communications have lead to the development of antennas those can support higher bit rates. Tapered slot antennas (TSA) are used for high rate communications at short distances. TSAs can be easily fabricated using standard lithography techniques and these antennas are not only light in weight and small in size but they also offer higher directivity and gain at the higher frequencies. In the case of tapered slot antennas bandwidth variation can also be obtained by varying the slot length and the opening angle. Besides these advantages tapered slot antennas exhibit few disadvantages, these antennas demonstrate a high cross polarization level in the diagonal plane and they also have a broad main beam in both azimuth and elevation planes, these antennas also contain high lack lobes. The versatility in terms of multifunction operation is lesser as compared with microstrip antennas. In certain situations, like during their use in a two dimensional array, the tapered slot antennas loose its planar architecture. 5.0 Limitation of TSA as Travelling Wave Antenna Tapered Slot Antennas belong to the category of traveling wave antennas, they are available in leaky wave and surface wave forms. Since in these types (slow wave) of antennas the wave radiates at the discontinuities, the radiation pattern arises from, the beginning and at the end of the structure. Hence these antennas are not highly directive. Leaky wave antennas and are not useful for point-to-point communications, mainly because the main beam produced by them is in the direction other than “end-fire”. Researchers have claimed that in order to remain as efficient travelling wave antenna, the effective dielectric thickness should be between 0.005 and 0.03 wavelengths for 4 to 10 antennas. 6.0 Feeding Procedures for Tapered Slot Antennas (TSAs) As far as the input impedance is concerned the TSAs are similar to the tapered transmission lines as they demonstrate high-pass properties and their input impedance is infinite. Their operational bandwidth is restricted by the transition between the feed and the antenna slot. Three main feeding procedures are used in the case of tapered slot antennas, these are: Microstrip line feed Coplanar waveguide (CPW) feed Coaxial feed Microstrip line feed In the case of tapered slot antenna, the most common form of tranmission line used for feeding is the microstrip line feed. Owing to its formation microstrip feed is an unbalanced line which is opposite to the slot line (balanced transmission line). For feeding the antenna, an electromagnetic coupling arrangement is required to permit signal transmission from the microstrip transmission line to the slot line. It has been observed that, the stronger is the electromagnetic coupling, the better the transition. Coplanar Waveguide (CPW) feed This is the type of feed that has been used in the present antenna. The CPW structure for feeding a TSA is shown in Fig.2. Figure 2. CPW to feed TSA The advantages of CPW feed include low radiation loss, less dispersion and uniplanar configuration, it also enables direct integration of active solid state devices with the antenna and can be etched on to the opposite side of the dielectric. Centre conductor of the coplanar waveguide which extends to form a crossover with the antenna slot line, is used to couple the signal to the antenna. Coaxial line feed Although this feed is non-planar and is unbalanced still the coaxial line feed can be used for most of the tapered slot antennas. It is so because the coaxial line feed is compatible with all the three, that is, the slot line, coplanar microstrip, or balanced microstrip. A very common way to excite tapered slot antenna by a coaxial line feed is by extending its conductor over the slot line section of the tapered slot antenna and anchor the coaxial feed with solder connection to the ground plane. 7.0 UWB applications Ultra-wide band or Ultraband is a wireless and a high bandwidth technology that is used at a very low energy levels and for short range distance, UWB systems use narrow pulses to transmit data and UWB transmission is difficult to intercept. Agencies like FCC have legalized use of UWB for commercial applications. Upcoming applications of UWB include tracking and data collection from sensors, traditionally UWB has been used for radars, radio astronomy and microwave imaging. Owing to the absence of carrier frequencies the design of UWB based transmitters and receivers is very simple but care needs to be taken for containing data in the sahepe of the UWB pulse and the antenna pulse distortion must also be kept to a minimum. Researchers and practitioners have posited that a small UWB TSA antenna can be used for portable devices including laptop computers or distributed sensor networks. UWB is already being used in MP3 players, tablets, 3G handsets, printers, speakers, scanners, DVD players etc. UWB has also been proposed for use in the state of the art networks called Wireless Personal Area Networks (WPANs) and it is already being used for Bluetooth devices. Besides relatively lower than expected outcomes, slower than expected progress in the domain of UWB standards development and high cost of initial implementations are the reasons for the limited adoption of UWB applications, owing to this few UWB vendors have ceased operations in the recent past. At present leading agencies and companies into UWB applications inluclude Defense Advanced Research Projects Agency (DARPA), Lawrence Livermore National Laboratories (LLNL), Wisair etc. Academically too UWB research groups and labs. have been established at University of California (Berkeley) , University of Massachusetts, Università di Roma etc. Although there are a few shortcomings exist with UWB, but the future seems to be bright for this upcoming modality of communications. References: 1. Australian Institute of Health and Welfare & National Breast and Ovarian Cancer Centre 2009. Breast Cancer in Australia: An overview, 2009. Cancer Series no. 50. Cat. no. CAN 46. Canberra: AIHW. 2. Foerster, J., Green, E., Somayazulu, S., & Leeper, D. (2001). Ultra-wideband Technology for Short- or Medium-range. Intel Technology Journal, (Q2), 1-11. Read More