bluuebunny/crossref_metadata_2025 huggingface dataset:
Short-Term Power Fluctuations of Large Wind Power Plants*
Authors: Wan Yih-huei , Bucaneg, Demy
Published: November 2002

To evaluate short-term wind power fluctuations and their impact on electric power systems, the National Renewable Energy Laboratory, in cooperation with Enron Wind, has started a project to record output power from several large commercial wind power plants at the 1-Hertz rate. This paper presents statistical properties of the data collected so far and discusses the results of data analysis. From the available data, we can already conclude that despite the stochastic nature of wind power fluctuations, the magnitudes and rates of wind power changes caused by wind speed variations are seldom extreme, nor are they totally random. Their values are bounded in narrow ranges. Power output data also show significant spatial variations within a large wind power plant. The data also offer encouraging evidence that accurate wind power forecasting is feasible. To the utility system, large wind power plants are not really random burdens. The narrow range of power level step changes provides a lot of information with which system operators can make short-term predictions of wind power. Large swings of wind power do occur, but those infrequent large changes (caused by wind speed changes) are always related to well-defined weather events, most of which can be accurately predicted in advance.
Analysis of the Structural and Inflow Data From the List Turbine
Authors: Sutherland Herbert J.
Published: November 2002

The Long-term Inflow and Structural Test (LIST) program is collecting long-term, continuous inflow and structural response data to characterize the spectrum of loads on wind turbines. A heavily instrumented Micon 65/13M turbine with Phoenix 8m blades is being used as the test turbine for the first measurement campaign of this program. This turbine is located in Bushland, TX, a test site that exposes the turbine to a wind regime representative of a Great Plains commercial site. The turbine and inflow are being characterized with 60 measurements: 34 to characterize the inflow, 19 to characterize structural response, and seven to characterize the time-varying state of the turbine. In this paper, an analysis of the structural and inflow data is presented. Particular attention is paid to the determination of the various structural loads on the turbine, long-term fatigue spectra and the correlation of various inflow descriptors with fatigue loads. For the latter analysis, the inflow is described by various parameters, including the mean, standard deviation, skewness and kurtosis of the hub-height horizontal wind speed, turbulence intensity, turbulence length scales, Reynolds stresses, local friction velocity, Obukhov length, and the gradient Richardson number. The fatigue load spectrum corresponding to these parameters is characterized as an equivalent fatigue load. A regression analysis is then used to determine which parameters are correlated to the fatigue loads. The results illustrate that the vertical component of the inflow is the most important of the secondary inflow parameters with respect to fatigue loads. Long-term fatigue spectra illustrate that extrapolation of relatively short-term data to longer times is consistent for the data reported here.
Analysis of Rotor-Rotor and Stator-Stator Interferences in Multi-Stage Turbomachines
Authors: He L. , Chen T. , Wells R. G. , Li Y. S. , Ning W.
Published: November 2002

A nonlinear harmonic methodology is adopted to analyze interactions between adjacent stages in multi-stage compressors. Of particular interest are the effects of circumferential “aperiodic” distributions and the relative circumferential positioning (‘clocking’) of blades. The main feature of the present approach is that both the aperiodic and clocking effects are very efficiently included with circumferential “steady” harmonic disturbances. Consequently, a single run of the nonlinear harmonic solver using a single-passage domain can produce the whole annulus unsteady and aperiodic time-averaged flow field. In addition, performance variation at any clocking position can be obtained simply by post-processing the result. A case study is presented for a two-and-half-stage transonic compressor, and the present results show much stronger rotor-rotor interaction than stator-stator interaction. A mechanism leading to strong rotor-rotor interference seems to be the interaction between upstream rotor wakes and the downstream rotor passage shock waves. A rotor-rotor clocking study illustrates a qualitatively different loss variation with respect to clocking position in a transonic flow compared to that in a subsonic flow.
A Fundamental Model of Mistuning for a Single Family of Modes
Authors: Feiner D. M. , Griffin J. H.
Published: November 2002

A new reduced order model of mistuned bladed disk vibration is presented. This new approach is shown to accurately represent the response of real turbine geometries when only a single family of modes is excited. Yet its mathematical form is even simpler than that of a mass-spring model. Because it requires only minimal input data, this model is much easier to use than previous reduced order methods. Furthermore, its simplicity allows the fundamental parameters that control mistuning to be readily identified.
Effects of the Nature of Excitation on the Response of a Mistuned Bladed Disk Assembly
Authors: Cha D. , Sinha A.
Published: November 2002

In this paper, responses of a mistuned bladed disk assembly are examined and compared for three types of excitations: uncorrelated narrow band random excitations, correlated narrow band random excitations and sinusoidal excitations with unknown (time-invariant and random) amplitudes. Analytical techniques are also developed to compute the statistics of responses for these types of excitations. Effects of correlations of narrow band excitations are investigated in details. It has been found that the response statistics for correlated narrow band random excitations can be viewed in terms of the concepts related to the response to a deterministic engine order excitation.
Experimental Investigation of Unsteady Flow Field Within a Two-Stage Axial Turbomachine Using Particle Image Velocimetry
Authors: Uzol Oguz , Chow Yi-Chih , Katz Joseph , Meneveau Charles
Published: November 2002

Detailed measurements of the flow field within the entire 2nd stage of a two-stage axial turbomachine are performed using particle image velocimetry. The experiments are performed in a facility that allows unobstructed view on the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. The entire flow field is composed of a “lattice of wakes,” and the resulting wake-wake and wake-blade interactions cause major flow and turbulence nonuniformities. The paper presents data on the phase averaged velocity and turbulent kinetic energy distributions, as well as the average-passage velocity and deterministic stresses. The phase-dependent turbulence parameters are determined from the difference between instantaneous and the phase-averaged data. The distributions of average passage flow field over the entire stage in both the stator and rotor frames of reference are calculated by averaging the phase-averaged data. The deterministic stresses are calculated from the difference between the phase-averaged and average-passage velocity distributions. Clearly, wake-wake and wake-blade interactions are the dominant contributors to generation of high deterministic stresses and tangential nonuniformities, in the rotor-stator gap, near the blades and in the wakes behind them. The turbulent kinetic energy levels are generally higher than the deterministic kinetic energy levels, whereas the shear stress levels are comparable, both in the rotor and stator frames of references. At certain locations the deterministic shear stresses are substantially higher than the turbulent shear stresses, such as close to the stator blade in the rotor frame of reference. The nonuniformities in the lateral velocity component due to the interaction of the rotor blade with the 1st-stage rotor-stator wakes, result in 13 percent variations in the specific work input of the rotor. Thus, in spite of the relatively large blade row spacings in the present turbomachine, the nonuniformities in flow structure have significant effects on the overall performance of the system.
Comparing Fatigue Strength From Full Scale Blade Tests With Coupon-Based Predictions
Authors: van Leeuwen Hans , van Delft Don , Heijdra John , Braam Henk , Jørgensen Eric R. , Lekou Denja , Vionis Pantelis
Published: November 2002

In order to get a deeper understanding of the blade-to-blade variations and to determine the statistical distribution of the fatigue strength of rotor blades, 37 small rotor blades have been tested in static and fatigue loading. The blades are 3.4 m commercially available blades adapted to the needs of the project. In addition to these blade tests, coupons of the blade material have been tested. The tests have encompassed static flapwise bending tests, flapwise fatigue tests at two different sections of the blade, and edgewise fatigue tests. Since some blades could be re-used after a first test, a total number of 42 blade tests has been carried out in three different testing laboratories. The blades showed large deformation, development of creep and stiffness reduction. After correction for these phenomena, the fatigue strength of the blades was predicted very well by the classical Goodman relation using the well-known slope parameter of 10.
Modal Response of 3-Bladed Wind Turbines
Authors: Malcolm David J.
Published: November 2002

A set of linear equations describing the motion of an operating 3-bladed HAWT is obtained from the dynamic characteristics of the stationary turbine by adding rotating frame effects. The approach makes use of the Coleman multi-blade transformation to present all results relative to the fixed frame. The formulation is in terms of a selected number of stationary, real mode shapes. The formulation is applied to the expression of both the aerodynamic loading and the displacement response in terms of the complex operating mode shapes. The technique is demonstrated on a hypothetical 46-m wind turbine subject to vertical wind shear. The principal objective of the paper is to enable the characteristics of the inflow to be related to the nature of the response. A second objective is to illustrate a method of extracting linearized models from general aeroelastic codes such as ADAMS™.
Predicting the Long Term Distribution of Extreme Loads From Limited Duration Data: Comparing Full Integration and Approximate Methods
Authors: Fitzwater LeRoy M. , Cornell C. Allin
Published: November 2002

In this paper, we present a methodology for proceeding from the short-term observations of extreme loads to the long-run load distribution of these extreme events, for both flap and edge loading in both operating and parked wind turbine conditions. First, a general approach utilizing full integration, where numerical routines are used to directly integrate the conditional short-term load distribution over the annual occurrence of wind speeds and turbulence intensities, is presented. Then starting from this general approach, a qualitative analysis is undertaken to explore the extent of the contribution of each of the three variables in the governing equation to the variability in the long-term extreme load distribution. From this analysis, lower-order models are considered, where instead of using the entire distribution of the variables, a constant fractile of the short-term extreme load distribution, turbulence intensity distribution, or both are used. Finally recommendations are given to guide the analyst to decide when simpler, yet robust, methods which account for sufficient variability in extreme load events may be employed with confidence.
Acoustic Emission Monitoring of Small Wind Turbine Blades
Authors: Joosse P. A. , Blanch M. J. , Dutton A. G. , Kouroussis D. A. , Philippidis T. P. , Vionis P. S.
Published: November 2002

Wind turbine blade certification tests often generate sudden audible cracking sounds from somewhere within the blade, without the operators being able to locate the noise source or to evaluate the existence or the extent of any damage. It would be beneficial to be able to detect any damage incurred by the blade, whether it is accompanied by audible noise or not. The current project, named AEGIS, is looking at the possibility of using acoustic emission monitoring during testing of fiber composite blades to detect the source of damage events and assess the blade condition. The test methodology is discussed in the context of the blade certification procedure and results are presented from a series of static and fatigue blade tests to failure in the laboratory.
Time-Domain Identification of Synchronous Generator Transfer Functions
Authors: Molenaar David-P. , Bosgra O. H. , Hoeijmakers M. J.
Published: November 2002

The design of a robust frequency converter controller for high dynamic performance of a synchronous generator requires an accurate dynamic model of the electromagnetic part. In this paper, a new procedure for identifying the transfer functions of Park’s dq-axis model of a synchronous generator has been developed. It will be shown that the parameters of this model can be easily identified from standstill time-domain data. The validity of the theoretical model has been verified by comparing time-domain simulations with measurements taken from the Lagerwey LW-50/750 direct-drive synchronous generator. It can be concluded that a consistent model estimate of the electromagnetic part of the LW-50/750 generator has been obtained. Ultimate validation, however, will follow after the implementation of the designed frequency converter controller in this wind turbine.
Effect of Turbulence Variation on Extreme Loads Prediction for Wind Turbines
Authors: Moriarty Patrick J. , Holley William E. , Butterfield Sandy
Published: November 2002

The effect of varying turbulence levels on long-term loads extrapolation techniques was examined using a joint probability density function of both mean wind speed and turbulence level for loads calculations. The turbulence level has a dramatic effect on the statistics of moment maxima extracted from aeroelastic simulations. Maxima from simulations at lower turbulence levels are more deterministic and become dominated by the stochastic component as turbulence level increases. Short-term probability distributions were calculated using four different moment-based fitting methods. Several hundred of these distributions were used to calculate a long-term probability function. From the long-term probability, 1- and 50-yr extreme loads were estimated. As an alternative, using a normal conditional distribution of turbulence level produced a long-term load comparable to that of a log-normal conditional distribution and may be more straightforward to implement. A parametric model of the moments was also used to estimate the extreme loads. The parametric model required less data, but predicted significantly lower loads than the empirical model. An input extrapolation technique was also examined. Extrapolating the turbulence level prior to input into the aeroelastic code simplifies the loads extrapolation procedure but, in this case, produces loads lower than most empirical models and may be non-conservative in general.
Bubble-Induced Unsteadiness on A Wind Turbine Airfoil
Authors: Mayda E. A. , van Dam C. P.
Published: November 2002

The effect of laminar separation bubbles on the surface pressure distribution and aerodynamic force characteristics of a horizontal axis wind turbine airfoil is studied numerically. The NREL S809 airfoil for stall-controlled horizontal-axis wind turbines is analyzed at a chord Reynolds number of 1.0×106. For all flow conditions involving laminar separation in the present study, bubble-induced vortex shedding is observed. This flow phenomenon causes significant oscillations in the airfoil surface pressures and, hence, in the airfoil-generated aerodynamic forces. The computed time-averaged pressures compare favorably with wind-tunnel measurements.
Cost Effective Large Blade Components by Using Carbon Fibers
Authors: Joosse P. A. , van Delft D. R. V. , Kensche Chr. , Soendergaard D. , van den Berg R. M. , Hagg F.
Published: November 2002

For large off-shore wind turbines, blades with relative low blade mass are becoming more important. The economic use of large-tow carbon fibers can help achieve lower blade masses. Basic material design data have been established for two promising material combinations, including the fatigue properties for Panex33/epoxy. Blade root joints have been developed in a carbon/glass combination, resulting in a better price performance ratio. The initial cost assessment on a blade dominated by severe fatigue loads shows that application of carbon fibers in the spar leads to cost reductions.
A Viscous Three-Dimensional Differential/Actuator-Disk Method for the Aerodynamic Analysis of Wind Farms
Authors: Ammara Idriss , Leclerc Christophe , Masson Christian
Published: November 2002

Computational Fluid Dynamics (CFD) is a promising tool for the analysis and optimization of wind turbine positioning inside wind parks (also known as wind farms) in order to maximize power production. In this paper, 3-D, time-averaged, steady-state, incompressible Navier-Stokes equations, in which wind turbines are represented by surficial forces, are solved using a Control-Volume Finite Element Method (CVFEM). The fundamentals of developing a practical 3-D method are discussed in this paper, with an emphasis on some of the challenges that arose during their implementation. For isolated turbines, results have indicated that the proposed 3-D method attains the same level of accuracy, in terms of performance predictions, as the previously developed 2-D axisymmetric method and the well-known momentum-strip theory. Furthermore, the capability of the proposed method to predict wind turbine wake characteristics is also illustrated. Satisfactory agreement with experimental measurements has been achieved. The analysis of a two-row periodic wind farm in neutral atmospheric boundary layers demonstrate the existence of positive interference effects (venturi effects) as well as the dominant influence of mutual interference on the performance of dense wind turbine clusters.
Turbofan Performance Deterioration Tracking Using Nonlinear Models and Optimization Techniques
Authors: Mathioudakis K. , Kamboukos Ph. , Stamatis A.
Published: November 2002

A method of identifying the gradual deterioration in the components of jet engines is presented. It is based on the use of an engine model which has the capability to adapt component condition parameters, so that measured quantities are matched. The main feature of the method is that it gives the possibility to identify performance deviations in a number of parameters larger than the number of measured quantities. This is achieved by optimizing a cost function which incorporates not only measurement matching terms, but also terms expressing various constraints resulting from the physical knowledge of the deterioration process. Time series of data representing deterioration scenarios are used to demonstrate the method’s capabilities. The test case considered is a twin spool partially mixed turbofan, representative of present-day large civil aeroengines. Implementation aspects, related to both the measurement set and the identification algorithms are discussed. An interpretation of the output of the method in function of different parameters entering the diagnostic problem is presented.
Light and electron radiation fields' coincidence at extended distance
Authors: Lief Eugene P.
Published: November 2002

Light field is routinely used for matching electron fields with adjacent photon fields and also positioning an ion chamber for cutout output measurements. At extended distance, however, the peripheral aspects of a radiation field are shifted with respect to their light shadow by up to 10 mm at SSD of 115 cm. This is mostly due to a difference in positions of light and virtual electron sources; however loss of the lateral scatter equilibrium, electron scatter in air, and partial transmission through cutout edges can also contribute to the effect. For the aperture sizes large enough for the lateral scatter equilibrium to be achieved, a simple geometrical formula allows one to calculate the misalignment at off‐axis locations if the virtual electron source position is known. If the source position is not known or the aperture is too small for the lateral scatter equilibrium to be established, then a light‐radiation coincidence test using a ready‐pack film at can be used to measure the light and electron radiation fields coincidence.
Improved treatment planning for boron neutron capture therapy for glioblastoma multiforme using fluorine‐18 labeled boronophenylalanine and positron emission tomography
Authors: Nichols Trent L. , Kabalka George W. , Miller Laurence F. , Khan Mohammad K. , Smith Gary T.
Published: November 2002

Boron neutron capture therapy (BNCT) is a cancer brachytherapy based upon the thermal neutron reaction: The efficacy of the treatment depends primarily upon two conditions being met: (a) the preferential concentration of a boronated compound in the neoplasm and (b) an adequate fluence of thermal neutrons delivered to the neoplasm. The boronated amino acid, para‐boronophenylalanine (BPA), is the agent widely used in clinical trials to deliver to the malignancy. Positron emission tomography (PET) can be used to generate in vivo boron distribution maps by labeling BPA with the positron emitting nuclide fluorine‐18. The incorporation of the PET‐derived boron distribution maps into current treatment planning protocols is shown to provide improved treatment plans. Using previously established protocols, six patients with glioblastoma had PET scans. The PET distribution maps obtained were used in the conventional BNCT treatment codes. The isodose curves derived from the PET data are shown to differ both qualitatively and quantitatively from the conventional isodose curves that were derived from calculations based upon the assumption of uniform uptake of the pharmaceutical in tumor and normal brain regions. The clinical course of each of the patients who eventually received BNCT (five of the six patients) was compared using both sets of isodose calculations. The isodose contours based upon PET derived distribution data appear to be more consistent with the patients' clinical course.
Evaluation of the imaging properties of two generations of a CCD‐based system for digital chest radiography
Authors: Båth Magnus , Sund Patrik , Månsson Lars Gunnar
Published: November 2002

Two generations of a CCD‐based detector system with lens‐based optical coupling for digital chest radiography were evaluated in terms of presampling MTF, NPS, NEQ, DQE, linearity in response, and SNR over the detector area. Measurements were performed over a wide exposure range and at several different beam qualities. Neither the presampling MTF nor the DQE showed any general strong beam quality dependence, whereas the NPS and NEQ did when compared at specific entrance air kerma values. The exposure dependency for the DQE was found to be considerable, with the detectors showing low DQE at low exposures, and higher DQE at higher exposures. It was found that the second generation has been substantially improved compared to its predecessor regarding all the relevant parameters. The DQE(0) at an entrance air kerma of 5 μGy increased from 9% to 15%, mainly due to a better system gain (including optical coupling efficiency and matching of the energy of the emitted light photons to the sensitivity of the CCD camera). The first generation of detectors was found to have problems with bad peripheral resolutionThis problem was nonexistent for the second generation for which uniform resolution has been obtainedA theoretical calculation of the DQE of two model systems similar to the ones evaluated was also performed, and the results were comparable to the experimentally determined data at high exposures. The model shows that both systems suffer from low optical coupling efficiency due to the large demagnification used. The main conclusion is that although the second generation has been improved, there is still a problem with low system gain leading to relatively modest DQE values, especially at low exposures.
An investigation of a new amorphous silicon electronic portal imaging device for transit dosimetry
Authors: Grein Ellen E. , Lee Richard , Luchka Kurt
Published: November 2002

The relationship between the pixel value and exit dose was investigated for a new commercially available amorphous silicon electronic portal imaging device. The pixel to dose mapping function was established to be linear for detector distances between 116.5 cm to 150 cm from the source, radiation field sizes from to and beam energies of 6 to 18 MV. Coefficients in the mapping function were found to be dependent on beam energy and field size. Open and wedged field profiles measured with the device showed agreement to a maximum of 5% and 8%, respectively, as compared to film. A comparison of relative transmission measurements between the EPID and ion chamber indicate a maximum deviation of 6% and 2% at 6 and 18 MV, respectively, for an attenuator thickness of 21 cm and SDD⩾130 cm. It was found that accuracies of better than 1% could be obtained if detector position and field size specific fitting parameters were used to generate unique mapping functions for each configuration.
Reshapable physical modulator for intensity modulated radiation therapy
Authors: Xu Tong , Shikhaliev Polad M. , Al‐Ghazi Muthana , Molloi Sabee
Published: November 2002

A new method of generating beam intensity modulation filters for intensity modulated radiation therapy (IMRT) is presented. The modulator was based on a reshapable material, which is not compressible but can be deformed under pressure. A two‐dimensional (2D) piston array was used to repeatedly shape the attenuating material. The material is a mixture of tungsten powder and a silicon‐based binder. The linear attenuation coefficient of the material was measured to be 0.409 cm−1 for a 6 MV x‐ray beam. The maximum thickness of the physical modulator is 10.2 cm, allowing a transmission of 1.5%. A square piston array was used to generate a depth pattern in the deformable attenuating material. Each piston has a cross section of The modulator was placed 65 cm from the radiation source of the linear accelerator in the position of the shielding tray. At this position, each piston projects to a area at the isocenter, giving a treatment field of The percent depth dose curve and output factor measurement show a slight beam hardening and a 1%–4% increase in scatter fraction when 2.2–4.4 cm uniform thickness filters are in the beam. The surface dose was decreased with the filter in the beam. Ion chamber and verification films were used to verify the entrance dose. The measured absolute and relative doses were compared with the calculated dose. The agreement of measurements and calculations is within 3%. In order to verify the spatial modulation of dose, 1‐D dose profiles were obtained using dose calculations. Calculated and measured profiles were compared. The 20%–80% penumbra of the modulator was measured to be 5.5–10 mm. The results show that a physical modulator formed using a piston array and a deformable attenuation material can provide intensity modulation for IMRT comparable with those provided by currently available commercial MLC techniques.
Intensity modulated arc deliveries approximated by a large number of fixed gantry position sliding window dynamic multileaf collimator fields
Authors: MacKenzie Marc A. , Robinson Donald M.
Published: November 2002

The intensity modulated arc has been proposed as an alternative to tomotherapy. Treatment planing systems more typically model the conventional step and shoot or sliding window dynamic multileaf collimator (DMLC) deliveries, and may not support intensity modulated arc therapy (IMAT). As well, another potential drawback to this technique is that increasing the number of intensity levels required to achieve certain dose distributions necessitates increasing the number of gantry passes, as may occur if the desired dose distribution is complex (e.g., concave or bifurcated), potentially increasing the overall treatment time. A technique is presented here for the delivery of tomotherapy like dose distributions in a single gantry pass by the use of a large number of fields modulated by a sliding window DMLC technique from fixed equally spaced gantry positions. This serves as a good approximation to either IMAT or tomotherapy deliveries. The planning of these fields is achieved using iterative filtered back projection. Measured results of deliveries of varying degrees of complexity on a homogeneous phantom are compared to desired distributions.
Matching PET and CT scans of the head and neck area: Development of method and validation
Authors: Klabbers Bianca M. , de Munck Jan C. , Slotman Ben J. , Langendijk Hans A. , de Bree Remco , Hoekstra Otto S. , Boellaard Ronald , Lammertsma Adriaan A.
Published: November 2002

Positron emission tomography (PET) provides important information on tumor biology, but lacks detailed anatomical information. Our aim in the present study was to develop and validate an automatic registration method for matching PET and CT scans of the head and neck. Three difficulties in achieving this goal are (1) nonrigid motions of the neck can hamper the use of automatic ridged body transformations; (2) emission scans contain too little anatomical information to apply standard image fusion methods; and (3) no objective way exists to quantify the quality of the match results. These problems are solved as follows: accurate and reproducible positioning of the patient was achieved by using a radiotherapy treatment mask. The proposed method makes use of the transmission rather than the emission scan. To obtain sufficient (anatomical) information for matching, two bed positions for the transmission scan were included in the protocol. A mutual information‐based algorithm was used as a registration technique. PET and CT data were obtained in seven patients. Each patient had two CT scans and one PET scan. The datasets were used to estimate the consistency by matching PET to to and to PET using the full circle consistency test. It was found that using our method, consistency could be obtained of 4 mm and 1.3° on average. The PET voxels used for registration were 5.15 mm, so the errors compared quite favorably with the voxel size. Cropping the images (removing the scanner bed from images) did not improve the consistency of the algorithm. The transmission scan, however, could potentially be reduced to a single position using this approach. In conclusion, the represented algorithm and validation technique has several features that are attractive from both theoretical and practical point of view, it is a user‐independent, automatic validation technique for matching CT and PET scans of the head and neck, which gives the opportunity to compare different image enhancements.
Clinical implementation of AAPM TG61 protocol for kilovoltage x‐ray beam dosimetry
Authors: Yoo Sua , Grimm Daniel , Zhu Ron , Jursinic Paul , Lopez Francisco , Rownd Jason , Gillin Michael
Published: November 2002

Historically, there have been a variety of dosimetry protocols used for kilovoltage x‐ray therapy beams with a set of conversion factors and correction factors taken from different references. Corresponding to the continued installation and use of kilovoltage machines, the American Association of Physicists in Medicine (AAPM) presented a unified protocol developed by Task Group 61 (TG61). TG61 determines the absorbed dose to water with an ionization chamber calibrated in air in terms of air kerma TG61 presents both an in‐air method and an in‐phantom method. In this work we only examine the TG61 in‐air method. Our traditional dosimetry procedure, which is based upon NCRP Report 69 and on material found in standard medical physics texts, has been compared to the TG61. A variety of kilovoltage beam energies were examined with a set of various field sizes and source to surface distances. TG61 published updated data for the mass absorption coefficient ratios, backscatter factors, and the average energy per ion pair factor. The following conclusions have been reached: (1) Our traditional procedures and the TG61 protocol for in‐air measurements are equivalent. (2) The conversion and correction factors used in TG61 are different by up to 4.5% compared to the old factors that we have used. (3) The application of the TG61 factors can result in up to 5% differences in the determination of the absorbed dose.
Evaluation of a miniature electromagnetic position tracker
Authors: Hummel Johann , Figl Michael , Kollmann Christian , Bergmann Helmar , Birkfellner Wolfgang
Published: November 2002

The advent of miniaturized electromagnetic digitizers opens a variety of potential clinical applications for computer aided interventions using flexible instruments; endoscopes or catheters can easily be tracked within the body. With respect to the new applications, the systematic distortions induced by various materials such as closed metallic loops, wire guides, catheters, and ultrasound scan heads were systematically evaluated in this paper for a new commercial tracking system. We employed the electromagnetic tracking system Aurora™ (Mednetix/CH, NDI/Can); data were acquired using the serial port of a PC running SuSE Linux 7.1 (SuSE, Gmbh, Nürnberg). Objects introduced into the digitizer volume included wire loops of different diameters, wire guides, optical tracking tools, an ultrasonic (US) scan head, an endoscope with radial ultrasound scan head and various other objects used in operating rooms and interventional suites. Beyond this, we determined the influence of a C‐arm fluoroscopy unit. To quantify the reliability of the system, the miniaturized sensor was mounted on a nonmetallic measurement rack while the transmitter was fixed at three different distances within the digitizer range. The tracker was shown to be more sensitive to distortions caused by materials close to the emitter (average distortion error 13.6 mm±16.6 mm for wire loops positioned at a distance between 100 mm and 200 mm from the emitter). Distortions caused by materials near the sensor (distances smaller than 100 mm) are small (typical error 2.2 mm±1.9 mm). The C‐arm fluoroscopy unit caused considerable distortions and limits the reliability of the tracker (distortion error 18.6 mm±24.9 mm). Distortions resulting from the US scan head are high at distances smaller than about 100 mm from the emitter. The distortions also increase when the scan head is positioned horizontally and close to the sensor (average error 4.1 mm±1.5 mm when the scan head is positioned within a distance of 100 mm from the sensor). The distortions are slightly higher when the ultrasound machine is switched on. We also evaluated the influence of common medical instruments on distance measurements. For these measurements the average deviation from the known distance of 200 mm amounted to 3.0 mm±1.5 mm (undistorted distance measurement 1.5 mm±0.3 mm). The deviations also depend on the relative orientation between emitter and sensor. The results demonstrate that the miniature tracking system opens up new perspectives with regard to surgery applications where a flexible instrument is to be tracked within the body. Significant distortions caused by metallic objects only occur in the worst cases, for example, in the presence of a closed, unisiolated wire loop or a C‐arm fluorescence unit close to the emitter and which can be avoided by suitable usage.
Dosimetry comparison of Sources
Authors: Papagiannis P. , Angelopoulos A. , Pantelis E. , Sakelliou L. , Baltas D. , Karaiskos P. , Sandilos P. , Vlachos L.
Published: November 2002

sources besides being widely utilized in the field of conventional brachytherapy also find use in contemporary peripheral and coronal intravascular applications. In this study, the same Monte Carlo simulation code and input data were used to investigate differences between the dose rate distributions of the most commonly used sources in the cm and mm distance range. Findings are discussed in view of differences in source and encapsulation dimensions as well as structural details. Results are presented in the AAPM TG‐43 formalism, as generalized by AAPM TG‐60, for five HDR source designs as well as an LDR seed and an LDR wire source. Dose rate constants of the sources at and were found proportional to the corresponding geometry factors along the transverse source bisectors and an equation of the form provides results within clinical accuracy (less than 2%) for any source. Radial dose functions do not depend significantly on source and encapsulation geometry and agree within 2% with that of a point source. Anisotropy is of importance for accurate dosimetry at the cm distance range but it does not affect dose rate in the mm distance range significantly. At such short radial distances the source geometry factor defines the shape of isodose lines. Dose uniformity at given distances from the sources is strongly dependent on source dimensions as indicated by dose rate profiles in polar and Cartesian coordinates.
Dosimetric aspects of a national survey of diagnostic and interventional radiology in Switzerland
Authors: Aroua A. , Decka I. , Burnand B. , Vader J.‐P. , Valley J.‐F.
Published: November 2002

The effective dose delivered to the patient was determined, by modeling, for 257 types of examinations covering the different modalities of diagnostic and interventional radiology. The basic operational dosimetric quantities considered were obtained from the parameters of the examinations on the basis of dosimetric models. These models required a precise characterization of each examination. The operational dosimetric quantities were converted into doses to organs and effective doses using appropriate conversion factors. The determination of the collective effective dose to the Swiss population requires a number of corrections to account for the variability of several parameters: sensitivity of the detection system, age, gender, and build of the patient. The use of various dosimetric models is illustrated in this paper for a limited number of examination types covering the different radiological modalities, for which the established typical effective doses are given. With regard to individual doses, the study indicated that the average effective doses per type of examination can be classified into three levels: (a) the weakly irradiating examinations (less than 0.1 mSv), which represent 78% of the examinations and 4% of the collective dose, (b) the moderately irradiating examinations (between 0.1 mSv and 10 mSv), which represent 21% of the examinations and 72% of the collective dose, (c) the strongly irradiating examinations (more than 10 mSv), which represent 1% of the examinations and 24% of the collective dose.
Choice of phantom materials for dosimetry of Leksell Gamma Knife unit: A Monte Carlo study
Authors: Cheung Joel Y. C. , Yu K. N. , Yu C. P. , Ho Robert T. K.
Published: November 2002

In calculations for the Leksell Gamma Knife, GammaPlan employs a tissue equivalent material without the presence of a skull bone, while dosimetry work is based on a polystyrene phantom. The compatibility of these dose distributions is uncertain. The Monte Carlo technique was employed to determine the radial dose distributions from a single 14 mm collimator helmet in 160 mm diam phantoms with different materials. The materials studied were polystyrene, perspex, water, and water with skull bone. Results showed no significant differences among the radial doses in different phantom materials for the 14 mm collimator helmet. The Monte Carlo simulation was repeated with the inclusion of all 201 sources. Again, no significant differences were observed.
Parameter dependence of the MCNP electron transport in determining dose distributions
Authors: Reynaert N. , Palmans H. , Thierens H. , Jeraj R.
Published: November 2002

In this paper, a detailed study of the electron transport in MCNP is performed, separating the effects of the energy binning technique on the energy loss rate, the scattering angles, and the sub‐step length as a function of energy. As this problem is already well known, in this paper we focus on the explanation as to why the default mode of MCNP can lead to large deviations. The resolution dependence was investigated as well. An error in the MCNP code in the energy binning technique in the default mode (DBCN 18 was revealed, more specific in the updating of cross sections when a sub‐step is performed corresponding to a high‐energy loss. This updating error is not present in the ITS mode (DBCN 18 and leads to a systematically lower dose deposition rate in the default mode. The effect is present for all energies studied (0.5–10 MeV) and depends on the geometrical resolution of the scoring regions and the energy grid resolution. The effect of the energy binning technique is of the same order of that of the updating error for energies below 2 MeV, and becomes less important for higher energies. For a 1 MeV point source surrounded by homogeneous water, the deviation of the default MCNP results at short distances attains 9% and remains approximately the same for all energies. This effect could be corrected by removing the completion of an energy step each time an electron changes from an energy bin during a sub‐step. Another solution consists of performing all calculations in the ITS mode. Another problem is the resolution dependence, even in the ITS mode. The higher the resolution is chosen (the smaller the scoring regions) the faster the energy is deposited along the electron track. It is proven that this is caused by starting a new energy step when crossing a surface. The resolution effect should be investigated for every specific case when calculating dose distributions around beta sources. The resolution should not be higher than where EFAC is the energy loss per energy step and CSDA a continuous slowing down approximation range. This effect could as well be removed by determining the cross sections for energy loss and multiple scattering at the average energy of an energy step and by sampling the cross sections for each sub‐step. Overall, we conclude that MCNP cannot be used without a caution due to possible errors in the electron transport. When care is taken, it is possible to obtain correct results that are in agreement with other Monte Carlo codes.
The effect of set‐up uncertainties, contour changes, and tissue inhomogeneities on target dose‐volume histograms
Authors: Cho B. C. John , van Herk Marcel , Mijnheer Ben J. , Bartelink Harry
Published: November 2002

Understanding set‐up uncertainty effects on dose distributions is an important clinical problem but difficult to model accurately due to their dependence on tissue inhomogeneities and changes in the surface contour (i.e., variant effects). The aims are: (1) to evaluate and quantify the invariant and variant effects of set‐up uncertainties, contour changes and tissue inhomogeneities on target dose–volume histograms (DVHs); (2) to propose a method to interpolate (variant) DVHs. We present a lung cancer patient to estimate the significance of set‐up uncertainties, contour changes and tissue inhomogeneities in target DVHs. Differential DVHs are calculated for 15 displacement errors (with respect to the isocenter) using (1) an invariant shift of the dose distribution at the isocenter, (2) a full variant calculation, and (3) a B‐spline interpolation applied to sparsely sampled variant DVHs. The collapsed cone algorithm was used for all dose calculations. Dosimetric differences are quantified with the root mean square (RMS) deviation and the equivalent uniform dose (EUD). To determine set‐up uncertainty effects, weighted mean EUDs, assuming normally distributed displacement errors, are used. The maximum absolute difference and RMS deviation in the integral DVHs' relative dose between (1) the invariant and calculated curves are 65.2% and 5.8% and (2) the interpolated and calculated curves are 16.9% and 2.5%. Similarly, the maximum absolute difference and RMS deviation in mean EUD as a function of the set‐up uncertainty's standard deviation between (1) the invariant and calculated curves are 0.02 and 0.01 Gy; and (2) the interpolated and calculated curves are 0.01 and 0.006 Gy. Since a “worst‐case” example is selected, we conclude that, in the majority of clinical cases, the variant effects of contour changes, tissue inhomogeneities and set‐up uncertainties on EUD are negligible. Interpolation is a valid, efficient method to approximate DVHs.
Energy‐loss straggling algorithms for Monte Carlo electron transport
Authors: Chibani Omar
Published: November 2002

A new method is presented for the modeling of the electron (positron) energy‐loss straggling in Monte Carlo transport simulations. First, the Vavilov energy‐loss distribution is calculated for electrons and positrons using the Møller and Bhabha collision cross‐sections, respectively. The maximum energy transfer in a single collision is considered as variable. Binding effects from low‐energy collisions are modeled using the Blunck and Westphal model. Secondly, new algorithms are developed to fit the Vavilov distribution. These algorithms are based on the first three moments of the energy‐loss distribution. They are suitable for rapid random sampling of the energy loss. The new algorithms are validated against the Vavilov distribution for electrons and positrons, water and lead, kinetic energy of 0.1, 1, and 10 MeV and several values of (10, 50, 100, and 200 keV). The developed algorithms are incorporated in a new version of the GEPTS Monte Carlo code called GEPTSIII. Collisions involving energy transfers larger than are simulated individually and the energy loss due to soft collisions (energy transfers less than is sampled using the new algorithms. The straggling effect is therefore taken into account whatever the chosen value. GEPTSIII and EGSnrc are used for the calculation of (1) electron dose distributions in water and (2) energy spectra for electrons passing through water and tungsten slabs. Electron beams of 1, 2, 5, 10, and 20 MeV along with varying values are considered. Electron dose distributions in water are rather insensitive to the soft collision straggling. The use of the new algorithms results in a slight gain in computation time when relatively large values are used (e.g., for 10 MeV electrons). However, the calculation of electron energy spectra is very sensitive to the soft collision straggling. GEPTSIII is about 5 and 11 times faster than EGSnrc for the case of 2 and 20 MeV electrons passing through 0.025 and 0.25 cm water slabs, respectively. Contrary to EGSnrc, GEPTSIII accounts for the energy‐spectrum broadening due to the binding effects. The resulting differences between the two codes are significant for 5 and 10 MeV electrons passing through a 0.01 cm tungsten slab. Gains in GEPTSIII computation times (∼a factor 5) are also observed for tungsten. In short, GEPTSIII provides significant advantages (rapidity and accuracy) for electron transport simulations, especially those dealing with energy‐spectrum calculations, as encountered in clinical electron beam modeling studies. In other respects, the developed approach is more suitable than class‐II codes for the use of accurate electron cross sections (numerical data) at low energy (<100 keV).