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This is an important statement.
In it, the AIUM provides two tables containing recommended maximum times for the duration of an ultrasound exposure (dwell time) at a given setting of the Thermal Index (TI).
The recommendations are intended to provide reasonable assurance that an ultrasound examination can be conducted without risk of producing an adverse effect due to a thermal mechanism. However, the ALARA principle should always be followed so that examination times are only as long as necessary.
Due to the higher outputs of acoustic radiation force impulse (ARFI) and pulsed Doppler, the dwell times in Table 1 should be reduced by 33% when bone is near the transducer focus.
Additionally, scanning febrile gravidas (pregnant patients who have a fever) should only be performed if clinically indicated and should be deferred until the temperature is normal, if possible.
Table 1. Pertains to obstetric (including gynecologic when pregnancy is possible), neonatal transcranial, and neonatal spinal examinations.
From this table, we can see that the higher the TI, the shorter the recommended dwell time. We can also see that if the TI can be kept at or below 0.7, there is no time limit to the recommended dwell time.
Table 2. Pertains to adult transcranial, general abdominal, peripheral vascular, neonatal (except head and spine), and other scanning examinations (except the eye).
From this table, we can see that for these less sensitive procedures, the TI values can be higher, and that if the TI can be maintained at or below 1.5, there is no time limit.
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