Whole-heart coronary MR angiography using a 3D cones phyllotaxis trajectory

Abstract

Purpose: To develop a 3D cones steady-state free precession sequence with improved robustness to respiratory motion while mitigating eddy current artifacts for free-breathing whole-heart coronary magnetic resonance angiography.

Method: The proposed sequence collects cone interleaves using a phyllotaxis pattern, which allows for more distributed k-space sampling for each heartbeat compared to a typical sequential collection pattern. A Fibonacci number of segments is chosen to minimize eddy current effects with the trade-off of an increased number of acquisition heartbeats. For verification, phyllotaxis-cones is compared to sequential-cones through simulations, phantom studies, and in vivo coronary scans with 8 subjects using 2D image-based navigators for retrospective motion correction.

Results: Simulated point spread functions and moving phantom results show less coherent motion artifacts for phyllotaxis-cones compared to sequential-cones. Assessment of the right and left coronary arteries using reader scores and the image edge profile acutance vessel sharpness metric indicate superior image quality and sharpness for phyllotaxis-cones.

Conclusion: Phyllotaxis 3D cones results in improved qualitative image scores and coronary vessel sharpness for free-breathing whole-heart coronary magnetic resonance angiography compared to standard sequential ordering when using a steady-state free precession sequence.

Keywords: 3D cones trajectory; coronary MRA; free-breathing; motion artifacts; phyllotaxis.

FIG 1.

The plots above show how the polar angle (azimuthal angle (φ) and elevation angle (θ)) is defined (a) and the 18 cone polar angles (b, c) on the unit sphere and readouts (d, e) acquired during the 606^th heartbeat for both methods: sequential (b, d) and phyllotaxis (c, e) respectively. In plots (e) and (d), phyllotaxis (e) is more distributed in k-space compared to sequential (d) per heartbeat.

FIG 2.

In the histogram plot (a), the readout distributions for both sequential and phyllotaxis are shown (using a bin width of π/250 radians) which correspond to the elevation functions in (b) and (c). The graph in (b) shows the elevation functions for sequential (blue) and phyllotaxis (red) for readouts 1–200. The graph shows how sequential-cones has coarser sampling in elevation angle while phyllotaxis-cones is more smoothly sampled. The graph in (c) shows the elevation function for all 10,980 readouts and example elevation angles for segments (heartbeats 1 and 305) in the phyllotaxis design.

FIG 3.

The plots above show a top view of the sequential (a) and the phyllotaxis (b) polar angle patterns on the unit sphere corresponding to each cone readout.

FIG 4.

The log scale PSFs without motion. The central axial and sagittal/coronal slices through the 3D PSF are shown for sequential (a, d) and phyllotaxis (b, e) acquisition methods. The central axial and sagittal/coronal profiles for both methods are shown in (c, f) which have similar central and side lobes.

FIG 5.

The log scale PSFs with S/I motion derived from (in vivo) 2D iNAVs (Fig. S3a). The central axial and sagittal/coronal slices through the 3D PSF are shown for sequential (a, d) and phyllotaxis (b, e) acquisition methods. In the sagittal and coronal slices, motion artifacts are more concentrated along the vertical axis (motion direction) for sequential whereas the motion is incoherently spread around the center of k-space for phyllotaxis. The central axial and sagittal/coronal profiles for both methods are shown in (c, f). In the sagittal/coronal profile (f), more energy is outside the central lobe for sequential (arrows) which corresponds to more blurring/streaking due to motion compared to phyllotaxis.

FIG 6.

Phantom images acquired on a stationary (a, d) and an oscillating (b, e) scanner table moving 25 mm in the z-direction at a peak velocity of 13 mm/s for both sequential (b, c) and phyllotaxis (e, f). Before motion correction, sequential (b) contains severe streaking artifacts while phyllotaxis (e) contains blurring and incoherent noise artifacts which are highlighted in the corresponding yellow and green boxes. The motion corrected images for sequential and phyllotaxis are shown in (c, f), respectively, after 2D iNAV-based translational motion correction. Small residual motion artifacts with similar structure as (b, e) are shown for sequential (yellow box) and phyllotaxis (green box) in (c, f) respectively.

FIG 7.

The images above show in vivo data for central sagittal, coronal, and axial slices when using sequential (a-c) and phyllotaxis (d-f) acquisition methods before correcting for motion. Similar to the PSFs, sagittal and the coronal slices are worse for sequential (a, b) compared to phyllotaxis (d, e). Coherent artifacts for sequential (dotted yellow boxes) are shown compared to less coherent artifacts for phyllotaxis (dotted green boxes). In the images, less streaking and better SNR are apparent for phyllotaxis. The corresponding images for sequential (g-i) and phyllotaxis (j-l) after 2D iNAV-based translational motion correction show improvements, but some coherent artifacts still remain for sequential (solid yellow boxes) while being less severe for phyllotaxis (solid green boxes). The corresponding enlarged regions of interest are shown on the right to highlight these artifacts with yellow and green arrows for sequential and phyllotaxis respectively.

FIG 8.

Reformatted maximum intensity projection images of the RCA for three healthy volunteers (a, b, c) after 2D iNAV-based translational motion correction. The yellow (sequential) and green (phyllotaxis) arrows show proximal (top) and distal (bottom) locations for both methods. The results show improved coronary artery depiction with phyllotaxis for both the proximal and distal regions.

FIG 9.

Reformatted maximum intensity projection images of the LCA for three healthy volunteers (a, b, c) after 2D iNAV-based translational motion correction. The yellow (sequential) and green (phyllotaxis) arrows show proximal (top) and distal (bottom) locations for both methods. Similar to the RCA images, phyllotaxis outperformed sequential for both the proximal and distal regions.

FIG 10.

Results of reader scores (a, c) and IEPA values (b, d) for all eight volunteers. The reader scores for each subject (a) and average scores (c) demonstrate higher values for the phyllotaxis acquisition. Higher reader scores and IEPA values correspond to better coronary image quality and sharper vessels respectively. The IEPA values for every subject (b) and average IEPA value (d) similarly show higher value trends for phyllotaxis. The statistical significance of the results for reader scores and IEPA values was P<0.001 for both the RCA and LCA when using the two-tailed Student’s t-test.