Photoacoustic imaging of muscle oxygenation during exercise

Monitoring muscle hemodynamics and oxygenation is important for studying muscle function and fatigue. Current state-of-the-art for noninvasive oximetry is near-infrared spectroscopy (NIRS), which provides relative oxygen saturation (SO2) with good sensitivity but has poor spatial resolution and sensing depth, and does not provide anatomical context. In this study, we demonstrated the utility of a dual-modality imaging system that could generate co-registered ultrasound (US) and photoacoustic (PA) images for real-time, functional imaging of human muscle. The system consisted of a wavelength-tunable pulsed laser (Opotek) integrated with a research ultrasound system (Verasonics). US images provided anatomical context and the PA images acquired at 690 nm and 830 nm were processed to estimate SO2 during a sustained isometric contraction and return to rest using a protocol approved by our Institutional Review Board. PA-based SO2 was compared to measurements acquired from a commercially available NIRS oximeter under the same conditions. Preliminary results showed good qualitative agreement between SO2 dynamics observed via PA-based approach and NIRS, with contraction coinciding with an exponential decay in SO2, and relaxation with a return to original levels. This in vivo study demonstrated the feasibility of PA imaging for measuring temporally and spatially resolved muscle oxygenation during functional tasks.