| Time-domain ultrasound and photoacoustic imaging of biological tissue have commonality in the aspect of detecting method. Ultrasound imaging detects the echo, while photoacoustic imaging detects laser-induced thermal acoustic. They both calculate tissue characteristics by detecting the ultrasonic pulse signal. In this thesis, a conditioning circuit was developed, which can be used for front-end pulsed ultrasound signal acquisition, according to biomedical photoacoustic signal spectrum, intensity and transmission attenuation characteristics. The circuit was applied to the photoacoustic scan experiments using phantom tissue samples, and the noise of acquired experimental data was analyzed in the meanwhile. Specifically, the work mainly includes the following contents:Firstly, for different effectiveness demands in wideband and small-signal medical photoacoustic testing, three classes of ultrasonic testing conditioning circuits are provided via theoretical analysis and simulation, according to the characteristics of medical ultrasound and biomedical photoacoustic signal. These circuits mainly consist of discrete components type, discrete components and integrated amplifiers combination type, specific integrated circuits type variable gain amplifier respectively. These circuits can comply with the design requirements include 0-80dB variable gain, 30MHz bandwidth@80dB, equivalent input noise voltage<5nV/(?). Moreover, its advantages and disadvantages, applicable scope are also discussed here. The designs may provide some useful references in the aspects of cost, flexibility, convenience concern in photoacoustic testing design.Secondly, based on the above designs and simulations, this section concentrates on time-domain photoacoustic signal. First of all, a differential amplifier AD8334-based wideband circuit for photoacoustic signal conditioning with large scale adjustable gain was designed. The design specifications of the circuit are 50kHz ～50MHz frequency reception range, 0～100dB gain adjustment range, equivalent input noise density <1nV/(?), more than 40dB(with 80dB gain) signal-to-noise improvement ratio,200ns(fastest) gain control response time. Next, a gain linear compensation theoretical basis was derived, and the gain of the circuit is controlled by FPGA and AD5207 digital potentiometer with linear dB dynamic gain control to achieve time gain compensation(TGC). TGC brings about correction of photoacoustic signal distortion results from different optical and ultrasonic attenuation in different propagation depth.Thirdly, in order to verify the functionality and performance of the circuit, some verification tests with AGAR phantom samples contained pencils were done in this section. The tests verified amplification, output SNR and SNIR, dynamic gain control of the circuit. At the same time, noise analysis of the acquired data from verification experiment was discussed, which identified three main sources of noise. Then, improvement measures were also proposed. Last, through the contrast of two wavelet filtering method -- multiscale wavelet transform and wavelet soft thresholding-using wavelet analysis, this thesis adopts the latter as filtering method of image reconstruction in application experiments.Finally, the mentioned circuit was applied to scan two phantom samples and reconstruct the images based on the existing photoacoustic imaging system in Lab. The first sample was a pie-like Agar and fat milk mixture with three pencils(absorbers) placed at 3mm intervals each other in it. The positions of three pencils were clearly distinguished after three-dimensional reconstruction. The second sample simulated prostate sample which contained foreign matter, and was irradiated by dispersion fiber. In this sample, two pencils(foreign matter) were placed forward and backward around the fiber, and the front and rear boundaries of the fiber were clearly distinguished after two-dimensional scan. |
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