bluuebunny/crossref_metadata_2025 huggingface dataset:
Stiffness and Efficiency Optimization of a Hydrostatic Laser Surface Textured Gas Seal
Authors: Feldman Y. , Kligerman Y. , Etsion I.
Published: April 2007

Microdimples generated by laser surface texturing (LST) can be used to enhance performance in hydrostatic gas-lubricated mechanical seals. This is achieved by applying microdimples with high area density over a certain portion of the sealing dam width adjacent to the high-pressure side, leaving the remaining portion untextured. The textured portion provides an equivalent larger gap that results in converging clearance in the direction of pressure drop and hence, hydrostatic pressure buildup, similar to that of a radial step seal. A mathematical model based on the solution of the Reynolds equation for compressible Newtonian fluid in a narrow gap between two nominally parallel stationary surfaces is developed. A detailed dimensionless analysis of the texturing parameters is performed to achieve maximum gas film stiffness with minimum gas leakage.
EHL Modeling for Nonhomogeneous Materials: The Effect of Material Inclusions
Authors: Slack Trevor S. , Raje Nihar , Sadeghi Farshid , Doll Gary , Hoeprich Michael R.
Published: April 2007

Inclusions are common in bearing materials and are a primary site for subsurface fatigue crack initiation in rolling element bearings. This paper presents a new approach for computing the pressure, film thickness, and subsurface stresses in an elastohydrodynamic lubrication (EHL) contact when inclusions are present in the elastic half-space. The approach is based on using the discrete element method to determine the surface elastic deformation in the EHL film thickness equation. The model is validated through comparison with the smooth EHL line contact results generated using linear elasticity. Studies are then carried out to investigate the effects of size, location, orientation, and elastic properties of inclusions on the EHL pressure and film thickness profiles. Both inclusions that are stiffer than and/or softer than the base material are seen to have effects on the pressure distribution within the lubricant film and to give rise to stress concentrations. For inclusions that are stiffer than the base material (hard inclusions), the pressure distribution within the lubricant film behaves as though there is a bump on the surface, whereas for inclusions that are less stiff than the base material (soft inclusions), the pressure distribution behaves in a manner similar to that of a dented surface. Inclusions close to the surface cause significant changes in the contact stresses that are very significant considering the stress life relationship. For inclusions that are located deep within the surface, there is little change in the EHL pressure and film thickness.
Assessment and Verification of a Novel Method for Near Surface Measurement of Mechanical Properties
Authors: Ozcan S. , Farhang K. , Filip P.
Published: April 2007

A novel two-parameter area function for determination of near surface properties of Young’s modulus of elasticity and hardness has shown promise for compensating for the imperfection of the tip-end in an instrumented indentation measurement. This paper provides a comprehensive study involving a Berkovitch tip. The tip is utilized in an MTS nanoindentation measurement machine and is used to establish load indentation information for fused silica samples. The geometry of the tip is then characterized independently using a highly accurate atomic force microscope. Using the indentation data along with the two-parameter area function methodology, the tip-end radius of curvature is found to provide the most consistent value of modulus of elasticity. Independently, the data from the scanning electron microscope measurement of the same tip is used to obtain the least-squares estimation of the tip curvature. The two approaches yield favorable agreement in the estimation of tip-end radius of curvature. Therefore, the validity of the two-parameter area function method is proved. The method is shown to provide a robust, reliable, and accurate measurement of modulus of elasticity and hardness in the nanoscale proximity of a surface.
Stability Characteristics of a Rigid Rotor Supported by a Gas-Lubricated Spiral-Groove Conical Bearing
Authors: Pan Coda H. T. , Kim Daejong
Published: April 2007

Five-degree-of-freedom (5-DOF) characterization of the stability of a gas-lubricated conical bearing of the spiral-groove design is presented for bearing numbers up to 500. Critical points in stability analysis are identified in impedance contour plots separately for axial, cylindrical, and conical modes. The stability thresholds with respect to each mode are graphed as functions of the bearing number. For axial and cylindrical modes, the threshold parameter is the rotor mass. For the conical mode, the threshold parameter is an equivalent mass that is dependent on both transverse and polar radii of gyration of the rotor. An application example illustrates a rational procedure to specify nominal bearing clearance and its allowable tolerance range.
Influence of Groove Size on the Static and Rotordynamic Characteristics of Short, Laminar-Flow Annular Seals
Authors: Childs Dara W. , Graviss Matthew , Rodriguez Luis E.
Published: April 2007

Test results are presented for a smooth seal and three centrally grooved seals that are representative of buffered-flow oil seals in centrifugal compressors. The seals are short (L∕D≅0.21), with a diameter of 117mm and a nominal radial clearance of 0.085mm, netting the clearance-to-radius ratio 0.0015. The grooves have groove depth to clearance ratios (Dg∕Cr) of 5, 10, and 15. Test conditions include three shaft speeds from 4000rpm to 10,000rpm, three inlet oil pressures from 24bar to 70bar, and seal eccentricity ratios from 0 (centered) to 0.7. Dynamic results include stiffness, damping, and added-mass coefficients; static results include stator position, attitude angles, and seal leakage. Stiffness, damping, and mass coefficients plus leakage are compared for the seal geometries. Results show that all rotordynamic coefficients consistently decrease with increasing seal groove depths, and seal leakage is largely unchanged. Comparisons are also made between experimental results and predictions from a computer program based on a Reynolds + energy equation model. The model includes the assumption that a groove is large enough to create separate lands within the seal, creating a zero or negligible pressure perturbation within the groove. Test results show that even the deepest groove depth tested is not deep enough to satisfy this assumption.
Residual Strain Measurements in Thermal Spray Cermet Coatings via Neutron Diffraction
Authors: Ahmed R. , Yu H. , Stewart S. , Edwards L. , Santisteban J. R.
Published: April 2007

The impact and fatigue resistance of overlay coatings is significantly influenced by the residual strain (or stress) field induced during coating deposition, post-treatment, and in-service loading. Optimization of the residual strain field is therefore critical to the life and performance of components. Nondestructive measurement of these strain fields in relatively thin (300-400μm) thermal spray coatings, however, poses a challenge because conventional techniques, such as deep hole drilling, x-ray diffraction, synchrotron diffraction, and changes in beam curvature either make these techniques destructive and/or provides only a very near-surface strain measurement. This particularly complicates the strain analysis in cermet coatings, e.g., WC-Co deposited by the thermal spraying process, where the low penetration depth of x-ray and synchrotron-diffraction ray can only provide a through thickness measurement of stress or strain profile via the destructive layer removal technique. Recent investigations have therefore concentrated on the use of neutron diffraction technique for such analysis, and this paper reports some of the early findings of the comparison of through thickness strain measurements in relatively thin (400μm) as-sprayed and post-treated WC-12wt.%Co coatings via the neutron diffraction technique. Since neutrons are not charged, they do not interact with the electron cloud surrounding the atom (unlike x-ray); hence, diffraction results from the interaction with the atomic nucleus. Neutrons therefore have greater penetration depth in most engineering materials, and therefore provide a nondestructive through thickness strain measurement. Results of strain measurement are discussed with the structure property relationships and contact fatigue performance, and indicate that post-treatment of these coatings results in harmonization of the strain field within the coating, and at the coating substrate interface. This significantly influences the contact fatigue performance by improving both the cohesive and adhesive strength of these coatings.
Numerical Modeling of Rotordynamic Coefficients for Deliberately Roughened Stator Gas Annular Seals
Authors: Chochua Gocha , Soulas Thomas A.
Published: April 2007

A method is proposed for computations of rotordynamic coefficients of deliberately roughened stator gas annular seals using computational fluid dynamics. The method is based on a transient analysis with deforming mesh. Frequency-dependent direct and cross-coupled rotordynamic coefficients are determined as a response to an assigned rotor surface periodic motion. The obtained numerical results are found to be in good agreement with the available test data and one-dimensional tool predictions. The method can be used as a research tool or as a virtual annular seal test rig for seal design and optimization.
Clarification of a Semi-Empirical Approach in Piston Ring-Cylinder Bore Conformability Prediction
Authors: Dunaevsky V. , Rudzitis J.
Published: April 2007

This technical brief clarifies origins of the piston ring conformability bounds presented by Dunaevsky (1990, Tribol. Trans. 33(1), pp. 33–40) and provides their correct expression. The subject has strong practical significance in the field of piston engines and compressors, and it is currently getting additional prominence due to stringent emission regulations. The bounds stem from an earlier version, developed by the authors using a semi-empirical approach, and published in the 1970s in Russian technical journals. A semi-empirical background of the bounds was inadvertently missed in the aforementioned Tribology Transactions’ publication. This fact, and the virtual inaccessibility of Russian sources to the English reader, interfere with the proper use of the bounds and cloud their authorship. In this technical brief, the correct expression of these conformability bounds, the development process, and uses are highlighted.
A Review of Dry Particulate Lubrication: Powder and Granular Materials
Authors: Wornyoh Emmanuel Y. A. , Jasti Venkata K. , Fred Higgs C.
Published: April 2007

Research efforts related to dry particulates in sliding contacts are reviewed. In the tribology community, there are primarily two types of dry particulate lubricants that are studied—granular and powder. Granular lubricants usually refer to dry, cohesionless, hard particles that transfer momentum and accommodate surface velocity differences through shearing and rolling at low shear rates, and collisions at high shear rates. Powder lubricants refer to dry, cohesive, soft particles that accommodate surface velocity differences mostly by adhering to surfaces and shearing in the bulk medium, in a manner similar to hydrodynamic fluids. Spanning the past five decades, this review proposes a classification system for the scientific works in the dry particulate tribology literature in terms of theory, experiments, and numerical simulations. It also suggests that these works can be further categorized based on their tribosystem geometry—annular, parallel, and converging.
Predictive sensor based x‐ray calibration using a physical model
Authors: de la Fuente Matías , Lutz Peter , Wirtz Dieter C. , Radermacher Klaus
Published: April 2007

Many computer assisted surgery systems are based on intraoperative x‐ray images. To achieve reliable and accurate results these images have to be calibrated concerning geometric distortions, which can be distinguished between constant distortions and distortions caused by magnetic fields. Instead of using an intraoperative calibration phantom that has to be visible within each image resulting in overlaying markers, the presented approach directly takes advantage of the physical background of the distortions. Based on a computed physical model of an image intensifier and a magnetic field sensor, an online compensation of distortions can be achieved without the need of an intraoperative calibration phantom. The model has to be adapted once to each specific image intensifier through calibration, which is based on an optimization algorithm systematically altering the physical model parameters, until a minimal error is reached. Once calibrated, the model is able to predict the distortions caused by the measured magnetic field vector and build an appropriate dewarping function. The time needed for model calibration is not yet optimized and takes up to on a CPU. In contrast, the time needed for distortion correction is less than and therefore absolutely acceptable for intraoperative use. First evaluations showed that by using the model based dewarping algorithm the distortions of an XRII with a FOV could be significantly reduced. The model was able to predict and compensate distortions by approximately 80% to a remaining error of (max) ( rms).
Hyperfast parallel‐beam and cone‐beam backprojection using the cell general purpose hardware
Authors: Kachelrieß Marc , Knaup Michael , Bockenbach Olivier
Published: April 2007

Tomographic image reconstruction, such as the reconstruction of computed tomography projection values, of tomosynthesis data, positron emission tomography or SPECT events, and of magnetic resonance imaging data is computationally very demanding. One of the most time‐consuming steps is the backprojection. Recently, a novel general purpose architecture optimized for distributed computing became available: the cell broadband engine (CBE). To maximize image reconstruction speed we modified our parallel‐beam backprojection algorithm [two dimensional (2D)] and our perspective backprojection algorithm [three dimensional (3D), cone beam for flat–panel detectors] and optimized the code for the CBE. The algorithms are pixel or voxel driven, run with floating point accuracy and use linear (LI) or nearest neighbor (NN) interpolation between detector elements. For the parallel‐beam case, 512 projections per half rotation, 1024 detector channels, and an image of sizewas used. The cone‐beam backprojection performance was assessed by backprojecting a full circle scan of 512 projections of sizeinto a volume of sizevoxels. The field of view was chosen to completely lie within the field of measurement and the pixel or voxel size was set to correspond to the detector element size projected to the center of rotation divided by. Both the PC and the CBE were clocked at. For the parallel backprojection of 512 projections into aimage, a throughput of(LI) and(NN) was measured on the PC, whereas the CBE achieved(LI) and(NN), respectively. The cone‐beam backprojection of 512 projections into thevolume tookon the PC and is as fast ason the cell. Thereby, the cell greatly outperforms today's top‐notch backprojections based on graphical processing units. Using both CBEs of our dual cell‐based blade (Mercury Computer Systems) allows to 2D backproject 330 images/s and one can complete the 3D cone‐beam backprojection in.
Quantifying lateral tissue heterogeneities in hadron therapy
Authors: Pflugfelder D. , Wilkens J. J. , Szymanowski H. , Oelfke U.
Published: April 2007

In radiotherapy with scanned particle beams, tissue heterogeneities lateral to the beam direction are problematic in two ways: they pose a challenge to dose calculation algorithms, and they lead to a high sensitivity to setup errors. In order to quantify and avoid these problems, a heterogeneity number as a method to quantify lateral tissue heterogeneities of single beam spot is introduced. To evaluate this new concept, two kinds of potential errors were investigated for single beam spots: First, the dose calculation error has been obtained by comparing the dose distribution computed by a simple pencil beam algorithm to more accurate Monte Carlo simulations. The resulting error is clearly correlated with . Second, the analysis of the sensitivity to setup errors of single beam spots also showed a dependence on . From this data it is concluded that can be used as a criterion to assess the risks of a compromised delivered dose due to lateral tissue heterogeneities. Furthermore, a method how to incorporate this information into the inverse planning process for intensity modulated proton therapy is presented. By suppressing beam spots with a high value of , the unfavorable impact of lateral tissue heterogeneities can be reduced, leading to treatment plans which are more robust to dose calculation errors of the pencil beam algorithm. Additional possibilities to use the information of are outlined in the discussion.
Evaluation of the accuracy of fetal dose estimates using TG‐36 data
Authors: Kry Stephen F. , Starkschall George , Antolak John A. , Salehpour Mohammad
Published: April 2007

The American Association of Physicists in Medicine Radiation Therapy Committee Task Group 36 report (TG‐36) provides guidelines for managing radiation therapy of pregnant patients. Included in the report are data that can be used to estimate the dose to the fetus. The purpose of this study is to evaluate the accuracy of these fetal dose estimates as compared to clinically measured values. TG‐36 calculations were performed and compared with measurements of the fetal dose made in vivo or in appropriately‐designed phantoms. Calculation and measurement data was collected for eight pregnant patients who underwent radiation therapy at the MD Anderson Cancer Center as well as for several fetal dose studies in the literature. The maximum measured unshielded fetal dose was , which was 1.5% of the prescription dose. For all cases, TG‐36 calculations and measured fetal doses differed by up to a factor of 3—the ratio of the calculated to measured dose ranged from 0.34 to 2.93. On average, TG‐36 calculations underestimated the measured dose by 31%. No significant trends in the relationship between the calculated and measured fetal doses were found based on the distance from, or the size of, the treatment field.
Monte Carlo simulation of x‐ray buildup factors of lead and its applications in shielding of diagnostic x‐ray facilities
Authors: Kharrati Hedi , Agrebi Amel , Karaoui Mohamed‐Karim
Published: April 2007

X‐ray buildup factors of lead in broad beam geometry for energies from 15 toare determined using the general purpose Monte Carlo‐particle radiation transport computer code (MCNP4C). The obtained buildup factors data are fitted to a modified three parameter Archeret al.model for ease in calculating the broad beam transmission with computer at any tube potentials/filters combinations in diagnostic energies range. An example for their use to compute the broad beam transmission at 70, 100, 120, andis given. The calculated broad beam transmission is compared to data derived from literature, presenting good agreement. Therefore, the combination of the buildup factors data as determined and a mathematical model to generate x‐ray spectra provide a computationally based solution to broad beam transmission for lead barriers in shielding x‐ray facilities.
Detection of coronary calcifications from computed tomography scans for automated risk assessment of coronary artery disease
Authors: Išgum Ivana , Rutten Annemarieke , Prokop Mathias , van Ginneken Bram
Published: April 2007

A fully automated method for coronary calcification detection from non‐contrast‐enhanced, ECG‐gated multi‐slice computed tomography (CT) data is presented. Candidates for coronary calcifications are extracted by thresholding and component labeling. These candidates include coronary calcifications, calcifications in the aorta and in the heart, and other high‐density structures such as noise and bone. A dedicated set of 64 features is calculated for each candidate object. They characterize the object's spatial position relative to the heart and the aorta, for which an automatic segmentation scheme was developed, its size and shape, and its appearance, which is described by a set of approximated Gaussian derivatives for which an efficient computational scheme is presented. Three classification strategies were designed. The first one tested direct classification without feature selection. The second approach also utilized direct classification, but with feature selection. Finally, the third scheme employed two‐stage classification. In a computationally inexpensive first stage, the most easily recognizable false positives were discarded. The second stage discriminated between more difficult to separate coronary calcium and other candidates. Performance of linear, quadratic, nearest neighbor, and support vector machine classifiers was compared. The method was tested on 76 scans containing 275 calcifications in the coronary arteries and 335 calcifications in the heart and aorta. The best performance was obtained employing a two‐stage classification system with a‐nearest neighbor (‐NN) classifier and a feature selection scheme. The method detected 73.8% of coronary calcifications at the expense of on average 0.1 false positives per scan. A calcium score was computed for each scan and subjects were assigned one of four risk categories based on this score. The method assigned the correct risk category to 93.4% of all scans.
Monte Carlo simulation of time‐dependent, transport‐limited fluorescent boundary measurements in frequency domain
Authors: Pan Tianshu , Rasmussen John C. , Lee Jae Hoon , Sevick‐Muraca Eva M.
Published: April 2007

Recently, we have presented and experimentally validated a unique numerical solver of the coupled radiative transfer equations (RTEs) for rapidly computing time‐dependent excitation and fluorescent light propagation in small animal tomography. Herein, we present a time‐dependent Monte Carlo algorithm to validate the forward RTE solver and investigate the impact of physical parameters upon transport‐limited measurements in order to best direct the development of the RTE solver for optical tomography. Experimentally, the Monte Carlo simulations for both transport‐limited and diffusion‐limited propagations are validated using frequency domain photon migration measurements for 1.0%, 0.5%, and 0.2% intralipid solutions containing indocyanine green in a cylindrical phantom corresponding to the small volume employed in small animal tomography. The comparisons between Monte Carlo simulations and the numerical solutions result in mean percent error in amplitude and the phase shift less than 5.0% and , respectively, at excitation and emission wavelengths for varying anisotropic factors, lifetimes, and modulation frequencies. Monte Carlo simulations indicate that the accuracy of the forward model is enhanced using (i) suitable source models of photon delivery, (ii) accurate anisotropic factors, and (iii) accurate acceptance angles of collected photons. Monte Carlo simulations also show that the accuracy of the diffusion approximation in the small phantom depends upon (i) the ratio , where is the phantom diameter and is the transport mean free path; and (ii) the anisotropic factor of the medium. The Monte Carlo simulations validates and guides the future development of an appropriate RTE solver for deployment in small animal optical tomography.
Variations in energy spectra and water‐to‐material stopping‐power ratios in three‐dimensional conformal and intensity‐modulated photon fields
Authors: Jang Si Young , Liu H. Helen , Mohan Radhe , Siebers Jeffrey V.
Published: April 2007

Because of complex dose distributions and dose gradients that are created in three‐dimensional conformal radiotherapy (3D‐CRT) and intensity‐modulated radiation therapy (IMRT), photon‐ and electron‐energy spectra might change significantly with spatial locations and doses. This study examined variations in photon‐ and electron‐energy spectra in 3D‐CRT and IMRT photon fields. The effects of spectral variations on water‐to‐material stopping‐power ratios used in Monte Carlo treatment planning systems and the responses of energy‐dependent dosimeters, such as thermoluminescent dosimeters (TLDs) and radiographic films were further studied. The EGSnrc Monte Carlo code was used to simulate megavoltage 3D‐CRT and IMRT photon fields. The photon‐ and electron‐energy spectra were calculated in 3D water phantoms and anthropomorphic phantoms based on the fluence scored in voxel grids. We then obtained the water‐to‐material stopping‐power ratios in the local voxels using the Spencer‐Attix cavity theory. Changes in the responses of films and TLDs were estimated based on the calculated local energy spectra and published data on the dosimeter energy dependency. Results showed that the photon‐energy spectra strongly depended on spatial positions and doses in both the 3D‐CRT and IMRT fields. The relative fraction of low‐energy photons increased inversely with the photon dose in low‐dose regions of the fields. A similar but smaller effect was observed for electrons in the phantoms. The maximum variation of the water‐to‐material stopping‐power ratio over the range of calculated dose for both 3D‐CRT and IMRT was negligible for ICRU tissue, cortical bone, and soft bone and less than 3.6% for dry air and lung. Because of spectral softening at low doses, radiographic films in the phantoms could over‐respond to dose by more than 30%, whereas the over‐response of TLDs was less than 10%. Thus, spatial variations of the photon‐ and electron‐energy spectra should be considered as important factors in 3D‐CRT and IMRT dosimetry.
Development of an inverse optimization package to plan nonuniform dose distributions based on spatially inhomogeneous radiosensitivity extracted from biological images
Authors: Chen Guang‐Pei , Ahunbay Ergun , Schultz Christopher , Li X. Allen
Published: April 2007

An inverse optimization package which is capable of generating nonuniform dose distribution with subregional dose escalation is developed to achieve maximum equivalent uniform dose (EUD) for target while keeping the critical structure doses as low as possible. Relative cerebral blood volume (rCBV) maps obtained with a dynamic susceptibility contrast‐enhanced MRI technique were used to delineate spatial radiosensitivity distributions. The voxel rCBV was converted to voxel radiosensitivity parameters (e.g., and ) based on previously reported correlations between rCBV, tumor grade, and radiosensitivity. A software package, DOSEPAINT, developed using MATLAB, optimizes the beamlet weights to achieve maximum EUD for target while limiting doses to critical structures. Using DOSEPAINT, we have generated nonuniform 3D‐dose distributions for selected patient cases. Depending on the variation of the pixel radiosensitivity, the subregional dose escalation can be as high as 35% of the uniform dose as planned conventionally. The target dose escalation comes from both the inhomogeneous radiosensitivities and the elimination of integral target dose constraint. The target EUDs are found to be higher than those for the uniform dose planned ignoring the spatial inhomogeneous radiosensitivity. The EUDs for organs at risk are found to be approximately equal to or lower than those for the uniform dose plans. In conclusion, we have developed a package that is capable of generating nonuniform dose distributions optimized for spatially inhomogeneous radiosensitivity. Subregional dose escalation may lead to increased treatment effectiveness as indicated by higher EUDs. The current development will impact biological image guided radiotherapy.
Rodent brain imaging with SPECT/CT
Authors: Seo Youngho , Gao Dong‐Wei , Hasegawa Bruce H. , Dae Michael W. , Franc Benjamin L.
Published: April 2007

We evaluated methods of imaging rat models of stroke in vivo using a single photon emission computed tomography (SPECT) system dedicated to small animal imaging (X‐SPECT™, Gamma Medica‐Ideas, Northridge, CA). An animal model of ischemic stroke was developed for in vivo SPECT/CT imaging using the middle cerebral artery occlusion (MCAO) technique. The presence of cerebral ischemia was verified in ex vivo studies using triphenyltetrazolium chloride (TTC) staining. In vivo radionuclide imaging of cerebral blood flow was performed in rats following MCAO using dynamic planar imaging of ‐exametazime with parallel hole collimation. This was followed immediately by in vivo radionuclide imaging of cerebral blood flow with ‐exametazime in the same animals using 1‐mm pinhole SPECT. Correlated computed tomography imaging was performed to localize radiopharmaceutical uptake. The animals were allowed to recover and ex vivo autoradiography was performed with separate administration of ‐exametazime. Time activity curve of ‐exametazime showed that the radiopharmaceutical uptake could be maintained for over . The activity would be expected to be relatively stable for a much longer period, although the data were only obtained for . TTC staining revealed sizable infarcts by visual observation of inexistence of TTC stain in infracted tissues of MCAO rat brains. In vivo SPECT imaging showed cerebral blood flow deficit in the MCAO model, and the in vivo imaging result was confirmed with ex vivo autoradiography. We have demonstrated a capability of imaging regions of cerebral blood flow deficit in MCAO rat brains in vivo using a pinhole SPECT dedicated to small animal imaging.
Build‐up and surface dose measurements on phantoms using micro‐MOSFET in 6 and x‐ray beams and comparisons with Monte Carlo calculations
Authors: Xiang Hong F. , Song Jun S. , Chin David W. H. , Cormack Robert A. , Tishler Roy B. , Makrigiorgos G. Mike , Court Laurence E. , Chin Lee M.
Published: April 2007

This work is intended to investigate the application and accuracy of micro‐MOSFET for superficial dose measurement under clinically used MV x‐ray beams. Dose response of micro‐MOSFET in the build‐up region and on surface under MV x‐ray beams were measured and compared to Monte Carlo calculations. First, percentage‐depth‐doses were measured with micro‐MOSFET under 6 and beams of normal incidence onto a flat solid water phantom. Micro‐MOSFET data were compared with the measurements from a parallel plate ionization chamber and Monte Carlo dose calculation in the build‐up region. Then, percentage‐depth‐doses were measured for oblique beams at onto the flat solid water phantom with micro‐MOSFET placed at depths of , , and below the surface. Measurements were compared to Monte Carlo calculations under these settings. Finally, measurements were performed with micro‐MOSFET embedded in the first layer of bolus placed on a flat phantom and a curved phantom of semi‐cylindrical shape. Results were compared to superficial dose calculated from Monte Carlo for a thin layer that extends from the surface to a depth of . Results were (1) Comparison of measurements with MC calculation in the build‐up region showed that micro‐MOSFET has a water‐equivalence thickness (WET) of for beam and for beam from the flat side, and a WET of for beam and for beam from the epoxy side. (2) For normal beam incidences, percentage depth dose agree within 3%–5% among micro‐MOSFET measurements, parallel‐plate ionization chamber measurements, and MC calculations. (3) For oblique incidence on the flat phantom with micro‐MOSFET placed at depths of , , and , measurements were consistent with MC calculations within a typical uncertainty of 3%–5%. (4) For oblique incidence on the flat phantom and a curved‐surface phantom, measurements with micro‐MOSFET placed at agrees with the MC calculation within 6%, including uncertainties of micro‐MOSFET measurements of 2%–3% (1 standard deviation), MOSFET angular dependence of 3.0%–3.5%, and 1%–2% systematical error due to phantom setup geometry asymmetry. Micro‐MOSFET can be used for skin dose measurements in 6 and beams with an estimated accuracy of .
Quantitative evaluation of free‐form deformation registration for dynamic contrast‐enhanced MR mammography
Authors: Tanner Christine , Schnabel Julia A. , Hill Derek L. G. , Hawkes David J. , Degenhard Andreas , Leach Martin O. , Hose D. Rodney , Hall‐Craggs Margaret A. , Usiskin Sasha I.
Published: April 2007

In this paper, we present an evaluation study of a set of registration strategies for the alignment of sequences of 3D dynamic contrast‐enhanced magnetic resonance breast images. The accuracy of the optimal registration strategies was determined on unseen data. The evaluation is based on the simulation of physically plausible breast deformations using finite element methods and on contrast‐enhanced image pairs without visually detectable motion artifacts. The configuration of the finite element model was chosen according to its ability to predict in vivo breast deformations for two volunteers. We computed transformations for ten patients with 12 simulated deformations each. These deformations were applied to the postcontrast image to model patient motion occurring between pre‐ and postcontrast image acquisition. The original precontrast images were registered to the corresponding deformed postcontrast images. The performance of several registration configurations (rigid, affine, B‐spline based nonrigid, single‐resolution, multi‐resolution, and volume‐preserving) was optimized for five of the ten patients. The images were most accurately aligned with volume‐preserving single‐resolution nonrigid registration employing 40 or control point spacing. When tested on the remaining five patients the optimal configurations reduced the average mean registration error from 1.40 to for the whole breast tissue and from 1.20 to for the enhancing lesion. These results were obtained on average within 26 for 40 control point spacing. The visual appearance of the difference images from 30 patients was significantly improved after volume‐preserving single‐resolution nonrigid registration in comparison to no registration or rigid registration. No substantial volume changes within the region of the enhancing lesions were introduced by this nonrigid registration.
Introducing gel dosimetry in a clinical environment: Customization of polymer gel composition and magnetic resonance imaging parameters used for 3D dose verifications in radiosurgery and intensity modulated radiotherapy
Authors: Crescenti Remo A. , Scheib Stefan G. , Schneider Uwe , Gianolini Stefano
Published: April 2007

Radiation sensitive gels have been used as dosimeters for clinical dose verification of different radiation therapy modalities. However, the use of gels is not widespread, because careful techniques are required to achieve the dose precision and accuracy aimed for in clinical dose verification. Here, the introduction of gel dosimetry in a clinical environment is described, including the whole chain of customizations and preparations required to introduce magnetic resonance (MR) based gel dosimetry into clinical routine. In order to standardize gel dosimetry in dose verifications for radiosurgery and intensity modulated radiotherapy (IMRT), we focused on both the customization of the gel composition and of the MR imaging parameters to increase its precision. The relative amount of the components of the normoxic, methacrylic acid based gel (MAGIC) was changed to obtain linear and steep dose response relationships. MR imaging parameters were customized for the different dose ranges used in order to lower the relative standard deviation of the measured transversal relaxation rate . An optimization parameter was introduced to quantify the change in the relative standard deviation of taking the increase in MR time into account. A 9% methacrylic acid gel customized for radiosurgery was found to give a linear dose response up to with a slope of , while a 6% methacrylic acid gel customized for IMRT had a linear range up to with a slope of . With the help of an introduced optimization parameter, the mean was improved by 13% for high doses and by 55% for low doses, without increasing MR time to unacceptable values. A mean dose resolution of less than has been achieved with the gel and imaging parameters customized for IMRT and a dose resolution from (at ) to (at ) for the radiosurgery dose range. The comparisons of calculated and measured relative 3D dose distributions performed for radiosurgery and IMRT showed an acceptable overall correlation. The gamma criterion for the radiosurgery verification with a voxel size of was passed by 96.8% of the voxels ( distance, 8% in dose). For the IMRT verification using a voxel size of the gamma criterion was passed by 50.3% of the voxels ( distance, 3% dose uncertainty). Using dedicated data analysis and visualization software, MR based normoxic gel dosimetry was found to be a valuable tool for clinically based dose verification, provided that customized gel compositions and MR imaging parameters are used. While high dose precision was achieved, further work is required to achieve clinically acceptable dose accuracy.
In vitro study of cell survival following dynamic MLC intensity‐modulated radiation therapy dose deliverya)
Authors: Moiseenko Vitali , Duzenli Cheryl , Durand Ralph E.
Published: April 2007

The possibility of reduced cell kill following intensity‐modulated radiation therapy (IMRT) compared to conventional radiation therapy has been debated in the literature. This potential reduction in cell kill relates to prolonged treatment times typical of IMRT dose delivery and consequently increased repair of sublethal lesions. While there is some theoretical support to this reduction in cell kill published in the literature, direct experimental evidence specific to IMRT dose delivery patterns is lacking. In this study we present cell survival data for three cell lines: Chinese hamster V79 fibroblasts, human cervical carcinoma, SiHa and colon adenocarcinoma, WiDr. Cell survival was obtained for delivered as acute dose with parallel‐opposed pair (POP), irradiation time , which served as a reference; regular seven‐field IMRT, irradiation time ; and IMRT with a break for multiple leaf collimator (MLC) re‐initialization after three fields were delivered, irradiation time . An actual seven‐field dynamic MLC IMRT plan for a head and neck patient was used. The IMRT plan was generated for a Varian EX or iX linear accelerator with 120 leaf Millenium MLC. Survival data were also collected for doses , , , , and to establish parameters of the linear‐quadratic equation describing survival following acute dose delivery. Cells were irradiated inside an acrylic cylindrical phantom specifically designed for this study. Doses from both IMRT and POP were validated using ion chamber measurements. A reproducible increase in cell survival was observed following IMRT dose delivery. This increase varied from small for V79, with a surviving fraction of 0.8326 following POP vs 0.8420 following uninterrupted IMRT, to very pronounced for SiHa, with a surviving fraction of 0.3903 following POP vs 0.5330 for uninterrupted IMRT. When compared to IMRT or IMRT with a break for MLC initialization, cell survival following acute dose delivery was significantly different, , in three out of six cases. In contrast, when cell survival following IMRT was compared to that following IMRT with a break for MLC initialization the difference was always statistically insignificant. When projected to a 30 fraction treatment, dose deficit to bring cell survival to the same value as in POP was calculated as 4.1, 24.9, and for V79, WiDr, and SiHa cell lines, respectively. The dose deficit did not relate to the ratio obtained in this study for the three cell lines. Clinical data do not show reduction in local control following IMRT. Possible reasons for this are discussed. The obtained data set can serve as a test data set for models designed to explore the effect of dose delivery prolongation/fractionation in IMRT on radiation therapy outcome.
Survey of computed tomography scanners in Taiwan: Dose descriptors, dose guidance levels, and effective doses
Authors: Tsai H. Y. , Tung C. J. , Yu C. C. , Tyan Y. S.
Published: April 2007

The IAEA and the ICRP recommended dose guidance levels for the most frequent computed tomography (CT) examinations to promote strategies for the optimization of radiation dose to CT patients. A national survey, including on‐site measurements and questionnaires, was conducted in Taiwan in order to establish dose guidance levels and evaluate effective doses for CT. The beam quality and output and the phantom doses were measured for nine representative CT scanners. Questionnaire forms were completed by respondents from facilities of 146 CT scanners out of 285 total scanners. Information on patient, procedure, scanner, and technique for the head and body examinations was provided. The weighted computed tomography dose index , the dose length product (DLP), organ doses and effective dose were calculated using measured data, questionnaire information and Monte Carlo simulation results. A cost‐effective analysis was applied to derive the dose guidance levels on and DLP for several CT examinations. The mean effective deviation distributes from for the routine head examination to for the examination of liver, spleen, and pancreas. The surveyed results and the dose guidance levels were provided to the national authorities to develop quality control standards and protocols for CT examinations.
Quantification of the impact of MLC modeling and tissue heterogeneities on dynamic IMRT dose calculations
Authors: Mihaylov I. B. , Lerma F. A. , Fatyga M. , Siebers J. V.
Published: April 2007

This study quantifies the dose prediction errors (DPEs) in dynamic IMRT dose calculations resulting from (a) use of an intensity matrix to estimate the multi‐leaf collimator (MLC) modulated photon fluence instead of an explicit MLC particle transport, and (b) handling of tissue heterogeneities by superposition/convolution (SC) and pencil beam (PB) dose calculation algorithms. Monte Carlo (MC) computed doses are used as reference standards. Eighteen head‐and‐neck dynamic MLC IMRT treatment plans are investigated. DPEs are evaluated via comparing the dose received by 98% of the GTV (GTV ), the CTV , the nodal , the cord and the brainstem , the parotid , the parotid mean dose , and generalized equivalent uniform doses (gEUDs) for the above structures. For the MC‐generated intensity grids, is within for all targets and critical structures. The SC algorithm is within for 98.3% of the indices tallied, and within for all of the tallied indices. The PB algorithm is within for 92% of the tallied indices. Statistical equivalence tests indicate that PB requires a interval to state equivalence with the MC standard, while the intervals are for SC and . Overall, these results indicate that SC and MC IMRT dose calculations which use MC‐derived intensity matrices for fluence prediction do not introduce significant dose errors compared with full Monte Carlo dose computations; however, PB algorithms may result in clinically significant dose deviations.
On the discrepancies between Monte Carlo dose calculations and measurements for the Varian photon beam
Authors: Chibani Omar , Ma C.‐M. Charlie
Published: April 2007

Significant discrepancies between Monte Carlo dose calculations and measurements for the Varian photon beam with a large field size were reported by different investigators. In this work, we investigated these discrepancies based on a new geometry model (“New Model”) of the Varian 21EX linac using the GEPTS Monte Carlo code. Some geometric parameters used in previous investigations (Old Model) were inaccurate, as suggested by Chibani in his AAPM presentation (2004) and later confirmed by the manufacturer. The entrance and exit radii of the primary collimator of the New Model are larger than previously thought. In addition to the corrected dimensions of the primary collimator, the New Model includes approximate models for the lead shield and the mirror frame between the monitor chamber and the jaws. A detailed analysis of the phase space data shows the effects of these corrections on the beam characteristics. The individual contributions from the linac component to the photon and electron fluences are calculated. The main source of discrepancy between measurements and calculations based on the Old Model is the underestimated electron contamination. The photon and electron fluences at the isocenter are 5.3% and 36% larger in the New Model in comparison with the Old Model. The flattening filter and the lead shield (plus the mirror frame) contribute 48.7% and 13% of the total electron contamination at the isocenter, respectively. For both open and filtered ( Pb) fields, the calculated (New Model) and measured dose distributions are within 1% for depths larger than . To solve the residual problem of large differences at shallow depths (8% at depth), the detailed geometry of an IC‐10 ionization chamber was simulated and the dose in the air cavity was calculated for different positions on the central axis including at the surface, where half of the chamber is outside the phantom. The calculated and measured chamber responses are within 3% even at the zero depth.
Effects of pulsatile blood flow in large vessels on thermal dose distribution during thermal therapy
Authors: Horng Tzyy‐Leng , Lin Win‐Li , Liauh Chihng‐Tsung , Shih Tzu‐Ching
Published: April 2007

The aim of this study is to evaluate the effect of pulsatile blood flow in thermally significant blood vessels on the thermal lesion region during thermal therapy of tumor. A sinusoidally pulsatile velocity profile for blood flow was employed to simulate the cyclic effect of the heart beat on the blood flow. The evolution of temperature field was governed by the energy transport equation for blood flow together with Pennes’ bioheat equation for perfused tissue encircling the blood vessel. The governing equations were numerically solved by a novel multi‐block Chebyshev pseudospectral method and the accumulated thermal dose in tissue was computed. Numerical results show that pulsatile velocity profile, with various combinations of pulsatile amplitude and frequency, has little difference in effect on the thermal lesion region of tissue compared with uniform or parabolic velocity profile. However, some minor differences on the thermal lesion region of blood vessel is observed for middle‐sized blood vessel. This consequence suggests that, in this kind of problem, we may as well do the simulation simply by a steady uniform velocity profile for blood flow.