Image-based computational simulation of flow dynamics in a giant intracranial aneurysm

Abstract

Background and purpose: Blood flow dynamics are thought to play an important role in the pathogenesis and treatment of intracranial aneurysms; however, hemodynamic quantities of interest are difficult to measure in vivo. This study shows that computational fluid dynamics (CFD) combined with computed rotational angiography can provide such hemodynamic information in a patient-specific and prospective manner.

Methods: A 58-year-old woman presented with partial right IIIrd cranial nerve palsy due to a giant carotid-posterior communicating artery aneurysm that was subsequently coiled. Computed rotational angiography provided high resolution volumetric image data from which the lumen geometry was extracted. This and a representative flow rate waveform were provided as boundary conditions for finite element CFD simulation of the 3D pulsatile velocity field.

Results: CFD analysis revealed high speed flow entering the aneurysm at the proximal and distal ends of the neck, promoting the formation of both persistent and transient vortices within the aneurysm sac. This produced dynamic patterns of elevated and oscillatory wall shear stresses distal to the neck and along the sidewalls of the aneurysm. These hemodynamic features were consistent with patterns of contrast agent wash-in during cine angiography and with the configuration of coil compaction observed at 6-month follow-up.

Conclusion: Anatomic realism of lumen geometry and flow pulsatility is essential for elucidating the patient-specific nature of aneurysm hemodynamics. Such image-based CFD analysis may be used to provide key hemodynamic information for prospective studies of aneurysm growth and rupture or to predict the response of an individual aneurysm to therapeutic options.

Fig 1.

Geometry of aneurysm and parent vessel, shown in anteroposterior (left column) and lateral (right column) views. A and B, Maximum intensity projections obtained through the CT reconstructed data illustrate the geometrical complexity of the aneurysm and parent vessel. Angled arrows identify blebs on the right and posterior sides of the aneurysm sac. Horizontal arrows point to an apparent stenosis of the petrous segment, which may be attributed to compression of the vessel against adjacent bone. C and D, Corresponding views of the finite element mesh show the geometric faithfulness and spatial resolution of the CFD model; for clarity, only the corner nodes of the quadratic finite elements are shown. Note the model coordinate system: ± x = left/right, ± y = anterior/posterior, ± z = superior/inferior.

Fig 2.

Virtual slipstreams, shown at selected times in lateral (top row) and anteroposterior (bottom row) oblique views, provide an overview of the aneurysm hemodynamics. A, Early systole (t = 50 ms). Slipstreams showed little mixing or spreading as they approached the neck. B, Peak systole (t = 150 ms). Slipstreams spread and began to mix as they entered both the proximal and distal ends of the neck. C, Early diastole (t = 300 ms). Slipstreams mixed as they impacted the posterior wall of the aneurysm and swirled in the right and inferior directions. D, Late diastole (t = 800 ms). Within 0.5 second, the aneurysm was almost entirely opacified by vigorously mixed slipstreams.

Fig 3.

More detailed views of the complex aneurysm hemodynamics are provided at selected times and for selected planes via conventional field plots of sagittal (top row), coronal (middle row), and axial (bottom row) planes, each from the nominal center of the aneurysm. Contours of velocity magnitude (V, in cm/s) are shown with vectors superimposed to indicate the magnitude and direction of in-plane flow components. White circles identify the approximate center of each vortex. S, superior; I, inferior; A, anterior; P, posterior; R, right; L, left. A, Peak systole (t = 150 ms). B, Early diastole (t = 300 ms). C, Mid-diastole (t = 450 ms). D, Late diastole (t = 600 ms).

Fig 4.

Computed wall shear stress patterns are shown in oblique posterior (left panels) and anterior (right panels) views. A and B, Contours of cycle-averaged wall shear stress magnitude (WSS, dynes/cm²). C and D, Contours of oscillatory shear index (OSI, dimensionless), a measure of the relative variability of the wall shear stress over the cardiac cycle.

Fig 5.

Two sequential frames from 2-Hz cine digital subtraction angiograms. These zoomed views correspond to the orientation and extent of the CFD model in the lower panels of Figure 2C and D. A, Aneurysm filling dynamics, shown approximately 0.5 second after selective injection into the internal carotid artery, show good general agreement with the corresponding virtual slipstreams shown in Figure 2C. B, One frame (ie, 0.5 second) later, the aneurysm is already largely opacified, which also is consistent with the corresponding virtual slipstreams shown in Figure 2D.

Fig 6.

Illustration of a possible relationship between coil compaction and computed flow dynamics. A, Lateral digital subtraction angiogram obtained at 6-month follow-up examination shows compaction of the coil mass away from the neck and toward the posterior wall of the aneurysm. B, High speed flow entering the aneurysm (shown by using a 15 cm/s isovelocity surface from the cycle-averaged velocity field superimposed on a projection of the aneurysm model approximately corresponding to that in shown in A) is also directed toward the posterior wall of the aneurysm.