Echocardiography is commonly the first-line investigation for shunts, with PC-MRI being used as an adjunct modality for certain shunts that are difficult to detect by echocardiography and also because of its quantitative abilities. PC-MRI evaluation of flow dynamics commonly involves the calculation of pulmonary to systemic shunt ratios (Qp/Qs). A shunt ratio >1.5 usually requires intervention.
4D flow has been shown to identify intracardiac shunts with accuracy equal to that of echocardiography and better than that of 2D cine PC-MRI.77 Extracardiac shunts can also be identified with sensitivity equal to that of echocardiography.77 4D flow may have a unique role in the evaluation of complex, multilevel shunts. Quantification of each individual component of flow has been described with 4D flow in the setting of anomalous pulmonary venous return and atrial septal defect.78 Total and partial anomalous pulmonary venous return can be identified by MRI with depiction of both anatomic detail and shunt quantification. Compared with conventional MRI, 4D flow allows for anomalous venous distinction from adjacent vessels without contrast and provides characterization of flow patterns and standard anatomic and shunt information.79
TOF is the most common cyanotic congenital cardiac defect. MRI is routinely used for monitoring patients after surgical repair. Analysis includes assessment of right ventricular volumes and function and quantification of pulmonary stenosis and regurgitation. In addition, asymmetric pulmonary flow can be quantified and may prompt endovascular intervention for pulmonary stenoses. 4D flow has been used to characterize abnormal flow patterns throughout the right ventricular outflow tract and pulmonary circulation after surgical repair.80,81 Even in patients without residual pulmonary stenosis after repair, elevated peak systolic velocity and abnormal vortex flow in the main pulmonary artery have been demonstrated.80 Given the previously demonstrated correlation between vortex flow and pulmonary hypertension, the presence of vortex flow in the TOF repair may offer a new risk stratification parameter beyond right ventricular function.
4D flow MRI is a rapidly advancing technique with tremendous clinical potential. New acceleration techniques allow for data acquisition in ≤15 minutes. Complex flow visualization is evolving, with consensus emerging on the best ways to characterize abnormal flow features. A range of quantitative hemodynamic markers can be calculated that extend the diagnostic possibilities of the technique. Focused clinical applications are being explored, with encouraging preliminary results. 4D flow evaluation of intracardiac flow may aid the diagnosis of diastolic dysfunction and early systolic heart failure. Valve-related aortic disease is another area of potential application, with the degree of eccentric systolic flow shown to correlate with progressive aortic dilation in patients with BAV. Larger studies are needed to convincingly demonstrate the unique abilities of 4D flow. One goal is to refine the assessment of cardiovascular disease so as to better identify responders to specific medical or surgical therapies. Another goal is to identify cardiovascular disease processes early in their course so that preemptive treatment can be undertaken.
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