Photoacoustic location of optical absorbers in phantom tissue

Pulsed photoacoustic (PA) signals may be used for the detection and imaging of blood vessels in tissue. A relatively strong absorption by red blood cells and low absorption by the surrounding tissue, combined with a reasonable penetration depth of the light is found at a wavelength of ca. 577 nm. Experiments were performed with a pulsed frequency doubled Nd:YAG laser which delivered 10 ns pulses at 532 nn wavelength. 10 % dilutions of India ink and 50 %, suspensions of red blood cells in PBS were used as optical absorbers. Blood vessels were simulated by hollow nylon fibers with an inner diameter of ca. 250 mu m through which these suspensions flew. The optical scattering of the surrounding tissue was simulated by a 12 % dilution of Intralipid-10 % to get a solution with a reduced scattering coefficient of 1.8 mm(-1). The PA signals were detected with a hydrophone that contained four wide band piezoelectric transducers made of 9 mu m thick PVdF film with an effective diameter of 200 mu m. Laser pulses with energies up to 8 mu J were delivered to the sample by a 50 or 100 mu m con diameter glass fiber. Pulsed optical heating of red blood cells up to 30-35 degrees for more then 12000 times did not affect the photoacoustic response of the cells. If a single fiber is used to illuminate the sample, then even at a depth of 1 mm the PA signals show that the volume that is effectively illuminated is laterally restricted to a diameter of ca. 1 mm. Vessels with blood or ink dilutions were detected up to a depth of more than 1 mm in the scattering medium. Monte-Carlo (MC) simulations were used to simulate the spatial distribution of light absorption in phantom tissue. From this distribution the PA response of blood vessels was simulated. A delay-and-sum beamforming algorithm was developed for 3-D near field configurations and applied to a PA image reconstruction program. The images based on MC simulations as well as experimental data show that the side of larger vessels that is facing the illuminating fiber can be located with a resolution that depends on the configuration and varies between 0.1 and 1 time the inner vessel diameter. This shows the principle and the feasibility of three dimensional photoacoustic dermal tissue imaging.