As carotid artery plaque cannot be diagnosed by simple ultrasound, researchers have come up with a dual-frequency intravascular ultrasound that helps detect microbubbles.

There are other methods to find out whether the plaque is “vulnerable” to embolism, including detecting microvessels that go deep inside the plaque and spot the molecular signatures of plaques that commonly disintegrate.
Both use microbubbles as contrast agents that are found out using intravascular ultrasound, but traditional ultrasound still doesn’t yield the required result.
Source-Medindia