(a) Schematic diagram of the imaging system. A tunable pulsed Quantum Cascade Laser (QCL) provides the excitation for optoacoustic imaging. An absorption intensity map is obtained by displacing the sample along the focal plane while simultaneously acquiring the optoacoustic signal. (b,c) Lateral and axial resolution test of the imaging system. (b, upper) Point spread function (PSF) using a 1 µm polystyrene sphere. (b, lower) Intensity profile along different orientations of the PSF. The mean full-width at the half maximum (FWHM) indicates a lateral resolution of 5.3 µm. (c, upper) Intensity projection in z of the same PSF shown in b. (c, lower) Intensity profile along a selected line in the z projection. The FWHM indicates an axial resolution of 42.2 µm. An acoustic reflection from the ZnSe window of the sample holder (FWHM of 58.1 µm) overlapping with the main signal was also observed. (d) Contrast test of the endogenous imaging system using a 10 µm polyamide suture and 50 µm polyethylene spheres; FOV 0.5 mm x 1.0 mm and 2 µm step size. (left) Image obtained at 2850 cm-1 showing a strong positive contrast for the spheres and a dim positive contrast for the suture; the background (agar) appears dark. (right) Image obtained at 1550 cm-1. Here the strong contrast comes from the suture, but also the water content in agar offers positive contrast. In this image, the polyethylene spheres appear dark, scale bars: 100 µm. Data in b-d are representative of three independent experiments.