bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of the Clinac‐35 for tissue activation in noninvasive measurement of capillary blood flow
Authors: Nussbaum Gilbert H. , Purdy James A. , Granda Conrad O. , Emami Bahman , Sapareto Stephen A.
Published: August 2003

Through adjustment of operating parameters responsible for the acceleration, steering, and focusing of electrons en route to the target, we have extracted a 30‐MV x‐ray beam from the Clinac‐35 linear accelerator that is suitable for use in photon activation −15O decay studies of tissue perfusion. This beam is significantly more efficient than the “standard” 25‐MV beam in producing 15O in situ through activation of tissue oxygen, thereby substantially reducing the dose to tissue required to yield a desired initial 15O activity. Production and characteristics of the higher energy beam are discussed, and data obtained from its application to measurement of capillary blood flow in animal tumors are presented and analyzed.
Effect of the accelerator design on the position of the effective electron source
Authors: Cecatti Eugenio Roberto , Gonçalves Joel Francisco , Cecatti Sonia Garcia Pereira , da Penha Silva Maria
Published: August 2003

An analysis of the effective source–surface distance was made taking into account the influence of energy, field size, equivalent square field size, and the accelerator design. Measurements made in three accelerators showed the influence of parameters like beam flattening and collimation systems on the effective source–surface distance. A brief review of the theory is made leading to a mathematical expression for the effective source–surface distance, showing a good agreement with the experimental results. An alternative method for correcting the dose rate, maintaining the nominal source–surface distance, and not using the inverse square law, but a greater power function, is suggested.
Acceptance testing and quality assurance of automated scanning film densitometers used in the dosimetry of electron and photon therapy beams
Authors: Holmes Timothy W. , McCullough Edwin C.
Published: August 2003

Scanning film densitometry routinely used for obtaining dosimetric information about therapy treatment beams is subject to several sources of inaccuracies. The most significant of these are described and appropriate methods of testing are presented. This establishes the need for adequate acceptance testing followed by a quality assurance program if these devices are to be used to provide accurate relative dose information over extended periods of time.
Geometric reconstruction of seed implants using a three‐film technique
Authors: Biggs P. J. , Kelley D. M.
Published: August 2003

A computer program has been written to reconstruct the three‐dimensional distribution of radioactive seed implants. The technique requires the use of three rotated films with random input of the seeds. This algorithm differs from previous ones in the constraints that are applied, since no correlation is made between distances on the three films from the plane of rotation through the isocenter nor is there a correlation between the distances from the axis of rotation. Instead, projections from the x‐ray source to the seed images on each of the three films are compared to see which combinations of seeds intersect at a point and therefore correspond to real seeds. This program has been written in Fortran IV for the PDP VAX 11/780 and 11/34 computers. Changes have been made to the TP11 program “QSEED” on the PDP 11/34 to accommodate the coordinate data from this program to produce the requisite isodose curves.
Determination of x‐ray spectral distribution from transmission measurements using K‐edge filters
Authors: Chu K. , Fenster A.
Published: August 2003

Accurate high‐resolution x‐ray spectral distributions can be obtained with an intrinsic germanium crystal. This technique is difficult to implement since it requires expensive, complicated, and bulky equipment. If energy resolution requirements are relaxed, then the above drawbacks can be overcome and an x‐ray spectrometer that is small, inexpensive could be built. A technique that meets these requirements is described here. X‐ray spectra with about 7‐keV resolution can be achieved by obtaining transmission measurements through a number of differentK‐edge filters. Using these measurements, a system of equations is set up and solved giving the required spectral distribution. The technique has been tested using 80, 100, and 120‐kVcp x‐ray beams with total filtrations of 3.78, 5.78, 9.06 mm Al, and 3.78 mm Al+0.137 mm Ho. The results show that the calculated spectra closely resemble tabulated spectra. The errors ranged from 3% to 13%. The half‐value layers (HVL) were also calculated and compared to the HVL obtained from tabulated spectra and were found to differ by about 7%.
Design and evaluation of a radio frequency coil for nuclear magnetic resonance imaging of fluorine and protons
Authors: Joseph Peter M. , Fishman Joel E.
Published: August 2003

A frequency‐switchable, homogeneous‐field rf saddle coil has been designed for imaging both protons (1H, 59.1 MHz) and fluorine (19F, 55.6 MHz) on a 1.4‐T superconducting small‐bore nuclear magnetic resonance imager. Frequency and impedance switching is accomplished by external capacitance and cable length changes; these operations permit imaging of both nuclei without perturbing the sample. The coil is optimized for 19F operation, yet performs better at the proton frequency than does the unswitched 19F coil. The angular distribution of the coil's wires and the use of distributed capacitors are designed to optimize field homogeneity and Q. A quantitative image of field homogeneity is presented. The coil is suitable for imaging small animals (7‐cm‐diam bore) and couples far better to small samples than does our standard receiver coil (15.2 cm in diameter). Images of phantoms and rats injected with a perfluorinated blood substitute are presented.
Noise in stenosis measurement using digital subtraction angiography
Authors: Wright G. A. , Taylor K. W. , Rowlands J. A.
Published: August 2003

This paper examines statistical errors in the measurement of arterial stenoses by digital videodensitometry. Images of vessel phantoms were acquired using digital subtraction angiographic techniques with low concentrations of an iodine contrast medium and low levels of x‐ray exposure. Effects of the spatial and temporal averaging of image information on signal‐to‐noise ratios in the stenosis measurement were of primary interest. The influences of iodine concentration, x‐ray scatter, veiling glare, x‐ray energy spectrum, x‐ray exposure, and detective quantum efficiency of the system were also included in the theoretical analysis. The agreement between theoretical calculation and experimental measurement of a simulated vessel was verified using measured values of the imaging system parameters. With a 14.2 mg/ml iodine concentration, using 20 mR per image at the entrance to a 13‐cm water phantom, and averaging over a 6‐mm length of a vessel 6.2 mm in diameter, the standard deviation in a measurement of a vessel's relative cross‐sectional area was about 0.05. The extension of these results to practical applications in vivo is discussed.
Investigation of basic imaging properties in digital radiography. 6. MTFs of II–TV digital imaging systems
Authors: Fujita Hiroshi , Doi Kunio , Giger Maryellen Lissak
Published: August 2003

We devised a new, simple technique for measuring the modulation transfer function (MTF) of a digital imaging system by using an image of an angulated slit. With this technique, the “presampling” analog MTF, which includes the geometric unsharpness, the detector unsharpness, and the unsharpness of the sampling aperture, can be measured even beyond the Nyquist frequency. A single‐frame image of a slightly angulated slit was employed in order to obtain Fourier transforms of line spread functions at different alignments. The presampling MTF was determined by averaging the two Fourier transforms which we obtained from two extreme alignments (center and shifted) of the slit relative to the sampling coordinate. The presampling MTFs of our digital subtraction angiographic system were determined in two orthogonal directions for three different image‐intensifier modes.
Validity of the concept of separating primary and scatter dose
Authors: Mohan Radhe , Chui Chen‐Shou
Published: August 2003

The technique of separating dose into primary and scatter components for calculating photon dose distributions is widely used. The primary and scatter dose models ignore the fact that electrons have a finite range. This may be a good approximation for 60Co photons but not for higher energies. For the latter, the range of electrons may be several centimeters in soft tissue and even longer in lungs and will lead to errors in computed dose in regions where electronic equilibrium does not exist. Ignoring the finite range of electrons will affect dose at points such as those near the beam boundaries, near inhomogeneity boundaries, and at bone–soft‐tissue interfaces. Other possible problems associated with the definition and use of “primary” and “scatter” dose in dose distribution calculations result from extrapolation of measured data to obtain data for zero and very large field sizes and from the use of these quantities, which are defined for central axis, for points at large distances from the central axis. This paper examines the limits of the validity of these assumptions.
Interaction of low‐energy photons with biological materials and the effective atomic number
Authors: Rao B. V. Thirumala , Raju M. L. N. , Narasimham K. L. , Parthasaradhi K. , Rao B. Mallikarjuna
Published: August 2003

The effective atomic numbers for total photon interaction in bone, muscle, liver, spleen, fat, and water are determined and found to decrease up to 50% as the energy increases from 10 to 200 keV. Muscle, spleen, liver, and water are found to behave in an approximately similar manner, in this energy region, as far as photon interaction is concerned.
Monte Carlo evaluation of Compton scatter subtraction in single photon emission computed tomography
Authors: Floyd C. E. , Jaszczak R. J. , Harris C. C. , Greer K. L. , Coleman R. E.
Published: August 2003

In single photon emission computed tomography (SPECT), Compton scattering produces a background that degrades the image quality and contributes erroneously to quantitative measurements. A clinically implementable compensation algorithm has previously been reported that subtracts a Compton scatter image, acquired in an energy window set below the energy of the photopeak, from the primary image acquired in the photopeak window. We present an evaluation and justification of the assumptions made in the previous empirical development of the subtraction algorithm. A Monte Carlo model of the SPECT system in which the Compton scattered events may be followed independently of the nonscattered events was used to evaluate this subtraction technique. Simulation shows that the assumptions made in the experimental application of this algorithm were valid. Specifically (1) the “scatter” energy window used in the experiment (91–125 keV for imaging Tc‐99m) contains only scattered events, (2) the shape of the line spread function (LSF) for the events in the scatter window is a reasonable approximation to the shape of the scatter in the photopeak window, and (3) the ratio of the number of scattered events in the photopeak window to the number of events in the scatter window is 0.57, close to the value of 0.5 derived heuristically. Thus, Monte Carlo simulation validates the basic assumptions underlying the empirical implementation of the scatter subtraction algorithm.
Polyacrylamide‐based phantoms as tissue substitute in experimental radiation physics
Authors: Wielopolski L. , Maryanski M. , Washington A. C. , Schidlovsky G. , Cohn S. H. , Reinstein L. E. , Kalend A. M. , Meek A. B.
Published: August 2003

Polyacrylamide‐based tissue‐equivalent phantoms simulating cortical bone and muscle are described. The equivalency is based upon similar elemental composition and density, and partial similarity in the morphology of bone. Satisfactory results were obtained when the phantoms were tested at low (20 keV) and high (15 MeV) gamma radiation. Applicability of this phantom material to neutron transport is discussed. The material can be molded and shaped and its composition is easily modified by altering the proportions of the constituents. Trace elements or radionuclides are easily added. Details of the physical and radiation characteristics of the formulated systems are given together with the manufacturing procedures.
The effect of anode curvature on radiographic heel effect
Authors: Fritz Steven L. , Livingston William H.
Published: August 2003

Previous computer modeling of radiographic heel effect has shown that a simple model can give an accurate representation of heel effect for flat anode surfaces. This paper details the extension of this model to allow curved anode surfaces. A fixed‐anode x‐ray tube with a demountable anode was constructed and used to test the predictions of the computational model at 25 kVp using stainless‐steel anodes. The computer model was also used to estimate the magnitude of the heel effect for a tungsten anode at 60 kVp. This work allows determination of the effect of anode curvature, which is an inherent feature of certain recently proposed tube designs for scanning slit radiography.
Performance of antiscatter grids in diagnostic radiology: Experimental measurements and Monte Carlo simulation studies
Authors: Chan Heang‐Ping , Higashida Yoshiharu , Doi Kunio
Published: August 2003

We have devised an experimental method with which one can accurately measure the transmission of primary radiation and the transmission of total radiation by an antiscatter grid in a setting similar to a practical radiographic examination. We measured the transmission values for 27 combinations of x‐ray tube potentials, phantom thicknesses, screen–film systems, and grid parameters. The standard deviation of one measurement was estimated to be 2.9% and 1.5% for the total and primary transmissions, respectively. The measured grid transmission was compared with results predicted by our Monte Carlo calculations; 92% of the measured and calculated values agree within two standard deviations. This close agreement indicates that our Monte Carlo calculation can accurately predict the performance of antiscatter grids under diagnostic imaging conditions.
Stereotaxic radiotherapy technique for small intracranial lesions
Authors: Houdek Pavel V. , Fayos Juan V. , Van Buren John M. , Ginsberg Murray S.
Published: August 2003

A stereotaxic radiotherapy technique that permits accurate delivery of highly localized dose to a small intracranial target has been developed. The technique facilitates precise integration of the diagnostic and therapeutic procedures including target localization, treatment planning, simulation, repetitive patient irradiation, and daily treatment verification. A conventional linear accelerator and computed tomography scanner as well as special diagnostic and therapeutic guides are used. A suitable dosimetric distribution is achieved using arc therapy with small radiation fields and 10‐MV x rays.
Microdensitometry using a slide projector
Authors: Maleki Nasser , Bjärngard Bengt E.
Published: August 2003

We have evaluated a simple microdensitometer concept by Stanton et al. for measurements of dose distributions with high spatial resolution. A magnified image of the film is created by a slide projector and its light intensity measured with a photomultiplier with a small aperture. The projector is placed on a movable stage to allow scanning of the film. A resolution of 17 μm (full width at half maximum of the line spread function) was achieved in the film plane.
Noise performance of surface coils for magnetic resonance imaging at 1.5 T
Authors: Hayes Cecil E. , Axel Leon
Published: August 2003

In this paper we analyze the signal‐to‐noise ratio (SNR) for surface coil magnetic resonance imaging at 1.5 T. We have applied the treatment of Hoult and Lauterbur to determine the factors that most affect coil performance. We have imaged lossy phantoms with 8‐, 10‐, and 14‐cm‐diam circular surface coils and compared the results to body and head coil images. Surface coils can improve SNR by a factor of 4 or more for regions close to the surface. Surface coils are effective for regions up to 6 cm deep in the head and about 12 cm deep in the body. Nonuniformity of image intensity is a necessary requirement for improved SNR in surface coils. Coil losses make only a small contribution to image noise compared to tissue losses at 1.5 T. Surface coils need not be placed in close contact with the patient at 1.5 T.
Computer dosimetry of 192Ir wire
Authors: Kline Robert W. , Gillin Michael T. , Grimm Daniel F. , Niroomand‐Rad Azam
Published: August 2003

The dosimetry of 192Ir linear sources with a commercial treatment planning computer system has been evaluated. Reference dose rate data were selected from the literature and normalized in a manner consistent with our clinical and dosimetric terminology. The results of the computer calculations are compared to the reference data and good agreement is shown at distances within about 7 cm from a linear source. The methodology of translating source calibration in terms of exposure rate for use in the treatment planning computer is developed. This may be useful as a practical guideline for users of similar computer calculation programs for iridium as well as other sources.
Two‐dimensional dose distribution of 125I seeds
Authors: Ling C. Clifton , Schell Michael C. , Yorke Ellen D. , Palos Bernice B. , Kubiatowicz David O.
Published: August 2003

Two‐dimensional dose distribution has been measured for the new (model 6711) 125I seeds used in interstitial implants. Two independent methods, using a silicon diode or thermoluminescent dosimeters, yielded identical results. At any given distance r from the seed center, the dose varies with θ, the angle relative to the seed's axis. Similarly, the r dependence of the dose distribution is different at various θ values. These observations can be qualitatively understood in terms of several factors, namely, source encapsulation, geometrical relationship, and attenuation and scatter. Empirical expressions which approximate the measured results have been developed to facilitate clinical dose distribution calculations.
Lung dose corrections for 6‐ and 15‐MV x rays
Authors: Mackie T. R. , El‐Khatib E. , Battista J. , Scrimger J. , Van Dyk J. , Cunningham J. R.
Published: August 2003

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue–air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer‐aided planning of radiation therapy, but have been validated primarily for cobalt‐60 radiation. Tests performed with 6‐ and 15‐MV x rays reveal that incorrect doses can be computed within or near to a low‐density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x‐ray interactions is considered directly.
Kinestatic charge detection
Authors: DiBianca Frank A. , Barker Marion D.
Published: August 2003

A new technique for detecting ionizing radiation in two dimensions, called “kinestatic charge detection,” is proposed and analyzed. This technique is useful when the signal photons must be integrated, as in computed tomography and digital radiography, rather than counted, as in nuclear medicine imaging. A generic treatment of the technique with gas‐, liquid‐, or solid‐state radiation detectors is presented. A theoretical analysis is given of the fundamental physical parameters required for kinestatic charge detection to be successful.
Effective wedge angles for 6‐MV wedges
Authors: Zwicker R. D. , Shahabi S. , Wu A. , Sternick E. S.
Published: August 2003

Measurements were undertaken with 30° and 45° large wedges on a 6‐MV linac to determine the effective wedge angle for various combinations of open and wedged fields. The validity of Tatcher's equation, relating effective wedge angle to maximum dose weightings, was examined over a range of field sizes from 5×5 to 20×20 cm. An alternative equation involving only central axis quantities was also investigated. The results obtained from an analysis of point dose measurements indicate that, for these wedges, either equation yields sufficient accuracy for clinical purposes.
Tissue equivalence in clinical neutron dosimetry: Comparison of dose distributions in nine tissue substitutes for a d(14)Be neutron beam
Authors: Hensley F. W. , Rassow J. , Temme A.
Published: August 2003

Depth dose distributions for a d(14)Be neutron beam, measured separately for total and gamma absorbed dose, are presented for nine tissue substitutes and for two field sizes. These data are used to examine methods to transform depth dose curves from one material to another. Best results are found when the local depths are transformed by constant empirical factors which are independent of depth and field size. As a physical interpretation of the empirical factors, kerma‐weighted mean‐free‐path lengths are calculated for the interaction of the Essen neutron beam with the materials. The ratios of these free path lengths agree with the empirical factors within ±10%. However, for clinical dosimetry, a direct comparability of spatial absorbed dose distributions measured in two different phantom materials is only given if their corresponding transformation factor is near unity.
The response of thermometer probes inserted into catheters
Authors: Waterman Frank M.
Published: August 2003

The time constants of three commercially available thermometer probes were measured when inserted into catheters commonly used in hyperthermia. The catheter typically increases the time constant by a factor of 3. Following the transition from a constant to continuously changing temperature, the rate of change of temperature indicated by such a probe is initially incorrect, but it attains the true rate of change of temperature after an elapsed time that depends on the rate of change of temperature and the time constant of the probe. This elapsed time varies from 3.5 to 8 time constants, as the rate of change of temperature increases from 0.05 to 10 °C/min. The specific absorption rate determined from a least‐squares fit to temperatures recorded during the first 30 s following such a transition, are typically in error by 1%, 3%, and 6% when thermometer probes having time constants of 1, 2, and 3 s are used. These errors can be reduced to less than 0.1% by excluding the data recorded during an initial period of the first 3 time constants following the transition. Errors of similar magnitude result when blood flow is determined from the first 30 s of thermal clearance. These errors are similarly reduced to typically less than 1% by excluding the data recorded during the first 3 time constants from the determination.
A multiple‐pulse sequence for improved selective excitation in magnetic resonance imaging
Authors: Nishimura Dwight G.
Published: August 2003

A new framework for selective excitation that offers simpler design and better performance than conventional excitation methods is introduced. The guidelines for choosing the appropriate radiofrequency (rf) pulse envelope in a conventional selective excitation sequence often rely on Fourier analysis, leading to less than desirable results. Although providing useful insight, Fourier analysis of the rf pulse envelope determines the resultant slice shape accurately only for small flip‐angle excitations, and not for larger flip‐angle excitations owing to the generally nonlinear behavior of the spin system. In the new excitation framework, additional excitation pulses (typically one) are applied in sequence with the conventional pulse to improve the performance (in phase characteristics and slice definition) over that achieved by the conventional pulse alone. Given a desired spatial spin distribution and an associated rf pulse (e.g., Fourier transform pairs), the Bloch equation is solved backwards to yield the starting distribution required for the conventional pulse to give exactly the desired output. If this residual distribution is a small flip angle away from the actual starting distribution, then Fourier analysis of the residual distribution leads to the necessary “setup” pulse. A gradient of opposite polarity during the setup obviates a refocusing interval after the setup pulse. Computer simulations have verified the efficacy of the multiple‐pulse excitation sequence for both 90° and 180° excitations.
Characteristics of cobalt‐60 penumbral regions
Authors: Dawson D. J. , Akinradewo A. C.
Published: August 2003

Ionization chambers with wall materials of various electron densities have been employed to investigate the penumbral region of a cobalt‐60 beam of side length 10 cm. Measurements were made in air with chambers of various inside diameters. The homogeneous walls of the ionization chambers were 0.5 g/cm2 in thickness. Extrapolation techniques to zero inside diameter and to infinite density were used, respectively, to eliminate the effect of the inside diameter of the chamber and to determine the penumbral photon‐fluence distribution. The method also quantitates both the penumbral primary‐dose profiles in water‐equivalent media and the degradation of these profiles in media of low electron density.
A thermoregulated enclosure for controlling thermal drift in a radiation calorimeter
Authors: Kubo Hideo , Brown Dennis E. , Russell Michael D.
Published: August 2003

The ability to control thermal drifts is essential in operating a calorimeter. We investigated a thermal enclosure, which envelops the calorimeter with temperature‐regulated air, thus thermally isolating the calorimeter from the room. The desired temperature in the enclosure is controlled by a control circuit and a thermoelectric device, which works as a Peltier effect heat pump. In this report, the details of the enclosure design and construction are presented with actual performance evaluations.
Dual‐energy mammography: Initial experimental results
Authors: Johns Paul C. , Drost Dick J. , Yaffe Martin J. , Fenster Aaron
Published: August 2003

Dual‐energy x‐ray techniques may be able to enhance the detectability of calcifications in mammographic examinations by removing the background “clutter” caused by contrast between adipose and glandular tissue. This hypothesis is examined experimentally by implementation of dual‐energy imaging on a prototype digital scanned projection radiography system developed in our laboratory. A model of the propagation of signal and noise in dual‐energy processing for a given radiation dose is validated by measurements from phantom images. The experimental imaging system has low spatial resolution and cannot be operated at dose‐optimum energies; however, since both the single‐ and dual‐energy images are subject to the same technical limitations, a comparison of such images allows an assessment of the benefits of dual energy. Experimental images of breast tissue specimens, showing improved detectability of calcifications when obscuring background clutter is removed, are presented. The dose required for a given signal‐to‐noise ratio can be reduced by smoothing the higher energy image prior to dual‐energy processing. For practical implementation, it is reasonable to smooth the higher energy image such that its variance is reduced fourfold.
Absolute radioassay of extended sources: An equivalent point‐source coincidence‐counting approach with application to the thyroid
Authors: Fymat Alain L. , Greenfield Moses A. , Lee W. N. Paul
Published: August 2003

A general methodology is provided for the absolute assay of radioisotopes decaying with coincident photons in an extended source. In the determination of the source activity, the method requires neither the detailed consideration of the geometric and self‐attenuation processes taking place between the source component points nor a knowledge of the distribution of activity across the source. It derives from the concept of the “equivalent point source,” that is a fictitious point source whose activity would equal that measured for the actual extended source. It has been developed for an arbitrary number of coincident photon types displaying an arbitrary degree of mutual correlation, and for arbitrary detection geometry. A unifying formalism is developed for both point and extended sources and for single and dual detecting systems. It is found that in all cases the various instrumental and spectroscopic uncertainties appear within a composite parameter (herein called F factor) that can be determined by standard calibration procedures; this factor is in turn only weakly dependent on its own component parameters. New expressions and relationships are obtained that provide a greater physical insight into coincidence‐counting methods.
Physical characteristics of scattered radiation in diagnostic radiology: Monte Carlo simulation studies
Authors: Chan Heang‐Ping , Doi Kunio
Published: August 2003

We applied Monte Carlo methods for the simulation of x‐ray scattering in water phantoms. The phantom thickness was varied from 5 to 20 cm, and the monoenergetic incident x rays were varied from 15 to 100 keV. Eight screen pairs and a total absorption system were used as x‐ray receptors. We determined the angular, spectral, and spatial distributions of the scattered radiation and the scatter fractions recorded in the image plane. The dependence of these properties on the incident x‐ray energy, the phantom thickness, and the energy response of the recording system was examined. The results of this study provide useful information for the development of antiscatter techniques and for the evaluation of radiographic procedures.
A convolution method of calculating dose for 15‐MV x rays
Authors: Mackie T. R. , Scrimger J. W. , Battista J. J.
Published: August 2003

Arrays were generated using the Monte Carlo method representing the energy absorbed throughout waterlike phantoms from charged particles and scatter radiation set in motion by primary interactions at one location. The resulting “dose spread arrays” were normalized to the collision fraction of the kinetic energy released by the primary photons. These arrays are convolved with the relative primary fluence interacting in a phantom to obtain three‐dimensional dose distributions. The method gives good agreement for the 15‐MV x‐ray dose in electronic disequilibrium situations, such as the buildup region, near beam boundaries, and near low‐density heterogeneities.
Comments on dry air or humid air values for physical parameters used in the AAPM protocol for photon and electron dosimetry
Authors: Mijnheer B. J. , Williams J. R.
Published: August 2003

In order to determine if values for physical parameters for dry air or humid air should be used, the expressions for Ngas and Dmed, as defined in the AAPM protocol for the determination of absorbed dose from high energy photon and electron beams, have been derived. Applying the proposed values for W/e and L̄/ρ would increase the absorbed dose determinations by 0.6% compared to applying the AAPM recommended values for these parameters. (AIP)
Reply to comments of Mijnheer and Williams
Authors: Schulz R. J.
Published: August 2003

The comments of Mijnheer and Williams on AAPM dosimetry protocol for the determination of absorbed doses from high energy x‐rays and electron beams are answered.(AIP)
Modeling the quasicompressional wave field of a rectangular transducer in a transversely isotropic solid
Authors: Gridin Dmitri
Published: August 2003

A fast model for simulating the transient quasicompressional wave field of a rectangular ultrasonic transducer directly coupled to a transversely isotropic elastic half-space of general orientation is developed. The so-called two-tier asymptotic approach and the uniform stationary phase method are used to derive the high-frequency asymptotics of time-harmonic displacements. Then, transient fields are modeled by means of harmonic synthesis. In geometrical regions, the formulas involve elementary and inside boundary layers, well-known special functions (Fresnel integral and generalized Fresnel integral), and are applicable in the radiating near field. The asymptotics elucidate the physics in terms of various arrivals and give explicit dependence of the radiated waves upon model parameters. The asymptotic code is tested against a direct numerical solution. It is at least a thousand times faster but describes accurately both arrival times and amplitudes of various pulses radiated by the transducer.
Modeling the effect of suppression on the periodicity of stimulus frequency otoacoustic emissions
Authors: Lineton Ben , Lutman Mark E.
Published: August 2003

The distributed roughness theory of the origins of spectral periodicity in stimulus frequency otoacoustic emissions (SFOAEs) predicts that the spectral period will be altered by suppression of the traveling wave (TW) [Zweig and Shera, J. Acoust. Soc. Am. 98, 2018–2047 (1995)]. In order to investigate this effect in more detail, simulations of the variation of the spectral period under conditions of self-suppression and two-tone suppression are obtained from nonlinear cochlear models based on this theory. The results show that during self-suppression the spectral period is increased, while during high-side two-tone suppression, the period is reduced, indicating that the detailed pattern of disruption of the cochlear amplifier must be examined if the nonlinear behavior of SFOAEs is to be understood. The model results suggest that the SFOAE spectral period may be sensitive to changes in the state of the cochlear amplifier. A companion paper [Lineton and Lutman, J. Acoust. Soc. Am. 114, 871–882 (2003)] presents experimental data which are compared with the results of the above models with a view to testing the underlying theory of Zweig and Shera.
Estimating bone conduction transfer functions using otoacoustic emissions
Authors: Purcell David W. , Kunov Hans , Cleghorn William
Published: August 2003

A technique for estimating the nonparametric bone conduction transfer function using distortion product otoacoustic emissions (DPOAEs) is presented. Individual transfer functions were obtained using DPOAEs recorded from a single ear of five normal-hearing adults. Repeatability of the technique was investigated by performing measurements on at least three dates. Functions were reasonably repeatable, and were unique to each individual as expected from subjective measurements. Input force and DPOAE measurements were made for each individual, and a model of the auditory periphery representative of an average person was employed. The technique is objective and requires only passive cooperation, but robust DPOAEs are needed and the measurement time can be onerous for a wide frequency band or fine frequency resolution. With appropriate adjustments to the model of the auditory periphery, the method could be applied with animal models.
The effect of suppression on the periodicity of stimulus frequency otoacoustic emissions: Experimental data
Authors: Lineton Ben , Lutman Mark E.
Published: August 2003

In a companion paper [Lineton and Lutman, J. Acoust. Soc. Am. 114, 859–870 (2003)], changes in the spectral period of stimulus frequency otoacoustic emissions (SFOAEs) during self-suppression and two-tone suppression were simulated using a nonlinear cochlear model based on the distributed roughness theory of otoacoustic emission generation [Zweig and Shera, J. Acoust. Soc. Am. 98, 2018–2047 (1995)]. The current paper presents the results of an experimental investigation of SFOAE suppression obtained from 20 human subjects. It was found that, in most subjects, the spectral period increased during self-suppression, but reduced during high-side two-tone suppression. This pattern of results is in close agreement with the predictions of the cochlear model, and therefore strongly supports the distributed roughness theory of Zweig and Shera. In addition, the results suggest that the SFOAE spectral period is sensitive to changes in the state of the cochlear amplifier.
Interaction of a bounded ultrasonic beam with a thin inclusion inside a plate
Authors: Vanaverbeke Sigfried , Leroy Oswald , Shkerdin Gennady
Published: August 2003

A theoretical study of the reflection of a two-dimensional Gaussian ultrasonic beam, incident at a Lamb angle of a plate containing a thin rectangular inclusion at an arbitrary position, is presented on the basis radiation mode theory. The inclusion is parallel to the plate surface and its thickness is assumed to be much smaller than the ultrasonic wavelength. It is shown that the amplitude and phase of the reflected beam profile can be used for accurate inclusion characterization. However, this only holds for certain internal positions of the inclusion and for material combinations that do not strongly perturb the excitation of Lamb waves in the plate. When these conditions are satisfied, it is possible to define the Lamb waves and the associated experimental conditions for which good estimates can be obtained of the position of the beginning point of the inclusion as well as of the length and the thickness of the inclusion.
Application of cylindrical near-field acoustical holography to the visualization of aeroacoustic sources
Authors: Lee Moohyung , Bolton J. Stuart , Mongeau Luc
Published: August 2003

The purpose of this study was to develop methods for visualizing the sound radiation from aeroacoustic sources in order to identify their source strength distribution, radiation patterns, and to quantify the performance of noise control solutions. Here, cylindrical Near-field Acoustical Holography was used for that purpose. In a practical holographic measurement of sources comprising either partially correlated or uncorrelated subsources, it is necessary to use a number of reference microphones so that the sound field on the hologram surface can be decomposed into mutually incoherent partial fields before holographic projection. In this article, procedures are described for determining the number of reference microphones required when visualizing partially correlated aeroacoustic sources; performing source nonstationarity compensation; and applying regularization. The procedures have been demonstrated by application to a ducted fan. Holographic tests were performed to visualize the sound radiation from that source in its original form. The system was then altered to investigate the effect of two modifications on the fan’s sound radiation pattern: first, leaks were created in the fan and duct assembly, and second, sound absorbing material was used to line the downstream duct section. Results in all three cases are shown at the blade passing frequency and for a broadband noise component. In the absence of leakage, both components were found to exhibit a dipole-like radiation pattern. Leakage was found to have a strong influence on the directivity of the blade passing tone. The increase of the flow resistance caused by adding the acoustical lining resulted in a nearly symmetric reduction of sound radiation.
A new microphone system for near whispering
Authors: Choi Sungjoon , Moon Wonkyu , Lee Jeong Hyun
Published: August 2003

A new microphone system was developed to monitor the human voice near the microphone in a noisy environment. The system is equipped with two special functions in addition to the usual microphone functions: reduction of air-blow effects by the mouth and focused reception to a sound source. A wind filter was developed to reduce the air-blow effects from the mouth during speaking. This filter is a plate perforated by an array of small holes; the method used to design the filter is also presented. To achieve focused reception, four microphones were used in conjunction with a new signal-processing method. The proposed signal-processing method effectively increases the directivity in the desired direction. Additionally, it provides the system with focusing on the source since the source is located adjacent to the system. A prototype of the proposed system was fabricated and subjected to performance tests. The results showed that air-blow effects can be reduced by up to 20 dB and the directional gain is more than 4 dB. The proposed microphone system shows such good performance that it can be used in mobile phones for whispering communication.
Bubble levitation and translation under single-bubble sonoluminescence conditions
Authors: Matula Thomas J.
Published: August 2003

Bubble levitation in an acoustic standing wave is re-examined for conditions relevant to single-bubble sonoluminescence. Unlike a previous examination [Matula et al., J. Acoust. Soc. Am. 102, 1522–1527 (1997)], the stable parameter space [Pa,R0] is accounted for in this realization. Forces such as the added mass force and drag are included, and the results are compared with a simple force balance that equates the Bjerknes force to the buoyancy force. Under normal sonoluminescence conditions, the comparison is quite favorable. A more complete accounting of the forces shows that a stably levitated bubble does undergo periodic translational motion. The asymmetries associated with translational motion are hypothesized to generate instabilities in the spherical shape of the bubble. A reduction in gravity results in reduced translational motion. It is hypothesized that such conditions may lead to increased light output from sonoluminescing bubbles.
The sonar beam pattern of a flying bat as it tracks tethered insects
Authors: Ghose Kaushik , Moss Cynthia F.
Published: August 2003

This paper describes measurements of the sonar beam pattern of flying echolocating bats, Eptesicus fuscus, performing various insect capture tasks in a large laboratory flight room. The beam pattern is deduced using the signal intensity across a linear array of microphones. The positions of the bat and insect prey are obtained by stereoscopic reconstruction from two camera views. Results are reported in the form of beam-pattern plots and estimated direction of the beam axis. The bat centers its beam axis on the selected target with a standard deviation (σ) of 3°. The experimental error is ±1.4°. Trials conducted with two targets show that the bat consistently tracks one of the targets with its beam. These findings suggest that the axis of the bat sonar beam is a good index of selective tracking of targets, and in this respect is analogous to gaze in predominantly visual animals.
Multiecho processing by an echolocating dolphin
Authors: Altes Richard A. , Dankiewicz Lois A. , Moore Patrick W. , Helweg David A.
Published: August 2003

Bottlenose dolphins (Tursiops truncatus) use short, wideband pulses for echolocation. Individual waveforms have high-range resolution capability but are relatively insensitive to range rate. Signal-to-noise ratio (SNR) is not greatly improved by pulse compression because each waveform has small time–bandwidth product. The dolphin, however, often uses many pulses to interrogate a target, and could use multipulse processing to combine the resulting echoes. Multipulse processing could mitigate the small SNR improvement from pulse compression, and could greatly improve range-rate estimation, moving target indication, range tracking, and acoustic imaging. All these hypothetical capabilities depend upon the animal’s ability to combine multiple echoes for detection and/or estimation. An experiment to test multiecho processing in a dolphin measured detection of a stationary target when the number N of available target echoes was increased, using synthetic echoes. The SNR required for detection decreased as the number of available echoes increased, as expected for multiecho processing. A receiver that sums binary-quantized data samples from multiple echoes closely models the N dependence of the SNR required by the dolphin. Such a receiver has distribution-tolerant (nonparametric) properties that make it robust in environments with nonstationary and/or non-Gaussian noise, such as the pulses created by snapping shrimp.
Maturation of cochlear nonlinearity as measured by distortion product otoacoustic emission suppression growth in humans
Authors: Abdala Caroline , Chatterjee Monita
Published: August 2003

The growth of distortion product otoacoustic emission (DPOAE) suppression follows a systematic, frequency-dependent pattern. The pattern is consistent with direct measures of basilar-membrane response growth, psychoacoustic measures of masking growth, and measures of neural rate growth. This pattern has its basis in the recognized nonlinear properties of basilar-membrane motion and, as such, the DPOAE suppression growth paradigm can be applied to human neonates to study the maturation of cochlear nonlinearity. The objective of this experiment was to investigate the maturation of human cochlear nonlinearity and define the time course for this maturational process. Normal-hearing adults, children, term-born neonates, and premature neonates, plus a small number of children with sensorineural hearing loss, were included in this experiment. DPOAE suppression growth was measured at two f2 frequencies (1500 and 6000 Hz) and three primary tone levels (55–45, 65–55, and 75–65 dB SPL). Slope of DPOAE suppression growth, as well as an asymmetry ratio (to compare slope for suppressor tones below and above f2 frequency), were generated. Suppression threshold was also measured in all subjects. Findings indicate that both term-born neonates and premature neonates who have attained term-like age, show non-adult-like DPOAE suppression growth for low-frequency suppressor tones. These age effects are most evident at f2=6000 Hz. In neonates, suppression growth is shallower and suppression thresholds are elevated for suppressor tones lower in frequency than f2. Additionally, the asymmetry ratio is smaller in neonates, indicating that the typical frequency-dependent pattern of suppression growth is not present. These findings suggest that an immaturity of cochlear nonlinearity persists into the first months of postnatal life. DPOAE suppression growth examined for a small group of hearing-impaired children also showed abnormalities.
Effect of the speed of a single-channel dynamic range compressor on intelligibility in a competing speech task
Authors: Stone Michael A. , Moore Brian C. J.
Published: August 2003

Using a “noise-vocoder” cochlear implant simulator [Shannon et al., Science 270, 303–304 (1995)], the effect of the speed of dynamic range compression on speech intelligibility was assessed, using normal-hearing subjects. The target speech had a level 5 dB above that of the competing speech. Initially, baseline performance was measured with no compression active, using between 4 and 16 processing channels. Then, performance was measured using a fast-acting compressor and a slow-acting compressor, each operating prior to the vocoder simulation. The fast system produced significant gain variation over syllabic timescales. The slow system produced significant gain variation only over the timescale of sentences. With no compression active, about six channels were necessary to achieve 50% correct identification of words in sentences. Sixteen channels produced near-maximum performance. Slow-acting compression produced no significant degradation relative to the baseline. However, fast-acting compression consistently reduced performance relative to that for the baseline, over a wide range of performance levels. It is suggested that fast-acting compression degrades performance for two reasons: (1) because it introduces correlated fluctuations in amplitude in different frequency bands, which tends to produce perceptual fusion of the target and background sounds and (2) because it reduces amplitude modulation depth and intensity contrasts.