Improved quantification and mapping of anomalous pulmonary venous flow with four-dimensional phase-contrast MRI and interactive streamline rendering

Abstract

Background: Cardiac MRI is routinely performed for quantification of shunt flow in patients with anomalous pulmonary veins, but can be technically-challenging to perform. Four-dimensional phase-contrast (4D-PC) MRI has potential to simplify this exam. We sought to determine whether 4D-PC may be a viable clinical alternative to conventional 2D phase-contrast MR imaging.

Methods: With institutional review board approval and HIPAA-compliance, we retrospectively identified all patients with anomalous pulmonary veins who underwent cardiac MRI at either 1.5 Tesla (T) or 3T with parallel-imaging compressed-sensing (PI-CS) 4D-PC between April, 2011 and October, 2013. A total of 15 exams were included (10 male, 5 female). Algorithms for interactive streamline visualization were developed and integrated into in-house software. Blood flow was measured at the valves, pulmonary arteries and veins, cavae, and any associated shunts. Pulmonary veins were mapped to their receiving atrial chamber with streamlines. The intraobserver, interobserver, internal consistency of flow measurements, and consistency with conventional MRI were then evaluated with Pearson correlation and Bland-Altman analysis.

Results: Triplicate measurements of blood flow from 4D-PC were highly consistent, particularly at the aortic and pulmonary valves (cv 2-3%). Flow measurements were reproducible by a second observer (ρ = 0.986-0.999). Direct measurements of shunt volume from anomalous veins and intracardiac shunts matched indirect estimates from the outflow valves (ρ = 0.966). Measurements of shunt fraction using 4D-PC using any approach were more consistent with ventricular volumetric displacements than conventional 2D-PC (ρ = 0.972-0.991 versus 0.929).

Conclusion: Shunt flow may be reliably quantified with 4D-PC MRI, either indirectly or with detailed delineation of flow from multiple shunts. The 4D-PC may be a more accurate alternative to conventional MRI.

Keywords: flow; pulmonary; shunt; structural; veins.

Figure 1. Representative images from two infants with unrepaired partial anomalous pulmonary venous return and sinus venosus atrial septal defects

Color velocity and vector-rendered images are shown in the upper and lower panels, respectively. Left: 7 minute 1.5T scan of 18 month old boy (10 kg, 0.5 m²) with 132 bpm heart rate. Right: 3.5 minute 3T scan of a 6 month old boy (8.33 kg, 0.4m²) with 126 bpm heart rate. Note the smoother color-rendering and qualitative consistency of vector field seen at 3T.

Figure 2. Interobserver consistency of flow measurements

Scatter plots display the consistency of flow measurements obtained by each observer at each location and of indirect measurements of shunt fraction and shunt volume. The mean of triplicate measurements by observer 1 (x-axis) were in close agreement with single measurements by observer 2 (y-axis).

Figure 3. Streamline mapping of pulmonary veins to their receiving atrial chambers

Right-ventricular 3-chamber and oblique coronal views in mid-diastole with (a) grayscale anatomic rendering, (b) color speed overlay, and (c, d) superimposed streamlines show the incremental value of each rendering mode for mapping flow from right upper pulmonary veins to the receiving chamber. Top: 3 year old boy with unrepaired anomalous right upper pulmonary veins, scanned at 3T. Bottom: 21 year old woman after surgical baffle of the anomalous right upper pulmonary veins to the left atrium, scanned at 1.5T. Note smoother streamlines with the higher signal-to-noise acquisition afforded by the scan performed at 3T.

Figure 4. Streamline mapping of pulmonary veins to their receiving atrial chambers

In (a), 4-chamber views during diastole show anomalous right pulmonary veins drain into the right atrium (arrows) and an intact atrial septum. In (b), 4-chamber views during diastole in another patient also show right pulmonary veins drain to the right atrium. The left pulmonary veins drain normally into the left atrium, though some flow arrives in the right atrium through a sinus venosus ASD (arrow). In (c), oblique coronal views from a third patient during systole (left) and diastole (middle) show blood from anomalous pulmonary veins and the SVC draining into both atria through a straddling sinus venosus defect (arrow). Each of the left and one right pulmonary vein drain into the left atrium (right).

Figure 5. Consistency of shunt fraction (top) and shunt volume (bottom) with ventricular volume displacements from SSFP imaging

Bland-Altman plots show that measurements of shunt fraction by conventional phase-contrast and 4D-PC by each observer (middle, right) correlated well with the ratio of right-to-left ventricular stroke volumes. Shunt fractions measured as a ratio of triplicate measurements on 4D-PC by observer 1 were more tightly correlated to ventricular displacements than 2D-PC (p<0.05, F-test). Single measurements on 4D-PC by observer 2 also had narrower limits of agreement to ventricular displacement than 2D-PC, but did not achieve statistical significance