Phase-contrast MRI (also referred to as velocity-encoded cine MRI) is used in select clinical scenarios to quantify blood velocity and flow in the cardiovascular system. For many patients, it is an adjunct to echocardiography, which is widely available and routinely performed. But echocardiography has weaknesses, including limited acoustic windows and quantitative abilities, and phase-contrast MRI has unique advantages. We will discuss how phase-contrast MRI is currently used, and its potential in the near future to inform the clinical management of patients with cardiovascular disease.

We will focus on the emerging applications of multidimensional MR flow imaging (4D Flow). The techniques and hemodynamic biomarkers that we will discuss can be applied broadly throughout the cardiovascular system. Two key issues must be addressed when considering these applications: 1) clear advantages over ultrasound/echocardiography and 2) matching advanced imaging capabilities with clinical questions that change the management of patients with cardiovascular disease. The goal is to provide a unique understanding of how abnormal flow promotes or exacerbates disease. This understanding, in turn, could allow patients to be risk-stratified based on flow, guide medical therapy, and identify new pathways to target with drug therapy and patients that may benefit from early intervention. The outline of the talk is 1) review of two current clinical applications for MRI flow imaging and 2) discussion of four emerging applications for 4D Flow.

Current Flow Applications A short list of current clinical uses of 2D phase-contrast imaging includes quantification of valvular regurgitation and differential pulmonary flow, assessment of vascular and vascular stenoses, and cardiac shunt quantification. The lesson to be learned from each of these applications is that the flow imaging is used to assess a specific data point that directly impacts clinical management. Here we will discuss aortic coarctation and left-to-right cardiac shunts.

1. Aortic Coarctation The term refers to congenital narrowing of the distal aortic arch causing flow obstruction. Before the surgical era, people died of this disorder. Now they are repaired. The main imaging question is whether a coarctation is hemodynamically significant and needs repair. If this cannot be determined with imaging, invasive catheterization is needed. We will review the ways that MRI blood flow imaging has been used to determine if a gradient of 20 mm Hg exists across a coarct, the threshold for determining if repair/re-repair is needed.

2. Cardiac Shunt Left-to-right cardiac shunts lead to pulmonary over-circulation, hypertension and cyanosis if flow reversal occurs. Repair is indicated if the ratio of pulmonary to systemic flow (referred to as Qp/Qs) exceeds 1.5. Phase contrast is routinely used to quantify this ratio.

Emerging Flow Applications There are many. We will review 2 promising cardiac applications and 2 promising aortic flow applications.

1. Mitral Regurgitation Mitral regurgitation is the most common valvular disease, and can cause heart failure when severe. Quantifying mitral regurgitation is challenging with echocardiography. Recent literature suggests that MRI may better select candidates for surgical repair than echo. Direct assessment with 2D Flow is limited because of the considerable movement of the mitral annulus with the cardiac cycle. (Because of this, the approach employed to demonstrate the superiority of MRI over echo (Uretsky S et al. JACC 2015) does not try to directly evaluate mitral flow.) 4D Flow may allow direct mitral flow calculation with mitral annulus tracking. This could streamline the evaluation of mitral regurgitation with a single sequence, and potentially more reproducible measurements.

2. Ventricular Flow Components This is a more forward thinking cardiac application. 4D Flow can reveal distinct components of ventricular flow such as residual ventricular flow or delayed outflow that cannot be seen with other techniques. The ratio of ventricular flow components shifts with heart failure. Ratio changes may be used as a subclinical marker of early heart failure that could guide medical therapy.

3. BAV Aortopathy Bicuspid aortic valve (BAV) is the most common congenital heart defect. Up to 1 in 50 people have it. Many patients with BAV develop ascending aortic dilation. Abnormal flow and intrinsic aortic disease likely contribute. Much work has gone into using 4D Flow to assess the degree of abnormal flow with BAV. The clinical application would be to use flow analysis to identify the subset of patients most likely to have rapid progression of aortopathy, and intervene on them early.

4. Type B Aortic Dissection The traditional teaching is that type B dissection is managed medically, whereas type A (with ascending aortic involvement) needs emergent repair. But a significant number of patients with type B die at follow up. Because of this, more patients with type B have undergone endovascular stent-graft repair. Hemodynamics undoubtedly contribute to disease progression. 4D Flow may reveal unique flow environments that could single out patients for early repair.