bluuebunny/crossref_metadata_2025 huggingface dataset:
The averaging lemma
Authors: DeVore Ronald , Petrova Guergana
Published: July 2002

Averaging lemmas deduce smoothness of velocity averages, such as
\[
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="ModifyingAbove f With bar left-parenthesis x right-parenthesis colon equals integral Underscript normal upper Omega Endscripts f left-parenthesis x comma v right-parenthesis d v comma normal upper Omega subset-of double-struck upper R Superscript d Baseline comma">

f
¯

(
x
)
:=

∫
Ω

f
(
x
,
v
)

d
v
,

Ω
⊂

R

d

,

\bar f(x):=\int _\Omega f(x,v)\, dv ,\quad \Omega \subset \mathbb {R}^d,

\]
from properties of

f
f

. A canonical example is that

f
¯

\bar f

is in the Sobolev space

W

1

/

2

(

L
2

(

R

d

)
)

W^{1/2}(L_2(\mathbb {R}^d))

whenever

f
f

and

g
(
x
,
v
)
:=
v
⋅

∇
x

f
(
x
,
v
)

g(x,v):=v\cdot \nabla _xf(x,v)

are in

L
2

(

R

d

×
Ω
)

L_2(\mathbb {R}^d\times \Omega )

. The present paper shows how techniques from Harmonic Analysis such as maximal functions, wavelet decompositions, and interpolation can be used to prove

L
p

L_p

versions of the averaging lemma. For example, it is shown that

f
,
g
∈

L
p

(

R

d

×
Ω
)

f,g\in L_p(\mathbb {R}^d\times \Omega )

implies that

f
¯

\bar f

is in the Besov space

B
p
s

(

L
p

(

R

d

)
)

B_p^s(L_p(\mathbb {R}^d))

,

s
:=
min
(
1

/

p
,
1

/

p
′

)

s:=\min (1/p,1/p^\prime )

. Examples are constructed using wavelet decompositions to show that these averaging lemmas are sharp. A deeper analysis of the averaging lemma is made near the endpoint

p
=
1

p=1

.
On spectra of geometric operators on open manifolds and differentiable groupoids
Authors: Lauter Robert , Nistor Victor
Published: July 2002

We use a pseudodifferential calculus on differentiable groupoids to obtain new analytical results on geometric operators on certain noncompact Riemannian manifolds. The first step is to establish that the geometric operators belong to a pseudodifferential calculus on an associated differentiable groupoid. This then leads to Fredholmness criteria for geometric operators on suitable noncompact manifolds, as well as to an inductive procedure to compute their essential spectra. As an application, we answer a question of Melrose on the essential spectrum of the Laplace operator on manifolds with multicylindrical ends.
TLCQA: quantitative study of thin-layer chromatography.
Authors: Chau F T , Chan T P , Wang J
Published: July 2002

SUMMARY: An accurate, simple and low-cost image-analysis system has been set up for capturing thin-layer chromatography images, and a software package, TLCQA, has also been developed for both qualitative and quantitative analyses using the acquired image. AVAILABILITY: Available upon request from the authors. CONTACT: bcftchau@poly.edu.hk
Single column discrepancy and dynamic max-mini optimizations
for quickly finding the most parsimonious evolutionary trees
Authors: Purdom, Jr P. W. , Bradford P. G. , Tamura K. , Kumar S.
Published: July 2002

Motivation: In the maximum parsimony (MP) method, the tree requiring the minimum number of changes (discrepancy) to explain the given set of DNA or amino acid sequences is chosen to represent their evolutionary relationships. To find the MP tree, the branch-and-bound algorithm is normally used. For a partial phylogenetic-tree (one that has a subset of the organisms) the traditional algorithm assigns a cost equal to the discrepancy of the partial phylogenetic-tree. We propose a single column discrepancy heuristic which increases this cost by predicting a minimum additional discrepancy needed to attach the sequences yet to be added to the partial phylogenetic-tree. A dynamic Max-mini order of sequence addition is also proposed to quickly terminate branch-and-bound search paths that are guaranteed to lead to suboptimal solutions.

Results: We studied the running time of 47 problems generated from 17 data sets. The use of single column discrepancy heuristic speeded up the computation to 2.4-fold for static and 18.2-fold for dynamic search order. The improvement appeared to increase exponentially with the number of sequences. The proposed strategies are also likely to be useful in speeding up the MP tree search using heuristic searches that are based on branch-and-bound-like algorithms.

Contact: s.kumar@asu.edu
Delayed activation of PPARγ by LPS and IFN‐γ attenuates the oxidative burst in macrophages
Authors: ANDREAS VON KNETHEN , BERNHARD BRÜNE
Published: July 2002

Desensitization of macrophages is important during the development of sepsis. It was our intention to identify mechanisms that promote macrophage deactivation upon contact with endotoxin (LPS) and interferon‐γ (IFN‐γ) in vitro. Macrophage activation was achieved with 12‐O‐tetradecanoylphorbol 13‐acetate (TPA), and the oxidative burst (i.e., oxygen radical formation) was followed by oxidation of the redox‐sensitive dyes hydroethidine and dichlorodihydrofluorescein diacetate. Prestimulation of macrophages for 15 h with a combination of LPS/IFN‐γ attenuated oxygen radical formation in response to TPA. Taking the anti‐inflammatory properties of the peroxisome proliferator‐activating receptorγ (PPARγ) into consideration, we established activation of PPARγ in response to LPS/IFN‐γ by an electrophoretic mobility shift, supershift, and a reporter gene assay. The reporter contains a triple PPAR‐responsive element (PPRE) in front of a thymidine kinase minimal promoter driving the luciferase gene. We demonstrated that PPRE decoy oligonucleotides, supplied in front of LPS/IFN‐γ, allowed a full oxidative burst to recover upon TPA addition. Furthermore, we suppressed the oxidative burst by using the PPARγ agonists 15‐deoxy‐Δ12,14‐prostaglandin J2, BRL 49653, or ciglitazone. No effect was observed with WY 14643, a PPARa agonist. We conclude that activation of PPARs, most likely PPARγ, promotes macrophage desensitization, thus attenuating the oxidative burst. This process appears important during development of sepsis.—von Knethen, A., Brüne, B. Delayed activation of PPARg by LPS and IFN‐γ attenuates the oxidative burst in macrophages. FASEB J. 15, 535‐544 (2001)
Natural and anthropogenic dispersal mechanisms in the marine environment: a study using cheilostome Bryozoa
Authors: Watts Phillip C. , Thorpe John P. , Taylor Paul D.
Published: July 2002

The global geographic ranges occupied by 197 species of cheilostomate Bryozoa found in British waters were obtained by a literature survey. Morphological grade, larval mode, environmental tolerance, species abundance and the ability to raft and to foul shipping were all investigated as traits potentially able to affect the geographic ranges of these bryozoan species. When considered independently all variables except larval mode had a significant correlation with the geographic range occupied by a species. However, when controlling for the potentially confounding effects of the other covariates, only the ability to foul or raft and species abundance had a significant effect on median geographic range and only fouling and abundance had a significant effect over global ranges. The strength of the association between fouling ability and range suggests that transport upon the hulls of ships is a very important dispersal mechanism for bryozoans, as it is thought to be also for various other marine taxa. Potential long-term (evolutionary) consequences of increased ranges brought about by anthropogenic mechanisms are discussed.
Genetic interactions between clock mutations inNeurospora crassa: can they help us to understand complexity?
Authors: Morgan Louis W. , Feldman Jerry F. , Bell-Pedersen Deborah
Published: July 2002

Recent work on circadian clocks inNeurosporahas primarily focused on thefrequency(frq) andwhite–collar(wc) loci. However, a number of other genes are known that affect either the period or temperature compensation of the rhythm. These include theperiod(no relationship to theperiodgene ofDrosophila) genes and a number of genes that affect cellular metabolism. How these other loci fit into the circadian system is not known, and metabolic effects on the clock are typically not considered in single–oscillator models. Recent evidence has pointed to multiple oscillators inNeurospora, at least one of which is predicted to incorporate metabolic processes. Here, theNeurosporaclock–affecting mutations will be reviewed and their genetic interactions discussed in the context of a more complex clock model involving two coupled oscillators: a FRQ/WC–based oscillator and a ‘frq–less’ oscillator that may involve metabolic components.
Monte Carlo dosimetry of a new high dose rate brachytherapy source
Authors: Angelopoulos A. , Baras P. , Sakelliou L. , Karaiskos P. , Sandilos P.
Published: July 2002

This work provides full dosimetric data for a new high‐strength source currently launched by Varian Oncology Systems for use in their high dose rate remote afterloading systems. The active core length of the new source is reduced to 5 mm compared to a value of 10 mm for the existing VariSource source design, with all other geometric source and encapsulation details being similar. Dose‐rate constant, radial dose functions, geometry factors, and anisotropy functions, utilized in the AAPM Task Group 43 dose calculation formalism, were calculated using Monte Carlo simulation. Results are compared with corresponding data published for the existing VariSource and microSelectron high dose rate sources. The dose‐rate constant for the new Varian source was found to be equal to compared to values of and calculated for the existing VariSource and microSelectron sources, respectively. The radial dose functions between the three sources are similar with the exception of their values at radial distances very close to the source where differences of are observed. The new Varian source demonstrates a smaller anisotropy relative to the existing VariSource source design for polar angles close to the source longitudinal axis, due to its smaller active core length.
Introduction of audio gating to further reduce organ motion in breathing synchronized radiotherapy
Authors: Kubo H. Dale , Wang Lili
Published: July 2002

With breathing synchronized radiotherapy (BSRT), a voltage signal derived from an organ displacement detector is usually displayed on the vertical axis whereas the elapsed time is shown on the horizontal axis. The voltage gate window is set on the breathing voltage signal. Whenever the breathing signal falls between the two gate levels, a gate pulse is produced to enable the treatment machine. In this paper a new gating mechanism, audio (or time‐sequence) gating, is introduced and is integrated into the existing voltage gating system. The audio gating takes advantage of the repetitive nature of the breathing signal when repetitive audio instruction is given to the patient. The audio gating is aimed at removing the regions of sharp rises and falls in the breathing signal that cannot be removed by the voltage gating. When the breathing signal falls between voltage gate levels as well as between audio‐gate levels, the voltage‐ and audio‐gated radiotherapy (ART) system will generate an AND gate pulse. When this gate pulse is received by a linear accelerator, the linear accelerator becomes “enabled” for beam delivery and will deliver the beam when all other interlocks are removed. This paper describes a new gating mechanism and a method of recording beam‐on signal, both of which are, configured into a laptop computer. The paper also presents evidence of some clinical advantages achieved with the ART system.
The effect of voxel size on the accuracy of dose‐volume histograms of prostate seed implants
Authors: Corbett Jean‐François , Jezioranski John , Crook Juanita , Yeung Ivan
Published: July 2002

Cumulative dose‐volume histograms (DVH) are crucial in evaluating the quality of radioactive seed prostate implants. When calculating DVHs, the choice of voxel size is a compromise between computational speed (larger voxels) and accuracy (smaller voxels). We quantified the effect of voxel size on the accuracy of DVHs using an in‐house computer program. The program was validated by comparison with a hand‐calculated DVH for a single 0.4‐U iodine‐125 model 6711 seed. We used the program to find the voxel size required to obtain accurate DVHs of five iodine‐125 prostate implant patients at our institution. One‐millimeter cubes were sufficient to obtain DVHs that are accurate within 5% up to 200% of the prescription dose. For the five patient plans, we obtained good agreement with the VariSeed (version 6.7, Varian, USA) treatment planning software's DVH algorithm by using voxels with a sup‐inf dimension equal to the spacing between successive transverse seed implant planes (5 mm). The volume that receives at least 200% of the target dose, calculated by VariSeed was 30% to 43% larger than that calculated by our program with small voxels. The single‐seed DVH calculated by VariSeed fell below the hand calculation by up to 50% at low doses (30 Gy), and above it by over 50% at high doses (>250 Gy).
Seed localization in eye plaque brachytherapy
Authors: Emery Richard S. , Szechter Andrzej
Published: July 2002

We describe a simple method for localizing seeds in eye plaques used for brachytherapy treatment of intraocular tumors and age‐related macular degeneration. The method is based on obtaining magnified photocopy images of plaques with seeds in place and then using simple geometrical considerations to reconstruct seed coordinates with about 2% accuracy. A spreadsheet program is used for reconstruction and dosimetric calculations.
Range precision of therapeutic proton beams
Authors: Schaffner Barbara
Published: July 2002

The precision of proton radiotherapy treatment planning is limited mainly by the precision achieved in the determination of relative stopping power values of human tissue and the accuracy of the analytical dose calculation model. A stoichiometric calibration curve for the conversion from CT values into relative proton stopping power values was defined theoretically and its accuracy was checked by measurements on biological tissue samples. Including the effects of beam hardening on the CT values an upper limit of 2–4 mm for the uncertainty in the position of the distal falloff of the dose distribution was estimated. Different dose calculation algorithms for the spot‐scanning technique have been developed. Their accuracy was evaluated by a comparison to Monte Carlo simulated dose distributions. Large errors due to inaccuracies of the planning algorithm are encountered mainly in the presence of density heterogeneities where the range of the protons is poorly defined. A quick estimation of the expected range dilution as a function of the beam incidence angle can be used to identify critical areas. Besides uncertainties in the treatment planning itself one has to consider the effects of setup uncertainties on the dose distribution. A suggestion is presented on how to deal with uncertainties during the treatment planning process.
A generalized film technique for the verification of vertex fields used in the treatment of brain tumors
Authors: Huq M. Saiful , Yue Ning , Suntharalingam N. , Curran Walter J.
Published: July 2002

With the availability of commercial three‐dimensional (3D)‐treatment planning systems, more and more treatment plans call for the use of noncoplanar conformal beams for the treatment of brain tumors. However, techniques for the verification of many noncoplanar beams, such as vertex fields which involve any combination of gantry, collimator, and table angles, do not exist. The purpose of this work is to report on the results of an algorithm and a technique that have been developed for the verification of noncoplanar vertex fields used in the treatment of brain tumors. This technique is applicable to any geometric orientation of the beam, i.e., a beam orientation that consists of any combination of gantry, table, and collimator rotations. The method consists of superimposing a central plane image of a correctly magnified vertex field on a lateral or oblique field port film. To achieve this, the 3D coordinates of the projection of the isocenter onto the film for lateral (or oblique) as well as the vertex fields are determined and then appropriately matched. Coordinate transformation equations have been developed that enable this matching precisely. A film holder has been designed such that a film cassette can be secured rigidly along the side rails of the treatment table. The technique for taking a patient treatment setup verification film consists of two steps. In the first step, the gantry, table, and collimator angles for the lateral (or oblique) field are set and the usual double exposures are made; the first exposure corresponds to that of the treatment portal with the isocenter clearly identified and the second one a larger radiation field so that the peripheral anatomy is visible on the film. In the next step, the gantry, table, and collimator angles are positioned for the vertex field and the table is moved laterally and vertically and the film longitudinally to a position that will enable precise matching of the isocenter on the film. A third exposure is then taken with the vertex portal. What is seen on the film is a superposition of a central plane image of the vertex field onto the image of the lateral or oblique field. This technique has been used on 60 patients treated with noncoplanar fields for brain tumors. In all of these cases, the coincidence of the projection of the isocenter for the lateral (or oblique) and the vertex fields was found to be within 3 mm.
Dosimetric characteristics of acrylic and stainless steel cones for electron beam therapy
Authors: Kakinohana Yasumasa , Toita Takafumi , Ogawa Kazuhiko , Kinjyou Tsukasa , Maeshiro Syukou , Sawada Satoshi
Published: July 2002

Dosimetric characteristics of acrylic and stainless steel cones for electron beam therapy were investigated. Acrylic and stainless steel cylindrical cones of 6, 7, and 8 cm in diameter and electron beams of energies 6, 9, 12, 15, 18, and 21 MeV were used for the measurements. Both acrylic and stainless steel cones showed high dose areas along the rim. The dose along the rim grew with increasing electron beam energy. The highest dose along the rim was 115% of the maximum dose on a central axis when a 6‐cm‐diameter acrylic cone and 21‐MeV electrons were combined.
The design and evaluation of interstitial ultrasound applicators for controlled thermal treatment of localized tumors
Authors: Deardorff Dana L.
Published: July 2002

This dissertation investigates the design and development of interstitial ultrasound applicators to provide localized and controlled heating of tissue for potential clinical implementation as an alternative thermal treatment of cancerous and benign diseases. Implantable ultrasound applicators were designed and fabricated using cylindrical piezoceramic transducers (1.5–2.5 mm o.d., frequency =6.5–7.5 MHz) in a direct‐coupled configuration. Transducers were longitudinally sectioned to produce specific angular sectors of acoustic output (90°–360°). Applicators were also constructed using multiple transducer elements (3) with individual power control. An internal cooling mechanism (air or water) was implemented to remove excessive thermal energy from the inner transducer surface (h=700–4200 Measurements of the relative acoustic intensity were used to characterize the efficiency, quality, and directivity of the ultrasound beam output. Thermal performance was experimentally evaluated through multiple heating trials in vivo (canine prostate, porcine liver, and thigh muscle), in vitro (porcine liver, muscle, and kidney), and in tissue‐mimicking phantom for both hyperthermia and coagulative thermal therapy. Theoretical capabilities of these applicators were evaluated using biothermal computer simulations to model the acoustic output and resultant heating in tissue. In general, results demonstrated excellent thermal performance and control of heating in tissue. Directed and collimated thermal coagulation was produced in both the angular and axial expanse, and found to correspond to the active acoustic sector of the transducer. The use of multiple transducer elements provided active control of axial heating in the tissue. Increased levels of applied power (12–48 W) with transducer cooling substantially improved radial depths of coagulation (8–22 mm) and decreased sonication times (0.5–5 min). This ability to control and direct heating shows great potential for the treatment of localized tumors in sites such as the prostate, brain, liver, and breast, without damaging the surrounding healthy tissue.
Predicted radiation dose and risk associated with pediatric diagnostic x‐ray procedures
Authors: Hintenlang Kathleen M.
Published: July 2002

Risks from exposure to radiation are assessed by calculating individual tissue absorbed doses and subsequently determining the effective dose. The absorbed dose to specific tissues has traditionally been difficult and time consuming to measure. This research describes a new methodology that permits rapid measurement of the effective dose and associated risks to pediatric patients for a variety of plain film diagnostic examinations. The methodology utilizes an anthropomorphic phantom of a one‐year‐old, incorporating direct reading MOSFET dosimetry that allows the rapid determination of absorbed dose to individual organs. The research integrated clinical examinations, dosimetry measurements, and BEIR V risk assessments for pediatric patients undergoing clinical radiology exams. Site surveys were performed at ten facilities using standardized exams to characterize variations in effective dose related to clinical practice and detailed quantitative studies for a variety of examinations were performed at Shands at UF. The effective dose for these procedures ranged from 0.1 mrem for an AP skull exam that utilized thyroid shielding, to 9.2 mrem for an AP upright abdomen exam. The relative leukemia carcinogenic risk ranged from 1.000 041 for the skull exam, to 1.002 732 for the abdomen exam, with corresponding excess risks from exams of 0.005 and 0.3 leukemias per year, respectively. These individual examinations provided a significant benefit to the patient with minimal risk. This dissertation provided the basis for continuing research to quantify long‐term risks, as well as providing a clinical tool to evaluate the effective dose delivered to pediatric patients from current and emerging radiology imaging modalities.
Dose perturbations produced by thermoluminescent dosimeters and ionization chambers in high energy photon beams (in French)
Authors: Ferreira Ivaldo Humberto
Published: July 2002

The thesis studied perturbation by thermoluminescent dosimeters (TLDs) and ionization chambers of the absorbed dose measured in a medium irradiated with photon beams from γ‐rays to 25‐MV x‐rays. The perturbations were evaluated using experimental and theoretical techniques involving analytical calculations and the Monte Carlo (MC) method (EGS4‐PRESTA I). The MC results agree with experimental data better than within 0.4%, with specific uncertainties (1 s.d.) less than 0.1% for MC and 0.3% for experimental values. We used Burlin's general cavity theory and MC to determine the influence of TLD detector material, its form, size, density, and dopant concentration in the energy correction factors. The size and density influence the energy correction factors about 2%. The dopant concentration does not have a significant effect. For the specific use of TLDs in radiotherapy postal quality checks by the Gustave–Roussy Institute, we investigated the influence on absorbed dose of the TLD holder (0.5% to 2%), the thermal conditioning of the TLD (1%), and heterogeneities around the dosimeters (2%). Uncertainties in the absorbed dose determination in water with TLD were evaluated to be about 1.7% for the and 2% for the x‐rays. In ionization chambers, correction factors were determined for the walls, waterproofing caps, and the central electrode from Exradin, Nuclear Enterprise, and Wellhöfer chambers. Global perturbation correction factors were evaluated for primary (BIPM) and secondary (IRD) standard ionization chambers. This determination validated the MC method for the calculation of global perturbation correction factors for the determination of the absorbed dose to water.
Effect of filtering on the detection and localization of small Ga‐67 lesions in thoracic single photon emission computed tomography images
Authors: Wells R. Glenn , Simkin Peter H. , Judy Philip F. , King Michael A. , Pretorius P. Hendrik , Gifford Howard C.
Published: July 2002

Tumor detection can be significantly affected by filtering so determining an optimal filter is an important aspect of establishing a clinical reconstruction protocol. The purpose of this study was to identify the cut‐off frequency of a Butterworth filter used in a filtered backprojection (FBP) reconstruction that maximized the detection and localization accuracy of 1 cm spherical lesions in Ga‐67 citrate, thoracic SPECT images. Image quality was evaluated by means of a localization receiver operating characteristic (LROC) study using computer simulated images. Projection data were generated using the mathematical cardiac‐torso digital phantom with a clinically realistic background source distribution. The images were reconstructed using FBP with multiplicative Chang attenuation correction and fifth‐order Butterworth filtering. The cut‐off frequencies considered were 0.25, 0.32, 0.47, and 0.79 cm−1 for the case of three‐dimensional (3D) post‐filtering and 0.25, 0.32, and 0.47 cm−1 for two‐dimensional (2D) post‐filtering. The images were read by three research scientists and one board certified nuclear medicine clinician. The area under the LROC curve and the localization accuracy for all test conditions were compared using Scheffé's multiple comparisons test. It was found that 3D post‐filtering using filters with cut‐off frequencies of 0.32 and 0.47 cm−1 resulted in the highest lesion detectability and localization accuracy. These two test conditions did not differ significantly from each other but were significantly better than all of the 2D, and the 3D 0.79 cm−1 cut‐off frequency cases.
Determination of the dose distribution of therapeutic electrons at interfaces
Authors: Ostwald Patricia M.
Published: July 2002

The variation of dose close to tissue/inhomogeneity or tissue/air interfaces was investigated in slab and anthropomorphic phantoms for electron beams between 4 and 20 MeV. Effective source‐surface distance (ESSD), beam obliquity, backscatter from internal interfaces, and dose to the sidewalls of air cavities were investigated using an Attix ion chamber, different thicknesses of thermoluminescent dosimeters, and EGS4 and MCNP Monte Carlo simulations. Surface dose was found to vary due to the SSD and beam obliquity. Relative surface dose decreased as the SSD increased due to the effect of low energy scattering from the cone and ESSD determined at the surface differed from the ESSD measured at The difference was found to be most significant for the combination of high energy beams and small field sizes. Surface dose increased with angle for obliquely incident beams from approximately 30° up to a maximum, which was found at approximately 80°, and then decreased rapidly. This dose maximum angle increased with increasing electron beam energy for both surface and near surface depths. Dose variations at internal tissue/inhomogeneity interfaces are due to disequilibrium of electron scatter. The backscatter of electrons (EBF) from inhomogeneities was found to be dependent on the effective atomic number and relative electron density of the scatterer. Where dose upstream of the scatterer was reduced due to a lack of backscattered electrons. EBF was dependent on the mean energy of the beam incident on the interface, but was independent of the shape of the energy spectrum. The energy of backscattered electrons at the interface was generally low, but increased with increasing atomic number of the inhomogeneity and was related to the upstream extent of backscattered electrons. Dose variation along the side wall of a cavity was dependent on the angular distribution of the beam entering the cavity and dimensions of the cavity.
Dose calculations of photon beams using a finite‐size pencil beam model
Authors: Hinson William Howell
Published: July 2002

This work investigates the finite‐size pencil beam (FSPB) model for calculating dose deposition from photon beams generated by a clinical linear accelerator. The FSPB model, first published by Bourland and Chaney, uses the superposition of preconvolved “finite‐size pencil beams” of small cross‐sectional area to determine the dose deposition in a uniform water phantom. In the dose computation, FSPBs for a range of energy bins are pieced together like mosaic tiles to collectively form the cross section of the full beam. Depending on the full beam resolution, the superposition calculation can be much faster than full convolution. The results stress the importance of a knowledge of the photon spectrum of the beam for accurate dose calculations. However, published methods of indirect spectral measurements using transmission measurements through beam attenuators require mathematical fits with a large number of parameters and constraints. A simple strategy is presented for fitting transmission data that models important physical characteristics of photon beams produced in linear accelerators. The fitting equation has these advantages over previous methods: (1) the equation describes the shape of a bremsstrahlung spectrum based on physical expectations; (2) only three fit parameters are required with a single constraint. Results are presented for 4 and 6 MV photon beams. Comparisons of calculated and measured TMRs and output factors of open fields show excellent agreement. Results include discussions of FSPB generation, the method of spectral measurement, the effect of beam softening across the field cross section, and the method for modeling this effect using different FSPB weighting factors as a function of energy and location.
Superficial doses from serial tomotherapy delivery
Authors: Mutic Sasa , Low Daniel A.
Published: July 2002

Serial tomotherapy patients are treated using a commercial computer‐controlled treatment planning and delivery system and a linear accelerator in arc mode. Target volumes are irradiated using sequential 1.68‐cm‐thick slices with the patient moved between each successive slice delivery. Due to the method of delivery, superficial doses from a course of intensity modulated radiation therapy may be significantly different from doses delivered with conventional radiation therapy. An experimental investigation was conducted to determine the superficial depth‐dose distribution for geometric target volumes. Treatment plans were conducted for 5‐cm‐diam, 5‐cm‐long right cylindrical target volumes embedded in a 16‐cm‐diam, 12‐cm‐long water‐equivalent cylindrical phantom. Three experiments were conducted with centrally located, off‐centered, and superficial targets. Doses at superficial depths were measured using thermoluminescent dosimetry chips. For all three targets, the measured doses were slightly greater than the calculated doses. Doses measured in the buildup region for the superficial target indicated that bolus would be required to provide the prescribed dose at the phantom surface.
The impact of fluctuations in intensity patterns on the number of monitor units and the quality and accuracy of intensity modulated radiotherapy
Authors: Mohan Radhe , Arnfield Mark , Tong Shidong , Wu Qiuwen , Siebers Jeffrey
Published: July 2002

The purpose of this work is to examine the potential impact of the frequency and amplitude of fluctuations (“complexity”) in intensity distributions on intensity‐modulated radiotherapy (IMRT) dose distributions. The intensity‐modulated beams are efficiently delivered using a multileaf collimator (MLC). Radiation may be delivered through a continuous (dynamic mode) or discrete (step‐and‐shoot) sequence of windows formed by the leaves. Algorithms and software that convert optimized intensity distributions into leaf trajectories apply approximate empirical corrections to account for the various effects associated with MLC characteristics, such as the rounded leaf tips, tongue‐and‐groove leaf design, leaf transmission, leaf scatter, and collimator scatter upstream from the MLC. Typically, the difference between inter‐ and intraleaf transmissions is ignored. In this paper, using a schematic example of IMRT for head and neck carcinomas, we demonstrate that complex anatomy and severe optimization constraints produce complex intensity patterns. Using idealized intensity patterns we also demonstrate that, for complex intensity patterns, the average window width tends to be smaller and, for the same dose received by the tumor, the number of MUs is larger. We found that as the complexity increases, so does the contribution of radiation transmitted through and scattered from the leaves (“indirect radiation”) to the total delivered dose. As a consequence, the lowest deliverable intensity in complex intensity patterns may be significantly greater than that required to provide adequate protection for some normal tissues. Furthermore, since corrections for leaf transmission and scatter effects are approximate and the difference between inter‐ and intraleaf transmission is ignored, the accuracy of the delivered dose may be affected. Using the results of a simple experiment and a typical intensity‐modulated beam for a head and neck case as examples, we show the effect of window width and complexity on the accuracy and deliverability of intensity patterns. Some possible strategies for improving the accuracy and for relaxing the lower limit on deliverable intensity are discussed.
The coefficient of restitution for collisions of happy balls, unhappy balls, and tennis balls
Authors: Cross Rod
Published: July 2002

A perfectly happy ball is one that bounces to its original height when dropped on a massive, rigid surface. A completely unhappy ball does not bounce at all. In the former case, the coefficient of restitution (COR) is unity. In the latter case, the COR is zero. It is shown that when an unhappy ball collides with a happy ball, the COR increases from zero to unity as the stiffness of the happy ball decreases from infinity to zero. The COR is independent of the mass of each ball. The implication of reducing the COR of a tennis ball, as a possible means of slowing the serve in tennis, is also considered. It is shown that (a) the COR for a collision with a racket varies with the impact point and is a maximum at the vibration node near the center of the strings, and (b) the serve speed is reduced by only about 20% if the COR for a bounce on the court is reduced to zero.
Polarization of light by reflection and the Brewster angle
Authors: Ouseph P. J. , Driver Kevin , Conklin John
Published: July 2002

An experiment to determine the Brewster angle and to study polarization of reflected light is discussed in this paper. The equipment used is commercially available from PASCO Scientific. A light sensor, which detects the intensity of the reflected light, is connected to a computer through a Pasco Science Workshop 750 interface and the intensity is read directly from a digital meter in the Science Workshop program. Intensities of the reflected light polarized in the plane of incidence and perpendicular to the plane were obtained as a function of the angle of incidence. The results are consistent with Fresnel’s laws of reflection. In addition, the refractive index calculated from the Brewster angle agrees with the accepted value for the dielectric reflector used in the experiment.
Impedance and admittance matrices of symmetric piezoelectric annular bimorphs and their applications
Authors: Ha Sung K. , Kim Young H.
Published: July 2002

The 5×5 impedance and admittance matrices of a symmetric triple-layer piezoelectric annular bimorph (PAB) are presented and the PAB with the piezoelectric layers partially covering the shim layer in harmonic motion is analyzed using the matrices. The electromechanical flow vectors are defined as the mechanical displacements and rotations at the inner and outer boundaries and the electric current of the piezoelectric layer; the conjugate parameters, i.e., the electromechanical effort vectors, are accordingly defined. The impedance matrix which relates the flow vector with the effort vector is divided into three matrices, the mechanical, electrical, and electromechanical coupling impedance matrices; each matrix is represented by a block network with five ports. The resonance and the antiresonance frequencies and the effective electromechanical coupling factors of the PAB excited by the partially covering piezoelectric layers are then calculated for various boundary conditions imposed at the inner and outer surfaces. The numerical results by the impedance matrix are compared with those by the finite element methods; they are in excellent agreement with each other. It is also shown that the matrix representations can be easily applied to the piezoelectric circular bimorph (PCB) with the extended shim layer.
A consideration of the normalization that is typically included in correlation-based models of binaural detection
Authors: van de Par Steven , Trahiotis Constantine , Bernstein Leslie R.
Published: July 2002

An analysis of binaural detection and new data that elucidate the nature and precision of normalization that must be assumed if binaural detection is accomplished via mechanisms that effectively compute the coefficient of cross correlation is presented. Based on that analysis, it is argued that the precision of normalization required to remove deleterious effects resulting from variations in the levels of the stimuli is so great that it is highly unlikely that normalization, per se, actually occurs as part of binarual processing. Instead, it appears more likely that binaural processing is accomplished via “subtractive” mechanisms, such as the one originally described by Durlach [J. Acoust. Soc. Am. 35, 1206–1218 (1963)]. Within that framework, deleterious effects that could result from variations in the levels of the stimuli simply do not arise.
Spatial resolution of time-reversal arrays in shallow water
Authors: Kim Seongil , Edelmann G. F. , Kuperman W. A. , Hodgkiss W. S. , Song H. C. , Akal T.
Published: July 2002

A series of time-reversal experiments was performed in shallow water including a range-dependent slope environment. Time-reversal arrays implemented with center frequencies of 445 and 3500 Hz achieved sharp focal regions up to ranges of 30 and 13 km, respectively in 110–130-m shallow water. In this paper, resolution expressions are derived using an image method to describe the focal sizes achieved with time-reversal arrays in various ocean environments. Analysis for the measured data indicates that the focal size approaches the diffraction limit of an array for given waveguide conditions, i.e., waveguide geometry and attenuation. The measured focal size has implications for the maximum achievable resolution of linear matched-field processing which is a computational implementation of the time-reversal process.
Quantifying uncertainty in geoacoustic inversion. I. A fast Gibbs sampler approach
Authors: Dosso Stan E.
Published: July 2002

This paper develops a new approach to estimating seabed geoacoustic properties and their uncertainties based on a Bayesian formulation of matched-field inversion. In Bayesian inversion, the solution is characterized by its posterior probability density (PPD), which combines prior information about the model with information from an observed data set. To interpret the multi-dimensional PPD requires calculation of its moments, such as the mean, covariance, and marginal distributions, which provide parameter estimates and uncertainties. Computation of these moments involves estimating multi-dimensional integrals of the PPD, which is typically carried out using a sampling procedure. Important goals for an effective Bayesian algorithm are to obtain efficient, unbiased sampling of these moments, and to verify convergence of the sample. This is accomplished here using a Gibbs sampler (GS) approach based on the Metropolis algorithm, which also forms the basis for simulated annealing (SA). Although GS can be computationally slow in its basic form, just as modifications to SA have produced much faster optimization algorithms, the GS is modified here to produce an efficient algorithm referred to as the fast Gibbs sampler (FGS). An automated convergence criterion is employed based on monitoring the difference between two independent FGS samples collected in parallel. Comparison of FGS, GS, and Monte Carlo integration for noisy synthetic benchmark test cases indicates that FGS provides rigorous estimates of PPD moments while requiring orders of magnitude less computation time.
Porous road pavements: Acoustical characterization and propagation effects
Authors: Bérengier M. C. , Stinson M. R. , Daigle G. A. , Hamet J. F.
Published: July 2002

Measurements of the acoustical properties of some porous road pavements are presented here and an acoustical method for monitoring the performance of these surfaces is presented. Porous road pavements have been used previously because of their driving qualities and drainage capacities during rainy days (i.e., the elimination of water splash and spray) but they have also been found to reduce traffic noise substantially. Reductions in A-weighted sound levels of 3–5 dB, compared to a dense pavement structure, have been measured. To study further their acoustical performance, measurements over real road surfaces have been carried out and the results compared to theoretical predictions based upon models describing the surface impedance and sound propagation. For the impedance characterization, both a phenomenological and a microstructural model were used. Both approaches introduce a viscous and a thermal dependence to account for the different phenomena inside the porous structure. By incorporating these models into the theoretical propagation predictions, it is possible to evaluate the impact of porous asphalt on highway noise levels. A nondestructive testing technique has been designed for determining in situ the noise reduction performance of porous road pavements.
Optimal selection of parameters for the angular spectrum approach to numerically evaluate acoustic fields
Authors: Wu Ping , Kazys Rymantas , Stepinski Tadeusz
Published: July 2002

Optimal selection of parameters is presented for the angular spectrum approach (ASA) to the numerical calculation of acoustic fields radiated by planar transducers and linear arrays with and without focusing. The parameters include spatial sampling interval, discretization size of a source plane in which a source is located, and the angular range over which the plane waves decomposed from the source using the discrete Fourier transform (DFT) are chosen to superimpose and construct the fields. The concept of instantaneous frequency is applied to the Fourier transformed Green’s function to determine the angular range of the plane waves used for the construction with minimal spatial aliasing error. Based on the minimization of spatial frequency and spatial aliasing errors in a constructed field, optimal selection of the parameters is worked out. The ASA with the optimal selection is then applied to computing the fields radiated by planar transducers and linear arrays into water and a layered (oil/water) medium. The results demonstrate that the methods proposed in this paper to select the parameters for the ASA allow the aliasing errors to effectively eliminate and thus yield the best computational accuracy for the parameters selected.
Experimental investigation of flow-excited cavity pressure fluctuations
Authors: Bezemek Jacklyn D. , Mongeau Luc
Published: July 2002

Cavity pressure fluctuations induced by flow grazing over a long, narrow, open cavity were investigated. The apparatus stimulated a typical road vehicle door gap/door cavity/primary bulb seal assembly, tested using a wind tunnel. The cavity pressure distribution was measured over a range of flow velocities. Pressure spectra were decomposed into a source spectral distribution function and an acoustic frequency response function, using a system identification method based on dimensional analysis and a linear source-filter model. The results indicated a weak coupling between the cavity response and the flow excitation. The excitation mechanism was dominated by flow-edge interactions, i.e., the ‘‘edge tone.’’ The characterization method was validated by varying the acoustic properties of the cavity and the excitation strength. The source function was independent of sensor location. The system response was shown to be associated with transverse standing waves. Tapering the orifice trailing edge simultaneously reduced the excitation strength, the excitation frequency, and the cavity response. The presence of flow aspiration through the seal raised the broadband noise floor of the system response function. This anomaly underlined one limitation of the spectral decomposition method.
Multi-variable structural acoustic control with static compensation
Authors: Clark Robert L. , Cox David E.
Published: July 2002

Structural acoustic control of stochastic disturbance sources based on static compensation of multiple-input, multiple-output colocated transducer arrays is addressed in this work. Prior feedback controller designs for structural acoustic systems have been based on linear quadratic regulator theory or linear quadratic Gaussian theory, however, the need for a state estimator limits the practical implementation of such control systems on plants characterized by model uncertainty and high modal density. Results presented in this work serve to demonstrate that direct output-feedback control can be successfully employed to attenuate the structural acoustic response of a panel subjected to exogenous input disturbances. Colocated transducer pairs were implemented with a fully populated constant feedback gain matrix from sensor outputs to control inputs. The colocated transducer array serves to enhance robust stability, while the constant feedback gain matrix provides a practical means of implementation. The effects of instrumentation density, control effort penalty, and model order on predicted performance are considered.
Speechreading and the structure of the lexicon: Computationally modeling the effects of reduced phonetic distinctiveness on lexical uniqueness
Authors: Auer Edward T. , Bernstein Lynne E.
Published: July 2002

A lexical modeling methodology was employed to examine how the distribution of phonemic patterns in the lexicon constrains lexical equivalence under conditions of reduced phonetic distinctiveness experienced by speechreaders. The technique involved (1) selection of a phonemically transcribed machine-readable lexical database, (2) definition of transcription rules based on measures of phonetic similarity, (3) application of the transcription rules to a lexical database and formation of lexical equivalence classes, and (4) computation of three metrics to examine the transcribed lexicon. The metric percent words unique demonstrated that the distribution of words in the language substantially preserves lexical uniqueness across a wide range in the number of potentially available phonemic distinctions. Expected class size demonstrated that if at least 12 phonemic equivalence classes were available, any given word would be highly similar to only a few other words. Percent information extracted (PIE) [D. Carter, Comput. Speech Lang. 2, 1–11 (1987)] provided evidence that high-frequency words tend not to reside in the same lexical equivalence classes as other high-frequency words. The steepness of the functions obtained for each metric shows that small increments in the number of visually perceptible phonemic distinctions can result in substantial changes in lexical uniqueness.