A CMOS-SPAD Array Line Scanning Imaging System for Biophotonic Applications

2D single photon imaging arrays have been used to study time-resolved light passage through scattering media [1], however in many cases the light capture ability of the system is heavily limited by the detector fill-factor and total area. This work describes the use of an alternate method, applying a line array scanned on one axis to form a 2D image. This can offer improved light capture ability and increased spatial resolution for time-correlated single
photon imaging. We implement the RaII sensor [2], a 512x1 pixel CMOS-based single-photon avalanche diode (CMOS-SPAD) line array, combined with a single mirror scanning system to form a camera-like imaging system. The single line detector scans across the desired field of view to create 2D image frames composing of a series of sequential slices. Single photon sensitivity in the near-infrared ”optical window” for biological tissue (around 800 nm) allows exploitation of this system in various biomedical applications.

We demonstrate the capability to image and analyse NIR photon transit through thick biological tissues ( 5cm) across a 30cm field of view. Each pixel exhibits time-resolved single photon capability meaning we can represent light pulse evolution as it passes through the scattering system over nanosecond timescales (with 50ps resolution). Using this information we can not only locate light sources from within biological media [1], but also study and recover optical properties of the biological system. Changes in tissue properties manifest themselves in perturbations of the measured time-resolved traces, meaning that it is possible to differentiate tissue based upon their optical signatures at our detector across the entire field of view. This may provide the framework for a novel diagnostic method to determine variations in tissue health based upon optical property variation.