Fast-scanning ultrasonic-photoacoustic biomicroscope: in vivo performance

摘要

The combination of ultrasonic and photoacoustic imaging modalities has yet to be realized in the high-frequency regime (>20MHz) where spatial resolution may permit visualization of the microvasculature. In this work, we characterize the in-vivo performance of a custom ultrasound-photoacoustic B-scanning imaging system. This system utilizes a combined ultrasound/photoacoustic probe attached to a voice-coil capable of approximately lcm lateral translation at a rate of up to 15Hz. The probe is comprised of a 25MHz ultrasound transducer, configured confocally with a conical mirror-based dark-field laser delivery system. The fast-scanning mode permits realtime ultrasound imaging. The imaging speed of the photoacoustic mode is limited by the repetition rate of the 532nm laser (up to 20Hz). Signals from the transducer are amplified by a 39dB preamp with an additional time-gain compensation stage of up to 24dB. Control of the system is through a digital input-output PCI card, which acts as a pulse-sequencer and permits software control of time-gain compensation. This setup permits interlaced pulse sequences for excellent registration of ultrasonic and photoacoustic data, as well as separate time-gain compensation curves for photoacoustic and ultrasound modalities. We have managed to achieve a lateral resolution of 155 μm and an axial resolution of 40μm. The system is used to visualize the finger and palm of a hand to almost lcm ultrasound depths and multiple millimeter-scale photoacoustic depths. Photoacoustic images are overlaid on the ultrasound images for simultaneous visualization of the microvasculature and surrounding tissue.