Ultrasound and Photoacoustic Molecular Imaging

• Author / Creator

  Forbrich, Alexander E

• Molecular imaging techniques are important for understanding fundamental biological processes and genetic functions in organisms. Non-invasive, high-resolution, deep imaging, as well as the ability to separate molecular species, are all critical to the success of a molecular imaging technique. Histology combined with optical imaging is often used as the gold standard; however, due to light scattering in tissues these techniques only provide high-resolution images in thin, transparent samples or superficially in tissues. Photoacoustic imaging combines the absorption-based contrast afforded by optical imaging techniques and the acoustic resolution afforded by the ultrasound detector. Since acoustic waves are scattered three orders of magnitude less than light in tissues, photoacoustic imaging allows high-resolution visualization in deep tissues. As complimentary imaging modalities, ultrasound and photoacoustic imaging can be combined to provide molecular information, as well as anatomical and structural information. Although ultrasound and photoacoustic imaging have great potential for non-invasively profiling the molecular makeup of tissues in vivo, their utility for these applications is still in its infancy and requires refinement, as well as further investigation. The work presented in this dissertation approaches molecular imaging using ultrasound and photoacoustic imaging from several different directions to answer the question: “How much new molecular information can be extracted using combined ultrasound and photoacoustic imaging systems?”. This dissertation begins with an investigation of how ultrasound alone can help profile the molecular nature of tissues. The final chapters investigate how photoacoustic imaging, combined with ultrasound, can be used to non-invasively detect molecular species. The concept of ultrasound inducing sonoporation and liberating biomarkers into the blood stream and surrounding media is explored. Using this technique, molecular profiles could be built up from blood samples in organisms reducing the requirement of highly-invasive biopsies. We demonstrated, for the first time, that nucleic acid biomarkers could be released using this technique and that phase-changing ultrasound contrast agents, such as microbubbles, could be used to substantially increase the release of biomarkers into the surrounding media. To enable visualization of these phase-change events, novel image sequences were developed for high-speed differential ultrasound imaging. These techniques isolated the signals from microbubble destruction events and nanodroplet vaporization events from background signals. We believe that this will help assess the efficacy of biomarker liberation surrounding tissues of interest. A novel photoacoustic screening technique is developed to evolve genetically-encoded reporter molecules optimized specifically for photoacoustic imaging. These new reporters were used in a number of protein constructs to increase the sensitivity of photoacoustic imaging. Additionally, proteins were developed to enable nonlinear photoacoustic imaging using the concept of Förster resonance energy transfer (FRET). With this screening technique, we were able to enhance the photoacoustic signal four-fold compared to the original gene and we were able to visualize, for the first time, the FRET effect using genetically-encodable proteins. Lifetime-weighted photoacoustic imaging, a novel nonlinear imaging technique, was investigated as another contrast mechanism for photoacoustic imaging. Using this technique, short-lifetime molecules, such as background signals from blood, could be nullified while the long-lifetime molecules could be highlighted. Finally, part of this work was conducted while developing ultrasound and photoacoustic imaging systems at FUJIFILM VisualSonics Inc. This work included implementing and advancing the molecular imaging capabilities of the commercial photoacoustic imaging system Vevo LAZR. Spectral demixing techniques, processing algorithms, and software developments were all done to enable new applications for photoacoustic molecular imaging. Imaging of lymphatic pumping is presented as an example of an application that became possible with these enhancements. With photoacoustic imaging, we demonstrated multiple new contrast mechanisms and techniques to enable molecular photoacoustic imaging in new applications. The work presented in this dissertation contributes to the field of molecular imaging using ultrasound and photoacoustic imaging and will help further the understanding of fundamental biological processes and genetic pathways.

• Subjects / Keywords

  • Directed Evolution
  • Photoacoustic
  • Ultrasound
  • Biomarkers
  • Molecular Imaging
  • Nanodroplets

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R32F7K51C

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.