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Current systems designed for deep photoacoustic (PA) imaging typically use a low repetition rate, high power pulsed
laser to provide a ns-scale pulse illuminating a large tissue volume. Acoustic signals recorded on each laser firing can be
used to reconstruct a complete 2-D (3-D) image of sources of heat release within that region. Using broad-beam
excitation, the maximum frame rate of the imaging system is restricted by the pulse repetition rate of the laser.
An alternate illumination approach is proposed based on fast scanning by a low energy (~ 1 mJ) high repetition rate (up
to a few kHz) narrow laser beam (~1 mm) along the tissue surface over a region of interest. A final PA image is
produced from the summation of individual PA images reconstructed at each laser beam position. This concept can take
advantage of high repetition rate fiber lasers to create PA images with much higher frame rates than current systems,
enabling true real-time integration of photoacoustics with ultrasound imaging. As an initial proof of concept, we compare
conventional broad beam illumination to a scanned beam approach in a simple model system.
Two transparent teflon tubes with diameters of 1.6 mm and 0.8 mm were filled with ink having an absorption coefficient
of 5 cm-1. These tubes were buried inside chicken breast tissue acting as an optical scattering medium. They were
separated by 3 mm or 10 mm to test spatial and contrast resolution for the two scan formats. The excitation wavelength
was 700 nm. The excitation source is a traditional OPO pumped by a
Q-switched Nd:YAG laser with doubler.
Photoacoustic images were reconstructed using signals from a small, scanned PVDF transducer acting as an acoustic
array. Two different illumination schemes were compared: one was 15 mm x 10 mm in cross section and acted as the
broad beam; the other was 5 mm x 2 mm in cross section (15 times smaller than the broad beam case) and was scanned
over an area equivalent to broad beam illumination. Multiple images obtained during narrow beam scanning were added
together to form one PA image equivalent to the single-shot broad beam one.
Results of the phantom study indicate that PA images formed by narrow beam scanning excitation can be equivalent to
one shot broad beam illumination in signal to noise ratio and spatial resolution. Future studies will focus on high
repetition-rate laser sources and scan formats appropriate for real-time, integrated deep photoacoustic/ultrasonic imaging.
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