Ioannis Pantos

Patient Radiation Doses in Interventional Cardiology Procedures

Interventional cardiology procedures result in substantial patient radiation doses due to prolonged fluoroscopy time and radiographic exposure. The procedures that are most frequently performed are coronary angiography, percutaneous coronary interventions, diagnostic electrophysiology studies and radiofrequency catheter ablation. Patient radiation dose in these procedures can be assessed either by measurements on a series of patients in real clinical practice or measurements using patient-equivalent phantoms. In this article we review the derived doses at non-pediatric patients from 72 relevant studies published during the last 22 years in international scientific literature. Published results indicate that patient radiation doses vary widely among the different interventional cardiology procedures but also among equivalent studies. Discrepancies of the derived results are patient-, procedure-, physician-, and fluoroscopic equipmentrelated. Nevertheless, interventional cardiology procedures can subject patients to considerable radiation doses. Efforts to minimize patient exposure should always be undertaken.

A new method of three‐dimensional coronary artery reconstruction from X‐ray angiography: Validation against a virtual phantom and multislice computed tomography

Objective: To develop and implement a method for three‐dimensional (3D) reconstruction of coronary arteries from conventional monoplane angiograms. Background: 3D reconstruction of conventional coronary angiograms is a promising imaging modality for both diagnostic and interventional purposes. Methods: Our method combines image enhancement, automatic edge detection, an iterative method to reconstruct the centerline of the artery and reconstruction of the diameter of the vessel by taking into consideration foreshortening effects. The X‐Ray‐based 3D coronary trees were compared against phantom data from a virtual arterial tree projected into two planes as well as computed tomography (CT)‐based coronary artery reconstructions in patients subjected to coronary angiography. Results: Comparison against the phantom arterial tree demonstrated perfect agreement with the developed algorithm. Visual comparison against the CT‐based reconstruction was performed in the 3D space, in terms of the direction angle along the centerline length of the left anterior descending and circumflex arteries relative to the main stem, and location and take‐off angle of sample bifurcation branches from the main coronary arteries. Only minimal differences were detected between the two methods. Inter‐ and intraobserver variability of our method was low (intra‐class correlation coefficients > 0.8). Conclusion: The developed method for coronary artery reconstruction from conventional angiography images provides the geometry of coronary arteries in the 3D space.

Simulation of cardiac motion on non-Newtonian, pulsating flow development in the human left anterior descending coronary artery.

This study aimed at investigating the effect of myocardial motion on pulsating blood flow distribution of the left anterior descending coronary artery in the presence of atheromatous stenosis. The moving 3D arterial tree geometry has been obtained from conventional x-ray angiograms obtained during the heart cycle and includes a number of major branches. The geometry reconstruction model has been validated against projection data from a virtual phantom arterial tree as well as with CT-based reconstruction data for the same patient investigated. Reconstructions have been obtained for a number of temporal points while linear interpolation has been used for all intermediate instances. Blood has been considered as a non-Newtonian fluid. Results have been obtained using the same pulse for the inlet blood flow rate but with fixed arterial tree geometry as well as under steady-state conditions corresponding to the mean flow rate. Predictions indicate that myocardial motion has only a minor effect on flow distribution within the arterial tree relative to the effect of the blood pressure pulse.

Flow Patterns at Stented Coronary Bifurcations Computational Fluid Dynamics Analysis

Background— The ideal bifurcation stenting technique is not established, and data on the hemodynamic characteristics at stented bifurcations are limited. Methods and Results— We used computational fluid dynamics analysis to assess hemodynamic parameters known affect the risk of restenosis and thrombosis at coronary bifurcations after the use of various single- and double-stenting techniques. We assessed the distributions and surface integrals of the time averaged wall shear stress (TAWSS), oscillatory shear index (OSI), and relative residence time (tr). Single main branch stenting without side branch balloon angioplasty or stenting provided the most favorable hemodynamic results (integrated values of TAWSS=4.13·10−4 N, OSI=7.52·10−6 m2, tr=5.57·10−4 m2/Pa) with bifurcational area subjected to OSI values >0.25, >0.35, and >0.45 calculated as 0.36 mm2,0.04 mm2, and 0 mm2, respectively. Extended bifurcation areas subjected to these OSI values were seen after T-stenting: 0.61 mm2, 0.18 mm2, and 0.02 mm2, respectively. Among the considered double-stenting techniques, crush stenting (integrated values of TAWSS=1.18·10−4 N, OSI=7.75·10−6 m2, tr=6.16·10−4 m2/Pa) gave the most favorable results compared with T-stenting (TAWSS=0.78·10−4 N, OSI=10.40·10−6 m2, tr=6.87·10−4 m2/Pa) or the culotte technique (TAWSS=1.30· 10−4 N, OSI=9.87·10−6 m2, tr=8.78·10−4 m2/Pa). Conclusions— In the studied models of computer simulations, stenting of the main branch with our without balloon angioplasty of the side branch offers hemodynamic advantages over double stenting. When double stenting is considered, the crush technique with the use of a thin-strut stent may result in improved immediate hemodynamics compared with culotte or T-stenting.

Vortex formation and recirculation zones in left anterior descending artery stenoses: computational fluid dynamics analysis

Flow patterns may affect the potential of thrombus formation following plaque rupture. Computational fluid dynamics (CFD) were employed to assess hemodynamic conditions, and particularly flow recirculation and vortex formation in reconstructed arterial models associated with ST-elevation myocardial infraction (STEMI) or stable coronary stenosis (SCS) in the left anterior descending coronary artery (LAD). Results indicate that in the arterial models associated with STEMI, a 50% diameter stenosis immediately before or after a bifurcation creates a recirculation zone and vortex formation at the orifice of the bifurcation branch, for most of the cardiac cycle, thus allowing the creation of stagnating flow. These flow patterns are not seen in the SCS model with an identical stenosis. Post-stenotic recirculation in the presence of a 90% stenosis was evident at both the STEMI and SCS models. The presence of 90% diameter stenosis resulted in flow reduction in the LAD of 51.5% and 35.9% in the STEMI models and 37.6% in the SCS model, for a 10 mmHg pressure drop. CFD simulations in a reconstructed model of stenotic LAD segments indicate that specific anatomic characteristics create zones of vortices and flow recirculation that promote thrombus formation and potentially myocardial infarction.

A new era in computed tomographic dose optimization: the impact of iterative reconstruction on image quality and radiation dose.

Abstract The ongoing evolution of computer technology has made the use of iterative reconstruction (IR) algorithms clinically applicable. We reviewed current literature on the clinical use of IR against filtered back projection algorithms in terms of image quality and radiation dose. Iterative reconstruction algorithms provide equal or better image quality compared with filtered back projection, with dose reduction ranging from 25% to 98.6%. However, several studies have reported that the superior results of IR regarding objective evaluation are not always favorably interpreted by radiologists. Further clinical evaluation is needed to certify the optimal tradeoff between imaging quality and radiation dose, and radiologists need to become more familiar with the new appearance of computed tomographic images.

Surgical ablation for atrial fibrillation

This paper reviews the history of surgical procedures developed for eradication of atrial fibrillation (AF) during cardiac surgery for structural heart disease, and in patients with AF without other indication for cardiac surgery. Current evidence indicates that, despite their proven efficacy, the Cox-Maze procedure and its modifications require cardiopulmonary bypass and cannot be easily justified in the case of AF without other indication for cardiac surgery. In patients undergoing cardiac surgery for mitral valve disease, concomitant ablation techniques using modifications of the Maze and alternative energy sources appear to be safe and effective in treating AF, especially in non-rheumatic disease. Minimally invasive epicardial ablation has been recently developed and can be performed on a beating heart through small access incision ports. Various techniques combining pulmonary vein isolation, ganglionated plexi ablation, and left atrial lines have been tried. Initial results are promising but further clinical experience is required to establish ideal lesion sets, appropriate energy sources, and the benefit-risk ratio of such an approach in patients without other indication for cardiac surgery. The role of surgical ablation in the current management of AF is under investigation.

In vivo wall shear stress measurements using phase-contrast MRI.

There is growing evidence to suggest that endothelial biology and atherosclerosis depend on arterial wall shear stress (WSS). We review the existing literature on in vivo measurements of WSS in healthy individuals using phase-contrast MRI, which is a promising, noninvasive technique for determinating various blood flow characteristics. WSS data exist for the following arteries: carotid, brachial, aorta and femoral. Measured values indicate that WSS is site specific, a finding which opposes the notion that physiological WSS values are maintained at a constant magnitude in all parts of the arterial system. Among the WSS values obtained at the same site by different investigators there is qualitative agreement; however, differences exist in absolute values mainly due to the dependence on the method used to obtain WSS values from velocity data.

Measurement of systolic and diastolic arterial wall shear stress in the ascending aorta

PURPOSE Wall shear stress (WSS) appears to contribute significantly in the initiation and progression of atherosclerotic disease. The purpose of this work is to present in vivo systolic and diastolic WSS calculations in the human ascending aorta by the application of three straightforward methodologies based on Poiseuilles theory of flow. MATERIALS AND METHODS Blood flow measurements were performed retrospectively in the ascending aorta of 20 non-atherosclerotic patients using phase-contrast MRI. WSS calculations were performed assuming Poiseuilles theory of flow based on average flow volume, average flow velocity and maximum flow velocity. Systolic and diastolic WSS values were calculated and compared with the calculated maximum and minimum values of WSS throughout the cardiac cycle. RESULTS Systolic WSS values calculated by average flow volume, average flow velocity and maximum flow velocity were similar (0.4+/-0.2N/m(2), 0.4+/-0.3N/m(2)and 0.4+/-0.2N/m(2), respectively). Diastolic WSS values calculated by maximum flow velocity were significantly higher (11.6+/-7.0x10(-2)N/m(2)) compared to values calculated by average flow volume (0.3+/-0.9x10(-2)N/m(2)) and average flow velocity (0.3+/-1.0x10(-2)N/m(2)). Comparison of systolic and diastolic WSS values with maximum and minimum WSS values showed that time instances of maximum and minimum blood flow velocities do not coincide with time instances of maximum and minimum blood flow volume. CONCLUSION In vivo calculation of WSS in the ascending aorta is feasible by phase-contrast MRI flow measurements and straightforward methodologies based on Poiseuilles theory of flow. However, measurements based on maximum flow velocity show larger deviations compared to measurements based on mean flow volume or mean flow velocity.

Wall shear stress calculation in ascending aorta using phase contrast magnetic resonance imaging. Investigating effective ways to calculate it in clinical practice

INTRODUCTION There is growing evidence that atherosclerosis, as well as endothelial biology, depend on arterial wall shear stress (WSS). Several methods of WSS calculation with varying degrees of complexity have been proposed. This study aimed at investigating whether the most straightforward and easier to apply of these methods give comparable results in clinical practice. METHODS Complete velocity encoding measurements using phase contrast magnetic resonance imaging were performed in 20 patients at a level perpendicular to the long axis of the ascending aorta approximately 2cm above the aortic valve. WSS was calculated at this location on maximum systole. MR imaging was accomplished on a 1.5T scanner. Four methods were applied to calculate WSS; three of them are based on the predictions of Poiseuilles theory of flow, while the last one is based on calculations resulting by the application of the definition of WSS. RESULTS WSS calculated with the above mentioned methods was found to be in the range 4.2+/-1.8 to 3.5+/-1.7dynes/m(2). The velocity profile at the site of measurements can be described with a parabolic equation of the form u=ar(2)+br+c with an average r(2)=0.83, which is in good agreement with Poiseuilles theory of flow. Comparison of the results shows no statistically significant differences between WSS measurements calculated with these methods. DISCUSSION The four methods are equivalent in calculating WSS at the ascending aorta when blood flow velocities have a good parabolic distribution.