Optimizing acoustic detection for deep-tissue LED-based photoacoustic imaging

Photoacoustic imaging is a hybrid technique which offers high contrast and specificity of optical imaging techniques and excellent spatial resolution of ultrasound imaging. In recent years, there has been tremendous developments in this field and photoacoustic imaging is now facing an exciting challenge of clinical translation. We developed a multispectral LED-based photoacoustic and ultrasound imaging system (AcousticX), which has been already proven to be useful in multiple preclinical and clinical superficial imaging applications. In the first version of AcousticX, acoustic detection was performed using linear array ultrasound probes with center frequencies of 5, 7 and 10 MHz. With these probes, we were able to achieve an imaging depth of 1 cm in tissue in vivo with spatial resolution of around 150-250 μm. However, it is important to improve the imaging depth for exploring wide range of deep-tissue applications. In this work, we developed a new broadband 2.5 MHz ultrasound probe to improve the imaging depth. Using this ultrasound probe and 850 nm LED arrays, we performed phantom, ex vivo and in vivo human volunteer studies and achieved an imaging depth of 2.5^-3 cm. We compared the imaging performance of the newly developed probe with the already available US probes and conformed that the 2.5 MHz probe is 3-4 times more sensitive with spatial resolution suitable for deep-tissue applications. When imaging the brachial artery of a human volunteer, we achieved an imaging depth of 2.6 cm at a frame rate of 6 Hz, which is the highest reported in vivo imaging depth in LED-based photoacoustic imaging. We believe that this is the first step towards real-time deep tissue LED-based photoacoustic imaging with potential in multiple clinical applications.