Abnormal Flow Dynamics Result in Low Wall Shear Stress and High Oscillatory Shear Index in Abdominal Aortic Dilatation: Initial in vivo Assessment with 4D-flow MRI

Figures

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  Fig. 1. The scheme of the measurements of the values reflecting geometrical features of the abdominal aortic aneurysm. The example of auxiliary lines delineated on the enhanced 3D MRA of a 68-year-old female. The maximum luminal diameter (40 mm) and the length of the dilated segment as well as the diameter of upstream non-dilated aorta (25 mm) were measured. The immediate upper bent angle created by the middle lines of the dilated segment and the undilated segment (93°) was also measured. MRA, magnetic resonance angiography.

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  Fig. 2. (a)–(c) The examples of the ratings for laminar or non-laminar flow observed on streamline analysis. (a) Streamline analysis for the AAA of a 84-year-old male at systole. Laminar flow at the upstream undilated aorta (white arrows) and the tiny vortices (red arrows) with diameter less than one-third of the aneurysmal diameter at the shoulder of the AAA are seen. The degree of non-laminar flow were both rated as none to mild. (b) Streamline analysis for the AAA of a 68-year-old male at diastole. A large helical flow (white arrows) occupying more than one-third of the AAA diameter was rated moderate (Although the helical flow was over one-third of the aneurysm, it was single). (c) Streamline analysis for the AAA of a 60-year-old male at diastole. There are more than two large vortices (white arrows and red arrows) with size over one-third of the aneurysmal diameter at the proximal portion of the AAA, which were rated as severe non-laminar flow. Supplementary movie shows hemodynamics over one cardiac cycle using streamline analysis. AAA, abdominal aortic aneurysm.

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  Fig. 3. A color coded 3D WSS map of a 84-year-old male AAA patient shows the range of abdominal aorta imaged on 4D-flow MRI. WSS was measured on the 3D map covering circumferential area of the dilated (surrounded by white line) vs. non-dilated upstream aorta (surrounded by yellow line). Note: dilated portion (WSS coded blue to green) is considerably lower WSS than the non-dilated segment (higher WSS coded red). AAA, abdominal aortic aneurysm; WSS, wall shear stress.

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  Fig. 4. Graph shows time-resolved mean velocity values perpendicular to the cross sections of the ROI in the aneurysm (closed circle) and upstream undilated aortic segment (open square) measured with 4D-flow MRI. The velocity in the upstream undilated aorta measured with time-resolved 2D PC cine MRI dotted line is also shown for reference. The agreements of the velocities measured using 4D-flow and the 2D PC cine MRI were reasonably good, being identical in the diastole and up to 8.6% difference in the mid-systole. The values within the undilated upstream aorta, systolic six phases (*) were significantly higher compared with end-diastole (10th phase). Similarly, the values within the aneurysm at the systolic five phases (#) were significantly higher than those of end-diastole. The mean velocities in the aneurysm are significantly lower than those in the undilated upstream aorta throughout the entire cardiac cycle. Bars are shown in SE. The original phase resolution for the 2D PC cine MRI (30 phases/s) was adapted to 12 phases to compare with that of 4D-flow MRI. 2D PC cine MRI, 2D phase contrast MRI; 4D-flow MRI, 3D phase-contrast MRI; ROI, region of interest; SE, standard error.

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  Fig. 5. (a) and (b) The appearances and the severity of the abnormal flow patterns observed within the aneurysm and the undilated upstream segment of the abdominal aorta. (a) Three grades of non-laminar flow dynamics (none to mild, moderate and severe) were rated by observer 1. (b) The same assessments rated by observer 2. *, #, $: significantly higher rates than undilated upstream segments (*: P = 0.0156 #: P = 0.0001, $: P = 0.0074). The comparisons were made using integrated data of observer 1 and 2. dia, diastole; sys, systole.

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  Fig. 6. (a)–(c) Color-coded 3D reconstruction images at systole of a 60-year-old male with AAA shown as a left anterior oblique projection. Flow analysis software was used to produce these 3D interposed images on 3D MRA model. There is a bending (bent angle of 38°) and an abrupt diameter increase (diameter ratio 1.7) in the lumen of the abdominal aorta. Note: upper bent angle created non-laminar flow in the left half of the aorta and consequent decreased WSS and the increased OSI of the corresponding aortic wall (arrows). (a) The streamline image shows fast laminar flow within the undilated upstream segment (arrowheads); however, the velocities decrease once the flow enters the aneurysm. The bending and dilatation of the flow area affect the integrity of the flow patterns. The lost momentum of the flow causes vortex flow (arrows), where it coincides with low WSS (b). (b) A WSS map shows the aneurysmal wall particularly on the inner curvature of the aneurysm, which is coded blue, has a lower WSS (arrows) than the upstream undilated segment, which is coded green (arrowheads). (c) An OSI map of the same case shows a high OSI (coded red to green) where vortex flow were observed in the…

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  Fig. 7. (a)–(c) Anterior projection of the color coded 3D image of a 68-year-old male with AAA at mid-systole. There is a subtle bending and abrupt diameter increase in the lumen of the abdominal aorta. In this particular case, there is a large helical flow flowing across the entire aortic width; therefore resulted in relatively homogenous reduction of the WSS and increased OSI. (a) The streamline image shows laminar flow within the undilated upstream segment (arrowheads); however, large helicities (arrows) dominates the entire aneurysmal cavity and partially of its neck (small arrow). Again, the bent and the dilatation of the flow area seem to be affecting the integrity of the flow pattern. (b) A WSS map shows diffuse low WSS of the aneurysmal wall (arrows) and a part of its neck (small arrow) which are coded blue as compared with the upstream undilated segment coded green (arrowheads). (c) An OSI map shows a high OSI (arrows; coded red to green) in the aneurysm and a part of its neck (small arrow) corresponding to the low WSS patterns and abnormal flow patterns. AAA, abdominal aortic aneurysm; OSI, oscillatory shear index; WSS, wall shear stress.

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  Fig. 8. The time (cardiac phase) course changes of spatially-averaged WSS through a cardiac cycle measured in the aneurysm and the undilated upstream aorta. The mean spatially-averaged WSSs of the undilated upstream aorta were significantly higher during systolic seven phases (*) than the end-diastole (10th phase). The spatially-averaged WSSs of the aneurysm during systolic seven phases (#) were significantly higher than end-diastole. The spatially-averaged WSSs in the aneurysms were significantly lower (P < 0.001) than those of the undilated aortic segments of the proximal aorta in all cardiac phases (not marked on the graph). WSS, wall shear stress.

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  Fig. 9. (a) and (b) The relationship between dilated/undilated aorta diameter ratio vs. WSS. (a) The maximum WSS (at mid-systole) showed moderately good correlation to the diameter ratio [r = −0.542, P = 0.0203, confidence interval (CI): −0.805, −0.010). (b) The amplitude of WSS, i.e., minimum WSS subtracted from maximum WSS showed a better inverse correlation to the diameter ratio (r = −0.694, P = 0.0014, CI: −0.877, −0.335). WSS, wall shear stress.

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  Fig. 10. OSI which reflects the fluctuation of the WSS during the cardiac cycle was higher in the dilated aorta (0.093 ± 0.010, CI: 0.080, 0.11) compared with the proximal undilated portion (0.041 ± 0.0089, CI: 0.012, 0.059; P = 0.013). OSI, oscillatory shear index; WSS, wall shear stress.