LAUSR

Magnetic resonance imaging (MRI) has been widely used for the assessment of the function and structure of the cardiovascular system. Compared with other noninvasive imaging techniques, MRI can provide both quantitative and qualitative information, resulting in superior diagnostic and prognostic accuracy. This is especially true for the right ventricle (RV) which, due to its complex three-dimensional (3D) shape and retrosternal location, is difficult to accurately assess with echocardiography. As such, MRI is considered the gold standard for both qualitative and quantitative RV analysis. Accurate RV analysis is critical in the setting of pulmonary hypertension (PH) as increases in normalized right ventricular volume indicate decoupling, a harbinger of impending RV failure and death. Four-dimensional (4D) flow based on time-resolved 3D phase-contrast MRI, as a new MRI sequence, offers the ability to measure and to visualize the temporal evolution of complex blood flow patterns within an acquired 3D volume, which helps one understand blood flow changes and provides accurate flow values. This modality has enabled to assess aortic and pulmonary artery flows, which is associated with many cardiopulmonary diseases. However, integrating 4D flow MRI into routine clinical practice for the diagnosis of PH still requires further research investigation or validation due to its longer acquisition time, complicated velocity encoding along all three dimensions, and motion-related artifacts. In this issue of JMRI, an article “Evaluation of pulmonary hypertension using 4D flow MRI” by Cerne et al, explored 4D flow MRI-derived pulmonary artery hemodynamics to diagnose and classify PH among WHO groups 1, 2, 3, and 4 PH patients, and controls by comparing the clinically established reference data, which were collected using the invasive right heart catheterization (RHC) method. RHC is currently a gold standard for the definitive diagnosis and follow-up of PH by measuring mean pulmonary arterial systolic pressure. Authors identified quantitative parameters, such as peak velocity and mean velocity, from 4D flow suitable to characterize and reliably diagnose PH and demonstrated that WHO group 1 patients had lower values than control subjects. The preliminary findings suggest a potential role for 4D flow MRI in distinguishing WHO group 1 patients from other WHO groups, as well as a potential ability to noninvasively monitor treatment response by using 4D flow velocities to track pulmonary vascular resistance (PVR). The importance of accurate, MRI-derived RV volume and function parameters in the setting of PH cannot be overstated. Other than MRI-derived biomarkers such as pulmonary artery stiffness, a growing list of 4D flow-derived parameters such as wall shear stress, vortical flow, turbulent kinetic energy, etc. and even machine-learning derived parameters, such as 3D RV motion phenotypes, are proving useful in PH patient management. MRI-derived biomarkers are a major step forward in the role of MRI in PH. This is not only the case because it allows for extraction of a variety of biomarkers, but also because the retrospective derivation of these parameters from a 4D flow data set suggests the possibility of a relatively quick, one-sequence PH MRI analysis. Although the 4D sequence itself takes several minutes to run, advances in technology such as compressed sensing/parallel imaging allow such sequences to be run in less than 10 minutes. Due to the obviation of running multiple sequences in multiple imaging planes, the overall scan time can be decreased at the expense of time required for postprocessing. Imagine what being able to scan three PH patients in a standard 45 minutes MRI time slot would do for availability of this modality for these patients. In addition to freeing up the scanner resource to scan more patients, moving this time penalty into the postprocessing space puts it into a space where advances in machine learning enabled automated and semi-automated postprocessing can reduce this time penalty and improve reproducibility. As 4D flow sequences and postprocessing software become more broadly available, 4D flow MRI will likely become an essential component of cardiac imaging for routine clinical practices involved in the management of PH and acquired structural heart disease. The publication by Cerne et al is an important step in that direction. The limitations of the study are the relatively low number of cases in each group being analyzed and the long-time interval between the RHC and the MRI acquisition. This paper would attract more readers and show great values with a higher volume of patients in each class. However, Cerne et al provided initial evidence and quantitative parameters for