Technical considerations in quantitative blood oxygenation measurement using photoacoustic microscopy in vivo

Using peak amplitude spectral PA measurements in the range of 570 - 600 nm, we found it possible to quantify blood oxygenation levels. Visible light illumination minimizes the inversion error of the PA measurements. Owing to high blood absorption in this optical regime, there is also an improved signal-to-noise ratio and less influence from optical scattering. To arrive at correct, and vessel size independent, SO_2 measurements, the central frequency of the ultrasonic transducer must be high enough to satisfy the relation µ_a Λ < 1, that is above 25 MHz for a chosen optical wavelength region, although lower frequency transducers may produce correct results after correction of the optical absorption spectra. However, additional efforts are needed to achieve accurate SO_2 values for in vivo measurements.