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At least 937 records · Page 52Linked to original sources
PubMed1999

Carbon dioxide angiography: effect of injection parameters on bolus configuration.

PURPOSE: Predict the intravascular distribution of carbon dioxide during angiography. MATERIALS AND METHODS: Mathematical modeling was used to predict the flow pattern of CO2 in a pulsatile system as a function of the CO2 flow rate. Findings were validated in an in vitro pulsatile circuit. RESULTS: The annular flow pattern with filling of nearly the entire lumen with CO2 is the most desirable, followed by intermittent bubble flow (provided individual bubbles are large). Stratified flow relates to a continuous floating CO2 bubble. Configuration of the CO2 bolus depends on fluid properties, fluid velocity, flow rates, mean intraluminal pressure, pressure amplitude, pulse rate, and vessel diameter. In vessels with less than 10-mm inner diameter, annular flow can be achieved relatively easily with injection rates above 20-30 mL/sec. Higher rates are not expected to produce superior results. When imaging a 2-cm artery, the best that can be realized clinically is intermittent flow with large bubbles. Bubbles size increases with increasing CO2 flow rate. In aneurysms, only stratified flow can be achieved with reasonable injection rates. Periodicity of the flow patterns is determined by the pulsatile circuit and can produce indentations in the CO2 bolus, which can be mistaken for stenoses. CONCLUSIONS: Flow regime maps can be used to optimize bolus configuration during CO2 angiography.

Algorithms↗
PubMed2003

Improving tissue segmentation of human brain MRI through preprocessing by the Gegenbauer reconstruction method.

The Gegenbauer image reconstruction method, previously shown to improve the quality of magnetic resonance images, is utilized in this study as a segmentation preprocessing step. It is demonstrated that, for all simulated and real magnetic resonance images used in this study, the Gegenbauer reconstruction method improves the accuracy of segmentation. Although it is more desirable to use the k-space data for the Gegenbauer reconstruction method, only information acquired from MR images is necessary for the reconstruction, making the procedure completely self-contained and viable for all human brain segmentation algorithms.

Algorithms↗
PubMed1997

Social factors predicting postpartum choice of Norplant among African-American and non-Hispanic white adolescents.

PURPOSE: This study was designed to determine the factors associated with Norplant choice for postpartum teens. METHODS: A total of 151 teenagers, ages 12-20 years, who delivered at the Medical Center of Delaware from July to December 1992, were offered insertion of Norplant within 48 h postpartum. A structured interview was conducted in the postpartum period after nondirective counseling sessions including a physical demonstration using anatomical models of various contraceptive methods. Student's t-tests, chi-square, and multivariate analyses were used. RESULTS: Eighty-six teenagers were African-American (mean age = 17.3 +/- 1.9 years) and 65 non-Hispanic white (NHW) (mean age = 18.3 +/- 1.5 years). The NHW teenagers were older (p < 0.001); the African-American teenagers were more likely to have Medicaid (49% vs. 14%; p < 0.001) and to have one or more friends who use Norplant (62% vs. 34%; p < 0.001). In multivariate analyses, NHW teenagers were more likely to choose Norplant if they had discussed their choice with a parent or guardian [adjusted odds ratio (AOR) = 14.6, 95% confidence interval (CI), 2.12-100.57]; had Medicaid funding (AOR = 12.1; 95% (CI), 10.6-91.34); and had any friends who used Norplant (AOR = 6.3; 95% (CI), 1.38-28.40). However, for African-American teenagers, the strongest predictor for choice of Norplant was number of prior children delivered. After two deliveries, there was a better than four-fold likelihood (AOR = 4.8; 95% (CI), 1.47-15.94) that African-American teenagers would choose Norplant. For the African-American teenagers, parental discussion, Medicaid status, and friends' use of Norplant were not as important as family size, but far greater percentages of the African-American teenagers had access to Medicaid funding and peers who used Norplant. CONCLUSIONS: NHW and African-American teenagers choose Norplant for different reasons. Lack of funding may have been a barrier to choosing Norplant. Discussions with parents and friends have a positive influence on choosing Norplant for NHW teenagers. African-American teenagers were more likely than NHW to have Medicaid coverage, and more frequently choose Norplant if the current birth was their third child.

Adolescent↗
PubMed2003

A new removable implant analog.

A new implant analog system that improves the efficiency of the implant restoration's fabrication process is described. This 2-piece analog contains an outer plastic sleeve that is retained within the stone cast. The inner portion, a metal analog, can be easily removed and replaced within the plastic sleeve. Because the metal analog is removable, the time-consuming process of screwing and unscrewing the prosthesis- or abutment-retaining screws during laboratory procedures can be minimized. The prosthesis can remain secured to the analog during waxing, metal finishing, and porcelain buildup, yet is easily removed from the cast for inspection.

Crowns↗
PubMed1994

An ellipsoidal shell model for volume estimation of the right ventricle from magnetic resonance images.

RATIONALE AND OBJECTIVES: I developed a volume estimation technique for the crescentic volume of the right ventricle (RV) of the heart. A geometric model was desired to avoid the lengthy data collection and reduction required by Simpson's rule. METHODS: An ellipsoidal shell model was developed that requires only simple mathematics and that resembles the RV shape. RV cast volumes were obtained by water displacement and Simpson's rule and model-based calculations using magnetic resonance (MR) imaging. RESULTS: Model-based estimates correlated well with water displacement volumes (r = 0.924), with a slope not significantly different from unity. Simpson's rule results showed a higher correlation (r = 0.994), but it required longer acquisition and processing. Geometric irregularity in the RV shape required no modification in mathematics. CONCLUSION: The model provides reliable RV volume estimates from two MR image planes. The mathematics provides a simple approach to a relatively complex, crescentic shape. Short times for data acquisition and analysis suggest the potential for time savings during routine clinical measurements.

Animals↗
PubMed1998

Impedance index measurements of in vitro PTFE end-to-side anastomoses: effect of angle and Miller cuff.

OBJECTIVE: To measure non-invasively the impedance index of a range of geometries of PTFE end-to-side anastomoses (ESA). DESIGN: In vitro experiments using a custom-built flow rig. SUBJECTS: Anastomoses constructed at each angle. (15 degrees, 30 degrees, 45 degrees, 60 degrees, 90 degrees, 120 degrees) for both standard and cuffed ESA. RESULTS: The impedance index of each ESA increased non-linearly with flow rate. The impedance index at a given flow rate was reduced by decreasing the anastomotic angle and further reduced by the addition of a Miller cuff. CONCLUSIONS: The reduction in impedance index achieved with a Miller cuff may help to explain the improved patency rates of femoropopliteal grafts incorporating a cuff.

Anastomosis, Surgical↗
PubMed2000

Influence of head constraint and muscle forces on the strain distribution within the intact femur.

The aim of this study was to analyse the influence of muscle action and a horizontally constrained femoral head on the strain distribution within the intact femur. The strain distribution was measured for three loading configurations: joint reaction force only, joint reaction force plus abductors, and joint reaction force plus the abductors, vastus lateralis and iliopsoas. In each case the strains were recorded from 20 uniaxial strain gauges placed on the medial, lateral, anterior and posterior aspects of the proximal femur. Application of the abductor muscle force produced a marginal decrease in the strain levels on all aspects of the femur as compared with the joint reaction force alone. This is in contrast with previous studies which have simulated an unconstrained femoral head. The inclusion of vastus lateralis and iliopsoas further reduced the strain levels. A horizontally constrained femoral head produces smaller variation in the strain levels when muscle forces are applied. In vivo data, demonstrating negligible movement of the femoral head in one-legged stance, support the results of this study and suggest that in the absence of comprehensive muscle force data, a constrained femoral head may provide a more physiologically relevant loading condition.

Elasticity↗
PubMed2002

An advanced computer-aided geometric modeling and fabrication method for human middle ear.

This paper presents a practical and systematic method for reconstructing accurate computer and physical models of the entire human middle ear. The proposed method starts with the histological section preparation of human temporal bone. Through tracing outlines of the middle ear components on the sections, a set of discrete points is obtained and employed to construct B-spline curves that represent the exterior contours of the components using a curve-fitting technique. The surface-skinning technique is then employed to quilt the B-spline curves for smooth boundary surfaces of the middle ear components using B-spline surfaces. The solid models of the middle ear components are constructed using these surfaces and then assembled to create the entire middle ear in a computer-aided design environment. This method not only provides an effective way to visualize and measure the three-dimensional structure of the middle ear, but also provides a detailed knowledge of middle ear geometry that is required for finite element analysis or multibody dynamic analysis of the human middle ear. In addition, the geometric model constructed using the proposed method is smooth and can be fabricated in various scales using solid freeform fabrication technology. The physical model of the human middle ear is extremely effective in realizing the middle ear anatomy and enhancing discussion and collaboration among researchers and physicians.

Anatomy, Cross-Sectional↗
PubMed2003

Particle-hemodynamics modeling of the distal end-to-side femoral bypass: effects of graft caliber and graft-end cut.

Late-stage occlusions of peripheral synthetic bypass grafts are frequently due to intimal hyperplasia and/or thrombosis at the distal anastomosis, resulting in unacceptably high failure rates. It has been widely established that hemodynamic and blood particle interactions with the vascular surface as well as surgical injury and compliance mismatch are inciting mechanisms capable of eliciting various cellular level responses associated with distal anastomotic intimal hyperplasia (IH) formation. Primary geometric factors influencing anastomotic hemodynamics include the graft-to-artery diameter ratio and graft-hood shape, which are determined by the graft caliber and initial graft-end cut selected by the vascular surgeon. In this study, the particle-hemodynamic effects of graft-end cuts (straight, curved, and S-shaped) and graft-to-artery diameter ratios (2:1 vs. 1.5:1) have been numerically assessed in four common unexpanded anastomotic configurations with respect to vortical flow patterns, wall shear stress based parameters, and platelet interactions with the vascular surface. Sites of significant platelet-wall interactions have been identified by a novel near-wall residence time (NWRT) model, which includes shear stress based factors for platelet activation and endothelial cell expression of anti-thrombogenic compounds. Of the configurations evaluated, straight and curved graft-end cuts with a graft-to-artery diameter ratio of 1.5:1 were found to reduce the particle-hemodynamic potential for IH development at locations critical to flow delivery. Nevertheless, the potential for significant IH occurrence via platelet and/or endothelial response pathways was highly evident in all conventional anastomoses considered, such that a decisively superior configuration was not determined. These results illustrate the need for alternative anastomotic designs with the intent of reducing critical hemodynamic wall parameters and mitigating regions of significant particle-wall interactions.

Anastomosis, Surgical↗
PubMed1998

Three-dimensional finite element analysis of thermal shock in a premolar with a composite resin MOD restoration.

A three-dimensional finite element model of a human premolar with a composite resin Class II MOD has been used to investigate the temperature changes and induced thermal stresses associated with the imbibing of a hot liquid. Regions of high tensile stress were revealed and their possible clinical significance with respect to microleakage and failure in the approximal region of the restoration are discussed.

Bicuspid↗
PubMed1998

Variability of the planarity of the human aortic bifurcation.

The susceptibility of vascular branches to atherosclerosis is believed to be due in part to the unusual fluid dynamic environments that the vessel wall experiences in these regions. As vascular geometry is a primary determinant of the local haemodynamic environment, it is of interest to quantitate the geometric features of vascular branches and their variability. The present research focusses on branch planarity, using axial magnetic resonance images of the aortic bifurcations of 20 healthy subjects. The in vivo images were processed to obtain vector representations of the vessel axes at the bifurcation, from which the planarity of the bifurcation was derived using a novel robust definition. Three-dimensional reconstructions of the bifurcations were rendered using computer graphics techniques to demonstrate the variability of the planarity of this region of the vasculature; this variability might be related to variable predispositions to atherosclerosis at the aortic bifurcation.

Aorta, Abdominal↗
PubMed1999

Effects of local skull inhomogeneities on EEG source estimation.

The accuracy of the head model affects the solutions of the EEG inverse problems. If a simple three-sphere model and standard conductivity values for brain, skull and scalp regions are used, significant errors may occur in the dipole localisation. One of the most sensitive head model parameters is the conductivity of the skull. A realistic three-dimensional finite-element model provides a method to study the effect of inhomogeneities of the skull on the solutions of EEG inverse problems. In this paper the effect of a local skull conductivity inhomogeneity on source estimation accuracy is analyzed by computer simulations for different numbers of electrodes. It is shown that if the inhomogeneity of the skull conductivity is not taken into account, localisation errors of approximately 1 cm can be encountered in the equivalent current dipole estimation. This modelling error introduces a bias to the solution which cannot be compensated by increasing the number of electrodes.

Biophysical Phenomena↗
PubMed2002

BrainSuite: an automated cortical surface identification tool.

We describe a new magnetic resonance (MR) image analysis tool that produces cortical surface representations with spherical topology from MR images of the human brain. The tool provides a sequence of low-level operations in a single package that can produce accurate brain segmentations in clinical time. The tools include skull and scalp removal, image nonuniformity compensation, voxel-based tissue classification, topological correction, rendering, and editing functions. The collection of tools is designed to require minimal user interaction to produce cortical representations. In this paper we describe the theory of each stage of the cortical surface identification process. We then present classification validation results using real and phantom data. We also present a study of interoperator variability.

Algorithms↗
PubMed2002

Nonrigid registration of 3D tensor medical data.

New medical imaging modalities offering multi-valued data, such as phase contrast MRA and diffusion tensor MRI, require general representations for the development of automated algorithms. In this paper we propose a unified framework for the registration of medical volumetric multi-valued data using local matching. The paper extends the usual concept of similarity between two pieces of data to be matched, commonly used with scalar (intensity) data, to the general tensor case. Our approach to registration is based on a multiresolution scheme, where the deformation field estimated in a coarser level is propagated to provide an initial deformation in the next finer one. In each level, local matching of areas with a high degree of local structure and subsequent interpolation are performed. Consequently, we provide an algorithm to assess the amount of structure in generic multi-valued data by means of gradient and correlation computations. The interpolation step is carried out by means of the Kriging estimator, which provides a novel framework for the interpolation of sparse vector fields in medical applications. The feasibility of the approach is illustrated by results on synthetic and clinical data.

Algorithms↗
PubMed2002

Fusing speed and phase information for vascular segmentation of phase contrast MR angiograms.

This paper presents a statistical approach to aggregating speed and phase (directional) information for vascular segmentation of phase contrast magnetic resonance angiograms (PC-MRA). Rather than relying on speed information alone, as done by others and in our own work, we demonstrate that including phase information as a priori knowledge in a Markov random field (MRF) model can improve the quality of segmentation. This is particularly true in the region within an aneurysm where there is a heterogeneous intensity pattern and significant vascular signal loss. We propose to use a Maxwell-Gaussian mixture density to model the background signal distribution and combine this with a uniform distribution for modelling vascular signal to give a Maxwell-Gaussian-uniform (MGU) mixture model of image intensity. The MGU model parameters are estimated by the modified expectation-maximisation (EM) algorithm. In addition, it is shown that the Maxwell-Gaussian mixture distribution (a) models the background signal more accurately than a Maxwell distribution, (b) exhibits a better fit to clinical data and (c) gives fewer false positive voxels (misclassified vessel voxels) in segmentation. The new segmentation algorithm is tested on an aneurysm phantom data set and two clinical data sets. The experimental results show that the proposed method can provide a better quality of segmentation when both speed and phase information are utilised.

Algorithms↗
PubMed1999

Alignment of magnetic-resonance brain datasets with the stereotactical coordinate system.

Neuroanatomical and neurofunctional studies are often referenced to high-resolution magnetic-resonance brain datasets. For the analysis of the cortical surface, mapping of functional information on to the cortex or visualization, it is necessary to remove the outer surfaces of the brain. For intersubject comparison, it is useful to align the dataset with a coordinate system and introduce a spatial normalization. We describe an image processing chain that combines all of these steps in an interaction-free procedure. We report on a period of 2 years of routine application of this procedure, with >250 successfully processed datasets from healthy subjects and patients with various forms of brain damage.

Brain Diseases↗
PubMed2002

Modeling of post-surgical brain and skull defects in the EEG inverse problem with the boundary element method.

OBJECTIVES: In order to obtain accurate EEG inverse solutions in patients subjected to surgery, we have studied the feasibility and influence of incorporating brain and skull defects in realistic head models. METHODS: We first measured the conductivity of the methacrylate used for cranioplasty. Then, we designed realistic boundary element method head models with a skull burr hole, a methacrylate plug or a temporal-lobe resection. We simulated the potentials that would be produced at 71 electrode locations (10/10 system) by dipoles located near the defects. Then, we fitted dipoles on these potentials using a defect-free head model. We also ran simulations in a noisy situation and with higher skull and cerebrospinal fluid (CSF) conductivity. RESULTS: The largest errors were found for burr holes, with a localization error up to 20 mm for a radial dipole located 30 mm below the hole and an amplification factor of 8. Methacrylate plugs lead to errors up to 5 mm and 0.5; the resection only lead to errors of 2 mm and 1.3. Results obtained with noise were consistent with those obtained without noise. Doubling the skull conductivity led to errors that were reduced by 10-20%, while doubling CSF conductivity increased the errors by up to 31%. CONCLUSIONS: We have shown that it is important to incorporate skull defects in realistic head models when sources are located near the defects and precision is sought. Brain cavities of the size of a typical anterior temporal lobe resection may be omitted without a significant impact on dipole localization.

Brain↗
PubMed1999

Computer simulation of trabecular remodeling using a simplified structural model.

A simplified three-dimensional simulation of trabecular bone remodeling has been developed. The model utilizes 441 planar structural units to represent approximately 50 mm3 of initial bone volume with 199 basic multicellular units (BMUs). The simulation takes into account trabecular perforation in the structural model. The cases of male bone remodeling with no menopause and female bone remodeling with menopause are examined from the period of simulated age 25-80 years. Menopause is arbitrarily started at age 45 and extends for 7.5 years. Zero-, first-, and second-order BMU activation responses are employed to examine how the bone would be affected by the method of increase of BMU activation during menopause. At age 80, the female bone remodeling simulation produced a bone volume loss of approximately 49% for all three activation responses. This compared to a 38% bone volume loss for the case of no menopause. For the menopause simulations, an average of about 40% of the total bone loss was due to perforation.

Adult↗