Modeling and evaluating selective-plane X-ray induced luminescence imaging

Objectives The goal of this study is to develop an imaging model for selective-plane x-ray induced luminescence (SPXIL) of lanthanide-based nanoparticles (NPs), which emit near-infrared light in response to stimulation with x-rays. Our current research seeks to model the physics of the x-ray luminescence imaging for simulations that evaluate potential applications in monitoring response to radiation therapy or NP-enhanced radiation therapy.

Methods We synthesized europium-doped yttrium oxide (Y2O3:Eu3+) NPs and dissolved them in an aqueous solution. We used a high resolution, cooled CCD camera to image the x-ray stimulated luminescence. We then modeled the physics of the experimental setup to determine the light yield of the NPs from the measurements. The light yield was then used in simulations to determine the minimum dose required for x-ray luminescence detection under a variety of conditions.

Results Our analysis determined the light yield of the NP to be 1.81×10^-5 photons per eV deposited in a single voxel for a NP concentration of 5 mg per mL. The simulations found that the minimum dose to achieve a detection SNR of 5 was 4.5 cGy for the experimental imaging voxel size of 20 µm x 20 µm X 2mm. The minimum dose decreased rapidly for simulations with larger imaging pixels, reaching 4 microGray for isotropic 2 mm voxels. .While these high concentrations of the NPs might be used for radiosensitization, even trace quantities could be imaged at relatively modest doses. The model indicates that 4 mGy would be required to image 5 micrograms/ml of NPs in 2 mm voxels.

Conclusions Minimum dose excitations on the order of mGy down to microGys for trace and therapeutic concentrations of NPs, respectively, imply that the technique has potential for use in planning and monitoring NP-enhanced radiation therapy. While self-absorption of emitted light has not been explicitly modeled, typical absorption coefficients are 0.15/cm at 610 nm, which implies the effect will be minimal for tumors at depths of 1-2 cm.