THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

504 17. Commercial and Medical Ultrasound Applicationsstones. Kidney stones are calcified particles that tend to block the urinary tract.In this type of treatment, the patient is immersed in water to equalize as much aspossible the acoustic impedances between the transducer and the patient’s body.A focused, high-pressure ultrasonic pulse is directed through the water and intothe patient’s torso to break the stone into small pieces. The pulverized materialcan now pass out of the body unhindered. Lithotripsy causes very little damage tokidney tissue.A promising procedure for therapeutic ultrasound is the laser-guided ablativeacoustic surgery, in which sound supersedes the scalpel in destroying benign ormalignant tissues. The ultrasound focused by a specially shaped set of transducersconverges inside the body to create a region of intense heat that can destroy tumorcells. The spot of destruction is so small that a boundary of only six cells liesbetween the destroyed tissue and completely unharmed tissue, which connotes aprecision far beyond any current method of surgical incision.Acoustical surgery offers a potentially better means of treating cancerous tumorsbecause it does not require an anesthetic, can be administered in a single treatment,and causes no observable side effects. In a Phase I clinical trial at Marsden Hospitalin London, focused sound waves destructed parts of liver, kidney, and prostate tumorsin 23 patients. In the next phase the researchers will attempt to fully destroytumors in the liver and prostate. Also under testing is the Sonablade TM systemby Focus Surgery of Indianapolis, IN, which incorporates proprietary transducertechnology in a transrectal probe that provides imaging for tissue targeting andhigh-intensity focused ultrasound (HIFU) for tissue ablation. After the operatordefines the area of periutheral tissue to be ablated, the treatment process beginsunder computer control, where the focus of the dual-function transducer is electromechanicallystepped through the designated volume of the tissue. HIFU resultsin thermally induced coagulative necrosis only in the intraprostatic tissue encompassedby the focal volume, with effect on intervening tissues. Confirmation oftargeting accuracy is provided through continuously updated images. The necrotictissue is either sloughed during urination or reabsorbed, along with the cessationof patient symptoms.Ultrasound can also be used to stop internal bleeding through an effect calledacoustic hemostatis. With sufficient power, ultrasonic pulses can elevate the bodytemperature at selected sites from 37 ◦ C to between 70 ◦ C and 90 ◦ C in an extremelyshort time, less than 1 s. This causes the tissue to undergo a series of phasetransitions, and the protein-based bodily fluids and blood coagulate as the resultof the proteins undergoing cross-linking (a process similar to cooking an egg).A research team at University of Washington’s Applied Physics Laboratory isinvestigating the use of ultrasound to stop internal bleeding during surgery and fortreating trauma cases. The present method of stemming bleeding in delicate organs,such as the liver, pancreas, or kidney, is through cauterization on the surface withion or microwave systems. The focused ultrasound waves, however, can penetratedeeply into the organ and “cook” the tissue in a layer as thin as 1 mm. It follows thattrauma patients could be treated without the need for a sterile environment of anoperating room and without the danger of infection that accompanies conventional