bluuebunny/crossref_metadata_2025 huggingface dataset:
State Space Implementation of the Algorithm of Mode Isolation
Authors: Drexel Michael V. , Ginsberg Jerry H. , Zaki Bassem R.
Published: April 2003

The Algorithm of Mode Isolation (AMI) extracts modal properties from complex frequency response data. Previous work used classic undamped modes as the analytical framework for the algorithm. The present work extends the algorithm to implement damped modal analysis, in which the eigenvalues and eigenvectors are complex. In order to assess how well this reformulation performs when natural frequencies are close and drive point mobilities are low, a prototypical system consisting of a cantilever beam with attached subsystems is introduced. One of these subsystems is selected to be a tuned vibration absorber for the isolated beam, so the system features a combination of modes whose natural frequencies are close and modes whose drive point mobility is low. The time domain response of this system is evaluated, contaminated with substantial white noise, and then FFT processed in order to obtain synthetic complex frequency response data. The performance of AMI is evaluated by comparing extracted values for natural frequency, modal damping ratio, and complex normal mode vectors to the analytical values. The results reveal that the pair of modes having proximite natural frequencies are accurately identified. Natural frequencies and damping ratios for those modes whose drive mobility is low are identified by processing the ensemble of frequency response functions, but identification of normal mode coefficients for such modes remains problematic.
Active Monitoring for On-Line Damage Detection in Composite Structures
Authors: Yuan Shenfang , Lei Wang , Shi Lihua
Published: April 2003

This study focuses on an active monitoring method for damage detection applied to composite structures. Honeycomb sandwich and carbon fiber composite structures are studied. Two kinds of damage are considered: delamination and impact damage. Wavelet analysis methods are adopted to postprocess the raw monitored signal. A new damage signature is introduced to determine the presence and extent of damage in composites, while eliminating the influence of different distances between the active actuator and active monitoring elements. The proposed method is shown to be effective, reliable, and straightforward for the specimens considered in the present study, which are composed of different materials and suffer various levels of damage. An online real-time active monitoring system for damage detection is described that is based on this research.
Time Domain Identification of Moving Loads on Bridge Deck
Authors: Zhu X. Q. , Law S. S.
Published: April 2003

A new time domain method is presented to identify moving loads on a bridge deck based on the measured responses. The bridge deck is modeled as an orthotropic plate and the loads are modeled as a group of four loads moving on top of the bridge deck at fixed distance apart. Dynamic behavior of the bridge deck is analyzed by the orthotropic plate theory and mode superposition technique. Like all inverse problems, this identification is an ill-conditioned problem, and a regularization technique is employed to stabilize the computations. The identified loads moving at different eccentricities are presented. Laboratory work on the force identification is also presented. The effect of incomplete measured modes in the responses is discussed, and an underestimation in the loads may result if the number of vibration mode for identification is larger than that in the responses. Computational simulations and laboratory tests show that the method is effective and practical for identification of individual wheel loads on bridge decks.
Compatibility Equations in the Theory of Elasticity
Authors: Andrianov Igor V. , Awrejcewicz Jan
Published: April 2003

It is shown by operational method that the boundary value problem of the theory of elasticity related to stresses, which can be reduced to three strains compatibility equations and to three equilibrium equations, in fact is of sixth order. Hence, it is not required to formulate additional boundary conditions.
A Novel Semi-Active Multi-Modal Vibration Control Law for a Piezoceramic Actuator
Authors: Corr Lawrence R. , Clark William W.
Published: April 2003

In this paper, a novel semi-active energy rate multi-modal vibration control technique is developed for a piezoceramic actuator coupled to a switching resistor/inductor shunt. The technique works by briefly connecting a resistor/inductor shunt to a piezoceramic actuator in order to apply the necessary signed charge to allow energy dissipation. The switch timing is determined by a control scheme that observes the rate of energy change in controlled modes. The control scheme is developed in the paper, and is simplified to enable practical implementation. This new multi-modal control law is applied to both a simple numerical and an experimental test structure. The results from the numerical and experimental tests show that the energy rate multi-mode control law is able to dissipate energy from one, two and three modes of the flexible structures using a single actuator.
Damping in High Transient Motion
Authors: Al Majid A. , Dufour R.
Published: April 2003

An original method for evaluating the dissipative effect in SDOF systems due to the transient phenomenon was presented in a previous article. This method based on the use of an additional dimension, and the general relativity concept was validated experimentally. However, the function of the forcing frequency required to establish the metric of the space was identified using an experimental transfer function. In the present paper the main objective is to solve the geodesic equations in order to avoid the experimental identification of the function contained in the metric. The variational problem of the metric of Riemannian space gives three geodesic equations for the SDOF system studied. Solving these equations gives in particular the transient forced response which, when compared with experimental results, permits validating the proposed method and therefore proving that the transient motion bends the space-time.
An Investigation of Coupled van der Pol Oscillators
Authors: Low Lesley Ann , Reinhall Per G. , Storti Duane W.
Published: April 2003

In this paper we present findings from an investigation of synchronization of linearly diffusively coupled van der Pol oscillators. The stability boundary of the in-phase mode of two identical oscillators in terms of the two coupling parameters is determined numerically. We show that in addition to the out-of-phase and in-phase motions of the oscillators there exist two other phase-locked motions and behavior that appears chaotic. The effect of detuning the oscillators from each other is also presented. Finally, the analytical and numerical results from an investigation of the in-phase mode system of n coupled oscillators is presented.
The Impact of Moments Near Discontinuities in Structural-Acoustic Systems
Authors: Shepard, W. Steve
Published: April 2003

The impact of a moment excitation on the vibrational and acoustical response of a fluid-loaded system containing a structural discontinuity is examined. The response metric of interest is the acoustic power radiated from a semi-infinite plate supported in a rigid baffle. The impact of a moment excitation located near a locally reacting distributed-mass discontinuity as well as near the simple supports is examined. Numerical results show that the addition of a moment component can have a dramatic impact on the radiated acoustic power. Both the magnitude and phase of the moment relative to the force can impact the radiated acoustic power.
Propagation of Sound in Inhomogeneous Media: Exact, Transient Solutions in Curvilinear Geometries
Authors: Subrahmanyam P. Bala , Sujith R. I. , Lieuwen Tim C.
Published: April 2003

Exact solutions for one-dimensional, transient acoustic wave propagation in curvilinear geometries with mean temperature variations are presented in this paper. These solutions are obtained by using a transformation of the spatial and acoustic variables in a manner suggested by the WKB approximation. The analysis is performed for spherical and cylindrical co-ordinate systems. Temperature profiles admitting exact traveling wave type solutions are derived. Although these solutions resemble the approximate, “high frequency,” WKB form of solution of the wave equation, they have the interesting property that they are exact, regardless of the scale of the acoustic disturbance relative to that of the inhomogeneity. Calculations showing the propagation of waves in cylindrical and spherical geometries are presented.
Free Flexural Vibrations of Orthotropic Composite Base Plates or Panels With a Bonded Noncentral (or Eccentric) Stiffening Plate Strip
Authors: Yuceoglu U. , O¨zerciyes V.
Published: April 2003

The problem of the free flexural (or bending) vibrations of a rectangular, composite base plate or panel stiffened by a bonded, noncentral stiffening plate strip is considered. The lower composite base plate and the upper stiffening plate strip are assumed as dissimilar Mindlin Plates connected by a very thin and deformable adhesive layer. In the formulation, the entire composite system is considered to have simply supported edges in one direction while the other two edges may have arbitrary boundary conditions. The set of governing partial differential equations is reduced to a “special form” of a system of the first order ordinary differential equations. Then, they are integrated by the “Modified Transfer Matrix Method (with Interpolation Polynomials).” The mode shapes and the natural frequencies of the composite system are investigated and presented in detail for several boundary conditions. It was also found that the “hardness” and the “softness” of the in-between adhesive layer have significant effects on the mode shapes and the natural frequencies.
Quantifying the effect of intrafraction motion during breast IMRT planning and dose delivery
Authors: George R. , Keall P. J. , Kini V. R. , Vedam S. S. , Siebers J. V. , Wu Q. , Lauterbach M. H. , Arthur D. W. , Mohan R.
Published: April 2003

Respiratory motion during intensity modulated radiation therapy (IMRT) causes two types of problems. First, the clinical target volume (CTV) to planning target volume (PTV) margin needed to account for respiratory motion means that the lung and heart dose is higher than would occur in the absence of such motion. Second, because respiratory motion is not synchronized with multileaf collimator (MLC) motion, the delivered dose is not the same as the planned dose. The aims of this work were to evaluate these problems to determine (a) the effects of respiratory motion and setup error during breast IMRT treatment planning, (b) the effects of the interplay between respiratory motion and multileaf collimator (MLC) motion during breast IMRT delivery, and (c) the potential benefits of breast IMRT using breath‐hold, respiratory gated, and 4D techniques. Seven early stage breast cancer patient data sets were planned for IMRT delivered with a dynamic MLC (DMLC). For each patient case, eight IMRT plans with varying respiratory motion magnitudes and setup errors (and hence CTV to PTV margins) were created. The effects of respiratory motion and setup error on the treatment plan were determined by comparing the eight dose distributions. For each fraction of these plans, the effect of the interplay between respiratory motion and MLC motion during IMRT delivery was simulated by superimposing the respiratory trace on the planned DMLC leaf motion, facilitating comparisons between the planned and expected dose distributions. When considering respiratory motion in the CTV‐PTV expansion during breast IMRT planning, our results show that PTV dose heterogeneity increases with respiratory motion. Lung and heart doses also increase with respiratory motion. Due to the interplay between respiratory motion and MLC motion during IMRT delivery, the planned and expected dose distributions differ. This difference increases with respiratory motion. The expected dose varies from fraction to fraction. However, for the seven patients studied and respiratory trace used, for no breathing, shallow breathing, and normal breathing, there were no statistically significant differences between the planned and expected dose distributions. Thus, for breast IMRT, intrafraction motion degrades treatment plans predominantly by the necessary addition of a larger CTV to PTV margin than would be required in the absence of such motion. This motion can be limited by breath‐hold, respiratory gated, or 4D techniques.
Modeling the anestheto‐dynamic phase transition of the cerebral cortex
Authors: Steyn‐Ross D. Alistair
Published: April 2003

This thesis examines a stochastic model for the electrical behavior of the cerebral cortex under the influence of a general anesthetic agent. The modeling element is the macrocolumn, an organized assembly of cooperating neurons within a small cylindrical volume of the cortex. The state variables are and the mean‐field average soma voltages for the populations of excitatory and inhibitory neurons comprising the macrocolumn; the random fluctuations of about steady state are taken as the source of the scalp‐measured EEG (electroencephalogram) signal. The effect of GABAergic anesthetic (e.g., propofol) is incorporated as a lengthening of the inhibitory postsynaptic impulse response. After performing a linear stability analysis of the anesthetic‐determined distribution of soma voltage steady states, we obtain a model prediction of a pair of first‐order discontinuities in soma voltage occurring at hysteretically separated critical values of anesthetic concentration: the first jump corresponding to induction (loss of consciousness), and the second corresponding to emergence (return of consciousness). Significant changes in EEG characteristics are predicted in the vicinity of both critical points: (1) strong increase in cortical power (“biphasic” effect); (2) pronounced spectral shift toward lower frequencies with concomitant decrease in spectral entropy; (3) pronounced increase in correlation time of EEG fluctuations (“critical slowing down”). There is excellent clinical support for all three predictions. When NMDA voltage‐gated receptors are included in the model, we find a prediction of a double biphasic peak for a slow GABAergic induction of anesthesia, and this finding has also been confirmed in a recent report in the anesthesiology literature.
Are we doing any good by doing really well? (Where's the Bacon?)
Authors: Herbert Donald
Published: April 2003

Francis Bacon, who with Rene Decartes laid the intellectual foundations for Western science in the seventeenth century, asserted that the purpose of all knowledge is “action in the production of works for…the relief of man's estate.” We assess briefly several aspects of a few of the current efforts directed to the production of such “works” with respect to such “relief ” as they may provide: cancer mortality, the medical literature, evidence‐based medicine, clinical trials, observational databases and criteria for the promotion and tenure of the medical faculty. We suggest why each of these efforts appears to have failed to some degree and then propose some measures that may possibly serve as correctives.
Comparison of the efficacy of intensity modulated radiotherapy delivered by competing technologies
Authors: Seco Joao Carlos
Published: April 2003

The project involved the study and comparison of the various intensity‐modulated radiation therapy (IMRT) delivery techniques. IMRT can be delivered via (i) the NOMOS MIMiC tomotherapy device, (ii) the dynamic multileaf collimator (DMLC), and (iii) the technique of multiple‐static fields (MSF) using a multileaf collimator (MLC). To evaluate the relative benefits and limitations of the different methods of delivering IMRT an inverse‐planning simulation code was developed. The simulation uses two distinct beam models: (a) the PEACOCK pencil‐beam model based on the double Gaussian convolution for the MIMiC, and (b) the macropencil beam model (with the extended source model included to correct for the output factor) which is used for the DMLC and MSF‐MLC delivery techniques. The process of delivering an IMRT treatment may involve various beam‐modifying techniques such as multileaf collimators, the NOMOS MIMiC, blocks, wedges, etc. The constraints associated with the IMRT delivery technique are usually neglected in the process of obtaining the “optimal” inverse treatment plan. Consequently, dose optimization may be significantly reduced when the “optimal” beam profiles are converted to leaf/diaphragm positions via a leaf‐sequencing interpreter. The work developed assessed the effects on the optimum treatment plan of the following leaf‐sequencing algorithms: MSF‐MLC, DMLC, and NOMOS MIMiC. An increase of 2.5%, 3.7% and 5.7% was observed for the PTV dose, when delivering a fluence profile with the DMLC, MSF, and NOMOS MIMiC techniques, respectively. An intensity‐modulated beam optimization algorithm was developed to incorporate the delivery constraints into the optimization cycle. The optimization algorithm was based on the quasi‐Newton method of iteratively solving minimization problems. The developed algorithm iteratively corrects the incident, pencil‐beam‐like fluence to incorporate the delivery constraints. In the case of the DMLC and MSF the optimization converged within 3–5 iterations to a mean PTV dose of 69.60±1.34 and 69.71±1.34 Gy, respectively, while for NOMOS MIMiC approximately 10 iterations were needed to obtain 69.68±1.55 Gy. Additionally, the effect of imposing “smooth” intensity modulated beam constraints on the inverse planning was studied for the various delivery techniques.
Photon beam relative dose validation of the DPM Monte Carlo code in lung‐equivalent media
Authors: Chetty Indrin J. , Charland Paule M. , Tyagi Neelam , McShan Daniel L. , Fraass Benedick A. , Bielajew Alex F.
Published: April 2003

Validation experiments have been conducted using 6 and 15 MV photons in inhomogeneous (water/lung/water) media to benchmark the accuracy of the DPM Monte Carlo code for photon beam dose calculations. Small field sizes (down to and low‐density media were chosen for this investigation because the intent was to test the DPM code under conditions where lateral electronic disequilibrium effects are emphasized. The treatment head components of a Varian 21EX linear accelerator, including the jaws (defining field sizes of 2×2, 3×3 and 10×10 were simulated using the BEAMnrc code. The phase space files were integrated within the DPM code system, and central axis depth dose and profile calculations were compared against diode measurements in a homogeneous water phantom in order to validate the phase space. Results of the homogeneous phantom study indicated that the relative differences between DPM calculations and measurements were within ±1% (based on the rms deviation) for the depth dose curves; relative profile dose differences were on average within ±1%/1 mm. Depth dose and profile measurements were carried out using an ion‐chamber and film, within an inhomogeneous phantom consisting of a 6 cm slab of lung‐equivalent material embedded within solid water. For the inhomogeneous phantom experiment, DPM depth dose calculations were within ±1% (based on the rms deviation) of measurements; relative profile differences at depths within and beyond the lung were, on average, within ±2% in the inner and outer beam regions, and within 1–2 mm distance‐to‐agreement within the penumbral region. Relative point differences on the order of 2–3% were within the estimated experimental uncertainties. This work demonstrates that the DPM Monte Carlo code is capable of accurate photon beam dose calculations in situations where lateral electron disequilibrium effects are pronounced.
Thresholds for detecting and characterizing focal lesions using steady‐state MR elastography
Authors: Doyley Marvin M. , Weaver John B. , Van Houten Elijah E. W. , Kennedy Francis E. , Paulsen Keith D.
Published: April 2003

An objective contrast‐detail analysis was performed in this study to assess the low contrast detectability of a clinical prototype harmonic magnetic resonance elastographic imaging system. Elastographic imaging was performed on gelatin phantoms containing spherical inclusions of varying size and modulus contrast. The results demonstrate that lesions as small as 5 mm can be detected with a minimum modulus contrast of 14 dB. However, the shear modulus of such small lesions was not accurately recovered. In general, the shear modulus of larger focal lesions was accurately (i.e., within 25% of the true value) recovered. The minimum modulus contrast needed to detect focal lesions was observed to decrease with increasing lesion size.
Image reconstruction with shift‐variant filtration and its implication for noise and resolution properties in fan‐beam computed tomography
Authors: Pan Xiaochuan , Yu Lifeng
Published: April 2003

In computed tomography (CT), the fan‐beam filtered backprojection (FFBP) algorithm is used widely for image reconstruction. It is known that the FFBP algorithm can significantly amplify data noise and aliasing artifacts in situations where the focal lengths are comparable to or smaller than the size of the field of measurement (FOM). In this work, we propose an algorithm that is less susceptible to data noise, aliasing, and other data inconsistencies than is the FFBP algorithm while retaining the favorable resolution properties of the FFBP algorithm. In an attempt to evaluate the noise properties in reconstructed images, we derive analytic expressions for image variances obtained by use of the FFBP algorithm and the proposed algorithm. Computer simulation studies are conducted for quantitative evaluation of the spatial resolution and noise properties of images reconstructed by use of the algorithms. Numerical results of these studies confirm the favorable spatial resolution and noise properties of the proposed algorithm and verify the validity of the theoretically predicted image variances. The proposed algorithm and the derived analytic expressions for image variances can have practical implications for both estimation and detection/classification tasks making use of CT images, and they can readily be generalized to other fan‐beam geometries.
Head scatter off‐axis for megavoltage x rays
Authors: Zhu Timothy C. , Bjärngard Bengt E.
Published: April 2003

Our objective in this study has been to investigate how head scatter varies with the off‐axis position in a 6 MV x‐ray beam. We define the head‐scatter off‐axis ratio, HOA, as the ratio of the kerma due to head‐scatter photons at the off‐axis position x to the kerma from direct primary photons on the central axis. “Direct primary” are those photons that come from the source without interactions in the intervening structures. We determined HOA from measurements with an ionization chamber in a miniphantom. Head‐scatter and direct primary photons contribute to a measurement of the ionization per mu at the off‐axis position x in the open field The ionization per mu measured in the same position but with the field collimated to the smallest possible opening is intended to include only direct primary photons. Head‐scatter photons cannot be completely eliminated, and the errors due to remaining head scatter and radiation backscattered by the movable collimators into the monitor were estimated. For normalization of the final results, ionization due to direct primary photons was also measured on the central axis, HOA was derived from these three measurements as On the central axis HOA represents the “scatter‐to‐primary ratio” between head scatter and the direct primary dose. Monte Carlo simulations were made to help with the interpretation and evaluation of the results. HOA could be fitted to a Gaussian model with two components corresponding to sources of widths 1.8 and 14 cm, projected on a plane 5 cm below the x‐ray source. The narrow Gaussian component is interpreted as the source of photons scattered in the flattening filter and the primary collimator. The broad component is attributed to photons scattered in the secondary (variable) collimators. Conventional head‐scatter models (e.g., a single Gaussian source model) do not fit the measured HOA data for large collimator settings or outside beam collimation. The full width at half‐maximum (FWHM) of across the field increased with the field width (cx) in the direction of the measurements in a manner consistent with the field of view of the two sources. It was not sensitive to the field measure in the orthogonal direction (cy). Head scatter outside the field also increased with field size, reflecting an increased contribution of photons scattered at large angles. It exceeds the leakage through the collimator 2 cm outside the edge for square fields Monte Carlo calculations showed considerably less head scatter outside the field than measurements.
IMRT dose shaping with regionally variable penalty scheme
Authors: Cotrutz Cristian , Xing Lei
Published: April 2003

A commonly known deficiency of currently available inverse planning systems is the difficulty in fine‐tuning the final dose distribution. In practice, it is not uncommon that just a few unsatisfactory regions in the planning target volume or an organ at risk prevent an intensity modulated radiation therapy (IMRT) plan from being clinically acceptable. The purpose of this work is to introduce a mechanism for controlling the regional doses after a conventional IMRT plan is obtained and to demonstrate its clinical utility. Two types of importance factors are introduced in the objective function to model the tradeoffs of different clinical objectives. The first is the conventional structure‐dependent importance factor, which quantifies the interstructure tradeoff. The second type is the voxel‐dependent importance factor which “modulates” the importance of different voxels within a structure. The planning proceeds in two major steps. First a conventional inverse planning is performed, where the structure‐dependent importance factors are determined in a trial‐and‐error fashion. The next level of planning involves fine‐tuning the regional doses to meet specific clinical requirements. To achieve this, the voxels where doses need to be modified are identified either graphically on the isodose layouts, or on the corresponding dose‐volume histogram (DVH) curves. The importance value of these voxels is then adjusted to increase/decrease the penalty at the corresponding regions. The technique is applied to two clinical cases. It was found that both tumor hot spots and critical structure maximal doses can be easily controlled by varying the regional penalty. One to three trials were sufficient for the conventionally optimized dose distributions to be adjusted to meet clinical expectation. Thus introducing the voxel‐dependent penalty scheme provides an effective means for IMRT dose distributions painting and sculpting.
Evaluation of a flat panel digital radiographic system for low‐dose portable imaging of neonates
Authors: Samei Ehsan , Hill Jeanne G. , Frey G. Donald , Southgate W. Michael , Mah Eugene , Delong David
Published: April 2003

The purpose of this study was to evaluate the clinical utility of an investigational flat‐panel digital radiography system for low‐dose portable neonatal imaging. Thirty image‐pairs from neonatal intensive care unit patients were acquired with a commercial Computed Radiography system (Agfa, ADC 70), and with the investigational system (Varian, Paxscan 2520) at one‐quarter of the exposure. The images were evaluated for conspicuity and localization of the endings of ancillary catheters and tubes in two observer performance experiments with three pediatric radiologists and three neonatologists serving as observers. The results indicated no statistically significant difference in diagnostic quality between the images from the investigational system and from CR. Given the investigational system's superior resolution and noise characteristics, observer results suggest that the high detective quantum efficiency of flat‐panel digital radiography systems can be utilized to decrease the radiation dose/exposure to neonatal patients, although post‐processing of the images remains to be optimized. The rapid availability of flat‐panel images in portable imaging was found to be an added advantage for timely clinical decision‐making.
The kinetic analysis of metastatic tumor formation and its clinical relevance
Authors: Lee Steve Pai‐hsun
Published: April 2003

Knowledge regarding the kinetics of metastatic tumor formation, as related to the growth of the primary tumor, represents a fundamental issue in cancer biology with significant clinical relevancy. By using an in vivo mammalian model, this work showed that one could obtain useful information from the frequency distribution of the sizes of metastatic colonies in distant organs after serial sectioning and image reconstruction. Tumor cells were implanted into the mice subcutaneously, and the subsequent presence of metastases in the lungs at various time points was assessed and their individual colony size measured. To explain the experimental finding, a quantitative model was constructed based on the growth patterns of the primary tumor and metastases and a stochastic process of metastatic colony formation. With this approach, the mechanistic parameters governing the process of metastatogenesis in the lungs for two murine tumor cell lines (KHT and MCaK) were determined. It was found that the elementary assumption of exponential forms of growth for the primary tumor and metastatic colonies predicts a linear relation on a log–log plot of a metastatic colony size distribution, which was consistent with the experimental results. The observed metastasis‐free probability also supported the assumption of a time‐dependent Poisson process of metastatic formation. More complicated tumor growth patterns, e.g., Gompertzian and logistic models, were included in the mathematical construct, with the attempt to explain possible deviation from the log–log linear relation. Heterogeneous distributions of various biological parameters were also introduced. After showing that the biological parameters in the model could be obtained in the laboratory (thus a workable metastatic “assay”), the theoretical model was expanded to interpret some clinically assessable quantities encountered in cancer medicine. These included metastasis‐free survival probability (derived by assuming certain limiting detection resolution of clinical diagnostic technology) and dose‐dependent control probability by a cytotoxic agent (e.g., radiation therapy). In this way, a more rational basis in designing treatment regimens for subclinical metastases could be established. Finally, ways to improve the efficiency of the metastatic “assay” developed in this work were explored, using sampling technique and the mathematics of stereology.
Application of measured pencil beam parameters for electron beam model evaluation
Authors: Higgins Patrick D. , Gerbi Bruce J. , Khan Faiz M.
Published: April 2003

Most current electron beam models, as are used in commercial treatment planning systems, combine measured broad beam central axis depth dose data with measured or modeled functions to approximate radial scatter and heterogeneity effects. In this paper, we extend a recently developed pencil beam model to calculate doses outside the field edge and doses in heterogeneous media. We have also explored use of this model as a tool for evaluating commercial electron planning programs. The algorithm we have developed, based on the concept of the lateral buildup ratio (LBR), enables calculation of dose at any point in an irregular electron field, and is capable of generating both on‐ and off‐axis depth dose curves and isodose profiles. This model includes the effects of density and mass‐angular scattering power in measured broad beam central axis depth dose data, which when combined with small field reference data, can be used to generate LBR ratios. From these ratios one can infer the depth dependent, effective pencil beam radial spread parameter σ in water or other materials, which can be used to model any arbitrary field. We have used this approach to calculate fractional depth doses for small fields incident on aluminum and cork, which we have then compared against measurements and the calculations of several commercial planning systems.
Quantifying the predictability of diaphragm motion during respiration with a noninvasive external marker
Authors: Vedam S. S. , Kini V. R. , Keall P. J. , Ramakrishnan V. , Mostafavi H. , Mohan R.
Published: April 2003

The aim of this work was to quantify the ability to predict intrafraction diaphragm motion from an external respiration signal during a course of radiotherapy. The data obtained included diaphragm motion traces from 63 fluoroscopic lung procedures for 5 patients, acquired simultaneously with respiratory motion signals (an infrared camera‐based system was used to track abdominal wall motion). During these sessions, the patients were asked to breathe either (i) without instruction, (ii) with audio prompting, or (iii) using visual feedback. A statistical general linear model was formulated to describe the relationship between the respiration signal and diaphragm motion over all sessions and for all breathing training types. The model parameters derived from the first session for each patient were then used to predict the diaphragm motion for subsequent sessions based on the respiration signal. Quantification of the difference between the predicted and actual motion during each session determined our ability to predict diaphragm motion during a course of radiotherapy. This measure of diaphragm motion was also used to estimate clinical target volume (CTV) to planning target volume (PTV) margins for conventional, gated, and proposed four‐dimensional (4D) radiotherapy. Results from statistical analysis indicated a strong linear relationship between the respiration signal and diaphragm motion over all sessions, irrespective of session number and breathing training type Using model parameters obtained from the first session, diaphragm motion was predicted in subsequent sessions to within 0.1 cm (1 σ) for gated and 4D radiotherapy. Assuming a 0.4 cm setup error, superior–inferior CTV–PTV margins of 1.1 cm for conventional radiotherapy could be reduced to 0.8 cm for gated and 4D radiotherapy. The diaphragm motion is strongly correlated with the respiration signal obtained from the abdominal wall. This correlation can be used to predict diaphragm motion, based on the respiration signal, to within 0.1 cm (1 σ) over a course of radiotherapy.
Optical‐CT gel‐dosimetry I: Basic investigations
Authors: Oldham Mark , Siewerdsen Jeffrey H. , Kumar Sai , Wong John , Jaffray David A.
Published: April 2003

Comprehensive verification of the intricate dose distributions associated with advanced radiation treatments is now an immediate and substantial problem. The task is challenging using traditional dosimeters because of restrictions to point measurements (ion chambers, diodes, TLD, etc.) or planar measurements (film). In essence, rapid advances in the technology to deliver radiation treatments have not been paralleled by corresponding advances in the ability to verify these treatments. A potential solution has emerged in the form of water equivalent three dimensional (3D) gel‐dosimetry. In this paper we present basic characterization and performance studies of a prototype optical‐CT scanning system developed in our laboratory. An analysis of the potential role or scope of gel dosimetry, in relation to other dosimeters, and to verification across the spectrum of therapeutic techniques is also given. The characterization studies enabled the determination of nominal operating conditions for optical‐CT scanning. “Finger” phantoms are introduced as a powerful and flexible tool for the investigation of optical‐CT performance. The modulation‐transfer function (MTF) of the system is determined to be better than 10% out to 1 mm−1, confirming sub‐mm imaging ability. System performance is demonstrated by the acquisition of a 1×1×1 mm3 dataset through the dose distribution delivered by an x‐ray lens that focuses x rays in the energy range 40–80 KeV. This 3D measurement would be extremely difficult to achieve with other dosimetry techniques and highlights some of the strengths of gel dosimetry. Finally, an optical Monte Carlo model is introduced and shown to have potential to model light transport through gel‐dosimetry systems, and to provide a tool for the study and optimization of optical‐CT gel dosimetry. The model utilizes Mie scattering theory and requires knowledge of the variation of the particle size distribution with dose. The latter was determined here using the technique of dynamic‐light‐scattering.
Review of the energy check of an electron‐only linear accelerator over a 6 year period: Sensitivity of the technique to energy shift
Authors: Biggs Peter J.
Published: April 2003

The calibration and monthly QA of an electron‐only linear accelerator dedicated to intra‐operative radiation therapy has been reviewed. Since this machine is calibrated prior to every procedure, there was no necessity to adjust the output calibration at any time except, perhaps, when the magnetron is changed, provided the machine output is reasonably stable. This gives a unique opportunity to study the dose output of the machine per monitor unit, variation in the timer error, flatness and symmetry of the beam and the energy check as a function of time. The results show that, although the dose per monitor unit varied within ±2%, the timer error within ±0.005 MU and the asymmetry within 1–2%, none of these parameters showed any systematic change with time. On the other hand, the energy check showed a linear drift with time for 6, 9, and 12 MeV (2.1, 3.5, and 2.5%, respectively, over 5 years), while at 15 and 18 MeV, the energy check was relatively constant. It is further shown that based on annual calibrations and RPC TLD checks, the energy of each beam is constant and that therefore the energy check is an exquisitely sensitive one. The consistency of the independent checks is demonstrated.
An experimental comparison of detector performance for direct and indirect digital radiography systems
Authors: Samei Ehsan , Flynn Michael J.
Published: April 2003

Current flat‐panel detectors either directly convert x‐ray energy to electronic charge or use indirect conversion with an intermediate optical process. The purpose of this work was to compare direct and indirect detectors in terms of their modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Measurements were made on three flat‐panel detectors, Hologic Direct‐Ray DR‐1000 (DRC), GE Revolution XQ/i (XQ/i), and Philips Digital Diagnost (DiDi) using the IEC‐defined RQA5 (∼74 kVp, 21 mm Al) and RQA9 (∼120 kVp, 40 mm Al) radiographic techniques. The presampled MTFs of the systems were measured using an edge method [Samei et al., Med. Phys. 25, 102 (1998)]. The NPS of the systems were determined for a range of exposure levels by two‐dimensional (2D) Fourier analysis of uniformly exposed radiographs [Flynn and Samei, Med. Phys. 26, 1612 (1999)]. The DQEs were assessed from the measured MTF, NPS, exposure, and estimated ideal signal‐to‐noise ratios. For the direct system, the MTF was found to be significantly higher than that for the indirect systems and very close to an ideal function associated with the detector pixel size. The NPS for the direct system was found to be constant in relation to frequency. For the XQ/i and DRC systems, the DQE results reflected expected differences based on the absorption efficiency of the different detector materials. Using RQA5, the measured DQE values in the diagonal (and axial) direction(s) at spatial frequencies of and were 64% (64%) and 20% (15%) for the XQ/i system, and 38% (38%) and 20% (20%) for the DRC, respectively. The DQE results of the DiDi system were difficult to interpret due to additional preprocessing steps in that system.
Temporal resolution and the evaluation of candidate algorithms for four‐dimensional CT
Authors: Taguchi Katsuyuki
Published: April 2003

The four‐dimensional computed tomography (“4D‐CT”) with area detector has been developed for dynamic volumetric imaging with large longitudinal coverage. In this paper one of the key technologies for 4D‐CT development is discussed: Image reconstruction algorithm with high temporal resolution. All of the cone‐beam algorithms investigated previously assume that the object is stationary. In this paper a new class of cone‐beam problem is addressed: a dynamic volumetric (4‐D) imaging. A continuously rotating circular (stationary couch) scanning is employed, and then, a generalized version of the well‐known Feldkamp algorithm with the following three steps is performed: (1) applying a weighting function (along the time axis) to projection data, (2) filtering the weighted data along the detector row direction, (3) cone‐beam backprojecting of the filtered data along the corresponding x‐ray path. The weighting function controls the time center, the temporal resolution, and the image quality. Four weighting functions developed for fan‐beam reconstruction were applied to the first step: (a) a constant weight fixed at 0.5 (FS‐FDK), (b) feathering both edges of the (time) window (OS‐FDK), (c) Parker's weight for a half‐scan (HF‐FDK), and (d) an extended Parker's weight, which allows us to use a larger range of projection data up to one rotation (NHS‐FDK). We evaluated them in terms of temporal resolution, image noise, and image quality. Also, the cause of the artifact has been investigated. The temporal resolution of NHF‐FDK equals that of HS‐FDK, which is half of the one rotation period. For the moving object, NHS‐FDK offers the best image quality. The images with FS‐FDK are degraded by streak artifacts; HS‐FDK provides poor image quality with good temporal resolution; and images by OS‐FDK are blurred due to insufficient temporal resolution. The cause of the artifact was found as an inconsistency of projection data due to object motion (in FS‐FDK) and lost 3‐D‐Radon data caused by applying Parker's weight (in HS‐FDK). A hand toy was employed for the preliminary evaluation of dynamic volumetric imaging with the real 256‐slice scanner. In an overall evaluation, NHS‐FDK provides the stable and the sufficient image quality both with moving and stationary objects.
Technical note: Fitted dosimetric parameters of high dose‐rate sources according to the AAPM TG43 formalism
Authors: Lliso F. , Pérez‐Calatayud J. , Carmona V. , Ballester F. , Puchades V. , Granero D.
Published: April 2003

Functional fits for the anisotrophy function and the radial dose function, have been studied, in a previous work, in order to characterize dose‐rate distributions around some of the high‐intensity sources. The purpose of the present work is to complete the previous one in order to include all the existing HDR and PDR sources. The sources addressed here are: the Buchler source from Amersham, the and Plus PDR sources and the and Plus HDR sources from GammaMed, and the new VariSource HDR source wire model VS2000 from Varian Oncology Systems.
Therapeutic treatment plan optimization with probability density‐based dose prescription
Authors: Lian Jun , Cotrutz Cristian , Xing Lei
Published: April 2003

The dose optimization in inverse planning is realized under the guidance of an objective function. The prescription doses in a conventional approach are usually rigid values, defining in most instances an ill‐conditioned optimization problem. In this work, we propose a more general dose optimization scheme based on a statistical formalism [Xing et al., Med. Phys. 21, 2348–2358 (1999)]. Instead of a rigid dose, the prescription to a structure is specified by a preference function, which describes the user's preference over other doses in case the most desired dose is not attainable. The variation range of the prescription dose and the shape of the preference function are predesigned by the user based on prior clinical experience. Consequently, during the iterative optimization process, the prescription dose is allowed to deviate, with a certain preference level, from the most desired dose. By not restricting the prescription dose to a fixed value, the optimization problem becomes less ill‐defined. The conventional inverse planning algorithm represents a special case of the new formalism. An iterative dose optimization algorithm is used to optimize the system. The performance of the proposed technique is systematically studied using a hypothetical C‐shaped tumor with an abutting circular critical structure and a prostate case. It is shown that the final dose distribution can be manipulated flexibly by tuning the shape of the preference function and that using a preference function can lead to optimized dose distributions in accordance with the planner's specification. The proposed framework offers an effective mechanism to formalize the planner's priorities over different possible clinical scenarios and incorporate them into dose optimization. The enhanced control over the final plan may greatly facilitate the IMRT treatment planning process.
Determining the incident electron fluence for Monte Carlo‐based photon treatment planning using a standard measured data set
Authors: Keall Paul J. , Siebers Jeffrey V. , Libby Bruce , Mohan Radhe
Published: April 2003

An accurate dose calculation in phantom and patient geometries requires an accurate description of the radiation source. Errors in the radiation source description are propagated through the dose calculation. With the emergence of linear accelerators whose dosimetric characteristics are similar to within measurement uncertainty, the same radiation source description can be used as the input to dose calculation for treatment planning at many institutions with the same linear accelerator model. Our goal in the current research was to determine the initial electron fluence above the linear accelerator target for such an accelerator to allow a dose calculation in water to within 1% or 1 mm of the measured data supplied by the manufacturer. The method used for both the radiation source description and the patient transport was Monte Carlo. The linac geometry was input into the Monte Carlo code using the accelerator's manufacturer's specifications. Assumptions about the initial electron source above the target were made based on previous studies. The free parameters derived for the calculations were the mean energy and radial Gaussian width of the initial electron fluence and the target density. A combination of the free parameters yielded an initial electron fluence that, when transported through the linear accelerator and into the phantom, allowed a dose‐calculation agreement to the experimental ion chamber data to within the specified criteria at both 6 and 18 MV nominal beam energies, except near the surface, particularly for the 18 MV beam. To save time during Monte Carlo treatment planning, the initial electron fluence was transported through part of the treatment head to a plane between the monitor chambers and the jaws and saved as phase‐space files. These files are used for clinical Monte Carlo‐based treatment planning and are freely available from the authors.