Abstract
Purpose :
Photoacoustic microscopy (PAM) is an emerging hybrid imaging technology which can non-invasively evaluate optical absorption with high spatial resolution and deep penetration. Laser safety remains a major concern for clinical translation of PAM. To improve the safety of PAM, we develop, validate, and demonstrate the safety of an ultra-low energy PAM retinal imaging system in rabbit eyes down to 1% of the ANSI safety limit.
Methods :
A high resolution multimodality PAM and OCT imaging system is modified to allow for ultra-low energy PAM using a tunable nanosecond pulsed OPO laser (405 – 2600 nm, pulse repetition rate 1 kHz, pulse duration 3 – 5 ns, NT-242, Ekspla) integrated into a Ganymede-II-HR Thorlabs OCT system with a custom-built needle ultrasound transducer with a central frequency of 25 MHz (Optosonic) . Following two-stage amplification and filtering, the signal was sent to 3 different channels of a multi-channel DAQ system (PX1500–4, Signatec) fully utilizing the dynamic range of the DAQ system. The 3 signals were averaged and normalized to the recorded laser energy to eliminate pulse energy fluctuation. To further enhance the SNR, a 3 by 3 median filter in the spatial domain was applied. The performance and safety of this system was evaluated on 5 Dutch-belted rabbits with ERG, OCT, photography, FAF, FA, ICGA, histology, and IHC for up to 1 month.
Results :
The performance of the ultra-low energy PAM system was tested by imaging the retinal vasculature. Pulse energy levels of 1.6 nJ, 3.2 nJ, and 4.8 nJ, which are at 1%, 2%, and 3% of the ANSI safety limit, respectively, were used in imaging. Ultra-low energy PAM can image the retinal vasculature with sufficient contrast-to-noise ratio at 2% of the ANSI safety limit whereas conventional PAM at this energy had no visible vasculature (Fig 1). Fundus photography, OCT, FAF, ERG, histology, and IHC demonstrate no retinal damage at 2% of the ANSI safety limit.
Conclusions :
By optimizing the imaging and data acquisition, the signal-to-noise ratio can be improved to allow for ultra-low energy PAM in rabbit eyes using 1% of the ANSI safety limit. A safety evaluation including functional and anatomic testing demonstrated no retinal damage after PAM imaging.
This is a 2020 Imaging in the Eye Conference abstract.