The effects of ultrasound waves on biological tissues are called bioeffects. Ultrasound is a non-ionizing form of radiation, they have an extremely low frequency which is why ultrasound waves are not known to cause harm to human tissue. There are different categories of bioeffects; these categories are known as thermal effects and mechanical effects. Thermal bioeffects depend on the duration of exposure and the frequency. In brightness modulation and in color mode a wider area is scanned so the heat can be distributed over that space. In amplitude modulation, motion mode, and Doppler modes, one line is scanned repeatedly to gather information which in turn makes that line have a concentrated amount of heat, increasing the likelihood of thermal bioeffects occurring. Thermal Index (TI) is the on screen measurement of the amount of energy absorbed by the body over a certain period of time. Thermal bioeffects are used as a form of medical heat therapy sometimes by applying low frequency ultrasound to joint and muscle sprains or tendonitis. Also thermal bioeffects have been used to thermally kill tumors or stop bleeding. Mechanical bioeffects are the effects of the ultrasound wave that are non-thermal, produced by force. The force that is usually produced is something called cavitation. Cavitation can occur if the Mechanical Index (MI) is the measure on screen of the potential for cavitation to begin in the tissue. There are two types of cavitation; stable cavitation that the gas bubbles expand and contract but do not explode, and transient cavitation where the gas bubbles expand and contract so quickly they just burst. Transient cavitation can cause a lot of harm to surrounding tissue and could possible cause high temperatures. Cavitation is medically used for lithotripsy, which breaks down kidney stones and as a sterilization technique. When ultrasound was beginning to be used in medicine, it was initially used as a form of therapy instead of a way to diagnose a patient. In the 1920’s the first person to realize that there was destruction at areas of very high intensity was a man named Langévin. The ultrasonic therapy was used in many areas; physical and rehabilitation medicine, arthritis, and stomach ulcers. Around the 1950’s people began using a variety of different machines and began to realize that they could use these waves to get images of organs and structures inside of the body and generate diagnosis from these images. The safety of diagnostic ultrasound has been questioned since the beginning of scanning, the fact that ultrasound was being used to destruct ulcers made people question if it was destructing other parts of the body in the process. Research studies have been done on pregnant rats, where they were exposed to continuous high intensities for three days after they had been fertilized with no adverse affects.
The American Institute of Ultrasound in Medicine (AIUM) is an institution that has been around since the origin of ultrasound in the medical field. This organization sets goals and standards that we must meet on a daily basis in our field, in the form of education on bioeffects and also giving us safety ranges that must be met by the ultrasound machines. The most relative intensity of bioeffects is SPTA; the spatial peak and the temporal average. SPTA is the most important in bioeffects as it is the needed range for tissue heating in diagnostic imaging. The AIUM sets these strength limitations so there is a measurable allowable exposure depending on what you are scanning. The AIUM’s main concern is making sure fetal exposure remains within the allowable range during scan. To make sure we aren’t exposing a patient for just enjoyment even though there are no known effects. The FDA has regulations on intensity maximums also, which are the same as the AIUM’s limitations.
We are taught to make sure we practice the ALARA (As Low As Reasonably Achievable) with every patient; which means adjusting different aspects of the machine to reduce what gets to the patient and also only exposing a person to needed medical exams. More education and information are becoming available to teach all sonographers about the potential effects and the acoustic output of the machines to keep all patients as safe as possible. Thermal and mechanical bioeffects are measured and kept on the screen so the Sonographer knows that the sound waves are within the allowable strengths in diagnostic imaging. The thermal index (TI) is the measurement of the potential for tissue heating. The three strengths TI measures for are Thermal Index for soft tissue (TIS), Thermal Index for Bone (TIB), and Thermal Index for cranium (TIC). AIUM & the FDA say that ISTPA levels should not be above 100mW/cm² in an unfocused beam and not above 1000mW/cm² with a TI of 2 in a focused beam. Mechanical Index (MI) is the measurement of the potential to induce cavitation in tissues. If the MI is above 0.3 there is a possibility of minor damage, and as a Sonographer we need to know to reduce the exposure time as short as possible. If the MI is above 0.7 there is a risk of cavitation if a contrast agent containing gas, and a potential for cavitation without use of a contrast agent. We need to remember that diagnostic imaging is only appropriate if a patient will benefit from the exam medically. There are no definite harmful bioeffects from exposure in diagnostic ultrasound it is accepted that the benefits to the patient outweighs the risks that the exam could potentially create.
References
Royal College of Obstetricians and Gynecologists. (2006). Publication: Is Ultrasound Safe? United Kingdom: Authors: Jolly Joy, Inez Cooke, and Mark Love. Retrieved from http://onlinelibrary.wiley.com/doi/10.1576/toag.8.4.222.27271/pdf

Ultrasound Obstetrics and Gynecology 2. (1992). Diagnostic Ultrasound: Bioeffects and Safety. New Orleans, LA, Winston-Salem, North Carolina, Denver, CO: C.R.B. Merritt, F.W. Kremkau, and J.C. Hobbins. Retrieved from http://onlinelibrary.wiley.com/doi/10.1046/j.1469-0705.1992.02050366.x/pdf

American Institute of Ultrasound in Medicine. (2008). Potential Bioeffects of Diagnostic Ultrasound. Retrieved from
http://www.jultrasoundmed.org/content/27/4/503.full.pdf