Statement on Mammalian Biological Effects of Heat

1. An excessive temperature increase can result in toxic effects in mammalian systems. The biological effects observed depend on many factors, such as the exposure duration, the type of tissue exposed, its cellular proliferation rate, and its potential for regeneration. Age and stage of development are important factors when considering fetal and neonatal safety. Temperature increases of several degrees Celsius above the normal core range can occur naturally. The probability of an adverse biological effect increases with both the duration and the magnitude of the temperature rise.

2. In general, adult tissues are more tolerant of temperature increases than fetal and neonatal tissues. Therefore, higher temperatures and/or longer exposure durations would be required for thermal damage. The considerable data available on the thermal sensitivity of adult tissues support the following inferences1:

a. For exposure durations up to 50 hours, there have been no significant adverse biological effects observed due to temperature increases less than or equal to 1.5°C above normal.2

b. For temperature increases between 1.5°C and 6°C above normal, there have been no significant adverse biological effects observed due to temperature increases less than or equal to 6 – [log10(t/60)]/0.6 where t is the exposure duration in seconds. For example, for temperature increases of 4°C and 6°C, the corresponding limits for the exposure durations t are 16 minutes and 1 minute, respectively.

c. For temperature increases greater than 6°C above normal, there have been no significant adverse biological effects observed due to temperature increases less than or equal to 6 – [log10(t/60)]/0.3 where t is the exposure duration in seconds. For example, for temperature increases of 9.6°C and 6.0°C, the corresponding limits for the exposure durations t are 5 and 60 seconds, respectively.

d. For exposure durations less than 5 seconds, there have been no significant, adverse biological effects observed due to temperature increases less than or equal to 9 – [log10(t/60)]/0.3 where t is the exposure duration in seconds. For example, for temperature increases of 18.3°C, 14.9°C, and 12.6°C, the corresponding limits for the exposure durations t are 0.1, 1, and 5 seconds, respectively.

3. Acoustic -output from diagnostic ultrasound devices is sufficient to cause temperature elevations in fetal tissue. Although fewer data are available for fetal tissues, the following conclusions are justified1,3:

e.  In general, temperature elevations become progressively greater from B-mode to color Doppler to spectral Doppler applications.

f. For identical exposure conditions, the potential for thermal bioeffects increases with the dwell time during examination.

g.  For identical exposure conditions, the temperature rise near bone is significantly greater than in soft tissues, and it increases with ossification development throughout gestation. For this reason, conditions in which an acoustic beam impinges on ossifying fetal bone deserve special attention due to its close proximity to other developing tissues.

h. The current US Food and Drug Administration regulatory limit for the derated spatial-peak temporal-average intensity (ISPTA.3) is 720 mW/cm2. For this exposure, the theoretical estimate of the maximum temperature increase in the conceptus may exceed 1.5°C.

i.  Although an adverse fetal outcome is possible at any time during gestation, most severe and detectable effects of thermal exposure in animals have been observed during the period of organogenesis. For this reason, exposures during the first trimester should be restricted to the lowest outputs consistent with obtaining the necessary diagnostic information.

j. Ultrasound exposures that elevate fetal temperature by 4°C above normal for 5 minutes or more have the potential to induce severe developmental defects. Thermally induced congenital anomalies have been observed in a large variety of animal species. In current clinical practice, using commercially available equipment, it is unlikely that such thermal exposure would occur at a specific fetal anatomic site provided that the thermal index (TI) is at or below 2.5 and the dwell time on that site does not exceed 4 minutes.

k. Transducer self-heating is a significant component of the temperature rise of tissues close to the transducer. This maybe of significance in transvaginal scanning, but no data for the fetal temperature rise are available.

4. The temperature increase during exposure of tissues to diagnostic ultrasound fields is dependent on: (1) output characteristics of the acoustic source, such as frequency, source dimensions, scan rate, output power, pulse repetition frequency, pulse duration, transducer self-heating, exposure time, and wave shape; and (2) tissue properties, such as attenuation, absorption, speed of sound, acoustic impedance, perfusion, thermal conductivity, thermal diffusivity, anatomic structure, and the non linearity parameter.

5. Calculations  of the maximum temperature increase resulting from ultrasound exposure in vivo are not exact because of the uncertainties and approximations associated with the thermal, acoustic, and structural characteristics of the tissues involved. However, experimental evidence shows that calculations are generally capable of predicting measured values within a factor of 2. Thus, such calculations are used to obtain safety guidelines for clinical exposures in which direct temperature measurements are not feasible. These guidelines, called thermal indices,* provide a real-time display of the relative probability that a diagnostic system could induce thermal injury in the exposed subject. Under most clinically relevant conditions, the soft tissue thermal index (TIS) and the bone thermal index (TIB) either overestimate or closely approximate the best available estimate of the maximum temperature increase (?Tmax). For example, if TIS = 2, then ?Tmax = 2°C; actual temperature increases are also dependent on dwell time.

However, in some applications, such as fetal examinations in which the ultrasound beam passes through a layer of relatively unattenuating liquid, such as urine or amniotic fluid, the TI can underestimate ?Tmax by up to a factor of 2.4,5

*The thermal indices are the non dimensional ratios of attenuated acoustic power at a specific point to the attenuated acoustic power required to raise the temperature at that point in a specific tissue model by 1°C.6

Reference

  1. O’Brien WD Jr, Deng CX, Harris GR, et al. The risk of exposure to diagnostic ultrasound in postnatal subjects: thermal effects. J Ultrasound Med 2008; 27:517–535.
  2. Kimmel GL, Cuff JM, Kimmel CA. Skeletal development following heat exposure in the rat. Teratology 1993; 47:229–242.
  3. Church CC, Barnett SB. Ultrasound-induced heating and its biological consequences. In: ter Haar G (ed). The Safe Use of Ultrasound in Medical Diagnosis. London, England: British Institute of Radiology; 2012:46–68.
  4. Shaw A, Pay NM, Preston RC. Assessment of the Likely Thermal Index Values for Pulsed Doppler Ultrasonic Equipment—Stages II and III: Experimental Assessment of Scanner/Transducer Combinations. NPL Report CMAM 12. Teddington, England: National Physical Laboratory; 1998.
  5. Jago JR, Henderson J, Whittingham TA, Mitchell G. A comparison of AIUM/NEMA thermal indices with calculated temperature rises for a simple third trimester pregnancy tissue model. Ultrasound Med Biol 1999; 25:623–628.
  6. International Electrotechnical Commission. IEC 60601-2-37. Medical Electrical Equipment, Part 2-37: Particular Requirements for the Basic Safety and Essential Performance of Ultrasonic Medical Diagnostic and Monitoring Equipment. 2nd ed. Geneva, Switzerland: International Electrotechnical Commission; 2007.

 


Approved: 10/07/1987; Reapproved: 03/26/1997, 04/06/2009, 03/25/2015, 10/30/2016

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