Preliminary Study of Tagged MR Image Velocimetry in a Replica of an Intracranial Aneurysm
Haruo Isodaa,
Shoichi Inagawaa,
Hiroyasu Takedaa,
Satoshi Isogaia,
Yasuo Takeharaa and
Harumi Sakaharaa
a From the Department of Radiology, Hamamatsu University School of Medicine, Handayama, Shizuoka, Japan
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FIG 1. Images of the replica of an anterior communicating artery aneurysm.
A, On an original axial image, transverse through the center of the aneurysm, an aneurysmal sac (left arrow) shows relatively low signal intensity compared with the A1 segments (arrowheads) of the anterior cerebral artery. This signal intensity is due to the relatively slow flow in the intracranial aneurysm. Stronger signal intensity (right arrow) at the left aspect of the replica is observed; this is due to stronger flow from the right A1 segment into the left aspect of the aneurysm.
B–D, Anteroposterior (B), inferosuperior (C), and left-to-right (D) projections of MIP MR angiograms show the anterior communicating artery aneurysm (arrow).
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FIG 2. Schematic diagram of the PIV system. The PIV technique is used to calculate the flow vector on the basis of the positional changes of the particles in the flowing fluid; for this, two sequential digital images are used.
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FIG 3. Tagged MR images and data processed with PIV software during the diastolic phase. Row A, Six tagged MR images, (1)–(6), were obtained with an interval of 9 milliseconds during the diastolic 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. The tag lines gradually disappear due to the longitudinal relaxation of the spin in the flowing fluid and due to the mixture of the spin. Rows B–D, Flow-vector (row B), vorticity (row C), and shear-strain (row D) images were processed with the PIV software by using the data of tagged MR images. 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. The flow-vector images reveal the flow vector (arrowheads) of the flowing fluid at each location in the replica every 9 ms. We could measure the quantitative velocity and the direction of flowing fluid. The maximum flow was 2.2 cm/s. 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. During the diastolic phase, the absolute value of vorticity and shear strain seem to be low, according to the color shown on the images.
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FIG 4. 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.
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AJNR - American Journal of Neuroradiology