Biologic Effects of Ultrasound and Safety

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Definition

Biological effects of ultrasound are the potential biological consequences due to the interaction between the ultrasound wave and the scanned tissues.

The use of ultrasound for cardiac imaging has not known significant adverse biological effects. Concern about the safety of ultrasound prompted several agencies to devise regulatory limits on the machine output intensities. The visual display of thermal and mechanical indices during ultrasound imaging provides an aid to limit the output of the machine [1]. Sonographic evaluation of the human body, including potentially sensitive tissues, such as developing fetus and the eye, have been performed on millions of patients without documentation of serious adverse events. However, ultrasound waves have the potential to cause significant biological effects, depending on ultrasound wave characteristics and scanned tissues sensitivity. Physicians and sonographers must be aware of these potential biological effects in assessing the overall safety of the procedure.

Types of biological effects

The biological effects of ultrasound depend on the total energy applied to a given region. Thus, varying duration of exposure to wave emission, intensity and frequency of the ultrasound beam, pulsed or continuous emission modality and acoustic power, may lead to significant biological effects, that are commonly divided in thermal and non-thermal effects.

Thermal

The biological effects of ultrasound energy are related primarily to the production of heat. Heat is generated whenever ultrasound energy is absorbed, and the amount of heat produced depends on the intensity of the ultrasound, the time of exposure, and the specific absorption characteristics of the tissue. As much as 70% of the total temperature increase associated with ultrasound occurs within the first minute of exposure [2], but temperature continues to rise as exposure time is prolonged. Minimizing the exposure time is probably the single most important factor for ensuring patient safety from thermal injury [3]. Other important parameters to be considered are:

  • The relative protein content of each tissue, since absorption coefficients of tissues are directly related to protein content; absorption coefficients vary between 1 (skin, tendon, spinal cord) and 10 (bone) dB/cm MHz.
  • The perfusion of the tissue, which has a dampening effect on heat generation and physically allows heat to be carried away from the point of energy transfer.
  • Emission modality, since pulsed-wave ultrasound is extremely unlikely to significantly heaten tissues.
  • Beam width, since a wider beam width reduces the rate and extent of temperature rise by permitting the energy to be distributed over a larger perfusion territory [4].

Non-thermal

Ultrasound energy creates also mechanical forces independent of thermal effects, thereby causing biologic effects that are not related to temperature rise alone, such as cavitation, torque forces, oscillatory shear, radiation, pressure and microstreaming.

Cavitation

The interaction of ultrasound with gas bubbles or contrast agents causes rapid and potentially large changes in bubble size. This process, termed cavitation, may increase temperature and pressure within the bubble and thereby cause mechanical stress on surrounding tissues, precipitate fluid microjet formation, and generate free radicals [5]. Gas-containing structures (e.g., lungs, intestines) are most susceptible to the effects of acoustic cavitation. Ultrasound wavelength has an important role in bubble formation and growth: short wavelength ultrasound (observed at higher frequencies) does not provide sufficient time for significant bubble growth; therefore, cavitation is less likely under these circumstances compared with long wavelengths. The short half-life of cavitation nuclei prevents most cavitation-related biological effects, unless ultrasound contrast agents are also present. Contrast agents markedly reduce the threshold intensity for cavitation. However, because of the relatively high viscosity of blood and soft tissue, significant cavitation is unlikely and cavitation has not been shown to occur with the ultrasound exposure commonly used during a diagnostic examination.

Other effects

A variety of other physical forces may also be produced by ultrasound energy. Although each of these effects can be demonstrated in vitro, there is no evidence that any of these physical phenomena has a significant biological effect on patients.


Measurement of biological effects

The biological effects of ultrasound are generally discussed in terms of power (the amount of acoustic energy per unit of time), and the units of power are in the milliwatt range. Intensity (acoustic power per unit of area) is usually expressed as watts per meter squared (W/m2) or in milliwatts per centimeter squared (mW/cm2). To calculate the energy from a pulsed ultrasonic beam, it is necessary to know the duty factor, which is a measure of the fraction of time during which the transducer emits ultrasound. The maximum overall intensity is then described as the highest exposure within the beam (spatial peak) averaged over the period of exposure (temporal average) and is known as the spatial peak temporal average (SPTA) intensity. Another common measure is the spatial peak pulse average (SPPA), defined as the average pulse intensity at the spatial location where the pulse intensity is maximum. Commercial ultrasound instruments operating in pulsed-wave modality for two-dimensional imaging have spatial peak, temporal averaged intensities ranging from 0.001 to more than 200 mW/cm2. Pulsed Doppler imaging, however, may have a spatial peak, temporal average as high as 1900 mW/cm2, considerably greater than 100 mW/cm2 level that has been most extensively studied and has never been shown to produce a biologic effect. The relatively short periods of pulsing, coupled with the fact that the transducer is constantly moving so that no single area is imaged for a long period, contribute to the low likelihood of delivering significant heat to the tissue.
A major limitation of measuring the intensity of ultrasound exposure is that estimating the actual tissue exposure is difficult, due to attenuation and other interactions with the tissue. Furthermore, tissue exposure is limited only to transmission periods and to the time the ultrasound beam dwells at a specific point, both of which are considerably shorter than the total examination time. Other indices that incorporate these factors have been developed to better define the exposure levels with diagnostic ultrasound. These measures include the mechanical index (MI) and tissue thermal index (TTI).
The thermal index (TI) and mechanical index (MI) were introduced to provide the operator with an indication of the potential for ultrasound-induced bioeffects. The TI provides an on-screen indication of the relative potential for a tissue temperature rise. MI provides an on-screen indication of the relative potential for ultrasound to induce an adverse bio-effect by a non-thermal mechanism such as cavitation. Thermal indices are conservatively determined to ensure patient safety. Under most clinical conditions, the thermal index closely approximates or overestimates the maximum temperature increase for ultrasound exposure. Three different thermal indices (depending on the structures encountered in the path of the ultrasound beam, soft tissue or TIs, bone or TIb and cranium or TIc) are used to estimate temperature increases associated with an ultrasound beam. In fact, thermal indices in soft tissue or bone provide fairly accurate in vivo estimates of ultrasound-related temperature rise in the tissue types [6]. Contemporary ultrasound equipment have the theoretic capability to cause a tissue temperature increase greater than 4°C at the focal point [7].
The MI describes the relationship between cavitation formation and acoustic pressure and is defined as the ratio of the peak rarefactional negative pressure adjusted for tissue attenuation and square root of the frequency [5]. The MI was originally formulated based on the threshold for acoustic cavitation in water and blood, and hence may not specifically consider the type of tissue in which this process occurs [8], [9].
The American Institute of Ultrasound in Medicine (AIUM) has proposed guidelines for limits below which ultrasound clearly has been demonstrated to be safe [10]. These guidelines include:

  • A diagnostic exposure that produces a 1°C or less temperature elevation above normal.
  • An exposure intensity less than 1 W/cm2 for focused ultrasound beams.

Current diagnostic ultrasound systems have outputs ranging from 10 mW/cm2 (SPTA) for imaging to as high as 430 mW/cm2 (SPTA) for pulsed Doppler ultrasound. There has been no evidence to date to suggest adverse effects of echocardiography at these ultrasonic outputs.

A small glossary of main terms is provided here.

Transoesophageal imaging

During transoesophageal imaging, especially during intraoperative imaging, the probe may remain nearly stationary for extended periods. The heat generated by the transducer itself must also be considered. Although there are no reports of significant injury resulting from even prolonged intraoperative transoesophageal echocardiography, attention to these issues is recommended. Limited imaging time, occasional repositioning of the probe, and constant monitoring of the probe temperature will all help to ensure an impeccable safety record.


All evidence to date suggests that diagnostic ultrasound, particularly that used in echocardiography, is an extremely safe tool with no demonstrated adverse effects even with the use of newer technology and more powerful instrumentation. Although this is reassuring and justifiably inspires continued confidence in ultrasound imaging, the desire for more and better diagnostic information should never occur at the expense of patient safety. Therefore, limiting the scan time to a minimum, knowing the power output and exposure intensity of different modalities of each instrument, and keeping up to date on any new scientific findings or data relating to possible adverse effects, should always be a consideration.

References

  1. Shankar H, Pagel PS. Potential adverse ultrasound-related biological effects: a critical review. Anesthesiology. 2011 Nov;115(5):1109-24.
  2. Doody C, Porter H, Duck FA, Humphrey VF: In vitro heating of human fetal vertebra by pulsed diagnostic ultrasound. Ultrasound Med Biol 1999; 25:1289–94.
  3. Deane C, Lees C: Doppler obstetric ultrasound: A graphical display of temporal changes in safety indices. Ultrasound Obstet Gynecol 2000; 15:418–23
  4. Horder MM, Barnett SB, Vella GJ, Edwards MJ, Wood AK: In vivo heating of the guinea-pig fetal brain by pulsed ultra- sound and estimates of thermal index. Ultrasound Med Biol 1998; 24:1467–74
  5. 5.0 5.1 Dalecki D: Mechanical bioeffects of ultrasound. Annu Rev Biomed Eng 2004; 6:229 – 48.
  6. Safety standard for diagnostic ultrasound equipment. J Ultrasound Med 1983; 2:S1–50
  7. O’Brien WD Jr: Ultrasound-biophysics mechanisms. Prog Biophys Mol Biol 2007; 93:212–55
  8. Section 7–discussion of the mechanical index and other exposure parameters: American Institute of Ultrasound in Medicine. J Ultrasound Med 2000; 19:143–8, 154–68
  9. Christopher T: Computing the mechanical index. J Ultrasound Med 1999; 18:63– 8
  10. Fowlkes JB, Bioeffects Committee of the American Institute of Ultrasound in Medicine: American Institute of Ultrasound in Medicine consensus report on potential bioeffects of diagnostic ultrasound: Executive summary. J Ultrasound Med 2008; 27:503–15


Further readings

  1. Feigenbaum H. Echocardiography, Sixth ed. Lea and Febiger, Malvern, PA. 2009.
  2. Otto C. Textbook of Clinical Echocardiography, 4th Ed. Elsevier, 2009.
  3. American Institute of Ultrasound in Medicine (AIUM). Bioeffects and safety of diagnostic ultrasound. Laurel, MD: AIUM Publications; 1993.
  4. Standard for Real-Time Display of Thermal and Mechanical Indices on Diagnostic Ultrasound Equipment. American Institute of Ultrasound in Medicine, Laurel, MD, 1992 and National Electrical Manufacturers Association, Rosslyn, VA.
  5. British Medical Ultrasound Society. Guidelines for the Safe Use of Diagnostic Ultrasound Equipment. 2009. Prepared by the Safety Group of the British Medical Ultrasound Society.
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