Critical Technologies Research Of The Microwave-Induced Thermo-Acoustic Tomography System

The accident and the mortality ratio of various cancer is increasing continually reported by a lot of medical literatures in every year, while the technical progress of cancriform detection and therapy is unsatisfactory; A commonsense opinion of cancriform field is that the more early diagnose more surviving, this is also the truth in the breast cancer detection and therapy field. Development a detection instrument about the early breast cancer is the aim of this paper.The microwave-induced thermo-acoustic tomography (MITAT) is an innovative technique for tumor's detection, it employees the modulated microwave pulses to irradiate the biologic tissue, there has thermo-deposition due to the electromagnetic energy absorbed by the tissue. Because the different tissue heating originated the electric difference, the detectable ultrasonic signals are generated by this procedure. Especially the malignant tumor with distinct permittivity and electrical conductivity versus the surrounding normal tissue, microwave-induced thermo-acoustic (MITA) signals can be obtained with very high signal-noise-ratio. For the microwave pulses used as the irradiating signals, while the received signals are ultrasound in microwave-induced thermo-acoustic tomography system, high resolve and contrast images can be got. So it has becoming a promising technology in medical imaging field.However, microwave-induced thermo-acoustic tomography system has very many difficulties. Firstly, the microwave-induced thermo-acoustic is an interdisciplinary field procedure. It includes the electromagnetic wave propagating in heterogeneous medium, the transform from the electromagnetic energy to the thermo-energy and the mechanical wave (ultrasound) generation due the tissue expanding. The analytic studies of the MITA procedure based on dynamics and the function of thermo-diffuse and thermo-conduction combine these three physic field systematically. Compared with situation function with thermo-balance method, the analytic studies based on dynamics have less parameters approximation, and more precision. Meanwhile, the discipline of the microwave energy absorbed by tissue is should studied also. Secondly, there has severe noise with width spectrum due to the powerful irradiating microwave pulses. This is a big challenge to design and realize the receiver subsystem to detect the weak MITA signals. How to improve the signal-noise-ratio of the received thermo-acoustic signals in terms that the irradiating microwave power should be decrease possibly is the keystone of a successful system. In order to balance the power of the irradiating and the receiver signals with optimal signal-noise-ratio, after the study of the noise sources in the MITAT system, an innovative ultrasonic receiver with perfect electromagnetic compatibly and pre-amplifier subsystem are designed and applied to the MITAT system. On the signal processing opinion, the received thermo-acoustic signals is deal with wavelet technique, the noise depressing and wavelet analyses method are very useful for the MITA signals. Meanwhile, in order to obtain uniform microwave energy distribution in biologic tissue and improve the effectiveness of the radiator, a circle horn antenna with low voltage-stand-wave-ratio is simulative studied to replace current rectangle waveguide radiator. Thirdly, it is also difficulty and focus in MITAT system that the imaging algorithm in terms that the biologic tissue is heterogeneous. Due to the heterogeneous and the targets located in the near field of the irradiating, the diffuseness and the multi-path effects can't be neglected. Otherwise, the resolve and contrast of the images generated by some imaging algorithm, such as back-projection, should be of low quality. The superior images can be got by the imaging algorithm of time reversal mirror technique based on pseudo-spectrum time domain method applied to MITAT system. Because that the all the multi-path information can be utilized by this imaging algorithm, and the targets can be refocused at original position correctly. By the way, the characteristics of the TRM noise depressing and statistic stability in terms that the model's parameters have random distribution are studied also. In additional, for the numerical pseudo-spectrum time domain method is employed as the forward core of the time reversal mirror technique, the Green's function required by the TRM can be realized by the numerical model easily and the larger scale biologic organ can be simulated quickly.The aim of this paper is focus on the critical hardware and software research of themicrowave-induced thermo-acoustic tomography system, the contents are:1. After discussing the character of the microwave energy absorbed by biologic tissue,the mechanism of the microwave-induced thermo-acoustic procedure are analyticstudied based on the dynamics and the function of the thermo-diffuses and thermo-conduction. The important conclusion can be draw that there has an equivalent relationship between the microwave energy absorption distribution and the induced thermo-acoustic sources distribution.2. The subsystems of the microwave-induced thermo-acoustic tomography system are studied systematically. The design criterions of the irradiating microwave pulses, detection subsystem and electromagnetic compatibility are studied. The technical standards of the MITAT prototype system are: center frequency 2.45 GHz, the width of the pulse 0.5 - 2.0 us and the peak value of the pulse 0.8-40 kW.3. The measured microwave-induced thermo-acoustic signals are studied in temporal-spectral domain. To explain the phenomenon that the thermo-acoustic signals delay error become larger when the distance of the sample and ultrasonic transducer is small in temporal domain, the model includes that the cross-section scale of the sample and the aperture scale of the ultrasonic transducer, and the synthetic ultrasonic pressure wave generated by the thermo-acoustic sources are considered. The amended thickness information error of the sample based on the model and the synthetic pressure theory can remarkably improve. From the signal processing point of view, the MITAT system is cascaded by each subsystems, the irradiating microwave pulses, sample with geometric information, and the response function of the ultrasonic transducer. According to this viewpoint, the spectral characteristic of the MITA signals are studied to explain how the geometric information of the sample to affect the spectral distribution of the measured signals, meanwhile the bandwidth of a very MITAT system can be evaluated by this analyses.4. The noise depressing and multi-resolution application of the wavelet technique in an MITAT system are studied.5. The imaging algorithms are studied base on the measured thermo-acoustic signals from the prototype MITAT system: back-projection algorithm and time reversal mirror technique based on the pseudo-spectral time domain method, the resolution of the image less than 3 mm, and the contrast large than 25 dB.6. Two-dimension images generated by some simple targets (simplex biologic tissue with high water content, such as pork muscle) immerged in a homogenous medium (mineral oil or porcine oil), the linear and circle scan Patten are used. Two-dimension images generated by complex targets (a biologic tissue with skin, fat and muscle) are studied also.