In vivo photoacoustic and pulse echo imaging of a pancreatic tumor using a hand held device

Ultrasonography and computed tomography are often used to diagnose pancreatic cancer. Using similar equipment as ultrasonography, photoacoustic (PA) imaging can provide vascular information over the same region of interest. Information about the vascularity in and around a lesion can be used to aid in the characterization and diagnosis of various cancers. Current PA imaging setups are restricted to bulky bench-top setups, limiting its practicality for clinical research. An ideal imaging platform would be non-invasive, real-time, portable, and inexpensive. We designed and fabricated an attachment to a clinical ultrasound probe which houses an optically transparent acoustic reflector in water. The design enables laser illumination in-line with the acoustic propagation path. The detector array simultaneously captures unbeamformed data on 64 elements at frequencies up to 10 MHz. We used the device to image in vivo a subcutaneous tumor in a SCID mouse implanted with Capan-2 pancreatic cancer cells and also a mouse pancreas embedded in a gel. Laser pulses (5 ns, 13 mJ/cm²) were transmitted through an optical window in the device, providing line illumination below the skin surface. Real time 2D PA images between 700 nm and 960 nm were captured along with conventional pulse echo (PE) ultrasound to examine different sources of contrast in the tumor. The PE image identifies the acoustic window, skin surface, and variation in acoustic impedance within the tumor. PA images visualized areas of near infrared light absorption 4 mm deep within the tumor. Co-registered PE images provided an anatomical reference. This in vivo study demonstrates how a simple adapter to a clinical ultrasound array can be used for real-time simultaneous PE and PA imaging of cancer. With this efficient and practical design, cancer research can capitalize on noninvasive optical contrast below the tissue surface. With an optimized design, clinicians can incorporate PA imaging with routine ultras- - ound exams.