PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Models, Anatomic”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 235 records · Page 13Linked to original sources
PubMed1998

Spinal cord localization in the treatment of lung cancer: use of radiographic landmarks.

PURPOSE: In the treatment of thoracic malignancies with radiotherapy, the critical dose-limiting structure is the spinal cord. Oblique fields typically are designed to exclude the spinal cord, and by convention, the field edge that shields the spinal cord is placed at the anterior border of the vertebral pedicles. Thus, the purpose of our study was to estimate the distance between the field edge and spinal cord in oblique fields that were designed by using the vertebral pedicle as a radiographic landmark. METHODS AND MATERIALS: The spinal cord of a cadaver was wrapped in wire, and oblique fields were simulated at 15 degree intervals. The distance from the spinal cord to a field edge placed at the anterior border of the pedicle was measured. In the second investigation, a three-dimensional treatment planning system was used to simulate hypothetical fields using actual patient data from computed tomography (n = 10), and measurements identical to those in the anatomical model were made (n = 1,100). RESULTS: The results of the anatomical and computed tomographic models were in close agreement (mean difference, 0.6 mm). The computed tomographic model predicted a mean field edge to spinal cord distance of 8.7 mm (95% confidence interval, 5.6-11.8 mm) for 30 degree/150 degree oblique fields and 8.0 mm (95% confidence interval, 4.7-11.7 mm) for 45 degree/135 degree oblique fields. This distance was greatest at levels T-1, T-2, and T-11 (8 to 20% greater). CONCLUSIONS: The mean distance from a field edge placed at the anterior border of a vertebral pedicle to the spinal cord for commonly used oblique angles constitutes a sufficient margin to account for expected differences in daily positional variations and mechanical uncertainties.

Cadaver↗
PubMed2002

Transition from ventricular fibrillation to ventricular tachycardia: a simulation study on the role of Ca(2+)-channel blockers in human ventricular tissue.

We study the effect of blocking the L-type Ca(2+)-channel on fibrillation in simulations in two-dimensional (2D) isotropic sheets of ventricular tissue and in a three-dimensional anisotropic anatomical model of human ventricles, using a previously developed model of human ventricular cells. Ventricular fibrillation (VF) was obtained as a result of spiral wave breakup and consisted of a varying number of chaotically wandering wavelets activating tissue at a frequency of about 6.0 Hz. We show that blocking the Ca(2+)-current by 75% can convert ventricular fibrillation into a periodic regime with a small number of stable spiral waves, ranging from six in 2D sheets of 25 x 25 cm to a single spiral in the anatomical model of human ventricles. The dominant frequency during this process changed to about 10.0 Hz in the 2D simulations, but to only 5.0 Hz in the whole heart simulations where a single spiral wave anchored around an anatomical obstacle. We show that the observed effects were due to a flattening of the electrical restitution curve, which prevented the generation of wave breaks and stabilized the activation patterns.

Calcium↗
PubMed1996

An O(N log N) algorithm for shape modeling.

We present a shape-recovery technique in two dimensions and three dimensions with specific applications in modeling anatomical shapes from medical images. This algorithm models extremely corrugated structures like the brain, is topologically adaptable, and runs in O(N log N) time, where N is the total number of points in the domain. Our technique is based on a level set shape-recovery scheme recently introduced by the authors and the fast marching method for computing solutions to static Hamilton-Jacobi equations.

Algorithms↗
PubMed1996

Dose uniformity in MECS interstitial hyperthermia: the impact of longitudinal control in model anatomies.

The quality of temperature distributions that can be generated with the multi-electrode current source (MECS) interstitial hyperthermia system, which allows 3D control of the spatial SAR distribution, has been investigated. For the investigations, computer models of idealized anatomies were used. These anatomical models did not contain discrete vessels. Binary-media anatomies, containing media interfaces oriented parallel, perpendicular or oblique with respect to the long axis of the implant, represent simple anatomies which can be encountered in the clinic. The implant volume was about 40 cm3. A seven-catheter hexagonal implant geometry with a nearest-neighbor distance of 15 mm was used. In each interstitial probe between one and four electrodes with a diameter of 2.1 mm were placed along an "active section' with a length of 50 mm. The electrode segments had lengths of 50, 20, 12 and 9 mm. This study shows that even with high contrasts in electrical and thermal conductivity in the implant it remains possible to obtain satisfactory temperature distributions with the MECS system. Due to its 3D spatial control the temperature homogeneity in the implant can be made quite satisfactory, with T10-T90 of the order of 2-3 K. Treatment planning must ensure that the placement of the current source electrodes is compatible with the media configuration.

Biophysical Phenomena↗
PubMed2004

Modeling the anatomical distribution of sunlights.

One of the major technical challenges in calculating solar irradiance on the human form has been the complexity of the surface geometry (i.e. the surface-normal vis-a-vis the incident radiation). Over 80% of skin cancers occur on the face, head, neck and back of the hands. The quantification, as well as the mapping of the anatomical distribution of solar radiation on the human form, is essential if we are to study the etiology of skin cancers or cataracts or immune system suppression. Using advances in computer graphics, including high-resolution three-dimensional mathematical representations of the human form, the calculation of irradiance has been attained to subcentimeter precision. Lighting detail included partitioning of direct beam and diffuse skylight, shadowing effects and gradations of model surface illumination depending on model surface geometry and incident light angle. With the incorporation of ray-tracing and irradiance algorithms, the results are not only realistic renderings but also accurate representations of the distribution of light on the subject model. The calculation of light illumination at various receptor points across the anatomy provides information about differential radiant exposure as a function of subject posture, orientation relative to the sun and sun elevation. The integration of a geodesic sun-tracking model into the lighting module enabled simulation of specific sun exposure scenarios, with instantaneous irradiance, as well as the cumulative radiant exposure, calculated for a given latitude, date, time of day and duration. Illustration of instantaneous irradiance or cumulative radiant exposure is achieved using a false-color rendering--mapping light intensity to color--creating irradiance or exposure isopleths. This approach may find application in the determination of the reduction in exposure that one achieves by wearing a hat, shirt or sunglasses. More fundamentally, such an analysis tool could provide improved estimates of scenario-specific dose (i.e. absorbed radiant exposure) needed to develop dose-response functions for sunlight-induced disease.

Adult↗
PubMedDate not supplied

The three-dimensional determination of internal loads in the lower extremity.

A three-dimensional model of the lower limb containing 47 muscles was developed to study the differences between a two- and three-dimensional approach for determining internal loads, the role of the dynamic joint representation, and the behavior of different load-bearing criteria in walking and running. The problem of redundancy of the musculo-skeletal system was resolved by applying inverse dynamics and static optimization methods. Different hypothetical load-bearing capabilities of hinge, spherical and intermediate joint types for the knee and the ankle joints were tested. It was found that even almost planar movements such as walking and running are associated with significant three-dimensional intersegment moments, especially in the frontal plane. Thus, a two-dimensional approach may underestimate internal loads up to 60%. It is shown that pure hinge joints are inappropriate for modeling the dynamical joint function of the knee and ankle joints. A more flexible joint representation in combination with a squared muscle stress minimization criterion predicted a lot of synergistic as well as antagonistic muscle activation which was also found in the EMG patterns. The results indicate the importance of muscular joint stabilization in natural human movements. Compared to in vivo measurements it is speculated that the predicted force magnitudes are considerably overestimated due to error propagation and still insufficient anatomical models. Thus, increased efforts to improve further the reliability of internal load calculations should be made in the future.

Adult↗
PubMed1994

Malignant melanoma incidence in Connecticut (United States): time trends and age-period-cohort modeling by anatomic site.

This study examined time trends and age-period-cohort patterns in the incidence of cutaneous malignant melanoma (CMM) by gender and anatomic site in Connecticut (United States) between 1950 and 1989, using data from the population-based Connecticut Tumor Registry. A total of 8,249 invasive CMM incident cases were included. Cases were grouped into melanomas of the head and neck, upper limb, lower limb, and trunk. Between 1950 and 1989, rates increased substantially for all sites. The largest relative increases occurred in melanoma of the upper limb for both males and females; the largest absolute increase occurred for melanoma of the trunk in males; and the smallest increase occurred in head and neck melanoma in females. Recent trends for time periods 1970-89 among birth cohorts 1930-69 indicated that the rate of increase of CMM is slowing substantially among males, but not among females. Nevertheless, continued overall increases in CMM incidence are likely in Connecticut in the 1990s in both genders, with a decrease in the male-female ratio. The age-period-cohort patterns were significantly different between the genders and among anatomic sites, suggesting different trends in carcinogenic exposures (mainly ultraviolet radiation from the sun) or etiologic distinctions between males and females and among the sites.

Adult↗
PubMed1991

Generalized Stein's model for anatomically complex neurons.

A neuron with a large dendritic structure is considered. The number of synapses located on the dendrites is substantially higher than on the soma. The synaptic input effect on the neuronal excitability decreases with distance between a synapse ending and the trigger zone. Two areas are distinguished in accordance with the effect of synaptic input--dendritic and somatic. The dendritic area, when compared to the soma, is characterized by much higher intensity of its activation but the amplitudes of synaptically evoked changes of the membrane potential at the trigger zone are in general small. This situation is suitable for a diffusion approximation. However, on the soma, especially in the proximity of the trigger zone, the membrane potential changes are a large fraction of the threshold depolarization. The membrane potential at the trigger zone is modelled by a one-dimensional stochastic process. The diffusion Ornstein-Uhlenbeck process serves as a basis of the model; however, at the moments of somatic synapses activation its voltage changes in jumps. Their sizes represent the amplitudes of the evoked postsynaptic potentials. The unimodal histograms of interspike intervals can be explained by the model. The values of the coefficient of variation greater than one are connected with substantial inhibition.

Animals↗
PubMed2006

Validation and re-evaluation of a discriminant model predicting anatomic suitability for biventricular repair in neonates with aortic stenosis.

OBJECTIVES: The purpose of this study was to validate and re-evaluate our previously reported scoring systems for predicting optimal management in neonates with aortic stenosis (AS). BACKGROUND: In 1991, we reported a multivariate discriminant equation and an ordinal scoring system for predicting which neonates with AS are suitable for biventricular repair and which are better served by single ventricle management. METHODS: Retrospective analysis was performed to: 1) validate our scoring systems in 89 additional neonates with AS and normal mitral valve area, 2) assess the effects of 5% measurement variation on predictive scores, 3) evaluate our cohort with the Congenital Heart Surgeons' Society scoring system, and 4) repeat the discriminant analysis on the basis of all 126 patients. RESULTS: The original scores each predicted outcome accurately in 68 patients (77%). Minor (5%) measurement variation changed the outcome predicted by the discriminant equation in 8 patients (9%) and by the threshold system in 13 patients (15%). The most accurate model for predicting survival with a biventricular circulation among the full cohort is: 10.98 (body surface area) + 0.56 (aortic annulus z-score) + 5.89 (left ventricular to heart long-axis ratio) - 0.79 (grade 2 or 3 endocardial fibroelastosis) - 6.78. With a cutoff of -0.65, outcome was predicted accurately in 90% of patients. CONCLUSIONS: Both of our original scoring systems are less accurate at predicting outcome than in our original analysis. Revised discriminant analysis yielded a model similar to our original equation that was 90% accurate at predicting survival with a biventricular circulation among neonates with AS and a mitral valve area z-score >-2.

Aortic Valve Stenosis↗
PubMed2006

Improved quantitation for PET/CT image reconstruction with system modeling and anatomical priors.

Accurate quantitation of positron emission tomography (PET) tracer uptake levels in tumors is important for staging and monitoring response to treatment. Quantitative accuracy in PET is particularly poor for small tumors because of system partial volume errors and smoothing operations. This work proposes a reconstruction algorithm to reduce the quantitative errors due to limited system resolution and due to necessary image noise reduction. We propose a method for finding and using the detection system response in the projection matrix of a statistical reconstruction algorithm. In addition, we use aligned anatomical information, available in PET/CT scanners, to govern the penalty term applied during each image update. These improvements are combined with Fourier rebinning in a clinically feasible algorithm for reconstructing fully three-dimensional PET data. Results from simulation and measured studies show improved quantitation of tumor values in terms of bias and variance across multiple tumor sizes and activity levels with the proposed method. At common clinical image noise levels for the detection task, the proposed method reduces the error in maximum tumor values by 11% compared to filtered back-projection and 5% compared to conventional iterative methods.

Algorithms↗
PubMed2001

A fast impingement detection algorithm for computer-aided orthopedic surgery.

OBJECTIVE: For simulation of computer-aided orthopedic interventions, the detection of impingement between parts of the patient's anatomy and/or implants is often of key importance. The impingement (collision) detection methods used in the existing literature seem to be unsuitable for two reasons. First, a polyhedral approximation of an anatomical model is not appropriate because medical images are quite irregular and are geometrically complex. Second, geometric and temporal coherences are not always available, because only the final results may be of interest. This article describes the development of a fast and accurate impingement detection algorithm for medical applications. MATERIALS AND METHODS: The presented algorithm takes implicit object models from reconstructions of anatomical CT data that represent complicated anatomical structures. To speed up the detection procedure, a lookup table and a linear transform are used so that searching for impingement between any two objects becomes a problem of calculating spatial indices and checking the lookup table. RESULTS: For any given transformation, the algorithm could perform impingement detection of two objects within 0.1 s on a 167 MHz Sun UltraSPARC1 workstation. Experimental results concerning accuracy, reliability and speed are given for a phantom and for a patient's data set. CONCLUSIONS: This algorithm provides a general-purpose impingement detection method in the sense that objects can be of any shape, and it can be extended to any number of objects in the scene.

Algorithms↗
PubMed2005

The effect of formulation variables and breathing patterns on the site of nasal deposition in an anatomically correct model.

PURPOSE: This study was conducted to evaluate the effect of formulation variables and breathing patterns on aerosol distribution in the nasal cavity. METHODS: Placebo nasal spray formulations containing 0.25% w/v Avicel CL611 (viscosity = 4 cP) and 2% w/v methylcellulose (MC; viscosity = 18.2 cP) were radiolabeled with (99m)Technicium. Following spraying into a silicone nose model, through which air was drawn at one of three constant rates (0, 10, and 20 L/min) or one of two breathing profiles (representing fast and slow inhalations), aerosol deposition in the model was quantified by gamma scintigraphy. Regional deposition was expressed as horizontal[inner, middle (h), outer] and vertical distribution [upper, middle (v), lower] in the nose model. RESULTS: Compared to 2% MC, Avicel CL611 significantly increased aerosol deposition in the middle (h) region of the nasal cavity under all breathing conditions, and in the inner region at 0 and 20 L/min and with a slow inhalation. The different breathing rates showed no effect on deposition of 2% MC. However, 10 L/min significantly increased the upper deposition of Avicel compared to 0 and 20 L/min. CONCLUSIONS: Nasal sprays with a low viscosity provided greater surface coverage of the nasal mucosa than higher viscosity formulations. Changes in breathing profiles did not affect aerosol deposition in this nose model.

Administration, Intranasal↗
PubMed2004

LUCY: a 3-D pelvic model for surgical simulation.

Development of 3-D models of human anatomy for use in virtual reality simulators is anticipated to enhance surgical training. These models may be a valuable resource for gaining mastery of minimal-access procedures. The pelvis portion (hip to upper-thigh) of a 32-year-old female cadaver was frozen and sectioned axially in approximately 2-mm increments as the first step in producing an accurately representative 3-D model of the human female pelvis. Photographic exposures of the entire series of 95 sections were then converted to digital format. Adobe PhotoShop masks for each structure were created and converted into wire-frame and surface-textured models; this aggregate model set was named "LUCY." To date, 3-D representations of 40 pelvic structures (over 2200 individual masks) have been modeled In conjunction with haptic technology, these virtual anatomic models will enable users to practice fundamental surgical manipulations and procedures such as tubal ligation and ovariectomy. The deployment of surgical-simulation models such as LUCY may facilitate technical-performance aspects of surgical training, particularly those associated with minimal-access procedures. Manipulations and procedures can be practiced over the Internet, providing a host of flexible options to enhance the surgical curricula.

Adult↗
PubMed1994

SAR calculations in an anatomically realistic model of the head for mobile communication transceivers at 900 MHz and 1.8 GHz.

A new mathematical model of the head has been constructed from a set of serial MRI slices from one subject. Finite-difference time-domain (FDTD) calculations of the specific energy absorption rate (SAR) have been performed on this model with a 2 mm resolution for a generic mobile communication transceiver represented by a quarter-wavelength monopole on a metal box. The antenna was mounted either at the centre or corner of the top face of the box. The frequencies considered were 900 MHz and 1.8 GHz. Three irradiation geometries were considered, a vertical handset in front of the eye and vertical and horizontal orientations at the side of the ear. The effect of a hand grasping the handset was considered. The head model was scaled to represent the head of an infant and a subset of calculations was performed to verify that the SAR deposited in the infant head did not exceed that in the adult. Results are also presented for a half-wavelength dipole. The maximum SAR values produced by the generic transceiver for the horizontal orientation at the side of the head which is the most typical position, averaged over 10 g of tissue at 900 MHz and 1.8 GHz, are 2.1 and 3.0 W kg(-1) per W of radiated power. The corresponding values over 1 g of tissue are 2.3 and 4.8 W kg(-1) per W at 900 MHz and 1.8 GHz. However, if one were to consider all possible operational conditions, the placement of the transceiver in front of the eye will give 3.1 and 4.6 W kg(-1) per W averaged over 10 g of tissue and 4.7 and 7.7 W kg(-1) per W over 1 g of tissue at 900 MHz and 1.8 GHz, respectively.

Adult↗
PubMed1995

Electric field and current density distributions induced in an anatomically-based model of the human head by magnetic fields from a hair dryer.

We have used the impedance method to calculate the induced electric (E) fields and current densities (J) for the spatially varying vector magnetic fields due to a hair dryer. In this method, applicable for low-frequency exposures where the quasi-static approximation may be made, the biological body or the exposed parts thereof are represented by a three-dimensional (3-D) network of impedances whose individual values are obtained from the electrical properties sigma, epsilon r for the various tissues. We have measured the 3-D variations of the 50-Hz magnetic fields from a typical hair dryer and found that the various components correlate well with those for a helical coil. The non-uniform magnetic fields thus obtained are used to calculate the induced E and J with a resolution of 1.31 cm for the model of the head and neck. The induced E values are compared with the fields endogenous to the body and the minimum detectable E-field limits based on the cellular thermal noise model proposed by Weaver and Astumian (1990, 1992).

Electromagnetic Fields↗
PubMed2006

Biventricular myocardial strains via nonrigid registration of anatomical NURBS model [corrected].

We present research in which both left and right ventricular deformation is estimated from tagged cardiac magnetic resonance imaging using volumetric deformable models constructed from nonuniform rational B-splines (NURBS). The four model types considered and compared for the left ventricle include two Cartesian NURBS models--one with a cylindrical parameter assignment and one with a prolate spheroidal parameter assignment. The remaining two are non-Cartesian, i.e., prolate spheroidal and cylindrical each with their respective prolate spheroidal and cylindrical parameter assignment regimes. These choices were made based on the typical shape of the left ventricle. For each frame starting with end-diastole, a NURBS model is constructed by fitting two surfaces with the same parameterization to the corresponding set of epicardial and endocardial contours from which a volumetric model is created. Using normal displacements of the three sets of orthogonal tag planes as well as displacements of contour/tag line intersection points and tag plane intersection points, one can solve for the optimal homogeneous coordinates, in a weighted least squares sense, of the control points of the deformed NURBS model at end-diastole using quadratic programming. This allows for subsequent nonrigid registration of the biventricular model at end-diastole to all later time frames. After registration of the model to all later time points, the registered NURBS models are temporally lofted in order to create a comprehensive four-dimensional NURBS model. From the lofted model, we can extract three-dimensional myocardial deformation fields and corresponding Lagrangian and Eulerian strain maps which are local measures of nonrigid deformation. The results show that, in the case of simulated data, the quadratic Cartesian NURBS models with the cylindrical and prolate spheroidal parameter assignments outperform their counterparts in predicting normal strain. The decreased complexity associated with the Cartesian model with the cylindrical parameter assignment prompted its use for subsequent calculations. Lagrangian strains in three canine data, a normal human, and a patient with history of myocardial infarction are presented. Eulerian strains for the normal human data are also included.

Algorithms↗