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  • 1
    Abstract: Michael Delles Karlsruhe Institute of Technology (Germany) Fabian Rengier Univ. Hospital Heidelberg (Germany) and German Cancer Research Ctr. (Germany) Sebastian Ley Univ. Hospital Heidelberg (Germany) Hendrik von Tengg-Kobligk Univ. Hospital Heidelberg (Germany) and German Cancer Research Ctr. (Germany) Hans-Ulrich Kauczor Univ. Hospital Heidelberg (Germany) Roland Unterhinninghofen and Rüdiger Dillmann Karlsruhe Institute of Technology (Germany) In cardiovascular diagnostics, the knowledge of blood pressure is essential for the physician. Nowadays, blood pressures are usually obtained by catheter measurements or sphygmomanometric methods. These techniques suffer from different drawbacks in terms of invasiveness, observable vessels and the resolution of the pressure values, respectively. Magnetic resonance imaging (MRI) offers a promising approach to establish a method for blood pressure measurements that is able to overcome these difficulties. Phase-contrast MRI is used to acquire velocity-encoded data. Fluid pressure gradients can be derived from the measured velocities using the Navier-Stokes equations. Unfortunately, this technique is known to suffer from a strong sensitivity to imaging quality. Especially the low signal-to-noise ratios (SNR) of phase contrast MRI data combined with the limited spatial and temporal resolution could severely reduce the reliability of computations. In this paper, we analyze computations of blood pressure gradients based on phase contrast MRI measurements of steady and pulsatile flow in a phantom. The influence of image quality of the velocity-encoded data as well as of different segmentation techniques is evaluated. In case of steady flow, the pressure gradient values computed via Navier-Stokes equations show good agreement with theoretical values if physical a-priori knowledge is incorporated. If a pulsatile aortic flow profile is applied, the computed pressure gradients generally match catheter measurements well. Nevertheless, an underestimation of pressure gradient peaks is observed. Different segmentation techniques influence the size of root mean squared errors between computation and measurement as well as their reduction by the use of higher SNRs.
    Type of Publication: Proceeding
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  • 2
    Keywords: SITE ; SURGERY ; BLOOD-PRESSURE ; PREVALENCE ; exercise ; ANGIOPLASTY ; TERM-FOLLOW-UP ; STENTS ; phase-contrast MRI ; NATIVE COARCTATION
    Abstract: To assess spatial and temporal pressure characteristics in patients with repaired aortic coarctation compared to young healthy volunteers using time-resolved velocity-encoded three-dimensional phase-contrast magnetic resonance imaging (4D flow MRI) and derived 4D pressure difference maps. After in vitro validation against invasive catheterization as gold standard, 4D flow MRI of the thoracic aorta was performed at 1.5T in 13 consecutive patients after aortic coarctation repair without recoarctation and 13 healthy volunteers. Using in-house developed processing software, 4D pressure difference maps were computed based on the Navier-Stokes equation. Pressure difference amplitudes, maximum slope of pressure amplitudes and spatial pressure range at mid systole were retrospectively measured by three readers, and twice by one reader to assess inter- and intraobserver agreement. In vitro, pressure differences derived from 4D flow MRI showed excellent agreement to invasive catheter measurements. In vivo, pressure difference amplitudes, maximum slope of pressure difference amplitudes and spatial pressure range at mid systole were significantly increased in patients compared to volunteers in the aortic arch, the proximal descending and the distal descending thoracic aorta (p 〈 0.05). Greatest differences occurred in the proximal descending aorta with values of the three parameters for patients versus volunteers being 19.7 +/- 7.5 versus 10.0 +/- 2.0 (p 〈 0.001), 10.9 +/- 10.4 versus 1.9 +/- 0.4 (p = 0.002), and 8.7 +/- 6.3 versus 1.6 +/- 0.9 (p 〈 0.001). Inter- and intraobserver agreements were excellent (p 〈 0.001). Noninvasive 4D pressure difference mapping derived from 4D flow MRI enables detection of altered intraluminal aortic pressures and showed significant spatial and temporal changes in patients with repaired aortic coarctation.
    Type of Publication: Journal article published
    PubMed ID: 25645544
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  • 3
    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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  • 4
    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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  • 5
    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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