Clinically translatable ultrasound/photoacoustic imaging for real-time needle biopsy guidance

Ultrasound (US) needle tracking with a real-time, portable imaging system is common for biopsies and therapeutic injections. However, proper US probe-needle orientation is required or tracking precision is lost because of poor US contrast and additional artifacts. In contrast, the strong photoacoustic (PA) signal from a needle is relatively independent of light orientation. Unfortunately, a bulky, slow (10s of Hz) laser is usually required for sufficient pulse energy at centimeters depth inside the body, hindering PA integration into a real-time US scanner. We employ a compact, low-cost, low pulse energy (1 mJ/pulse) laser that can operate at 10s of kHz to construct an integrated US/PA system with frame rates higher than 30 Hz. A scanning approach with a rotating galvo mirror was used by combining multiple laser shots covering a large scan region to form an integrated image. Multiple pulse-echo US focused beams were formed between laser firings. Insertion of an 18-gauge needle in a piece of chicken breast tissue, and subsequent injection of an absorptive agent through the needle, was imaged. The PA image displays the needle at higher contrast (> 30 dB) than the US image, even at a depth of 12 mm where light energy has largely decayed, demonstrating the potential of the current scanning system for real-time needle guidance. An improved frame rate is envisioned with an optimized scan scheme.