4D flow MRI: impact of region of interest size, angulation and spatial resolution on aortic flow assessment

Objectives.In cardiovascular magnetic resonance, the 3D time-resolved phase-contrast technique, also known as 4D flow, is gaining increasing attention due to applications that exploit three-directional velocity encoding throughout the cardiac cycle. Blood flow volume assessment usually requires an expert to draw regions of interest (ROI) around the vessel cross section, whereas the errors involved in this estimation have not been thoroughly investigated. Our objective is to quantify the influence of ROI sizing, angulation and spatial resolution of the reconstructed plane employed in blood flow measurements using 4D flow.Approach.Three circular ROIs were drawn around the ascending, arch and descending aorta of healthy volunteers (n= 27) and patients with a dilated ascending aorta or bicuspid valve (n= 37). We applied systematic changes of ROI diameter (up to ±10%), tilt angle (up to ±25°) and spatial resolution (from 0.25 to 2 mm) of the reconstructed oblique planes, calculating the effects on net, forward and backward blood flow volumes.Main results.Patients had a larger ascending aorta than healthy volunteers with similar ages and male sex proportion (60 ± 15 y.o. vs 58 ± 16 y.o. and 84% vs 70%, respectively). Higher forward and backward flow volumes were observed in the ascending aorta and the aortic arch of the patients with respect to controls (p< 0.001), whereas net volumes were similar: 74.0 ± 20.8 ml versus 75.7 ± 21.8 ml (p= 0.37), respectively. The ascending aorta was the most sensitive to ROI modifications. Changes of ±10% in the ROI diameter and ±25° in tilt angles produced flow volume differences of up to 9 ml (10%) and 18 ml (15%) in controls and patients, respectively. Modifying the reconstructed planes spatial resolution produced flow volume changes below 2 ml.Significance.Since the setting of the ROI size and plane angle could produce errors that represent up to 20% of the forward and/or backward aortic flow volume, a good standardization for vessel segmentation and plane positioning is desirable.

High-resolution analysis with multiphase computed tomography of the left atrio-ventricular junction in mitral annular disjunction

Background

Mitral annular disjunction (MAD) has been associated with myxomatous mitral valve disease (MMVD) but functional analyses of the mitral annulus (MA) and the entire left atrio-ventricular junction (LAVJ) are lacking from high-resolution imaging to understand MAD physiopathology.

Purpose

Using multiphase cardiac computed tomography imaging (4D-MCCTI), we aimed at characterizing the morphological changes of MA and LAVJ throughout the cardiac cycle.

Methods

Volume imaging data sets through time were obtained from 4D-MCCTI of 2 groups of 10 patients as defined by echocardiographic diagnosis of MMVD and/or MAD: MMVD+MAD+, MMVD+MAD− and 20 case-controls without mitral regurgitation (MMVD−). Data were implemented in a custom software for 3D semi-automated delineation of 18 points around MA perimeter. Coordinates of these points in each of the 10 time-phases within an RR interval were used for MA reconstruction and calculation of areas, perimeters, 360°-diameters and parameters of planar/vertical deformation. Subsequently, left ventricle (LV) and left atrium (LA) inner contours were delineated.

Results

Groups were similar for age, body surface area and LV ejection fraction. Compared to the 2 other groups, MA expansion in MMVD+MAD+ started early after end-diastole, following LA expansion, and reached a maximum at end-systole (ES): 3D-area ES: 13.9±4.0cm2/m2, 3D-perimeterE S: 10.4±1.6cm/m2 versus respectively 8.8±3.0cm2/m2, 7.9±1.4cm/m2 in MMVD+MAD− (p&lt;0.001) and 6.1±1.3cm2/m2, 7.0±1.1cm/m2 in MVVD− (p&lt;0.001). However throughout the cardiac cycle, MA perimeter reduction was not altered in MMVD+MAD+: −14.0±6.3%, compared to MMVD+MAD−: −10.1±5.6% and MMVD−: −8.7±2.3% (p=0.09). MA shape modification was different in MMVD+MAD+ group, MA becoming more elliptical during systole, whereas MA reached a minimal eccentricity index (EcI) at ES in both MMVD+MAD− and MMVD− groups: EcIES = 0.63±0.09 versus 0.52±0.15 and 0.49±0.13 (p=0.04), respectively (Figure). Surprisingly, vertical deformation of MA increased during early systole in MMVD+MAD+ group when MA became more planar in the MMVD+MAD− and MMVD− groups.

Conclusions

MA function was globally preserved in MAD but was desynchronized with respect to LV contraction.

Eccentricity index of mitral annulus

Funding Acknowledgement

Type of funding source: None

Dynamics of the right atrio-ventricular junction in human: a spatial high-resolution study with multiphase computed tomography

Introduction

In order to improve knowledge of the tricuspid valve (TV) function and its coupling with the right atrio-ventricular junction (RAVJ) and right ventricle (RV), new four-dimensional high-definition imagery methods are mandatory (3D+t).

Purpose

Using an innovative reconstruction method based on multiphase cardiac computed tomography imaging (4D-MCCTI), we finely analyzed the morphological & dynamical features of tricuspid annulus (TA) and RAVJ components in order to assess new functional parameters of TV and RV functions.

Methods

Volume imaging data sets through time were obtained from 4D-MCCTI of 30 subjects (sex ratio 1, mean age 57±11y.) with no rhythm, valvular or ventricular abnormalities on echocardiography and implemented in a custom software for 3D semi-automated delineation of 18 points around TA perimeter. Coordinates of these points in each of the 10 time-phases within an RR interval were used to calculate specific geometrical features of TA such as 3D/2D areas, perimeters, 360°-diameters and vertical deformation. Subsequently, RV and Right Atrium (RA) inner contours were also delineated (Figure).

Results

TA shape was elliptical in horizontal projection with a mean eccentricity index (EcI) of 0.58±0.12; and saddle-shapped in vertical projection with a horn nearby the antero-septal commissure. This feature remained throughout the cardiac cycle but TA was more planar (minimal TA-height: 4.47±1.04 mm) and circular (minimal EcI=0.44±0.14) in mid-diastole when TA-3Darea and TA-3Dperimeter reached a maximum of 6.98±1.21 cm2/m2 and 7.41±0.91 cm, respectively. Correlation between TA-3Darea, TA-2Darea and latero-septal diameter (LSD) were R2=0.99 and R2=0.73, respectively. LSD was minimal in early-systole (18.83±3.04 mm/m2) and maximal in mid-diastole (20.04±3.05 mm/m2). Correlation of TA-3Darea with RV and RA cross-sectional areas were R2=0.82 and R2=0.71, respectively. Conversely, there was no significant correlation between TA, RV and RA concentric contractions.

Conclusions

Our method for 4D-MTCCI analysis has allowed confirming the shape and dynamics function of RAVJ throughout the cardiac cycle in healthy subjects, and giving new reference parameters for TV and RV evaluation.

Software multiplanar view of TA

Funding Acknowledgement

Type of funding source: None

Continuous assessment of carotid intima-media thickness applied to estimate a volumetric compliance using B-mode ultrasound sequences

Recent reports have shown that the carotid artery wall had significant movements not only in the radial but also in the longitudinal direction during the cardiac cycle. Accordingly, the idea that longitudinal elongations could be systematically neglected for compliance estimations became controversial. Assuming a dynamic change in vessel length, the standard measurement of cross-sectional compliance can be revised. In this work, we propose to estimate a volumetric compliance based on continuous measurements of carotid diameter and intima-media thickness (IMT) from B-mode ultrasound sequences. Assuming the principle of conservation of the mass of wall volume (compressibility equals zero), a temporal longitudinal elongation can be calculated to estimate a volumetric compliance. Moreover, elongations can also be estimated allowing small compressibility factors to model some wall leakage. The cross-sectional and the volumetric compliance were estimated in 45 healthy volunteers and 19 asymptomatic patients. The standard measurement underestimated the volumetric compliance by 25% for young volunteers (p < 0.01) and 17% for patients (p < 0.05). When compressibility factors different from zero were allowed, volunteers and patients reached values of 9% and 4%, respectively. We conclude that a simultaneous assessment of carotid diameter and IMT can be employed to estimate a volumetric compliance incorporating a longitudinal elongation. The cross-sectional compliance, that neglects the change in vessel length, underestimates the volumetric compliance.

Three-dimensional evaluation of thoracic aorta enlargement and unfolding in hypertensive men using non-contrast computed tomography

Aging produces a simultaneous thoracic aorta (TA) enlargement and unfolding. We sought to analyze the impact of hypertension on these geometric changes. Non-contrast computed tomography images were obtained from coronary artery calcium scans, including the entire aortic arch, in 200 normotensive and 200 hypertensive asymptomatic men. An automated algorithm reconstructed the vessel in three-dimensions, estimating orthogonal aortic sections along the whole TA pathway, and calculated several geometric descriptors to assess TA morphology. Hypertensive patients were older with respect to normotensive (P<0.001). Diameter and volume of TA ascending, arch and descending segments were higher in hypertensive patients with respect to normotensive (P<0.001) and differences persisted after adjustment for age. Hypertension produced an accelerated unfolding effect on TA shape. We found increments in aortic arch width (P<0.001), radius of curvature (P<0.001) and area under the arch curve (P<0.01) with a concomitant tortuosity decrease (P<0.05) and no significant change in aortic arch height. Overall, hypertension produced an equivalent effect of 2-7-years of aging. In multivariate analysis adjusted for age and hypertension treatment, diastolic pressure was more associated to TA size and shape changes than systolic pressure. These data suggest that hypertension accelerates TA enlargement and unfolding deformation with respect to the aging effect.

Association between mechanics and structure in arteries and veins: theoretical approach to vascular graft confection

Biomechanical and functional properties of tissue engineered vascular grafts must be similar to those observed in native vessels. This supposes a complete mechanical and structural characterization of the blood vessels. To this end, static and dynamic mechanical tests performed in the sheep thoracic and abdominal aorta and the cava vein were contrasted with histological quantification of their main constituents: elastin, collagen and muscle cells. Our results demonstrate that in order to obtain adequate engineered vascular grafts, the absolute amount of collagen fibers, the collagen/elastin ratio, the amount of muscle cells and the muscle cells/elastic fibers ratio are necessary to be determined in order to ensure adequate elastic modulus capable of resisting high stretches, an adequate elastic modulus at low and normal stretch values, the correct viscous energy dissipation, and a good dissipation factor and buffering function, respectively.

Arterial complex elastic modulus was preserved after an intercontinental cryoconserved exchange

There is a pressing need to obtain adequate vascular substitutes for arterial by-pass or reconstruction. Since the performance of venous and commercially prosthetic grafts is not ideal and the availability of autologous arteries is limited, the use of cryopreserved arteries has emerged as a very attractive alternative. In this sense, the development of an inter-continental network for cryopreserved tissue exchange would improve international cooperation increasing the possibilities of obtaining the requested materials. In this work, the effects of an inter-continental shipment, which includes cryopreservation, on the biomechanical properties of sheep aortas were evaluated by means of the arterial complex elastic modulus. It is shown that these properties were preserved after the shipment. The actual possibilities of establishing a network for arterial exchange for the international cooperation are discussed.

Upgrading BME studies in Buenos Aires, Argentine. Favaloro University

The Favaloro University initiated the academic path in biomedical sciences in Buenos Aires, Argentina. As a very promising area, the biomedical world offers the integration between several domains, complementing medicine with engineering topics. The profile of the graduated students proved to be very versatile, allowing their instantaneous incorporation to the competitive market in this vast field. The university strategy is to concentrate basic science in the first 3 years and add the specific biological/medical concepts in the last 2 years. The students achieve a bachelor degree that integrates their knowledge in basic sciences. Afterwards, they attend a professional 2 year cycle, choosing between biomedical, physics and medical computing branches. The results proved to be effective in terms of professional and academic quality, market insertion and even the creation of promising seeds for future enterprises.

Dynamics of cryopreserved human carotid arteries

The viscoelastic properties of the arterial wall are responsible for their functional role in the arterial system. Cryopreservation is widely used to preserve blood vessels for vascular reconstruction but is controversially suspected to affect the dynamic behaviour of these allografts. The aim of this study was to determine whether differences in the dynamic behaviour exist or not between fresh and cryopreserved human common carotid arteries (CCA). Using a previously developed mock circulation system, dynamic pressure-diameter tests were performed on segments of human fresh (n=10) and cryopreserved arteries (n=7). A diameter-pressure transfer function was designed to evaluate the wall dynamics. An adaptive model was fit to obtain its frequency response. Three models were tested. Results show that non-significant differences exist between wall dynamics of fresh and cryopreserved segments of human CCA.

Smooth muscle role on pulmonary arterial function during acute pulmonary hypertension in sheep

AIM

We determined the wall mechanical response of the pulmonary artery (PA) to acute pulmonary hypertension induced pharmacologically and by an occlusion maneuver, to study the vascular response of the local segment and its influence in the whole pulmonary circulation.

METHODS

Pulmonary pressure and diameter were measured in six anaesthetized sheep under steady-state conditions. Transient hypertension in the PA was induced by phenylephrine (PHE) and a high pressure (HP) mechanical occlusion aimed at producing the same pulse and mean pressure responses. A viscoelastic arterial wall model was applied and the elastic (E(pd)) and viscous (micro) indexes were obtained. The micro/E(pd) ratio was adopted to quantify the damping performance of the arterial wall segment. The diastolic time constant was used as an indicator of the whole pulmonary buffering function. The systemic pressure was always measured.

RESULTS

The pulmonary mean, systolic and pulse pressure increases (P < 0.05) were similar during PHE and HP, with respect to control. PHE also induced a systemic pressure rise (P < 0.05). The E(pd) elastic index increased during HP (P < 0.05) and tended to increase during PHE with respect to control. The viscous index micro only increased with PHE (P < 0.05) with respect to control and occlusion. The diastolic time constant increased with PHE with respect to control (P < 0.05).

CONCLUSIONS

A pressure rise in the PA, induced by an occlusion maneuver, increased local stiffness. Similar pressure rises with smooth muscle activation (PHE), produced both a stiffness and viscous index increase. In PHE resistance increases more than compliance decreases so that the global net effect is a longer decay time. Smooth-muscle activation enhances the local damping effect (micro/E(pd)), concomitant with the buffering function improvement.