## 2 Materials and Methods
### MRI Acquisition and Data Analysis  
A 1.5 Tesla MR system (SIGNA Artist, GE Healthcare) was used to acquire all CMR images. The routine measurement was carried out using a standard balanced steady state-free precession (SSFP) cine image technique with a retrospective electrocardiogram (ECG) gating and the calculation of cardiac output using a segmented free-breathing fast imaging employing steady-state acquisition (FIESTA) sequence. Free-breathing sequences were used instead of breath-holding sequences wherever possible due to the difficulty of breath-holding for children, especially those with DMD [28]. The parameters of the scan were as follows: repetition time (TR)/echo time (TE) = 4.1 ms/1 ms, flip angle = 50\({}^{\circ}\), slice thickness = 8 mm, matrix = 192 \(\times\) 160 pixels, and field of view (FOV) = 270 \(\times\) 100 mm\({}^{2}\). Late gadolinium enhancement (LGE) sequences were realized 10 min after gadolinium injection using two LGE sequences to maximize the detection of myocardial fibrosis. Single-shot myocardial delayed enhancement (SS-MDE) sequence in short-axis view and free-breathing phase-sensitive inversion-recovery (PSIR) views were used (Figure 1). For the SS-MDE sequence, the used parameters were TR/TE = 3.8 ms/1 ms, flip angle = 40\({}^{\circ}\), slice thickness = 8 mm; matrix = 160 \(\times\) 128 pixels and field of view 280 \(\times\) 100 mm\({}^{2}\); for the PSIR sequence, TR/TE = 3.9 ms/1 ms, flip angle = 45\({}^{\circ}\), slice thickness = 8 mm, matrix = 160 \(\times\) 128 pixels, and field of view 280 \(\times\) 100 mm\({}^{2}\).  
The images underwent analysis utilizing Circle Cardiovascular Imaging (CVI42) software (Calgary, Alberta, Canada). The structure and function of both the left and right ventricles were measured on short-axis cine images. Endocardial and epicardial contours  
Figure 1: On the top row: (**A**) free-breathing FIESTA in the LV short-axis view, (**B**) **4**-chamber PSIR, and (**C**) short-axis SS-MDE showing a moderate LGE in the epicardial layer of the basal inferolateral segment (arrow). On the bottom row: strain analysis of LV endocardi (in red) and epicardium (in green) in short-axis view; (**D**) limit points of the myocardium, (**E**) tracking mesh, and (**F**) circumferential strain map.  
were applied during the end-systole (ES) and end-diastole (ED) phases. Manual verification and correction of phases and contours were then performed. Papillary muscles were not considered in the assessment of ventricular volume. The following cardiac measurements were obtained: LV volumes and ejection fraction.  
CMR-FT measurements were performed in the post-processing phase after the specified study was finished using CVI42. Thus, using cine images and endocardium and epicardium contours were identified along the cardiac cycle for short- and long-axis slices (Figure 1). The basal level was obtained by slicing the heart horizontally near its base, including the ventricles' upper sections. The mid-ventricular level was taken at a specific cross-section of the heart, usually around the papillary muscles. The apical level was taken at the apex, the lower tip of the heart. The GCS and GRS were analyzed in short-axis slices (basal, mid-ventricular, and apical), and the GLS was assessed in long-axis slices (2-chamber, 3-chamber, 4-chamber).