Development of active microwave imaging techniques for applications in mammography

Breast cancer is the most common cause of non-skin malignancy in women and a leading cause offemale mortality. A potentially important strategy for reducing this menace is the 'detection at an early stage. However, conventional mammographic detection techniques have many drawbacks and often cause harm to patients. The invention of non-invasive and nonionizing microwave technique, to reveal the internal structure of biological objects thus became a break through in the field of medical diagnostics. When exposed to microwaves, breast tumor exhibits electrical properties that are significantly different from that of healthy breast tissue and hence false free detection of the tumor in terms of dielectric contrast is possible. An introductory chapter which deals with various imaging techniques, their advantages and disadvantages is given as Chapter 1 of this thesis. The advantages of microwave imaging for breast cancer detection, its classifications and the two approaches of active microwave imaging microwave tomographic imaging with measured scattered signals and confocal microwave imaging with measured reflected signals are also addressed here. The variation of dielectric properties of tissues upon exposure to microwaves is the underlying principle of microwave imaging. Hence a good understanding of the behavior of biological tissues when interacted with microwave energy is essential to study the variations of the electrical properties of the tissues. This is explained in Chapter 2 of the thesis. Also the various techniques to study the dielectric properties such as rectangular and coaxial cavity perturbation techniques, their theoretical analysis, and the calibration of the experimental set up for data acquisition are also explained in this chapter. The design of a single transmitter-receiver combination suitable for both microwave tomographic imaging and confocal microwave imaging is a challenging task in microwave breast imaging. As microwave tomographic imaging is a frequency domain approach, the antenna should exhibit good matching at the desired resonant frequency. More over the antenna should exhibit less edge reflections for better coupling of electro magnetic energy into the tissue and should be compact enough for the ease in system implementation and data acquisition. Time domain confocal microwave technique requires wide band antenna to transmit short transient pulses. The antenna should efficiently focus the microwave signal towards the target and collect the back-scattered energy. A novel coplanar strip line fed bow tie antenna capable of using for both these applications is designed and developed and is discussed in Chapter 3. An in depth study of the new antenna is performed in terms of radiation characteristics, return loss and bandwidth to assess the feasibility of using the antenna for microwave breast imaging. Theoretical analysis of the antenna is performed using FDTD method and good agreement is observed. The chapter also gives a literature survey of the various antennas that are tried for microwave imaging. The literature review shows that even though microstrip bow tie antennas with coaxial feed have been simulated for microwave imaging, their experimental feasibility is not reported. Also some of these antennas reported erratic response when immersed in water. Evaluation of the performance of microwave imaging prototypes requires imaging and interpretation of test objects or phantoms. The dielectric properties of the breast equivalent material should yield close match to the actual tissue conditions. In microwave imaging, the object immersed in a coupling medium is subjected to microwave illumination and the reflected / transmitted energy is analyzed to study the dielectric profile of the object. The specific use of suitable coupling medium enhances the coupling of electromagnetic energy to the object as well as increases the resolution. Chapter 4 discusses the development of a single material to be suitable as both phantom and coupling medium in microwave breast imaging. The dielectric parameters of the material are studied in detail and the performance of the antenna when immersed in this medium is also studied. The new material exhibits matching values of dielectric constant with that of the breast tissues with less propagation loss. Chapter 5 discusses the development of 2-D microwave tomographic images on breast phantoms and breast tissues in the presence of matching coupling medium. Here the object is illuminated by a microwave signal and 2D images are reconstructed from the collected scattered fields using distorted Born iterative method in terms of dielectric constant. A detailed literature survey of the various algorithms used in microwave tomographic breast imaging is also discussed in the chapter. To the best of our knowledge and belief, ours is the only 2-D microwave tomographic approach that has been tried on an actual breast model in the presence of a matching coupling medium and accurate determination of the presence ofturnor is achieved. Time domain Confocal Microwave Technique implemented on real time breast model to locate the position of the tumor is discussed in Chapter 6. Here the object is illuminated by a microwave signal and the time domain response of the reflected signal is analyzed to locate the tumor. The point of dielectric contrast is represented as a sharp peak in the response. Experimental analysis is substantiated by FDTD method and good agreement is observed. A detailed literature survey of the various time domain approaches used in microwave breast imaging is also discussed in this chapter. For the techniques discussed in Chapters 5 and 6, same breast tissue samples are used. Microwave tomographic imaging of the samples provides valuable diagnostic information about the shape and quantitative description of the dielectric constant profile of the object, rather than identifying the location of the tumor. The time domain CMT identifies the presence and location of regions of dielectric contrast of the object rather than completely reconstructing the dielectric property profile. Thus the advantages of two techniques are explored and are put together in this doctoral work for a reliable and assertive detection of breast cancer. Conclusion and further extension of the work to 3-D analysis is discussed in Chapter 7. Three Appendices are included showing the associated studies performed by the author. They are microwave tomographic imaging of wax samples, microwave studies of poly vinyl acetate based phantom and details of the driver circuit used for data acquisition. The thesis concludes with the index, list of publications of the author and a brief resume.