Diagnostic Ultrasound Megahertz Signal Detection with Multi-bounce Laser Microphone

Diagnostic ultrasound is one of the most common imaging modalities in the medical imaging field. One critical component of this imaging technology is designing sensitive receivers for detecting megahertz ultrasound signals. Currently, piezoelectric transducers have been the most widely used detectors for ultrasound signal. However, newer imaging techniques, such as photoacoustic imaging, require broader detection bandwidth and more sensitive detectors in order to work with significantly weaker signal. Conventional piezoelectric transducers may be limited in their ability to detect the full frequency bandwidth with high sensitivity, due to restricted transducer size and material composition. Sensitive and broadband ultrasound signal detecting techniques are needed for the development of advanced ultrasound imaging. The multi-bounce laser microphone utilizes optical methods to detect the displacement of a gold-covered thin film diaphragm caused by the ultrasound signal pressure waves. This sensitive all-optical sensing technique would provide new opportunities for advanced ultrasound imaging as it is expected to achieve a higher signal-to-noise ratio (SNR) in a broader spectrum when compared with conventional transducers. Each laser beam bounce interrogating on the diaphragm would take a different optical path-length that reflects the displacement and be involved in accumulation of the ultimate phase difference between the reference and signal beams for the detector to recognize. The system was previously developed for detecting acoustic signatures generated by explosives below 10 kHz. To demonstrate its feasibility in biomedical imaging, experimental design in this work has applied this technique to detect ultrasound signals ranging from 100kHz to 1MHz. The results of the experiments conducted in this work prove that the multi-bounce laser microphone system is capable of detecting megahertz range ultrasound signal. As compared to the conventional fibre-optic hydrophone, the laser microphone system achieves a higher SNR as the number of bounces increases. In the test of detecting 500kHz ultrasound signal, the SNR provided by the fibre-optic hydrophone is 38dB while the one achieved by the13-bounce laser microphone is 54dB. This is a promising sensitivity enhancement with a number of potential applications for detecting signals of significantly lower power, that would otherwise be very difficult for conventional piezoelectric ultrasound transducers to detect