Detection and quantification of bubble activity in therapeutic ultrasound: Magnetic resonance imaging for cavitation detection and quantification

Abstract

For some applications of therapeutic ultrasound, magnetic resonance imaging (MRI) may serve as a useful tool for detecting, measuring, and quantifying cavitation activity—especially in cases where direct sampling of acoustic emissions are difficult. Doing so can be challenging because the physical phenomena that drive ultrasonic cavitation (e.g., pressure, surface tension, micro-second time scales) have little overlap with the phenomena that drive MRI (e.g., quantum spin, Faraday induction, milli-second time scales). However, this same principle also protects MRI-based cavitation detection from common confounds that plague traditional direct acoustic detection. The proposed paper will first review known methods for encoding cavitation behavior into MR images. These methods generally employ one of two strategies: (1) Encode transient cavitation activities into otherwise approximately static MR signal sources.; and (2) Leverage the time-average effects of many, many cycles of cavitation activity to alter MR signal sources. This paper will then discuss how these methods may be useful to therapeutic ultrasound applications. Finally, this paper will pose opportunities for future development.