Assessment of coronary blood flow in humans using phase difference MR imaging. Comparison with intracoronary Doppler flow measurement

Semi-Automatic Reconstruction of Patient-Specific Stented Coronaries based on Data Assimilation and Computer Aided Design

PURPOSE

The interplay between geometry and hemodynamics is a significant factor in the development of cardiovascular diseases. This is particularly true for stented coronary arteries. To elucidate this factor, an accurate patient-specific analysis requires the reconstruction of the geometry following the stent deployment for a computational fluid dynamics (CFD) investigation. The image-based reconstruction is troublesome for the different possible positions of the stent struts in the lumen and the coronary wall. However, the accurate inclusion of the stent footprint in the hemodynamic analysis is critical for detecting abnormal stress conditions and flow disturbances, particularly for thick struts like in bioresorbable scaffolds. Here, we present a novel reconstruction methodology that relies on Data Assimilation and Computer Aided Design.

METHODS

The combination of the geometrical model of the undeployed stent and image-based data assimilated by a variational approach allows the highly automated reconstruction of the skeleton of the stent. A novel approach based on computational mechanics defines the map between the intravascular frame of reference (called L-view) and the 3D geometry retrieved from angiographies. Finally, the volumetric expansion of the stent skeleton needs to be self-intersection free for the successive CFD studies; this is obtained by using implicit representations based on the definition of Nef-polyhedra.

RESULTS

We assessed our approach on a vessel phantom, with less than 10% difference (properly measured) vs. a customized manual (and longer) procedure previously published, yet with a significant higher level of automation and a shorter turnaround time. Computational hemodynamics results were even closer. We tested the approach on two patient-specific cases as well.

CONCLUSIONS

The method presented here has a high level of automation and excellent accuracy performances, so it can be used for larger studies involving patient-specific geometries.

Double-Kissing Nanocrush for Bifurcation Lesions: Development, Bioengineering, Fluid Dynamics, and Initial Clinical Testing

BACKGROUND

When possible, a single-stent technique to treat coronary bifurcation disease is preferable. However, when 2 stents are required, there is scope to improve on existing techniques. The crush technique has already been improved with the introduction of double-kissing (DK) and minicrush. We sought to refine and simplify the minicrush technique, retaining its advantages while avoiding its disadvantages, by developing a DK nanocrush technique.

METHODS

The DK nanocrush method allows complete lesion coverage of a bifurcation lesion without excessive metal layers. This is achieved by positioning the side branch (SB) stent with minimal protrusion into the main branch (MB), implantation of the SB stent with an undeployed balloon in the MB, immediate kissing-balloon inflation with formation of a minimal neocarina, stenting the MB, recrossing the proximal part of the SB without crossing a double metal layer, and final kissing. We demonstrate this technique with benchtop implantation, microscopic computed tomographic reconstruction, computational fluid dynamics (CFD) modelling, and clinically with the use of angiographic and intravascular imaging.

RESULTS

The DK nanocrush was practically feasible and resulted in full ostial coverage. CFD analysis demonstrated minimally disturbed blood flow. The technique was successfully utilised in 9 patients with bifurcation lesions with excellent angiographic outcomes and no adverse events over 12 months.

CONCLUSIONS

The DK nanocrush technique may represent the ultimate refinement of the original crush technique with a number of practical and theoretical advantages. It remains to be tested against other bifurcation techniques in prospective trials.

A framework for computational fluid dynamic analyses of patient-specific stented coronary arteries from optical coherence tomography images

The clinical challenge of percutaneous coronary interventions (PCI) is highly dependent on the recognition of the coronary anatomy of each individual. The classic imaging modality used for PCI is angiography, but advanced imaging techniques that are routinely performed during PCI, like optical coherence tomography (OCT), may provide detailed knowledge of the pre-intervention vessel anatomy as well as the post-procedural assessment of the specific stent-to-vessel interactions. Computational fluid dynamics (CFD) is an emerging investigational tool in the setting of optimization of PCI results. In this study, an OCT-based reconstruction method was developed for the execution of CFD simulations of patient-specific coronary artery models which include the actual geometry of the implanted stent. The method was applied to a rigid phantom resembling a stented segment of the left anterior descending coronary artery. The segmentation algorithm was validated against manual segmentation. A strong correlation was found between automatic and manual segmentation of lumen in terms of area values. Similarity indices resulted >96% for the lumen segmentation and >77% for the stent strut segmentation. The 3D reconstruction achieved for the stented phantom was also assessed with the geometry provided by X-ray computed micro tomography scan, used as ground truth, and showed the incidence of distortion from catheter-based imaging techniques. The 3D reconstruction was successfully used to perform CFD analyses, demonstrating a great potential for patient-specific investigations. In conclusion, OCT may represent a reliable source for patient-specific CFD analyses which may be optimized using dedicated automatic segmentation algorithms.

Healthy and diseased coronary bifurcation geometries influence near-wall and intravascular flow: A computational exploration of the hemodynamic risk

Local hemodynamics has been identified as one main determinant in the onset and progression of atherosclerotic lesions at coronary bifurcations. Starting from the observation that atherosensitive hemodynamic conditions in arterial bifurcation are majorly determined by the underlying anatomy, the aim of the present study is to investigate how peculiar coronary bifurcation anatomical features influence near-wall and intravascular flow patterns. Different bifurcation angles and cardiac curvatures were varied in population-based, idealized models of both stenosed and unstenosed bifurcations, representing the left anterior descending (LAD) coronary artery with its diagonal branch. Local hemodynamics was analyzed in terms of helical flow and exposure to low/oscillatory shear stress by performing computational fluid dynamics simulations. Results show that bifurcation angle impacts lowly hemodynamics in both stenosed and unstenosed cases. Instead, curvature radius influences the generation and transport of helical flow structures, with smaller cardiac curvature radius associated to higher helicity intensity. Stenosed bifurcation models exhibit helicity intensity values one order of magnitude higher than the corresponding unstenosed cases. Cardiac curvature radius moderately affects near-wall hemodynamics of the stenosed cases, with smaller curvature radius leading to higher exposure to low shear stress and lower exposure to oscillatory shear stress. In conclusion, the proposed controlled benchmark allows investigating the effect of various geometrical features on local hemodynamics at the LAD/diagonal bifurcation, highlighting that cardiac curvature influences near wall and intravascular hemodynamics, while bifurcation angle has a minor effect.

Influence of Variable Native Arterial Diameter and Vasculature Status on Coronary Diagnostic Parameters

In current practice, diagnostic parameters, such as fractional flow reserve (FFR) and coronary flow reserve (CFR), are used to determine the severity of a coronary artery stenosis. FFR is defined as the ratio of hyperemic pressures distal (p˜rh) and proximal (p˜ah) to a stenosis. CFR is the ratio of flow at hyperemic and basal condition. Another diagnostic parameter suggested by our group is the pressure drop coefficient (CDP). CDP is defined as the ratio of the pressure drop across the stenosis to the upstream dynamic pressure. These parameters are evaluated by invasively measuring flow (CFR), pressure (FFR), or both (CDP) in a diseased artery using guidewire tipped with a sensor. Pathologic state of artery is indicated by lower CFR (<2). Similarly, FFR lower than 0.75 leads to clinical intervention. Cutoff for CDP is under investigation. Diameter and vascular condition influence both flow and pressure drop, and thus, their effect on FFR and CDP was studied. In vitro experiment coupled with pressure-flow relationships from human clinical data was used to simulate pathophysiologic conditions in two representative arterial diameters, 2.5 mm (N1) and 3 mm (N2). With a 0.014 in. (0.35 mm) guidewire inserted, diagnostic parameters were evaluated for mild (∼64% area stenosis (AS)), intermediate (∼80% AS), and severe (∼90% AS) stenosis for both N1 and N2 arteries, and between two conditions, with and without myocardial infarction (MI). Arterial diameter did not influence FFR for clinically relevant cases of mild and intermediate stenosis (difference < 5%). Stenosis severity was underestimated due to higher FFR (mild: ∼9%, intermediate: ∼ 20%, severe: ∼ 30%) for MI condition because of lower pressure drops, and this may affect clinical decision making. CDP varied with diameter (mild: ∼20%, intermediate: ∼24%, severe: by 2.5 times), and vascular condition (mild: ∼35%, intermediate: ∼14%, severe: ∼ 9%). However, nonoverlapping range of CDP allowed better delineation of stenosis severities irrespective of diameter and vascular condition.

Sequential Structural and Fluid Dynamic Numerical Simulations of a Stented Bifurcated Coronary Artery

Despite their success, stenting procedures are still associated to some clinical problems like sub-acute thrombosis and in-stent restenosis. Several clinical studies associate these phenomena to a combination of both structural and hemodynamic alterations caused by stent implantation. Recently, numerical models have been widely used in the literature to investigate stenting procedures but always from either a purely structural or fluid dynamic point of view. The aim of this work is the implementation of sequential structural and fluid dynamic numerical models to provide a better understanding of stenting procedures in coronary bifurcations. In particular, the realistic geometrical configurations obtained with structural simulations were used to create the fluid domains employed within transient fluid dynamic analyses. This sequential approach was applied to investigate the final kissing balloon (FKB) inflation during the provisional side branch technique. Mechanical stresses in the arterial wall and the stent as well as wall shear stresses along the arterial wall were examined before and after the FKB deployment. FKB provoked average mechanical stresses in the arterial wall almost 2.5 times higher with respect to those induced by inflation of the stent in the main branch only. Results also enlightened FKB benefits in terms of improved local blood flow pattern for the side branch access. As a drawback, the FKB generates a larger region of low wall shear stress. In particular, after FKB the percentage of area characterized by wall shear stresses lower than 0.5 Pa was 79.0%, while before the FKB it was 62.3%. For these reasons, a new tapered balloon dedicated to bifurcations was proposed. The inclusion of the modified balloon has reduced the mechanical stresses in the proximal arterial vessel to 40% and the low wall shear stress coverage area to 71.3%. In conclusion, these results show the relevance of the adopted sequential approach to study the wall mechanics and the hemodynamics created by stent deployment.

Evaluation of coronary blood flow velocity during cardiac arrest with circulation maintained through mechanical chest compressions in a porcine model

Background

Mechanical chest compressions (CCs) have been shown capable of maintaining circulation in humans suffering cardiac arrest for extensive periods of time. Reports have documented a visually normalized coronary blood flow during angiography in such cases (TIMI III flow), but it has never been actually measured. Only indirect measurements of the coronary circulation during cardiac arrest with on-going mechanical CCs have been performed previously through measurement of the coronary perfusion pressure (CPP). In this study our aim was to correlate average peak coronary flow velocity (APV) to CPP during mechanical CCs.

Methods

In a closed chest porcine model, cardiac arrest was established through electrically induced ventricular fibrillation (VF) in eleven pigs. After one minute, mechanical chest compressions were initiated and then maintained for 10 minutes upon which the pigs were defibrillated. Measurements of coronary blood flow in the left anterior descending artery were made at baseline and during VF with a catheter based Doppler flow fire measuring APV. Furthermore measurements of central (thoracic) venous and arterial pressures were also made in order to calculate the theoretical CPP.

Results

Average peak coronary flow velocity was significantly higher compared to baseline during mechanical chests compressions and this was observed during the entire period of mechanical chest compressions (12 - 39% above baseline). The APV slowly declined during the 10 min period of mechanical chest compressions, but was still higher than baseline at the end of mechanical chest compressions. CPP was simultaneously maintained at > 20 mmHg during the 10 minute episode of cardiac arrest.

Conclusion

Our study showed good correlation between CPP and APV which was highly significant, during cardiac arrest with on-going mechanical CCs in a closed chest porcine model. In addition APV was even higher during mechanical CCs compared to baseline. Mechanical CCs can, at minimum, re-establish coronary blood flow in non-diseased coronary arteries during cardiac arrest.

Blood Flow in Coronary Artery Bypass Vein Grafts: Volume versus Velocity at Cardiovascular MR Imaging

Forty-nine patients with previous bypass surgery underwent coronary angiography and cardiovascular magnetic resonance (MR) imaging of single-vein bypass grafts. Volume flow and velocity analyses were performed and compared on MR velocity maps. Bland-Altman analysis showed close agreement between the two types of analysis. Comparison of areas under the receiver operating characteristic curve revealed no significant differences between the analyses for detection of stenoses of 70% or greater. Diagnostic accuracy for volume flow and velocity parameters was 92% and 93%, respectively. Velocity analysis appears to be the preferred method, because it is less time-consuming and has a similar diagnostic accuracy to volume flow analysis.

MR flow mapping in coronary artery bypass grafts: a validation study with Doppler flow measurements

PURPOSE

To validate fast magnetic resonance (MR) flow mapping with intravascular Doppler flow measurements in vitro and in patients with nonstenotic and stenotic coronary artery bypass grafts.

MATERIALS AND METHODS

MR and Doppler flow measurements were performed in a small-diameter flow phantom with physiologic flow conditions and at baseline and during adenosine stress in 27 grafts in 23 patients, who were scheduled for cardiac catheterization. At invasive analysis, the grafts were divided into those with stenosis of less than 50% (nonstenotic) and those with stenosis greater than or equal to 50% (stenotic). In vitro velocity values and velocity values in nonstenotic and stenotic grafts were compared with linear regression analysis, and the in vitro interstudy variability was determined.

RESULTS

Excellent correlations in average peak velocity (r = 0.99, P <.001) and diastolic peak velocity (r = 0.99, P <.001) were demonstrated in vitro between MR and Doppler flow measurements, with less than 5% interstudy variability. MR and Doppler flow measurements revealed good correlations in peak velocity and velocity reserve both in nonstenotic (n = 20) (average peak velocity: r = 0.81, P <.001; diastolic peak velocity: r = 0.83, P <.001; velocity reserve: r = 0.56, P =.010) and stenotic (n = 7) (average peak velocity: r = 0.83, P <.001; diastolic peak velocity: r = 0.78, P =.001; velocity reserve: r = 0.70, P =.078) grafts.

CONCLUSION

Fast MR flow mapping provides noninvasive measures of peak velocity and velocity reserve, which closely correlate with Doppler values both in vitro and in nonstenotic and stenotic grafts.

Accuracy of segmented MR velocity mapping to measure small vessel pulsatile flow in a phantom simulating cardiac motion

The purpose of this study was to investigate the accuracy of conventional, segmented, and echo-shared MR velocity mapping sequences to measure pulsatile flow in small moving vessels using a phantom with simulated cardiac motion. The phantom moved either cyclically in-plane, through-plane, in- and through-plane, or was stationary. The mean error in average flow was -2% +/- 3% (mean +/- SD) for all sequences under all conditions, with or without background correction, as long as the region of interest (ROI) size was equal to the vessel cross-sectional size. Overestimation of flow as a result of an oversized ROI was less than 20%, and independent of field of view (FOV) and matrix, as long as the offset in angle between the imaging plane and flow direction was less than 10 degrees. Segmented velocity mapping sequences are surprisingly accurate in measuring average flow and render flow profiles in small moving vessels despite the blurring in the images due to vessel motion. J. Magn. Reson. Imaging 2001;13:722-728.

Evaluation of coronary artery bypass grafts by magnetic resonance imaging

Magnetic resonance (MR) angiography and flow mapping have the potential to become a major noninvasive diagnostic tool for the assessment of coronary artery bypass graft morphology and function. Several MR sequences, such as conventional non-respiratory compensated methods, and phase contrast cine flow sequences have been reported for the evaluation of bypass graft patency. However the visualization of different graft segments and the detection of graft stenosis remains difficult. Recent advances in MR coronary angiography and flow mapping are volume coronary angiongraphy with targeted scans, navigator gated angiography, contrast-enhanced angiography, and breath-hold or navigator gated flow sequences. Future approaches, such as navigator gated fast MR techniques resulting in high-resolution angiography in combination with breath-hold MR flow mapping with high temporal resolution, might allow a comprehensive evaluation of bypass graft stenosis and function. This review article will address the major issues concerning the MR evaluation of bypass grafts.