The system consists of a commercial handheld US imager (MyLab_One, Esaote Europe BV, Maastricht, The Netherlands) with a linear array probe SL3323 (128 elements, 7.5 MHz, 100% bandwidth).11 The system is used in the research mode with 64 elements where US transmission, laser pulse transmission, and data acquisition are controlled using custom-made software running on a PC. Illumination uses an Nd:YAG laser (Quanta-Ray Pro 250, Spectra Physics, Mountain View, California) pumping an optical parametric oscillator (VersaScan-L532, GWU, Erftstadt, Germany) at 10 Hz. The energy per pulse is monitored using a photodetector. A multimode optical fiber (NA 0.22, diameter ) coupled to the laser delivers 10 ns laser light with at 800 nm. For the multiwavelength studies, the range of 720 to 860 nm was covered with 5 nm steps; single wavelength studies used 800 nm. The fiber is held in a biopsy needle (, Bard Ltd., UK) and slide-fits into the 14 G cutting cannula [Fig. 1(a)]. The PA frame rate is 10 Hz and images can be displayed online using a GPU-based delay and sum reconstruction algorithm. Images presented here are averaged over 100 frames and reconstructed offline using a frequency domain reconstruction algorithm,12 assuming a constant speed of sound in the medium for all measurements (phantom measurement: , ex vivo measurement: ).