Fast simulation of stent deployment with plastic beam elements

Coronary stent deployment is a reference cardiology intervention, used to treat atherosclerosis and prevent heart attacks. The outcomes of the intervention highly depend on the accuracy of the stent apposition, which could benefit from per-operative prediction tools. In this paper, we propose a fast and mechanically realistic 3D simulation of a coronary stent expansion. Our simulation relies on the finite element method and involves serially linked beam elements to model the slender geometry of a stent. The elements are implemented with a non-linear elasto-plastic behavior, describing realistically the complex deformation of a balloon-expandable stent. As a proof of concept, we simulated the free expansion of a coronary stent. The simulation output was compared with micro-CT data, acquired experimentally during the device expansion. Results show that the plastic beam model is able to reproduce successfully the final geometry of the stent. In addition, the use of 1D elements allows to achieve a significantly lower computational time than for equivalent literature simulations, based on 3D elements. This preliminary work highlights the compatibility of our method with clinical routine in terms of execution time. Further developments include the application of the method to more advanced simulation scenarios, with the addition of a personalized artery model.

Validation of cadmium-zinc-telluride camera for measurement of left ventricular systolic performance

BACKGROUND

There are paucity of data comparing measurements of left ventricular systolic performance using cadmium-zinc-telluride (CZT) semiconductor cameras with other imaging modalities. This study compared the new system with echocardiography (echo) and cardiac magnetic resonance (CMR) imaging.

METHODS

60 Patients presenting with ST-elevated myocardial infarction (MI) were included. Each patient underwent echo, myocardial perfusion imaging using Spectrum Dynamics D-SPECT(r) (CZT-SPECT), and CMR 6 weeks after MI. The primary endpoint was the agreement between CZT-SPECT and CMR for left ventricular ejection fraction (LVEF) measurement.

RESULTS

48 of the 60 patients underwent all 3 studies (echo, CMR, and CZT-SPECT) 40 days after admission. CZT-SPECT and CMR LVEF were well correlated (r = .79, P < .0001), as well as CZT-SPECT vs echo and CMR vs echo (r = .79 and .84, respectively, P < .0001). The segmental LV wall thickening and wall motion also showed good concordance between three techniques.

CONCLUSIONS

CZT-SPECT is reliable for LVEF measurement.

Contour segmentation of the intima, media, and adventitia layers in intracoronary OCT images: application to fully automatic detection of healthy wall regions

PURPOSE

Quantitative and automatic analysis of intracoronary optical coherence tomography images is useful and time-saving to assess cardiovascular risk in the clinical arena.

METHODS

First, the interfaces of the intima, media, and adventitia layers are segmented, by means of an original front propagation scheme, running in a 4D multi-parametric space, to simultaneously extract three non-crossing contours in the initial cross-sectional image. Second, information resulting from the tentative contours is exploited by a machine learning approach to identify healthy and diseased regions of the arterial wall. The framework is fully automatic.

RESULTS

The method was applied to 40 patients from two different medical centers. The framework was trained on 140 images and validated on 260 other images. For the contour segmentation method, the average segmentation errors were [Formula: see text] for the intima-media interface, [Formula: see text] for the media-adventitia interface, and [Formula: see text] for the adventitia-periadventitia interface. The classification method demonstrated a good accuracy, with a median Dice coefficient equal to 0.93 and an interquartile range of (0.78-0.98).

CONCLUSION

The proposed framework demonstrated promising offline performances and could potentially be translated into a reliable tool for various clinical applications, such as quantification of tissue layer thickness and global summarization of healthy regions in entire pullbacks.

Estimation of Myocardial Strain and Contraction Phase From Cine MRI Using Variational Data Assimilation

This paper presents a new method to estimate left ventricle deformations using variational data assimilation that combines image observations from cine MRI and a dynamic evolution model of the heart. The main contribution of the model is that it embeds parameters modeling the contraction / relaxation process. It estimates myocardial motion and contraction parameters simultaneously, providing accurate complementary information for diagnosis. The method was applied to synthetic datasets with known ground truth motion and to 47 patients MRI datasets acquired at three slice locations (base, mid-ventricle and apex). Radial and circumferential strain components were compared to those obtained with a reference tag tracking software, exhibiting good agreement with intraclass correlation coefficients (ICC) above 0.8. Results were also evaluated against wall motion score indices used to assess cardiac kinetics in clinical practice. The assimilation process overcame issues caused by temporal artifacts as a result of the dynamic model, compared to using the observation term alone. Moreover we found that the new dynamic model, consisting of a piecewise transport model acting independently on systole and diastole performed better than the standard continuous transport model, which oversmooths temporal variations. Estimated strain and contraction parameters significantly correlated to clinical scores, making them promising features for diagnosing not only hypokinesia but also dyskinesia.

Improved Estimation of Cardiac Function Parameters Using a Combination of Independent Automated Segmentation Results in Cardiovascular Magnetic Resonance Imaging

This work aimed at combining different segmentation approaches to produce a robust and accurate segmentation result. Three to five segmentation results of the left ventricle were combined using the STAPLE algorithm and the reliability of the resulting segmentation was evaluated in comparison with the result of each individual segmentation method. This comparison was performed using a supervised approach based on a reference method. Then, we used an unsupervised statistical evaluation, the extended Regression Without Truth (eRWT) that ranks different methods according to their accuracy in estimating a specific biomarker in a population. The segmentation accuracy was evaluated by estimating six cardiac function parameters resulting from the left ventricle contour delineation using a public cardiac cine MRI database. Eight different segmentation methods, including three expert delineations and five automated methods, were considered, and sixteen combinations of the automated methods using STAPLE were investigated. The supervised and unsupervised evaluations demonstrated that in most cases, STAPLE results provided better estimates than individual automated segmentation methods. Overall, combining different automated segmentation methods improved the reliability of the segmentation result compared to that obtained using an individual method and could achieve the accuracy of an expert.

Nonsupervised ranking of different segmentation approaches: application to the estimation of the left ventricular ejection fraction from cardiac cine MRI sequences

A statistical methodology is proposed to rank several estimation methods of a relevant clinical parameter when no gold standard is available. Based on a regression without truth method, the proposed approach was applied to rank eight methods without using any a priori information regarding the reliability of each method and its degree of automation. It was only based on a prior concerning the statistical distribution of the parameter of interest in the database. The ranking of the methods relies on figures of merit derived from the regression and computed using a bootstrap process. The methodology was applied to the estimation of the left ventricular ejection fraction derived from cardiac magnetic resonance images segmented using eight approaches with different degrees of automation: three segmentations were entirely manually performed and the others were variously automated. The ranking of methods was consistent with the expected performance of the estimation methods: the most accurate estimates of the ejection fraction were obtained using manual segmentations. The robustness of the ranking was demonstrated when at least three methods were compared. These results suggest that the proposed statistical approach might be helpful to assess the performance of estimation methods on clinical data for which no gold standard is available.

Biomechanical wall properties of human intracranial aneurysms resected following surgical clipping (IRRAs Project)

BACKGROUND AND PURPOSE

Individual rupture risk assessment of intracranial aneurysms is a major issue in the clinical management of asymptomatic aneurysms. Aneurysm rupture occurs when wall tension exceeds the strength limit of the wall tissue. At present, aneurysmal wall mechanics are poorly understood and thus, risk assessment involving mechanical properties is inexistent. Aneurysm computational hemodynamics studies make the assumption of rigid walls, an arguable simplification. We therefore aim to assess mechanical properties of ruptured and unruptured intracranial aneurysms in order to provide the foundation for future patient-specific aneurysmal risk assessment. This work also challenges some of the currently held hypotheses in computational flow hemodynamics research.

METHODS

A specific conservation protocol was applied to aneurysmal tissues following clipping and resection in order to preserve their mechanical properties. Sixteen intracranial aneurysms (11 female, 5 male) underwent mechanical uniaxial stress tests under physiological conditions, temperature, and saline isotonic solution. These represented 11 unruptured and 5 ruptured aneurysms. Stress/strain curves were then obtained for each sample, and a fitting algorithm was applied following a 3-parameter (C(10), C(01), C(11)) Mooney-Rivlin hyperelastic model. Each aneurysm was classified according to its biomechanical properties and (un)rupture status.

RESULTS

Tissue testing demonstrated three main tissue classes: Soft, Rigid, and Intermediate. All unruptured aneurysms presented a more Rigid tissue than ruptured or pre-ruptured aneurysms within each gender subgroup. Wall thickness was not correlated to aneurysmal status (ruptured/unruptured). An Intermediate subgroup of unruptured aneurysms with softer tissue characteristic was identified and correlated with multiple documented risk factors of rupture.

CONCLUSION

There is a significant modification in biomechanical properties between ruptured aneurysm, presenting a soft tissue and unruptured aneurysms, presenting a rigid material. This finding strongly supports the idea that a biomechanical risk factor based assessment should be utilized in the to improve the therapeutic decision making.

Comparison of different segmentation approaches without using gold standard. Application to the estimation of the left ventricle ejection fraction from cardiac cine MRI sequences

A statistical method is proposed to compare several estimates of a relevant clinical parameter when no gold standard is available. The method is illustrated by considering the left ventricle ejection fraction derived from cardiac magnetic resonance images and computed using seven approaches with different degrees of automation. The proposed method did not use any a priori regarding with the reliability of each method and its degree of automation. The results showed that the most accurate estimates of the ejection fraction were obtained using manual segmentations, followed by the semiautomatic methods, while the methods with the least user input yielded the least accurate ejection fraction estimates. These results were consistent with the expected performance of the estimation methods, suggesting that the proposed statistical approach might be helpful to assess the performance of estimation methods on clinical data for which no gold standard is available.

High-resolution coronary imaging by optical coherence tomography: Feasibility, pitfalls and artefact analysis

BACKGROUND

Optical coherence tomography is an imaging method that enables cardiologists to study atheromatous plaques, and to check the implantation and evolution of coronary stents. It is an invasive technique, providing high-resolution (10 microm) in vivo images, but with limitations and artefacts that need to be understood before the field of application can be extended.

AIM

To determine the feasibility and limitations of optical coherence tomography coronary imaging from a single-centre experience.

METHODS

We analysed the first 301 optical coherence tomography (version M2, LightLab Imaging) sequences obtained in our department from examination of 73 patients.

RESULTS

Results showed that 92% of sequences for selected lesions were usable, with a mean examination time of 17 min. Only one complication occurred (ventricular fibrillation, reduced by external electroshock). In our registry, sequence quality depended on operator experience (improving after 20 examinations), and was impaired by artefacts, especially in right coronary analysis and in arteries of greater than 3.5 mm calibre.

CONCLUSIONS

Proximal coronary occlusion and the distal flush quality currently required for quality imaging should no longer be indispensable with the new generation of optical coherence tomography systems.

In vivo supervised analysis of stent reendothelialization from optical coherence tomography

The aim of this study is to interactively assess reendothelialization of stents at an accuracy of down to a few micrometer by analyzing endovascular optical coherence tomography (OCT) sequences. Vessel wall and stent struts are automatically detected by using morphological, gradient, and symmetry operators coupled with active contour models; alerts are issued to ask for user supervision over some extreme irregular geometries caused by thrombotic lesions or dissections. A complete distance map is then computed from sparse distances measured between wall and struts. Missing values are interpolated by thin-plate spline (TPS) functions. Accuracy and robustness are increased by taking into account the inhomogeneity of data points and integrating in the same framework orthogonalized forward selection of support points, optimal selection of regularization parameters by generalized cross-validation, and rejection of detection outliers. Validation is performed on simulated data, phantom acquisitions and 11 typical in vivo OCT sequences. The comparison against manual expert measurements demonstrates a bias of the order of OCT resolution (less than 10 microm) and a standard deviation of the order of the strut width (less than 150 microm).

Evaluation of a stochastic approach to 2D-3D intensity-based registration

this paper assesses a new 2D-3D rigid registration method based on stochastic clustering and an enhanced estimator for mutual information. It combines precision, accuracy and acceptable computation time. Spine datasets (fluoroscopy and computed tomography) with their gold standard transformations are used. Both optimization method and similarity measure are assessed separately using standardized evaluation methodologies. Sub-millimeter accuracy and the high convergence rate obtained within one minute are compared to other quasi-global optimization processes such as particle filtering.

Early detection and monitoring of cartilage alteration in the experimental meniscectomised guinea pig model of osteoarthritis by 99mTc-NTP 15-5 scintigraphy

PURPOSE

This study in the meniscectomised guinea pig aimed to demonstrate that the radiotracer (99m)Tc-NTP 15-5 would have pathophysiological validity for in vivo osteoarthritis imaging.

METHODS

The specificity of (99m)Tc-NTP 15-5 for cartilage was determined in healthy animals (n = 13), by tissue radioactivity counting, joint autoradiography and scintigraphy. (99m)Tc-NTP 15-5 scintigraphy was performed at 20, 50, 80, 115, 130, 150 and 180 days after medial meniscectomy (n = 10 MNX) or sham operation (n = 5), and scintigraphic ratios (operated/contralateral) were calculated for femoral (F) and tibial (T) areas. F and T ratios were compared with those of (99m)Tc-MDP bone scintigraphy. At the study end-point, autoradiographic analysis of joint (99m)Tc-NTP 15-5 distribution and macroscopic scoring of cartilage integrity were performed.

RESULTS

The high and specific accumulation of (99m)Tc-NTP 15-5 in normal cartilage (about 5.5 +/- 1.7 % of injected dose/g of tissue), which permitted joint imaging with high contrast, was affected by osteoarthritis. In the MNX group, (99m)Tc-NTP 15-5 accumulation in cartilage within the operated joint, relative to the contralateral joint, was observed to change in the same animals as pathology progressed. Although F and T ratios were significantly higher in MNX (F = 1.7 +/- 0.2; T = 1.6 +/- 0.1) than in shams (F = 1.0 +/- 0.1; T = 1.0 +/- 0.1) at day 50, they were significantly lower in MNX (F = 0.6 +/- 0.1; T = 0.7 +/- 0.1) than in shams (F = 1.0 +/- 0.1; T = 0.9 +/- 0.1) at day 180. No change in (99m)Tc-MDP uptake was observed over 6 months. Macroscopic analysis confirmed features of osteoarthritis only in MNX knees.

CONCLUSION

These results in MNX guinea pigs provide additional support for the use of (99m)Tc-NTP 15-5 for in vivo imaging of osteoarthritis.

In vitro evaluation of 2D-digital subtraction angiography versus 3D-time-of-flight in assessment of intracranial cerebral aneurysm filling after endovascular therapy

BACKGROUND AND PURPOSE

The aim of this study was to evaluate 2D-digital subtraction angiographic (DSA) and 3D-time-of-flight (TOF) MR imaging in assessment of aneurysmal residue by using a pulsating silicon aneurysm model. For each imaging system, we studied intra- and interobserver reproducibility and the agreement between interpretations and reference measurements. We also examined how each imaging technique affected the operator's therapeutic decision.

METHODS

Two silicon aneurysm models depicting subarachnoidal aneurysms were used, one with a wide neck and one with a narrow neck. Each aneurysm model was placed in series on a pulsed flow circuit and was filled with Guglielmi detachable coils to simulate a clinical case. Each aneurysm was then gradually filled with silicon gel in increments of 10%, up to 100% to simulate different levels of occlusion (residual neck or dog ear, partial, complete) at each filling level. For each level of filling, we performed conventional 2D-DSA and 3D-TOF MR imaging. We submitted the images for examination by 2 senior medical staff with 2 readings per image. A combined reading of the 2 images was submitted to each expert to determine whether the 2 examinations were complementary.

RESULTS

The 2D-DSA analysis showed good reproducibility (k = 0.8 and k = 0.57) and agreement (k = 0.71) in describing "complete" treatments. The distinction between a "residual neck" and "partial treatment," however, was not reliable. The 2D-DSA provided a good description of the coil and silicon protrusion into the parent artery. The 3D-TOF analysis of the residual aneurysm, however, was not reproducible, though it was more effective than the 2D-DSA in evaluation of partially wide-necked aneurysms (k = 0.68 MR imaging vs k = 0.041 2D-DSA; P = .018). At the same filling level, the 2D-DSA analysis indicated repeat treatment more often than 3D-TOF analysis (P = .059).

CONCLUSION

The 2D-DSA remains the gold standard, but MR imaging is more effective in evaluating a "partial treatment." The 2D-DSA analysis indicated repeat treatment more often than the 3D-TOF for the same occlusion level. The distinction between "partial treatment" and a "residual neck" was not reliable with either method of evaluation.

Radiographic stereometry for non-metal-backed acetubular cups: 3D wear estimation and related uncertainty

The aim of this work was to assess the 3D wear of non-metal-backed acetubular cups from two conventional digitized radiographs. The centers of the femoral head and the metal ring in the equatorial plane of the cup are located using 3D pose techniques for spheres and circles from sampled points in the images. The method used to locate these points of interest and also estimate their covariance is fully described in previous work. The covariance is used to decrease the bias of the pose estimation, while bootstrapping decreases its variance and gives access to the directions of minimal variability between the two centers for each image. These directions are used to produce the final distance after reconstruction. Results are compared with the 2D technique working on a single anteroposterior (AP) radiograph and assuming null lateral wear. Validation is performed on acetubular cups: (a) simulated by Monte-Carlo, (b) implanted on a pelvic model, and (c) acquired on patients just after arthroplasty. The accuracy in wear for simulation increases from about one to four hundredths of a millimeter as caudal and cranial absolute angulations decreases from 45 degrees to 30 degrees . It is more difficult to assess for real prostheses, but results are shown to lie within the manufacturer's dimensional tolerances. Globally, the access to the lateral wear is obtained at the expense of confidence in the global linear wear (0.06 mm), which is doubled with regard to 2D technique but still satisfying for clinical use, whereas the bias of the measurement is decreased.

Monitoring of polyethylene wear in nonmetal-backed acetubular cups by digitized anteroposterior pelvic radiography

The aim of this study was to assess polyethylene wear in a total hip prosthesis by digitized radiography of the whole pelvis in the anteroposterior (AP) plane. The three-dimensional (3-D) pose of the nonmetal-backed acetubular cup, materialized by its metal ring and the femoral head made of metal or ceramic, was estimated using iterative algebraic algorithms with inner bias correction and bootstrapping for variance reduction. Points of interest were obtained by maximizing the correlation between sampled density profiles and 3-D geometric models degraded by the modulation transfer function (MTF) of the radiographic system and the film scanner. The error in the maximal correlation estimate were inferred from noise power spectra (NPS) and allowed the calculation of the point covariance matrix. Both NPS and MTF were modeled for each stage and estimated using least-square fitting of the overall NPS model to the autospectral density function calculated in stationary regions. Comparison of the radiographic time series was made possible by the high accuracy level and 3-D matching from the cup orientation. The feasibility of the full 3-D measurement, the assumption of negligible lateral wear and its influence on AP wear are discussed on simulated and real radiographic data.

Blood flow velocity estimation from x-ray densitometric data: an efficient numerical scheme for the inverse advection problem

In previously published studies, blood flow velocity from x-ray biplane angiography was measured by solving an inverse advection problem, relating velocity to bolus densities summed across sections. Both spatial and temporal velocity variations were recovered through a computationally expensive parameter estimation algorithm. Here we prove the existence and uniqueness of the solution on three sub-domains of the plane defined by the axial position along the vessel and the time of the angiographic sequence. A fast direct scheme was designed in conjunction with a regularization step stemming from the volume flow conservation law applied on consecutive segments. Its accuracy and immunity towards noise were tested on both simulated and real densitometric data. The relative error between the estimated and expected velocities was less than 5% for more than 90% of the points of the spatiotemporal plane with simulated densities normalized to 1.0 and a Gaussian additive noise of standard deviation 0.01. For densities reconstructed from a biplane angiographic sequence, increase in velocity is used as a functional index for the stenosis ratio and to characterize the sharing of flow at bifurcation.

Three-dimensional tracking of coronary arteries from biplane angiographic sequences using parametrically deformable models

A new method for coronary artery tracking in biplane digital subtraction is presented. The dynamic tracking of nonrigid objects from two views is achieved using a generalization of parametrically deformable models. Three-dimensional (3-D) Fourier descriptors used for shape representation are obtained from the two-dimensional (2-D) descriptors of the projections. A new constraint inferred from epipolar geometry is applied to the contour model. Direct 3-D tracking is compared with the classical approach in two steps: independent 2-D tracking in each of the two projection planes; 3-D reconstruction using the epipolar constraint. Convergence quality and accuracy of the 3-D reconstruction are analyzed for several sequences showing different displacement amplitudes, deformation rates and image contrasts.

Assessment of stenosis severity using a novel method to estimate spatial and temporal variations of blood flow velocity in biplane coronarography

The authors present a novel method to estimate absolute blood flow velocity in coronary arteries from biplane angiograms. Spatial and temporal velocity variations are derived giving simultaneously a direct geometric and an indirect functional index of stenosis severity, stenosis ratio and coronary flow reserve. No prior assumption concerning stenosis geometry is made. Deformable models are used to track a coronary artery segment dynamically in three dimensions. A densitometric map is obtained by summing densities across sections at every position along the previously calculated path and at every time of the cardiac cycle. An advection relationship between density and velocity is observed. The spatiotemporal velocity map is a solution of a nonlinear least-squares scheme. A simulation protocol based on simple geometric conformations and blood flow properties is used to assess numerical stability and immunity towards noise. Predicted results for temporal velocity variations are compared with the intracoronary Doppler recordings to test the model assumptions for basal state and hyperaemia examinations of the same patient. The stenosis ratio was accurate to within 3% for a simulated additive Gaussian noise with a standard deviation of 0.14. The limits of agreement between angiographic and Doppler velocities were -11.4 and 11.8 cm s-1 for a peak value of 23 cm s-1 (basal state) and -16.8 and 13.5 cm s-1 for a peak value of 52 cm s-1 (hyperaemia), corresponding to 18 and 3.5% errors on the average peak values and a 16% error on the coronary flow reserve. To summarize, the advection model derivation and its solution are presented. Simulated and experimental results corroborate the validity of the numerical schemes and support clinical applicability.