Multi-source electromagnetic focusing control for thermoacoustic imaging in cylindrical tissue model

The breast cancer is the number one at cancer caused deaths at women and similar to all other cancer types early diagnosis has a critical importance in dealing with breast cancer and to decrease the number of breast cancer related deaths. Risk of getting breast cancer increases by age and the diagnosis of breast cancer ratio is almost 70 percent. Today, mammography is most widely used medical imaging devices for breast cancer diagnosis and follow. Mammography is a very cost-effective technic but the x-rays used for imaging has cancer-triggering harmful side effect. Also there are some difficulties about diagnosing the glandular tissue with mammography. Computerized tomography which uses higher dosages of x-ray is not used for diagnosis of breast cancer. Ultrasonic imaging has no harmful side-effect, produces high point-resolution; it is a low-cost modality but has poor contrast resolution which complicates early diagnosis of some cancers. Magnetic resonance imaging with contrast agents gives good results in diagnosis of malign cancer, but it is relatively expensive and has no standard application protocols yet. Ultrasonic imaging is an imaging technic which is low-cost and has no harmful side-effects. Point resolution of this technique is high but contrast resolution is poor; this complicates the early diagnosis of some cancers and distinguishing the tumor if it is benign and malign. RF and microwave breast tomography is a technique based on different electrical properties of tissues, having potentially high contrast resolution and no ionozing harmfull side effect, but having poor point resolution. Microwave or laser induced thermoacoustic imaging is a technique combining contrast advantage of microwave tomography and high point resolution of ultrasonic imaging. Since imaging depth is limited by frequency dependent wave attenuation in tissue, use of microwave band is a proper chocie for deep tissue structure such as breast and brain. When objects with electrical conductivity are stimulated by high-power electromagnetic (RF, microwave or laser) pulses, the electromagnetic energy absorbed in the object creates impulsive temperature rise, this temperature increase causes pulse expansion the object; which produces a high frequency acoustic (ultrasonic) wave. This physical principle is called the photoacoustic effect. Thermoacoustic imaging is created by the reconstruction technique, in which acoustic waves produced by the photoacoustic effect are collected by ultrasonic receiver sensors. The thermoacoustic image is related to the electromagnetic absorption, thermal and acoustic parameters of the environment. With this feature, thermoacoustic imaging provides different, complementary information than existing imaging methods. Thermoacoustic imaging is mainly based on analytical solution of acoustic wave equation with boundary conditions. The quality of thermoacoustic imaging is based on intensity and uniform distribution of microwave, sufficent spatial sampling of measurement surface, the realistic boundary conditions used for solving the thermoacoustic wave equation as well as the performance of numerical implementation of the techniques. Thermoacoustic imaging is a new hybrid imaging approach that combines the high contrast resolution feature of laser and electromagnetic based imaging with the high-point resolution properties of pulse-reflected ultrasound imaging. Thermoacoustic imaging modality has no harmful side effect, and thus arising as a new emerging imaging modality for early diagnosis and follow up of breast cancer. In this study, we work on developing microwave antenna part of microwave induced thermoacoustic imaging system. In the thermoacoustic imaging system, it is possible to stimulate the tissue using one microwave antenna as well as using phased array antenna. The electromagnetic wave emitted in relation to the absorption coefficient of the tissue and the cavity due to absorption and interference, and the electric field intensity in an absorbing medium is reduced exponentially. Therefore, they observe high acoustic attenuation in the tissue. One of the problems with single source stimulation is occured low amplitude of TA signal. The thermoacoustic pressure wave is related to the absorbed microwave power, received thermoacoustic signal is proportional to the microwave power absorbed by the medium and also to the local heating and expansion coefficients. The higher it get a amplitude of signal, the higher it requires a microwave power. This regional focussing approach provides high local power and increases the level of the produced acoustic signal. Compared with single-source excitation, the local power is increased in the focus region of the medium by the proposed multi-element phase array antennas and local excitation results directly in the acquisition of thermoacoustic images with a higher signal-to-noise ratio (SNR) of the target region. The most suitable approach to focus the electromagnetic energy is to use the phase array antenna system and the position of the focusing region can be controlled by adjusting the phase of the array antennas. In this study, multiple excitation with phase array antennas is proposed as a solution to the problem of low amplitude thermoacoustic signal, which is one of the problems with single source illumination. A 12 element array antenna is used for this system. This system adjusts the phases of each channel, allowing microwave energy to concentrate in a desired area with cross-section control in the horizontal direction. With this method, higher signal-to-noise ratio and higher resolution thermoacoustic signals can be obtained. There are some analytical and numerical approaches to this problem. Analytically, the solution of the electromagnetic equation was obtained in the cylindrical coordinates using the layered tissue models.This solution was tested with a one, two and three-layer numeric tissue model, respectively. In this setup, incident and scattered electromagnetic fields have been tried to be calculated with various approaches. Besides, all related phase information is obtained comparetively by using several methods in different domains. Thus localization problem and penetration through the different layers are researched extensively. It is difficult to solve the problem analytically in a lossy medium. Because of the loss of the tissue, numerical approaches are prefered. At first, Direct Solution Method and other Iterative Methods (least squares method etc.) have been investigated. These results were found to be inadequate when compared with HFSS results. For this reason, for the first time in litratarure as far as we known, an algorithm based on the Fast Marching Method (FMM) was developed for phase calculations. The reason, FMM is selected due to its adaptive and iterative nature. The results obtained from FMM and HFSS results are compromised. In addition, the results of the Fast Marching Method are also compatible with the analytical solutions obtained in the frequency domain. In this thesis study, a road map was sought for the implementation of future integrated TA imaging system or the steps to be followed in prototype production. Adjusting the phases of array antennas to take advantage of multiple excitation helps to focus locally.