Cardiac Magnetic Resonance and Feature Tracking

All CMR examinations were performed with a 1.5T scanner (Signa HDxt and MRI 450, GE Medical Systems, Milwaukee, WI, USA) using a 32-channel phased array cardiac coil. For measuring ventricular function, steady-state free precession (SSFP) cine images were acquired in a horizontal and vertical long-axis plane, as well in a short-axis plane covering the entire single ventricle. The SSFP parameters were as follows: retrospective cardiac gating, 40 phases/cardiac cycle, TE 1.5–1.8 ms, TR 2.8–3.1 ms, flip angle 45°, bandwidth 125 kHz, matrix 224 × 224, number of excitations 1, field of view 250–350, views per segment 4–10 according to heart rate. In-plane resolution was 1–1.5 mm and, true temporal resolution was < 25 ms.

Myocardial strain was measured by applying feature tracking on SSFP cine images during post-processing with a dedicated software (Qstrain, Medis Version 3.3, Leiden, Netherlands). Circumferential strain (CS) was measured on short-axis images and longitudinal strain (LS) was measured on long-axis images. The endocardial borders of the SV were first segmented manually in the end-diastolic phase and subsequently expanded to all phases using an automatic border detection algorithm. The endocardial borders were checked for adequacy in all cardiac phases and manually corrected if necessary. The software provided endocardial peak systolic strain.

Measurement of radial strain has previously not been recommended due to inaccuracy and variability of the measurements [12]. Measurement of global circumferential strain (GCS) has been recommended at mid-ventricular level [13]. In our cohort, we performed preliminary analysis of 40 patients and have observed a large variability of the measurements at the basal or apical level, but more robust data at mid-ventricular level. Thus, all values of GCS used for the study were taken at mid-ventricular level. Reference values for both LV [14] and RV [15] are available for adults, while pediatric reference values are available only for the LV [16].