Soft optically-tuneable fluorescence phantoms based on gel wax and quantum dots: a tissue surrogate for fluorescence imaging validation

Fluorescence-guided brain tumour resection, notably using 5-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) for high-grade gliomas, has been demonstrated to provide better tissue differentiation, thereby improving patient outcomes when compared to white-light guidance. Novel fluorescence imaging devices aiming to increase detection specificity and sensitivity and targeting applications beyond high-grade gliomas are typically assessed by measurements using tissue-mimicking optical phantoms. The field currently lacks adequate phantoms with well-characterised tuneable optical properties. In this study, we developed soft tissue-mimicking fluorescence phantoms (TMFP) highly suitable for this purpose. We investigated: 1) the ability to independently tune optical and fluorescent properties; 2) the stability of the fluorescence signal over time; and 3) the potential of the proposed phantoms for imaging device validation. The TMFP is based on gel-wax which is an optically transparent mineral-oil based soft material. We embedded TiO2 as scattering material, carbon black oil-paint as background absorber, and CdTe Quantum Dots (QDs) as fluorophore because of its similar fluorescence spectrum to PpIX. Scattering and absorption properties were measured by a spectrophotometer, while the fluorescence was assessed by a wide-field fluorescence imaging system (WFFI) and a spectrometer. We demonstrated that: 1) the addition of QDs didn't alter the phantom's scattering which was only defined by the concentration of TiO2, whereas its absorption was defined by both QDs and colour oil paint; 2) the measured fluorescence intensity was linearly-proportional to the concentration of QDs; 3) the fluorescence intensity was stable over time (up to eight months); and 4) the fluorescence signal measured by the WFFI were strongly correlated to spectrometer measurements.