Kyungeun Lee

A patient-specific virtual stenotic model of the coronary artery to analyze the relationship between fractional flow reserve and wall shear stress

OBJECTIVE As the stenotic severity of a patient increases, fractional flow reserve (FFR) decreases, whereas the maximum wall shear stress (WSSmax) increases. However, the way in which these values can change according to stenotic severity has not previously been investigated. The aim of this study is to devise a virtual stenosis model to investigate variations in the coronary hemodynamic parameters of patients according to stenotic severity. METHODS To simulate coronary hemodynamics, a three-dimensional (3D) coronary artery model of computational fluid dynamics is coupled with a lumped parameter model of the coronary micro-vasculature and venous system. RESULTS To validate the present method, we first simulated 13 patient-specific models of the coronary arteries and compared the results with those obtained clinically. Then, virtually narrowed coronary arterial models derived from the patient-specific cases were simulated to obtain the WSSmax and FFR values. The variations in FFR and WSSmax against the percentage of diameter stenosis in clinical cases were reproducible by the virtual stenosis models. We also found that the simulated FFR values were linearly correlated with the WSSmax values, but the linear slope varied by patient. CONCLUSION We implemented 130 additional virtual models of stenosed coronary arteries based on data from 13 patients and obtained statistically meaningful results that were identical to the large-scale clinical studies. And the slope of the correlation line between FFR and WSSmax may help clinicians to design treatment plans for patients.

Estimation of the flow resistances exerted in coronary arteries using a vessel length-based method.

Flow resistances exerted in the coronary arteries are the key parameters for the image-based computer simulation of coronary hemodynamics. The resistances depend on the anatomical characteristics of the coronary system. A simple and reliable estimation of the resistances is a compulsory procedure to compute the fractional flow reserve (FFR) of stenosed coronary arteries, an important clinical index of coronary artery disease. The cardiac muscle volume reconstructed from computed tomography (CT) images has been used to assess the resistance of the feeding coronary artery (muscle volume-based method). In this study, we estimate the flow resistances exerted in coronary arteries by using a novel method. Based on a physiological observation that longer coronary arteries have more daughter branches feeding a larger mass of cardiac muscle, the method measures the vessel lengths from coronary angiogram or CT images (vessel length-based method) and predicts the coronary flow resistances. The underlying equations are derived from the physiological relation among flow rate, resistance, and vessel length. To validate the present estimation method, we calculate the coronary flow division over coronary major arteries for 50 patients using the vessel length-based method as well as the muscle volume-based one. These results are compared with the direct measurements in a clinical study. Further proving the usefulness of the present method, we compute the coronary FFR from the images of optical coherence tomography.

Spatial pattern and association of tree species in a mixed Abies holophylla-broadleaved deciduous forest in Odaesan National Park

The mixed Abies holophylla-broadleaved deciduous forest is mature relative to other forest types in the midland of South Korea. The spatial distribution patterns of eight dominant canopy tree species were analyzed using Ripley’s K function. This study was conducted to clarify interspecific and intraspecific associations among growth stages and to interpret the coexistence mechanism among such species, by extension, to forecast their future. Disturbance-driven site heterogeneity has spatially separated disturbance-resistant Magnolia sieboldii from the other seven species. Spatial distribution of other species is affected by dispersal mechanisms and interspecific and intraspecific competition. These species were classified into three groups. The first group, composed of A. holophylla, Tilia amurensis, Acer pseudo-sieboldianum, and Quercus mongolica, was the most dominant and intraspecifically affinitive. Additionally, it seemed that they were established before the others. Q. mongolica and T. amurensis are poorly resistant to shade and are likely to be crowded out. In contrast, the other two species may continue, as they are highly resistant to shade and have high reproductivity. The second group was composed of Carpinus cordata, Acer tegmentosum, and Acer mono, i.e., late-successional species that wait for chances with shade tolerance and high reproductivity. These species are expected to occupy much of the Q. mongolica and T. amurensis space. M. sieboldii, i.e., the third group, were negatively related with other species and have dominated the valleys where intense disturbances are repeated. Understories have poor reproductivity, but a stationary population is expected to be maintained if canopy gaps are created by occasional disturbances.

Classifying plant species indicators of eutrophication in Korean lakes

The objectives of this study were to establish a method of classifying plants as indicator species of eutrophication, as a key metric for assessing lake ecosystem health, and to select sensitive and tolerant plant species among aquatic macrophytes and hygrophytes. Thus, 38 natural and artificial lakes throughout Korea were investigated. The distribution and abundance of plant species were investigated. As a measure of eutrophication, the modified trophic state index (TSIKO), derived from total phosphorus and chlorophyll a, was used. Using TSIKO criteria and the response curves of plants to TSIKO, a selection method of classifying sensitive and tolerant species was developed. Overall, 15 sensitive and 11 tolerant species among 117 macrophytes and hygrophytes were classified. The applicability of the selected indicator species was verified by a comparison with the results of a multimetric analysis using eight variables. The results suggest that the selected indicator species is expected to be useful as a metric for assessing lake ecosystem health.

A vessel length-based method to compute coronary fractional flow reserve from optical coherence tomography images

BackgroundHemodynamic simulation for quantifying fractional flow reserve (FFR) is often performed in a patient-specific geometry of coronary arteries reconstructed from the images from various imaging modalities. Because optical coherence tomography (OCT) images can provide more precise vascular lumen geometry, regardless of stenotic severity, hemodynamic simulation based on OCT images may be effective. The aim of this study is to perform OCT–FFR simulations by coupling a 3D CFD model from geometrically correct OCT images with a LPM based on vessel lengths extracted from CAG data with clinical validations for the present method.MethodsTo simulate coronary hemodynamics, we developed a fast and accurate method that combined a computational fluid dynamics (CFD) model of an OCT-based region of interest (ROI) with a lumped parameter model (LPM) of the coronary microvasculature and veins. Here, the LPM was based on vessel lengths extracted from coronary X-ray angiography (CAG) images. Based on a vessel length-based approach, we describe a theoretical formulation for the total resistance of the LPM from a three-dimensional (3D) CFD model of the ROI.ResultsTo show the utility of this method, we present calculated examples of FFR from OCT images. To validate the OCT-based FFR calculation (OCT–FFR) clinically, we compared the computed OCT–FFR values for 17 vessels of 13 patients with clinically measured FFR (M-FFR) values.ConclusionA novel formulation for the total resistance of LPM is introduced to accurately simulate a 3D CFD model of the ROI. The simulated FFR values compared well with clinically measured ones, showing the accuracy of the method. Moreover, the present method is fast in terms of computational time, enabling clinicians to provide solutions handled within the hospital.

THE INFLUENCES OF SWIRL FLOW ON FRACTIONAL FLOW RESERVE IN MILD/MODERATE/SEVERE STENOTIC CORONARY ARTERIAL MODELS

Swirl flow is often found in proximal coronary arteries, because the aortic valves can induce swirl flows in the coronary artery due to vortex formation. In addition, the curvature and tortuosity of arterial configurations can also produce swirl flows. The present study was performed to investigate fractional flow reserve alterations in a post-stenotic distal part due to the presence of pre-stenotic swirl flow by computational fluid dynamics analysis for virtual stenotic models by quantifying fractional flow reserve(FFR). Simplified stenotic coronary models were divided into those with and without pre-stenotic swirl flow. Various degrees of virtual stenosis were grouped into three grades: mild, moderate, and severe, with degree of stenosis of 0 ~ 40%, 50 ~ 60%, and 70 ~ 90%, respectively. In this study, three-dimensional computational hemodynamic simulations were performed under hyperemic conditions in virtual stenotic coronary models by coupling with a zero-dimensional lumped parameter model. The results showed that the influence of pre-stenotic swirl inflow is dominant on FFR alteration in mild stenosis, whereas stenosis is dominant on FFR alteration in moderate/severe stenosis. The decrease in FFR caused by swirl flow is more significant in mild stenosis than moderate/severe stenosis. Biomechanical modeling is useful for clinicians to provide insight for medical intervention strategies. This hemodynamic-based parameter study could play a critical role in the development of a non-invasive imaging-based strategy-support system for percutaneous transluminal angioplasty in cases of mild/moderate stenosis.

Corrigendum to “Physiological indices for the categorization of Mibyeong severity” [Integr. Med. Res. 2017; 6: 88-92]

[This corrects the article DOI: 10.1016/j.imr.2017.01.005.].

Physiological indices for the categorization of Mibyeong severity

Background Individuals with Mibyeong are difficult to identify. Although extensive research has attempted to introduce an easy and clear method for Mibyeong diagnosis, the indices used to categorize Mibyeong severity are unclear. We hypothesized that individuals with severe Mibyeong have reduced physiological function, thus activating homeostatic regulatory functions and inducing alterations in vascular resistance and capacitance. Methods Novel indices used to categorize Mibyeong severity based on the cardiovascular system model are described. We analyzed resistance and capacitance values using a simple cardiovascular system model optimally satisfying the measured systolic and diastolic pressures, heart rate, and age. Results Clinical data from 509 individuals were examined to test our hypothesis. A statistical analysis revealed that the vascular resistance was lower in individuals with severe Mibyeong symptoms and decreased with increasing Mibyeong severity, whereas the vascular capacitance showed an opposite trend. Conclusion We derived indices to categorize Mibyeong severity and tested 509 individuals. An epidemiological analysis revealed that the vascular resistance decreased while the capacitance increased with increasing Mibyeong severity, indicating the validity of the values as Mibyeong indices.