Reflectance confocal microscopy (RCM) has become a standard method for skin diagnostics. It is based on tightly focusing a continuous laser beam with an objective into the skin and collecting the back scattered light through a pinhole for generating images of selected planes to a depth of several 100 micrometers. For enhancing its diagnostic capability, the RCM can be combined with optical resolution photoacoustic microscopy (OR-PAM), providing strong optical absorption contrast for melanocytic lesions. We have developed a compact add-on to common optical objectives that is able to detect the photoacoustically generated transients with high bandwidth, using four piezoelectric elements made of poly-vinylidene fluoride (PVDF). The elements are arranged on four quadrants of a conical surface around the objective, taking advantage of the focusing effect of the slightly curved surface. For taking an image, the pulsed and continuous excitation beams are simultaneously scanned over the sample using a pair of galvanometric mirrors. Photoacoustic images of the selected plane are then generated for each separate sensing element and are subsequently added in order to achieve an enhanced signal-to-noise ratio. Simultaneously recorded back scattered light provides the input for the RCM mode. We present a characterization of the sensors and provide experimental results on phantoms.
A device for acquiring co-registered reflectance confocal microscopy (RCM) and optical-resolution photoacoustic microscopy (OR-PAM) images is presented. It uses a pulsed laser for photoacoustic excitation and a continuous laser for the confocal mode, co-aligned, focused and scanned over planes within the object. The photoacoustic part uses a probe beam deflection method, where a continuous laser beam is bent in the refractive index gradient generated by the pressure wave. First images on a carbon microfiber phantom demonstrate the capability of generating co-registered images, where the RCM mode displays back-scattering contrast, while the OR-PAM mode creates images with optical absorption contrast. In the temporal signals of both methods, typical signatures of transient cavitation were visible in the carbon fiber experiment. A lateral resolution of the setup of about 1.6 μm and a noise equivalent pressure amplitude of 90 Pa for the probe beam deflection method were measured.
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