MR phase-contrast imaging in pulmonary hypertension

Abstract

Pulmonary hypertension (PH) is a life-threatening, multifactorial pathophysiological haemodynamic condition, diagnosed when the mean pulmonary arterial pressure equals or exceeds 25 mmHg at rest during right heart catheterization. Cardiac MRI, in general, and MR phase-contrast (PC) imaging, in particular, have emerged as potential techniques for the standardized assessment of cardiovascular function, morphology and haemodynamics in PH. Allowing the quantification and characterization of macroscopic cardiovascular blood flow, MR PC imaging offers non-invasive evaluation of haemodynamic alterations associated with PH. Techniques used to study the PH include both the routine two-dimensional (2D) approach measuring predominant velocities through an acquisition plane and the rapidly evolving four-dimensional (4D) PC imaging, which enables the assessment of the complete time-resolved, three-directional blood-flow velocity field in a volume. Numerous parameters such as pulmonary arterial mean velocity, vessel distensibility, flow acceleration time and volume and tricuspid regurgitation peak velocity, as well as the duration and onset of vortical blood flow in the main pulmonary artery, have been explored to either diagnose PH or find non-invasive correlates to right heart catheter parameters. Furthermore, PC imaging-based analysis of pulmonary arterial pulse-wave velocities, wall shear stress and kinetic energy losses grants novel insights into cardiopulmonary remodelling in PH. This review aimed to outline the current applications of 2D and 4D PC imaging in PH and show why this technique has the potential to contribute significantly to early diagnosis and characterization of PH.

Figures

Figure 1.

Schematic drawing indicating haemodynamic parameters typically assessed by right heart catheterization (RHC) and clinical PH groups. Arrows sketch the localization of the origin of disease. CO, cardiac output; CTEPH, chronic thromboembolic pulmonary hypertension; LA, left atrium; mPA, main pulmonary artery; PAH, pulmonary arterial hypertension; PAP, pulmonary arterial pressure; PAWP, pulmonary arterial wedge pressure; PH-LHD, pulmonary hypertension due to left heart disease; PH-lung, PH due to lung diseases; PV, pulmonary veins; PVR, pulmonary vascular resistance; RA, right atrium; RAP, right atrial pressure; RV, right ventricle; TPG, transpulmonary pressure gradient.

Figure 2.

Principle of an electrocardiographically (ECG)-gated two-dimensional phase-contrast sequence. Segmentation is usually performed by interleaved acquisition of more (here three) data lines with and without velocity encoding per heartbeat. The principle of reconstruction of velocity (red) and magnitude (blue) images is indicated for three pixels (P) in one frame.

Figure 3.

Impact of the choice of velocity-encoding value (VENC). Three two-dimensional phase-contrast measurements through the main pulmonary artery were performed consecutively in a healthy volunteer; the same systolic phase is shown. VENC = 300 cm s−1 (left panel): maximum velocities are much smaller than VENC and cause consequently only small phase differences, low contrast to stationary tissue and bad signal-to-noise ratio. VENC = 90 cm s−1 (mid panel): maximum velocities are close to VENC and cause phase differences close to 180°, high contrast to stationary tissue and good signal-to-noise ratio. VENC = 60 cm s−1 (right panel): maximum velocities are higher than VENC and cause phase differences larger 180°, which are erroneously represented as velocities in the opposite direction. This phenomenon is called aliasing.

Figure 4.

Principle of three-directional phase-contrast imaging data acquisition. A velocity vector is specified by its two in-plane and one through-plane components.

Figure 5.

Examples of a vector plot, streamline and a particle trace visualization of the same four-dimensional phase-contrast data set of a healthy volunteer displayed on maximum intensity projection for anatomic orientation. Vector plot showing three-dimensional velocity vectors projected on a multiplanar reconstructed cut-plane through the superior vena cava (SVC), inferior vena cava (IVC), right atrium (RA) and right ventricle (RV) in end-systole. Velocity magnitude is colour encoded (a). Streamline visualization demonstrating the three-dimensional tangent curves to the velocity vectors in end-systole. Seeding points were placed in the IVC, SVC and right ventricular outflow tract; velocity magnitude is colour encoded (b). Particle trace visualization showing the three-dimensional paths of particles in end-systole, which were seeded in the IVC (blue) and SVC (red) in end-diastole.

Figure 6.

Planning of a through-plane two-dimensional phase-contrast imaging acquisition plane (a, b) in the main pulmonary artery (solid line) together with the resulting magnitude (c) and velocity (d) images. For evaluation of pulmonary arterial blood flow, the vessel cross-sectional area is usually segmented on the magnitude (solid contour) and transferred to the phase image (dotted contour).

Figure 7.

Evaluation of a through-plane two-dimensional phase-contrast measurement in the main pulmonary artery. Time course of the vessel cross-sectional area (a): area change (AC) and relative area change (RAC) are calculated from minimum cross-sectional area (Amin) and maximum cross-sectional area (Amax); time course of the maximum velocity (b): peak velocity (vpeak) is indicated; time course of the mean velocity (c): time to peak (TTP) is defined as the time interval between onset of blood flow and maximum mean velocity (vmean,max); the ejection time (ET) is time interval between onset and end (vmean = 0 cm s−1) of systolic pulmonary arterial blood flow; time course of the pulmonary blood flow (d): acceleration time (AT) is defined as time interval between onset of blood flow and peak flow (Qpeak); maximum change in flow rate during ejection (dQ/dtmax) and acceleration volume (AccV) are indicated. The subscript avg denotes the average with respect to all cardiac phases. RR, cardiac interval.

Figure 8.

Retrograde blood flow (RF) in a through-plane two-dimensional phase-contrast measurement in the main pulmonary artery (mPA) of a patient with pulmonary arterial hypertension (a). Onset time (ROT) of RF as well as the relative onset time of retrograde flow (rROT) with respect to cardiac interval (RR) can be evaluated by segmentation of RF in the velocity images (b).

Figure 9.

Vector plots of mid-systolic blood flow velocity fields in a healthy volunteer (a) and a patient with pulmonary hypertension (PH) (b) in orientation of the right ventricular outflow tract. Whereas blood motion is uniformly directed forward in the main pulmonary artery of the healthy volunteer, a vortical blood flow pattern along the main pulmonary artery is observed in the main pulmonary artery in the patient with PH.

Figure 10.

Early diastolic streamline visualizations of main pulmonary artery blood flow in two different patients with pulmonary arterial hypertension. A parallel appearance of vortical flow in the main pulmonary artery and helical flow into the right pulmonary artery (a). Pulmonary regurgitation does not hinder formation of vortical blood flow (b).

Figure 11.

Assessment of pulse-wave velocity (PWV) in the pulmonary artery by transit-time approach (a) and flow-area method (b). Δx denotes the distance between measurement positions, Δt is difference of velocity onsets. Black dots in the vessel area—flow plot display measured data points in early systole. RR, cardiac interval.

Figure 12.

Schematic drawing illustrating the definition of wall shear stress (WSS). Δv/Δx denotes the radial velocity gradient at the vessel wall. ηblood is the viscosity of blood.

Figure 13.

Planning of the through-plane two-dimensional phase-contrast imaging acquisition plane of tricuspid inflow (solid line) on early diastolic images in 4-chamber (left) and right ventricular 2-chamber (right) view (a) and delineation of the tricuspid inflow (yellow region of interest) on resulting magnitude (left) and velocity (right) images (b). Planning of the tricuspid regurgitation jet velocity on systolic images in 4-chamber (left) and right ventricular 2-chamber (right) view (c) and determination of the peak velocity (vpeak) (d). (Acquisition plane 1 cm proximal to the tricuspid valve). VENC, velocity-encoding value.