Magnetic Resonance Imaging in Pulmonary Arterial Hypertension

During the last few years, magnetic resonance imaging (MRI) has emerged as a modality with enormous potential for the noninvasive evaluation of pulmonary hypertension (PH). It does not involve ionizing radiation or nephrotoxic contrast agents (unlike computed tomography or nuclear techniques) and does not have imaging window limitations (as is the case with echocardiography). These MRI techniques are being applied to assessment of the vascular structure as well as the myocardium.

High resolution MRI and magnetic resonance angiography (MRA) have been used in the imaging of atherothrombotic disease with the advantage of imaging the entire vessel wall, not just the luminal surface as is possible with conventional angiography. The combined use of these imaging modalities has been used to image the aorta and cerebral circulations to characterize plaque size and structure and ultimately regression after interventions such as statin therapy. In the peripheral arteries, MRI techniques have been used to assess response and complications of balloon angioplasty of stenotic lesions. These imaging techniques may also allow for high fidelity imaging of the coronary circulation as well.1

The fidelity of MRI techniques to assess the vasculature may also be further enhanced by the use of cell-specific contrast agents that identify a specific cell type in the vessel wall. A wide range of cellular and molecular targets including adhesion molecules, inflammatory cells, apopotic cells, matrix proteins, angiogenic proteins, and thrombosis-related proteins have been suggested to further characterize vascular lesions.1

Evolving MRI techniques may also allow for the assessment of flow through a vessel. By tagging the blood cells as they flow through a specific vessel, these techniques would allow for assessment of flow patterns in healthy as well as diseased vessels.1

MRI is rapidly evolving as a comprehensive approach to evaluating both the structure and function of the heart. This has obvious implications in the identification of left-sided cardiac disease and congenital heart lesions in the pathogenesis of PH. MRI of the right ventricle (RV) can assess mass, volume, and contractility, yielding accurate measurements that do not depend on geometrical assumptions and that correlate strongly with pulmonary pressures obtained by conventional modalities such as echo and right heart catheterization. Quantification of the anomalous interventricular septal curvature associated with RV pressure overload is highly accurate for the determination of systolic pulmonary pressure and the detection of PH.23

Contrast-enhanced magnetic resonance angiography (MRA) can be used to visualize the complete pulmonary tree and may be useful in detecting the typical changes of chronic thromboembolic PH.45 Time-resolved MRA additionally provides physiologic information of lung circulatory physiology. Phase-contrast imaging can be employed for accurate flow quantifications and calculations of QP/QS ratios.6 In addition, the flow profile in the complete cross-section of the pulmonary artery or its branches can be noninvasively evaluated, and various parameters derived from these measurements correlate with the degree of hemodynamic impairment.6

MRI techniques have also been used to determine the extent of fibrotic remodeling following myocardial infarction. MRI may also be useful in determining the extent of myocardium that is currently ischemic or at risk of ischemia. This may be important in the identification of exercise-induced PH. Both of these may have important roles in determining the degree of RV impairment in PH as MRI demonstrates the presence of fibrosis in the insertion sites of the RV in the interventricular septum, a finding correlated with the severity of PH.78 Recent advances in interventional MRI, allowing for the simultaneous quantification of pulmonary pressures with the use of MRI-compatible catheters, promise to further increase the utility of this versatile technique.

MRI has much promise in the further structural and functional characterization of the pulmonary circulation and will likely evolve as an additional important diagnostic and prognostic tool.