Fifty-seventh annual meeting of the American association of physicists in medicine
TH-AB-204-02: Preliminary Study of Up-Converting Nanocrystal (UCNC) Imaging in Optical-ECT
Optical-emission computed tomography (optical- ECT) enables high-resolution 3D imaging of emission in optically cleared tissues on the order of 1 cm3. This work demonstrates capabilities of optical-ECT to quantify the red fluorescent protein (RFP) tdTomato in rat lung, and presents preliminary data on the feasibility of imaging near-IR absorptive up-converting nanocrystals (UCNCs) with optical-ECT.
The nanocrystal NaYF4:Yb/Er studied in this work up-converts 980nm light to visible light peaking sharply at ∼530nm. Collimation of light using telecentric lenses coupled with a CCD array allows for inverse radon 3D reconstruction of fluorescent molecules in optically cleared tissue or whole-organ samples in emission mode. As demonstration of the typical capability of the system, tdTomato RFP emission in the lung of a Cre mouse model is reconstructed in 3D. Compatibility of UCNC with optical clearing is assessed with agarose phantom, to which 1mg/mL of 100-nm diameter NaYF4:Yb/Er coated with polyethyleneimine was injected at two different locations before the agarose was fixed with ethanol and cleared with methyl salicylate.
Emission mode imaging of the tdTomato-expressing mouse lung reveals in high-resolution (25.8µm3/voxel) information unseen in transmission imaging. UCNCs are shown to emit in ethanol and methyl salicylate, detected by fluorimetry. Projection images of injected UCNC crystals in agarose confirm that the crystal is unaffected by optical clearing. One-second exposure images of the excited nanocrystals yield maximum CCD counts of ∼3800 with a 530nm filter, a significant increase compared to three-second exposure image of tdTomato yielding ∼1200 counts.
Optical-ECT imaging enables 3D emission mapping of un-sectioned tissue or whole organ on the order of 1cm3. UCNCs are not only confirmed to be unaffected by optical clearing but also yield high intensity fluorescence. Since UCNC is biocompatible, 3D imaging of the behavior of cancer cells in small animal models with UCNCs is currently under exploration.