Recommended Maximum Scanning Times for Displayed Thermal Index (TI) Values

The following two tables contain recommended maximum times for the duration of an ultrasound exposure at a given setting of the Thermal Index (TI).¹ These recommendations for maximum dwell time* versus TI 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 under any scanning conditions. If it is necessary to exceed the recommendations, the occurrence of an adverse thermal effect is still unlikely under most scanning situations due to mitigating factors such as transducer movement and tissue perfusion. However, the principle of ALARA should be followed so that examination times are only as long as necessary to produce a useful diagnostic result. See the AIUM Statement “ As Low as Reasonably Achievable (ALARA) Principle.”

For acoustic radiation force impulse (ARFI) and pulsed Doppler modes, clinical ultrasound scanners tend to underestimate TIB (thermal index for bone near focus) due to limitations in standard methods for measuring acoustic output (insufficient hydrophone spatial averaging correction). Actual values of TIB are typically higher than displayed values.^2–4 Since thermal dose is directly proportional to dwell time, this effect may be countered by reducing maximum scanning times in Tables 1 and 2 by 33% for ARFI and pulsed Doppler examinations when bone is near the transducer focus.^2–4 In addition, scanning febrile gravidas should only be performed if clinically indicated and should be deferred until the temperature is normal, if possible.

Table 1. Recommended maximum dwell time and TI ranges for obstetric (including gynecologic when pregnancy is possible), neonatal transcranial, and neonatal spinal examinations. For obstetric exams, monitoring the TIS is recommended up to 10 weeks from the last menstrual period (LMP) or a crown-rump length (CRL) of about 33–34mm, and TIB thereafter. Dwell times should be reduced by 33% for ARFI and pulsed Doppler examinations when bone is near the transducer focus.

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TI Range                                                                   Max Dwell Time

                                                                                    (minutes)

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TI > 3.0                                                                    Not recommended

2.5 < TI ≤ 3.0                                                           <1

2.0 < TI ≤ 2.5                                                           <4

1.5 < TI ≤ 2.0                                                           <15

1.0 < TI ≤ 1.5                                                           <30

0.7 < TI ≤ 1.0                                                           <60

TI ≤ 0.7                                                                    No time limit

Table 2. Recommended dwell time and TI ranges for adult transcranial, general abdominal, peripheral vascular, neonatal (except head and spine), and other scanning examinations (except the eye⁵). Dwell times should be reduced by 33% for ARFI and pulsed Doppler examinations when bone is near the transducer focus.

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TI Range                                                                   Max Dwell Time

                                                                                    (minutes)

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TI > 6.0                                                                    Not recommended

5.0 < TI ≤ 6.0                                                            <0.25 (15 s)

4.0 < TI ≤ 5.0                                                            <1

3.0 < TI ≤ 4.0                                                            <4

2.5 < TI ≤ 3.0                                                            <15

2.0 < TI ≤ 2.5                                                            <60

1.5 < TI ≤ 2.0                                                            <120

TI ≤ 1.5                                                                     No time limit

*Dwell time - The total time that a specified anatomic structure is continuously exposed to the ultrasound beam during an ultrasound examination.

1. Harris GR, Church CC, Dalecki D, Ziskin MC, Bagley JE. Comparison of thermal safety practice guidelines for diagnostic ultrasound exposures.Ultrasound Med Biol 2016; 42:345–357.

2. Wear KA, Vaezy S. Note to physicians and sonographers on potential underestimation of acoustic safety indexes for diagnostic array transducers. IEEE Trans Ultrason Ferroelectr Freq Control 2021; 68:357.

3. Wear KA. Hydrophone spatial averaging correction for acoustic exposure measurement from arrays-Part I: Theory and impact on diagnostic safety indexes. IEEE Trans Ultrason Ferroelectr Freq Control 2021;  68:358–375.

4. Wear KA. Hydrophone spatial averaging correction for acoustic exposure measurements from arrays—Part II: Validation for ARFI and pulsed Doppler waveforms. IEEE Trans Ultrason Ferroelectr Freq Control 2021; 68:376–388.

5. Abramowicz JS, Adhikari S, Dickman E, et al. Ocular ultrasound: Review of bioeffects and safety, including fetal and point of care perspective: Review of bioeffects and safety, including fetal and point-of-care perspective [published online ahead of print November 1, 2021]. J Ultrasound Med 2022; 41:1609–1622. doi: 10.1002/jum.15864.

6. Recommended Ultrasound Terminology. 4th ed. Laurel, MD: American Institue of Ultrasound in Medicine; 2019. http://aium.s3.amazonaws.com/resourceLibrary/rut.pdf