Abstract
.Significance: Deep-tissue penetration by x-rays to induce optical responses of specific molecular reporters is a new way to sense and image features of tissue function in vivo. Advances in this field are emerging, as biocompatible probes are invented along with innovations in how to optimally utilize x-ray sources.Aim: A comprehensive review is provided of the many tools and techniques developed for x-ray-induced optical molecular sensing, covering topics ranging from foundations of x-ray fluorescence imaging and x-ray tomography to the adaptation of these methods for sensing and imaging in vivo.Approach: The ways in which x-rays can interact with molecules and lead to their optical luminescence are reviewed, including temporal methods based on gated acquisition and multipoint scanning for improved lateral or axial resolution.Results: While some known probes can generate light upon x-ray scintillation, there has been an emergent recognition that excitation of molecular probes by x-ray-induced Cherenkov light is also possible. Emission of Cherenkov radiation requires a threshold energy of x-rays in the high kV or MV range, but has the advantage of being able to excite a broad range of optical molecular probes. In comparison, most scintillating agents are more readily activated by lower keV x-ray energies but are composed of crystalline inorganic constituents, although some organic biocompatible agents have been designed as well. Methods to create high-resolution structured x-ray-optical images are now available, based upon unique scanning approaches and/or a priori knowledge of the scanned x-ray beam geometry. Further improvements in spatial resolution can be achieved by careful system design and algorithm optimization. Current applications of these hybrid x-ray-optical approaches include imaging of tissue oxygenation and pH as well as of certain fluorescent proteins.Conclusions: Discovery of x-ray-excited reporters combined with optimized x-ray scan sequences can improve imaging resolution and sensitivity.
Highlights
Optical molecular imaging and sensing from x-ray excitation utilizes a fundamentally different type of interaction and sensing approach to excite optical reporters in biological tissues and detect and localize the emission
The ability to sense through tissue using traditional x-ray sources, while still using optical molecular contrast, presents potential advantages in depth penetrance and spatial resolution
Optical molecular sensing from x-ray excitation describes a range of technologies and research studies where an incident x-ray beam is used for deep tissue sensing
Summary
Optical molecular imaging and sensing from x-ray excitation utilizes a fundamentally different type of interaction and sensing approach to excite optical reporters in biological tissues and detect and localize the emission. When compared to other methods of molecular sensing in tissue, the strengths of x-ray-based molecular sensing may be less clear, because this methodology only began emerging during the last decade.[1,2,3] the ability to sense through tissue using traditional x-ray sources, while still using optical molecular contrast, presents potential advantages in depth penetrance and spatial resolution. Optical sensing provides superior molecular sensitivity to x-raybased contrast methods, because x-ray contrast is generally based upon the photoelectric effect, with peak attenuation in the keV energy range,[4] and x-ray contrast agents usually need to be present in high levels, near millimolar quantities in tissue making them only useful for imaging blood volume and leakage. The field of x-ray-based molecular sensing benefits from an extraordinarily large range of detectors and sensors for optical emission that have sensitivity to the single photon level, making the detection side of the sampling potentially very efficient
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