High-speed photoacoustic microscopy in vivo

The overarching goal of this research is to develop a novel photoacoustic microscopy (PAM) technology capable of high-speed, high-resolution 3D imaging in vivo. PAM combines the advantages of optical absorption contrast and ultrasonic resolution for deep imaging beyond the quasi-ballistic regime. Its high sensitivity to optical absorption enables the imaging of important physiological parameters, such as hemoglobin concentration and oxygen saturation, which closely correlate with angiogenesis and hypermetabolism---two hallmarks of cancer.;To translate PAM to the clinic, both high imaging speed and high spatial resolution are desired. With high spatial resolution, PAM can detect small structural and functional changes early; whereas, high-speed image acquisition helps reduce motion artifacts, patient discomfort, cost, and potentially the risks associated with minimally invasive procedures such as endoscopy and intravascular imaging.;To achieve high imaging speed, we have constructed a PAM system using a linear ultrasound array and a kHz-repetition-rate tunable laser. The system has achieved a 249-Hz B-scan rate and a 0.5-Hz 3D imaging rate (over ∼6 mm x 10 mm x 3 mm), over 200 times faster than existing mechanical scanning PAM using a single ultrasonic transducer. In addition, high-speed optical-resolution photoacoustic microscopy (OR-PAM) technology has been developed, in which the spatial resolution in one or two dimension(s) is defined by the diffraction-limited optical focus. Using section illumination, the elevational resolution of the system has been improved from ∼300 mum to ∼28 mum, resulting in a significant improvement in the 3D image quality. Furthermore, multiple optical foci with a microlens array have been used to provide finer than 10-mum lateral resolution---enabling the system to image capillary-level microvessels in vivo---while offering a speed potentially 20 times faster than previously existing single-focus OR-PAM. Finally, potential biomedical applications of the developed technology have been demonstrated through in vivo imaging of murine sentinel lymph nodes, microcirculation dynamics, and human pulsatile dynamics. In the future, this high-speed PAM technology may be adapted for clinical imaging of diabetes-induced vascular complications or tumor angiogenesis, or miniaturized for gastrointestinal or intravascular applications.