Ultrasound and microbubbles: Their generation, detection and potential utilization in tissue and organ therapy—Experimental

Ultrasound-induced cavitation in tissue and organs has been well recognized and documented. Generally, this phenomenon has been seen as something to be avoided except in cases such as lithotripsy, where its production is considered an essential part of the treatment process or as a desirable contrast media in some areas of visualization enhancement. This article covers theree areas in which the phenomenon has been observed, and shows how the effect can or may be therapeutically beneficial. Studies in the pig show that implanted human gallstones and the gallbladder itself can be eliminated in a nonsurgical procedure using ultrasound-induced cavitation in the gallbladder. In the dog brain, relatively stable cavitation-induced microbubbles have been transported through the vascular system to regions outside a focal seeding site. These bubbles produce ablation of tissue volumes at a remote site when irradiated with appropriate ultrasound. The cavitation phenomenon has been observed in the dog and human prostate. In the human prostate, microbubbles transported from ultrasound-induced focal seeding sites can be readily visualized with ultrasound and may be potentially useful under controlled conditions in tissue debulking for the treatment of benign prostatic hyperplasia (BPH). A similar microbubble transport has not been seen in the dog prostate under similar ultrasound treatment parameters. The ability to detect cavitation-induced microbubbles, follow their transportation through the vascular system and excite them at the appropriate time and place provides interesting possibilities for therapy. Of course, the entire microbubble process can be avoided by working below the cavitation threshold, thereby using only the absorption of ultrasound in tissue to produce focal thermal lesions. The term microbubble is used here in the context of those bubbles which can be transported in the vascular system down to vessels diameters below the 100-μm range. This is the vessel size in the vascular field into which microbubbles are transported and can be both visualized as well as disrupted with ultrasound.