Rapid pediatric cardiac assessment of flow and ventricular volume with compressed sensing parallel imaging volumetric cine phase-contrast MRI

Abstract

Objective: The quantification of cardiac flow and ventricular volumes is an essential goal of many congenital heart MRI examinations, often requiring acquisition of multiple 2D phase-contrast and bright-blood cine steady-state free precession (SSFP) planes. Scan acquisition, however, is lengthy and highly reliant on an imager who is well-versed in structural heart disease. Although it can also be lengthy, 3D time-resolved (4D) phase-contrast MRI yields global flow patterns and is simpler to perform. We therefore sought to accelerate 4D phase contrast and to determine whether equivalent flow and volume measurements could be extracted.

Materials and methods: Four-dimensional phase contrast was modified for higher acceleration with compressed sensing. Custom software was developed to process 4D phase-contrast images. We studied 29 patients referred for congenital cardiac MRI who underwent a routine clinical protocol, including cine short-axis stack SSFP and 2D phase contrast, followed by contrast-enhanced 4D phase contrast. To compare quantitative measurements, Bland-Altman analysis, paired Student t tests, and F tests were used.

Results: Ventricular end-diastolic, end-systolic, and stroke volumes obtained from 4D phase contrast and SSFP were well correlated (ρ = 0.91-0.95; r(2) = 0.83-0.90), with no statistically significant difference. Ejection fractions were well correlated in a subpopulation that underwent higher-resolution compressed-sensing 4D phase contrast (ρ = 0.88; r(2) = 0.77). Four-dimensional phase contrast and 2D phase contrast flow rates were also well correlated (ρ = 0.90; r(2) = 0.82). Excluding ventricles with valvular insufficiency, cardiac outputs derived from outlet valve flow and stroke volumes were more consistent by 4D phase contrast than by 2D phase contrast and SSFP.

Conclusion: Combined parallel imaging and compressed sensing can be applied to 4D phase contrast. With custom software, flow and ventricular volumes may be extracted with comparable accuracy to SSFP and 2D phase contrast. Furthermore, cardiac outputs were more consistent by 4D phase contrast.

Fig. 1

Screen captures from 4DPC custom processing software that was created for this work and enables calculation of flows and ventricular volumes. Reformatted views from a 5-year old patient with partial anomalous pulmonary venous return (high-resolution group) show multiple right-sided pulmonary veins draining into the superior vena cava (left). From the same patient, short-axis views at several cardiac phases (right) are readily reformatted with velocity overlay to facilitate identification of end-systole and end-diastole and delineate the ventricular lumen. High-velocity (150 cm/s) is color-coded in red, intermediate-velocity in green, and low-velocity in blue.

Fig. 2

Correlation of measured volumes between 4DPC and SSFP. Scatter (left) and Bland-Altman plots (right) show agreement of the two methods. Systemic measurements are displayed in red diamonds and pulmonary measurements in blue squares. End-systolic volumes are shown with closed symbols and end-diastolic volumes are shown with open symbols.

Fig. 3

Correlation of stroke volumes between 4DPC and SSFP. Scatter (left) and Bland-Altman plots (right) show agreement of the two methods. Systemic measurements are displayed in red diamonds and pulmonary measurements in blue squares. Measurements from the early population are shown with closed symbols and measurements from the later population with open symbols.

Fig. 4

Correlation of ejection fraction between 4DPC and SSFP. Scatter (left) and Bland-Altman plots (right) show modest correlation in the early population (top-right) and better correlation in the later, higher-resolution population (bottom-right), a difference that was statistically significant (p<0.05, F-test). Systemic measurements are displayed in red diamonds and pulmonary measurements in blue squares.

Fig. 5

Reformatted short axis and 3-chamber images in mid-systole from 4DPC for two matched patients with repaired tetralogy of Fallot. Images from 2009 patient #3, (top) are sufficient for characterization of left ventricular luminal size, but comparable images from the 2010 patient #2 (bottom) demonstrate improved spatial resolution and overall image quality, and allow for better delineation of the right ventricular wall. High-velocity (150 cm/s) is color-coded in red, intermediate-velocity in green, and low-velocity in blue.

Fig. 6

Correlation of flow measurements between 4DPC and 2DPC. Scatter (left) and Bland-Altman plots (right) show agreement of the two methods. On average, 2DPC measurements slightly exceeded 4DPC by 12%. Systemic measurements are displayed in red diamonds and pulmonary measurements in blue squares. Measurements from the early population are shown with closed symbols and measurements from the later population with open symbols.

Fig. 7

Comparison of cardiac outputs by phase-contrast flow and ventricular volumes. Systemic measurements are displayed in red diamonds and pulmonary measurements in blue squares. On the left, scatter plots show the correlation of measurements in the absence of significant (>10%) valvular insufficiency (closed symbols) or with insufficiency (open symbols). All open symbols are expected to be seen well-below the line of identity. On the right, Bland-Altman plots show the strength of correlation in the absence of valvular insufficiency. By the 4D method (a), all open symbols are seen below the line of identity. By the conventional method (b), two patients with significant valvular insufficiency showed outlet valve flow rates exceeding ventricular outputs. 2DPC flow rates slightly exceeded SSFP volumes by an average of 8%. Limits of agreement of cardiac output were also significantly wider by 2DPC and SSFP (p<0.05, F-test).

Fig. 7

Comparison of cardiac outputs by phase-contrast flow and ventricular volumes. Systemic measurements are displayed in red diamonds and pulmonary measurements in blue squares. On the left, scatter plots show the correlation of measurements in the absence of significant (>10%) valvular insufficiency (closed symbols) or with insufficiency (open symbols). All open symbols are expected to be seen well-below the line of identity. On the right, Bland-Altman plots show the strength of correlation in the absence of valvular insufficiency. By the 4D method (a), all open symbols are seen below the line of identity. By the conventional method (b), two patients with significant valvular insufficiency showed outlet valve flow rates exceeding ventricular outputs. 2DPC flow rates slightly exceeded SSFP volumes by an average of 8%. Limits of agreement of cardiac output were also significantly wider by 2DPC and SSFP (p<0.05, F-test).