High-resolution ultrasound imaging of micro-bubble cavitation using separate emission / reception transducers

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The use of microbubbles or cavitation agents has enabled a wide and rapidly expanding field of research, with their benefits being repeatedly demonstrated, both in localized drug delivery applications and ultrasound image enhancement. In both cases, the oscillations of microbubbles are exploited but their effects may vary depending on the ultrasound frequency and intensity. At low ultrasound intensities, the microbubbles oscillate in a stable motion, also known as stable cavitation, inducing linear reflection and harmonic response to an incident ultrasound field. In contrast, at higher ultrasound intensities, inertial cavitation characterized by rapid growth and collapse of the microbubbles occur, and is thus characterized by an extended bandwidth. Since high intensity ultrasound can cause irreversible thermal damage to tissue, inertial cavitation should be prevented, and the local intensity thus controlled. For this purpose, a dual system capable of focusing and monitoring the local intensity is required at the targeted area.Existing dual systems are based on the use of classical beamforming algorithms and dedicated ultrasound probe used for both cavitation and imaging with a central frequency selected to induce stable cavitation. However, due to the limited bandwidth of existing ultrasound probes, the harmonic imaging is often limited to the second harmonic. Other technologies based on the separation of therapeutic and imaging transducers have been proposed in the literature for low- and high-intensity focused ultrasound. Those assemblies allow separation of emission and reception but are cumbersome and limited in resolution, and thus not adapted to drug delivery or imaging within complex loci where the access window is limited to 1 cm2 at best (spinal cord or temporal bone window for instance). Moreover, in the case of constrained areas, surroundings organs (bones, interfaces) induce reflections, refraction and diffusion of ultrasound waves that may influence the focusing and imaging performance. Based on these observations, two challenges should be addressed in order to better exploit contrast agents in the case of complex loci for both focalization and imaging: 1) the design and manufacturing of dedicated compact transducers and 2) the adaptation of imaging algorithms for cavitation regime tracking with a limited data bandwidth. This research project is thus organized following three distinct thrusts, conducted in parallel by 2 PhD and 1 MASc students, that aim 1) to propose novel compact transducer design based on separate emission / reception and spatial compounding, 2) to adapt existing micro-machining techniques for compact theranostic transducer assemblies and 3) to develop high-resolution correlation-based imaging techniques for contrast agent imaging in complex media. Different applications will be considered using numerical simulations and in-vitro phantoms such as: spinal cord for which multiple reflections at the vertebra impair the imaging and focusing, and contrast agent-based transcranial applications for which the phase and amplitude aberrations induced by the transmission through the skull will be considered. Real-time implementation for clinical and pre-clinical researchers on a commercial ultrasound prototyping platform (Verasonics Vantage) will be achieved for use in a further project.