Flow-dependent photothermal modulation of the photoacoustic response

The temperature dependence of photoacoustic generation is utilized for monitoring the temperature in flowing blood. A phantom blood vessel is probed with photoacoustic (PA) excitation from a 830nm laser diode whose intensity is sinusoidally modulated at ultrasound frequencies. A second laser diode at the same wavelength is used to photothermally (PT) induce sinusoidal temperature fluctuations in the probed volume. The temperature oscillations lead to modulation sidebands in the PA response. Measurement of the sidebands amplitude as a function of the PT modulation frequency, for different flow rates, reveals a strong dependence of the PT modulation frequency response (MFR) on the flow rate. This is attributed to the thermal properties of the volume under test, and in particular to the heat clearance rate, which is strongly affected by the flow. A simplified lumped model based on the similarity between the system temporal behavior and that of an RC circuit is used to analyze the resulting MFR's. With the addition of an appropriate calibration protocol and by using multispectral PA and/or PT excitation the proposed approach can be used for simultaneous in-vivo measurement of both flow and oxygenation level.