X-ray luminescence computed tomography (XLCT) is proposed as a new molecular imaging modality for imaging X-ray-excitable phosphorescent nanoparticles three-dimensionally, in small animals. Some of these nano-sized particles can emit near-infrared (NIR) light when excited with X-rays and be functionalized to target specific biological processes in vivo. XLCT enables anatomical images to be acquired simultaneously with molecular images via standard X-ray computed tomography (CT). The imaging mechanism used in XLCT consists in irradiating the subject using a sequence of X-ray beams while sensitive photo-detectors measure the light diffusing out of the subject. For each beam position, the production of light is constrained to the narrow volume of the beam, hence, the collection of optical measurements forms parallel-beam projections. An XLCT system was simulated using Monte-Carlo. Preliminary experiments were also conducted in phantoms using a 50 kvP treatment X-ray generator and an EM-CCD camera. Images were reconstructed using a maximum-likelihood iterative algorithm. From simulations, tracer uptake in 2 mm-diameter targets can be detected and quantified with sub-picomolar sensitivity with less than 1 cGy of average radiation dose. Provided sufficient signal-to-noise ratio, the spatial resolution of the system can be made arbitrarily small by narrowing the beam aperture. In particular, 1 mm uniform spatial resolution was achieved for a 1 mm-wide X-ray beam. Images reconstructed from experimental XLCT measurements showed good agreement with the simulation model. In particular, the reconstructed signal was linear with phosphor concentration. Preliminary simulations and experiments show that XLCT is a feasible approach for imaging small animals or dedicated organs. With the next version of our experimental set-up, we expect improved spatial resolution and molecular sensitivity.

A: Proposed design for an XLCT system. B: Gradient phantom. C: Nanophosphor X-ray-stimulated emission spectrum. D: Optically diffusive phantom.