Sentinel lymph node biopsy (SLNB) has become the standard method for axillary staging in breast cancer patients,
relying on invasive identification of sentinel lymph nodes (SLNs) following injection of blue dye and radioactive tracers.
While SLNB achieves a low false negative rate (5-10%), it is an invasive procedure requiring ionizing radiation. As an
alternative to SLNB, ultrasound-guided fine needle aspiration biopsy has been tested clinically. However, ultrasound
alone is unable to accurately identify which lymph nodes are sentinel. Therefore, a non-ionizing and noninvasive
detection method for accurate SLN mapping is needed.
In this study, we successfully imaged methylene blue dye accumulation in vivo in rat axillary lymph nodes using a
Phillips iU22 ultrasound imaging system adapted for photoacoustic imaging with an Nd:YAG pumped, tunable dye
laser. Photoacoustic images of rat SLNs clearly identify methylene blue dye accumulation within minutes following
intradermal dye injection and co-registered photoacoustic/ultrasound images illustrate lymph node position relative to
surrounding anatomy. To investigate clinical translation, the imaging depth was extended up to 2.5 cm by adding
chicken breast tissue on top of the rat skin surface. These results raise confidence that photoacoustic imaging can be used
clinically for accurate, noninvasive SLN mapping.
A one of a kind photoacoustic system has been built around a Philips iU22 ultrasound scanner. The modified channel
board architecture allows access to the raw per-channel photoacoustic data, while keeping all of the imaging capabilities
of an actual commercial ultrasound scanner. A captured photoacoustic data frame is Fourier beamformed to generate a
single laser shot photoacoustic image. In addition to the photoacoustic data, the system supplies the beamformed
ultrasound data, providing a truly dual-modality imaging capability. A tunable OPO laser system (700-900nm), pumped
by an Nd:YAG solid state laser, is used as an illumination source with 5ns long pulses. An FPGA-based electronic
board synchronizes the iU22 start of frame with the laser firing, currently permitting photoacoustic imaging at a rate of
10 Hz (laser repetition rate limit). At that imaging frame rate the photoacoustic system, consisting of a PC modified
with 32 Gbytes of acquisition memory and an FPGA array, is able to store several minutes of continuously captured
data, enabling monitoring and off-line analysis of dynamic photoacoustic events and/or fast scanning for performing
pseudo-3D imaging. The system can use all of the standard iU22 array transducers both for photoacoustic imaging, and
in all of the ultrasound imaging modes.