In vivo photoacoustic imaging of blood vessels using a homodyne interferometer with zero-crossing triggering.

We demonstrate a quasinoncontact photoacoustic imaging method using a homodyne interferometer with a long coherence length laser. The generated photoacoustic signal is detected by a system that is locked at its maximum sensitivity through the use of balanced detection and zero-crossing triggering. The balanced detector is substantially equalized, so its output is zero when the system reaches the maximum sensitivity. The synchronization approach is used to trigger the excitation and detection of the photoacoustic signal. The system is immune to ambient vibrations. A thin water layer on the sample surface is used to reduce the effect of the rough tissue surface. The performance of the system is demonstrated by in vivo imaging of the microvasculature in mouse ears.