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Three dimensional finite difference time domain simulations on harmonic motion microwave doppler imaging method using realistic tissue models

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2019
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Imaging of electromagnetic and elastic properties of the breast tissue can be used to identify cancerous regions in the dense fibro glandular breast tissue at the early stage. Numerical simulations are useful in predicting the performance of the imaging method. In Harmonic Motion Microwave Doppler Imaging Method (HMMDI), tissue is vibrated locally, microwave signals illuminate the tissue and the back scattered Doppler signal component is used to form images. In this thesis, forward problem of HMMDI method is solved with various numerical tissue models using the 3-D (Three dimensional) Finite Difference Time Domain (FDTD) method. In the forward problem solution, mechanical and electromagnetic problems are examined separately. In the mechanical problem, the acoustic radiation force of amplitude modulated focused ultrasound waves is used to produce local vibrations inside the tissue. According to mechanical simulation results, displacement values in tumor region are smaller compared to the breast tissue region as expected and increasing frequency decreases the displacement values. In electromagnetic problem, transmitter antenna sends a monochromatic signal during focused ultrasound excitation, and a receiver antenna is used to receive the amplitude and phase modulated reflected signal. In the forward problem calculations, computation speed is an important issue since a large number of iterations (>1000) is needed in the FDTD solution. In this thesis, the forward problem is solved with parallel programming in Matlab. 19 times acceleration is achieved when GPU is employed compared to CPU case. Using the developed simulation tools, the received HMMDI signal is analyzed using three types of breast tissue models. Firstly, tumor inside a homogenous breast tissue problem is solved. The received Doppler signal amplitude increases in the tumor region in this case. In the second type, a tumor is located inside the fibro glandular tissue. Tumor and fibro glandular tissue have the same electrical properties but their mechanical properties are different. The received Doppler amplitude decreases in tumor region, since tumor is stiffer. In the third case, a tumor in realistic breast tissue model is tested. Simulations are run with and without the tumor in the breast tissue. The results show that the Doppler signal levels change in the tumor region by 0.5 to 2 dB, compared to the case without the tumor. The difference in the Doppler signal amplitude increase with decreasing mechanical vibration frequency. In conclusion, the tumor is detectable in all cases.