2D backward-mode photoacoustic imaging system for NIR (650-1200nm) spectroscopic biomedical applications

A 2D photoacoustic imaging system for spectroscopic biomedical applications is reported, based on a Fabry-Perot (FP) polymer film ultrasound sensor. A variety of broadband sensors have been developed with bandwidths from 20MHz to 50MHz. These ultrasound sensors have a unique dichroic design which has an optical transmission window from 650nm to 1200nm and can be interrogated in the 1520-1610nm wavelength region. This enables the system to operate in backward mode with a tunable Optical Parametric Oscillator (OPO) as the excitation source for near infrared (NIR) spectroscopic applications such as the measurement of blood oxygenation. The area over which the photoacoustic signals can be mapped is 4cm × 2.5cm with an optically defined element size of 64μm diameter. The system's noise-equivalent pressure (NEP) is 0.3kPa over a 20MHz bandwidth without signal averaging. The photoacoustic signals are mapped by rapidly scanning a focused laser beam over the surface of the sensor with a point to point acquisition time of 100ms. The spatial resolution of the imaging system, evaluated from tests on phantoms using a 50MHz FP sensor, is 37μm (lateral) × 27μm (vertical) FWHM. It is considered that this system has the potential to be used in applications that require high resolution 3D imaging of the structure and oxygenation status of the microvasculature.

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