Hydrogel based depth standards and phantoms for optical imaging applications

Abstract

Medical imaging technology is advancing rapidly making disease diagnostics faster, efficient, and detailed. Currently much effort is focused on designing and fabricating new NIR laser sources and detectors, devising novel label free imaging methodologies, which along with applications of machine learning are helping to optimise imaging at ever greater depths. To aid the development of deeper tissue optical imaging methodologies and to allow system validation and robust comparison of imaging methods and techniques, there is a need for the generation of readily available, robust, and reliable standards and phantoms. Human tissue, although a natural and most realistic model for this purpose, possesses heterogeneity among samples, lacks long term stability and is unsuitable for system calibration or comparison. Alternatively, synthetic phantoms constructed using materials ranging from solids, semi-solids and liquids, incorporating molecules that respond to different imaging modalities, can overcome the limitations human tissues. Here, a new material/construct that can be used in the fabrication of tissue phantoms is introduced. The material was comprised of a double network hydrogel matrix made using two interpenetrating polymers: agarose and polyacrylamide. The double network hydrogel was robust and was used for the fabrication of stable multi-layered depth phantoms incorporating specific imaging modality markers between the layers. The generated phantoms ranged from single layers to more complex constructs consisting of up to seven layers, each layer being variable in depth and tuneable in terms of scattering and absorbance properties. Once fabricated, the phantoms were found to be stable for several months. These phantoms allowed a comparison of imaging depths with different imaging modalities including conventional one photon fluorescence, two photon fluorescence, second harmonic generation and coherent anti-Stokes Raman scattering allowing imaging at depths of 1550 µm, 1550 µm, 1240 µm, and 1240 µm, respectively. These standards/phantoms also proved useful to understand the light interaction with biologically relevant material, resulting in the determination of an axial scaling factor for light microscopy. The ability to image at depth, the phantom’s robustness and their flexible layered structure and the ready incorporation of “optical markers” make these ideal depth standards for the validation of a variety of novel imaging modalities.