Photoacoustic waves generated in blood studied using pulsed digital holography.

We studied photoacoustic waves using pulsed digital holography. The acoustic waves were generated in a reindeer blood target by absorption of an IR laser pulse, lambda=1064 nm and pulse length=12 ns. The acoustic pressure waves were then imaged in water using a second collimated laser pulse at lambda=532 nm 2 micros after the first IR pulse. Quantitative information on acoustic wave properties such as three-dimensional shape and pressure distribution was calculated by applying the inverse Radon transform on the recorded projection. The pressure pulse had a flat and sharp front parallel with the blood surface, which indicates that the pressure was generated at the blood surface. The generated pressure was proportional to the laser fluence with the proportionality constant equal to 1.8+/-0.3 cm(-1). According to existing data, the proportionality constant should be 1.4 cm(-1) for oxygenated human blood, which made our calculations probable.