Tim Stiles: Ultrasound Safety

Whenever I meet new people and tell them about my work and research, they invariably ask, "So, is ultrasound safe?" I thought I'd take a moment and create a short page about ultrasound safety for the layman. As time goes on, this may grow into a significant portion of my website, so check back often for updates and new information.

Note: I want to be especially clear that the information on this page only expresses the views of the author and that I am not a doctor nor a licensed sonographer. I am a physicist that has seven years of experience working with ultrasound physics. If you have any questions or concerns about future or past ultrasound exams, please consult with your physician.

When examining the safety of ultrasound, there are two questions that need to be addressed. The first is, can ultrasound cause harm, that is, is it possible for extremely intense ultrasound to cause significant biological effects in humans, and if so, is there a threshold above which ultrasound causes damage? Once this has been answered, we can then ask the second questions, which is, do current ultrasound scanners exceed the threshold of harmful effects? I'll address each of these questions in turn.

Can Ultrasound Cause Harm?

The short answer to this question is yes. Extremely intense ultrasound has been shown to have the potential for causing harm to biological tissues in either of two ways.

The first pathway for generating biological effects is the generation of heat within tissue. As ultrasound propagates through tissue, some of the sound energy present in the beam is absorbed by the tissue and transformed into heat. The source of this heat is the friction that is present in tissue. Sound waves alternately compress and expand the material they are moving through, so as they move through tissue, the tissue is compressed and expanded. In the same way that rubbing your hands against each other causes them to warm, the ultrasound passing through tissue can cause it to warm as well. If the temperature within tissue is increased enough, permanent damage to that tissue can occur. In fact, investigations are currently ongoing into the possibility of using this heat created by extremely intense ultrasound to treat cancer by significantly heating a tumor without heating nearby tissue.

The other method by which ultrasound could cause harm is termed mechanical effects. Potentially the most serious mechanical effect is cavitation, in which small gas bubbles that could be present in tissue could be imploded by the passage of ultrasound through them. This implosion can be extremely violent, with temperatures reaching several thousands degrees and pressures of several hundreds times atmospheric pressure. However, the effected regions would be microscopic both in time and space. Currently, there is no concrete evidence that cavitation occurs in living tissue. It is much more likely to occur in water, which does not have the same resistance to motion that tissue has.

Do current scanners exceed the threshold?

The FDA and other regulations currently require strict measurements to be performed on every ultrasound scanner before it is sold that demonstrate that the output of the scanner is below the thresholds associated with either of these types of damage from ultrasound. Current ultrasound scanners are required to display two indices, the Mechanical Index and Thermal Index that tell the operator the likelihood of damage from cavitation or tissue heating for the current settings of the scanner. There are strict upper limits on the values that these indices can have, and every sonographer is urged to use the least amount of ultrasound necessary for a specific exam.

My personal research has focused on the accuracy of these measurements and finding methods to increase that accuracy. More information on my research can be found on my "Research" page.

Keepsake Ultrasound

In recent years, quite a few businesses have started operation across the U.S. offering the opportunity to have ultrasound scans performed and offer "photographs" and "videos" of the the scan for sale. These business generally state that they do nothing different than scans at hospitals and even use licensed sonographers. However, many organizations and myself feel that this use of ultrasound should be discontinued. Diagnostic ultrasound is a medical procedure and should only be done at the request of a medical professional. While much work is performed to ensure ultrasound scanners operate below the thresholds that may cause damage, there is no guarantee that ultrasound scans are completely without risks. Because of this, I feel that it is prudent that such use of ultrasound be discontinued.

Updated: July 12, 2006
Note: The views expressed on this page are those of the author, Timothy Stiles, alone and do not reflect in any way upon the University of Wisconsin nor the Department of Medical Physics.