FDA Recommendations for the Safe Use of
Ultrasound in Obstetrics
Author: Sherri A. Longo, M.D.
Objectives: Upon the completion of this CNE article, the reader will be able to:
- Define thermal index, mechanical index, absorption coefficient, spatial-peak temporal-average, and spatial-peak pulse-average.
- State the United States Food and Drug Administration, American Institute of Ultrasound in Medicine, and American College of Obstetrics and Gynecology recommendations for safety levels in obstetrical ultrasound.
- Describe the methods that can be used to minimize fetal exposure during obstetrical ultrasound.
No longer can an ultrasound department defer safety issues to the regulation of the United States Food and Drug Administration (FDA). Today, ultrasound safety is the responsibility of the diagnostician, who is expected to perform an optimal ultrasound examination with safety awareness.
In the past, safety issues were monitored by the FDA who provided safety regulations via the 1976 (510-k) Guide published in 1985. In 1976, the Medical Device Amendments to the Food, Drug and Cosmetic Act were added requiring the FDA to regulate all medical devices, which included diagnostic ultrasound equipment. The 1976 output levels were historically based and not scientific. In addition to the FDA recommendations, the American Institute of Ultrasound in Medicine (AIUM) made ultrasound safety statements that reported no adverse patient bioeffects with diagnostic ultrasound exposure or non-human mammal effects when the spatial-peak temporal-average intensity was below 100mW/cm2.
However, studies appeared, which identified adverse bioeffects confirmed in mammals exposed to ultrasound that had spatial-peak temporal-average intensities of less than 100mW/cm2. These studies caused the AIUM to adjust its non-human mammalian statement to reflect the in vivo bioeffects. In addition, since 1992, many ultrasound devices are being manufactured with the capability of exceeding the acoustic output levels allowable for obstetrical use per the 510-k Guide. Therefore, the devices are required to have output screen displays that include indices that predict the potential for adverse bioeffects. Those indices include the thermal index (TI) that predicts tissue heating and the mechanical index (MI) that reflects the potential for tissue cavitation.
The necessity for ultrasound safety also captured the attention of the American College of Obstetrics and Gynecology (ACOG), as evidenced by their Committee Opinion # 180 entitled "New Ultrasound Output Display Standard" issued in November 1996. This publication described the on screen display standards (as determined by the FDA, the AIUM, ACOG, and the National Electrical Manufacturers Association – NEMA) for commercially available ultrasound systems. The ACOG Committee Opinion stressed the necessity for user awareness of new ultrasound display standards recommending acoustic output limits for obstetrical use to minimize the potential for adverse ultrasound bioeffects. For those applicable ultrasound devices (manufactured after 1992), one of two acoustic output indices (MI or TI) is required to appear on the screen.
The Thermal Index (TI)
Thermal index (TI) is a calculated estimate of temperature increase with tissue absorption of ultrasound and is determined by the ratio of the total acoustic power to the acoustic power required to raise the tissue temperature by 1oC. Some devices further subcategorize the TI according to the insonated tissue: soft tissue thermal index (TIS) for soft homogeneous tissues, cranial bone thermal index (TIC) for bone at or near the surface, and bone thermal index (TIB) for bone after the beam has passed through soft tissue.
The largest temperature rise occurs in tissue between entry of the ultrasound beam and the focal region. The temperature rise is affected by the ultrasound power and the exposed tissue volume. Scanned modes that include B-mode and color flow Doppler allow a greater volume of exposed tissue to the ultrasound energy. Whereas, unscanned modes, M-mode and spectral Doppler, use a stationary ultrasound beam that distributes the energy over a narrower volume of tissue. For these unscanned modes, the greatest temperature increase is between the focal point and the surface. Several factors affect the exact location of the "hottest point" and include tissue attenuation, absorption, and the focal length of the ultrasound beam.
Attenuation is the energy lost from the ultrasound wave by one of two causes, which are scattering and absorption. Scattering refers to the redirection of the ultrasound beam and absorption is ultrasound energy converted into heat. The extent of ultrasound absorption depends on the type of tissue and its absorption characteristics, which can be quantified by the absorption coefficient (dB/cm).
The absorption coefficient is directly proportional to the frequency of the ultrasound and therefore is often normalized to the frequency, expressed in decibels per centimeter per megahertz. With a higher frequency there is greater absorption and less depth resolution. Often, the diagnostician resorts to increasing the acoustic output to improve image resolution and risks the potential for an increase in tissue temperature.
The various tissues (blood, amniotic fluid, cerebrospinal fluid, urine, soft tissue, and bone) have different absorption capabilities and therefore diverse absorption coefficients. Bone is the body tissue that absorbs the most ultrasound energy and has a very high absorption coefficient. However, even fetal bone will have varying absorption coefficients depending on gestational age and the degree of ossification. Amniotic fluid, blood, cerebrospinal fluid, and urine (because they are fluids) absorb very little ultrasound energy and therefore have an absorption coefficient of close to zero predicting minimal temperature rise. Soft tissue has an absorption coefficient that falls somewhere between fluid and bone.
In practical terms, there is concern regarding a thin pregnant woman who undergoes an ultrasound examination of her fetus. The patient’s thin abdominal wall and lack of adipose tissue, places the fetus at risk for increased absorption of ultrasonic energy during a meticulous evaluation of fetal anatomy, especially an intracranial evaluation. Adipose tissue and a thick abdominal wall provide a potentially protective effect for the fetus. These tissues absorb the ultrasonic energy and decrease fetal tissue absorption, which potentially would decrease any potential temperature increase in the fetus.
Several other factors affect temperature including ultrasound beam focus, the type of ultrasound transmission, and duration of ultrasound exposure. When an ultrasound beam is focused, image resolution is improved but the intensity also increases, which could cause a rise in temperature.
The type of ultrasound transmission is another factor that must be considered with regard to temperature. Ultrasound waves are transmitted in pulsed and continuous waveforms. When ultrasound is transmitted in a pulsed waveform, the acoustic intensity is high during the pulse and zero during the period between pulses. When the intensity is averaged over a given interval of time (pulse and no pulse), the temporal-average intensity is determined. The temporal-average intensity is important when determining bioeffects for higher temperatures. The spatial-peak temporal-average intensity (SPTA) is defined as the intensity at the maximum temporal-average intensity and is often used for ultrasound output specifications.
Another factor that may affect temperature rise of tissue is time or duration of ultrasound exposure. Temperature is affected by energy absorption during ultrasound exposure. Therefore, the longer the tissue is exposed to the ultrasound energy, the greater the absorption, thus increasing the risk for potential biological effects.
In practical terms, the potential for a temperature rise in the fetus is dependent on several factors that must be simultaneously present during an ultrasound examination. Those factors would include a thin abdominal wall that would decrease ultrasound absorption prior to fetal exposure, a prolonged and continuous evaluation of fetal tissue such as bone, and the capability of the ultrasound equipment to exceed the temporal-average intensity limit for obstetrical application. Thus far, there have been no significant thermal effects documented in humans and at this time the possibility of having all the factors present is highly unlikely. However, we are unsure of what the output levels will be for future technology and its applications.
The Mechanical Index (MI)
The mechanical effects of ultrasound absorption are also known as non-thermal effects and are represented by the mechanical index (MI), which is a relative measure. The MI is calculated by dividing the spatial-peak value of the peak rarefractional pressure (rated by 0.3 dB/cm-MHz at each point along the beam axis) by the square root of the center frequency. Specifically, the mechanical effects are the result of compression and decompression of insonated tissue with the formation of microbubbles, also referred to as cavitation.
Cavitation is related to the peak rarefractional pressure or peak negative pressure during a pulse, which is related to the pulse-average intensity. Therefore, the spatial-peak pulse-average (SPPA) intensity is related to cavitation. Many ultrasound products use SPPA intensity for specifications and therefore, user awareness is a necessity.
There are two categories of cavitation, which are "stable" and "inertial". With stable cavitation, gaseous bubbles vibrate yet the gaseous body pulsates or oscillates because of the ultrasound field and remains stable. A liquid medium eventually forms around the gas bubbles and, with the oscillations, begins to stream or flow called micro-streaming. Cell membranes can undergo disruption because of the stress of micro-streaming.
With inertial cavitation, the pressure from the ultrasound field causes pre-existing bubbles or cavitation nuclei to expand and then collapse in an implosion that can result in the production of reactive chemicals. The implosion can cause damage to the tissue and eventually cause cell death. Although cavitation effects have not been demonstrated in human tissue, there exists a theoretical risk. That risk could potentially become reality if future technology has the capability to exceed peak allowable output limits.
Current standards recommend that if an ultrasound device is capable of achieving a TI or MI greater than 1.0, then the output display screen must show the appropriate index value for the user to predict the potential for adverse bioeffects. The specific recommendation for obstetrical ultrasonography is for the MI or TI not to exceed 1.0.
The acoustic output is the intensity at the location of the greatest temporal-average intensity (the average of intensity over time) or the spatial-peak temporal-average. The FDA has limited acoustic output for ultrasound exposure of the fetus to less than 100mW/cm2 (specifically a maximum of 94mW/cm2). However, the power outputs of ultrasound systems are only required to be limited to 720mW/cm2. As you can see, the acoustic output of an ultrasound device could potentially be 8 times that allowed for fetal exposure.
In practical terms, a diagnostician should minimize the acoustic output and maximize the gain when attempting to optimize image resolution. Gain does not affect ultrasound energy and therefore does not affect temperature. Whereas, if acoustic output is increased, the risk for temperature rise in the insonated tissue increases, and the risk for adverse bioeffects increases.
Finally, diagnosticians are expected to administer prudent use of ultrasound with the ALARA principle that mandates that ultrasound exposure (in terms of TI, MI, and exposure time) be kept "as low as reasonably achievable" without compromising diagnostic capability or resolution. The AIUM in March 1993 issued the following official statement on clinical safety. "Although the possibility exists that such biological effects may be identified in the future, current data indicate that the benefits to the patient of the prudent use of diagnostic ultrasound outweigh the risks, if any, that may be present."
A diagnostician is expected to follow the general principles set forth by the FDA to reduce the potential for adverse bioeffects. One must learn to balance the risks and benefits of ultrasound use. Currently, many manufactured ultrasound devices exceed the allowable acoustic output for fetal exposure by almost eight-fold when compared with the FDA standards and therefore the safety responsibility falls on the individual user.
Ultrasonography is an integral diagnostic device in the field of obstetrics and gynecology among other specialties of medicine. Epidemiologically, its safety record has been unblemished, but historically, the regulation of acoustic output levels has been based upon the FDA’s 510-k Guide published in 1985. Today, new ultrasound technology and its applications may not only increase the benefits of ultrasound but also the potential risks. Therefore, being knowledgeable about ultrasound safety will hopefully decrease the potential for adverse bioeffects.
References or Suggested Reading:
- American Institute Of Ultrasound In Medicine. Bioeffects And Safety Of Diagnostic Ultrasound. Laurel, Maryland: AIUM, 1993.
- American Institute Of Ultrasound In Medicine. Medical Ultrasound Safety: Bioeffects and Biophysics. Laurel, Maryland: AIUM, 1994.
- American Institute Of Ultrasound In Medicine. Standard For Real-Time Display Of Thermal And Mechanical Acoustic Output Indices On Diagnostic Ultrasound Equipment. Laurel, Maryland: AIUM, 1992.
- American College of Obstetrics and Gynecology Committee Opinion: New Ultrasound Output Display Standard. Number 180, November 1996.
- Miller MW, Brayman AA, and Abramowicz JS. Obstetric Ultrasonography: A Biophysical Consideration of Patient Safety—The "Rules" Have Changed. Am J Obstet Gynecol 1998;179:241-54.
Dr. Sheri Longo is an Assistant Professor in the Department of Obstetrics and Gynecology at Tulane University School of Medicine in New Orleans, Louisiana. She is an active Perinatologist in the Division of Maternal-Fetal Medicine. She has received several teaching awards and has lectured on numerous topics at various conferences.
Dr. Longo has also published several articles in peer-review medical journals and has an interest in the area of safety of ultrasound in pregnancy. She has studied the awareness of users (of ultrasound equipment) as to their knowledge base regarding these safety standards.