Correlation analysis of stenotic aortic valve flow patterns using phase contrast MRI

Phase contrast MRI is an emerging tool for evaluating valvular pathology. However, the effects of variable image position and valvular pathology on velocity measurements have not been explored. We compare velocity maps with correlation methods, used in image processing to align images and quantify their similarity, to define these effects on calculations of valve orifice area. Quantitative flow images were acquired in four parallel planes (2 in aortic root, 2 in outflow tract), in patients (n = 22) with aortic stenosis. Velocity-time integrals (VTIs) were computed and cross-correlations were performed to quantitatively compare the shapes and relative positions of three-dimensional flow profiles between scans at various positions. Supravalvular VTIs correlated well with one another (R = 0.96), with comparable values. The two subvalvular VTIs exhibited a linear relationship (R = 0.93) but with a 23% difference in mean values. Cross-correlations between supravalvular levels were maximized at (0, 0) offset (indicating concentrically aligned jets) for 19/23 patients, with an average maximum value of 0.957 +/- 0.028; the average for the remainder was 0.800 +/- 0.037. For subvalvular levels, all cross-correlations were maximized at (0, 0) with average maximum 0.968 +/- 0.160. The aortic VTI measurements were comparable, indicating relative insensitivity to the position of the imaging plane; in the LVOT, measurements were only somewhat position-dependent. We conclude that phase contrast MRI is a robust tool for the evaluation of aortic stenosis.