Haemodynamics in a patient-specific intracranial aneurysm according to experimental and numerical approaches: A comparison of PIV, CFD and PC-MRI.

BACKGROUND: The haemodynamics determined by different approaches for studying fluid dynamics - i.e. computational fluid dynamics (CFD), particle image velocimetry (PIV), and phase-contrast magnetic resonance imaging (PC-MRI) - have rarely been thoroughly compared; nor have the factors that affect accuracy and precision in each method. As each method has its own advantages and limitations, this knowledge is important for future studies to be able to achieve valid analyses of fluid flows. OBJECTIVE: To gauge the capacity of these methods for analysing aneurysmal flows, we compared the haemodynamic behaviours determined by each method within a patient-specific aneurysm model. METHODS: An in vitro silicone aneurysm model was fabricated for PIV and PC-MRI, and an in silico aneurysm model with the same geometry was reconstructed for CFD. With the same fluid model prepared numerically and physically, CFD, PIV and PC-MRI were performed to study aneurysmal haemodynamics. RESULTS: 2D velocity vectors and magnitudes show good agreement between PIV and CFD, and 3D flow patterns show good similarity between PC-MRI and CFD. CONCLUSIONS: These findings give confidence to future haemodynamic studies using CFD technology. For the first time, the morphological inconsistency between the PCMRI model and others is found to affect the measurement of local flow patterns.

Evaluation of magnetic resonance angiography as a possible alternative to rotational angiography or computed tomography angiography for assessing cerebrovascular computational fluid dynamics.

The aim of this study was to conduct a flow experiment using a cerebrovascular phantom and investigate whether magnetic resonance angiography (MRA) could replace three-dimensional rotational angiography (RA) and computed tomography angiography (CTA) to construct vascular models for computational fluid dynamics (CFD). We performed MRA and 3D cine phase-contrast (PC) MR imaging with a silicone cerebrovascular phantom of an internal carotid artery-posterior communicating artery aneurysm with blood-mimicking fluid, and controlled flow with a flowmeter. We also obtained RA and CTA data for the phantom. Four analysts constructed vascular models based on the three different modalities. These 12 constructed models used flow information based on 3D cine PC MR imaging for CFD. We compared RA-, CTA-, MRA-based CFD results using the micro-CT-based CFD result as the criterion standard to investigate whether MRA-based CFD was not inferior to RA- or CTA-based CFD. We also analyzed the inter-analyst variability. Wall shear stress (WSS) distributions and streamlines of RA- or MRA-based CFD and those of micro-CT-based CFD were similar, but the vascular models and WSS values were different. Accuracy in measurements of blood vessel diameter, cross-sectional maximum velocity, and spatially averaged WSS was the highest for RA-based CFD, followed by MRA-based and CTA-based CFD using micro-CT-based CFD result as the reference. Except maximum velocity from CTA, all other parameters had good inter-analyst agreement using different modalities. The results demonstrated that non-invasive MRA can be used for cerebrovascular CFD models with good inter-analyst agreements.

Assessing the Risk of Intracranial Aneurysm Rupture Using Morphological and Hemodynamic Biomarkers Evaluated from Magnetic Resonance Fluid Dynamics and Computational Fluid Dynamics.

PURPOSE: Evaluate in vivo hemodynamic and morphological biomarkers of intracranial aneurysms, using magnetic resonance fluid dynamics (MRFD) and MR-based patient specific computational fluid dynamics (CFD) in order to assess the risk of rupture. METHODS: Forty-eight intracranial aneurysms (10 ruptured, 38 unruptured) were scrutinized for six morphological and 10 hemodynamic biomarkers. Morphological biomarkers were calculated based on 3D time-of-flight magnetic resonance angiography (3D TOF MRA) in MRFD analysis. Hemodynamic biomarkers were assessed using both MRFD and CFD analyses. MRFD was performed using 3D TOF MRA and 3D cine phase-contrast magnetic resonance imaging (3D cine PC MRI). CFD was performed utilizing patient specific inflow-outflow boundary conditions derived from 3D cine PC MRI. Univariate analysis was carried out to identify statistically significant biomarkers for aneurysm rupture and receiver operating characteristic (ROC) analysis was performed for the significant biomarkers. Binary logistic regression was performed to identify independent predictive biomarkers. RESULTS: Morphological biomarker analysis revealed that aneurysm size [P = 0.021], volume [P = 0.035] and size ratio [P = 0.039] were statistically significantly different between the two groups. In hemodynamic biomarker analysis, MRFD results indicated that ruptured aneurysms had higher oscillatory shear index (OSI) [OSI.max, P = 0.037] and higher relative residence time (RRT) [RRT.ave, P = 0.035] compared with unruptured aneurysms. Correspondingly CFD analysis demonstrated significant differences for both average and maximum OSI [OSI.ave, P = 0.008; OSI.max, P = 0.01] and maximum RRT [RRT.max, P = 0.045]. ROC analysis revealed AUC values greater than 0.7 for all significant biomarkers. Aneurysm volume [AUC, 0.718; 95% CI, 0.491-0.946] and average OSI obtained from CFD [AUC, 0.774; 95% CI, 0.586-0.961] were retained in the respective logistic regression models. CONCLUSION: Both morphological and hemodynamic biomarkers have significant influence on intracranial aneurysm rupture. Aneurysm size, volume, size ratio, OSI and RRT could be potential biomarkers to assess aneurysm rupture risk.

Accuracy of the Flow Velocity and Three-directional Velocity Profile Measured with Three-dimensional Cine Phase-contrast MR Imaging: Verification on Scanners from Different Manufacturers.

PURPOSE: The accuracy of flow velocity and three-directional velocity components are important for the precise visualization of hemodynamics by 3D cine phase-contrast MRI (3D cine PC MRI, also referred to as 4D-flow). The aim of this study was to verify the accuracy of these measurements of prototype or commercially available 3D cine PC MRI obtained by three different manufactures' MR scanners. METHODS: The verification of the accuracy of flow velocity in 3D cine PC MRI was performed by circulating blood mimicking fluid through a straight-tube phantom in a slanting position, such that the three-directional velocity components were simultaneously measurable, using three 3T MR scanners from different manufacturers. The data obtained were processed by phase correction, and the velocity and three-directional velocity components in the center of the tube on the central cross section of a slab were calculated. The velocity profile in each three directions and the composite velocity profiles were compared with the calculated reference values, using the Hagen-Poiseuille equation. In addition, velocity profiles and the spatially time-averaged velocity perpendicular to the tube were compared with the theoretical values and measured values by a flowmeter, respectively. RESULTS: An underestimation of the maximum velocity in the center of the tube and an overestimation of the velocity near the tube wall due to partial volume effects were observed in all three scanners. A roughening and flattening of profiles in the center of the tube were observed in one scanner, due, presumably, to the low signal-to-noise ratio. However, the spatially time-averaged velocities corresponded well with the measured values by the flowmeter in all three scanners. CONCLUSION: In this study, we have demonstrated that the accuracy of flow velocity and three-directional velocity components in 3D cine PC MRI was satisfactory in all three MR scanners.

Myocardial motion analysis based on an optical flow method using tagged MR images.

We developed a method of velocimetry based on an optical flow method using quantitative analyses of tagged magnetic resonance (MR) images (tagged MR-optical flow velocimetry, tMR-O velocimetry). The purpose of our study was to examine the accuracy of measurement of the proposed tMR-O velocimetry. We performed retrospective pseudo-electrocardiogram (ECG) gating tagged cine MR imaging on a rotating phantom. We optimized imaging parameters for tagged MR imaging, and validated the accuracy of tMR-O velocimetry. Our results indicated that the difference between the reference velocities and the computed velocities measured using optimal imaging parameters was less than 1%. In addition, we performed tMR-O velocimetry and echocardiography on 10 healthy volunteers, for four sections of the heart (apical, midventricular, and basal sections aligned with the short-axis, and a four-chamber section aligned with the long-axis), and obtained radial and longitudinal myocardial velocities in these sections. We compared the myocardial velocities obtained using tMR-O velocimetry with those obtained using echocardiography. Our results showed good agreement between tMR-O velocimetry and echocardiography in the radial myocardial velocities in three short-axial sections and longitudinal myocardial velocities on the midventricular portion of the four-chamber section in the long-axis. In the study conducted on the rotating phantom, tMR-O velocimetry showed high accuracy; moreover, in the healthy volunteers, the myocardial velocities obtained using tMR-O velocimetry were relatively similar to those obtained using echocardiography. In conclusion, tMR-O velocimetry is a potentially feasible method for analyzing myocardial motion in the human heart.

Hemodynamic vascular biomarkers for initiation of paraclinoid internal carotid artery aneurysms using patient-specific computational fluid dynamic simulation based on magnetic resonance imaging.

PURPOSE: We performed computational fluid dynamics (CFD) for patients with and without paraclinoid internal carotid artery (ICA) aneurysms to evaluate the distribution of vascular biomarkers at the aneurysm initiation sites of the paraclinoid ICA. METHODS: This study included 35 patients who were followed up for aneurysms using 3D time of flight (TOF) magnetic resonance angiography (MRA) and 3D cine phase-contrast MR imaging. Fifteen affected ICAs were included in group A with the 15 unaffected contralateral ICAs in group B. Thirty-three out of 40 paraclinoid ICAs free of aneurysms and arteriosclerotic lesions were included in group C. We deleted the aneurysms in group A based on the 3D TOF MRA dataset. We performed CFD based on MR data set and obtained wall shear stress (WSS), its derivatives, and streamlines. We qualitatively evaluated their distributions at and near the intracranial aneurysm initiation site among three groups. We also calculated and compared the normalized highest (nh-) WSS and nh-spatial WSS gradient (SWSSG) around the paraclinoid ICA among three groups. RESULTS: High WSS and SWSSG distribution were observed at and near the aneurysm initiation site in group A. High WSS and SWSSG were also observed at similar locations in group B and group C. However, nh-WSS and nh-SWSSG were significantly higher in group A than in group C, and nh-SWSSG was significantly higher in group A than in group B. CONCLUSION: Our findings indicated that nh-WSS and nh-SWSSG were good biomarkers for aneurysm initiation in the paraclinoid ICA.

Influence of Spatial Resolution in Three-dimensional Cine Phase Contrast Magnetic Resonance Imaging on the Accuracy of Hemodynamic Analysis.

INTRODUCTION: We aim to elucidate the effect of spatial resolution of three-dimensional cine phase contrast magnetic resonance (3D cine PC MR) imaging on the accuracy of the blood flow analysis, and examine the optimal setting for spatial resolution using flow phantoms. MATERIALS AND METHODS: The flow phantom has five types of acrylic pipes that represent human blood vessels (inner diameters: 15, 12, 9, 6, and 3 mm). The pipes were fixed with 1% agarose containing 0.025 mol/L gadolinium contrast agent. A blood-mimicking fluid with human blood property values was circulated through the pipes at a steady flow. Magnetic resonance (MR) images (three-directional phase images with speed information and magnitude images for information of shape) were acquired using the 3-Tesla MR system and receiving coil. Temporal changes in spatially-averaged velocity and maximum velocity were calculated using hemodynamic analysis software. We calculated the error rates of the flow velocities based on the volume flow rates measured with a flowmeter and examined measurement accuracy. RESULTS: When the acrylic pipe was the size of the thoracicoabdominal or cervical artery and the ratio of pixel size for the pipe was set at 30% or lower, spatially-averaged velocity measurements were highly accurate. When the pixel size ratio was set at 10% or lower, maximum velocity could be measured with high accuracy. It was difficult to accurately measure maximum velocity of the 3-mm pipe, which was the size of an intracranial major artery, but the error for spatially-averaged velocity was 20% or less. CONCLUSIONS: Flow velocity measurement accuracy of 3D cine PC MR imaging for pipes with inner sizes equivalent to vessels in the cervical and thoracicoabdominal arteries is good. The flow velocity accuracy for the pipe with a 3-mm-diameter that is equivalent to major intracranial arteries is poor for maximum velocity, but it is relatively good for spatially-averaged velocity.