I grant this work to the public domain. My name is Garth D. Wiebe. I am a college educated, degreed, career electrical/computer engineer (early-retired) with over 30 years of active, professional experience in that industry. I have no connection with the medical or biomedical engineering industry. I do also have extensive practical experience in professional audio production, in terms of working with sound power and intensity levels within the range of human hearing, but not ultrasound. However, I do in concept understand all of the scientific and engineering parameters discussed in the literature that I have read, having to do with medical ultrasound.
The two issues of concern with medical ultrasound are cavitation and localized heating of tissue. Cavitation is when tissues and membranes are physically separated by the mechanical force of the ultrasound vibrations to produce bubbles and voids ("cavities") in that tissue. Heating simply occurs when the kinetic energy of ultrasound vibrations translates into corresponding thermal energy in the form of heat by simple friction.
I examined a publication entitled Output Measurements for Medical Ultrasound, by Roy C. Preston, BSc, PhD, C Phys, F Inst P, Division of Radiation Science and Acoustics, National Physical Laboratory, Teddington, Middlesex, TW11 OLW, UK, Copyright ©1991, ISBN 3-540-19692-7, https://www.npl.co.uk/output-measurements-for-medical-ultrasound (from archive.org). On page 11 of that paper, section 1.4.6, Table 1.1, "Characteristics of medical ultrasound fields," the Ispta power levels cited for diagnostic ultrasound (1-30 mW/cm2) overlap that of lithotripter (kidney stone busting) ultrasound (10-100 mW/cm2). Pulsed Doppler ultrasound (300-5000 mW/cm2), as cited, is 30 to 50 times that of lithotripter ultrasound. The pressure level range given for diagnostic ultrasound of 1-8 MPa is equivalent in identical units to 145-1160 psi (pounds per square inch), which approaches lithotripter levels of 10-100 MPa. On page 15, section 1.6.6, acoustic pressure is that which is considered the parameter significant in exceeding cavitation thresholds, causing "harmful effects...if bubble activity leads to collapsed transient cavitation events. Such events can produce local high temperatures and pressures and be the source of chemical free-radicals."
Note that I converted the cited ultrasound pressure level range of "1-8 MPa" ("M" = "Million" = 1,000,000 and "Pa" = "Pascal") to equivalent psi (pounds per square inch) units above, coming up with 145-1160 psi, since a "pound" and an "inch" are down-to-earth units that everyone intuitively understands, not a "Pascal." Here are some practical comparisons using those units: A loud sound of 94 dB SPL in the human hearing range (below that of ultrasound) corresponds to 0.000145 psi of back-and-forth vibrational pressure on your body (and eardrum), which is miniscule. The pressure of the atmosphere on your skin is 14.7 psi. The air pressure of your car's tires is around 30 psi. The pressure of water coming out of your household faucets is around 40-80 psi. The pressure of an average human bite that smashes food in your mouth is around 160 psi. The pressure of carbon dioxide propellant in a paintball gun is 400-1200 psi. The pressure washers that you can get at a hardware store clean by spraying water at 1000-2000 psi to blast dirt and debris off of whatever you are washing.
The point of this illustration is that, from a standpoint of pure physics, the back-and-forth vibrational forces in medical ultrasound are substantial, and certainly not negligible. One can see from the above comparisons that the vibrational pressures involved in medical ultrasound are a million times that of audible sound in the range of human hearing. "Ultrasound" in this case is not to be thought of as "just like normal sound, except above the human hearing range." It is not like ultrasound coming from a dog whistle, or produced by a bat to locate its prey, sounds that are just like normal sounds, except too high in frequency for us to hear. And again, pressure levels associated with diagnostic ultrasound, as cited in the 1991 document, reach 8 MPa (1160 psi) in the high end, where lithotripter (kidney stone busting) levels start at 10 MPa (1450 psi). A dog whistle, a bat, or even loud music from a rock concert cannot break up anything like a kidney stone.
It is not as if to suggest that diagnostic ultrasound will break up human flesh in the same way that ultrasound is used to break up a kidney stone. Obviously, if that were the case, patients would routinely be transported from their ultrasound scans directly to the emergency room for treatment from bleeding and internal injury, and all the first trimester babies would immediately die. Although the vibrational forces are great, the amount that the flesh moves is miniscule, microscopic in fact, and the flesh is elastic, whereas a kidney stone is brittle. The point is simply that the physical forces in medical ultrasound are substantial and should not be thought of like sound in the human hearing range.
In a National Institutes of Health (NIH) technical article, Safety Assurance in Obstetrical Ultrasound, published in Semin Ultrasound CT MR. 2008 April; 29(2): 156-164, by Douglas L Miller, Ph.D., Research Professor, Department of Radiology, University of Michigan Medical Center, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2390856/pdf/nihms45739.pdf, it is noted that the 1985 limit of 46 mW/cm2 (SPTA) for diagnostic ultrasound was increased to 720 mW/cm2 (SPTA) in 1992. Since that time, the article notes, "there has been little or no subsequent research with the modern obstetrical machines to systematically assess potential risks to the fetus using either relevant animal models of obstetrical exposure or human epidemiology studies" (Abstract, page 1). With regard to cavitation, he notes that "ultrasonic cavitation can cause mechanical damage and can even generate free radicals and sonochemicals capable of DNA damage. For example, single strand breaks from hydrogen peroxide production and genetic mutation have been demonstrated in in vitro studies of cultured cells subjected to ultrasonic cavitation" (page 7). [Note: To be fair, it must be pointed out that the author of the above article cautiously concluded that obstetric ultrasound should be considered safe at this time, only recommending that it should not be used without a valid and compelling medical reason. The article is excellent and unbiased, however, fairly presenting all the factual data, which would allow the reader to come to his own conclusions.]
The FDA guidelines for the manufacture of medical ultrasound equipment are "Nonbinding Recommendations" and can be found in a document entitled Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems and Transducers, Sept. 9, 2008, U.S. Department of Health and Human Services, Food and Drug Administration, Center for Devices and Radiological Health, https://www.fda.gov/downloads/UCM070911.pdf (from archive.org). "Non-binding" is troubling, since the FDA recommendations can be legally exceeded at the discretion of the manufacturer. Also, the increased upper limit of 720 mW/cm2 (SPTA) is specified as the recommended design limit for those devices that display the exposure on-screen according to an "Output Display Standard (ODS)" (page 4), such that the operator can quantitatively view the exposure levels while the ultrasound is in progress. In this, responsibility is deferred to the discretion of the operator of the device regarding how much to expose the patient to ultrasound at what levels. An appropriate anecdote here is in considering the analogy of a simple surgical knife, which is not regulated by any government agency as to how sharp, long, or pointy it is, deferring to the discretion of the surgeon as to how to use it on a patient, how much to cut, how deep, how fast, the surgeon himself being relied upon to consider the patient's best interests. But a knife is a far simpler and much more easily understood tool than an ultrasound machine. Now every radiologist and ultrasound technician must make a determination about what is safe, based on technical scientific and engineering principles that only a few medical and scientific researchers would have a solid grasp of, since the clinical practitioners' education, training, and aptitude is primarily centered around human physiology, anatomy, epidemiology, and so on. Then, as the previous article noted, there is a lack of research substantiating either the safety or lack thereof. So, how are they to discern and judge what levels to use, besides choosing levels that give them good results on the display?
Personally, I would have little concern about these ultrasound levels as applied to most parts of an adult human body, given the power levels. A "Watt" is a "Watt," a standard measure of power across all scientific and engineering disciplines, including the scientific and engineering disciplines with which I am familiar in a professional capacity. Even if minor damage or effects occurred to the comparatively very small volume of bodily flesh exposed to the adult by the ultrasound transducer, I would expect the effects to be temporary, or at least easily healed in a short amount of time in most parts of the body. However, exposing the comparatively miniscule, developing, human baby in a womb to such effects from the same amount of power levels per unit area (and hence, volume) would be unacceptable to me. A fitting anecdote here is that the unborn baby can be as small as a kidney stone, which ultrasound at similar levels is used to break apart.
On the spiritual side, I am a Christian and, although faith in God would put me at ease with any situation that would otherwise be troubling, such as the fear of damage due to ultrasound, it also negates the need for it altogether, if one wholeheartedly trusts God for the health of the developing baby to begin with. That said, few in reality have or practice such faith, in which case I would not advise considering obstetric ultrasound as something having no potential effect on the developing baby.
I grant this work to the public domain.