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 goal of the talk is 1) to review of current clinical applications for MRI flow imaging and 2) to discuss 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 flow imaging is used to assess a specific data point that directly impacts clinical management.
1. Cardiac Shunts 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.
2. Complex Congenital Heart Disease Fontan palliation for single ventricle physiology and repair of Tetralogy of Fallot are two of the more common indications for imaging in this category. Multiple 2D phase contrast planes are needed to characterize and quantify flow through conduits and post-surgical anatomy. 4D Flow may improve the efficiency of this analysis.
3. Valvular Disease Regurgitation can be more reliably quantified with MR flow imaging than with echocardiography. Data has emerged that conventional phase contrast imaging has an important clinical role for assessment of both aortic and mitral regurgitation (Myerson SG et al. Circulation 2012; Uretsky S et al. JACC 2015). Mitral regurgitation is promising target for 4D Flow. Direct assessment with conventional phase contrast imaging is limited because of the considerable movement of the mitral annulus with the cardiac cycle. 4D Flow may allow direct mitral flow calculation with mitral valve tracking. We will review the two approaches to dynamic valve tracking: annulus versus flow tracking. Applying valve tracking clinically could streamline the evaluation of mitral regurgitation with a single sequence, and potentially more reproducible measurements.
Emerging Flow Applications There are many. We will only have time to review the role of eccentric flow in risk stratifying patients with BAV Aortopathy.
1. 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 heart failure that could guide medical therapy.
2. 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.
3. 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.
5 Review Articles to Check Out:
1. Dyverfeldt P, Bissell M, Barker AJ, Bolger AF, Carlhäll CJ, Ebbers T, Francios CJ, Frydrychowicz A, Geiger J, Giese D, Hope MD, Kilner PJ, Kozerke S, Myerson S, Neubauer S, Wieben O, Markl M. 4D flow cardiovascular magnetic resonance consensus statement. J Cardiovasc Magn Reson. 2015; 17(1):72. PMID: 26257141.
2. Burris NS, Hope MD. 4D flow MRI applications for aortic disease. Magn Reson Imaging Clin N Am. 2015 Feb; 23(1):15-23. PMID: 25476670.
3. Hope MD, Sedlic T, Dyverfeldt P. Cardiothoracic magnetic resonance flow imaging. J Thorac Imaging. 2013 Jul; 28(4):217-30. PMID: 23708687.
4. Hope MD, Wrenn SJ, Dyverfeldt P. Clinical Applications of Aortic 4D Flow Imaging. Curr Cardiovasc Imaging Rep. 2013; 6(2):128-39.
5. Markl M, Frydrychowicz A, Kozerke S, Hope M, Wieben O. 4D flow MRI. J Magn Reson Imaging. 2012 Nov; 36(5):1015-36. PMID: 23090914.