Preliminary Study of Tagged MR Image Velocimetry in a Replica of an Intracranial Aneurysm

Tagged MR images and data processed with PIV software during the systolic phase. Row A, Six tagged MR images, (1)–(6), were obtained with an interval of 9 milliseconds during the systolic phase. In the aneurysm replica, counterclockwise rotation of the tag lines is observed. The quantitative velocity is not known; however, we can estimate the hemodynamics in the aneurysm. Tag lines move more quickly in systolic phase than in the diastolic phase (Fig 3A). They also disappear sooner. Rows B–D, Flow vector images (B), vorticity images (C) and shear strain images (D) processed with PIV software from the data of tagged MR images (A) are shown. Each image—a, b, c, d, and e—was obtained from the following datasets, respectively: (1) and (2), (2) and (3), (3) and (4), (4) and (5), and (5) and (6). The aneurysm is displayed in the same orientation as in Figure 1A. Flow-vector images reveal the flow vector (arrowheads) of the flowing fluid at each location in the replica every 9 milliseconds. These velocities obtained in systolic phase (row B) are faster than those in the diastolic phase (Fig 3B). The maximum velocity is 4.5 cm/s, which is slower than expected. This velocity is due to the short neck of the aneurysm. Fewer flow vectors were noted in the left aspect of the aneurysm, where stronger inflow was estimated to be present; the tag lines were not clearly shown because of faster flow. Vorticity and shear-strain images are shown in gray scale. In the scalar legends depicted, gray at the midpoint means no vorticity or shear strain. Some vorticity images show black areas in the center of the aneurysm; these indicate counterclockwise vorticity. Vorticity is stronger during the systolic phase (row C) than during the diastolic phase (Fig 3C). The inflow portion of the left aspect of the replica shows a darker area, which indicates stronger shear strain (row D). On the other hand, the contralateral side of the replica demonstrated a lighter area, which indicates stronger shear strain. On the shear-strain images, differences in color reflect the differences in the direction of shear deformation. The difference in direction is not important because it is not related to the absolute value of the shear strain. Shear strains are stronger during the systolic phase (row D) than during diastolic phase (Fig 3D). Shear strain multiplied by viscosity equals shear stress. Therefore, the aneurysmal wall near the site with strong shear strain is estimated to have strong shear stress.