Aichi Chien

Quantitative hemodynamic analysis of brain aneurysms at different locations.

BACKGROUND AND PURPOSE: Studies have shown that the occurrence of brain aneurysms and risk of rupture vary between locations. However, the reason that aneurysms at different branches of the cerebral arteries have different clinical presentations is not clear. Because research has indicated that aneurysm hemodynamics may be one of the important factors related to aneurysm growth and rupture, our aim was to analyze and compare the flow parameters in aneurysms at different locations. MATERIALS AND METHODS: A total of 24 patient-specific aneurysm models were constructed by using 3D rotational angiographic data for the hemodynamic simulation. Previously developed computational fluid dynamics software was applied to each aneurysm to simulate the blood-flow properties. Hemodynamic data at peak pulsatile flow were recorded, and wall shear stress (WSS) and flow rate in the aneurysms and parent arteries were quantitatively compared. To validate our method, a comparison with a previously reported technique was also performed. RESULTS: WSS and flow rate in the aneurysms at the peak of the cardiac cycle were found to differ in magnitude between different locations. Multiple comparisons among locations showed higher WSS and flow rate in middle cerebral artery aneurysms and lower WSS and flow rate in basilar artery and anterior communicating artery aneurysms. CONCLUSIONS: We observed changes in hemodynamic values that may be related to aneurysm location. Further study of aneurysm locations with a large number of cases is needed to test this hypothesis.

An L 1 -based variational model for Retinex theory and its application to medical images

Human visual system (HVS) can perceive constant color under varying illumination conditions while digital images record information of both reflectance (physical color) of objects and illumination. Retinex theory, formulated by Edwin H. Land, aimed to simulate and explain this feature of HVS. However, to recover the reflectance from a given image is in general an ill-posed problem. In this paper, we establish an L1-based variational model for Retinex theory that can be solved by a fast computational approach based on Bregman iteration. Compared with previous works, our L1-Retinex method is more accurate for recovering the reflectance, which is illustrated by examples and statistics. In medical images such as magnetic resonance imaging (MRI), intensity inhomogeneity is often encountered due to bias fields. This is a similar formulation to Retinex theory while the MRI has some specific properties. We then modify the L1-Retinex method and develop a new algorithm for MRI data. We demonstrate the performance of our method by comparison with previous work on simulated and real data.

Patient-specific hemodynamic analysis of small internal carotid artery-ophthalmic artery aneurysms

BACKGROUND Prophylactic treatment of unruptured small brain aneurysms is still controversial due to the low risk of rupture. Distinguishing which small aneurysms are at risk for rupture has become important for treatment. Previous studies have indicated a variety of hemodynamic properties that may influence aneurysm rupture. This study uses hemodynamic principles to evaluate these in the context of ruptured and unruptured small aneurysms in a single location. METHODS Eight small internal carotid artery-ophthalmic artery (ICA-Oph) aneurysms (<10 mm) were selected from the University of California, Los Angeles, database. We analyzed rupture-related hemodynamic characteristics including flow patterns, wall shear stress (WSS), and flow impingement using previously developed patient-specific computational fluid dynamics software. RESULTS Most ruptured aneurysms had complicated flow patterns in the aneurysm domes, but all of the unruptured cases showed a simple vortex. A reduction in flow velocity between the parent artery and the aneurysm sac was found in all the cases. Inside the aneurysms, the highest flow velocities were found either at the apex or neck. We also observed a trend of higher and more inhomogeneous WSS distribution within ruptured aneurysms (10.66 +/- 5.99 Pa) in comparison with the unruptured ones (6.31 +/- 6.47 Pa) (P < .01). CONCLUSION A comparison of hemodynamic properties between ruptured and unruptured small ICA-Oph aneurysms found that some hemodynamic properties vary between small aneurysms although they are similar in size and share the same anatomical location. In particular, WSS may be a useful hemodynamic factor for studying small aneurysm rupture.

Enlargement of small, asymptomatic, unruptured intracranial aneurysms in patients with no history of subarachnoid hemorrhage: the different factors related to the growth of single and multiple aneurysms

OBJECT This study was performed to investigate the risk factors related to the growth of small, asymptomatic, unruptured aneurysms in patients with no history of subarachnoid hemorrhage (SAH). METHODS Between January 2005 and December 2010, a total of 508 patients in whom unruptured intracranial aneurysms were diagnosed at the University of California, Los Angeles medical center did not receive treatment to prevent rupture. Of these, 235 patients with no history of SAH who had asymptomatic, small, unruptured aneurysms (<7 mm) were monitored with 3D CT angiography images. Follow-up images of the lesions were used to measure aneurysm size changes. Patient medical history, family history of SAH, aneurysm size, and location were studied to find the risk factors associated with small aneurysm growth. RESULTS A total of 319 small aneurysms were included, with follow-up durations of 29.2±20.6 months. Forty-two aneurysms increased in size during the follow-up; 5 aneurysms grew to become ≥7 mm within 38.2±18.3 months. A trend of higher growth rates was found in single aneurysms than in multiple aneurysms (p=0.07). A history of stroke was the only factor associated with single aneurysm growth (p=0.03). The number of aneurysms (p=0.011), number of aneurysms located within the posterior circulation (p=0.030), and patient history of transient ischemic attack (p=0.044) were related to multiple aneurysm growth. CONCLUSIONS Multiple small aneurysms are more likely to grow, and multiple aneurysms located in the posterior circulation may require additional attention. Although single aneurysms have a lower risk of growth, a trend of higher growth rates in single aneurysms was found.

Comparative morphological analysis of the geometry of ruptured and unruptured aneurysms.

BACKGROUND:The risk of aneurysm rupture appears to be related to multiple factors such as topology, morphology, size, perianeurysmal environment, and blood flow hemodynamics. OBJECTIVE:To evaluate aneurysm morphology and to quantitatively compare the volumetric parameters between ruptured and unruptured aneurysms from our clinical database at the UCLA Medical Center. METHODS:Novel algorithms that automatically compute aneurysm geometry were tested on the basis of voxel data obtained from angiographic images, and measurements of aneurysm morphology were automatically recorded. We studied a total of 50 aneurysms (25 ruptured and 25 unruptured) with sizes ranging from 3 to 26 mm. To compare the geometric characteristics between ruptured and unruptured groups, we examined measurements, including volume and surface area, and the ratios of these measurements to the minimal bounding sphere around each aneurysm. RESULTS:More than 65% of ruptured aneurysms had a ratio of aneurysm volume to bounding sphere volume (AVSV) of > 0.5. More than 70% of ruptured aneurysms had a ratio of aneurysm surface to bounding sphere surface (AASA) of < 1. A trend differentiating ruptured and unruptured aneurysms was observed in AVSV (P = .07) and AASA (P = .04). Classification and regression trees analysis showed 68% correct classification with rupture for AVSV and 70% for AASA. CONCLUSION:By comparing aneurysm geometry with the bounding sphere, we found a trend associating the ratios of aneurysm volume and surface area with rupture. These geometric parameters may be useful for understanding the influence of morphology on the risk of aneurysm rupture.

Morphologic and Hemodynamic Risk Factors in Ruptured Aneurysms Imaged before and after Rupture

BACKGROUND AND PURPOSE: Due to limited information about aneurysm natural history, choosing the appropriate management strategy for an unruptured aneurysm is challenging. By comparing unruptured and ruptured cases, studies have identified a variety of aneurysm morphologic and hemodynamic properties as risk factors for rupture. In this study, we investigated changes in 4 ruptured aneurysms before and after rupture and tested whether previously published risk factors identified a risk before rupture. MATERIALS AND METHODS: A retrospective review of ruptured aneurysms based on the inclusion criteria of documenting angiographic images before and after rupture was performed. Such cases are extremely rare. To minimize hemodynamic influence due to location, we selected 4 cases at the posterior communicating artery. 3D morphologic and hemodynamic analyses were applied to examine qualitative and quantitative risk factors in aneurysms before and after rupture. RESULTS: When we compared aneurysms before and after rupture, all increased in size. Volume, surface area, and morphology changed in both high and low wall shear stress areas. Aneurysm surface ratio, nonsphericity index, and pulsatility index were the only risk factors to consistently identify risk before and after aneurysm rupture for all aneurysms. CONCLUSIONS: Although changes in shape and flow properties were found before and after aneurysm rupture, in this small study, we found that some risk factors were evident as early as 2 years before rupture.

Distinct trends of pulsatility found at the necks of ruptured and unruptured aneurysms

Background Aneurysm hemodynamics has been shown to be an important factor in aneurysm growth and rupture. Although pulsatility is essential for blood flow and vascular wall function, studies of pulsatile flow properties in brain aneurysm disease are limited. Objective To investigate differences in pulsatility within a group of ruptured and unruptured aneurysms by implementing patient-specific pulsatile flow simulation. Methods 41 of 311 internal carotid artery aneurysms were selected from an aneurysm database (29 unruptured and 12 ruptured) and used for patient-specific hemodynamic simulations of pulsatile flow. Flow pulsatility changes in ruptured and unruptured groups were analyzed by comparing different components of blood flow. Pulsatility index (PI) was used to quantify the pulsatility of blood flow in each group at the aneurysm neck, body, dome, and parent artery. Results Within the parent artery, PI did not significantly differ between ruptured and unruptured groups (0.58). Within unruptured aneurysms, values of PI similar to that of the parent artery were found (0.61). Trends of significantly higher PI (1.99) were found within ruptured aneurysms (p<0.001). These differences were localized at the aneurysm neck, where PI in ruptured (1.93) and unruptured (0.59) aneurysms was significantly different (p<0.001). Conclusions A trend towards a lower PI, similar to that in the parent artery, was found in unruptured aneurysms, while ruptured aneurysms followed a trend of higher pulsatility. The difference was significant at the aneurysm neck, indicating that pulsatility and this location may be important aspects of aneurysm rupture and a useful predictor of the risk of aneurysm rupture.

Topology Preserving Linear Filtering Applied to Medical Imaging

One of the central problems of medical imaging is the three-dimensional (3D) visualization of body parts. The 3D volume can be viewed in slices, but the extraction of a part requires a segmentation process. Inasmuch as body parts are distinguishable by their various densities, a widely accepted method for extracting an organ is to extract isodensity surfaces by a simple threshold. Unfortunately, the density of organs, arteries, etc. varies spatially due to morphology, and no unique threshold allows one to extract the organs boundaries. The snake or active contour methods have attempted to capture these boundaries as smooth and overall contrasted surfaces. The snake method suffers, however, from severe drawbacks. The contour has to be initialized near the boundary. In addition, many body parts have too complex a topology. In this paper we focus on another idea, which is to preprocess the image before thresholding. The preprocessing aims at the homogeneity of the different parts while preserving small features. Starting from a recent seminal work by Grady and Funka-Lea [in Computer Vision and Mathematical Methods in Medical and Biomedical Image Analysis: ECCV 2004 Workshops CVAMIA and MMBIA, Prague, Czech Republic, May 2004, Revised Selected Papers, Springer, Berlin, 2004, pp. 230-245], several linear heat equations on images will be compared. They stem from nonlinear partial differential equations or from their associated nonlinear filters. By linearizing these processes one obtains more accurate topology preserving methods. These linear filters will be tested comparatively to visualize challenging angiography images of arteries. A salient fact of the method will emerge. By a concentration phenomenon, peaks in the image histogram become much more concentrated under the linear heat equations, thus permitting us to fix the thresholds defining the surfaces without supervision. Automatic extraction can be performed in this way for angiography images taken at a one-year or longer delay.

Level Set Based Surface Capturing in 3D Medical Images

Brain aneurysm rupture has been reported to be directly related to the size of aneurysms. The current method used to determine aneurysm size is to manually measure the width of the neck and height of the dome on a computer screen. Because aneurysms usually have complicated shapes, using the size of the aneurysm neck and dome may not be accurate and may overlook important geometrical information. In this paper we present a level set based illusory surface algorithm to capture the aneurysms from the vascular tree. Since the aneurysms are described by level set functions, not only the volume but also the curvature of aneurysms can be computed for medical studies. Experiments and comparisons with models used for capturing illusory contours in 2D images are performed. This includes applications to clinical image data demonstrating the procedure of accurately capturing a middle cerebral artery aneurysm.

An integrated, ontology-driven approach to constructing observational databases for research

The electronic health record (EHR) contains a diverse set of clinical observations that are captured as part of routine care, but the incomplete, inconsistent, and sometimes incorrect nature of clinical data poses significant impediments for its secondary use in retrospective studies or comparative effectiveness research. In this work, we describe an ontology-driven approach for extracting and analyzing data from the patient record in a longitudinal and continuous manner. We demonstrate how the ontology helps enforce consistent data representation, integrates phenotypes generated through analyses of available clinical data sources, and facilitates subsequent studies to identify clinical predictors for an outcome of interest. Development and evaluation of our approach are described in the context of studying factors that influence intracranial aneurysm (ICA) growth and rupture. We report our experiences in capturing information on 78 individuals with a total of 120 aneurysms. Two example applications related to assessing the relationship between aneurysm size, growth, gene expression modules, and rupture are described. Our work highlights the challenges with respect to data quality, workflow, and analysis of data and its implications toward a learning health system paradigm.