11,077 research outputs found

    Estimation of wall shear stress using 4D flow cardiovascular MRI and computational fluid dynamics

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    Electronic version of an article published as Journal of mechanics in medicine and biology, 0, 1750046 (2016), 16 pages. DOI:10.1142/S0219519417500464 © World Scientific Publishing CompanyIn the last few years, wall shear stress (WSS) has arisen as a new diagnostic indicator in patients with arterial disease. There is a substantial evidence that the WSS plays a significant role, together with hemodynamic indicators, in initiation and progression of the vascular diseases. Estimation of WSS values, therefore, may be of clinical significance and the methods employed for its measurement are crucial for clinical community. Recently, four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has been widely used in a number of applications for visualization and quantification of blood flow, and although the sensitivity to blood flow measurement has increased, it is not yet able to provide an accurate three-dimensional (3D) WSS distribution. The aim of this work is to evaluate the aortic blood flow features and the associated WSS by the combination of 4D flow cardiovascular magnetic resonance (4D CMR) and computational fluid dynamics technique. In particular, in this work, we used the 4D CMR to obtain the spatial domain and the boundary conditions needed to estimate the WSS within the entire thoracic aorta using computational fluid dynamics. Similar WSS distributions were found for cases simulated. A sensitivity analysis was done to check the accuracy of the method. 4D CMR begins to be a reliable tool to estimate the WSS within the entire thoracic aorta using computational fluid dynamics. The combination of both techniques may provide the ideal tool to help tackle these and other problems related to wall shear estimation.Peer ReviewedPostprint (author's final draft

    Three-dimensional structure of the flow inside the left ventricle of the human heart

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    The laboratory models of the human heart left ventricle developed in the last decades gave a valuable contribution to the comprehension of the role of the fluid dynamics in the cardiac function and to support the interpretation of the data obtained in vivo. Nevertheless, some questions are still open and new ones stem from the continuous improvements in the diagnostic imaging techniques. Many of these unresolved issues are related to the three-dimensional structure of the left-ventricular flow during the cardiac cycle. In this paper we investigated in detail this aspect using a laboratory model. The ventricle was simulated by a flexible sack varying its volume in time according to a physiologically shaped law. Velocities measured during several cycles on series of parallel planes, taken from two orthogonal points of view, were combined together in order to reconstruct the phase averaged, three-dimensional velocity field. During the diastole, three main steps are recognized in the evolution of the vortical structures: i) straight propagation in the direction of the long axis of a vortex-ring originated from the mitral orifice; ii) asymmetric development of the vortex-ring on an inclined plane; iii) single vortex formation. The analysis of three-dimensional data gives the experimental evidence of the reorganization of the flow in a single vortex persisting until the end of the diastole. This flow pattern seems to optimize the cardiac function since it directs velocity towards the aortic valve just before the systole and minimizes the fraction of blood residing within the ventricle for more cycles

    Assessment of hemodynamic conditions in the aorta following root replacement with composite valve-conduit graft

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    This paper presents the analysis of detailed hemodynamics in the aortas of four patients following replacement with a composite bio-prosthetic valve-conduit. Magnetic resonance image-based computational models were set up for each patient with boundary conditions comprising subject-specific three-dimensional inflow velocity profiles at the aortic root and central pressure waveform at the model outlet. Two normal subjects were also included for comparison. The purpose of the study was to investigate the effects of the valve-conduit on flow in the proximal and distal aorta. The results suggested that following the composite valve-conduit implantation, the vortical flow structure and hemodynamic parameters in the aorta were altered, with slightly reduced helical flow index, elevated wall shear stress and higher non-uniformity in wall shear compared to normal aortas. Inter-individual analysis revealed different hemodynamic conditions among the patients depending on the conduit configuration in the ascending aorta, which is a key factor in determining post-operative aortic flow. Introducing a natural curvature in the conduit to create a smooth transition between the conduit and native aorta may help prevent the occurrence of retrograde and recirculating flow in the aortic arch, which is particularly important when a large portion or the entire ascending aorta needs to be replaced

    Improvements In computed tomography perfusion output using complex singular value decomposition and the maximum slope algorithm

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    OBJECTIVE: Determine if complex singular value decomposition (cSVD) used as preprocessing in the maximum slope algorithm reduces image noise of resultant physiologic parametric images. Noise will be decreased in the parametric maps of cerebral blood flow (CBF), cerebral blood volume (CBV) as compared to the same algorithm and data set with no cSVD applied. MATERIALS AND METHODS: A set of 10 patients (n=15) underwent a total combined 15 CT perfusion studies upon presenting with stroke symptoms. It was determined these patients suffered from occlusions resulting in a prolonged arrival time of blood to the brain. DICOM data files of these patients scans were selected based on this increased arrival delay. We compared the output of estimation calculations for cerebral blood flow (CBF), and cerebral blood volume (CBV), using preprocessing cSVD against the same scan data with no preprocessing cSVD. Image noise was assessed through the calculation of the standard deviation within specific regions of interest copied to specific areas of grey and white matter as well as CSF space. A decrease in the standard deviation values will indicate improvement in the noise level of the resultant images.. Results for the mean value within the regions of interest are expected to be similar between the groups calculated using cSVD and those calculated under the standard method. This will indicate the presence of minimal bias. RESULTS: Between groups of the standard processing method and the cSVD method standard deviation (SD) reductions were seen in both CBF and CBV values across all three ROIs. In grey matter measures of CBV, SD was reduced an average of 0.0034 mL/100g while measures of CBF saw SD reduced by an average of 0.073 mL/100g/min. In samples of white matter, standard deviations of CBV values were reduced on average by 0.0041mL/100g while CBF SD's were reduced by 0.073 mL/100g/min. CSF ROIs in CBV calculations saw SD reductions averaging 0.0047 mL/100g and reductions of 0.074 mL/100g/min in measures of CBF. Bias within CBV calculations was at most minimal as determined by no significant changes in mean calculated values. Calculations of CBF saw large downward bias in the mean values. CONCLUSIONS: The application of the cSVD method to preprocessing of CT perfusion imaging studies produces an effective method of noise reduction. In calculations of CBV, cSVD noise reduction results in overall improvement. In calculations of CBF, cSVD, while effective in noise reduction, caused mean values to be statistically lower than the standard method. It should be noted that there is currently no evaluation of which values can be considered more accurate physiologically. Simulations of the effect of noise on CBF showed a positive correlation suggesting that the CBF algorithm itself is sensitive to the level of noise

    High-density speckle contrast optical tomography (SCOT) for three dimensional tomographic imaging of the small animal brain

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    High-density speckle contrast optical tomography (SCOT) utilizing tens of thousands of source-detector pairs, was developed for in vivo imaging of blood flow in small animals. The reduction in cerebral blood flow (CBF) due to local ischemic stroke in a mouse brain was transcanially imaged and reconstructed in three dimensions. The reconstructed volume was then compared with corresponding magnetic resonance images demonstrating that the volume of reduced CBF agrees with the infarct zone at twenty-four hours.Peer ReviewedPostprint (author's final draft

    Heart applications of 4D flow

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    Four-dimensional (4D) flow sequences are an innovative type of MR sequences based upon phase contrast (PC) sequences which are a type of application of Angio-MRI together with the Time of Flight (TOF) sequences and Contrast-Enhanced Magnetic Resonance Acquisition (CE-MRA). They share the basic principles of PC, but unlike PC sequences, 4D flow has velocity encoding along all three flow directions and three-dimensional (3D) anatomic coverage. They guarantee the analysis of flow with multiplanarity on a post-processing level, which is a unique feature among MR sequences. Furthermore, this technique provides a completely new level to the in vivo flow analysis as it allows measurements in never studied districts such as intracranial applications or some parts of the heart never studied with echo-color-doppler, which is its sonographic equivalent. Furthermore, this technique provides a completely new level to the in vivo flow analysis as it allows accurate measurement of the flows in different districts (e.g., intracranial, cardiac) that are usually studied with echo-color-doppler, which is its sonographic equivalent. Of note, the technique has proved to be affected by less inter and intra-observer variability in several application. 4D-flow basic principles, advantages, limitations, common pitfalls and artefacts are described. This review will outline the basis of the formation of PC image, the construction of a 4D-flow and the huge impact the technique is having on the cardiovascular non-invasive examination. It will be then studied how this technique has had a huge impact on cardiovascular examinations especially on a central heart level

    Semiautomated Skeletonization of the Pulmonary Arterial Tree in Micro-CT Images

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    We present a simple and robust approach that utilizes planar images at different angular rotations combined with unfiltered back-projection to locate the central axes of the pulmonary arterial tree. Three-dimensional points are selected interactively by the user. The computer calculates a sub- volume unfiltered back-projection orthogonal to the vector connecting the two points and centered on the first point. Because more x-rays are absorbed at the thickest portion of the vessel, in the unfiltered back-projection, the darkest pixel is assumed to be the center of the vessel. The computer replaces this point with the newly computer-calculated point. A second back-projection is calculated around the original point orthogonal to a vector connecting the newly-calculated first point and user-determined second point. The darkest pixel within the reconstruction is determined. The computer then replaces the second point with the XYZ coordinates of the darkest pixel within this second reconstruction. Following a vector based on a moving average of previously determined 3- dimensional points along the vessel\u27s axis, the computer continues this skeletonization process until stopped by the user. The computer estimates the vessel diameter along the set of previously determined points using a method similar to the full width-half max algorithm. On all subsequent vessels, the process works the same way except that at each point, distances between the current point and all previously determined points along different vessels are determined. If the difference is less than the previously estimated diameter, the vessels are assumed to branch. This user/computer interaction continues until the vascular tree has been skeletonized

    4D reduced TE (RTE) spiral phase contrast NRI for rapid quantification and visualization of blood flow and hemodynamics.

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    4D flow Phase Contrast MRI is a relatively new technique in MRI which is capable of deriving time-resolved three-dimensional velocity fields in a 3D volume noninvasively. 4D flow imaging is a 3D k-space acquisition where for the third dimension, an additional phase-encoding step is required. The velocity field can then be used to obtain flow waveform, wall shear stress, vascular compliance, blood pressure, and other hemodynamic information. A significant limitation of 4D flow methods has been the requisite long scan times, requiring the patient to remain motionless at times on the order of 10-20 minutes, depending on scan parameters. The scan times may become more prohibitive in case of 4D cardiovascular studies where respiratory gating with navigator echoes is required. In this thesis the feasibility of using a reduced TE stack of spirals k-space acquisition for 4D flow imaging are investigated. Starting with fundamentals of MRI, the basics of Phase contrast and 4D flow MRI are thoroughly discussed in Chapter 1-3 and subsequently experimental phantom results are reported in Chapter 4, pointing to the feasibility of performing highly accurate 4D velocity and flow measurement with the proposed pulse sequence under a variety of flow conditions and with substantial reductions in scan time when compared to conventional 4D flow. In phantom studies, results based on the root mean square error criterion indicate that 4D Reduced TE (RTE) Spiral PC MRI is capable of providing the same level of accuracy as conventional 4D conventional PC MRI but in a much shorter scan time (30% reduction in scan time when imaging an FOV of 100*100*60 mm3 and spatial resolution of 1.5*1.5*3 mm3). Moreover, the proposed method has the added advantage of achieving the shorter echo time of 2 ms versus 3.6 ms for conventional 4D flow at Q=50ml/s and 1.57 ms versus 3.2 ms at the higher flow rate of Q=150 ml/s leading to more accurate assessment of flow distal to narrowings. Statistical results indicate that at low flow rates performance of both methods are similar. At higher flow rates, however, 4D RTE spiral flow achieves better accuracy. Qualitative results in phantom studies also revealed that at higher flow rates, better flow visualization was achieved with4D RTE spiral flow compared with conventional 4D flow. In the second part of Chapter 4, we also report on application of the proposed sequence, in-vivo, to 5 healthy volunteers and 5 subjects with mild to moderate Aortic Stenosis (AS) disease. Results from the proposed method were statistically correlated with velocity profiles derived from conventional 4D flow and Doppler Ultrasound. Results indicate that 4D RTE Spiral is capable of providing the same level of accuracy in flow measurement as Conventional 4D flow MRI for imaging of the aortic valve, but on average resulted in a 30% reduction in scan time and 45% reduction in echo time. 4D RTE Spiral was also able to achieve an echo time of 1.68 ms versus 2.9 ms for that of conventional 4D flow MRI, permitting less signal dephasing in the presence of jet flows distal to occlusions. With Doppler Ultrasound adopted as the reference method, 4D RTE Spiral flow measured peak velocity and maximum pressure gradient with a higher level of accuracy when compared to Conventional 4D flow MRI. Both methods measured left-ventricular out flow tract (LVOT) diameter, Aortic Valve (AV) eject time and time to AV peak with same accuracy. It is concluded that 4D RTE Spiral flow MRI is an excellent technique for flow measurement in cardiac patients who are unable to tolerate longer scan times, currently required by conventional 4D flow methods
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