Diagnostic ultrasound is an imaging modality that is usefulnin a wide range of clinical applications, and in particular,nprenatal diagnosis. There is, to date, no evidence that diagnosticnultrasound has produced any harm to humansn(including the developing fetus).nDespite its apparent excellent safety record, ultrasoundnimaging involves the deposition of energy in the body,nand should only be used for medical diagnosis, with thenequipment only being used by people who are fullyntrained in its safe and proper operation. It is the scan operatornwho is responsible for controlling the output of thenultrasound equipment. This requires a good knowledge ofnscanner settings, and an understanding of their effect onnpotential thermal and mechanical bio-effects.nA fundamental approach to the safe use of diagnosticnultrasound is to use the lowest output power and the shortestnscan time consistent with acquiring the required diagnosticninformation. This is the ALARA principle (i.e. asnlow as reasonably achievable). It is acknowledged that innsome situations it is reasonable to use higher output ornlonger examination times than in others: for example, thenrisks of missing a fetal anomaly must be weighed againstnthe risk of harm from potential bio-effects. Consequently,nit is essential for operators of ultrasound scanners to benproperly trained and fully informed when making decisionsnof this nature.nThe thermal index (TI) and mechanical index (MI) werenintroduced to provide the operator with an indication ofnthe potential for ultrasound-induced bio-effects. TI providesnan on-screen indication of the relative potential for a tissuentemperature rise. MI provides an on-screen indication ofnthe relative potential for ultrasound to induce an adversenbio-effect by a non-thermal mechanism such as cavitation.nThree forms of the TI may be displayed:
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