Acoustic output as measured by mechanical and thermal indices during routine obstetric ultrasound examinations

The purpose of this study was to quantify the acoustic output of clinical ultrasound instruments, as expressed by the thermal index (TI) and mechanical index (MI), during routine obstetric examinations.

A prospective, observational study was conducted. Sonographers were unaware of the data being sought. Data were collected regarding duration of the examination and specific duration spent at each MI and TI.

A total of 11 first-trimester, 14 second-trimester, and 12 third-trimester examinations were evaluated. The mean duration of the first-trimester examination was 8.9 minutes. The mean MI was 0.73 (range, 0.3-1.3), and the mean TI was 0.34 (0.1-1.7). The mean duration of the second-trimester examination was 31.8 minutes. The mean MI was 1.04 (0.5-1.5), and the mean TI was 0.28 (0.1-2.4). The mean duration of the third-trimester examination was 16.3 minutes. The mean MI was 1.06 (0.2-1.5), and the mean TI was 0.32 (0.1-2.4). Statistical significance existed across trimesters with regard to examination durations and MI (P < .001). However, no statistical significance existed in the TI across trimesters. During the third trimester, 3.5% of the examinations had a TI of greater than 1.0. Of these, 2.4% were between 1.0 and 1.49 and 1.1% were greater than 1.5. These changes (of TI > or = 1) were brief (mean +/- SD, 0.17 +/- 0.08 minutes) and were observed during the short periods of color Doppler imaging.

Output levels during routine obstetric ultrasound examinations, as expressed by the MI and TI, are generally low. However, higher output levels, particularly TI levels of greater than 1.5, can be achieved, although they account for only a very small proportion of examination time.