Sebastian Graf

Université René Descartes - Paris 5, Lutetia Parisorum, Île-de-France, France

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Publications (61)47.36 Total impact

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    ABSTRACT: Diagnosis and management of thoracic aorta (TA) disease demand the assessment of accurate quantitative information of the aortic anatomy. We investigated the principal modes of variation in aortic 3-dimensional geometry paying particular attention to the curvilinear portion. Images were obtained from extended noncontrast multislice computed tomography scans, originally intended for coronary calcium assessment. The ascending, arch, and descending aortas of 500 asymptomatic patients (57±9 y, 81% male) were segmented using a semiautomated algorithm that sequentially inscribed circles inside the vessel cross-section. Axial planes were used for the descending aorta, whereas oblique reconstructions through a toroid path were required for the arch. Vessel centerline coordinates and the corresponding diameter values were obtained. Twelve size and shape geometric parameters were calculated to perform a principal component analysis. Statistics revealed that the geometric variability of the TA was successfully explained using 3 factors that account for ∼80% of total variability. Averaged aortas were reconstructed varying each factor in 5 intervals. Analyzing the parameter loadings for each principal component, the dominant contributors were interpreted as vessel size (46%), arch unfolding (22%), and arch symmetry (12%). Variables such as age, body size, and risk factors did not substantially modify the correlation coefficients, although some particular differences were observed with sex. We conclude that vessel size, arch unfolding, and symmetry form the basis for characterizing the variability of TA morphology. The numerical data provided in this study as supplementary material can be exploited to accurately reconstruct the curvilinear shape of normal TAs.
    Journal of thoracic imaging 11/2013; · 1.42 Impact Factor
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    ABSTRACT: Wave reflection is an important factor that influences pressure wave morphology and becomes more significant with aging, when cardiovascular risk increases. A pressure wave, measured at any location in the arterial tree, can be decomposed into its forward and backward components and depends on the corresponding amplitude and shifting time delays. Fractal dimension (FD) quantifies the time series complexity defined by its geometrical representation. Objective: The aim of this study was to evaluate the arterial pressure and diameter time series in order to assess the relationship between wave reflection and arterial pressure fractal dimension (FD). Methods: Simultaneous aortic pressure and diameter were measured in 14 conscious dogs. A pair of ultrasonic crystals, a pressure microtransducer and a pneumatic cuff occluder were positioned in the upper third of the descending aorta. Results: Total reflection induced by the occlusion maneuver decreased FD concomitant to the aortic stiffening. Conclusion: Arterial pressure fractality is highly dependent on wave reflection.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 07/2013; 2013:1960-1963.
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    ABSTRACT: Abstract Arterial vascular beds can be characterized considering the arterial segments in terms of their physical properties. These and other trees have an open structure based on repeated bifurcations, following fractal rules. Fractal dimension (FD) quantifies the time series complexity defined by its geometrical representation. Objective: To evaluate the arterial pressure and diameter time series in order to assess the influence of arterial tree structure in arterial pressure fractal dimension (FD). Methods: Simultaneous aortic pressure and diameter were measured in 14 conscious dogs. A pair of ultrasonic crystals, a pressure microtransducer and a pneumatic cuff occluder were positioned in the upper third of the descending aorta. Results: Total reflection induced by the occlusion maneuver decreased FD concomitant to the aortic stiffening and early wave reflection. Conclusion: Arterial pressure fractality is highly dependent on the arterial tree structure.
    Journal of Physics Conference Series 01/2013; 477(012030):1-8.
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    ABSTRACT: The simultaneous measurement of pressure and diameter in a central artery during 24 hours could bring new information about the variability of the arterial mechanical properties. In order to perform these measurements in an animal, it is essential to count with a device that could acquire the signals from the sensors with a proper preconditioning circuit and then transfer them wirelessly to be stored and processed remotely. In this paper we present the prototype of a new portable and autonomous device, capable of being hosted on the animal's back. Preliminary results obtained from a pilot experience in a sheep using the prototype are detailed and limitations are discussed.
    IEEE Latin America Transactions 01/2013; 11(1):126-129. · 0.22 Impact Factor
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    ABSTRACT: Measures of atherosclerosis burden like coronary artery calcification are performed using non-contrast heart CT. However, additional information can be derived from these studies, looking beyond the coronary arteries without exposing the patients to further radiation. We present a semi-automated method to assess ascending, arch and descending aorta geometry from non-contrast CT datasets in 250 normotensive patients. We investigated the effect of aging on thoracic aorta morphometry. The algorithm identifies the aortic centerline coordinates following a toroidal path for the curvilinear portion and axial planes for descending aorta. Then it reconstructs oblique planes orthogonal to the centerline direction and a circle fitting process estimates the vessel cross-section. Finally, global thoracic aorta dimensions (diameter, volume and length) and shape (vessel curvature and tortuosity, aortic arch width and height) are calculated. From a multivariate analysis, adjusted for gender and body-size area, aortic volume and arch width were the descriptors that better represented the aortic size and shape alterations with aging. The thoracic aorta suffers an expanding and unfolding process with aging that deserves further attention to prevent aortic aneurisms.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 08/2012; 2012:4986-9.
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    ABSTRACT: Non invasive local arterial blood pressure measurement has become a challenge over recent years. The aim of this study was to evaluate in a general population the validity of an alternative method to assess systolic local arterial blood pressure, from the analysis of B-mode diameter waveforms, and to estimate the accuracy when compared to carotid and femoral arterial tonometry. In 190 asymptomatic subjects (51±11 years, range: 24-73; pulse pressure: 51±11 mmHg, range: 31-93) systolic arterial pressure was obtained at the left carotid and left femoral artery by applanation tonometry (SBP(Car)_Ton and SBP(Fem)_Ton) and by automatic analysis of B-mode echographic images, calibrated using an iterative exponential model. Tonometry and echocardiography-derived pressure estimates correlated significantly (R=0.99, p<0.05). Mean difference between the two methods was only -2.5±5.0 mmHg for carotid artery (SBP(Car)_Ton: 122±18 mHg), and -2.1±5.7 mmHg for femoral artery (SBP(Fem)_Ton: 134±21 mmHg), independent of pressure level. In conclusion, alternative method was found to allow an accurate and precise estimation of systolic local arterial pressure, with an underestimation error of ∼ 2%.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 08/2012; 2012:5610-3.
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    ABSTRACT: Purpose. To evaluate in chronically haemodialysed patients (CHPs), if: (1) the vascular access (VA) position (upper arm or forearm) is associated with differential changes in upper limb arterial stiffness; (2) differences in arterial stiffness exist between genders associated with the VA; (3) the vascular substitute (VS) of choice, in biomechanical terms, depends on the previous VA location and CHP gender. Methods. 38 CHPs (18 males; VA in upper arm: 18) were studied. Left and right carotid-brachial pulse wave velocity (PWV(c-b)) was measured. In in vitro studies, PWV was obtained in ePTFE prostheses and in several arterial and venous homografts obtained from donors. The biomechanical mismatch (BM) between CHP native vessel (NV) and VS was calculated. Results/Conclusions. PWV(c-b) in upper limbs with VA was lower than in the intact contralateral limbs (P < 0.05), and differences were higher (P < 0.05) when the VA was performed in the upper arm. Differences between PWV(c-b) in upper limbs with VA (in the upper arm) with respect to intact upper limbs were higher (P < 0.05) in males. Independently of the region in which the VA was performed, the homograft that ensured the minimal BM was the brachial artery. The BM was highly dependent on gender and the location in the upper limb in which the VA was performed.
    International journal of nephrology. 01/2012; 2012:598512.
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    ABSTRACT: Previous experiences in animals showed a different behavior between the variability of pressure, arterial diameter and elasticity when they were registered for a couple of hours. To better understand arterial mechanics variability, we propose to measure simultaneously aortic pressure and diameter during 24 hours in a sheep. For that purpose, we developed a portable prototype device. It allows continuously recording physiological signals throughout the day and storing them in a solid state memory for later analysis. Pulse wave velocity and Peterson modulus were assessed beat-to-beat as arterial stiffness indexes. We identified 53,762 heart beats during 24 hours that were separated into 2 groups: below or above median mean pressure (71 mmHg). Mean diameter, pulse wave velocity and Peterson modulus increased for higher pressure values (p<0.05) whereas heart rate slowed down (p<0.05). Pressure-diameter loops were successfully recreated all along the experience. This new methodology sets the basis for further experiences involving the estimation of 24 hours arterial mechanics variability.
    Journal of Physics Conference Series 12/2011; 332(1):012008.
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    ABSTRACT: Traditional risk factors are involved in the development of coronary artery disease (CAD), but geometrical risk factors have also proved to be determinant. In this work we present a new method to construct a 3D map of the left coronary artery tree using CT image processing and skeletonization techniques. We computed cumulative length-volume functions through the coronary tree bifurcations and also the relationships between total tree volumes, total length, number of segments and bifurcations with respect to the presence and severity of atherosclerotic plaques. A total of 65 patients, 40 with and 20 without CAD were recruited. We found more vascular segments and bifurcations per patient in the CAD group. Accordingly, total cumulative length was longer in CAD patients (p<0.01) whereas total cumulative volume was similar between groups. Cumulative length-volume (L-V) relationships conformed to an allometric function L = KV.Vβ. The allometric slope β≈0.75 did not change with disease, whereas the allometric constant KV was lower in the healthy group (p<0.05) consistent with the literature. These results suggest that CAD patients follow a compensatory mechanism, dilating the vessels to maintain a normal coronary flow. The 3D coronary map offers useful quantitative information of the coronary morphometry.
    Journal of Physics Conference Series 12/2011; 332(1):012029.
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    ABSTRACT: Increased arterial stiffness is associated with an increased risk of cardiovascular events. Estimation of arterial stiffness using local pulse wave velocity (PWV) promises to be very useful for noninvasive diagnosis of arteriosclerosis. In this work we estimated in an instrumented sheep, the local aortic pulse wave velocity using two sonomicrometry diameter sensors (separated 7.5 cm) according to the transit time method (PWVTT) with a sampling rate of 4 KHz. We simultaneously measured aortic pressure in order to determine from pressure-diameter loops (PWVPDLoop), the "true" local aortic pulse wave velocity. A pneumatic cuff occluder was implanted in the aorta in order to compare both methods under a wide range of pressure levels. Mean pressure values ranged from 47 to 101 mmHg and mean proximal diameter values from 12.5. to 15.2 mm. There were no significant differences between PWVTT and PWVPDLoop values (451±43 vs. 447±48 cm/s, p = ns, paired t-test). Both methods correlated significantly (R = 0.81, p<0.05). The mean difference between both methods was only −4±29 cm/s, whereas the range of the limits of agreement (mean ± 2 standard deviation) was −61 to +53 cm/s, showing no trend. In conclusion, the diameter waveforms transit time method was found to allow an accurate and precise estimation of the local aortic PWV.
    Journal of Physics Conference Series 12/2011; 332(1):012010.
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    ABSTRACT: Endothelial function can be assessed non-invasively with ultrasound, analyzing the change of brachial diameter in response to transient forearm ischemia. We propose a new technique based in the same principle, but analyzing a continuous recording of carotid-radial pulse wave velocity (PWV) instead of diameter. PWV was measured on 10 healthy subjects of 22±2 years before and after 5 minutes forearm occlusion. After 59 ± 31 seconds of cuff release PWV decreased 21 ± 9% compared to baseline, reestablishing the same after 533 ± 65 seconds. There were no significant changes observed in blood pressure. When repeating the study one hour later in 5 subjects, we obtained a coefficient of repeatability of 4.8%. In conclusion, through analysis of beat to beat carotid-radial PWV it was possible to characterize the temporal profiles and analyze the acute changes in response to a reactive hyperemia. The results show that the technique has a high sensitivity and repeatability.
    Journal of Physics Conference Series 09/2011; 313(1):012009.
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    ABSTRACT: Assessing the geometry of the coronary arteries in a patient can help to better explain coronary artery disease (CAD) development. Allometric scaling functions were successfully applied to describe how essential materials are transported through recursive networks, also observed in the coronary tree. In this work we used skeletonization methods on multislice computed tomography (MSCT) images to render the coronary tree in 3D. Fifty subjects were recruited in two groups: 1) free from plaques and 2) with focal lesions. The left coronary tree was segmented using a custom algorithm with minimum user intervention. Vessels were separated using a stem-crown architecture. Cumulative arterial length (L) with volume (V) and vessel stem diameter (DSTEM) with distal L functions were analysed. In the allometric functions L = kvVβ and DSTEM = kLLγ, no significant differences were found between groups. The scaling exponent range for β was 0.6–0.9 and for γ was 0.1–0.4. Values were not different from other studies in pigs. In a Log–Log scatter plot in all patients, lines were parallel, confirmed with an ANCOVA. In other words, the allometric function stood for all patients. The idea that parameters from diseased hearts are expected to deviate from normal was not revealed in this study. We believe that CAD did not disturb the allometric relations due to the focal nature of the lesions and the absence of diffuse CAD. Further combination of MSCT with 3D morphological extraction algorithms can help to overcome the qualitative analysis of a patient vasculature and advance into new clinical quantitative perspectives.
    Artery Research 01/2011; 5(1):15-23.
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    ABSTRACT: Type 2 Diabetes Mellitus (DM), or adult-onset diabetes, is being considered as a new pandemic. Cardiovascular disease is the major cause of morbidity and mortality in type 2 DM, due to arterial structure and functional changes. Assessment of arterial structure and biomechanics, by non-invasive methods and parameters, can be used to detect early alterations related to DM. Three markers of vascular disease may help to a better evaluation of vascular dysfunction in type 2 DM patients: carotid intima-media thickness (IMTc), arterial stiffness, assessed by pulse wave velocity (PWV), and endothelial function, evaluated through the brachial artery flow-mediated dilation (FMD). Among these parameters, IMTc is considered a marker of structural vessel wall properties, and arterial stiffness reflects functional wall properties. Endothelial function represents the arterial way to actively regulate its diameter (smooth muscle-dependent actions) and its visco-elastic properties (wall elasticity and viscosity). IMTc is increased in patients with type 2 DM and other independent risk factors, such as: age, hyperlipidemia and duration of DM. Subjects with DM have shown increased arterial stiffness. Type 2 DM is associated with reductions in FMD (endothelial dysfunction), which has already been reported to be inversely and strongly related to the extent of hyperglycemia. The underlying patho-physiological mechanisms are complex and remain to be fully elucidated. A complete understanding of the association between arterial alterations and early detection, and type 2 DM, may be critical for the primary prevention of DM-related macro-vascular disease.
    Current Diabetes Reviews 10/2010; 6(6):367-377.
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    ABSTRACT: Type 2 Diabetes Mellitus (DM), or adult-onset diabetes, is being considered as a new pandemic. Cardiovascular disease is the major cause of morbidity and mortality in type 2 DM, due to arterial structure and functional changes. Assessment of arterial structure and biomechanics, by non-invasive methods and parameters, can be used to detect early alterations related to DM. Three markers of vascular disease may help to a better evaluation of vascular dysfunction in type 2 DM patients: carotid intimamedia thickness (IMTc), arterial stiffness, assessed by pulse wave velocity (PWV), and endothelial function, evaluated through the brachial artery flow-mediated dilation (FMD). Among these parameters, IMTc is considered a marker of structural vessel wall properties, and arterial stiffness reflects functional wall properties. Endothelial function represents the arterial way to actively regulate its diameter (smooth muscle-dependent actions) and its visco-elastic properties (wall elasticity and viscosity). IMTc is increased in patients with type 2 DM and other independent risk factors, such as: age, hyperlipidemia and duration of DM. Subjects with DM have shown increased arterial stiffness. Type 2 DM is associated with reductions in FMD (endothelial dysfunction), which has already been reported to be inversely and strongly related to the extent of hyperglycemia. The underlying patho-physiological mechanisms are complex and remain to be fully elucidated. A complete understanding of the association between arterial alterations and early detection, and type 2 DM, may be critical for the primary prevention of DM-related macro-vascular disease.
    Current diabetes reviews 09/2010; 6(6):367-77.
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    ABSTRACT: The ambulatory arterial stiffness index (AASI) is claimed to be a new estimator for arterial rigidity. It was recently defined as one minus the slope of the linear regression of systolic to diastolic ambulatory pressure during 24 h. Although several reports testify its clinical relevance, the explanation of how this new index is conceptually associated with arterial stiffness remains controversial. In this work we hypothesize that nonlinear arterial elasticity is behind AASI physiological principles. To that end, random number generators were used to emulate arterial cross-sectional area (CSA) during 24 h. Pressure values were calculated using linear and nonlinear elasticity models for rigid and compliant arteries. The AASI was calculated from simulated pressures and also analytically predicted for each model. Additionally, invasive aortic pressure and CSA were continuously measured in a conscious sheep during 24 h to test the nonlinear model. We found that analytical solutions agreed with simulation outcomes; for the nonlinear model, the AASI was higher in rigid arteries with respect to compliant arteries (0.51 versus 0.38) and the linear model systematically predicted AASI = 0. For in vivo pressure measurements, AASI was 0.31. Using the measured pulsatile CSA and an estimation of the elastic constant for the nonlinear model, the AASI was accurately predicted with errors below 5%. We conclude that the AASI is higher in stiffer arteries due to the nonlinear behavior of the arterial wall. With a nonlinear arterial function, the slope of the linear regression of diastolic to systolic pressures during 24 h depends on the product of an elastic constant by the pulsatile CSA. As the elastic constant dominates the product, the reported associations between the AASI and arterial stiffness indices now have a consistent explanation.
    Physiological Measurement 07/2010; 31(7):1037-46. · 1.50 Impact Factor
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    ABSTRACT: Assessing the individual geometry of the coronary arteries in a patient can help to explain diffuse artery disease. Some allometric functions, relating arterial length and volume, were verified in porcine arteries and human autopsies but not in vivo. In this work we use skeletonization methods on MSCT images to render the whole coronary tree in healthy and cardiovascular patients. Twenty patients with and without coronary artery disease were recruited. The coronary was segmented with minimum user intervention. Vessels were separated and measured. A 3D coronary map was individually calculated. The allometric length-volume function L=k(v)V(β) was evaluated in each patient and plotted in a Log-Log scale. The coefficient k(v) ranged 1.00 ± 0.35. Slopes ranged β = 0.69-0.88 and seemed to overlap in the scatter Log plot. The analysis of covariance verified this perception and concluded that lines were parallel. In other words, the allometric function stood for all patients. Values were not different from other studies in humans and pigs. The combination of multislice CT with morphological extraction algorithms was effective to extract allometric functions from coronary arteries in patients and can be easily applied in the clinic.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 01/2010; 2010:5716-9.
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    ABSTRACT: The value of mean arterial pressure (MBP) is of clinical importance, and is required for peripheral resistance calculation as well as for central blood pressure calibration procedures. MBP is usually estimated at the upper arm using systolic and diastolic sphygmomanometers pressure values, as 33% of pulse pressure (PP) above diastolic pressure. In a previous work, we proposed to use 38%. The aim of this study was to investigate the validity of the proposed formula to calculate MBP, when assessing subjects with a wide range of blood pressure and pulse wave velocity (PWV) levels. In 73 volunteers (56 ± 10 years, range: 27-82; pulse pressure: 59 ± 12 mmHg, range: 43-86; PWV: 10 ± 2 m/s, range: 8-17) arterial pressure waveforms were obtained at the left brachial artery by applanation tonometry. Diastolic (DBP) and systolic (SBP) brachial pressure were obtained with oscillometric device. Brachial-radial PWV was obtained at the same arm using mechano-transducers. MBP computed as 38% of PP above diastolic pressure, introduces an error of only 0.1% in brachial MBP estimation, independent of pressure and PWV levels.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 01/2010; 2010:2861-4.
  • Journal of Hypertension - J HYPERTENSION. 01/2010; 28.
  • S. Graf, R. Armentano
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    ABSTRACT: Applanation tonometry or arterial distension waves has been proposed to assess non-invasive recording of the arterial pressure waveform and magnitude in both central and peripheral arteries. The value of mean arterial pressure (MAP) that is used in the calibration procedure is of vital importance. The aim of this study was to investigate the differences in carotid and femoral arterial pressure values after calibration using MAP calculated from brachial tonometry and from approximate formula. In 30 volunteers (56±10 years, range: 38-73; pulse pressure: 57±15 mmHg, range: 31-93) arterial pressure waveforms were consecutively obtained at the left brachial, left carotid and left femoral arteries by applanation tonometry. Diastolic (DBPbra) and systolic (SBPbra) brachial pressure were obtained with oscillometric device. Calibration of carotid and femoral waveforms was assessed assuming MAP minus diastolic pressure constant throughout the arterial tree. Calibrated tonometric signals were used as reference values. Error in estimating MAP from SBPbra and DBPbra values using the traditional formula resulted in an underestimation of MAP of 3% when compared with the brachial MAP obtained by applanation tonometry. This error tended to an underestimation of carotid and femoral systolic pressure of 5%. Moreover, absolute error increases with increasing pulse pressure (PP) levels. By other side, MAP computed as 38% of PP above diastolic pressure, introduces an error of 0.1% in brachial MAP and of 0.2% in carotid and femoral systolic pressure estimation, independent of PP levels.
    12/2009: pages 793-796;
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    ABSTRACT: Echographic studies of the composition of atheromatous plaque make use of the median gray level, which provides an overall measure of echogenicity. We propose adding an additional dimension to this approach by dividing the lesion into layers and generating a curve that shows the variation in echolucency with depth. Femoral and carotid plaque in asymptomatic patients was investigated using both the median gray level and new layer Interobserver variability was assessed for both Three risk factors were studied: age, gender and smoking status. Differences in echogenicity-depth curves between different groups were assessed using two-way ANOVA. The two methods gave similar results for the mean echogenicity of carotid and femoral plaque. Echogenicity increased as a function of depth (r=0.96; P< .001). With the median gray level method, none of the risk factors produced a change in echogenicity. However, with the layer method, the echogenicity of femoral plaque was found to increase with age (P< .001), though gender had no effect. Moreover, the echogenicity of superficial layers was less in smokers than nonsmokers (P< .01). In carotid plaque, echogenicity increased with age (P< .01) and was higher in men (P< .01). The echogenicity of deep plaque was greater in smokers than nonsmokers (P< .05). The reproducibility of the two methods was similar. The layer method was more effective than the median gray level method for identifying the effect of age, sex and smoking status on the echogenicity of atheromatous plaque.
    Revista Espa de Cardiologia 10/2009; 62(9):984-91. · 3.20 Impact Factor

Publication Stats

287 Citations
47.36 Total Impact Points

Institutions

  • 2013
    • Université René Descartes - Paris 5
      • Faculté de Médecine
      Lutetia Parisorum, Île-de-France, France
  • 2006–2012
    • University of the Republic, Uruguay
      • Departamento de Fisiopatología
      Ciudad de Montevideo, Montevideo, Uruguay
  • 1998–2012
    • Favaloro University
      • Facultad de Ciencias Exactas y Naturales
      Buenos Aires, Buenos Aires F.D., Argentina
  • 2002–2009
    • University of Buenos Aires
      Buenos Aires, Buenos Aires F.D., Argentina