NIR Fluorescence lifetime macroscopic imaging with a time-gated SPAD camera.

Area of Science:

• Photonics and Imaging Technology
• Biomedical Optics
• Fluorescence Lifetime Imaging

Background:

• SwissSPAD2 (SS2) is a large-scale, time-gated CMOS SPAD imager.
• Previous studies focused on visible range and longer dye lifetimes (2.5-4 ns).
• Near-infrared (NIR) imaging is crucial for small animal studies but presents challenges like lower sensitivity and shorter dye lifetimes (≤1 ns).

Purpose of the Study:

• To evaluate the performance of the SwissSPAD2 imager in the NIR regime for small animal imaging.
• To assess its capability in accurately measuring short fluorescence lifetimes (≤1 ns).
• To determine its suitability for preclinical molecular imaging applications.

Main Methods:

• Utilized a macroscopic imaging setup with a compact NIR picosecond pulsed laser and NIR-optimized optics.
• Employed a frequency-division scheme to synchronize the SS2 imager with varying laser repetition rates.
• Acquired data with different time gate widths and analyzed using nonlinear least-square fit (NLSF) and phasor analysis.

Main Results:

• Demonstrated that SS2 can accurately extract fluorescence lifetimes in the 1 ns range, even with gate widths exceeding the laser period.
• Showed that SS2 can effectively distinguish between closely spaced fluorescence lifetimes.
• Confirmed the successful operation of SS2 with different laser repetition rates via frequency division.

Conclusions:

• SwissSPAD2 is a versatile detector for NIR fluorescence lifetime imaging, suitable for small animal applications.
• The imager's large gate widths and fixed synchronization rate do not impede accurate lifetime measurements in the 1 ns range.
• SS2 offers a viable alternative to existing widefield time-resolved detectors for preclinical imaging.