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

17.4 Medical Uses of Ultrasound 503possibility of subtle or transient effects are still at an early stage. Consequentlydiagnostic ultrasound can only be considered safe if used prudently. Thermal hazard exists with some diagnostic ultrasound equipment, if used imprudently.A temperature elevation of less than 1.5 ◦ C is considered to presentno hazard to human or animal tissue, including a human embryo or fetus, evenif maintained indefinitely. Temperature elevations in excess of this may causeharm, depending on the time for which they are maintained. A temperature elevationof 4 ◦ C, maintained for 5 minutes or more, is considered to be potentiallyhazardous to a fetus or embryo. Some diagnostic ultrasound equipment, operatingin spectral pulsed Doppler mode, can produce temperature rises in excess of4 ◦ C in bone, with an associated risk of high temperatures being produced in adjacentsoft tissues by conduction. With some machines colour Doppler imagingmodes may also produce high temperature rises, particularly if a deep focus or anarrow colour box is selected. In other modes, temperature elevations in excessof 1 ◦ C are possible, but are unlikely to reach 1.5 ◦ C with equipment currently inclinical use, except where significant self-heating of the transducer occurs. Non-thermal damage has been demonstrated in animal tissues containing gaspockets, such as lung and intestine, using diagnostic levels of ultrasound (mechanicalindex values of 0.3 or more). In view of this, it is recommended thatcare should be taken to avoid unnecessary exposure of neonatal lung , andto maintain MI as low as possible when this is not possible. In other tissuesthere is no evidence that diagnostic ultrasound produces nonthermal damage,in the absence of gas-filled contrast agents. However, in view of the difficultyof demonstrating small, localised, regions of damage in vivo, the possibility ofthis cannot be excluded. The Mechanical Index, if displayed, acts as a guide tothe operator. The use of contrast agents in the form of stabilised gas bubblesincreases the probability of cavitation. Single beam modes (A-mode, M-mode,and spectral pulsed Doppler) have a greater potential for nonthermal hazardthan scanned modes (B-mode, Colour Doppler), although the use of a narrowwrite-zoom box increases this potential for scanning modes.Therapeutic Uses of UltrasoundWe may very well be witnessing at this time only the beginning of the use ofultrasound for therapeutic purposes. Some techniques, such as the use of 25 kHzultrasound combined with a water jet to remove plaque from teeth and the cleaningof dental and medical tools with ultrasound, have been well established for a numberof years. Ultrasonic nebulizers of pharmaceuticals operate without producingdestructive temperature levels. Athletic centers and sports medicine specialistsmake use of ultrasound devices to heat sore muscles. Newer techniques are arrivingon the market or are still in the testing stages. One example is the use ofultrasound in catheters to ream out arteriosclerositic deposits in arteries that is stillvery much in the experimental stage.One therapeutic use of ultrasound already in widespread clinical use, extracorporealshock wave lithotripsy, has completely changed the treatment of kidney