Impact of tissue harmonic imaging on measurement of nuchal translucency thickness

To evaluate the effect of tissue harmonic imaging (THI) on the measurement of fetal nuchal translucency thickness (NT).

One-hundred and three pregnant women underwent first-trimester NT measurement according to The Fetal Medicine Foundation criteria. NT was evaluated using conventional ultrasonography (CUS) and THI without modifying any other ultrasound parameter (e.g. gain). For each patient three images with CUS and three images with THI were stored and then measured independently on the ultrasound system by two different operators. The maximum measurements using CUS and THI were compared. Differences between the values of CUS and THI NT measurements were tested using the Wilcoxon signed-rank test. Bland-Altman plots were constructed, and intraobserver and interobserver variabilities were assessed by calculation of the intraclass correlation coefficient. Probability values of < 0.05 were considered significant.

Mean maternal age was 32.8 (range, 20-42) years, mean gestational age at examination was 12 + 1 (11 + 0 to 13 + 6) weeks and mean crown-rump length (CRL) was 55.8 (SD, 7.2) mm. Median fetal NT was 1.4 (0.8-3.5) mm using CUS and 1.2 (0.6-3.3) mm using THI (P < 0.001) for the first operator. A second operator remeasured the first 51 cases: median fetal NT was 1.4 (0.8-3.8) mm using CUS and 1.1 (0.6-3.1) mm using THI (P < 0.001). Fetal NT measurements were above the expected median value according to CRL in 49 cases (47.6%) using CUS and in only 24 cases (23.3%) using THI.

THI leads to a small, but significant, reduction of the NT measurement and this could reduce the sensitivity of screening for Down syndrome.