Near-infrared (NIR) fluorescence has emerged as a safe tool in various stages of research and clinical translation for applications such as tumor imaging. To suppress unwanted light or to image multiple dyes simultaneously, conventional spectral systems rely on optical filters and multiple light sources and cameras. Fluorescence, however, also exhibits a specific time-domain behavior which can be used as an additional filter. We present a new time-domain fluorescence camera system which improves background suppression and can distinguish multiple fluorescent dyes in the same NIR channel.
The system is built around a novel 64x64-pixel VUB CAPS fast-gated CMOS image sensor. This sensor can image the time-behavior of fluorescence by synchronizing to a pulsed excitation source and capturing light in nanosecond gate-windows that can be controlled with picosecond precision. The system is optimized for the 800 nm NIR fluorescence channel employing a 775 nm picosecond LASER by NKT Photonics and 70 nm optical bandpass filter around 835 nm. The capabilities of the system are demonstrated using custom-made phantoms with clinically relevant organic NIR dyes such as ICG and IRDye800CW and a phantom developed by Helmholtz Zentrum for fluorescence-guidance system evaluation which uses stable Quantum Dots (QD) emitting in the 800 nm channel.
Imaging both phantoms we demonstrate the capabilities of the time-domain fluorescence camera. Using the Helmholtz phantom, we show how scattered or reflected excitation light can be suppressed by time-gating fluorescence emission and how fluorescence-lifetime (FLT) can offer an absolute measurable quantity in cases of varying fluorescence intensities. The QDs, however, exhibit an FLT which is much longer than those seen for the organic NIR dyes. Imaging the FLT of our custom phantom, we demonstrate that the system is also capable of distinguishing multiple clinically relevant organic NIR dyes with very short FLTs near 1 ns. Phantom images are compared with a conventional fluorescence imaging system (Fluobeam800) and accuracy of the FLT imaging is demonstrated by comparing the measurements with a reference time-correlated single-photon counting fluorescence spectrometer (Edinburgh instruments Mini-tau). In-vivo experiments are currently in preparation.
The new VUB time-domain NIR fluorescence imaging system is capable of producing accurate FLT images for a wide range of FLTs and offers promising advantages over conventional systems. These advantages have been demonstrated using phantoms but are expected to improve in-vivo imaging of NIR fluorescent contrast agents.