Ultrasound mediated bioluminescence tomography for high sensitivity, high spatial resolution 3D imaging

Lead Research Organisation: University of Nottingham
Department Name: Faculty of Engineering

Abstract

Optical imaging is a unique and powerful technique for implementation of 'refinement' and 'reduction' within the Principles of Humane Experimental Techniques. Using traditional disease models, infected animals (ranging between 3-10) are sacrificed at defined time points and tissues are excised for determination of pathogen numbers and localization. For example, a six time point experiment would result in the use of 18-60 animals. In contrast, the non-invasive nature of optical imaging allows the course of an infection to be monitored simply by imaging the visible or near infrared signal detected from within the same group of animals, typically six to eight in total. Importantly, multiple imaging of the same animal throughout an experiment allows disease progression to be followed with extreme accuracy and consistency, while allowing each animal to act as its own control.

There is a major drawback in using optical imaging as light is heavily scattered by tissue which results in poor quality images. For example in tracking stem cells in tissue, optical scattering means that we cannot tell where precisely where the cells go in the body, how many are at a particular and what their action is. This severely hampers research into understanding the use of stem cells in aiding the body's immune response.

We propose to develop an imaging system that combines ultrasound and optical techniques to significantly improve the spatial resolution and sensitivity of bioluminescence imaging. Information from the ultrasound will be used in two ways to address this problem. Firstly, as ultrasound makes a small change to the mechanical properties of the tissue, this can be used to modulate the bioluminescent light produced within the tissue. This provides a modulated light 'beacon' which can be used to precisely probe different regions of the tissue, thus overcoming the effects of light scattering and improving spatial resolution. Secondly we will use the ultrasound image to provide 3D maps of tissue structure.

Both modulated light beacons and structural information will be used to inform an image reconstruction algorithm based on our widely used NIRFAST software (www.nirfast.org). The system will be demonstrated in studies of nude mice during the course of the project. Based on our proof of concept data, we anticipate that the spatial resolution will be a maximum of 0.5mm, compared to the current state of the art of 2.5mm. This will contribute to a significant impact on the 3Rs. Replacement: better imaging will inform more accurate computational models; Reduction: imaging enables the same animals to used over time and more accurate quantitative imaging allows fewer animals to be used in a single study; Refinement: through the improved quality of research findings.

Technical Summary

An imaging system that combines ultrasound and optical techniques will be developed with the capability to significantly improve the spatial resolution and quantitative accuracy of bioluminescence imaging (BLI). Enhancement of the information obtainable from current BLI systems will be achieved by first modulating the bioluminescent light emitted in the tissue with a focused ultrasound beam. This produces an ultrasound modulated light 'beacon' within the tissue in the region of the ultrasound focus which can be used to reduce the effects of light scattering and improve the image spatial resolution. To provide an added layer of information conventional ultrasound imaging will be carried out enabling images of tissue structure to be co-registered. Both modulated light beacons and structural information will inform a reconstruction algorithm based on our widely used NIRFAST reconstruction code enabling the two datasets to be merged. The potential of the system to outperform current state of the art BLI will be demonstrated through a number of exemplar pre-clinical 3D imaging studies, including tracking of mesenchymal stem cells in nude mice. Based on our proof of concept data the 3D image spatial resolution will be improved by at least a factor of 5 (500 um compared with 2.5 mm) enabling more accurate and consistent clinical data to be obtained from a smaller set of animals. This will contribute to a significant impact on the 3Rs. Replacement: better imaging will inform more accurate computational models; Reduction: imaging enables longitudinal studies on the same cohort, more accurate quantitative imaging allows fewer animals to be used in a study; Refinement: through the improved quality of research findings.

Publications

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