METHODS

Patient population

We searched our CMR database to identify all patients younger than 18 years of age who underwent a CMR exam before 2018. Patients were included only if they had a normal RVOT anatomy, if they had static balanced steady state free precession (bSSFP) images performed in the strict transverse and sagittal views, and if they had 2D TTE images of the RVOT in the PSAX and PLAX views within 6 months of their CMR exam. The local ethics committee approved the study and written informed consent was waived. The study was performed in accordance with the ethical standards as laid down in the Helsinki Declaration as revised in 2013.

CMR measurements

CMR was performed on a 1.5 Tesla scanner (Magnetom Aera, Siemens Medical Systems, Erlangen, Germany). Static bSSFP images were acquired during free breathing and triggered to end-diastole. The typical imaging parameters were as follows: in-plane spatial resolution 2.0 x 1.5 mm2, slice thickness 5 mm, interslice gap 0 mm, 1 average. The RVOT diameter was measured on the strict transverse and sagittal views, perpendicular to the anterior wall of the RVOT, at the level of the aortic valve, from inner edge to inner edge, as illustrated in Figure 1.

TTE measurements

2D TTE was performed with commercially available echocardiography machines (Sonos iE33, Philips, Andover, MA, USA; Vivid E9, GE Healthcare, Milwaukee, WI, USA) using transducers of 5 and 8 MHz according to patient size. The RVOT diameter was measured at end-diastole in the PSAX and PLAX views, from inner edge to inner edge, according to the published guidelines, as illustrated in Figure 2.[2]

Statistical analysis

The measurements were indexed to the body surface area (BSA) using the Mosteller formula. The indexed diameters of the RVOT were confirmed to be normally distributed using the Kolmogorov-Smirnov test. The mean, standard deviation (SD) and reference range, using two SDs on either side of the mean, were calculated for all measurements. The RVOT diameter in the strict transverse view by CMR was compared with the RVOT diameter in the similar PSAX view by 2D TTE, and the RVOT diameter in the strict sagittal view by CMR was compared with the RVOT diameter in the similar PLAX view by 2D TTE. The correlation between the measurements was evaluated using the Pearson correlation coefficient, and the strength of the relationship was interpreted according to the published recommendations.[3] Agreement between the measurements was evaluated using the Bland-Altman plot. Significant relationships between the measurements were sought using the paired Student’s t -test.P values <0.05 were considered statistically significant.