Pulmonary Arterial Hypertension: Noninvasive Detection with Phase-Contrast MR Imaging

Pulmonary MR angiography techniques and applications

This article discusses the role of magnetic resonance angiography (MRA) in evaluating the pulmonary arterial system. For depiction of pulmonary arterial anatomy and morphology, MRA techniques are compared with CT angiography and digital subtraction x-ray angiography. Perfusion, flow, and function are emphasized, as the integrated MR examination offers a comprehensive assessment of vascular morphology and function. Advances in MR technology that improve spatial and temporal resolution and compensate for potential artifacts are reviewed as they pertain to pulmonary MRA. Current and emerging gadolinium contrast-enhanced and non-contrast-enhanced MRA techniques are discussed. The role of pulmonary MRA, clinical protocols, imaging findings, and interpretation pitfalls are reviewed for clinical indications.

[Interstitial lung diseases and pulmonary hypertension]

The prevalence of pulmonary hypertension in interstitial lung disease (ILD) is high (30-40%). However, diagnosis of pulmonary hypertension in ILD is often delayed. Pulmonary hypertension can occur in the absence of advanced pulmonary dysfunction or severe hypoxia and is associated with a worse prognosis. A number of pathogenic mechanisms such as oxidative stress, cytokines, and the endothelin system have been implicated in remodeling of both the lung parenchyma and the vessels. In addition, hypoxic vasoconstriction, vascular destruction and progressive fibrosis play an important role. Since clinical signs are often non-specific echocardiography, radiology and laboratory parameters such as NT-proBNP may be helpful. However, the definitive diagnosis of pulmonary hypertension is still confirmed by right heart catheterization. Treatment options of pulmonary hypertension in ILD are limited to the treatment of the underlying diseases. Newer vasodilating drugs may improve the prognosis but have first to be evaluated in clinical trials. Lung or lung and heart transplantation is the therapeutic option in end stage disease.

Evaluation of pulmonary artery stiffness in pulmonary hypertension with cardiac magnetic resonance

OBJECTIVES

This study sought to evaluate indexes of pulmonary artery (PA) stiffness in patients with pulmonary hypertension (PH) using same-day cardiac magnetic resonance (CMR) and right heart catheterization (RHC).

BACKGROUND

Pulmonary artery stiffness is increased in the presence of PH, although the relationship to PH severity has not been fully characterized.

METHODS

Both CMR and RHC were performed on the same day in 94 patients with known or suspected PH. According to the RHC, patients were classified as having no PH (n = 13), exercise-induced PH (EIPH) only (n = 6), or PH at rest (n = 75). On CMR, phase-contrast images were obtained perpendicular to the pulmonary trunk. From CMR and RHC data, PA areas and indexes of stiffness (pulsatility, compliance, capacitance, distensibility, elastic modulus, and the pressure-independent stiffness index beta) were measured at rest.

RESULTS

All quantified indexes showed increased PA stiffness in patients with PH at rest in comparison with those with EIPH or no PH. Despite the absence of significant differences in baseline pressures, patients with EIPH had lower median compliance and capacitance than patients with no PH: 15 (interquartile range: 9 to 19.8) mm2/mm Hg versus 8.4 (interquartile range: 6 to 10.3) mm2/mm Hg, and 5.2 (interquartile range: 4.4 to 6.3) mm3/mm Hg versus 3.7 (interquartile range: 3.1 to 4.1) mm3/mm Hg, respectively (p < 0.05). The different measurements of PA stiffness, including stiffness index beta, showed significant correlations with PA pressures (r2 = 0.27 to 0.73). Reduced PA pulsatility (<40%) detected the presence of PH at rest with a sensitivity of 93% and a specificity of 63%.

CONCLUSIONS

Pulmonary artery stiffness increases early in the course of PH (even when PH is detectable only with exercise and before overt pressure elevations occur at rest). These observations suggest a potential contributory role of PA stiffness in the development and progression of PH.

Role of cardiac magnetic resonance imaging in the management of patients with pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disorder characterized by abnormally elevated blood pressure of the pulmonary circulation that results, over time, from extensive vascular remodeling and increased pulmonary vascular resistance. Recent advances in magnetic resonance imaging (MRI) technology have led to the development of techniques for noninvasive assessment of cardiovascular structure and function, including hemodynamic parameters in the pulmonary circulation, which are superior in their identification of right ventricular morphologic changes. These advantages make cardiac MRI an attractive modality for following up and providing prognoses in patients with PAH. In this review, we summarize recent developments in the use of MRI for the diagnosis, assessment, and ongoing monitoring of patients with PAH. Over the coming decade, it can be anticipated that continued improvements in MRI image acquisition, spatial and temporal resolution, and analytical techniques will result in improved understanding of PAH pathophysiology, diagnosis, and prognostic variables, and will supplement, and may even replace, some of the invasive procedures currently applied routinely to the evaluation of PAH.

Role of cardiac magnetic resonance imaging in assessment of nonischemic cardiomyopathies

Diagnosis of nonischemic cardiomyopathy is a challenging process that influences patient morbidity and mortality. Currently, the well known World Health Organization classification has been revisited by an American Heart Association expert consensus panel. The contemporary classification is compatible with the rapid evolution in molecular genetics and evolving diagnostic tools such as cardiac magnetic resonance imaging (MRI). Magnetic resonance imaging is a robust diagnostic tool that offers various techniques to assess the function, morphology, perfusion, and scarring of myocardial tissue thus providing better understanding of the underlying causes of nonischemic cardiomyopathies. In this review, we discuss the current role of cardiac MRI in the evaluation of nonischemic cardiomyopathy, in the context of the current American Heart Association classification of these disorders.

Nonenhanced MR angiography

While nonenhanced magnetic resonance (MR) angiographic methods have been available since the earliest days of MR imaging, prolonged acquisition times and image artifacts have generally limited their use in favor of gadolinium-enhanced MR angiographic techniques. However, the combination of recent technical advances and new concerns about the safety of gadolinium-based contrast agents has spurred a resurgence of interest in methods that do not require exogenous contrast material. After a review of basic considerations in vascular imaging, the established methods for nonenhanced MR angiographic techniques, such as time of flight and phase contrast, are considered and their advantages and disadvantages are discussed. This article then focuses on new techniques that are becoming commercially available, such as electrocardiographically gated partial-Fourier fast spin-echo methods and balanced steady-state free precession imaging both with and without arterial spin labeling. Challenges facing these methods and possible solutions are considered. Since different imaging techniques rely on different mechanisms of image contrast, recommendations are offered for which strategies may work best for specific angiographic applications. Developments on the horizon include techniques that provide time-resolved imaging for assessment of flow dynamics by using nonenhanced approaches.

Prevalence and correlates of septal delayed contrast enhancement in patients with pulmonary hypertension

Using cardiac magnetic resonance, the presence of myocardial delayed contrast enhancement (DCE) has been described in the ventricular septum at the level of the right ventricular insertion points in patients with pulmonary hypertension (PH). The aim of this study was to investigate the prevalence, extent, and correlates of this finding. Septal DCE was evaluated in 55 patients with known or suspected PH of various causes. The extent of DCE was estimated visually with an insertion enhancement score (range 0 to 4) and quantified as DCE mass. The results were correlated with cine magnetic resonance and right-sided cardiac catheterization. Predictors of DCE were investigated using multivariate analysis. PH at rest was present in 42 patients (group 1) and absent in 13 (group 2). DCE was noted in 41 patients (97%) in group 1 and 3 (23%) in group 2 (p <0.0001). The extent of DCE was higher in group 1 than group 2 (median insertion enhancement score 3 vs 0, median DCE mass 8.7 vs 0 g, respectively; p <0.0001 for both). The extent of DCE showed moderate to good univariate correlations (r = 0.5 to 0.73) with pulmonary pressures and with right ventricular volumes, mass, and ejection fractions. In multivariate analysis, systolic pulmonary pressure was the only predictor of DCE. In conclusion, the presence of septal DCE at the right ventricular insertion points is common in PH of different causes, and the level of systolic pulmonary pressure elevation appears to be the main determinant of this finding.