In vivo functional brain imaging by using a broadband fiber optic photoacoustic probe

Oxygen saturation (sO2) imaging can offer useful information for pathological studies and clinical diagnostics. Here we report on a fiber-based photoacoustic microscope for functional brain imaging. The critical element is a fiber-optic ultrasound sensor, which translates the megahertz acoustic waves into detectable lasing frequency variations. Its working bandwidth has been extended to 40 MHz, by reducing the fiber diameter to 58 μm via chemical etching. As a result, in vivo sO2 imaging of a mouse brain can be performed by using a dual-wavelength excitation scheme, taking advantage of the difference in the absorption spectrum between the oxygenated (HbO2) and deoxygenated hemoglobin (HbR). The broadband ultrasound detection can effectively overcome the saturation effect originated from the strong optical absorption. The fiber optic photoacoustic probe paves the way for the implementation of a light-weight, wearable brain imager for awake, freely-moving animals.