bluuebunny/crossref_metadata_2025 huggingface dataset:
Visualization of the Contact Force Solution Space for Multi-Limbed Robots
Authors: Hong Dennis W. , Cipra Raymond J.
Published: December 2005

A new analytical method for determining, describing, and visualizing the solution space for the contact force distribution of multi-limbed robots with three feet in contact with the environment in three-dimensional space is presented. The foot contact forces are first resolved into strategically defined foot contact force components to decouple them, and then the static equilibrium equations are applied. Using the friction cone equation at each foot contact point, the problem is then transformed into a geometrical one. Using geometric properties of the friction cones and by simple manipulation of their conic sections, the entire solution space which satisfies the static equilibrium and friction constraints at each contact point can be found. Two representation schemes, the “force space graph” and the “solution volume representation,” are developed for describing and visualizing the solution space which gives an intuitive visual map of how well the solution space is formed for the given conditions of the system.
Analytic Form of the Six-Dimensional Singularity Locus of the General Gough-Stewart Platform
Authors: Li Haidong , Gosselin Clément M. , Richard Marc J. , St-Onge Boris Mayer
Published: December 2005

The determination of the 6D singularity locus of the general Gough-Stewart platform is discussed in this article. The derivation of the velocity equation and the corresponding Jacobian matrices is first presented. Then a new procedure is introduced to obtain the analytical expression of the singularity locus, which is a function of six variables (x,y,z,ϕ,θ,ψ), using the velocity equation. Examples are also given to illustrate the results obtained. Gough-Stewart platforms can be used in several robotic applications as well as in flight simulators. The determination of the singularity locus is a very important design and application issue.
Adaptive Robotic Mechanical Systems: A Design Paradigm
Authors: Gosselin Clément M.
Published: December 2005

This paper aims at developing a general framework for the use of the concept of adaptive mechanical system in the design of advanced robotic devices. The concept of adaptive mechanical system is first formalized. A design methodology is then proposed in order to formulate the associated design paradigm, based on the fundamental principles of mechanics. Finally, examples of adaptive robotic mechanical systems taken from the literature are presented in order to illustrate the application of the general design methodology.
On a Rigid Body Subject to Point-Plane Constraints
Authors: Wampler Charles W.
Published: December 2005

This paper investigates the location of a rigid body such that N specified points of the body lie on N given planes in space. Variants of this problem arise in kinematics, metrology, and computer vision, including some, such as the motion of a spherical four-bar, that are not at first glance point-plane contact problems. The case N=6, the minimum number to fully constrain the body, is of special interest: We give an eigenvalue method for finding all solutions, which may number up to eight. For N⩾7 there are, in general, no solutions, but if the constraints are compatible and not degenerate, we show how to find the unique solution by a linear least-squares method. For N⩽5, the body is underconstrained, having in general 6−N degrees of freedom; we determine the degree of the general motion for each case. We also examine the workspace of a particular three-degree-of-freedom parallel-link tripod mechanism.
Jacobian, Manipulability, Condition Number, and Accuracy of Parallel Robots
Authors: Merlet J. P.
Published: December 2005

Although the concepts of Jacobian matrix, manipulability, and condition number have existed since the very early beginning of robotics their real significance is not always well understood. In this paper we revisit these concepts for parallel robots as accuracy indices in view of optimal design. We first show that the usual Jacobian matrix derived from the input-output velocities equations may not be sufficient to analyze the positioning errors of the platform. We then examine the concept of manipulability and show that its classical interpretation is erroneous. We then consider various common local dexterity indices, most of which are based on the condition number of the Jacobian matrix. It is emphasized that even for a given robot in a particular pose there are a variety of condition numbers and that their values are not coherent between themselves but also with what we may expect from an accuracy index. Global conditioning indices are then examined. Apart from the problem of being based on the local accuracy indices that are questionable, there is a computational problem in their calculation that is neglected most of the time. Finally, we examine what other indices may be used for optimal design and show that their calculation is most challenging.
Workspaces of Cable-Actuated Parallel Manipulators
Authors: Stump Ethan , Kumar Vijay
Published: December 2005

This paper develops analytical techniques to delineate the workspace boundaries for parallel mechanisms with cables. In such mechanisms, it is not only necessary to solve the closure equations but it is also essential to verify that equilibrium can be achieved with non-negative actuator (cable) forces. We use tools from convex analysis and linear algebra to derive closed-form expressions for the workspace boundaries and illustrate the applications using planar and spatial examples.
Topological Synthesis of a Parallel Wrist Mechanism
Authors: Hess-Coelho Tarcisio A.
Published: December 2005

To determine a suitable kinematic structure for a specific application is a complex task. Some available methods may provide a list of possible architectures for a robot wrist. However, a systematic method for selecting one among others is still an open problem. The paper presents a qualitative procedure for topological synthesis based on criteria that consider wrist design requirements and parallel mechanism peculiarities. The performance of chosen topology is evaluated according to desired requirements.
Step Function Representation of Solid Models and Application to Mesh Free Engineering Analysis
Authors: Kumar Ashok V. , Lee Jongho
Published: December 2005

Numerical methods for solving boundary value problems that do not require generation of mesh to approximate the analysis domain have been referred to as mesh-free methods. While many of these are “mesh less” methods that do not have connectivity between nodes, a subset of these methods uses a structured mesh or grid for the analysis that does not conform to the geometry of the domain of analysis. Instead the geometry is represented using implicit equations. In this paper we present a method for constructing step functions of solids whose boundaries are represented using implicit equations. Step functions can be used to compute volume integrals over the solid that are needed for mesh free analysis. The step function of the solid has a unit value within the solid and zero outside. A level set of this step function can then be defined as the boundary of the solid. Boolean operators are defined in this paper that enable step functions of half-spaces and primitives to be combined to construct a single step function for more complex solids. Application of step functions to analysis using nonconforming mesh is illustrated.
Heat Partition in Rolling/Sliding Elastohydrodynamic Contacts
Authors: Clarke A. , Sharif K. J. , Evans H. P. , Snidle R. W.
Published: December 2005

The paper presents the results of a thermal analysis of a set of disk experiments carried out by Patching et al. to investigate scuffing. The experiments used crowned steel disks at 76-mm centers with maximum Hertzian contact pressures of up to 1.7 GPa. Experimental measurements of contact friction were used as the basis for a thermal analysis of the disks and their associated support shafts. Temperatures measured by embedded thermocouples 3.2 mm below the running tracks of the disks were used to determine the heat partition between the faster and slower running disks in order to match the experimental with calculated temperatures. This partition was found to vary approximately as a function of the product of sliding speed and surface temperature difference. A transient (flash) temperature analysis of one of the experiments was also carried out. This shows large differences between the disk transient surface temperatures. These surface temperature distributions were compared with those obtained from corresponding elastohydrodynamic lubrication (EHL) analyses using two different non-Newtonian lubricant formulations. The EHL analyses show that the heat partition obtained depends on the form of non-Newtonian behavior assumed, and that to achieve the same partition as is evident in the experiment a limiting shear stress formulation is necessary. It is suggested that the combination of heat transfer and EHL analysis presented in the paper could be used as a sensitive tool for distinguishing between different non-Newtonian lubricant models under realistic engineering loads and with high sliding speeds.
A Combined Optical-Ultrasonic Method of Establishing the Compressibility of High-Pressure Oil and Grease Films Entrapped in a Ball on Flat Contact
Authors: Kondo S. , Sayles R. S. , Lowe M. J. S.
Published: December 2005

The paper describes an experimental technique combining optical interferometry and ultrasonic attenuation to measure the bulk modulus (compressibility) and film thickness profiles of oil and grease films entrapped between a stationary steel ball and a sapphire plate. Results are presented for lithium- and urea-based greases, and their base oils alone, at contact pressures up to and around 3 GPa, and are compared with some models and results from other studies, extrapolated to the higher pressures achieved here. Results show that the bulk modulus of a particular grease and its base oil are found to be similar, at a given pressure, and largely dependent on the base-oil properties, which is demonstrated to be what is expected with the concentration levels of fillers and solid lubricants present. However, the film shapes measured for the grease and its base oil can be quite different. This is seemingly due to the manner in which the lubricant samples “solidify”, due to the high pressures, and also to the way in which the edges of the squeeze-film entrapped fluid are sealed. This latter sealing effect seems to be assisted by secondary phases, particularly with greases where fillers and solid lubricants are present, creating a very effective sealed contact where the entrapped lubricant samples could be observed to remain stable for several hours.
Thermal Distortion of an Anisotropic Elastic Half-Plane and its Application in Contact Problems Including Frictional Heating
Authors: Lin Yuan , Ovaert Timothy C.
Published: December 2005

The thermal surface distortion of an anisotropic elastic half-plane is studied using the extended version of Stroh’s formalism. In general, the curvature of the surface depends both on the local heat flux into the half-plane and the local temperature variation along the surface. However, if the material is orthotropic, the curvature of the surface depends only on the local heat flux into the half-plane. As a direct application, the two-dimensional thermoelastic contact problem of an indenter sliding against an orthotropic half-plane is considered. Two cases, where the indenter has either a flat or a parabolic profile, are studied in detail. Comparisons with other available results in the literature show that the present method is correct and accurate.
On the Hysteresis Phenomenon Associated With Instability of Rotor-Bearing Systems
Authors: Wang J. K. , Khonsari M. M.
Published: December 2005

A method is presented for predicting the occurrence of a hysteresis phenomenon, which is observed in experimental results dealing with the instability of rotor-bearing systems. The method is based on the Hopf bifurcation theory. It is shown that the existence of a hysteresis phenomenon is dependent upon the system’s operating parameters. To this end, the effect of oil viscosity on the hysteresis phenomenon and its implications on the rotor-bearing instability are investigated. The results of a series of experiments illustrating the effectiveness of the prediction of the hysteresis phenomenon using the Hopf bifurcation theory are also presented.
Compressibility of Oil/Refrigerant Lubricants in Elasto-Hydrodynamic Contacts
Authors: Tuomas Roger , Isaksson Ove
Published: December 2005

A high-pressure chamber is used to study lubricant compressibility when refrigeration oil is diluted by refrigerant. The tested lubricant in this work is a POE (polyol ester) oil, POE diluted with nonchlorinated (HFC) refrigerant R-134a, a naphthenic mineral oil, and the mineral oil diluted with the chlorinated (HCFC) refrigerant R-22. The high-pressure chamber experiments show that by adding 20 wt% of R-134a to the polyol ester oil, the stiffness of the lubricant increases by approximately 38 wt% at 1 GPa and is much higher than for R-22 and mineral oil.
A Parametric Analysis of the Thermal Shear Localization in Elastohydrodynamic Lubrication Films
Authors: Chang L.
Published: December 2005

Experiments and computer simulations have revealed some unusual results of elastohydrodynamic lubrication (EHL) associated with a high degree of thermally induced inhomogeneous shear across the lubricant film, or thermal shear localization. The results include the development of a sizable film dimple in the central EHL region and a dramatic reduction in EHL traction. In this study, a theoretical analysis is carried out to determine the conditions under which the thermal shear localization may develop in EHL films. For a Newtonian lubricant obeying the Barus law of viscosity, a dimensionless group-parameter is identified that fully governs the degree of the thermal inhomogeneous shear. Results are presented that show the critical range of values of this parameter corresponding to the onset of the shear localization. The analysis is also extended to lubricants with non-Newtonian behavior. Results suggest that the same dimensionless group-parameter may be used to measure the degree of the shear localization when the lubricant viscosity in the parameter is replaced by an effective viscosity that accounts for the non-Newtonian effect.
Investigation on Effect of Surface Roughness Pattern to Drag Force Reduction Using Rotary Rheometer
Authors: Haosheng Chen , Darong Chen , Yongjian Li
Published: December 2005

A drag force reduction experiment is performed on polyoxyethylene (PEO) water solution using a rotary rheometer. Longitudinal, transverse, and isotropic grooves are notched on the rotor’s surface to investigate the effect of surface roughness patterns. The experiment results show that higher drag force is generated on the surface with transverse grooves compared with the drag force on a smooth surface, while lower drag force is generated on the surface with longitudinal grooves. The drag force on the surface with isotropic grooves is between them. Forces on the side wall of the grooves play important roles in drag force reduction. They are numerically analyzed based on the Navier-Stokes equation using the finite volume method. The viscous force on the groove’s bottom plane is far less than the force on the corresponding plane of the smooth surface, but the drag force on the groove’s surface is compensated either by the pressure drag on the side walls of the transverse groove or by the viscous force on the side walls of the longitudinal groove. The pressure drag on side wall of transverse groove is always higher than the viscous force on side wall of longitudinal groove. The numerical results cope with the experiment results that only the surface with longitudinal grooves can reduce the drag force.
Forward Position Analysis of the 3-DOF Module of the TriVariant: A 5-DOF Reconfigurable Hybrid Robot
Authors: Li Meng , Huang Tian , Chetwynd Derek G. , Hu S. Jack
Published: December 2005

This paper deals with the forward position analysis of a 3-DOF parallel mechanism module, which forms the main body of a 5-DOF reconfigurable hybrid robot named TriVariant. The TriVariant is a modified version of the Tricept robot, achieved by integrating one of the three active limbs into the passive one. The analytical method is employed to obtain the forward position solutions. It shows that the forward position analysis of the TriVariant is much simpler than that of the Tricept.
Efficiency Analysis of Rapid Turbocharger With Alternative Bearing Design
Authors: Liazid Abdelkrim , Izidi Lahouari , Bencherif Mohamed
Published: December 2005

This work presents the possibility to adapt self-acting air bearing to a rapid diesel engine turbocharger originally designed with oil journal bearings. The aim of this innovative idea is to estimate a global evolution of efficiency which is a parameter that strongly influences the inlet air mass of the diesel engine. This study deals with radial bearings and illustrates the advantages and the limits of the aerodynamic bearing application. First, the computing concerns the determination of the bearing load by resolving the Reynolds equation. So, the determination of the neat turbine power by a mathematical model using some experimental data is also made. Lastly, a comparison with the original hydrodynamic bearings is presented. Some discussions are also developed.
Comparison of Nucleation Site Density for Pool Boiling and Gas Nucleation
Authors: Qi Yusen , Klausner James F.
Published: December 2005

It has been well established that the rate of heat transfer associated with boiling systems is strongly dependent on the nucleation site density. Over many years attempts have been made to predict nucleation site density in boiling systems using a variety of techniques. With the exception of specially prepared surfaces, these attempts have met with little success. This paper presents an experimental investigation of nucleation site density measured on roughly polished brass and stainless steel surfaces for gas nucleation and pool boiling over a large parameter space. A statistical model used to predict the nucleation site density in saturated pool boiling is also investigated. The fluids used for this study, distilled water and ethanol, are moderately wetting and highly wetting, respectively. Using distilled water it has been observed that the trends of nucleation site density versus the inverse of the critical radius are similar for pool boiling and gas nucleation. The nucleation site density is higher for gas nucleation than for pool boiling. An unexpected result has been observed with ethanol as the heat transfer fluid, which casts doubt on the general assumption that heterogeneous nucleation in boiling systems is exclusively seeded by vapor trapping cavities. Due to flooding, few sites are active on the brass surface and at most two are active on the stainless steel surface during gas nucleation experiments. However, nucleation sites readily form in large concentration on both the brass and stainless steel surfaces during pool boiling. The pool boiling nucleation site densities for ethanol on rough and mirror polished brass surfaces are also compared. It shows that there is not a significant difference between the measured nucleation site densities on the smooth and rough surfaces. These results suggest that, in addition to vapor trapping cavities, another mechanism must exist to seed vapor bubble growth in boiling systems.
Film Condensation of R-134a on Tube Arrays With Plain and Enhanced Surfaces: Part I—Experimental Heat Transfer Coefficients
Authors: Gstoehl D. , Thome J. R.
Published: December 2005

The aim of the present investigation was to study the effect of condensate inundation on the thermal performance of a vertical array of horizontal tubes with plain and enhanced surfaces. Refrigerant R-134a was condensed at a saturation temperature of 304K on tube arrays with up to ten tubes at pitches of 25.5,28.6,and44.5mm. Notably, local condensing heat transfer coefficients were measured at the midpoint of each tube, as opposed to mean values. Four commercially available copper tubes with a nominal diameter of 19.05mm(0.75in.) were tested: a plain tube, a 26fpi∕1024fpm low finned tube, and two tubes, with three-dimensional (3D) enhanced surface structures. At low liquid inundation rates, the tubes with 3D enhanced surface structures significantly outperformed the low finned tube. Increasing liquid inundation deteriorated the thermal performance of the 3D enhanced tubes, whereas it had nearly no affect on the low finned tube, resulting in a higher heat transfer coefficients for the low finned tube at high liquid film Reynolds numbers. All the tests were performed with minimal vapor shear.
Thermal Conductivity Measurements of Ultra-Thin Single Crystal Silicon Layers
Authors: Liu Wenjun , Asheghi Mehdi
Published: December 2005

Self-heating in deep submicron transistors (e.g., silicon-on-insulator and strained-Si) and thermal engineering of many nanoscale devices such as nanocalorimeters and high-density thermomechanical data storage are strongly influenced by thermal conduction in ultra-thin silicon layers. The lateral thermal conductivity of single-crystal silicon layers of thicknesses 20 and 100nm at temperatures between 30 and 450K are measured using joule heating and electrical-resistance thermometry in suspended microfabricated structures. In general, a large reduction in thermal conductivity resulting from phonon-boundary scattering is observed. Thermal conductivity of the 20nm thick silicon layer at room temperature is nearly 22Wm−1K−1, compared to the bulk value, 148Wm−1K−1. The predictions of the classical thermal conductivity theory that accounts for the reduced phonon mean free paths based on a solution of the Boltzmann transport equation along a layer agrees well with the experimental results.
Turbulent Heat Transfer in Plane Couette Flow
Authors: Le Phuong M. , Papavassiliou Dimitrios V.
Published: December 2005

Heat transfer in a fully developed plane Couette flow for different Prandtl number fluids was studied using numerical simulations. The flow field was created by two infinite planes moving at the same velocity, but in opposite directions, forming a region of constant total shear stress. Heat markers were released into the flow from the channel wall, and the ground level temperature was calculated for dispersion from continuous line sources of heat. In addition, the temperature profile across the channel was synthesized from the behavior of these continuous line sources. It was found that the heat transfer coefficient for Couette flow is higher than that in channel flow for the same Prandtl numbers. Correlations were also obtained for the heat transfer coefficient for any Prandtl number ranging from 0.1 to 15,000 in fully developed turbulence.
Entropy Generation in Counter Flow Gas to Gas Heat Exchangers
Authors: Mohamed Hany Ahmed
Published: December 2005

Analysis of heat transfer and fluid flow thermodynamic irreversibilities is realized on an example of a counter flow double pipe heat exchanger utilizing turbulent air flow as a working fluid. During the process of mathematical model creation and for different working and constructing limitations, total thermodynamic irreversibility is studied. The present work proves that the irreversibility occurred due to unequal capacity flow rates (flow imbalance irreversibility). It is concluded that the heat exchanger should be operated at effectiveness, ε, greater than 0.5 and the well operating conditions will be achieved when ε approaches one where low irreversibility is expected. A new equation is adopted to express the entropy generation numbers for imbalanced heat exchangers of similar design with smallest deviation from the exact value. The results obtained from the new equation are compared with the exact values and with those obtained by Bejan (Bejan, A., 1997, Advanced Engineering Thermodynamics, Wiley, New York).
Mechanistic Prediction of Nucleate Boiling Heat Transfer–Achievable or a Hopeless Task?
Authors: Dhir Vijay K.
Published: December 2005

Over the last half of the twentieth century, a number of purely empirical and mechanism-based correlations have been developed for pool nucleate boiling. Empirical correlations differ from each other substantially with respect to the functional dependence of heat flux on fluid and surface properties, including gravity. The mechanism-based correlations require knowledge of the number density of active sites, bubble diameter at departure, and bubble-release frequency. However, because of the complex nature of the subprocesses involved, it has not been possible to develop comprehensive models or correlations for these parameters. This, in turn, has led to the pessimistic view that mechanistic prediction of nucleate boiling is a hopeless task. However, there is an alternative to the past approaches—complete numerical simulation of the boiling process. Value of this approach for bubble dynamics and associated heat transfer is shown through excellent agreement of predictions with data obtained on microfabricated surfaces on which active nucleation sites can be controlled.
Natural Convection From Two Thermal Sources in a Vertical Porous Layer
Authors: Saeid Nawaf H.
Published: December 2005

Numerical study of natural convection flow induced by two isothermally heated elements located on adiabatic vertical plate immersed in a Darcian porous medium is carried out in the present article. The natural convection is affected by the Rayleigh number, the separation distance between the elements, their temperature ratio, and the length of the upper element. The numerical results are presented as average Nusselt number versus Rayleigh number for wide ranges of the governing parameters. It is found that the heat transfer from the lower element is not affected by the presence of the upper element for equal temperatures of the elements. The heat transfer from the lower element can be enhanced by increasing the temperature of the upper element due to the suction effect. The average Nusselt number along the upper heated element is found to increase with the increase of any of the governing parameters.
Dual‐energy digital mammography for calcification imaging: Scatter and nonuniformity corrections
Authors: Kappadath S. Cheenu , Shaw Chris C.
Published: December 2005

Mammographic images of small calcifications, which are often the earliest signs of breast cancer, can be obscured by overlapping fibroglandular tissue. We have developed and implemented a dual‐energy digital mammography (DEDM) technique for calcification imaging under full‐field imaging conditions using a commercially available flat‐panel based digital mammography system. The low‐ and high‐energy images were combined using a nonlinear mapping function to cancel the tissue structures and generate the dual‐energy (DE) calcification images. The total entrance‐skin exposure and mean‐glandular dose from the low‐ and high‐energy images were constrained so that they were similar to screening‐examination levels. To evaluate the DE calcification image, we designed a phantom using calcium carbonate crystals to simulate calcifications of various sizes overlaid with breast‐tissue‐equivalent material thick with a continuously varying glandular‐tissue ratio from 0% to 100%. We report on the effects of scatter radiation and nonuniformity in x‐ray intensity and detector response on the DE calcification images. The nonuniformity was corrected by normalizing the low‐ and high‐energy images with full‐field reference images. Correction of scatter in the low‐ and high‐energy images significantly reduced the background signal in the DE calcification image. Under the current implementation of DEDM, utilizing the mammography system and dose level tested, calcifications in the size range were clearly visible in DE calcification images. Calcification threshold sizes decreased to the size range when the visibility criteria were lowered to barely visible. Calcifications smaller than were usually not visible in most cases. The visibility of calcifications with our DEDM imaging technique was limited by quantum noise, not system noise.
Effect of correlation on combining diagnostic information from two images of the same patient
Authors: Liu Bei , Metz Charles E. , Jiang Yulei
Published: December 2005

We have shown previously, in the context of computer‐aided diagnosis (CAD), that information derived from multiple images of the same patient can be used to improve diagnostic performance. In that work, we ignored the correlation among multiple images of the same patient. In the present study, we investigate theoretically, within the framework of receiver operating characteristic (ROC) analysis, the effect of correlation on three methods for combining quantitative diagnostic information from two images: taking the average, the maximum, and the minimum of a pair of normally distributed decision variables. We assume, as in our previous work, that the quantitative diagnostic information obtained from the two images of a given patient can be transformed monotonically to two latent decision variables that are normally distributed. Similar to the situation of uncorrelated images, we found that (1) the average always improves the area under the ROC curve (AUC) compared to the single‐view image; (2) the maximum and the minimum can also, but not always, improve the AUC; and (3) each method can be the best method in certain situations. In addition, as the correlation strength increases, the average performs the best less often, whereas the maximum and the minimum perform the best more often. These theoretical results are illustrated with analysis of a mammography study.
An alternative solution to the nonuniform noise propagation problem in fan‐beam FBP image reconstruction
Authors: Wang Jing , Lu Hongbing , Li Tianfang , Liang Zhengrong
Published: December 2005

It has been observed that the variances in reconstructed images from stationary noisy projection data by fan‐beam filtered backprojection (fFBP) algorithm with ramp in the filtering step and linear interpolation in the backprojection step are nonuniform across the field‐of‐view. This is believed to be caused by the distance‐dependent factor in the fFBP reconstruction formula. Shift‐variant filtration approach in the filtering step has been investigated to address the nonuniform noise propagation problem in fFBP. In this work, we present an alternative solution by the use of spatially variant weighting, instead of spatially invariant interpolation, in the backprojection step, while maintaining the homogeneous ramp filtering. Both theoretically predicted and empirically determined results concur and demonstrate the effectiveness of the presented alternative solution.
Modeling skin collimation using the electron pencil beam redefinition algorithm
Authors: Chi Pai‐Chun M. , Hogstrom Kenneth R. , Starkschall George , Antolak John A. , Boyd Robert A.
Published: December 2005

Skin collimation is an important tool for electron beam therapy that is used to minimize the penumbra when treating near critical structures, at extended treatment distances, with bolus, or using arc therapy. It is usually made of lead or lead alloy material that conforms to and is placed on patient surface. Presently, commercially available treatment‐planning systems lack the ability to model skin collimation and to accurately calculate dose in its presence. The purpose of the present work was to evaluate the use of the pencil beam redefinition algorithm (PBRA) in calculating dose in the presence of skin collimation. Skin collimation was incorporated into the PBRA by terminating the transport of electrons once they enter the skin collimator. Both fixed‐ and arced‐beam dose calculations for arced‐beam geometries were evaluated by comparing them with measured dose distributions for 10‐ and 15‐MeV beams. Fixed‐beam dose distributions were measured in water at 88‐cm source‐to‐surface distance with an air gap of 32 cm. The field (dimensions projected to isocenter) had a 10‐mm thick lead collimator placed on the surface of the water with its edge 5 cm inside the field's edge located at . Arced‐beam dose distributions were measured in a 13.5‐cm radius polystyrene circular phantom. The beam was arced 90° ( to ), and 10‐mm thick lead collimation was placed at . For the fixed beam at 10 MeV, the PBRA‐calculated dose agreed with measured dose to within 2.0‐mm distance to agreement (DTA) in the regions of high‐dose gradient and 2.0% in regions of low dose gradient. At 15 MeV, the PBRA agreed to within a 2.0‐mm DTA in the regions of high‐dose gradient; however, the PBRA underestimated the dose by as much as 5.3% over small regions at depths less than 2 cm because it did not model electrons scattered from the edge of the skin collimation. For arced beams at 10 MeV, the agreement was 1‐mm DTA in the high‐dose gradient regions, and 2% in the low‐dose gradient regions. For arced beams at 15 MeV, the agreement was 1 mm in the high‐dose gradient regions, and in the low‐dose gradient region at depth less than 2 cm, as much as 5% dose difference was observed. This study demonstrated the ease with which skin collimation can be incorporated into the PBRA. The good agreement of PBRA calculated with measured dose shows that the PBRA is likely sufficiently accurate for clinical use in the presence of skin collimation for electron arc therapy. To further improve the accuracy of the PBRA in regions having significant electrons scattered from the edge of the skin collimation would require transporting the electrons through the lead skin collimation near its edges.
A three‐dimensional reconstruction algorithm for an inverse‐geometry volumetric CT system
Authors: Schmidt Taly Gilat , Fahrig Rebecca , Pelc Norbert J.
Published: December 2005

An inverse‐geometry volumetric computed tomography (IGCT) system has been proposed capable of rapidly acquiring sufficient data to reconstruct a thick volume in one circular scan. The system uses a large‐area scanned source opposite a smaller detector. The source and detector have the same extent in the axial, or slice, direction, thus providing sufficient volumetric sampling and avoiding cone‐beam artifacts. This paper describes a reconstruction algorithm for the IGCT system. The algorithm first rebins the acquired data into two‐dimensional (2D) parallel‐ray projections at multiple tilt and azimuthal angles, followed by a 3D filtered backprojection. The rebinning step is performed by gridding the data onto a Cartesian grid in a 4D projection space. We present a new method for correcting the gridding error caused by the finite and asymmetric sampling in the neighborhood of each output grid point in the projection space. The reconstruction algorithm was implemented and tested on simulated IGCT data. Results show that the gridding correction reduces the gridding errors to below one Hounsfield unit. With this correction, the reconstruction algorithm does not introduce significant artifacts or blurring when compared to images reconstructed from simulated 2D parallel‐ray projections. We also present an investigation of the noise behavior of the method which verifies that the proposed reconstruction algorithm utilizes cross‐plane rays as efficiently as in‐plane rays and can provide noise comparable to an in‐plane parallel‐ray geometry for the same number of photons. Simulations of a resolution test pattern and the modulation transfer function demonstrate that the IGCT system, using the proposed algorithm, is capable of isotropic resolution. The successful implementation of the reconstruction algorithm is an important step in establishing feasibility of the IGCT system.
Treatment delivery software for a new clinical grade ultrasound system for thermoradiotherapy
Authors: Novák Petr , Moros Eduardo G. , Straube William L. , Myerson Robert J.
Published: December 2005

A detailed description of a clinical grade Scanning Ultrasound Reflector Linear Array System (SURLAS) applicator was given in a previous paper [Med. Phys., 230–240 (2005)]. In this paper we concentrate on the design, development, and testing of the personal computer (PC) based treatment delivery software that runs the therapy system. The SURLAS requires the coordinated interaction between the therapy applicator and several peripheral devices for its proper and safe operation. One of the most important tasks was the coordination of the input power sequences for the elements of two parallel opposed ultrasound arrays (eight elements/array, array 1 and 2 operate at 1.9 and 4.9 MHz, respectively) in coordination with the position of a dual‐face scanning acoustic reflector. To achieve this, the treatment delivery software can divide the applicator's treatment window in up to 64 sectors (minimum size of ), and control the power to each sector independently by adjusting the power output levels from the channels of a 16‐channel radio‐frequency generator. The software coordinates the generator outputs with the position of the reflector as it scans back and forth between the arrays. Individual sector control and dual frequency operation allows the SURLAS to adjust power deposition in three dimensions to superficial targets coupled to its treatment window. The treatment delivery software also monitors and logs several parameters such as temperatures acquired using a 16‐channel thermocouple thermometry unit. Safety (in particular to patients) was the paramount concern and design criterion. Failure mode and effects analysis (FMEA) was applied to the applicator as well as to the entire therapy system in order to identify safety issues and rank their relative importance. This analysis led to the implementation of several safety mechanisms and a software structure where each device communicates with the controlling PC independently of the others. In case of a malfunction in any part of the system or a violation of a user‐defined safety criterion based on temperature readings, the software terminates treatment immediately and the user is notified. The software development process consisting of problem analysis, design, implementation, and testing is presented in this paper. Once the software was finished and integrated with the hardware, the therapy system was extensively tested. Results demonstrated that the software operates the SURLAS as intended with minimum risk to future patients.
Evaluation of precalibrated implantable MOSFET radiation dosimeters for megavoltage photon beams
Authors: Briere Tina Marie , Beddar A. Sam , Gillin Michael T.
Published: December 2005

We have studied the response of factory calibrated implantable MOSFET detectors to absorbed doses from 100 to 400 cGy. The average measured dose is quite close to the true delivered dose, with the standard deviation falling between 1.4 and 3.6%. The measured dose tends to be slightly underestimated for smaller doses, while it tends to be slightly overestimated for larger doses. Thus, although the calibration of the detector is most accurate for doses close to the calibration dose of 200 cGy, it may be used over the range of commonly used doses in fractionated radiotherapy.
Concomitant GRID boost for Gamma Knife radiosurgery
Authors: Ma Lijun , Kwok Young , Chin Lawrence S. , Simard J. Marc , Regine William F.
Published: December 2005

We developed an integrated GRID boost technique for Gamma Knife radiosurgery. The technique generates an array of high dose spots within the target volume via a grid of 4‐mm shots. These high dose areas were placed over a conventional Gamma Knife plan where a peripheral dose covers the full target volume. The beam weights of the 4‐mm shots were optimized iteratively to maximize the integral dose inside the target volume. To investigate the target volume coverage and the dose to the adjacent normal brain tissue for the technique, we compared the GRID boosted treatment plans with conventional Gamma Knife treatment plans using physical and biological indices such as dose‐volume histogram (DVH), DVH‐derived indices, equivalent uniform dose (EUD), tumor control probabilities (TCP), and normal tissue complication probabilities (NTCP). We found significant increase in the target volume indices such as mean dose (5%–34%; average 14%), TCP (4%–45%; average 21%), and EUD (2%–22%; average 11%) for the GRID boost technique. No significant change in the peripheral dose coverage for the target volume was found per RTOG protocol. In addition, the EUD and the NTCP for the normal brain adjacent to the target (i.e., the near region) were decreased for the GRID boost technique. In conclusion, we demonstrated a new technique for Gamma Knife radiosurgery that can escalate the dose to the target while sparing the adjacent normal brain tissue.
Exact fan‐beam and ‐acquisition cone‐beam SPECT algorithms with uniform attenuation correction
Authors: Tang Qiulin , Zeng Gengsheng L. , Wu Jiansheng , Gullberg Grant T.
Published: December 2005

This paper presents analytical fan‐beam and cone‐beam reconstruction algorithms that compensate for uniform attenuation in single photon emission computed tomography. First, a fan‐beam algorithm is developed by obtaining a relationship between the two‐dimensional (2D) Fourier transform of parallel‐beam projections and fan‐beam projections. Using this relationship, 2D Fourier transforms of equivalent parallel‐beam projection data are obtained from the fan‐beam projection data. Then a quasioptimal analytical reconstruction algorithm for uniformly attenuated Radon data, developed by Metz and Pan, is used to reconstruct the image. A cone‐beam algorithm is developed by extending the fan‐beam algorithm to solid angle geometry. The cone‐beam algorithm is also an exact algorithm.
3D MRI‐based tumor delineation of ocular melanoma and its comparison with conventional techniques
Authors: Daftari Inder k , Aghaian Elsa , O'Brien Joan M. , Dillon William , Phillips Theodore L.
Published: December 2005

The aim of this study is to (1) compare the delineation of the tumor volume for ocular melanoma on high‐resolution three‐dimensional (3D) T2‐weighted fast spin echo magnetic resonance imaging (MRI) images with conventional techniques of A‐ and B‐scan ultrasound, transcleral illumination, and placement of tantalum markers around tumor base and (2) to evaluate whether the surgically placed marker ring tumor delineation can be replaced by 3D MRI based tumor delineation. High‐resolution 3D T2‐weighted fast spin echo (3D FSE) MRI scans were obtained for 60 consecutive ocular melanoma patients using a 1.5 T MRI (GE Medical Systems, Milwaukee, WI), in a standard head coil. These patients were subsequently treated with proton beam therapy at the UC Davis Cyclotron, Davis, CA. The tumor was delineated by placement of tantalum rings (radio‐opaque markers) around the tumor periphery as defined by pupillary transillumination during surgery. A point light source, placed against the sclera, was also used to confirm ring agreement with indirect ophthalmoscopy. When necessary, intraoperative ultrasound was also performed. The patients were planned using EYEPLAN software and the tumor volumes were obtained. For analysis, the tumors were divided into four categories based on tumor height and basal diameter. In order to assess the impact of high‐resolution 3D T2 FSE MRI, the tumor volumes were outlined on the MRI scans by two independent observers and the tumor volumes calculated for each patient. Six (10%) of 60 patients had tumors, which were not visible on 3D MRI images. These six patients had tumors with tumor heights . A small intraobserver variation with a mean of was seen in tumor volumes delineated by 3D T2 FSE MR images. The ratio of tumor volumes measured on MRI to EYEPLAN for the largest to the smallest tumor volumes varied between 0.993 and 1.02 for 54 patients. The tumor volumes measured directly on 3D T2 FSE MRI ranged from 4.03 to with a mean of . The tumor shapes obtained from 3D T2 FSE MR images were comparable to the tumor shapes obtained using EYEPLAN software. The demonstration of intraocular tumor volumes with the high‐resolution 3D fast spin echo T2 weighted MRI is excellent and provides additional information on tumor shape. We found a high degree of accuracy for tumor volumes with direct MRI volumetric measurements in uveal melanoma patients. In some patients with extra large tumors, the tumor base and shape was modified, because of the additional information obtained from 3D T2 FSE MR images.
Final Aperture Superposition Technique applied to fast calculation of electron output factors and depth dose curves
Authors: Faddegon B. A. , Villarreal‐Barajas J. E.
Published: December 2005

The Final Aperture Superposition Technique (FAST) is described and applied to accurate, near instantaneous calculation of the relative output factor (ROF) and central axis percentage depth dose curve (PDD) for clinical electron beams used in radiotherapy. FAST is based on precalculation of dose at select points for the two extreme situations of a fully open final aperture and a final aperture with no opening (fully shielded). This technique is different than conventional superposition of dose deposition kernels: The precalculated dose is differential in position of the electron or photon at the downstream surface of the insert. The calculation for a particular aperture (x‐ray jaws or MLC, insert in electron applicator) is done with superposition of the precalculated dose data, using the open field data over the open part of the aperture and the fully shielded data over the remainder. The calculation takes explicit account of all interactions in the shielded region of the aperture except the collimator effect: Particles that pass from the open part into the shielded part, or visa versa. For the clinical demonstration, FAST was compared to full Monte Carlo simulation of , and inserts. Dose was calculated to 0.5% precision in voxels, spaced at 0.2 cm depth intervals along the central axis, using detailed Monte Carlo simulation of the treatment head of a commercial linear accelerator for six different electron beams with energies of 6–21 MeV. Each simulation took several hours on a personal computer with a 1.7 Mhz processor. The calculation for the individual inserts, done with superposition, was completed in under a second on the same PC. Since simulations for the pre calculation are only performed once, higher precision and resolution can be obtained without increasing the calculation time for individual inserts. Fully shielded contributions were largest for small fields and high beam energy, at the surface, reaching a maximum of 5.6% at 21 MeV. Contributions from the collimator effect were largest for the large field size, high beam energy, and shallow depths, reaching a maximum of 4.7% at 21 MeV. Both shielding contributions and the collimator effect need to be taken into account to achieve an accuracy of 2%. FAST takes explicit account of the shielding contributions. With the collimator effect set to that of the largest field in the FAST calculation, the difference in dose on the central axis (product of ROF and PDD) between FAST and full simulation was generally under 2%. The maximum difference of 2.5% exceeded the statistical precision of the calculation by four standard deviations. This occurred at 18 MeV for the field. The differences are due to the method used to account for the collimator effect.