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  • BLOOD-FLOW  (7)
  • 1
    Keywords: MODEL ; VISUALIZATION ; DISEASE ; BLOOD-FLOW ; MAGNETIC-RESONANCE ; magnetic resonance imaging ; SURVEILLANCE ; ANGIOGRAPHY ; endovascular repair ; endovascular ; ANEURYSM REPAIR ; Aorta ; stent graft ; phase-contrast flow ; blood flow velocity
    Abstract: PURPOSE: To validate flow measurements within an aortic nickel-titanium (nitinol) stent graft using velocity-encoded cine magnetic resonance imaging (VEC MRI) and to assess intraobserver agreement of repeated flow measurements. MATERIALS AND METHODS: An elastic tube phantom mimicking the descending aorta was developed with the possibility to insert an aortic nitinol stent graft. Different flow patterns (constant, sinusoidal and pulsatile aortic flow) were applied by a gear pump. A two-dimensional phase-contrast sequence was used to acquire VEC perpendicular cross-sections at six equidistant levels along the phantom. Each acquisition was performed twice with and without stent graft, and each dataset was analysed twice by the same reader. The percental difference of the measured flow volume to the gold standard (pump setting) was defined as the parameter for accuracy. Furthermore, the intraobserver agreement was assessed. RESULTS: Mean accuracy of flow volume measurements was -0.45+/- 1.63% without stent graft and -0.18+/- 1.45% with stent graft. Slightly lower accuracy was obtained for aortic flow both without (-2.31%) and with (-1.29%) stent graft. Accuracy was neither influenced by the measurement position nor by repeated acquisitions. There was significant intraobserver agreement with an intraclass correlation coefficient of 0.87 (without stent graft, p〈0.001) and 0.80 (with stent graft, p〈0.001). The coefficient of variance was 0.25% without stent graft and 0.28% with stent graft. CONCLUSION: This study demonstrated high accuracy and excellent intraobserver agreement of flow measurements within an aortic nitinol stent graft using VEC MRI. VEC MRI may give new insights into the haemodynamic consequences of endovascular aortic repair.
    Type of Publication: Journal article published
    PubMed ID: 20888719
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  • 2
    Keywords: ACCURACY ; BLOOD-FLOW ; PATTERNS ; HEALTHY-VOLUNTEERS ; phase-contrast MRI ; CARDIOVASCULAR MAGNETIC-RESONANCE ; TO-NOISE RATIOS ; WHOLE HEART ; 4D FLOW ; COARCTATION
    Abstract: Three-dimensional velocity-encoded cine magnetic resonance imaging (3D VEC MRI) allows for calculation of secondary flow parameters that may be used to estimate prognosis of individual cardiovascular diseases. However, its accuracy has not been fully investigated yet. The purpose of this study was to validate aortic flow quantification by 3D VEC MRI in vitro and in vivo using stacked two-dimensional acquisitions. Time-resolved stacks of two-dimensional planes with three-directional velocity-encoding (stacked-2D-3dir-MRI) were acquired in an elastic tube phantom with pulsatile flow simulating aortic flow as well as in 11 healthy volunteers (23 +/- 2 years). Previously validated two-dimensional through-plane VEC MRI at six equidistant levels in vitro and three locations in vivo (ascending aorta/aortic arch/descending aorta) was used as reference standard. The percentage difference of the stacked-2D-3dir-MRI measurement to the reference standard was defined as the parameter for accuracy. For in vitro aortic flow, stacked-2D-3dir-MRI underestimated average velocity by -6.8% (p 〈 0.001), overestimated average area by 13.6% (p 〈 0.001), and underestimated average flow by -7.4% (p 〈 0.001). Accuracy was significantly higher in the field of view centre compared to off-centre (p = 0.001). In vivo, stacked-2D-3dir-MRI underestimated average velocity (all three locations p 〈 0.001) and overestimated average area at all three locations (p = n.s./〈0.001/〈0.001). Average flow was significantly underestimated in the ascending aorta (p = 0.035), but tended to be overestimated in the aortic arch and descending aorta. In conclusion, stacked-2D-3dir-MRI tends to overestimate average aortic area and to underestimate average aortic velocity, resulting in significant underestimation of average flow in the ascending aorta.
    Type of Publication: Journal article published
    PubMed ID: 22362096
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  • 3
    Keywords: MODELS ; BLOOD-FLOW ; PATTERNS ; WALL ; VELOCITY ; THORACIC AORTA ; STENT-GRAFT PLACEMENT ; DIAMETER ; NUMERICAL-SIMULATION ; FLUID-DYNAMICS
    Abstract: Conservative medical treatment is commonly first recommended for patients with uncomplicated Type-B aortic dissection (AD). However, if dissection-related complications occur, endovascular repair or open surgery is performed. Here we establish computational models of AD based on radiological three-dimensional images of a patient at initial presentation and after 4-years of best medical treatment (BMT). Computational fluid dynamics analyses are performed to quantitatively investigate the hemodynamic features of AD. Entry and re-entries (functioning as entries and outlets) are identified in the initial and follow-up models, and obvious variations of the inter-luminal flow exchange are revealed. Computational studies indicate that the reduction of blood pressure in BMT patients lowers pressure and wall shear stress in the thoracic aorta in general, and flattens the pressure distribution on the outer wall of the dissection, potentially reducing the progressive enlargement of the false lumen. Finally, scenario studies of endovascular aortic repair are conducted. The results indicate that, for patients with multiple tears, stent-grafts occluding all re-entries would be required to effectively reduce inter-luminal blood communication and thus induce thrombosis in the false lumen. This implicates that computational flow analyses may identify entries and relevant re-entries between true and false lumen and potentially assist in stent-graft planning.
    Type of Publication: Journal article published
    PubMed ID: 23523079
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  • 4
    Keywords: MODEL ; VISUALIZATION ; HEART ; BLOOD-FLOW ; blood flow ; magnetic resonance imaging ; PATTERNS ; 2D ; three-dimensional imaging ; 3D ; endovascular repair ; ANEURYSM REPAIR ; phase-contrast MRI ; Aorta ; stent graft
    Abstract: RATIONALE AND OBJECTIVES: Three-dimensional (3D) velocity-encoded cine (VEC) magnetic resonance imaging (MRI) has the potential to quantify 3D hemodynamic aspects known from computational fluid dynamics and to be used to identify hemodynamic risk factors for complications of endovascular aortic repair. The purpose of this study was to investigate the impact of an aortic nickel-titanium (nitinol) stent graft on the accuracy of flow measurements by 3D VEC MRI. MATERIALS AND METHODS: A pump generated pulsatile aortic flow in an elastic tube phantom mimicking the aorta. Stacked two-dimensional three-directional VEC MRI (stacked-2D-3dir-MRI), 3D three-directional VEC MRI (3D-3dir-MRI), and gold-standard 2D through-plane VEC MRI were applied before and after the insertion of an aortic nitinol stent graft. Six equidistant levels were analyzed twice by the same reader. The percentage difference of the measured flow rate from the gold standard was defined as the parameter of accuracy. RESULTS: The overall accuracy of in-stent flow measurements related to the gold standard was -5.4% for stacked-2D-3dir-MRI and -4.1% for 3D-3dir-MRI, demonstrating significant overall underestimation compared to the gold standard (P = .016 and P = .013). However, flow measurements with the stent graft were significantly overestimated by 4.1% using stacked-2D-3dir-MRI (P 〈 .001) and by 5.4% using 3D-3dir-MRI (P = .003) compared to identical measurements without the stent graft. In stacked-2D-3dir-MRI, this positive bias was significantly greater at the proximal and distal ends of the stent graft (P = .025). In 3D-3dir-MRI, measurements along the whole length of the stent graft were affected (P = .006). Intraobserver agreement was excellent, with intraclass correlation coefficients of 0.94 for stacked-2D-3dir-MRI (P 〈 .001) and 0.90 for 3D-3dir-MRI (P 〈 .001). CONCLUSIONS: Flow measurements within an aortic nitinol stent graft by 3D VEC MRI are feasible, but stent grafts may cause a significant positive bias.
    Type of Publication: Journal article published
    PubMed ID: 22177284
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  • 5
    Keywords: BLOOD-FLOW ; PATHOGENESIS ; MR-ANGIOGRAPHY ; CT ANGIOGRAPHY ; MANAGEMENT ; aneurysm ; ENDOVASCULAR TREATMENT ; STENT-GRAFT REPAIR ; ASCENDING AORTA ; FLOW PATTERNS
    Abstract: To characterize the heartbeat-related distension of dissected and non-dissected thoracic aortic segments in chronic aortic dissection type b (CADB) ECG-gated computed tomography angiography was performed in ten CADB patients. For 20 time points of the R-R interval, multiplanar reformations were taken at non-dissected (A, B) and dissected (C) aorta: ascending aorta (A), aortic vertex (B), 10 cm distal to left subclavian (Ct, true channel; Cf, false channel). Relative amplitudes of aortic area and major and minor axis diameter changes were quantified. Area amplitudes were 12.9 +/- 3.7%, 11.4 +/- 1.8%, 16.5 +/- 5.9% and 10.5 +/- 5.7% at A, B, Ct and Cf, respectively. Area amplitudes were significantly greater at Ct than at Cf and B (p 〈 0.05). Major axis diameter amplitudes were 7.7 +/- 1.9%, 6.2 +/- 1.3%, 5.9 +/- 2.0% and 6.1 +/- 3.6% at A, B, Ct and Cf, respectively. There were no differences in major axis diameter amplitudes. Minor axis diameter amplitudes were 6.7 +/- 2.1%, 8.4 +/- 1.9%, 12.7 +/- 6.3% and 6.0 +/- 2.2% at A, B, Ct and Cf, respectively. Minor axis diameter amplitudes were significantly the greatest at Ct (p 〈 0.05). In CADB, the heartbeat-related distension of aortic area and diameter is evenly distributed over the non-dissected aortic arch. As a result from different blood flow properties, there are significantly greater conformational changes in the true channel of the dissected aorta
    Type of Publication: Journal article published
    PubMed ID: 18648819
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  • 6
    Keywords: QUANTIFICATION ; BLOOD-FLOW ; DISSECTION ; DISTENSIBILITY ; 4-DIMENSIONAL COMPUTED-TOMOGRAPHY ; ANEURYSMS ; ENDOLUMINAL ARCH REPLACEMENT ; ROOT MOTION ; STENT-GRAFT MODELS ; WALL SHEAR-STRESS
    Abstract: The purpose of this study was to assess the magnitude and direction of respiratory displacement of the ascending and descending thoracic aorta during breathing maneuvers. In 11 healthy nonsmokers, dynamic magnetic resonance imaging was performed in transverse orientation at the tracheal bifurcation during maximum expiration and inspiration as well as tidal breathing. The magnitude and direction of aortic displacement was determined relatively to resting respiratory position for the ascending (AA) and descending (DA) aorta. To estimate a respiratory threshold for occurrence of distinct respiratory aortic motion, the latter was related to the underlying change in anterior-posterior thorax diameter. Compound displacement between maximum expiration and inspiration was 24.3 +/- A 6.0 mm for the AA in the left anterior direction and 18.2 +/- A 5.5 mm for the DA in the right anterior direction. The mean respiratory thorax excursion during tidal breathing was 8.9 +/- A 2.8 mm. The respiratory threshold, i.e., the increase in thorax diameter necessary to result in respiratory aortic displacement, was estimated to be 15.7 mm. The data suggest that after a threshold of respiratory thorax excursion is exceeded, respiration is accompanied by significant displacement of the thoracic aorta. Although this threshold may not be reached during tidal breathing in the majority of individuals, segmental differences during forced respiration impact on aortic geometry, may result in additional extrinsic forces on the aortic wall, and may be of significance for aortic prostheses designed for thoracic endovascular aortic repair.
    Type of Publication: Journal article published
    PubMed ID: 19434450
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  • 7
    Keywords: BLOOD-FLOW ; DYNAMICS ; ATHEROSCLEROSIS ; WALL ; aneurysm ; PULSATILE FLOW ; SHEAR-STRESS ; ARCH ; FLUID-STRUCTURE INTERACTION ; BIFURCATION
    Abstract: Purpose of this computational study is to examine the hemodynamic parameters of velocity fields and shear stress in the thoracic aorta with and without aneurysm, based on an individual patient case and virtual surgical intervention. These two cases, case I (with aneurysm) and II (without aneurysm), are analyzed by computational fluid dynamics. The 3D Navier-Stokes equations and the continuity equation are solved with an unsteady stabilized finite element method. The vascular geometries are reconstructed based on computed tomography angiography images to generate a patient-specific 3D finite element mesh. The input data for the flow waveforms are derived from MR phase contrast flow measurements of a patient before surgical intervention. The computed results show velocity profiles skewed towards the inner aortic wall for both cases in the ascending aorta and in the aortic arch, while in the descending aorta these velocity profiles are skewed towards the outer aortic wall. Computed streamlines indicate that flow separation occurs at the proximal edge of the aneurysm, i.e. computed flow enters the aneurysm in the distal region, and that there is essentially a single, slowly rotating, vortex within the aneurysm during most of the systole. In summary, after virtual surgical intervention in case II higher shear stress distribution along the descending aorta could be found, which may produce more healthy reactions in the endothelium and benefit of vascular reconstruction of an aortic aneurysm at this particular location.
    Type of Publication: Journal article published
    PubMed ID: 21316789
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