Statistical analysis
Image-quality scores were expressed as mean ± standard deviation (SD) or
mean (95% confidence interval). Subgroups defined by trimester, time
period, center, operator and cardiac position were compared using
t-tests or ANOVA. When the ANOVA test was significant, a post-hoc test
with Bonferroni correction was used for pairwise comparisons. When
studied separately, quality criteria were expressed as a percentage for
the three time periods and according to cardiac position.
To estimate intra-rater and inter-rater variability of the image-quality
score, the intraclass correlation coefficient (ICC [95% confidence
interval]) was estimated based on single-rating, absolute-agreement,
2-way mixed- (intra-rater) and random- (inter-rater) effects models. To
estimate intra-rater and inter-rater variability of the individual
(binary) quality criterion, Kappa coefficient was used.
All statistical analysis were carried out using IBM SPSS 20.0 for
Windows software (IBM Inc., New York, USA). A value of p<0.05
was considered statistically significant.
Funding and conflicts of
interest
The authors confirm that they have no conflicts of interest in this
study.