Multispectral functional photoacoustic imaging of goat eyes for estimation of tissue chromophores

This work demonstrates multi-spectral functional photoacoustic microscopy (PAM) for imaging the molecular distributions of tissue chromophores (HbR, HbO2, sO2%, and melanin) non-destructively and non-invasively. Fundamentally, the optical absorption coefficient (μa) of soft tissue is predominantly contributed by the constituent chromophores. Measuring the photoacoustic (PA) signals – that are induced by a transient electromagnetic beam from a laser source (wavelengths ~700–850 nm) – using a focused transducer (operating frequency ~30 MHz), we determine the concentration of the individual (dominant) chromophores. We employed a spectral unmixing technique that involves solution to a system of linear simultaneous equations using the measurements of μa’s at different wavelengths. Validation studies were conducted in ex-vivo tissues (goat eyes). The results demonstrate that we can not only achieve functional oxygen saturation maps, but also quantitative overlays of HbR and HbO2 concentration maps. The experimental results reveal distinct chromophore profiles across the entire region of the eye encompassing retina, choroid, iris, cornea, and ciliary body. This novel technique of whole-eye quantitative tissue imaging at sub-millimetre resolution highlights the potential of PAM for non-destructive and non-invasive clinical applications, particularly in ocular diagnostics and therapeutic treatments.