Research progress of X-ray luminescence optical tomography

Abstract

Recently, X-ray luminescence optical tomography (XLOT) has been proposed as a promising optical molecular imaging modality. XLOT, or equivalently, X-ray luminescence computed tomography (XLCT), is a tomography imaging technique based on the X-ray excited nanophosphors (NPs). Briefly, when NPs in imaged object are irradiated with X-ray, NPs will emit visible or near infrared (NIR) luminescence. By mathematically modeling the light transportation and solving an inverse problem, the three dimensional (3D) distribution of NPs in imaged object can be recovered. Compared with the conventional optical molecular imaging modalities, XLOT has ability to eliminate the autofluorescence and provide the increased penetration depth in tissues. As a result, the imaging technique is well suited for biomedical researches. Especially, XLOT provides the potential for clinical applications due to the increased penetration depth. Based on the excitation patterns of X-ray beams, the current XLOT imaging systems can be classed into three types, i.e., the pencil-beam system, the fan-beam system, and the cone-beam system. Among these systems, the pencil-beam XLOT has the highest spatial resolution and the lowest time resolution. In contrast, the cone-beam XLOT system, especially the single-view cone-beam XLOT system, has the shortest data acquisition time. But, the imaging quality is relatively lower compared to the pencil-beam system. Hence, there is a tradeoff between the spatial resolution and time resolution for XLOT imaging technique. After acquring the measurement data by the above XLOT system, the 3D distribution of NPs in imaged object can be recovered by the reconstruction methods. For XLOT reconstruction, there are two types of methods, i.e., the filtered back projection (FBP) method and the reconstruction method based on photon migration model. For the pencil-beam XLOT system, the reconstruction is generally performed by FBP method, which is similar to XCT reconstruction method. For the cone-beam XLOT system, the NPs in imaged object are generally resolved by using the method based on photon migration model. It is worth noting that the reconstruction based on migration mode is a high ill-posed problem due to the high scattering of light in biological tissues. To alleviate the ill-posedness, the compressive sensing technique and a priori information method (e.g., excitation a priori information) can be used. In addition, the imaging probes play an important role in XLOT imaging. Currently, the rare-earth nanophosphors have be widely used in XLOT due to their optical properties. Of course, the rare-earth nanophosphors are not sole NPs for XLOT imaging. Other NPs, e.g., quantum dots and gold nanoclusters, have also been used as the imaging probes. Further, with the advances in NPs, more applications would be expected in bio-medical researches. To sum up, for XLOT excellent performances, during the last few years, the continuous research efforts have been made to develop new imaging systems, build robust reconstruction methods, design efficient NPs, and expand the applications of XLOT. This paper reviews the developments of XLOT, including the basic imaging principles, system compositions, and advantages and disadvantages of different XLOT imaging systems. Subsequently, the corresponding reconstruction methods and the imaging probes are classified and summarized. Finally, we discuss the applications of XLOT in the preclinical diagnosis, and the future research direction of XLOT.