Ultrasonic needle tracking system, shown schematically (a), with a photograph of the needle tip with the fibre-optic beacon at the distal end (b). Parallel multi-channel photoacoustic (PA) data and conventional B-mode ultrasound (US) data were acquired sequentially, and post-processed separately (c). As illustrated with acquisitions from the needle tip in water, raw PA data (d) were reconstructed to obtain a PA tracking image of the fibre-optic beacon (e; arrow). In a time series from transducer element 64, the onset of the beacon signal is apparent at t = 26 μs (f). The location of the beacon in the PA tracking image was coincident with that of the needle tip in the US image (g). PC: personal computer; MWCNT: multi-walled carbon nanotube; PDMS: polydimethylsiloxane; SVD: singular value decomposition; a.u.: arbitrary units.