bluuebunny/crossref_metadata_2025 huggingface dataset:
Dosimetric characteristics of the DRAXIMAGE model LS‐1 I‐125 interstitial brachytherapy source design: A Monte Carlo investigation
Authors: Williamson Jeffrey F.
Published: July 2002

We have used Monte Carlo photon transport simulations to evaluate dosimetric characteristics of a new seed, the DRAXIMAGE BrachySeed™ (model LS‐1) source for interstitial brachytherapy and to calculate the dosimetric parameters recommended by the AAPM Task Group 43 (TG‐43). To eliminate thick end welds, preformed closed‐ended Ti shells are welded together near the transverse plane. The radioactivity is localized near the seed ends in order to maximize isotropy at typical prescription distances. Transmission radiography reveals that its internal components can move by as much as 200 μm under the influence of gravitational forces affecting the dose‐rate constant and relative dose distribution by as much as 5% and 10%, respectively. However, the clinical dose distribution can be well represented by a weighted average of dose distributions corresponding to three different orientation‐dependent internal geometry configurations. This average dose distribution agrees closely (within 1% at 1 cm) with that derived from the geometry specified by the vendor model (seed components uniformly spaced), from which the recommended relative dosimetry parameters were derived. An average dose‐rate constant of 0.935 cGy h−1 U−1 is recommended for clinical application and values of 0.919, 0.963, and 0.920 for vertical orientation, horizontal orientation, and the vendor specified geometries, respectively. The NIST wide‐angle free‐air chamber realization of the air‐kerma strength primary standard, was explicitly simulated. We show that the output of Ag characteristic x rays is about half that of the model 6711 source resulting in a relative dose distribution between that of the model 6711 source and pure x‐ray emitters. Finally, it is demonstrated that the TG‐43 point‐source formalism more accurately estimates solid‐angle weighted dose at small distances if one uses a radial dose function data prepared using a point‐source, rather than line‐source, geometry factor.
High energy electron beams in intensity modulated radiation therapy
Authors: Korevaar Erik
Published: July 2002

The aims of the thesis were the development of the phase space evolution (PSE) model for electron beam treatment planning, investigation of techniques to improve electron beam dose distributions, and investigation of the added value of high energy (⩽50 MeV) electron beams in intensity modulated radiotherapy. The PSE model is capable of calculating dose distributions by taking into account density variations in full three dimensions and is accurate and fast compared to Fermi–Eyges based pencil beam models or Monte Carlo models. A relatively simple model that requires only measurements as input is adequate to describe the initial electron beam. The PSE model can also be used for the calculation of pencil beam kernels for optimization of intensity modulated electron beam treatments. Techniques, such as intensity modulation, energy modulation, and mixing with photons are suggested to improve dose distributions resulting from electron beam irradiations from a single direction. It is shown that, by the combination of intensity modulated electron and photon beams from one direction, dose distributions can have steep dose falloffs similar to unmodulated electron beams, and narrow penumbras similar to photon beams. The added value of high energy electron beams was investigated in a comparative treatment planning study using optimized intensity modulated electron and photon beams. For the studied clinical cases use of intensity modulated photon beams gives significant improvements over conventional treatments. Inclusion of electron beams could only marginally improve the expected treatment outcome, although dose volume histograms of organs at risk show improvements, especially at lower dose levels (e.g., <40 Gy) in the treatments with electrons.
Techniques in dosimetry and 3‐D treatment planning for stereotactic radiosurgery/radiotherapy
Authors: Heydarian Mostafa
Published: July 2002

The goal of this work was to improve stereotactic radiosurgery (SRS) dose calculation and dosimetry techniques.\sMonte Carlo techniques were used to calculate and PTW diamond detectors were used to measure SRS dose distributions.\sA method of calculating dose distribution using sector‐integration of Monte Carlo‐generated pencil beam kernels is proposed.\sThis technique can be used both for circular and irregular fields.\sIt is especially useful in calculating the dosimetry for irregular fields, where the shape of the treatment field is determined during planning.\sMonte Carlo techniques were confirmed as reliable references for calculation of the dose distribution of SRS fields.\sDiamond detectors were found to be potentially ideal for SRS.\sThey yield more accurate results than the commonly used silicon diodes, ionization chambers, film or thermoluminescence dosimetry.\sBioeffect planning, as a rather new approach, was introduced in which treatment plans can be evaluated according to the bioeffect distribution rather than the dose distribution.\sDose can be converted to biologically effective dose using the alpha/beta ratio from the linear‐quadratic model.\sEstimates of TCP and NTCP can also be obtained using the Alpha, Beta values, dose‐volume histogram data, and in the case of normal tissue the partial volume of the organ irradiated.
A self‐collimating convolution backprojection algorithm for optimizing dose distributions of I‐125 prostate implants
Authors: Siochi R. Alfredo C. , Elson Howard R. , Foster Alvis E. , Lamba Michael A.
Published: July 2002

The algorithm presented here for optimizing brachytherapy dose distributions is based on the idea that the seed distribution can be modeled as an activity distribution determined analogously to gamma camera imaging. The peripheral dose to the tumor is converted to a set of uncollimated projection data that are then filtered and backprojected to produce an initial seed distribution. The actual doses resulting from the seed placement are used to correct the initial projection data for attenuation, scatter, and lack of collimation. The corrected projection data are backprojected a second time to yield the optimized but unconstrained seed distribution. Clinical constraints such as the number of different seed activities, the maximum seed activity, the minimum peripheral tumor dose, and the minimum percentage of the volume which receives less than a specified dose are then applied to the unconstrained solution. Through the entire process, the dose calculations are functions of source anisotropy, scatter, and attenuation. When applied to a set of elliptical contours, the algorithm produces elliptical peripheral dose isodose contours and reasonable dose volume histograms for a constrained solution. The results for actual patient prostate contours were not as good, primarily because of the difficulties encountered in dealing with the irregular geometry of the prostate. However, the algorithm shows promise for further research.
Computer‐aided gynecologic intracavitary brachytherapy
Authors: Li Shidong
Published: July 2002

The focus of this thesis is to address ways in which we can more accurately determine and assess the 3D dose in patients. Specifically, methods are developed to (a) reconstruct source positions in a Fletcher‐Suit applicator which do not depend on the visualization of the individual dummy sources, but use applicator geometry; (b) reconstruct 3D locus and sources without requirement of one‐to‐one point correspondence between films; (c) detect, quantify, and adjust for patient motion during acquisition of biplane radiographs; (d) localize source positions from CT ScoutView images; (e) calculate dose and overlay on tomographic images with correction of shielding effect; (f) estimate error in the dose‐volume (or surface) histogram calculation; and (g) investigate methods to combine external and intracavitary dose distributions which provides TCP and NTCPs for the tumor and organs at risk, respectively.
Confirmation of target localization and dosimetry for three‐dimensional treatment planning by magnetic resonance scanning of a ferrous sulfate gel head phantom
Authors: Chan Maria Fan
Published: July 2002

A detailed model of a FeMRI gel head phantom has been developed to verify the 3D radiation dose mapping of conformal therapy treatment planning. The phantom consists of a human skull filled with a solution that is 1 mM in ferrous sulfate, 1 mM in sodium chloride, and 0.1 N in sulfuric acid, gelled with 7.5% by weight gelatin. The phantom was irradiated and the absorbed dose from multiple non‐coplanar fields was measured using the MRI dose‐dependent variation of ferric sulfate. These studies were carried out using a 1.5 T MR imager and detailed analysis was performed on a UNIX workstation. The phantom and the calibration vials were scanned simultaneously using a multiple‐scan inversion recovery sequence. The values of were evaluated on a pixel‐by‐pixel basis for entire volume through the use of custom‐built software on the workstation and were converted to dose using calibration data. Comparisons of dose distributions between those measured by FeMRI and those calculated by 3D treatment planning show good agreement.
Uncertainty of calibrations at the accredited dosimetry calibration laboratories
Authors: Ibbott G. S. , Attix F. H. , Slowey T. W. , Fontenla D. P. , Rozenfeld M.
Published: July 2002

The American Association of Physicists in Medicine, through a subcommittee (formerly Task Group 3) of the Radiation Therapy Committee, has accredited five laboratories to perform calibrations of instruments used to calibrate therapeutic radiation beams. The role of the accredited dosimetry calibration laboratories (ADCLs) is to transfer a calibration factor from an instrument calibrated by the National Institute of Standards and Technology (NIST) to a customer's instrument. It is of importance to the subcommittee, to physicists using the services of the ADCLs, and to the ADCLs themselves, to know the uncertainty of instrument calibrations. The calibration uncertainty has been analyzed by asking the laboratories to provide information about their calibration procedures. Estimates of uncertainty by two procedures were requested: Type A are uncertainties derived as the standard deviations of repeated measurements, while type B are estimates of uncertainties obtained by other methods, again expressed as standard deviations. Data have been received describing the uncertainty of each parameter involved in calibrations, including those associated with measurements of charge, exposure time, and air density, among others. These figures were combined with the uncertainty of NIST calibrations, to arrive at an overall uncertainty which is expressed at the two‐standard deviation level. For cable‐connected instruments in gamma‐ray and x‐ray beams of HVL >1 mm Al, the figure has an upper bound of approximately 1.2%.
A digital method for computing target margins in radiotherapy
Authors: Bedford J. L. , Shentall G. S.
Published: July 2002

Computer methods for determining the planning target volume from the gross tumor volume for both conventional and conformal radiotherapy are presented. Production of a two‐dimensional (2D) treatment plan is assisted by projecting outlines of the gross tumor onto a single transverse plane, so that the total extent of the tumor can be easily visualized. A 2D margin can then be added to the resulting outline, so as to account for microscopic tumor spread, organ motion, and setup uncertainty. The margin may be anisotropic to account for the known differences in setup accuracy in the anterior, posterior, left and right directions. For three‐dimensional (3D) treatment planning, it is necessary to add a 3D margin to the gross tumor volume to define the planning target volume, the anisotropy of the margin now being allowed to extend to the superior and inferior directions also. Robust methods for automatically calculating these regions are described, and illustrated for the case of a prostate tumor. It is demonstrated that while a slicewise 2D margin is adequate for 2D planning, a fully 3D margin must be used for 3D conformal planning to avoid underdosing the superior and inferior extremities of the clinical target volume.
Computer‐aided diagnosis: Automatic detection of malignant masses on digital mammograms
Authors: Mendez Arturo J.
Published: July 2002

A computerized method to automatically detect malignant masses on digital mammograms based on bilateral subtraction to identify asymmetries between left and right breast images was developed. After the digitization, the breast border and nipple are automatically detected in order to align left and right mammograms. Images are corrected to avoid differences in brightness due to the recording procedure. Left and right mammograms are subtracted, and a threshold is applied to obtain a binary image with the information of suspicious areas. The asymmetries are extracted by region growing. Size and eccentricity tests are used to eliminate false‐positive responses and texture features are extracted from suspicious regions to reject normal tissue regions. The scheme was tested in 70 pairs of digital mammograms and a true‐positive rate of 71% with an average number of 0.67 false‐positives per image was achieved. Computerized detection was evaluated by free‐response operating characteristic analysis (FROC). An area under the AFROC (A1) of 0.667 was obtained. Our results show that the scheme could help radiologists as a second reader in mammographic screening. The low number of false‐positives indicates that our scheme would not add confusion to radiologists, detecting normal regions as suspicious.
Measurement of backscatter to the monitor chamber of medical accelerators using target charge
Authors: Lam Kwok L. , Muthuswamy Moorthy S. , Ten Haken Randall K.
Published: July 2002

A simple noninvasive method is described for determining the backscatter to a monitor chamber of a medical accelerator based on the measurement of charge deposited in the target. This method is compared quantitatively to the more elaborate telescopic method for photon beams of 6 MV and 15 MV on linear accelerators having mica and Kapton monitor chambers. The new target charge method gives results consistent with the telescopic method to within 0.3%.
Relation between radiographic mottle for double and single emulsions
Authors: Arimura Hidetaka , Okawa Tomohiko , Okamoto Mitsuhide , Ikari Takeshi , Nakamori Nobuyuki , Kubota Hideaki , Matsumoto Masao , Takigawa Atsushi , Kanamori Hitoshi
Published: July 2002

The radiographic density fluctuations produced by using dual screen‐film systems are designated as the radiographic mottle. The density fluctuation of the radiographic mottle for the double emulsions at a density of the double emulsions consists of those for the front and back emulsions on a radiograph. However, the relation between the Wiener spectra of the radiographic mottle for the double and single emulsions had not been studied. Hence we compared the Wiener spectra of the radiographic mottle for the double emulsions with the sum of those for the front and back emulsions on the same radiographs and with the sum of those for the emulsions at the same densities. At all densities of more than 0.62 for lower spatial frequencies (), the Wiener spectral values of the radiographic mottle for the double emulsions were greater than the sum of those for the front and back emulsions for both comparisons on the same radiographs and at the same densities. In order to investigate the reason of the above phenomena, we separated the Wiener spectral values of the radiographic mottle for various densities into those of the three factors, i.e., quantum mottle, structure mottle, and film granularity, and performed the same comparisons as the radiographic mottle. Also, to explain the results for the three factors, we obtained the Wiener spectral values of the spatial fluctuations of the light exposure or the fluorescence intensity and the gradients of the characteristic curves of the film for the double and single emulsions of the x‐ray film. As a result of the investigation, we found that the phenomena on the radiographic mottle were caused by that (1) on the same radiographs the squares of the gradients of the characteristic curves for the double emulsions were about 5.3 times as great as those for the single emulsion at densities of more than 0.62 of the double emulsions, and (2) at the same density of more than 0.62 those were more than about 2.2 times as great as those for the single emulsion.
The use of an electronic portal imaging system to measure portal dose and portal dose profiles
Authors: Parsaei Homayoon , El‐Khatib Ellen , Rajapakshe Rasika
Published: July 2002

The dosimetric characteristics of a scanning liquid‐filled ionization chamber (SLIC) electronic portal imaging device have been investigated. To assess the system's response in relation to incident radiation beam intensity, a series of characteristic curves are obtained for various field sizes and nominal energies of 6 and 10 MV photons. The response of the imaging system is dependent on incident radiation intensity and can be described to within 1% accuracy on central axis using a square root function. Portal dose measurements with the SLIC at the plane of the detector, on central axis of the beam using homogeneous attenuating phantom materials show that the imaging system is capable of measuring the portal (transmission) dose to within 3% of the ionization chamber results for homogeneous material. For two‐dimensional dosimetry applications, the system is calibrated with a 10 cm Perspex™ block used as beam flattening material on the detector cassette to correct for variations in individual ion chamber sensitivity and the effect of nonuniform beam profiles produced by the flattening filter. Open and wedged dose profiles measured with the SLIC agreed with ion chamber measured profiles to within 3.5% accuracy.
Rapid portal imaging with a high‐efficiency, large field‐of‐view detector
Authors: Mosleh‐Shirazi M. A. , Evans P. M. , Swindell W. , Symonds‐Tayler J. R. N. , Webb S. , Partridge M.
Published: July 2002

The design, construction, and performance evaluation of an electronic portal imaging device (EPID) are described. The EPID has the same imaging geometry as the current mirror‐based systems except for the x‐ray detection stage, where a two‐dimensional (2D) array of 1 cm thick CsI(Tl) detector elements are utilized. The x‐ray quantum efficiency of the scintillation detector and its field‐of‐view at the isocenter are greater than other area‐imaging EPIDs. The imaging issues addressed are theoretical and measured signal‐to‐noise ratio, linearity of the imaging chain, influence of frame‐summing on image quality and image calibration. Portal images of test objects and a humanoid phantom are used to measure the performance of the system. An image quality similar to the current devices is achieved but with a lower dose. With dose delivered by a 6 MV beam, a 2 mm diam structure of 1.3% contrast and an 18 mm diam object of 0.125% contrast can be resolved without using image‐enhancement methods. A spatial resolution of about 2 mm at the isocenter is demonstrated. The capability of the system to perform fast sequential imaging, synchronized with the radiation pulses, makes it suitable for patient motion studies and verification of intensity‐modulated beams as well as its application in cone‐beam megavoltage computed tomography.
A bent Laue–Laue monochromator for a synchrotron‐based monochromatic x‐ray computed tomography system
Authors: Ren Baorui
Published: July 2002

A bent Laue–Laue monochromator for wide, fan‐shaped synchrotron x‐ray beams was designed and constructed for the program Multiple Energy Computed Tomography (MECT) at the National Synchrotron Light Source (NSLS). MECT uses monochromatic x‐ray beams from the NSLS's X17B beamline for CT with improved image contrast‐to‐noise ratio, image quantification, and the efficacy for dual‐energy imaging. The new monochromator uses two Czochralski‐grown Si〈111〉 crystals, which are 0.7 and 1.4 mm thick, respectively, and are cut with thick ribs on their upper and lower ends. The crystals are bent cylindrically, using 4‐rod benders employing two fixed and two movable rods. The bent‐crystal method was needed to solve the difficulties we had with the flat Laue–Laue monochromator previously used in MECT; these included: (a) insufficient beam intensity, (b) excessive beam‐intensity fluctuations, and (c) instability of the beam's horizontal profile shape. Compared with flat Laue–Laue monochromator, the bent Laue–Laue device tested at 42 keV and 108 keV showed ∼10‐fold increase in the beam flux, ∼5‐fold improvement of beam‐profile stability, ∼10‐fold suppression of beam harmonic contamination at the bending radius of 15 m, and a smaller energy bandwidth at certain bending radii. The new monochromator should reduce MECT's image noise and artifacts, bringing its performance close to that of the ideal CT.
Moving segments region of interest attenuator for x‐ray fluoroscopy
Authors: Sassi Salem A. , Britten Alan J.
Published: July 2002

Region of interest (ROI) imaging has previously been proposed as a means of reducing x‐ray fluoroscopy radiation dose. Previous ROI attenuators made of partially attenuating metal plates change beam quality, which may lead to uncertainty in image restoration procedures. The design and construction of a prototype moving segments ROI attenuator (MS‐ROI), which maintains beam quality across the whole field of view is described. The x‐ray beam intensity is halved by 36 lead segments which are rapidly rotated between the x‐ray tube and patient, with a central hole projecting a circular ROI at full intensity. Image processing techniques with automatic detection of the ROI boundary were used to homogenize image brightness across the whole image. Images restored using these techniques were judged to be visually acceptable, with a good match between pixel values inside and outside the ROI. Image contrast within the ROI was improved by 18% due to reduced scatter and veiling glare from the periphery. The introduction of the MS‐ROI attenuator also results in a 48% increase in statistical noise in the area outside the ROI, with no significant change in object contrast. The patient entrance dose measured using the dose area product (DAP) method was reduced by 53.4% under manual exposure control, with the dose to operators reduced by 48.4% under automatic brightness control. Further work is needed to determine whether the attenuator can be used with pulsed fluoroscopy, and to reduce vibrational effects on the ROI boundary. The MS‐ROI attenuator provides a more constant ratio of central‐to‐peripheral image intensity, and maintains uniform beam quality and image contrast across the whole image in comparison to simple metal plate attenuators.
Tomographic convolutional dosimetry of interstitial brachytherapy implants
Authors: Ove Roger
Published: July 2002

A method for approximation of the dose distribution generated by an interstitial brachytherapy implant is introduced. The method employs convolution of raw tomographic data, as a measurement of attenuation of CT pencil beams by the metallic substrate and casing of radioactive seeds. A convolution kernel is introduced in the algorithm for CT image reconstruction such that the tomographic computation results in an approximation of absorbed dose. Error estimates are discussed, and limitations of the method and the domain of validity are discussed. Equivalence with a technique of employing convolution methods to the CT image itself is demonstrated. Calibration and testing on phantom implants is underway.
Iterative image warping technique for temporal subtraction of sequential chest radiographs to detect interval change
Authors: Ishida Takayuki , Katsuragawa Shigehiko , Nakamura Katsumi , MacMahon Heber , Doi Kunio
Published: July 2002

A temporal subtraction technique has been developed to assist radiologists in the detection of interval changes on chest radiographs. Although the overall performance of the current temporal subtraction technique is relatively good, severe misregistration errors, mainly due to AP inclination and/or rotation, are observed in some cases. In order to reduce these errors, we attempted to improve the subtraction scheme by applying an iterative image warping technique. In cases obtained with the new temporal subtraction technique 177 (97.8%) of 181 showed adequate, good, or excellent quality. We also found that 156 (86.2%) of cases obtained with the new scheme showed improvements in the quality of the subtraction images compared with the previous scheme. The results indicate that the performance of the temporal subtraction technique was greatly improved by use of the iterative image warping technique.
Performance analysis of a new semiorthogonal spline wavelet compression algorithm for tonal medical images
Authors: Thompson Stephen K. , Hazle John D. , Schomer Donald F. , Elekes Almos A. , Johnston Dennis A. , Huffman John , Chui Charles K.
Published: July 2002

Lossy image compression is thought to be a necessity as radiology moves toward a filmless environment. Compression algorithms based on the discrete cosine transform (DCT) are limited due to the infinite support of the cosine basis function. Wavelets, basis functions that have compact or nearly compact support, are mathematically better suited for decorrelating medical image data. A lossy compression algorithm based on semiorthogonal cubic spline wavelets has been implemented and tested on six different image modalities (magnetic resonance, x‐ray computed tomography, single photon emission tomography, digital fluoroscopy, computed radiography, and ultrasound). The fidelity of the reconstructed wavelet images was compared to images compressed with a DCT algorithm for compression ratios of up to 40:1. The wavelet algorithm was found to have generally lower average error metrics and higher peak‐signal‐to‐noise ratios than the DCT algorithm.
Oximetry based on diffuse photon density wave differentials
Authors: Ntziachristos Vasilis , Kohl Matthias , Ma Honyan , Chance Britton
Published: July 2002

The quantification of tissue optical properties for calculating blood saturation and hemoglobin concentration using measurements of diffuse photon density waves at some distance away from an intensity‐modulated light source, generally requires the determination of the amplitude and phase of this light source. This determination may become a severe impediment for measurements performed in the clinical environment. In this work we extend a self‐calibrating methodology developed for constant wave and modulation depth‐phase measurements, to include amplitude and phase measurements of diffuse photon density waves. The method uses amplitude and phase changes of intensity modulated light, under the assumption of known index of refraction and invariant reduced scattering coefficient to quantify the absorption coefficient without requiring initial amplitude and phase knowledge. Quantification of the at selected time points during a measurement can then be employed to calibrate numerical solutions of the diffusion equation and compute the for the remaining time points of the experiment. It is shown that the method is quite insensitive to the knowledge of the exact value so that an assumption on the average value for the tissue measured may be employed. The sensitivity of calculating blood saturation and hemoglobin concentration, as a function of the deviation of the used in the calculation versus the real value is investigated using simulated data. It is also demonstrated that the saturation calculation is especially insensitive to the guess. The performance of the method to quantify blood oxygen saturation and the concentrations of oxy‐ and deoxy‐hemoglobin is examined with experimental measurements at two wavelengths on specially constructed blood model phantoms. To validate the method the measurements are monitored by a time‐resolved spectrometer. The method is shown to be accurate to within ±5% in calculating blood saturation and to within ±10% in calculating hemoglobin concentration compared to the results obtained with the time‐resolved spectrometer and the expected theoretical values.
Partially wedged radiation beams
Authors: Muren Ludvig Paul , Hafslund Rune , Murman Anders , Valen Harald , Dahl Olav
Published: July 2002

To increase dose homogeneity within certain radiotherapy targets, we defined a partially wedged radiation beam as a beam with wedge modification in one part of the field only. Partially wedged beams may be beneficial in cases with curved surfaces inside parts of the beam only, where they may compensate for missing tissue and/or for variations in depth to the target region. Possible sites suitable for partially wedged beams include urinary bladder and tangential breast irradiation. Customized partially wedged beams were delivered applying dynamic collimation techniques. Two different linear detector arrays, a semiconductor diode array and an ionization chamber array, were used independently in the same standard water tank to verify that the partially wedged beams were delivered according to the definition. Dose calculations of partial wedge fields were implemented in our treatment planning system and compared with the measured dose distributions. We re‐planned a representative treatment plan for both advanced urinary bladder cancer and tangential breast irradiation using partially wedged beams. For both patients the target dose homogeneity was improved, and the doses to surrounding critical normal tissues were reduced.
Lens distortion in optically coupled digital x‐ray imaging
Authors: Liu Hong , Jiang Hangyi , Fajardo Laurie L. , Karellas Andrew , Chen Wei R.
Published: July 2002

The objectives of this research are to analyze geometrical distortions introduced by relay lenses in optically coupled digital x‐ray imaging systems and to introduce an algorithm to correct such distortions. Methods: The radial and tangential errors introduced by a relay lens in digital x‐ray imaging were experimentally measured, using a lens‐coupled CCD (charge coupled device) prototype. An algorithm was introduced to correct these distortions. Based on an x‐ray image of a standard calibration grid, the algorithm first identified the location of the optical axis, then corrected the radial and tangential distortions using polynomial transformation technique. Results: Lens distortions were classified and both radial and tangential distortions introduced by lenses were corrected using polynomial transformation. For the specific lens‐CCD prototype investigated, the mean positional error caused by the relay lens was reduced by the correction algorithm from about eight pixels (0.69 mm) to less than 1.8 pixels (0.15 mm). Our investigation also shows that the fourth order of polynomial for the correction algorithm provided the best correction result. Conclusions: Lens distortions should be considered in position‐dependant, quantitative x‐ray imaging and such distortions can be minimized in CCD x‐ray imaging by appropriate algorithm, as demonstrated in this paper.
Monte Carlo modeling of radiation dose distributions in intravascular radiation therapy
Authors: Stabin Michael G. , Konijnenberg Mark , Knapp F. F. , Spencer Robert H.
Published: July 2002

Radiation dose distributions are developed for balloon and wire sources of radioactivity within coronary arteries. The Monte Carlo codes MCNP 4B and EGS4 were used to calculate dose distributions for photons and electrons at discrete energies around such sources, with and without the presence of a high‐density atherosclerotic plaque. An interactive computer program was developed which then calculates dose distributions for many radionuclides by applying the emission spectra to the discrete energy grids calculated by the Monte Carlo codes, weighting appropriately for electron energy and abundance. Results for Re‐186 and Re‐188 balloon sources are shown in comparison to an Ir‐192 wire source. The program provides dose distributions as well as estimates of activity levels needed to deliver prescribed doses to the vessel wall at selected distances from the lumen in a selected time interval. In addition, dose calculations are presented in this paper for other organs in the body, from photon radiation as well as from possible loss of liquid activity into the bloodstream in the case of a balloon rupture. These results, especially the interactive computer program permitting easy comparison of various radionuclides and their physical characteristics, will greatly facilitate the comparison process and aid in the selection of the best candidate(s) for clinical use.
Quantitative assessment of vocal development in the zebra finch using self-organizing neural networks
Authors: Janata Petr
Published: July 2002

To understand the mechanisms of song learning by songbirds it is necessary to have in hand tools for extracting, describing, and quantifying features of the developing vocalizations. The extremely large number of vocalizations produced by juvenile zebra finches and the variability in these vocalizations during the sensorimotor learning period preclude manual scoring methods. Here we describe an approach for classification of vocalizations produced during sensorimotor learning based on self-organizing neural networks. This approach allowed us to construct probability distributions of spectrotemporal features recorded on each day. By training the network with samples obtained across the course of vocal development in individual birds, we observed developmental trajectories of these features. The emergence of stereotypy in sequences of song elements was captured by computing the entropy in the matrices of first- and second-order transition probabilities. Self-organizing maps may assist in classifying large libraries of zebra finch vocalizations and shedding light on mechanisms of vocal development.
Lateral resonances in 1–3 piezoelectric periodic composite: Modeling and experimental results
Authors: Certon Dominique , Patat Frédéric , Levassort Franck , Feuillard Guy , Karlsson Brynjar
Published: July 2002

The objective of this work is to provide an accurate model of the lateral resonance modes in 1–3 piezoelectric composite materials. These materials are widely used in ultrasonic transducers and the lowest lateral mode frequency gives the upper limit for the usable transducer bandwidth. Considering the propagation of purely transverse waves in a 2-D periodic medium of infinite thickness, two different approaches for obtaining the solutions are presented and compared. The first approach is based on the use of the Bloch waves theory. The second is a straightforward method (a so-called membrane method) which consists in numerically solving the propagation equation in the two-phase medium while taking into account the periodic boundary conditions. Methods based on both models are described that allow the calculation of the dispersion curves and the stop band limits, as well as the frequencies and the displacement fields of the lateral modes. A test case is used to compare and discuss the theoretical predictions provided by each model. The calculations of the first lateral mode frequency are compared with experimental values obtained for samples with different ceramic volume fractions. The conclusion reached indicates that the infinite thickness assumption is valid for plates of practical interest and that the membrane model enables the prediction of lateral mode frequency with low computation effort and an accuracy better than 5%.
Scattering of interface waves from pointlike obstacles
Authors: Smith Eric
Published: July 2002

A method is presented for efficiently computing or estimating low-order elastic wave scattering, from a pointlike inhomogeneity in a bounded medium, into interface as well as bulk modes. The particular system considered is a half-space of fluid overlying a half-space of a linear elastic solid, with the obstacle placed in the solid very near the interface. Boundary constraints are enforced as an improvement to the bulk scattering vertex, so that the stratified-medium Green’s function is used to propagate scattered waves at each perturbative order. Analytic approximation of the first-order scattering amplitude, based on the symmetries of the scattering vertex and the interface-wave pole structure, makes it easy to identify the qualitatively different components of the scattered wave, and their dependence on medium properties.
The level dependence of response phase: Observations from cochlear hair cells
Authors: Cheatham M. A. , Dallos P.
Published: July 2002

Hair cell responses are recorded from third turn of the guinea pig cochlea in order to define the relationship between hair cell depolarization and position of the basilar membrane. Because the latter is determined locally, using the cochlear microphonic recorded in the organ of Corti (OC) fluid space, no corrections are required to compensate traveling wave and/or synaptic delays. At low levels, inner hair cells (IHC) depolarize near basilar membrane velocity to scala vestibuli reflecting the free standing nature of their stereocilia. At high levels, the time of depolarization changes rapidly from velocity to scala vestibuli to the scala tympani phase of the basilar membrane response. This change in response phase, recorded in the fundamental component of the IHC response, is associated with a decrease in response magnitude. The absence of this behavior in OC and outer hair cell responses implies that basilar membrane mechanics may not be responsible for these response patterns. Because these features are reminiscent of the magnitude notches and the large phase shifts observed in single unit responses at high stimulus levels, they provide the IHC correlates of these phenomena.
High-resolution velocity images of targets buried in soils
Authors: Sabatier James , Xiang Ning , Craig Ron
Published: July 2002

Acoustic-to-seismic coupling of airborne sound into the ground is used to detect objects buried in air-filled soils. Target sizes of 0.1–0.25 m are buried in various soils to depths of several centimeters. A sound source suspended in the air above the ground is used to excite vibrations in the poro-elastic soil. Audio frequencies of 1–10 kHz are broadcast. The acoustically induced seismic vibrations are measured using an off-the-shelf scanning laser Doppler vibrometer (LDV). Previous measurements with buried microphones and geophones and modeling show the Biot type II wave carries most of the energy near the soil surface. This wave is locally coupled into the soil, scattered from near-surface buried targets, and then returns to the soil surface. The scattered acoustic velocity field on the surface (summed with the incident and reflected fields) is imaged using the LDV. Using a high spatial resolution scan of 0.003 m, scattered wavefronts from buried targets are observed. These images facilitate visual determination of the target shape. Targets used thus far include solid iron circles, squares, and triangles. [Work supported by CECOM, Night Vision & Electronic Sensor Directorate.]
A summary of target strength observations on fishes from the shelf off West Africa
Authors: Svellingen Ingvald , Ona Egil
Published: July 2002

In many areas, and in tropical waters in particular, it may be difficult to fulfill the resolution criteria for high-quality target strength measurements both with respect to density and species mixing. Since 1985 target strength data have been collected periodically during cruises with R/V DR. FRIDTJOF NANSEN in West African waters. As a first attempt to establish a reasonably correct target strength for important species in the area, swimbladder morphology was studied and compared with species from the North Atlantic. After 1986, in situ target strength data was collected off Morocco and in Namibian waters using split-beam echo sounders. Data from the following species have been analyzed and discussed: Pilchard (Sardina Pilchardus), horse mackerel (Trachurus Capensis), hake (Merluccius Capensis), and myctophids. Only data which were considered to be of high quality have been analyzed. The calculated average target strength for pilchard are several decibels higher than the currently applied target strength of North Sea herring, but not very different from other reported target strength of sardine in other areas. Also, the target strength of horse mackerel is high compared to the presently applied target strength. For hake, the measured target strength are comparable to reported data for gadoids.
Perceptual segregation of competing speech sounds: The role of spatial location
Authors: Drennan Ward , Gatehouse Stuart , Lever Catherine
Published: July 2002

Culling and Summerfield [J. Acoust. Soc. Am. 92, 785–797 (1995)] showed that listeners could not use ITDs (defined as differences in interaural phase) to perceptually segregate the sounds. The present studies tested a free-field analog of their conditions. Listeners’ ability to identify one of two competing whispered ‘‘vowels’’ was determined. The ‘‘vowels’’ consist of narrow bands of noise, which represent the first and second formants of ‘‘ar,’’ ‘‘ee,’’ ‘‘er,’’ and ‘‘oo.’’ A target noise band pair (vowel) was presented at various angles on the listener’s left and a distracter was presented on the listener’s right. Listeners correctly identified the vowels in the free-field well above chance. To control for head movements, the stimuli were recorded through a head and torso simulator and presented to the listeners over headphones. Performance remained well above chance. The results show that, although a phase shift alone is not sufficient to achieve segregation, cues provided in real-world listening (ITD, ILD and onset-asynchrony) can provide sufficient information for this purpose. Further experiments systematically remove these cues evaluating their contribution to the task.
Talker variability effects in auditory-visual speech perception
Authors: Carney Arlene E. , Clement Bart R. , Cienkowski Kathleen M.
Published: July 2002

Talker variability effects have been demonstrated for many aspects of syllable, word, and sentence recognition for stimuli presented in the auditory-only modality. In the current investigation, talker variability effects were examined in multiple modalities—visual-only, auditory-only, and auditory-visual. Auditory-visual presentations were of two types: consonant [auditory and visual were of the same token—/visual bi and auditory bi/] or disparate [/visual gi and auditory bi or visual bi and auditory gi/]. The disparate condition has elicited ‘‘the McGurk effect,’’ in which listeners may report a fused response that is neither the auditory or the visual component of the stimulus. Eleven talkers (five male and six female) of varying cultural backgrounds were videotaped. Their productions of CV syllables were presented to adult listeners who reported their percepts. Individual talkers elicited different degrees of syllable fusion among listeners, despite their equivalent auditory intelligibility. In addition, individual listeners varied considerably in their overall perceptions, from complete fusion for all talkers to almost no fusion for any talker. Results suggest that listeners may employ different types of processing strategies for bimodal stimuli. [Research supported by NIDCD.]
Decay of Normal Stresses Developed in Capillary Flows of Viscoelastic Fluids
Authors: Powell Robert L. , Middleman Stanley
Published: July 2002

The expansion of a jet of viscoelastic fluid issuing from a capillary is measured. Two methods are suggested for estimating a relaxation time of the fluid. In one the dependence of the final jet diameter on residence time in the capillary is determined, and the calculated primary normal stress differences are interpreted through a model for the response of a Bogue fluid to the sudden imposition of simple shear. In the other, the shape of the extrudate is used to give an estimate of the normal stress difference as a function of distance from the capillary exit, and the data are interpreted through the solution for the stress decay in a Bogue fluid in steady shear, subject to a sudden cessation of shear. The resulting values of relaxation times as a function of shear rate are in good agreement for these two types of experiments, and agree well with values of the relaxation time based on the equilibrium normal stress difference as a function of shear rate.
Rheological Properties of Molten Polymers. III. Shear-Dependent Melt Viscosity
Authors: Hyun Kun S. , Karam Henry J.
Published: July 2002

The steady-state capillary flow data for several commercial polystyrene melts have been analyzed, and the shear-dependency of viscosity for these melts has been compared with theoretical results from Pao’s nonlinear hydrodynamic theory. Pao’s theory allows one to calculate the shear-dependent viscosity in terms of viscoelastic constants, such as retardation times and shear compliances which can be obtained from tensile creep experiments. Using the viscoelastic data obtained in Part II of this series in Pao’s equation, a semiquantitative agreement has been obtained. In most cases, the agreement was within a factor of two in the range where experimental data were available. Introducing a modifying factor common to several commercial polystyrene melts in calculating the retardation spectrum has improved the agreement between experimental data and theoretical results.
Mill Processing Behavior of Ethylene-Propylene-Hexadiene Terpolymers
Authors: Shih Chi-Kai
Published: July 2002

The mill processing behavior of ethylene/propylene/hexadiene terpolymers, one type of EPDM, has been described. In agreement with the general observations of Tokita and White, the milling characteristics of the polymers change from an elastic band (region 2) to a crumbling bag (region 3) and then to a viscoelastic fluid (region 4) as the temperature is increased from 5°C to 175°C. The 2–3 transition can be treated as a failure process which is time and temperature dependent. Hence, a continuous curve relating the transition nip setting; and the temperature exists for each polymer sample. The transition nip setting at a given temperature (or the transition temperature at a given nip width) was found to be wider (or higher) for samples of broader molecular weight distribution.
Molecular Motion in Block Copolymers
Authors: Legrand D. G.
Published: July 2002

It has been found that high-resolution proton magnetic resonance spectra can be obtained on swollen polymers and copolymers as well as plasticized polymers. In block copolymers of bisphenol-A polycarbonate and polydimethyl siloxane of constant composition and molecular weight which have been swollen with deuterated cyclohexane, the mobility of the aromatic protons is dependent upon block length, while the mobility of the gem dimethyl and the siloxane methyl protons is not. This information will be discussed and used to interpret the morphology and properties of such copolymers.
Capillary thinning of polymeric filaments
Authors: Kolte Mette Irene , Szabo Peter
Published: July 2002

The capillary thinning of filaments of a Newtonian polybutene fluid and a viscoelastic polyisobutylene solution are analyzed experimentally and by means of numerical simulation. The experimental procedure is as follows. Initially, a liquid sample is placed between two cylindrical plates. Then, the bottom plate is lowered under gravity to produce a specified strain. The sample is thereby stretched into a filament. Provided the filament is sufficiently long, surface tension will induce a thinning of the filament until breakup in finite time. The numerical simulations are performed with a Lagrangian finite element method and show good agreement with the experiments. A comparison of the results with existing theory in the literature reveals differences between the theoretical predictions and the real behavior, both for Newtonian and viscoelastic fluids. The origin to the divergence is analyzed and quantified.