The system (Fig. 1) consists of a US imager (Picus, Esaote Europe BV, Maastricht) with a linear array transducer L10-5 (128 elements, 7.5 MHz, 75% bandwidth).7 A multimode optical fiber (NA 0.22, diameter 600 μm) delivers 10 ns laser light at 1064 nm (Diny pQ, IB laser, Berlin) with at 250 Hz. The PA frame rate is 2 Hz and images are reconstructed online with a phased-array algorithm. The fiber is held in a metal housing (biopsy needle , Bard Ltd, UK) and fixed with Stycast discs (Fig. 1). Finally it slide-fits into a 14 G cutting cannula. A 2-mm cube of poly(vinyl alcohol) (PVA) (Table 1) is used as insert, held on a nylon thread and submerged in a liquid bath at 5 cm from the transducer. The bath contains solutions of Intralipid 20% (Fresenius Kabi, Germany) and Intralipid/Ecoline Black (nr. 700, Royal Talens, The Netherlands), with optical properties shown in Table 1. The PA needle is moved at a 45-deg orientation towards the insert. PA frames are acquired at several distances (18 averages, 10 to 20 frames, 0.5 mm step) between the fiber tip and insert.