Speaker: Prof. Olivier Couture

Time and Date: 9:00-11:00 am, October 26, 2018

Place: Room 423 of Scientific Building, Handan Campus,Fudan University

Abstract:

In ultrasound imaging, the trade-off between penetration and resolution was deemed incompressible until recently. Due to diffraction, the echoes from scatterers, such as microbubbles flowing in large concentrations in blood vessels, are undistinguishable from each other’s. In optics, this limit was bypassed using localization microscopy techniques, such as FPALM. It exploits the blinking of individual sources on thousands of images to separate fluorophores and determine their actual location with a resolution better than wavelength/10 [Betzig et al., Science, 2006].
We have shown that such phenomenon can also be obtained in ultrasound imaging by observing large concentrations of microbubbles at ultrafast frame rates. This lead us to introduce the concept of ultrafast ultrasound localization microscopy [uULM, Couture et al. Patent 2010, Couture et al. IEEE IUS Orlando 2011, Couture et al. IEEE UFFC 2018], which allows super-resolution imaging by reconstructing the precise position of individual microbubbles flowing into microvessels. We initially tested ULM in 3D in a microvessel phantom [Desailly et al. APL, 2013], before moving to the rat brain
[Errico et al. Nature, 2015]. This technique has recently been applied by Dayton’s group to map angiogenesis in a tumor model and assess its tortuosity [Lin et al. 2016].
Other approaches for ultrasound super-resolution have since been proposed. Microbubble Accumulation Super Resolution, which relies on low dilution of microbubbles observed at conventional frame rates has been applied both in-vitro [O’Reilly et al, Medical Physics 2013; Viessmann et al., PMB, 2013] and in-vivo [Siepmann et al., IEEE IUS 2011, Christensen-Jeffries et al., IEEE UFFC, 2015]. More recently, Super-Resolution Acoustical Fluctuation Imaging, which relies on the higher-order statistics of the echoes temporal fluctuation, has been demonstrated by Bar-Zion et al. [ArXiv 2016].
In general, the position of individual microbubbles can be determined with a resolution defined by the signal to noise ratio and the geometry of the system [Desailly et al., PMB, 2015]. The maximum resolution attainable with current programmable scanners was predicted to be 5 microns at 15 MHz, or wavelength/20. In uULM, this was confirmed in-vivo by injecting boluses of microbubbles in rat brain and performing ultrafast imaging coronally (500 fps) for 150 seconds. The resulting map of microbubble positions was reconstructed with a resolution of λ/12 (8 μm), down to 12 mm in depth.
Ultrasound localization microscopy brings new parameters in the trade-off between penetration and resolution, such as the concentration of microbubbles, frame rate, signal-to-noise ratio and acquisition time. This approach allows us to map vessels that are not visible by other imaging modalities in-depth. However, it remains limited by micrometric motion which can drastically affect image quality. Subwavelength image registration has been proposed to solve this issue [Hingot et al., Ultrasonics, 2016a]. Data overload, challenging probe design and electronics, currently restrict the use of 3D uULM, which is a conditio sine qua non for the mapping of complex microvasculature. Finally, we consider that ultrasound therapy should also benefit from the evolution toward a subwavelength precision, which was recently demonstrated for drug-delivery [Hingot et al., APL, 2016b].

Biography:

Olivier Couture was born in Quebec City (Canada) in 1978. He received his B.Sc degree in physics from McGill University, Montreal, Canada, in 2001, and his Ph.D degree from the department of Medical Biophysics, University of Toronto, Canada, in 2007. After a postdoctoral fellowship at ESPCI in Paris (France), he was hired as a tenured research associate at CNRS, based within the Langevin Institute. He was awarded the “2017 IEEE Ultrasonics Early Career Investigation Award” for the development of ultrasound super-resolution and plane-wave contrast imaging and the Sylvia Sorkin Greenfield Award of the American Association of Physicists in Medicine for the Best Paper published in Medical Physics in 2011. He received the prestigious ERC Consolidator Grant for the application of ultrasound localization microscopy to medical diagnosis. His current research interests include ultrasound localization microscopy, ultrafast ultrasound imaging, drug delivery, velocipede oscillations, contrast agents, diabetes and stroke.