Combined Photoacoustic and High-Frequency Power Doppler Ultrasound Imaging

Photoacoustic imaging has emerged as a promising technique for visualizing optically absorbing structures with ultrasonic spatial resolution. Since it relies on optical absorption of tissues, photoacoustic imaging is particularly sensitive to vascular structures even at the micro-scale. Power Doppler ultrasound can be used to detect moving blood irrespective of Doppler angles. However, the sensitivity may be inadequate to detect very small vessels with slow flow velocities. In this work, we merge these two synergistic modalities and compare power Doppler ultrasound images with high-contrast photoacoustic images. We would like to understand the advantages and disadvantages of each technique for assessing microvascular density, an important indicator of disease status. A combined photoacoustic and high-frequency ultrasound system has been developed. The system uses a swept-scan 25 MHz ultrasound transducer with confocal dark-field laser illumination optics. A pulse-sequencer enables ultrasonic and laser pulses to be interlaced so that photoacoustic and Doppler ultrasound images are co-registered. Experiments have been performed on flow phantoms to test the capability of our system and signal processing methods. Work in progress includes in vivo color flow mapping. This combined system will be used to perform blood oxygen saturation and flow estimations, which will provide us with the parameters to estimate the local rate of metabolic oxygen consumption, an important indicator for many diseases.