Comparison of respiratory motion suppression techniques for 4D flow MRI

Abstract

Purpose: The purpose of this work was to assess the impact of respiratory motion and to compare methods for suppression of respiratory motion artifacts in 4D Flow MRI.

Methods: A numerical 3D aorta phantom was designed based on an aorta velocity field obtained by computational fluid mechanics. Motion-distorted 4D Flow MRI measurements were simulated and several different motion-suppression techniques were evaluated: Gating with fixed acceptance window size, gating with different window sizes in inner and outer k-space, and k-space reordering. Additionally, different spatial resolutions were simulated.

Results: Respiratory motion reduced the image quality. All motion-suppression techniques improved the data quality. Flow rate errors of up to 30% without gating could be reduced to less than 2.5% with the most successful motion suppression methods. Weighted gating and gating combined with k-space reordering were advantageous compared with conventional fixed-window gating. Spatial resolutions finer than the amount of accepted motion did not lead to improved results.

Conclusion: Respiratory motion affects 4D Flow MRI data. Several different motion suppression techniques exist that are capable of reducing the errors associated with respiratory motion. Spatial resolutions finer than the degree of accepted respiratory motion do not result in improved data quality. Magn Reson Med 78:1877-1882, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Keywords: 4D flow MRI; MR flow imaging; artifacts; image quality; motion suppression; navigator gating.

Figure 1

Mean intensity projections of velocity images for reference image without motion (A), no gating (B), gating with 10 mm window (C), weighted gating with 10 mm inner window no outer window (D), ROPE (E), gating with 10 mm window combined with ROPE (F), gating with 10 mm window and voxel size reduced to 10 mm in feet‐to‐head direction (G), and no gating and voxel size reduced to 10 mm in feet‐to‐head direction (H).

Figure 2

Root mean square error (RMSE) versus scan time for magnitude (A) and velocity (B), and velocity gradient (C) data relative to the reference image without motion. Different scan times were obtained by using different motion suppression techniques and gating window sizes; tighter windows are associated with the longer scan times. Red line: Conventional fixed‐window gating for gating windows of 2.2.5, 7.5, 12.5, 17.5, and 22.5. Green line with asterisks: Weighted gating for gating windows of 2.5, 7.5, 12.5, 17.5, and 22.5 in the inner 50% of k‐space and 3 × the inner gate in the outer 50% of k‐space. Green line with squares: Weighted gating for gating windows of 2.5, 7.5, 12.5, 17.5, and 22.5 in the inner 9% of k‐space and 3 × the inner gate in the outer 91% of k‐space. Blue line: Fixed‐window gating combined with k‐space‐reordering method ROPE for gating windows of 2.5, 7.5, 12.5, 17.5, and 22.5. Magenta asterisk: ROPE. Black asterisk: No gating.

Figure 3

Aorta peak velocity (A) and flow rate estimates in the ascending aorta (B), aortic arch (C) and descending aorta (D) plotted versus scan time. Different scan times were obtained by using different motion‐suppression techniques and gating thresholds or voxel sizes. Red line: Conventional fixed‐window gating for gating windows of 2.5, 7.5, 12.5, 17.5, and 22.5. Green line with asterisks: Weighted gating for gating windows of 2.5, 7.5, 12.5, 17.5, and 22.5 in the inner 50% of k‐space and 3 × the inner gate in the outer 50% of k‐space. Green line with squares: Weighted gating for gating windows of 2.5, 7.5, 12.5, 17.5, and 22.5 in the inner 9% of k‐space and 3 × the inner gate in the outer 91% of k‐space. Dark blue line: Fixed‐window gating combined with k‐space‐reordering method ROPE for gating windows of 2.5, 7.5, 12.5, 17.5, and 22.5. Magenta asterisk: ROPE. Black asterisk: No gating. Light blue line: Fixed‐window gating with 10‐mm gating window and feet‐to‐head voxel sizes of 2.5, 5, 10, 15, and 20 mm.

Figure 4

Effect of spatial resolution on root mean square error (RMSE) of magnitude (A), velocity (B), and velocity gradient (C) data relative to the reference image without motion for different motion suppression methods. The horizontal axis denotes voxel size in the feet‐to‐head direction. Red lines: Conventional fixed‐window gating for gating windows of 5 (diamonds), 10 (circles), 15 (squares), and 20 (asterisks). Green line: Weighted gating for gating windows of 10 mm the inner 50% of k‐space and 30 mm in the outer 50% of k‐space. Magenta line: ROPE. Dark blue line: Fixed‐window gating with 10 mm window combined with k‐space‐reordering method ROPE. Black line: No motion.