Orestis Vardoulis

On the Estimation of Total Arterial Compliance from Aortic Pulse Wave Velocity

Total arterial compliance (CT) is a main determinant of cardiac afterload, left ventricular function and arterio-ventricular coupling. CT is physiologically more relevant than regional aortic stiffness. However, direct, in vivo, non-invasive, measurement of CT is not feasible. Several methods for indirect CT estimation require simultaneous recording of aortic flow and pressure waves, limiting CT assessment in clinical practice. In contrast, aortic pulse wave velocity (aPWV) measurement, which is considered as the “gold standard” method to assess arterial stiffness, is noninvasive and relatively easy. Our aim was to establish the relation between aPWV and CT. In total, 1000 different hemodynamic cases were simulated, by altering heart rate, compliance, resistance and geometry using an accurate, distributed, nonlinear, one-dimensional model of the arterial tree. Based on Bramwell–Hill theory, the formula

Spatio-temporal regularization of flow-fields

C_{\text{T}} = k \cdot {\text{aPWV}}^{ - 2}

Validation of Algorithms for the Estimation of Pulse Transit Time: Where do We Stand Today?

was found to accurately estimate CT from aPWV. Coefficient k was determined both analytically and by fitting CT vs. aPWV data. CT estimation may provide an additional tool for cardiovascular risk (CV) assessment and better management of CV diseases. CT could have greater impact in assessing elderly population or subjects with elevated arterial stiffness, where aPWV seem to have limited prognostic value. Further clinical studies should be performed to validate the formula in vivo.

Improved Variational Denoising of Flow Fields with Application to Phase-Contrast MRI Data

Glaucoma results in an increase in the resistance of the aqueous humor outflow, which in turn leads to an increase of the intraocular pressure (IOP). Several treatments are proposed to reduce and stabilize the IOP that include medications, filtering surgery and glaucoma drainage devices (GDD). So far computational fluid dynamics (CFD) modeling of the eye drainage system has not yet been well studied. Therefore our goal was to provide a 3D CFD model of the eye based on the anatomy of a real human eye. Such a tool would serve for future evaluation of new glaucoma surgical techniques involving, for example, GDD. The model was based on stacks of microphotographs from human eye slides from which digital processing of the images of the eye structure and 3D reconstruction of the model were performed. Simulations of the distribution of pressure and flow velocity in the model of a healthy eye gave results comparable to physiology references. Mimicking glaucoma conditions led to an increase of the IOP from normal range, which went down to lower values after a filtering procedure. Further refinements in the boundary conditions for the filtering procedure shall improve the accuracy of this innovative tool for modeling glaucoma surgery.

Single breathhold, three-dimensional measurement of left atrial volume and function using sparse CINE CMR imaging with iterative reconstruction

We introduce a novel variational framework for the regularized reconstruction of time-resolved volumetric flow fields. Our objective functional takes the physical characteristics of the underlying flow into account in both the spatial and the temporal domains. For an efficient minimization of the objective functional, we apply a proximal-splitting algorithm and perform parallel computations. To demonstrate the utility of our variational method, we first denoise a simulated flow-field in the human aorta and show that our method outperforms spatial-only regularization in terms of signal-to-noise ratio (SNR). We then apply the scheme to a real 3D+time phase-contrast MRI dataset and obtain high-quality visualizations.

Impact of aortic grafts on hemodynamics: A 1D computational assessment

Recent advance in imaging modalities used frequently in clinical routine can provide description of the geometrical and hemodynamical properties of the arterial tree in great detail. The combination of such information with models of blood flow of the arterial tree can provide further information, such as details in pressure and flow waves or details in the local flow field. Such knowledge maybe be critical in understanding the development or state of arterial disease and can help clinicians perform better diagnosis or plan better treatments. In the present review, the state of the art of arterial tree models is presented, ranging from 0-D lumped models, 1-D wave propagation model to more complex 3-D fluid–structure interaction models. Our development of a generic and patient-specific model of the human arterial tree permitting to study pressure and flow waves propagation in patients is presented. The predicted pressure and flow waveforms are in good agreement with the in vivo measurements. We discuss the utility of these models in different clinical application and future development of interest.

An integrated self-healable electronic skin system fabricated via dynamic reconstruction of a nanostructured conducting network

Reference EPFL-ARTICLE-199594doi:10.1007/s10439-014-1006-8View record in Web of Science Record created on 2014-06-16, modified on 2017-05-10

A New Pulse Contour Analysis for Cardiac Output Estimation: The Systolic Volume Balance Method

We propose a new variational framework for the problem of reconstructing flow fields from noisy measurements. The formalism is based on regularizers penalizing the singular values of the Jacobian of the field. Specifically, we rely on the nuclear norm. Our method is invariant with respect to fundamental transformations and can be efficiently solved. We conduct numerical experiments on several phantom data and report improved performance compared to existing vectorial extensions of total variation and curl-divergence regularizations. Finally, we apply our reconstruction method to an experimentally-acquired phase-contrast MRI recording for enhancing the data visualization.