bluuebunny/crossref_metadata_2025 huggingface dataset:
Modal Properties of Planetary Gears With an Elastic Continuum Ring Gear
Authors: Wu Xionghua , Parker Robert G.
Published: May 2008

The distinctive modal properties of equally spaced planetary gears with elastic ring gears are studied through perturbation and a candidate mode method. All eigenfunctions fall into one of four mode types whose structured properties are derived analytically. Two perturbations are used to obtain closed-form expressions of all the eigenfunctions. In the discrete planetary perturbation, the unperturbed system is a discrete planetary gear with a rigid ring. The stiffness of the ring is perturbed from infinite to a finite number. In the elastic ring perturbation, the unperturbed system is an elastic ring supported by the ring-planet mesh springs; the sun, planet and carrier motions are treated as small perturbations. A subsequent candidate mode method analysis proves the perturbation results and removes any reliance on perturbation parameters being small. All vibration modes are classified into rotational, translational, planet, and purely ring modes. The well defined properties of each type of mode are analytically determined. All modal properties are verified numerically.
A Micromechanics-Based Elastoplastic Model for Amorphous Composites With Nanoparticle Interactions
Authors: Liu H. T. , Sun L. Z.
Published: May 2008

A constitutive model is proposed to investigate the strengthening mechanism and the relationship between nanostructures and effective mechanical properties of the aluminum-based amorphous nanocomposites. A continuum micromechanics-based, three-phase composite model comprises of Al particles, rare-earth enriched interlayers, and the amorphous aluminum matrix. The local stress field and deformation are formulated based on the concept of eigenstrain and equivalent inclusion method with consideration of both the particle-interlayer-matrix interaction and the particle-particle interaction. An ensemble-volume averaging technique is conducted to obtain the overall elastoplastic constitutive behavior for amorphous nanocomposites with randomly distributed spherical nanoparticles. Explicit expressions of the effective elastic stiffness and yield function in terms of the constituent properties and nanostructures are obtained. The effective elastoplastic stress-strain curves for uniaxial loading and the initial yield surfaces for axisymmetric loading are calculated. Simulations are conducted to investigate the effects of the particle size and pairwise particle interaction on the effective mechanical properties.
Dynamic Response of Rapidly Heated Cylindrical Rods: Longitudinal and Flexural Behavior
Authors: Bertarelli Alessandro , Dallocchio Alessandro , Kurtyka Tadeusz
Published: May 2008

A very fast temperature increase, produced by a nonuniform heat generation, induces in a simply supported, isotropic, cylindrical rod both longitudinal and flexural vibrations. This paper presents an analytical method to study these vibrations and determine the stresses they provoke. The proposed procedure relies on three main steps: an exact solution for the temperature field is first obtained, by means of Fourier–Bessel expansions; quasistatic thermal stresses are then computed as a function of the calculated temperature distribution, making use of the thermoelastic displacement potential and of the solution to the equivalent isothermal two-dimensional stress problem; finally, longitudinal and flexural vibrations excited by an equivalent thermal force and thermal bending moment are determined using the mode-summation method. The influence of thermal shock duration on the maximum value of the longitudinal dynamic stress and of the ratio between the characteristic thermal time and structural response time on the dynamic bending deflection is analyzed and discussed. Finally, a comparison between the analytical model and experimental measurements is presented. The analytical model described in this paper allows the complete evaluation, within the linear elastic domain, of quasistatic and dynamic thermal stresses induced in an isotropic cylindrical rod by rapid internal heating.
On Colles’ Fracture: An Experimental Study Involving Structural and Material Testing
Authors: Gdela K. , Pietruszczak S. , Lade P. V. , Tsopelas P.
Published: May 2008

A two-stage experimental program was conducted, which was aimed at examining the process of initiation/propagation of fracture in human radii under the conditions simulating a fall onto an outstretched hand. It involved a number of destructive tests on dried cadaver bones. The bones were first subjected to DXA as well as spiral CT measurements to establish the density properties and the details of geometry. Subsequently, the specimens were tested under controlled boundary conditions, to induce Colles’ type of fracture. Following these tests, samples of cortical bone tissue were extracted at different orientations with respect to the direction of osteons and tested in axial tension. The results of material tests were used to verify the performance of an anisotropic fracture criterion for the cortical tissue. It has been demonstrated that the proposed criterion can reproduce the basic trends in the directional dependence of the tensile strength characteristics. For the structural tests, a correlation was established between the geometric characteristics of the cortex, the strength properties and the fracture load for individual radii that were tested. It was shown that the morphological traits and/or the strength properties alone are not adequate predictors of the fracture load of intact radii. A rational assessment of the fracture load requires a mechanical analysis that incorporates the key elements of the experimental program outlined here, i.e., the information on bone geometry, material properties of the bone tissue, and the static/kinematic boundary conditions. A preliminary example of a finite element analysis, for one of the radii bones tested, has been provided.
Spectral and Perturbation Analysis of First-Order Beams With Notch Damage
Authors: Apetre N. , Ruzzene M. , Hanagud S. , Gopalakrishnan S.
Published: May 2008

The influence of damage on waves propagating in beam structures is investigated through a numerical model formulated by combining spectral finite elements and perturbation techniques. The resulting numerical tool allows for an efficient computation of the wave propagation response and the analysis of the effects of localized damages of various extents and locations. The dynamic behavior of damaged beams is described through a first-order model, which couples bending and axial behavior, thus allowing the prediction of mode conversion phenomena. Damage is modeled as a small, localized reduction of the beam thickness which, allows for an application of perturbation theory. Numerical examples in the time and frequency domains are presented to illustrate the model capabilities.
Flow Through a Lens-Shaped Duct
Authors: Wang C. Y.
Published: May 2008

The flow through a symmetric, lens-shaped duct is solved by accurate Ritz and perturbation methods. The flow rate (resistance) is found for various thickness ratios. The flow rate is much better than friction factor–Reynolds number product as an index for duct flows, especially for the lens duct studied in this paper. The results are also important for the torsion of lens-shaped bars.
Nonisothermal
Transient Flow in Natural Gas Pipeline
Authors: Abbaspour M. , Chapman K.
S.
Published: May 2008

The fully implicit finite-difference method is used to solve the continuity, momentum, and energy equations for flow within a gas pipeline. This methodology (1) incorporates the convective inertia term in the conservation of momentum equation, (2) treats the compressibility factor as a function of temperature and pressure, and (3) considers the friction factor as a function of the Reynolds number and pipe roughness. The fully implicit method representation of the equations offers the advantage of guaranteed stability for a large time step, which is very useful for gas pipeline industry. The results show that the effect of treating the gas in a nonisothermal manner is extremely necessary for pipeline flow calculation accuracies, especially for rapid transient process. It also indicates that the convective inertia term plays an important role in the gas flow analysis and cannot be neglected from the calculation.
Homogenization Based 3D Continuum Damage Mechanics Model for Composites Undergoing Microstructural Debonding
Authors: Jain Jayesh R. , Ghosh Somnath
Published: May 2008

This paper develops a microscopic homogenization based continuum damage mechanics (HCDM) model framework for fiber reinforced composites undergoing interfacial debonding. It is an advancement over the 2D HCDM model developed by Raghavan and Ghosh (2005, “A Continuum Damage Mechanics Model for Unidirectional Composites Undergoing Interfacial Debonding,” Mech. Mater., 37(9), pp. 955–979), which does not yield accurate results for nonproportional loading histories. The present paper overcomes this shortcoming through the introduction of a principal damage coordinate system (PDCS) in the HCDM representation, which evolves with loading history. The material behavior is represented as a continuum constitutive law involving a fourth order orthotropic tensor with stiffness characterized as a macroscopic internal variable. The current work also extends the model of Raghavan and Ghosh to incorporate damage in 3D composites through functional forms of the fourth order damage tensor in terms of macroscopic strain components. The model is calibrated by homogenizing the micromechanical response of the representative volume element (RVE) for a few strain histories. This parametric representation can significantly enhance the computational efficiency of the model by avoiding the cumbersome strain space interpolations. The proposed model is validated by comparing the CDM results with homogenized micromechanical response of single and multiple fiber RVEs subjected to arbitrary loading history.
Analytical Solution of the Perturbed Magnetic Fields of Plates Under Tensile Stress
Authors: Qin Fei , Yan Dongmei
Published: May 2008

Development of magnetism based nondestructive testing technology and the Microelectronic mechanical system require accurate computation of perturbed magnetic fields generated by mechanical stress. In this paper, based on the linearized magnetoelastic theory, the governing equations and continuity conditions to determine the perturbed magnetic fields were formulated for the case of weak external magnetic fields such as the earth’s magnetic field. Under those weak magnetic fields, the effect of the magnetic fields on mechanical deformation was neglected. As a result, the interaction between the deformation and the magnetic field was simplified. The effect of deformation on the perturbed magnetic field was taken into account by introducing the displacement gradient into the boundary conditions that the perturbed field should satisfy. As examples, analytic solutions of the perturbed magnetic field of infinite plates with and without a round hole, which are subjected to tensile stresses and weak external magnetic fields, were obtained by the approach presented. The results show that the perturbed magnetic fields induced by stress are three orders less in magnitude of intensity than that of magnetic fields without stress, and some prominent local features such as that has more peaks and decays more rapidly in the radial direction than the case of stress free that are predicted by the solutions.
A Comparison of Coulomb Friction and Friction Stress Models Based on Multidimensional Nanocontact Experiments
Authors: Gao Y. F. , Xu H. T. , Oliver W. C. , Pharr G. M.
Published: May 2008

The accuracy of the Oliver–Pharr approach for nanoindentation experiments critically depends on the interfacial friction condition. Although Coulomb friction is often used in finite element simulations for the correction, the friction stress model may give a more appropriate physical scenario. The measurement of the tangential contact stiffness by a recently developed multidimensional nanocontact system provides a direct verification of these two friction models. Both friction models will predict the tangential stiffness reduction as the consequence of interface microslip, but quantitative comparison to the experiments supports the friction stress model.
Implementation of a computationally efficient least‐squares algorithm for highly under‐determined three‐dimensional diffuse optical tomography problems
Authors: Yalavarthy Phaneendra K. , Lynch Daniel R. , Pogue Brian W. , Dehghani Hamid , Paulsen Keith D.
Published: May 2008

Three‐dimensional (3D) diffuse optical tomography is known to be a nonlinear, ill‐posed and sometimes under‐determined problem, where regularization is added to the minimization to allow convergence to a unique solution. In this work, a generalized least‐squares (GLS) minimization method was implemented, which employs weight matrices for both data‐model misfit and optical properties to include their variances and covariances, using a computationally efficient scheme. This allows inversion of a matrix that is of a dimension dictated by the number of measurements, instead of by the number of imaging parameters. This increases the computation speed up to four times per iteration in most of the under‐determined 3D imaging problems. An analytic derivation, using the Sherman–Morrison–Woodbury identity, is shown for this efficient alternative form and it is proven to be equivalent, not only analytically, but also numerically. Equivalent alternative forms for other minimization methods, like Levenberg–Marquardt (LM) and Tikhonov, are also derived. Three‐dimensional reconstruction results indicate that the poor recovery of quantitatively accurate values in 3D optical images can also be a characteristic of the reconstruction algorithm, along with the target size. Interestingly, usage of GLS reconstruction methods reduces error in the periphery of the image, as expected, and improves by 20% the ability to quantify local interior regions in terms of the recovered optical contrast, as compared to LM methods. Characterization of detector photomultiplier tubes noise has enabled the use of the GLS method for reconstructing experimental data and showed a promise for better quantification of target in 3D optical imaging. Use of these new alternative forms becomes effective when the ratio of the number of imaging property parameters exceeds the number of measurements by a factor greater than 2.
6 MV dosimetric characterization of the 160 MLC™, the new Siemens multileaf collimator
Authors: Tacke Martin B. , Nill Simeon , Häring Peter , Oelfke Uwe
Published: May 2008

New technical developments constantly aim at improving the outcome of radiation therapy. With the use of a computer‐controlled multileaf collimator (MLC), the quality of the treatment and the efficiency in patient throughput is significantly increased. New MLC designs aim to further enhance the advantages. In this article, we present the first detailed experimental investigation of the new 160 MLCTM, Siemens Medical Solutions. The assessment included the experimental investigation of typical MLC characteristics such as leakage, tongue‐and‐groove effect, penumbra, leaf speed, and leaf positioning accuracy with a 6 MV treatment beam. The leakage is remarkably low with an average of due to a new design principle of slightly tilted leaves instead of the common tongue‐and‐groove design. But due to the tilt, the triangular tongue‐and‐groove effect occurs. Its magnitude of approximately is similar to the dose defect measured for MLCs with the common tongue‐and‐groove design. The average longitudinal penumbra measured at depth with standard fields is for the central range and increases to for the entire field range of . The increase is partly due to the single‐focusing design and the large distance between the MLC and the isocenter enabling a large patient clearance. Regarding the leaf speed, different velocity tests were performed. The positions of the moving leaves were continuously recorded with the kilovoltage‐imaging panel. The maximum leaf velocities measured were . In addition, several typical intensity‐modulated radiation therapy treatments were performed and the delivery times compared to the Siemens OPTIFOCUS™ MLC. An average decrease of in delivery time was observed. The experimental results presented in this article indicate that the dosimetric characteristics of the 160 MLC™ are capable of improving the quality of dose delivery with respect to precision and dose conformity.
A prototype instrument for single pinhole small animal adaptive SPECT imaging
Authors: Freed Melanie , Kupinski Matthew A. , Furenlid Lars R. , Wilson Donald W. , Barrett Harrison H.
Published: May 2008

The authors have designed and constructed a small‐animal adaptive SPECT imaging system as a prototype for quantifying the potential benefit of adaptive SPECT imaging over the traditional fixed geometry approach. The optical design of the system is based on filling the detector with the region of interest for each viewing angle, maximizing the sensitivity, and optimizing the resolution in the projection images. Additional feedback rules for determining the optimal geometry of the system can be easily added to the existing control software. Preliminary data have been taken of a phantom with a small, hot, offset lesion in a flat background in both adaptive and fixed geometry modes. Comparison of the predicted system behavior with the actual system behavior is presented, along with recommendations for system improvements.
Fast reconstruction of digital tomosynthesis using on‐board images
Authors: Yan Hui , Godfrey Devon J. , Yin Fang‐Fang
Published: May 2008

Digital tomosynthesis (DTS) is a method to reconstruct pseudo three‐dimensional (3D) volume images from two‐dimensional x‐ray projections acquired over limited scan angles. Compared with cone‐beam computed tomography, which is frequently used for 3D image guided radiation therapy, DTS requires less imaging time and dose. Successful implementation of DTS for fast target localization requires the reconstruction process to be accomplished within tight clinical time constraints (usually within 2 min). To achieve this goal, substantial improvement of reconstruction efficiency is necessary. In this study, a reconstruction process based upon the algorithm proposed by Feldkamp, Davis, and Kress was implemented on graphics hardware for the purpose of acceleration. The performance of the novel reconstruction implementation was tested for phantom and real patient cases. The efficiency of DTS reconstruction was improved by a factor of 13 on average, without compromising image quality. With acceleration of the reconstruction algorithm, the whole DTS generation process including data preprocessing, reconstruction, and DICOM conversion is accomplished within 1.5 min, which ultimately meets clinical requirement for on‐line target localization.
Management of the interplay effect when using dynamic MLC sequences to treat moving targets
Authors: Court Laurence E. , Wagar Matthew , Ionascu Dan , Berbeco Ross , Chin Lee
Published: May 2008

Interplay between organ motion and leaf motion has been shown to generally have a small dosimetric impact for most clinical intensity‐modulated radiation therapy treatments. However, it has also been shown that for some MLC sequences there can be large daily variations in the delivered dose, depending on details of patient motion or the number of fractions. This study investigates guidelines for dynamic MLC sequences that will keep daily dose variations due to the interplay between organ motion and leaf motion within 10%. Dose distributions for a range of MLC separations and displacements between adjacent MLCs were exported from ECLIPSE to purpose‐written software, which simulated the dose distribution delivered to a moving target. Target motion parallel and perpendicular to the MLC motion was investigated for a range of amplitudes , periods , and MLC speeds with target motions modeled as . Results were confirmed experimentally by measuring the dose delivered to an ion chamber array in a moving phantom for different MLC sequences. The simulation results were used to identify MLC sequences that kept dose variations within 10% compared to the dose delivered with no motion. The maximum allowable MLC speed, when target motion is parallel to the MLC motion, was found to be a simple function of target period and MLC separation. When the target motion is perpendicular to MLC motion, the maximum allowable MLC speed can be described as a function of MLC separation and the displacement of adjacent MLCs. These guidelines were successfully applied to two‐dimensional motion, and a simple program was written to import MLC sequence files and evaluate whether the maximum daily dose discrepancy caused by the interplay effect will be larger than 10%. This software was experimentally evaluated, and found to conservatively predict whether a given MLC sequence could give large daily dose discrepancies.
Quantifying the interplay effect in prostate IMRT delivery using a convolution‐based method
Authors: Li Haisen S. , Chetty Indrin J. , Solberg Timothy D.
Published: May 2008

The authors present a segment‐based convolution method to account for the interplay effect between intrafraction organ motion and the multileaf collimator position for each particular segment in intensity modulated radiation therapy (IMRT) delivered in a step‐and‐shoot manner. In this method, the static dose distribution attributed to each segment is convolved with the probability density function (PDF) of motion during delivery of the segment, whereas in the conventional convolution method (“average‐based convolution”), the static dose distribution is convolved with the PDF averaged over an entire fraction, an entire treatment course, or even an entire patient population. In the case of IMRT delivered in a step‐and‐shoot manner, the average‐based convolution method assumes that in each segment the target volume experiences the same motion pattern (PDF) as that of population. In the segment‐based convolution method, the dose during each segment is calculated by convolving the static dose with the motion PDF specific to that segment, allowing both intrafraction motion and the interplay effect to be accounted for in the dose calculation. Intrafraction prostate motion data from a population of 35 patients tracked using the Calypso system (Calypso Medical Technologies, Inc., Seattle, WA) was used to generate motion PDFs. These were then convolved with dose distributions from clinical prostate IMRT plans. For a single segment with a small number of monitor units, the interplay effect introduced errors of up to in the mean CTV dose compared against the planned dose evaluated by using the PDF of the entire fraction. In contrast, the interplay effect reduced the minimum CTV dose by , and the CTV generalized equivalent uniform dose by , in single fraction plans. For entire treatment courses delivered in either a hypofractionated (five fractions) or conventional ( fractions) regimen, the discrepancy in total dose due to interplay effect was negligible.
Calculation of the replacement correction factors for ion chambers in megavoltage beams by Monte Carlo simulation
Authors: Wang L. L. W. , Rogers D. W. O.
Published: May 2008

This article describes four methods of calculating the replacement correction factor, (or the product in the IAEA's notation), for a plane‐parallel chamber in both electron and photon beams, and for a Farmer chamber in photon beams, by using the EGSnrc Monte Carlo code. The accuracy of underlying assumptions and relative merits of each technique are assessed. With careful selection of parameters it appears that all four methods give reasonable answers although the direct methods are more intellectually satisfying and more accurate in some cases. The direct methods are shown to have an accuracy of when appropriate calculation parameters are selected. The depth dependence of for the NACP02 plane‐parallel chamber has been calculated in both 6 and 18 MeV electron beams. At the reference depth is 0.9964 for the 6 MeV beam and 1.0005 for the 18 MeV beam for this well‐guarded chamber; at the depth of maximum dose for the 18 MeV beam, is 1.0010. is also calculated for the NACP02 chamber and a Farmer chamber (diameter 6 mm) at a depth of 5 cm in a photon beam, giving values of 1.0063 and 0.9964, respectively. For the Farmer chamber, is about half a percent higher than the value (0.992) recommended by the AAPM dosimetry protocol. It is found that the dosimetry protocols may have adopted an incorrect value of for cylindrical chambers in photon beams. The nonunity values of for plane‐parallel chambers in lower energy electron beams imply a variety of values used in dosimetry protocols must be reassessed.
Measurement of the neutron leakage from a dedicated intraoperative radiation therapy electron linear accelerator and a conventional linear accelerator for 9, 12, 15(16), and 18(20) MeV electron energies
Authors: Jaradat Adnan K. , Biggs Peter J.
Published: May 2008

The issue of neutron leakage has recently been raised in connection with dedicated electron‐only linear accelerators used for intraoperative radiation therapy (IORT). In particular, concern has been expressed about the degree of neutron production at energies of 10 MeV and higher due to the need for additional, perhaps permanent, shielding in the room in which the device is operated. In particular, three mobile linear accelerators available commercially offer electron energies at or above the neutron threshold, one at 9 MeV, one at 10 MeV, and the third at 12 MeV. To investigate this problem, neutron leakage has been measured around the head of two types of electron accelerators at a distance of 1 m from the target at azimuthal angles of , , , , and . The first is a dedicated electron‐only (nonmobile) machine with electron energies of 6 (not used here), 9, 12, 15, and 18 MeV and the second a conventional machine with electron energies of 6 (also not used here), 9, 12, 16, and 20 MeV. Measurements were made using neutron bubble detectors and track‐etch detectors. For electron beams from a conventional accelerator, the neutron leakage in the forward direction in Sv/Gy is at 12 MeV, at 16 MeV, and at 20 MeV, assuming a quality factor (RBE) of 10. For azimuthal angles , the leakage is almost angle independent [ at 12 MeV; at 16 MeV, and at 20 MeV]. For the electron‐only machine, the neutron leakage was lower than for the conventional linac, but also independent of azimuthal angle for angles : {[: at 12 MeV; at 15 MeV; at 18 MeV]; [other angles: at 12 MeV; at 15 MeV; at 18 MeV]}. Using the upper limit of at 12 MeV for the IORT machine for azimuthal angles and assuming a workload of 200 Gy/wk and an inverse square factor of 10, the neutron dose equivalent is calculated to be 0.012 mSv/wk. For the primary beam at 12 MeV , the higher dose would be compensated by the attenuation of a primary beam stopper in a mobile linear accelerator. These neutron radiation levels are below regulatory values (National Council on Radiation Protection and Measurements, “Limitation of exposure to ionizing radiation,” NCRP Report No. 116, NCRP Bethesda, MD, 1993).
Photon spectral characteristics of dissimilar linear accelerators
Authors: Hinson William H. , Kearns William T. , deGuzman Allan F. , Bourland J. Daniel
Published: May 2008

This work measures and compares the energy spectra of four dosimetrically matched beams, generated from four physically different linear accelerators. The goal of this work is twofold. First, this study determines whether the spectra of dosimetrically matched beams are measurably different. This study also demonstrates that the spectra of clinical photon beams can be measured as a part of the beam data collection process for input to a three‐dimensional (3D) treatment planning system. The spectra of beams that are dosimetrically matched for clinical use were studied to determine if the beam spectra are similarly matched. Each of the four accelerators examined had a standing waveguide, but with different physical designs. The four accelerators were two Varian 2100C/Ds (one waveguide and one waveguide), one Varian with a vertically mounted waveguide and no bending magnet, and one Siemens MD 6740 with a waveguide. All four accelerators had percent depth dose curves for the beam that were matched within 1.3%. Beam spectra were determined from narrow beam transmission measurements through successive thicknesses of pure aluminum along the central axis of the accelerator, made with a graphite Farmer ion chamber with a Lucite buildup cap. An iterative nonlinear fit using a Marquardt algorithm was used to find each spectrum. Reconstructed spectra show that all four beams have similar energy distributions with only subtle differences, despite the differences in accelerator design. The measured spectra of different beams are similar regardless of accelerator design. The measured spectra show excellent agreement with those found by the auto‐modeling algorithm in a commercial 3D treatment planning system that uses a convolution dose calculation algorithm. Thus, beam spectra can be acquired in a clinical setting at the time of commissioning as a part of the routine beam data collection.
Transurethral ultrasound applicators with dynamic multi‐sector control for prostate thermal therapy: In vivo evaluation under MR guidance
Authors: Kinsey Adam M. , Diederich Chris J. , Rieke Viola , Nau William H. , Pauly Kim Butts , Bouley Donna , Sommer Graham
Published: May 2008

The purpose of this study was to explore the feasibility and performance of a multi‐sectored tubular array transurethral ultrasound applicator for prostate thermal therapy, with potential to provide dynamic angular and length control of heating under MR guidance without mechanical movement of the applicator. Test configurations were fabricated, incorporating a linear array of two multi‐sectored tubular transducers (7.8–8.4 MHz, 3 mm OD, 6 mm length), with three independent active sectors per tube. A flexible delivery catheter facilitated water cooling within an expandable urethral balloon . An integrated positioning hub allows for rotating and translating the transducer assembly within the urethral balloon for final targeting prior to therapy delivery. Rotational beam plots indicate acoustic output patterns from each transducer sector, negligible coupling between sectors, and acoustic efficiencies between and . Experiments were performed within in vivo canine prostate , with real‐time MR temperature monitoring in either the axial or coronal planes to facilitate control of the heating profiles and provide thermal dosimetry for performance assessment. Gross inspection of serial sections of treated prostate, exposed to TTC (triphenyl tetrazolium chloride) tissue viability stain, allowed for direct assessment of the extent of thermal coagulation. These devices created large contiguous thermal lesions (defined by maximum temperature, thermal dose contours, and TTC tissue sections) that extended radially from the applicator toward the border of the prostate during a short power application ( per active sector, 8–15 min), with or sector coagulation demonstrated depending upon the activation scheme. Analysis of transient temperature profiles indicated progression of lethal temperature and thermal dose contours initially centered on each sector that coalesced within to produce uniform and contiguous zones of thermal destruction between sectors, with smooth outer boundaries and continued radial propagation in time. The dimension of the coagulation zone along the applicator was well‐defined by positioning and active array length. Although not as precise as rotating planar and curvilinear devices currently under development for MR‐guided procedures, advantages of these multi‐sectored transurethral applicators include a flexible delivery catheter and that mechanical manipulation of the device using rotational motors is not required during therapy. This multi‐sectored tubular array transurethral ultrasound technology has demonstrated potential for relatively fast and reasonably conformal targeting of prostate volumes suitable for the minimally invasive treatment of BPH and cancer under MR guidance, with further development warranted.
Imaging performance of an amorphous selenium digital mammography detector in a breast tomosynthesis system
Authors: Zhao Bo , Zhao Wei
Published: May 2008

In breast tomosynthesis a rapid sequence of images is acquired when the x‐ray tube sweeps through different angular views with respect to the breast. Since the total dose to the breast is kept the same as that in regular mammography, the exposure used for each image of tomosynthesis is . The low dose and high frame rate pose a tremendous challenge to the imaging performance of digital mammography detectors. The purpose of the present work is to investigate the detector performance in different operational modes designed for tomosynthesis acquisition, e.g., binning or full resolution readout, the range of view angles, and the number of views . A prototype breast tomosynthesis system with a nominal angular range of was used in our investigation. The system was equipped with an amorphous selenium (a‐Se) full field digital mammography detector with pixel size of . The detector can be read out in full resolution or binning (binning in the tube travel direction). The focal spot blur due to continuous tube travel was measured for different acquisition geometries, and it was found that pixel binning, instead of focal spot blur, dominates the detector modulation transfer function (MTF). The noise power spectrum (NPS) and detective quantum efficiency (DQE) of the detector were measured with the exposure range of 0.4–6 mR, which is relevant to the low dose used in tomosynthesis. It was found that DQE at 0.4 mR is only 20% less than that at highest exposure for both detector readout modes. The detector temporal performance was categorized as lag and ghosting, both of which were measured as a function of x‐ray exposure. The first frame lags were 8% and 4%, respectively, for binning and full resolution mode. Ghosting is negligible and independent of the frame rate. The results showed that the detector performance is x‐ray quantum noise limited at the low exposures used in each view of tomosynthesis, and the temporal performance at high frame rate (up to 2 frames per second) is adequate for tomosynthesis.
Evaluation of guidewire path reproducibility
Authors: Schafer Sebastian , Hoffmann Kenneth R. , Noël Peter B. , Ionita Ciprian N. , Dmochowski Jacek
Published: May 2008

The number of minimally invasive vascular interventions is increasing. In these interventions, a variety of devices are directed to and placed at the site of intervention. The device used in almost all of these interventions is the guidewire, acting as a monorail for all devices which are delivered to the intervention site. However, even with the guidewire in place, clinicians still experience difficulties during the interventions. As a first step toward understanding these difficulties and facilitating guidewire and device guidance, we have investigated the reproducibility of the final paths of the guidewire in vessel phantom models on different factors: user, materials and geometry. Three vessel phantoms (vessel diameters ) were constructed having tortuousity similar to the internal carotid artery from silicon tubing and encased in Sylgard elastomer. Several trained users repeatedly passed two guidewires of different flexibility through the phantoms under pulsatile flow conditions. After the guidewire had been placed, rotational c‐arm image sequences were acquired ( II mode, pixel size), and the phantom and guidewire were reconstructed (, voxel size). The reconstructed volumes were aligned. The centerlines of the guidewire and the phantom vessel were then determined using region‐growing techniques. Guidewire paths appear similar across users but not across materials. The average root mean square difference of the repeated placement was (plastic‐coated guidewire), (steel guidewire) and (steel versus plastic‐coated). For a given guidewire, these results indicate that the guidewire path is relatively reproducible in shape and position.
Spill‐to‐spill and daily proton energy consistency with a new accelerator control systema)
Authors: Moyers M. F. , Ghebremedhin A.
Published: May 2008

The Loma Linda University proton accelerator has had several upgrades installed including synchrotron dipole power supplies and a system for monitoring the beam energy. The consistency of the energy from spill‐to‐spill has been tested by measuring the depth ionization at the distal edge as a function of time. These measurements were made with a minimally equipped beamline to reduce interference from confounding factors. The consistency of the energy over several months was measured in a treatment room beamline using an ionization chamber based daily quality assurance device. The results showed that the energy of protons delivered from the accelerator (in terms of water equivalent range) was consistent from spill‐to‐spill to better than at 70, 155, and 250 MeV and that the energy check performed each day in the treatment room over a several month period was within at 149 MeV. These results are within the tolerances required for the energy stacking technique.
Automated selection of trabecular bone regions in knee radiographs
Authors: Podsiadlo P. , Wolski M. , Stachowiak G. W.
Published: May 2008

Osteoarthritic (OA) changes in knee joints can be assessed by analyzing the structure of trabecular bone (TB) in the tibia. This analysis is performed on TB regions selected manually by a human operator on x‐ray images. Manual selection is time‐consuming, tedious, and expensive. Even if a radiologist expert or highly trained person is available to select regions, high inter‐ and intraobserver variabilities are still possible. A fully automated image segmentation method was, therefore, developed to select the bone regions for numerical analyses of changes in bone structures. The newly developed method consists of image preprocessing, delineation of cortical bone plates (active shape model), and location of regions of interest (ROI). The method was trained on an independent set of 40 x‐ray images. Automatically selected regions were compared to the “gold standard” that contains ROIs selected manually by a radiologist expert on 132 x‐ray images. All images were acquired from subjects locked in a standardized standing position using a radiography rig. The size of each ROI is . The automated method results showed a good agreement with the gold standard [similarity index (medial) and 0.81 (lateral) and the (medial) and (lateral)]. Bland and Altman plots were constructed for fractal signatures, and changes of fractal dimensions (FD) to region offsets calculated between the gold standard and automatically selected regions were calculated. The plots showed a random scatter and the 95% confidence intervals were (, 0.008) and (, 0.011). The changes of FDs to region offsets were less than 0.035. Previous studies showed that differences in FDs between non‐OA and OA bone regions were greater than 0.05. ROIs were also selected by a second radiologist and then evaluated. Results indicated that the newly developed method could replace a human operator and produces bone regions with an accuracy that is sufficient for fractal analyses of bone texture.
Improving intensity‐modulated radiation therapy using the anatomic beam orientation optimization algorithm
Authors: Potrebko Peter S. , McCurdy Boyd M. C. , Butler James B. , El‐Gubtan Adel S.
Published: May 2008

A novel, anatomic beam orientation optimization (A‐BOO) algorithm is proposed to significantly improve conventional intensity‐modulated radiation therapy (IMRT). The A‐BOO algorithm vectorially analyses polygonal surface mesh data of contoured patient anatomy. Five optimal (5‐opt) deliverable beam orientations are selected based on (1) tangential orientation bisecting the target and adjacent organ's‐at‐risk (OARs) to produce precipitous dose gradients between them and (2) parallel incidence with polygon features of the target volume to facilitate conformal coverage. The 5‐opt plans were compared to standard five, seven, and nine equiangular‐spaced beam plans (5‐equi, 7‐equi, 9‐equi) for: (1) gastric, (2) Radiation Therapy Oncology Group (RTOG) P‐0126 prostate, and (3) RTOG H‐0022 oropharyngeal (stage‐III, IV) cancer patients. In the gastric case, the noncoplanar 5‐opt plan reduced the right kidney V by 32.2%, 23.2%, and 20.6% compared to plans with five, seven, and nine equiangular‐spaced beams. In the prostate case, the coplanar 5‐opt plan produced similar rectal sparing as the 7‐equi and 9‐equi plans with a reduction of the V 75, V 70, V 65, and V of 2.4%, 5.3%, 7.0%, and 9.5% compared to the 5‐equi plan. In the stage‐III and IV oropharyngeal cases, the noncoplanar 5‐opt plan substantially reduced the V and mean dose to the contralateral parotid compared to plans with five, seven, and nine equiangular‐spaced beams: (stage‐III) 7.1%, 5.2%, 6.8%, and 5.1, 3.5, and (stage‐IV) 10.2%, 10.2%, 9.8% and 7.0, 7.1, . The geometry‐based A‐BOO algorithm has been demonstrated to be robust for application to a variety of IMRT treatment sites. Beam orientations producing significant improvements in OAR sparing over conventional IMRT can be automatically produced in minutes compared to hours with existing dose‐based beam orientation optimization methods.
The performance of interventional loopless MRI antennae at higher magnetic field strengths
Authors: El‐Sharkawy AbdEl‐Monem M. , Qian Di , Bottomley Paul A.
Published: May 2008

Interventional, “loopless antenna” MRI detectors are currently limited to . This study investigates whether loopless antennae offer signal‐to‐noise ratio (SNR) and field‐of‐view (FOV) advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to is investigated both analytically, using electromagnetic (EM) dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low‐loss loopless antennae fabricated for 1.5, 3, and , immersed in physiologically comparable saline. Gains of ‐ and ‐fold in SNR, and approximately 10‐ and 50‐fold gains in the useful FOV area are observed at 3 and , respectively, compared to . Heat testing of a biocompatible nitinol‐antenna fabricated with a redesigned decoupling circuit shows maximum heating of for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the antenna versus an equivalent commercial device in vivo. This work is the first to study the performance of experimental internal MRI detectors above . The large SNR and FOV gains realized present a major opportunity for high‐resolution imaging of vascular pathology and MRI‐guided intervention.
Software‐based respiratory gating for small animal conebeam CT
Authors: Farncombe T. H.
Published: May 2008

In volumetric CT imaging of small animals, the breathing motion of the lungs during the image acquisition process results in inconsistent projection data being acquired. When reconstructed, these inconsistent data may produce images with reduced spatial resolution and image contrast. In order to minimize these effects, various approaches have been utilized to capture the respiratory signal of the animals under study and to only obtain CT data at specific moments in the respiratory cycle. These approaches typically utilize hardware‐based physiological monitoring equipment in order to record the respiratory signal and either prospectively or retrospectively correlate this signal with acquired CT projection data. In this work, a new method of CT respiratory gating is described that does not rely on external physiological monitoring. Rather, determination of the respiratory phase of the animal is performed by postprocessing the acquired projection data. With this approach, any CT projection data can be respiratory gated with minimal effort. Validation of the method has been performed using a dynamic phantom and accuracy in tidal volumes determined to be within 16%. Rats and mice have been scanned and processed using the proposed method and compared to physiological‐based measurement. With the proposed method, image quality is significantly improved in addition to providing quantitative information regarding tidal lung volumes.
Fast internal marker tracking algorithm for onboard MV and kV imaging systems
Authors: Mao W. , Wiersma R. D. , Xing L.
Published: May 2008

Intrafraction organ motion can limit the advantage of highly conformal dose techniques such as intensity modulated radiation therapy (IMRT) due to target position uncertainty. To ensure high accuracy in beam targeting, real‐time knowledge of the target location is highly desired throughout the beam delivery process. This knowledge can be gained through imaging of internally implanted radio‐opaque markers with fluoroscopic or electronic portal imaging devices (EPID). In the case of MV based images, marker detection can be problematic due to the significantly lower contrast between different materials in comparison to their kV‐based counterparts. This work presents a fully automated algorithm capable of detecting implanted metallic markers in both kV and MV images with high consistency. Using prior CT information, the algorithm predefines the volumetric search space without manual region‐of‐interest (ROI) selection by the user. Depending on the template selected, both spherical and cylindrical markers can be detected. Multiple markers can be simultaneously tracked without indexing confusion. Phantom studies show detection success rates of 100% for both kV and MV image data. In addition, application of the algorithm to real patient image data results in successful detection of all implanted markers for MV images. Near real‐time operational speeds of for the detection of five markers in a image are accomplished using an ordinary PC workstation.