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Guidelines for the Safe Use of Diagnostic Ultrasound - Conclusions

3. Conclusions

3.1 General

• Although there are many exposure conditions for which the risk of injury during a diagnostic ultrasound examination is negligible, this is not the case for every possible exposure conditionusing currently available equipment.Therefore, the persons responsible for the ultrasonic exposure must ensure that the exposure is justified, i.e., that reliable diagnostic information can be achieved and that the benefits outweigh the risk.
• The conclusions listed below provide guidance as to the risks due to thermal and mechanical effects arising from ultrasound exposure. To be useful, all the conclusions need to be taken into consideration. Guidance as to the benefit of a diagnostic ultrasound examination can be obtained from clinical practiceguidelines availablefrom medicalsocietiesand associations in Canada.

3.2 Thermal Effects

• At the time of writing, the information published on output levels during B-mode imaging indicatesthat the risk of injury from ultrasonic heating is negligible during this type of examination. At this time, there appears to be no reason on thermalgrounds to limit such scanningfor any clinicalindica-tion, including ultrasound examination of normal pregnant women.
• In all other operating modes, especially those used for Doppler blood flow examinations, risk of injury from ultrasonic heating depends on the temperature elevation and the dwell time, as indicated by the conclusions given below.
• If the ThermalIndex (TI) does not exceed 1, currently available evidence indicates that the risk of an injury due to ultrasonic heating is negligible for the vast majority of conditions of the diagnostic ultrasound examination.
• For first trimester transabdominal fetal examinations through a bladder path greater than 5 cm in length, evidence indicates that it is possible that the maximum temperature elevation which could be obtained is as much as 2-3 times that of the displayed Soft Tissue Thermal Index (TIS). More caution may be warranted in these situations, particularly if the TIS exceeds 1.
• The Soft Tissue Thermal Index (TIS) is the appropriate indicatorof the potential for ultrasonic heating for examinations in which the ultrasound beam travels a path which is made up principally of homogeneous soft tissue or a soft tissue/fluid path, as in a first trimester fetal examination or an abdominal examination.
• If bone, including 2^ndor 3^rdtrimester fetal bone is within the ultrasound beam, then the Bone Thermal Index (TIB) is often the appropriate indicator, except as noted in the next conclusion.
• If bone is in contact with the transducer then the Cranial Thermal Index (TIC) is the appropriate indicator. If bone is within about 1 cm of the transducer and this is closer than the nearest focal zone, the Cranial Thermal Index (TIC) is the appropriate indicator. More caution may be warranted in these cases because of the potential for transducer self-heating; heating of the transducer may add significantly to any ultrasonic heating which may occur.
• Generally, more caution may be warranted for transvaginal, transesophegeal and transrectal examinations because heating of the transducer has the potential to produce additional heat to adjacent tissue.
• This conclusion and the following one provide guidance to the user if the temperatureelevation in the fetus could exceed
  1 °C as a result of a diagnostic ultrasound exposure. If the exposure produces a maximum in situ temperature of no more than 38.5 °C (1.5 °C above normal physiological levels) then it may be used clinically without reservation on thermal grounds.
• To be considered potentiallyhazardouson thermal grounds, it appears that a diagnostic ultrasound exposure must elevate embryonicand fetal in situ temperaturesto the following tem-peraturesfor approximately the corresponding durations (see Section 4.1.2):
  
  
  39 °C (2 degrees above normal), 60 minutes;
  40 °C,(3 degrees above normal), 15 minutes;
  41 °C,(4 degrees above normal), 4 minutes;
  42 °C,(5 degrees above normal), 1 minute;
  43 °C (6 degrees above normal), 0.25 minutes.

3.3 Mechanical Effects

• At exposures that do not exceed the output limits recommended in Section 2.4, there is no demonstrated risk of clinically significant damage in humans from mechanical effects of ultrasound exposure during a diagnostic examination. However, capillary hemorrhaging has been observed in lung and the intestine of mammals at diagnostically relevant exposures. This effect has also been observed in other soft tissues if gas contrast agents are used. For the most part, thresholds are just as likely to be exceeded for B-mode as for pulsed Doppleror colour flow Doppler modes. However,thresholds are lower for pulsed Doppler modes with relatively long pulses.
• If the Mechanical Index (MI) exceeds 1, there is a small risk of capillary hemorrhaging in the lung during ultrasound examinations involving exposure of the neonatal and infant chest. The risk may increase in more unusual exposures where the surfaceof the lung is near the focus. Although clinically significant hemorrhaging is unlikely, in part because of the small volume of tissue that is affected, the potential for achievingclinical significance may increase in the premature infant.
• At the currentmaximum values for the MI of 1.9, it is unlikely that diagnostic ultrasound exposure would lead to clinically significant intestinal hemorrhage in humans. However, the likelihood may increase for pathologic conditions inhibiting intestinal peristalsis and promoting intra-luminal and submucosal gas collections.
• A limited number of experimental studies suggests that use of ultrasound gas contrast agents (GCAs) (microbubbles) during a diagnostic examination has the potentialto increase the likelihood of capillary hemorrhaging in tissues other than lung. In experiments on animals, the risk of significant hemorrhaging from lithotripter fields is increased for several hours after injection.
• As long as the recommended output limits of Section 2.4 are not exceeded, mechanical effects are far less likely to be important in obstetrical ultrasound because of the absence of gas bodies.