Odile Bonnefous

A DSA-Based Method Using Contrast-Motion Estimation for the Assessment of the Intra-Aneurysmal Flow Changes Induced by Flow-Diverter Stents

BACKGROUND AND PURPOSE: Flow-diverter stents modify intra-aneurysmal blood flow and induce the progressive thrombosis of intracranial aneurysms followed by stable vascular reconstruction. The aim of this study was to report a new method for the appraisal of intracranial blood flow from DSA performed during endovascular treatment procedures. MATERIALS AND METHODS: A cohort of 24 patients with unruptured IAs who underwent FDS implantation was prospectively recruited. Pre- and post-DSA sequences in combination with 3D rotational angiography were acquired. The quantification of arterial and intra-aneurysmal flow was accomplished by using an optical flow approach. Flow reduction was assessed by using a new metric termed the mean aneurysm flow amplitude ratio. The correlation between the MAFA ratio and the incidence of aneurysm thrombosis was assessed by using receiver operating characteristic analysis and the Fisher exact test when the optimum Youden index was found. RESULTS: The quantification of flow was successfully achieved in 21 of 24 patients (87.5%). On the imaging follow-up, 18 aneurysms developed complete thrombosis (87.5%) and 3 displayed residual circulation (12.5%). The threshold analysis of the MAFA ratio significantly predicted thrombosis at 12 months below a threshold of 1.03 (P = .035). There was no significant correlation between the time for complete occlusion of the aneurysm and contrast stagnation inside the aneurysm after treatment (P > .05). CONCLUSIONS: The MAFA ratio based on DSA flow quantification appears to be a reliable predictor for the assessment of stent treatment outcomes in this small study. These results open the door for perioperative flow quantification and provide indices that may help clinicians make appropriate intraprocedural decisions.

Examining moving objects by ultrasound echograpy

An apparatus for scanning moving objects, notably flowing blood, by means of ultrasound echography in order to determine movement parameters of such objects. The apparatus comprises at least one ultrasound transducer which is connected to a stage for the periodic transmission of a pulse signal having a predetermined recurrent frequency F=1/T and to a stage for receiving echograpic signals returned to the transducer and for processing the signals received. In accordance with the invention, the apparatus is characterized in that it comprises a digital processing channel which is successively composed of a circit (320) for suppressing fixed echoes, a circuit for estimating flow parameters, a discriminator circuit (360), a device (370) for storage, scan conversion and color encoding, and a display device (312).

Quantification of arterial flow using digital subtraction angiography.

PURPOSE In this paper, a method for the estimation of arterial hemodynamic flow from x-ray video densitometry data is proposed and validated using an in vitro setup. METHODS The method is based on the acquisition of three-dimensional rotational angiography and digital subtraction angiography sequences. A modest contrast injection rate (between 1 and 4 ml/s) leads to a contrast density that is modulated by the cardiac cycle, which can be measured in the x-ray signal. An optical flow based approach is used to estimate the blood flow velocities from the cyclic phases in the x-ray signal. RESULTS The authors have validated this method in vitro, and present three clinical cases. The in vitro experiments compared the x-ray video densitometry results with the gold standard delivered by a flow meter. Linear correlation analysis and regression fitting showed that the ideal slope of 1 and intercept of 0 were contained within the 95 percentile confidence interval. The results show that a frame rate higher than 50 Hz allows measuring flows in the range of 2 ml/s to 6 ml/s within an accuracy of 5%. CONCLUSIONS The in vitro and clinical results indicate that it is feasible to estimate blood flow in routine interventional procedures. The availability of an x-ray based method for quantitative flow estimation is particularly clinically useful for intra-cranial applications, where other methods, such as ultrasound Doppler, are not available.

Quantification of Internal Carotid Artery Flow with Digital Subtraction Angiography: Validation of an Optical Flow Approach with Doppler Ultrasound

BACKGROUND AND PURPOSE: Digital subtraction angiography is the reference standard technique to evaluate intracranial vascular anatomy and used on the endovascular treatment of vascular diseases. A dedicated optical flow-based algorithm was applied to DSA to measure arterial flow. The first quantification results of internal carotid artery flow validated with Doppler sonography are reported. MATERIALS and METHODS: We included 22 consecutive patients who underwent endovascular procedures. To assess the sensitivity of the algorithm to contrast agent-blood mixing dynamics, we acquired high-frame DSA series (60 images/s) with different injection rates: 1.5 mL/s (n = 19), 2.0 mL/s (n = 18), and 3.0 mL/s (n = 13). 3D rotational angiography was used to extract the centerline of the vessel and the arterial section necessary for volume flow calculation. Optical flow was used to measure flow velocities in straight parts of the ICAs; these data were further compared with Doppler sonography data. DSA mean flow rates were linearly regressed on Doppler sonography measurements, and regression slope coefficient bias from value 1 was analyzed within the 95% confidence interval. RESULTS: DSA mean flow rates measured with the optical flow approach significantly matched Doppler sonography measurements (slope regression coefficient, b = 0.83 ± 0.19, P = .05) for injection rate = 2.0 mL/s and circulating volumetric blood flow <6 mL/s. For injection rate = 1.5 mL/s, volumetric blood flow <3 mL/s correlated well with Doppler sonography (b = 0.67 ± 0.33, P = .05). Injection rate = 3.0 mL/s failed to provide DSA–optical flow measurements correlating with Doppler sonography because of the lack of measurable pulsatility. CONCLUSIONS: A new model-free optical flow technique was tested reliably on the ICA. DSA-based blood flow velocity measurements were essentially validated with Doppler sonography whenever the conditions of measurable pulsatility were achieved (injection rates = 1.5 and 2.0 mL/s).

Unraveling the relationship between arterial flow and intra-aneurysmal hemodynamics

Arterial flow rate affects intra-aneurysmal hemodynamics but it is not clear how their relationship is. This uncertainty hinders the comparison among studies, including clinical evaluations, like a pre- and post-treatment status, since arterial flow rates may differ at each time acquisition. The purposes of this work are as follows: (1) To study how intra-aneurysmal hemodynamics changes within the full physiological range of arterial flow rates. (2) To provide characteristic curves of intra-aneurysmal velocity, wall shear stress (WSS) and pressure as functions of the arterial flow rate. Fifteen image-based aneurysm models were studied using computational fluid dynamics (CFD) simulations. The full range of physiological arterial flow rates reported in the literature was covered by 11 pulsatile simulations. For each aneurysm, the spatiotemporal-averaged blood flow velocity, WSS and pressure were calculated. Spatiotemporal-averaged velocity inside the aneurysm linearly increases as a function of the mean arterial flow (minimum R(2)>0.963). Spatiotemporal-averaged WSS and pressure at the aneurysm wall can be represented by quadratic functions of the arterial flow rate (minimum R(2)>0.996). Quantitative characterizations of spatiotemporal-averaged velocity, WSS and pressure inside cerebral aneurysms can be obtained with respect to the arterial flow rate. These characteristic curves provide more information of the relationship between arterial flow and aneurysm hemodynamics since the full range of arterial flow rates is considered. Having these curves, it is possible to compare experimental studies and clinical evaluations when different flow conditions are used.

Blood flow and tissue motion with ultrasound for vascular applications

Abstract This paper provides a simple description of the fundamental principles of ultrasound vascular imaging. The mechanisms which govern the flow/pressure relationship are presented. The various components of the Doppler systems, namely continuous Doppler, pulse Doppler, color flow imaging and a potential new application called arterial wall motion are described. The time-domain approach to the Doppler effect is shown to be efficient for understanding of the signal processing path. The capabilities of the ultrasound system are illustrated with several clinical examples dealing with the particular problem of carotid stenoses.

DEVICE FOR MEASURING THE SPEED OF BLOOD FLOWS BY ULTRASONIC ECHOGRAPHY AT AN INCREASED MEASURING SPEED

A device for the measurement of the speed of blood flows on the basis of a sequence of N successive echographic signals, comprising an M-point fixed-echo elimination device (200), followed by a unit (300) for measuring the speed by correlation/summation/interpolation of the N-M+1 independent signals supplied by the fixed-echo elimination device (220). In accordance with the invention, the measuring unit (300) comprises N F parallel processing channels (310j), each of which is formed by a filter (311j), a memory (312j) for storing the filtered N-M+1 signals, and a corrlation device (313j) which supplies N-M intercorrelation functions, and on the other hand an adder (320) which forms the mean value of the N F (N-M) intercorrelation functions obtained, an estimate of the relevant speed being supplied by an interpolation circuit (330) acts on the mean intercorrelation function.

Intra-Aneurysmal Flow Patterns: Illustrative Comparison among Digital Subtraction Angiography, Optical Flow, and Computational Fluid Dynamics

BACKGROUND AND PURPOSE: Digital subtraction angiography is the gold standard vascular imaging and it is used for all endovascular treatment of intracranial anerysms. Optical flow imaging has been described as a potential method to evaluate cerebral hemodynamics through DSA. In this study, we aimed to compare the flow patterns measured during angiography, by using an optical flow method, with those measured by using computational fluid dynamics in intracranial aneurysms. MATERIALS AND METHODS: A consecutive series of 21 patients harboring unruptured saccular intracranial aneurysms who underwent diagnostic angiography before treatment was considered. High-frame-rate digital subtraction angiography was performed to obtain an intra-aneurysmal velocity field by following the cardiac-modulated contrast wave through the vascular structures by using optical flow principles. Additionally, computational fluid dynamics modeling was performed for every case by using patient-specific inlet-boundary conditions measured with the optical flow method from both DSA and 3D rotational angiography datasets. Three independent observers compared qualitatively both the inflow direction and the apparent recirculation in regular DSA, optical flow images, and computational fluid dynamics flow patterns for each patient; κ statistics were estimated. RESULTS: We included 21 patients. In 14 of these 21, the flow patterns were conclusive and matching between the optical flow images and computational fluid dynamics within the same projection view (κ = .91). However, in only 8 of these 14 patients the optical flow images were conclusive and matching regular DSA images (observer κ = 0.87). In 7 of the 21 patients, the flow patterns in the optical flow images were inconclusive, possibly due to improper projection angles. CONCLUSIONS: The DSA-based optical flow technique was considered qualitatively consistent with computational fluid dynamics outcomes in evaluating intra-aneurysmal inflow direction and apparent recirculation. Moreover, the optical flow technique may provide the premises for new solutions for improving the visibility of flow patterns when contrast motion in DSA is not apparent. This technique is a diagnostic method to evaluate intra-aneurysmal flow patterns and could be used in the future for validation and patient evaluation.

Peak systolic or maximum intra-aneurysmal hemodynamic condition? Implications on normalized flow variables

BACKGROUND CFD has been used to assess intra-aneurysmal hemodynamics. Nevertheless, the lack of patient-specific flow information has triggered the possibility of implementing a wide variety of physiological flow conditions. Due to these uncertainties in the patient flow conditions, the normalization of the intra-aneurysmal hemodynamics is generally conducted. PURPOSE To investigate how intra-aneurysmal and arterial hemodynamics change over time when different physiological flow conditions are imposed. MATERIAL AND METHOD Eleven image-based aneurysm models were used in this study. CFD simulations were performed under pulsatile flows. Velocity magnitude and wall shear stress (WSS) were calculated during one cardiac cycle. RESULTS Maximum hemodynamic condition does not necessarily occurred at peak systole. The shifted time from peak systole to the time where the maximum hemodynamic condition occurs inside the aneurysm depends on the aneurysm size, flow rate, surrounding vasculature and the stabilities of flow patterns. Larger shifted times were observed with increasing aneurysm size as well as with reducing the flow rate. Moreover, the maximum hemodynamic condition can occur earlier than peak systole if flow patterns at parent artery change. Differences between peak systolic WSS and maximum WSS can be up to 65%. Moreover, the velocity magnitude and WSS depend on the selected segment of the parent artery, with relatively larger variability near peak systole than the rest of the cardiac cycle. More than 50% of differences were found between two arterial segments arbitrary selected for a given flow rate. CONCLUSIONS Our results indicate that if the highest intra-aneurysmal stress is calculated, then it is preferable to use the time instance where the maximum WSS occurred instead of the peak systolic WSS. Additionally, the normalization of intra-aneurysmal hemodynamics should be done with variables that do not depend on any arbitrary segment of the parent artery.

Modeling hemodynamics after flow diverter with a porous medium

In this work, we propose a novel approach for modeling hemodynamics after flow diverter (FD) stent in cerebral aneurysms. One image-based aneurysm model was used. The stented portion at the parent artery was modeled as a porous medium. Cell size, porous medium thickness and FD porosity were evaluated. Velocity magnitude and wall shear stress (WSS) inside the aneurysm were reduced after FD placement. Bigger cells compared to the stent strut diameter can be used. Thicker porous medium (which is equivalent of inserting multiple FDs) induces lower intra-aneurysmal velocity and WSS. Lower FD porosities produce higher reductions of intra-aneurysmal velocities, which diminish the contrast concentration inside the aneurysm and increase its residence time. Device design and multiple FD placements can be evaluated without remeshing the fluid domain.