Photoacoustic Imaging in Optically Diffusive Media Using Structured Illumination

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

Part 1: Optical imaging of objects inside or behind turbid media is exceptionally challenging. Multiple scattering scrambles the object information and hides features from view. This project aims to develop and characterize a technique for imaging deep inside of optically scattering media with unprecedented spatial resolution. The approach combines photoacoustic imaging, an emerging modality that uses light and ultrasound to image absorbing structures within tissue, with structured illumination microscopy, an established means of improving imaging resolution by spatially patterning the illumination. Mathematical and experimental techniques are proposed that exploit the optical speckle field that naturally emerges as light propagates through scattering media to enhance the resolution of photoacoustic imaging such that it approaches optical resolution, even deep within scattering media. The proposed research aims to transform biomedical imaging, allowing physicians to see through biological tissue to detect and diagnose disease states or conditions with exceptional resolution. It also enables nondestructive testing applications such as detecting structural defects in highly scattering polymer matrix composites and ceramic samples. The interdisciplinary research effort will support graduate students in applied mathematics and mechanical engineering and provide training in engineering, applied mathematics, and biology. Undergraduate researchers will also be supported for the duration of the project. An outreach program to a local high school is proposed and the research results will be incorporated into new and existing undergraduate classes. Part 2: The objective of this proposal is to develop an experimental technique and associated mathematical framework to image inside of and through multiply scattering media using random speckle illumination and photoacoustic detection. The goal is to achieve optical resolution imaging in the multiple scattering regime. The speckle patterns that naturally emerge as light propagates through diffuse media provide structured illumination to an object placed behind a scattering wall. The photoacoustic signal produced by such illumination is detected using a focused ultrasound transducer. Blind structured illumination photoacoustic microscopy (BSIPAM) offers a fundamentally new approach to imaging in turbid media, where image resolution is enhanced by making use of speckle patterns that are a result of the scattering process itself. BSIPAM exploits the fact that the photoacoustic signal generated by an absorbing object subjected to pulsed laser illumination is proportional to the product of the object optical absorption distribution and the illumination pattern. In the case of non-uniform illumination by a random speckle pattern, frequency mixing between the illumination and object encodes the high spatial frequencies of the object in a low spatial frequency photoacoustic response that can be detected with an ultrasound transducer. Furthermore, if the photoacoustic response generated by multiple random speckle patterns is recorded then there is sufficient information to reconstruct an image of the object with a spatial resolution approaching the speckle size. This project develops experimental techniques and reconstruction algorithms that allow for optical resolution imaging at unprecedented depths within static and dynamic diffusive media using the BSIPAM concept. The fundamental resolution enhancement that can be achieved using BSIPAM is explored and the performance of image reconstruction algorithms as a function of speckle size, imaging geometry, number of speckle patterns, imaging depth, and signal to noise ratio are determined. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.