Fig. 5.

Coregistered photoacoustic and ultrasound images were acquired, with the distance between the distal end of the fiber optic and the polymer tube filled with India ink, d, varied from 1 to 10 mm. Examples are shown for d = 5 mm [(a) and (b)] and for 1 mm [(c) and (d)]. The colorbars in (a) and (c), and (b) and (d) correspond to photoacoustic and ultrasound image amplitudes, respectively. Average photoacoustic amplitudes (S_ROI) in a 4 mm × 4 mm region of interest indicated in (a) for wavelengths 750, 800, and 850 nm are plotted for d from 1 to 10 mm in (e). The data in (e) represent the average values from 15 image frames; the error bars represent the standard deviations. Equation (4) is used to fit the measured S_ROI values for each wavelength and compare with the measured data. The intersections of the fitted curves and the noise floor indicate a maximum imaging depth of ∼15 mm for the detection of blood vessels. The estimated μ_eff values from curve fitting are compared with literature values in Table 1.