Near infrared spectroscopy (NIRS) in tissue is known to be insensitive to oxygenation changes inside blood vessels. Measurements are dominated by changes in the surrounding tissue, which has significantly lower optical absorption. A hybrid technique of NIRS with focused ultrasound (US) allows spectroscopic measurements to be collected from the acoustic focal region. This technique is currently limited by the low signal-to-noise ratio of this US-modulated signal relative to the background of unmodulated photons. We are investigating the use of microbubbles (a widely used clinical US contrast agent) as a means of amplifying this acousto-optic (AO) signal. Here we present a Monte Carlo model of light transport including US and microbubbles: analytical acoustic modelling of microbubbles is based on the Rayleigh–Plesset equation, which describes a bubble oscillating under applied US. The results of this model demonstrate that AO techniques are more sensitive to changes in oxygen saturation (SO2) in a deep highly absorbing blood vessel than conventional optical methods. AO measurements are also less sensitive to changes in the surrounding tissue SO2. This is a promising candidate for non-invasive measurements of SO2 in blood vessels such as the pulmonary artery.
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The authors would like to thank Samuel Powell for assistance in implementing the GPU-based simulations and Dr. Jing Deng for useful discussions. This work was funded by the Centre for Mathematics and Physics in the Life Sciences and Experimental Biology (CoMPLEX) Doctoral Training Centre at UCL, through a studentship, the British Heart Foundation, the Medical Research Council and the Engineering and Physical Sciences Research Council (Grant Code EP/G005036/1).
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