Photoacoustic Microscopy (PAM)
Photoacoustic microscopy (PAM)
In photoacoustic microscopy (PAM), depending on the type of light and ultrasound focusing one can achieve superior spatial resolution (lateral resolution). The axial resolution is dependent of the bandwidth of the ultrasound detection system. However, the lateral resolution is determined by either the ultrasound focusing (in case of acoustic resolution photoacoustic microscopy - ARPAM) or optical focusing (in case of optical resolution photoacoustic microscopy - ORPAM). Typically the optical and ultrasound are confocal, making the SNR high. Typical spatial resolution for PAM system varies from 0.2-2 micron depending on the system configuration. The imaging depth can go up to several mm. There are several mode of imaging either in the transmission or in the reflection mode. Most in vivo imaging application requires reflection mode operation. In our lab we are working on building various PAM imaging system and their application in the clinical and pre-clinical areas.
AR-PAM in the second near-infrared window (NIR-II):
Photoacoustic imaging is a non-invasive imaging technique having the advantages of high optical contrast and good acoustic resolution at improved imaging depths. Light transport in biological tissues is mainly characterized by strong optical scattering and absorption. Photoacoustic microscopy is capable of achieving high resolution images at greater depth compared to conventional optical microscopy methods. We have developed a high-resolution, acoustic resolution photoacoustic microscopy (AR-PAM) system in the near infra-red (NIR) window II (NIR-II, e.g., 1064 nm) for deep tissue imaging. 11 mm imaging depth is achieved as the tissue scattering at 1064 nm is lesser compared to visible or near infrared window-I (NIR-I). The system has lateral resolution of 130 µm, axial resolution of 57 µm. Sentinel lymph node and the lymph vessel, urinary bladder imaging was performed. More details can be found in Periyasamy et al., Journal of Biophotonics 12 (2019).
PAM is a scalable bioimaging modality; one can choose low acoustic resolution with deep penetration depth or high optical resolution with shallow imaging depth. High spatial resolution and deep penetration depth is rather difficult to achieve using a single system. We first built a switchable acoustic resolution and optical resolution photoacoustic microscopy (AR-OR-PAM) in a single imaging system capable of both high resolution and low resolution on the same sample. Lateral resolution of 4.2 µm (with ~1.4 mm imaging depth) and lateral resolution of 45 µm (with ~7.6 mm imaging depth) was successfully demonstrated using a switchable system. In vivo blood vasculature imaging was also performed for its biological application. More details can be found in Moothanchery et al., Sensors 17 (2017), and Moothanchery et al., Journal of Visualized Experiments 124, (2017).
Photoacoustic microscopy for microneedle drug delivery:
Microneedle technology allows micron-sized conduits to be formed within the outermost skin layers for both localized and systemic delivery of therapeutics including nanoparticles. Histological methods are often employed for characterization, and unfortunately do not allow for the in vivo visualization of the delivery process. We used optical resolution-photoacoustic microscopy (ORPAM) to characterize the transdermal delivery of nanoparticles using microneedles. Specifically, we observed the in vivo transdermal delivery of gold nanoparticles using microneedles in mice ear and study the penetration, diffusion, and spatial distribution of the nanoparticles in the tissue. The promising results reveal that photoacoustic microscopy can be used as a potential imaging modality for the in vivo characterization of microneedles based drug delivery. More details can be found in Moothanchery et al., Biomedical Optics Express 8 (2017).
Photoacoustic imaging of Lamina Cribrosa (LC):
Due to the embedded nature of the lamina cribrosa (LC) microcapillary network, conventional imaging techniques have failed to obtain high resolution images needed to assess the perfusion state of the LC. Both OR- and AR- PAM can be used to obtain static and dynamic information about LC perfusion in ex-vivo porcine eyes. The OR-PAM system could resolve a perfused LC microcapillary network and also provided good depth information to visualize through-thickness vascular variations. The AR-PAM system is capable of detecting time-dependent perfusion variations. This study represents the first step towards using PAM to study the LC’s perfusion, which could be a basis for further investigation of the hemodynamic aspects of glaucomatous optic neuropathy. More details can be found in Chuangsuwanich et al., Applied Optics 57 (2018).