## 3 Topic II: Advances in Magnetic Resonance Imaging  
The magnetic field properties of non-invasive MRIs cause the body's hydrogen atoms to align temporarily. Radiofrequency pulses are then applied to stimulate the protons out of equilibrium. The frequential response signal is collected and used to reconstruct an image of the region of interest. Cardiovascular MRIs provide detailed images to examine the heart's function, structure, cardiac motion, and tissue damage of the cardiac muscle. An additional advantage is the lack of radiation and use of iodine contrast. Left ventricular hypertrophy (LVH) is presented in between 15% and 20% of the population. LVH is more frequent in patients with hypertension and obesity and in Afro-American people. Cardiac MRIs provide a comprehensive assessment of myocardial hypertrophy. Cardiac MRI has become the gold standard for ventricular volume quantification. Accurate measurements of the wall thickness, hypertrophy pattern, chamber size, and ventricular function are necessary for the proper characterization of LVH. These measurements often require the delineation of the region of interest in a large number of images per patient. In recent years, machine learning (ML) methods have been used to accelerate the segmentation of the ventricles for LV function quantification. In this Special Issue, [PERSON] et al. aimed to automatically detect LVH in cardiac MRI scans by applying ML methods [6]. The authors developed an ensemble model based on a 3D ResNet. The study included 428 patients with LVH under various conditions and 234 healthy controls without any known cardiovascular diseases. The developed model achieved a 92% F1 score and 97% recall using the testing dataset, which are similar to those of expert reports. This study is an excellent example of the clinical cardiac applications of ML. Standard-of-care cardiac MRI protocols typically use 2D phase contrast (PC) to assess heart hemodynamics. However, 2D PC only acquires the through-plane velocity for the flow quantification in the region/vessel of interest. A whole heart 3D hemodynamic acquisition of the velocity field can be achieved using a 4D-flow MRI. This Special Issue collected four studies investigating the clinical applications, validation, and novel approaches for 4D-flow MRI. The study by [PERSON] et al. [7] aimed to investigate the hemodynamic changes within the PA and its association with ventricular remodeling and vascular architecture. This study included 33 patients with PAH and 17 healthy controls. The patients presented alterations in the PA, showing larger distances of PA bifurcation. In addition, the wall shear stress (WSS), kinetic energy (KE), and energy loss (EL) values decreased further in the PAH patients compared to those of the controls. The PA 4D-flow derived velocities were associated with RV ejection fraction. This study illustrated the potential usefulness of 4D-flow MRI for the hemodynamic assessment of the right heart, which is often more challenging to assess than the left heart. One important aspect in the 4D-flow acquisition planning is the subject's respiratory motion. Respiratory gating is typically used to reduce the effects of respiratory motion and artifacts. [PERSON] et al. [8] performed a comparison of 4D-flow acquisitions with and without respiratory self-gating and its impact on the 4D-flow derived parameters. This study was conducted with 15 healthy controls. Their findings showed that the acquisition without respiratory self-gating underestimated the values in the aortic arch, the descending aorta, and diaphragmatic aorta. Both the acquisitions with and without respiratory self-gating showed a significant variability in the 4D-flow-derived parameters. However, respiratory compensation provided a more reliable quantification. Advanced cardiac hemodynamic characterization ideally requires high temporal and spatial resolution datasets. A standard 4D-flow MRI acquisition can have >4000 images with an isotropic spatial resolution of 2-3 mm per voxel and 25-45 ms of temporal resolution.