Onset delay of acoustic second harmonic backscatter from bubbles or microspheres

Area of Science:

• Medical Imaging
• Biomedical Engineering
• Acoustics

Background:

• Ultrasound contrast agents, microbubbles or microspheres injected into the bloodstream, enhance blood detectability in ultrasound imaging.
• Harmonic imaging, a technique that detects the second harmonic of incident ultrasound, is commonly used for improved contrast agent visualization.

Purpose of the Study:

• To investigate a newly observed delay in the onset of the backscattered second harmonic signal relative to the backscattered first harmonic from ultrasound contrast agents.
• To determine the impact of this delay on the axial resolution achievable in harmonic imaging.
• To establish the dependence of this delay effect on incident ultrasonic amplitude and microsphere parameters.

Main Methods:

• The phenomenon was investigated using both computational simulations and experimental setups.
• Experiments involved injecting ultrasound contrast agents (bubbles or microspheres) into blood.
• Measurements focused on the timing difference between backscattered first and second harmonic signals.

Main Results:

• A delay in the onset of the backscattered second harmonic signal was observed for ultrasound contrast agents.
• This delay was found to be dependent on the amplitude of the incident ultrasound wave.
• The delay also showed a dependence on the physical parameters of the microspheres used as contrast agents.

Conclusions:

• The observed delay in second harmonic backscatter presents a limitation for achieving optimal axial resolution in harmonic ultrasound imaging.
• Understanding this delay is crucial for optimizing contrast agent design and imaging parameters.
• Further research is warranted to fully characterize and potentially mitigate this resolution-limiting effect.