Super-resolution tomographic imaging in centimeter-deep tissue - Light and sound waves have been widely used in biomedical imaging. An intrinsic physical property, diffraction, is the fundamental factor that eventually limits the size of the point-spread function (PSF) of the system and therefore the image’s spatial resolution. It is commonly called the diffraction limit. To break this limit, super- resolution (SR) imaging technologies have been intensively developed during the past decades, such as SR fluorescence microscopy, SR ultrasound, and SR photoacoustic imaging. While these technologies have significantly improved spatial resolution, they are also limited by their fundamentals. For example, SR- fluorescence microscopy is limited to super-thin samples (tens or hundreds of microns). Microbubble-based SR ultrasound is limited to only image vascular space because microbubbles are too big to enter extra-vascular spaces. Similarly, the major SR photoacoustic methods are limited to vascular imaging because they need to repeatedly record data sets for statistical analyses of the individual signal emitters. Other SR photoacoustic methods require coherent light for generating speckles or shaping wavefront, which is challenging in deep, dynamic, and live tissues because coherence can quickly and significantly degrade. In this proposal, we plan to develop a fundamentally different SR imaging technology, which is a general and cost-effective methodology for super-resolution imaging in centimeters-deep tissues and can be used for structural, functional, and molecular imaging. Its success will push human capabilities beyond the sub-millimeter imaging depth of SR fluorescence microscopy, beyond the vascular imaging limit of SR ultrasound and photoacoustic imaging (without requiring coherence light), beyond the limits of the high cost and system complexity of all current SR technologies including micro-CT and micro-MRI, and beyond the limit of using ionizing radiations in micro-CT. This technology is based on our previous ultrasound-switchable fluorescence (USF) imaging method and is denoted as SR-USF. It can resolve fine structures within a diffraction-limited ultrasound focus in centimeters-deep tissue by converting spatially indistinguishable structures into temporally differentiable and spatially weighted signals via a much simpler tomographic method than existing ones. The capability to break the diffraction limit is achieved by only requiring two items: (1) a signal emitter with a step-like-function between its input (controlling) energy and output (emitting) signal strength, and (2) a focused input (controlling) energy, which has a gradient of the energy distribution at the focus. The principle is not only simple but also can be implemented via different energy formats, such as acoustically, thermally, optically, or mechanically triggered, depending on the signal emitters. The specific aims include developing a SR-USF tomographic system and an image reconstruction method (Aim 1), testing and characterizing the method in tissue phantoms and ex vivo tissues (Aim 2), and testing and validating them in live animal tumors (Aim 3). The success of SR-USF will be a powerful tool for many fields, such as oncology, biology, physiology, biomaterials, tissue engineering, and others.
