Prediction of the extent of thrombus formation in the parent artery after endovascular occlusion of a distal anterior cerebral artery aneurysm using computational fluid dynamics.

Endovascular coiling of a cerebral aneurysm and coil occlusion of the parent artery have been occasionally performed to treat cerebral aneurysms; however, it is difficult to predict the accurate extent of thrombus formation in the parent artery proximal to the coiled aneurysm and the coil-occluded parent artery preoperatively, and unexpected occlusion of the arterial branches can occur by thrombus extension into or in the parent artery. The authors describe a case of a distal anterior cerebral artery (ACA) aneurysm treated by endovascular parent artery occlusion (PAO) with preoperative computational fluid dynamics (CFD) prediction of the extent of thrombus formation. A 73-year-old woman presented with subarachnoid hemorrhage and an aneurysm that was located on the right pericallosal artery distal to the paracentral artery bifurcation. Endovascular coiling of the aneurysm and the pericallosal artery was planned. In advance of the treatment, CFD was performed to predict the extent of thrombus formation with specific wall shear stress and shear rate thresholds. The hemodynamic results indicated that coiling of the aneurysm resulted in thrombus formation in the pericallosal artery up to just distal to the paracentral artery ostium; therefore, the treatment was implemented according to the CFD prediction. Postoperative digital subtraction angiography revealed that the extent of thrombus formation was consistent with the preoperative CFD prediction. This technique may prevent unexpected occlusion of arterial branches.

Quantification of hemodynamic irregularity using oscillatory velocity index in the associations with the rupture status of cerebral aneurysms.

BACKGROUND: Complex and unstable flow patterns are reported to be associated with the rupture status of cerebral aneurysms, while their evaluation depends on qualitative analysis of streamlines of bloodflow. Oscillatory velocity index (OVI) is a hemodynamic parameter to quantify flow patterns. The aim of this study is to elucidate the associations between OVI and the rupture status of cerebral aneurysms. METHODS: One hundred and twenty-nine ruptured and unruptured cerebral aneurysms were analyzed with computational fluid dynamics under pulsatile flow conditions. With the use of median value of OVI, all aneurysms were divided into high and low OVI groups. Statistical analysis was performed to compare rupture status, and morphological and hemodynamic parameters between the two groups. RESULTS: The median value of OVI was 0.006. High OVI was more likely observed in ruptured aneurysms (P=0.028) and associated with irregular shape, complex flow patterns, and unstable flow patterns (P<0.001, respectively). In morphological parameters, maximum size, aspect, projection, size, and volume-to-ostium area ratios were significantly higher in the high OVI group (P<0.001, respectively). In hemodynamic parameters, wall shear stress and wall shear stress gradient were significantly lower, and oscillatory shear index and gradient oscillatory number were significantly higher in the high OVI group (P<0.001, respectively). CONCLUSION: High OVI was associated with rupture status, and morphological and hemodynamic characteristics of ruptured aneurysms. These results indicate that OVI may serve as a valuable hemodynamic parameter for diagnosing rupture status and risks of aneurysms.

A Novel Radiologic Assessment of Screw Loosening Focusing on Spatial Position Change of Screws Using an Iterative Closest Point Algorithm with Stereolithography Data: Technical Note.

OBJECTIVE: In thoracolumbar spinal instrumentation surgery, pedicle screw fixation is widely used, whereas screw loosening occurs only occasionally over time. It is common to evaluate screw loosening by the radiographic lucent zone around screws, which can neither evaluate loosening quantitatively nor detect slight screw loosening. In the present technical note, we describe a novel assessment technique of screw loosening by generating 3-dimensional screw images from computed tomography data and superposing them in time series. METHODS: Computed tomography data were exported in digital imaging and communications in medicine dataset and imported to the 3-dimensional computer-aided designing software, by which screws and rods were segmented and outputted in stereolithography (STL) format. The STL files were imported to the software, and registration based on iterative closest point algorithm was performed to assess screw position changes. RESULTS: Positional changes on STL in time series were classified into 3 types: 1) no position changes existed in the entire system of screws and rods; 2) position changes existed in the entire system of screws and rods, but no position changes were shown when the left-sided and right-sided screws and rod were separately evaluated; and 3) position changes existed in the left and/or right-sided screws and rods even when evaluated separately. CONCLUSIONS: This technique enables the quantitative evaluation of screw loosening and loosening between screws and rods. In conjunction with conventional methods of assessing radiographic lucent zone, we are able to obtain more accurate information regarding screw loosening after spinal instrumentation surgery.

One-stage Stent-assisted Coil Embolization for Rupture-side-unknown Bilateral Vertebral Artery Dissecting Aneurysms in an Acute Stage: A Case Report.

Bilateral vertebral artery dissecting aneurysms (VADAs) with subarachnoid hemorrhage (SAH) are rare and their management is still challenging. In this report, we successfully performed one-stage stent-assisted coil embolization (SAC) for bilateral VADAs with SAH in an acute stage, because the ruptured side could not be diagnosed. A 47-year-old woman presented with a sudden onset of headache without laterality, and left-side dominant SAH with bilateral VADAs was noted on computed tomography (CT) scans. The size of aneurysmal dome and neck was similar between the two VADAs, and a bleb was observed only on the right VADA. In computational fluid dynamics (CFD) simulations, findings of wall shear stress (WSS), normalized WSS, and WSS gradient suggested that the left VADA was ruptured, while the oscillatory shear index and aneurysm formation indicator suggested the opposite-side one to be ruptured. Thus, we could not determine which VADA was ruptured by clinical data and CFD analyses. Therefore, we performed simultaneous treatment for the bilateral VADAs by using SAC technique 8 h after the onset under dual antiplatelet and anticoagulation therapies. There was no evidence of rebleeding and stent thrombosis. Stent thrombosis was monitored by duplex color-coded ultrasonography after the intervention. She was discharged without neurological deficits, and 6-month follow-up cerebral angiography demonstrated no recanalization of VADAs. This is the first report showing bilateral VADAs with SAH treated by one-stage SAC within 24 h of SAH, and the potential risks are discussed.

Hemodynamic characteristics of hyperplastic remodeling lesions in cerebral aneurysms.

BACKGROUND & PURPOSE: Hyperplastic remodeling (HR) lesions are sometimes found on cerebral aneurysm walls. Atherosclerosis is the results of HR, which may cause an adverse effect on surgical treatment for cerebral aneurysms. Previous studies have demonstrated that atherosclerotic changes had a correlation with certain hemodynamic characteristics. Therefore, we investigated local hemodynamic characteristics of HR lesions of cerebral aneurysms using computational fluid dynamics (CFD). METHODS: Twenty-four cerebral aneurysms were investigated using CFD and intraoperative video recordings. HR lesions and red walls were confirmed on the intraoperative images, and the qualification points were determined on the center of the HR lesions and the red walls. The qualification points were set on the virtual operative images for evaluation of wall shear stress (WSS), normalized WSS (NWSS), oscillatory shear index (OSI), relative residence time (RRT), and aneurysm formation indicator (AFI). These hemodynamic parameters at the qualification points were compared between HR lesions and red walls. RESULTS: HR lesions had lower NWSS, lower AFI, higher OSI and prolonged RRT compared with red walls. From analysis of the receiver-operating characteristic curve for hemodynamic parameters, OSI was the most optimal hemodynamic parameter to predict HR lesions (area under the curve, 0.745; 95% confidence interval, 0.603-0.887; cutoff value, 0.00917; sensitivity, 0.643; specificity, 0.893; P<0.01). With multivariate logistic regression analyses using stepwise method, NWSS was significantly associated with the HR lesions. CONCLUSIONS: Although low NWSS was independently associated with HR lesions, OSI is the most valuable hemodynamic parameter to distinguish HR lesions from red walls.

A Case of Vertebral Artery Fusiform Aneurysm Treated by Flow Alteration: Successful Prediction of Therapeutic Effects Using Computational Fluid Dynamics.

The treatment of intracranial complicated aneurysms remains challenging. In patients with complicated aneurysms that are neither clippable nor coilable, flow alteration treatment (FAT) with a combined procedure of proximal/distal occlusion or trapping of an aneurysm with bypass surgery has been reported. However, it is difficult to predict whatever FAT can achieve aneurysmal obliteration without ischemic complications. A 69-year-old female was incidentally diagnosed with a left vertebral artery (VA) fusiform aneurysm distal to the left posterior inferior cerebellar artery (PICA). Because one-year follow-up three-dimensional computed tomography angiography showed that the aneurysm grew significantly, surgical management was considered the therapy of choice. For determining treatment strategies, we assumed left VA occlusion at the proximal to the left PICA as a FAT model and performed computational fluid dynamics (CFD) analyses. The FAT model had much lower wall shear stress and shear rate at the aneurysm dome than presumed thresholds necessary to thrombus formation, while those at the PICA were obviously higher than the thresholds, and streamlines into the left PICA from the distal VA were preserved. These findings theoretically meant that surgical occlusion of the left VA proximal to the left PICA and aneurysm would induce intra-aneurysmal thrombus formation with preservation of the left PICA flow. The treatment was performed successfully and achieved the predicted results. CFD simulations may be useful to predict effects of FAT for complicated aneurysms.

Hemodynamic Differences Between Ruptured and Unruptured Cerebral Aneurysms Simultaneously Existing in the Same Location: 2 Case Reports and Proposal of a Novel Parameter Oscillatory Velocity Index.

BACKGROUND: Studies have demonstrated certain hemodynamic characteristics featuring the rupture status of cerebral aneurysms using computational fluid dynamics. These studies were conducted based on the comparison of a large number of ruptured and unruptured aneurysms. However, not only aneurysm size and location but also perianeurysm environment, such as hemorrhage and intracranial pressure, affect hemodynamic changes. We hypothesized that a case in which ruptured and unruptured cerebral aneurysms simultaneously exist in the same location would be an ideal model to demonstrate hemodynamic characteristics of the rupture status. CASE DESCRIPTION: We report 2 rare cases with subarachnoid hemorrhage, each of which involved 2 aneurysms at the common parent artery. One patient had 2 anterior communicating artery aneurysms, and the other patient had 2 middle cerebral artery aneurysms. Preoperative morphologic and hemodynamic examinations were performed to diagnose the rupture status of the 2 aneurysms, and each ruptured aneurysm was then confirmed during surgical clipping. Morphologic evaluation revealed higher shape indexes in both ruptured aneurysms. Lower wall shear stress, wall shear stress gradient, and aneurysm formation indicator were observed in both ruptured aneurysms. In contrast, ruptured aneurysms had a higher oscillatory shear index and oscillatory velocity index, which was the novel hemodynamic parameter to quantify the fluctuation of flow velocity vector. CONCLUSIONS: Quantitative characterization of the hemodynamic environment can distinguish the rupture status by using appropriate models minimizing certain bias caused by subarachnoid hemorrhage and aneurysm location.

Stagnation and complex flow in ruptured cerebral aneurysms: a possible association with hemostatic pattern.

OBJECT Histopathological examination has revealed that ruptured cerebral aneurysms have different hemostatic patterns depending on the location of the clot formation. In this study, the authors investigated whether the hemostatic patterns had specific hemodynamic features using computational fluid dynamics (CFD) analysis. METHODS Twenty-six ruptured middle cerebral artery aneurysms were evaluated by 3D CT angiography and harvested at the time of clipping. The hemostatic patterns at the rupture points were assessed by means of histopathological examination, and morphological parameters were obtained. Transient analysis was performed, and wall shear stress-related hemodynamic parameters and invariant Q (vortex core region) were calculated. The morphological and hemodynamic parameters were compared among the hemostatic patterns. RESULTS Hematoxylin and eosin staining of the aneurysm wall showed 13 inside-pattern, 9 outside-pattern, and 4 other-pattern aneurysms. Three of the 26 aneurysms were excluded from further analysis, because their geometry models could not be generated due to low vascular CT values. Mann-Whitney U-tests showed that lower dome volume (0.04 cm3 vs 0.12 cm3, p = 0.014), gradient oscillatory number (0.0234 vs 0.0289, p = 0.023), invariant Q (-0.801 10-2/sec2 vs -0.124 10-2/sec2, p = 0.045) and higher aneurysm formation indicator (0.986 vs 0.963, p = 0.041) were significantly related to inside-pattern aneurysms when compared with outside-pattern aneurysms. CONCLUSIONS Inside-pattern aneurysms may have simpler flow patterns and less flow stagnation than outside-pattern aneurysms. CFD may be useful to characterize the hemostatic pattern of ruptured cerebral aneurysms.

Using computational fluid dynamics analysis to characterize local hemodynamic features of middle cerebral artery aneurysm rupture points.

OBJECTIVE: Although rupture of cerebral aneurysms typically occurs at the fragile wall at the apex or pole, some aneurysms rupture through the body or the neck. The purpose of this study was to clarify the association between aneurysm rupture points and hemodynamic features through the use of computational fluid dynamics (CFD) analysis. METHODS: Twelve ruptured middle cerebral artery bifurcation aneurysms were analyzed by 3-dimensional computed tomographic angiography and CFD. Rupture points were evaluated on intraoperative videos by 3 independent neurosurgeons. Wall shear stress (WSS) was calculated at the rupture point, aneurysm dome, and parent artery. Intra-aneurysmal flow patterns were evaluated with cross-sectional velocity vector planes that included the rupture points. RESULTS: The mean WSS at the rupture point (0.29 Pa) was significantly lower than that at the dome (2.27 Pa) and the parent artery (8.19 Pa) (P < .01). All rupture points were located within the area of WSS ≤ 11.2% of the WSS at the parent artery. WSS at the rupture point was correlated with the minimum WSS at the dome (r = 0.64, P < .05), but not with aneurysm size (r = 0.26) or the aspect ratio (r = 0.16). Flow patterns revealed that all rupture points were located in lower-velocity area, which was associated with complex flow patterns and/or deviating necks. CONCLUSIONS: This study highlights the relationship between the local hemodynamic features and the rupture points observed during the microsurgical clipping. CFD may determine a rupture point of aneurysms using the feature of markedly low WSS.

Changes in hemodynamics after placing intracranial stents.

Stent-assisted coil embolization has enabled the endovascular treatment of wide-necked cerebral aneurysms. Moreover, recent reports demonstrated that stent-assisted coil embolization was associated with a significant decrease in angiographic recurrences of coiled cerebral aneurysms. One of the possible explanations for this adjunctive effect of stent-assisted coil embolization is changes in the local hemodynamics caused by placing intracranial stents. This study investigated the hemodynamic effect of intracranial stents using computational fluid dynamics (CFD) analysis. The geometry of the intracranial stent, Enterprise(TM) VRD, was acquired by using micro computed tomography and virtually placed across the aneurysm orifice of a saccular aneurysm model (saccular model) and a blister-like aneurysm model (blister-like model) constructed from patient-specific three-dimensional (3D) rotational angiography data. Transient CFD analysis was performed with these models with and without stents. Stent placement induced no significant changes in the 3D streamline in the saccular model and slight changes in the blister-like model. Both saccular and blister-like models with stents had lower wall shear stress (WSS) and flow velocity, and higher oscillatory shear index, WSS gradient, and relative residence time than the equivalent models without stents, indicating the possibility that stent placement induced stagnant and disturbed blood flow. Cross-sectional vector velocity around the stent strut revealed complex blood flow patterns with variable direction and velocity. Although this study was a simulation under limited conditions, similar hemodynamic changes might be induced in the neck remnants treated with stent-assisted coil embolization.

Low wall shear stress is independently associated with the rupture status of middle cerebral artery aneurysms.

BACKGROUND AND PURPOSE: We determined which hemodynamic parameter independently characterizes the rupture status of middle cerebral artery (MCA) aneurysms using computational fluid dynamics analysis. METHODS: In 106 patient-specific geometries of MCA aneurysms (43 ruptured, 63 unruptured), morphological and hemodynamic parameters were compared between the ruptured and unruptured groups. Multivariate logistic regression analysis was performed to determine parameters that independently characterized the rupture status of MCA aneurysms. RESULTS: Univariate analyses showed that the aspect ratio, wall shear stress (WSS), normalized WSS, oscillatory shear index, WSS gradient, and aneurysm-formation index were significant parameters. The size of the aneurysmal dome and the gradient oscillatory number were not significantly different between the 2 groups. With multivariate analyses, only lower WSS was significantly associated with the rupture status of MCA aneurysms. CONCLUSIONS: WSS may be the most reliable parameter characterizing the rupture status of MCA aneurysms.

Novel dynamic four-dimensional CT angiography revealing 2-type motions of cerebral arteries.

BACKGROUND AND PURPOSE: We developed a novel dynamic 4-dimensional CT angiography to accurately evaluate dynamics in cerebral aneurysm. METHODS: Dynamic 4-dimensional CT angiography achieved high-resolution 3-dimensional imaging with temporal resolution in a beating heart using dynamic scanning data sets reconstructed with a retrospective simulated R-R interval reconstruction algorithm. RESULTS: Movie artifacts disappeared on dynamic 4-dimensional CT angiography movies of 2 kinds of stationary phantoms (titanium clips and dry bone). In the virtual pulsating aneurysm model, pulsation on the dynamic 4-dimensional CT angiography movie resembled actual movement in terms of pulsation size. In a clinical study, dynamic 4-dimensional CT angiography showed 2-type motions: pulsation and anatomic positional changes of the cerebral artery. CONCLUSIONS: This newly developed 4-dimensional visualizing technique may deliver some clues to clarify the pathophysiology of cerebral aneurysms.