超声换能器带宽对光声成像的影响

研究了不同尺寸吸收体产生的光声压的频谱特性：对于厘米量级、毫米量级和几百个微米量级的吸收体，产生光声压频谱的主要范围分别约为20～300kHz、70kHz～2．5MHz和400kHz～20MHz；讨论了不同频率范围的光声信号对重建图像的影响，低频段的光声信号能反映物体的非边界区域，而高频段的光声信号能突出物体的细微结构，尤其是物体的边界特征。提出了不同尺寸的吸收体要选用或设计不同带宽范围的探测器进行检测的方法．当探测器的带宽范围与光声压频谱范围基本吻合时，损失的频率成份较少，重建的光声图像效果较好，这一结论在仿真和实验结果中都得到了证明。实验用的光源为YAG激光器，波长为532nm，重复频率为30Hz，脉宽为7ns，探测器为针状的PVDF膜水听器，接收面积的直径为1mm。

Influence of Bandwidth of Ultrasonic Transducer on Photoacoustic Imaging

Photoacoustic signals induced by a short pulse laser cover a wide spectral range. The spectral characteristics of absorbers with different sizes and the influence of photoacoustic signals with different spectral components on photoacoustic imaging are explored. The main frequency ranges of photoacoustic pressures of absorbers with diameters of ～cm, ～mm and hundreds of μm are about 20～300 kHz, 70 kHz～2.5 MHz and 400 kHz～20 MHz, respectively. The low spectral components of photoacoustic signals contributed to the non-boundary region of absorbers, and the high spectral components contributed to the small structure, especially, to the boundary. It is demonstrated that the ultrasonic transducers used to detect photoacoustic pressures should be designed and selected according to the frequency ranges of absorbers. When the frequency response range of transducers accords to that of absorbers, almost the whole frequency components of photoacoustic pressure can be detected. That produces good reconstructed images. A Q-switched Nd∶YAG laser operating at 532 nm was used as light source. The laser had a pulse width of 7 ns and a repetition frequency of 30 Hz, and a needle PVDF hydrophone with diameter of 1 mm was used to detect photoacoustic signals.