- The latest ASA meeting was held virtually June 8-10 with the theme “Acoustics in Focus.”
- There were seven presentations relevant to focused ultrasound research.
- We thank LabTAU’s Maxime Lafond, PhD, for his comments on the sessions.
With a theme of “Acoustics in Focus,” the 180th meeting of the Acoustical Society of America was held as a virtual meeting from June 8-10, 2021. Along with the technical program, the conference included two keynote lectures, presentation sessions with prerecorded talks within a specific topic or focus followed by live discussion and interaction, tutorial sessions with invited speakers, panel discussions, and lightning round sessions.
There were seven focused ultrasound presentations, and these are listed below along with comments from Maxime Lafond, PhD, a researcher from LabTau, a focused ultrasound Center of Excellence in France.
“The five-minute-long lightning round sessions also included focused ultrasound,” said Dr. Lafond. “The video content is in the process of being uploaded for any registered attendees who want to review them.”
The first four presentations highlighted the progress that has been made in recent years in high-level modeling. Rather than moving toward a complexification of the theoretical models, the current trend is to provide approximations and computational solutions to allow widespread use of these models. This is encouraging, because ultrasound propagation through microbubble suspensions is highly relevant in multiple therapeutic and diagnostic applications and could also involve safety issues — notably regarding heating.
You can search for the following presentations within the online portal.
1aPA1: Contrast agent microbubble scattering in response to sub-resonant ultrasonic driving. Paul Prentice, Centre for Medical and Industrial Ultrasonics, University of Glasgow.
Clinical focused ultrasound in transcranial brain applications tends to be below 1 MHz, which is below the resonant frequency of most ultrasound contrast agents. With 692 kHz focused ultrasound, the researchers showed that at low amplitude (459 kPa), shock wave emission contributed to harmonic peaks in the cavitation noise, while at higher pressure (1.14 MPa), period-doubled shock wave response induced emissions in the sub- and ultra-harmonics. This is an important finding because those peaks usually tend to be associated with stable cavitation rather than inertial.
1aPA2: Pulsed ultrasound propagation through polydisperse contrast agent suspensions. Nicholas Ovenden, Dept of Mathematics, University College London.
This group presented approximations that can help reduce the computational complexity of ultrasound propagation through a polydisperse contrast agent suspension.
1aPA4: The Iterative Nonlinear Contrast Source method for simulating ultrasound propagation through a polydisperse microbubble population. Agisilaos Matalliotakis, Imaging Physics, Applied Sciences, Delft University of Technology.
The iterative nonlinear contrast source method was adapted to the propagation of ultrasound through a microbubble suspension by adding a contrast source term in the source Green’s function. Multiple scattering is then accounted iteratively. This group obtained good agreement with the experiment (less than 2% error up to 6th harmonic).
1aPA6: Theoretical analysis on thermal effect of weakly nonlinear propagation of focused ultrasound toward medical applications. Shunsuke Kagami, Engineering Mechanics and Energy, Tsukuba University.
This group incorporated the thermal effects of bubble oscillations in the Khokhlov–Zabolotskaya–Kuznetzov (KZK) equation using the energy equation proposed by Prosperetti (J Fluid Mech 1991). They found that the thermal conduction of gas inside the bubbles was an important parameter that strongly affects the dissipation of ultrasound.
3aBA8: Focused ultrasound–mediated intranasal delivery. Dezhuang Ye, Washington University in Saint-Louis.
This group presented intranasal drug delivery to achieve enhanced delivery of anti-programmed cell death-ligand 1 antibody (aPD-L1), a checkpoint inhibitor, to intracranial murine glioma. The intranasal approach is an interesting way to bypass the blood-brain barrier (BBB). However, it is non-targeted and not efficient. The researchers showed a four-fold increase in aPD-L1 delivery (as measured by fluorescence) in brainstem gliomas in mice. In my opinion, this is a great way to reduce ultrasound exposure to the brain when delivering drugs.
3aBA11: Real-time cavitation control and monitoring for focused ultrasound blood-brain barrier opening via dB-based subject calibrated feedback controller. Chih-Yen Chien, Washington University in Saint-Louis.
With the proposed dB-based subject calibrated feedback controller, this group was able to achieve different levels of blood-brain barrier (BBB) opening by adjusting the desired dB level. While this is an important finding that BBB opening extent can be varied based on an acoustic feature, a more quantitative approach of cavitation exposure would increase repeatability by other groups and would be more robust to changes in equipment. The stable cavitation index that the researchers chose was based on the sub- and ultra-harmonic levels above the surrounding broadband noise, a classic approach. In my opinion, the impact of microbubble concentration on BBB opening and the stable cavitation index should be investigated.
3aBA18: Bubble-enhanced HIFU. Mike Averkiou, University of Washington.
In this interesting study, the researchers found that the effect of microbubble (MB)-enhanced heating was more prominent at low pressure. Of course, for a given configuration, a higher pressure will still result in slightly more heating, but this means that lowering amplitude could help mitigate deleterious effects in tissues on the acoustic path while maintaining most of the MB-enhanced heating effect at the focus.
3pBAa: Instrumentation and Simulation in Biomedical Acoustics: Rapid Prototyping for Focused Ultrasound Sources. Panel Discussion.
I was not able to attend this session.