bluuebunny/crossref_metadata_2025 huggingface dataset:
Dose corrections for low‐density tissue inhomogeneities and air channels for 10‐MV x rays
Authors: Young M. E. J. , Kornelsen R. O.
Published: August 2003

Methods of correcting for tissue inhomogeneities which consider only changes in the photon fluence are of limited usefulness for 10‐MV x rays. Although there is normally transient electronic equilibrium on the central axis beyond the depth of maximum buildup in soft tissue, when the beam enters a low‐density material the beam profile is degraded and there is a loss of lateral electron equilibrium which reduces the dose both within and beyond the inhomogeneity. This paper describes experimental measurements of the dose within and near large, low‐density inhomogeneities simulating lungs and also behind air channels (where similar effects occur). Methods of calculating correction factors are discussed.
Some phantom designs for radiation dosimetry and CT applications
Authors: Thomadsen B. R. , Paliwal B. R. , Laursen J. F. , Filamor C. O. , van de Geijn P.
Published: August 2003

Solid phantoms designed with nesting, rotatable cylinders, allow increased flexibility in the positioning of dosimeters or test objects, permitting continuous movement as in a water phantom, but with increased ease of handling. Using such phantoms, measurements can be made at almost any point in the cross section of a phantom, an ability not achievable with currently existing solid or water phantoms. A separate phantom design allows easy measurement at points along circles or radii in a phantom.
Electron beam simulation applicators
Authors: Purdy James A.
Published: August 2003

A system for simulating electron beam treatment portals using low‐temperature melting point alloy is described. Special frames having the same physical dimensions as the electron beam applicators used on the Varian Clinac® 20 linear accelerator were designed and constructed.
Prediction of transient temperature fields and cumulative tissue destruction for radio frequency heating of a tumor
Authors: Hayes Linda J. , Diller Kenneth R. , Pearce John A. , Schick Mark R. , Colvin David P.
Published: August 2003

A therapeutic hyperthermia protocol using a radio frequency (rf) electrode placed adjacent to a bronchial wall tumor has been modeled using the finite element technique. Variable physical properties and variable blood perfusion have been assigned to the tumor and to the surrounding normal lung tissue. The Laplace equation was solved on a curvilinear grid for a single rf source electrode to determine the steady‐state electric field, which in turn governs the energy deposition function. The heat generation in the tumor and in the lung tissue is then calculated from the energy deposition profile, and the bioheat equation is solved on the same finite element mesh to determine the transient temperature history. The temperatures are displayed as isothermal contours at designated times during the protocol and as temperature histories at selected points. In addition, an Arrhenius‐type injury model has been implemented to predict thermally induced damage, from which injury contours and injury histories are produced. Several protocols are investigated in which equal total amounts of energy are deposited into the tissue using a constant power density for an appropriate time or using a cyclic heating pattern. The cyclic heating pattern consisted of a series of equal duration time periods during which the rf current source is alternately turned on and off (50% duty cycle). This study illustrates how a finite element model could be used to evaluate alternative protocols for heating a tumor of a specific geometry and to evaluate thermally induced damage to surrounding normal tissue.
Simulation of automatic temperature control in tissue hyperthermia calculations
Authors: Doss James D.
Published: August 2003

Calculations of transient tissue temperature distributions are enhanced by the use of an algorithm to simulate automatic temperature regulation by negative feedback. This feature is quite useful in treatment planning, enabling the physicist to predict the effects on the treatment of variations in temperature regulation system parameters, such as temperature sensor placement and feedback loop gain.
A method of calculating high‐energy photon primary absorbed dose in water using forward and backward spread dose‐distribution functions
Authors: Iwasaki Akira
Published: August 2003

The complete distribution of high‐energy photon primary absorbed dose along the beam path within a semi‐infinite or finite water phantom can be expressed by one functional equation. The derivation is based on a technique of convoluting a primary collision kerma distribution function with a pair of forward and backward spread dose‐distribution functions. In particular, the characteristic patterns of primary dose in the regions near the beam entrance and exit surfaces can be obtained. The calculated ratio of primary dose to primary collision kerma in the electron‐equilibrium region in water for 60Co radiation is 1.0060 and that for 25‐MV radiation is 1.0444.
Obliquity correction for electron beams
Authors: Khan Faiz M. , Deibel Firmin C. , Soleimani‐Meigooni Ali
Published: August 2003

Uneven air gaps, beam obliquity, or sloping contours create complex dosimetry problems in electron beam treatment planning. We find that the depth dose distribution in an obliquely incident beam can be calculated from the data for a normally incident beam by using inverse square law and obliquity factors. The obliquity factors have been determined for various energy beams in the range of 6–22 MeV and can be used for manual treatment planning.
The perturbation of electron beam dose distributions at medium interfaces
Authors: Werner Barry L.
Published: August 2003

The perturbation of electron beam dose distributions in the vicinity of medium interfaces is calculated. Two different solutions to the Boltzmann equation, one an energy averaged solution and the other a diffusion approximation solution, are presented for the calculation of the electron distribution in the vicinity of interfaces. An extension of the energy averaged formalism is presented for media with high atomic number. A new effective depth approximation is introduced to calculate the beam conditions beneath the interface. The transformation from electron distribution to dose distribution is made and dose calculations are compared with published electron beam dose measurements.
A study of the characteristics of radiation contaminants within a clinically useful photon beam
Authors: Krithivas G. , Rao S. N.
Published: August 2003

Contaminant radiation within a therapeutic beam has been studied for accelerator‐produced 24‐MV x rays by a direct measurement utilizing independent jaws. A carefully positioned diode was exposed to secondary radiation for various collimator settings that project field sizes between [10×(0+5)] and [26×(0+13)] cm at a source‐to‐axis distance of 1 m. [The notation [L×(W1+W2)] means the projected field area due to lower (L) and upper (W1, W2) jaws.] Measurements were taken by placing polystyrene sheets with density thicknesses ranging from 0.21 to 4.53 g cm−2 in front of the detector. The data strongly demonstrate that with increasing field size, the ratio of the dose due to electrons to that due to photons increases rapidly. The characteristic feature of the electron depth dose curves may be linked to the observed shift in the depth of dose maximum (dmax) with field size. Data taken with a magnetic field of 0.15 T permit analysis of photons and electrons with regard to their intensity, energy, and behavior in a phantom. From an analysis of 12.5×12.5 cm data, various radiation components have been studied and are correlated with the observed contaminants.
A beam‐edge modifier for abutting electron fields
Authors: Kalend A. M. , Zwicker R. D. , Wu Andrew , Sternick E. S.
Published: August 2003

Abutment of unmodified electron fields to irradiate large areas can lead to significant dose inhomogeneities in the region of junction of the fields. In this paper we describe the design and dosimetric characteristics of a device developed to broaden the electron beam penumbra and thereby to improve the dose uniformity in the overlap region. The device is a high‐density triangular‐toothed comb capable of reducing the beam intensity without seriously degrading the beam energy. The effect of the comb is such that a single device will generate a beam penumbra which is broad and very nearly linear at all depths for all clinically used beam energies. Results are shown for various field configurations and energies. With a gap of 5.0 cm between the treatment cone and phantom surface the dose “ripple” in the region beneath the teeth was found not to exceed ±5% at 0.5‐cm depth.
Quantitative assessment of bone mineral by photon scattering: Accuracy and precision considerations
Authors: Shukla S. S. , Karellas A. , Leichter I. , Craven J. D. , Greenfield M. A.
Published: August 2003

A method to determine the bone mineral density of the calcaneum has been reported earlier by our laboratory. In this method, the calcaneum is irradiated by a 60‐keV photon beam from 241Am source and both the coherent and Compton scattered photons are detected by a high‐purity Ge detector. The bone mineral density is determined by measuring the ratio of coherent‐to‐Compton scattered photons. The accuracy and the precision (in vitro) of the method are reported in this paper. The accuracy was determined to be 5%. This was obtained by comparing the bone mineral density values of cadaver calcanea measured directly by Archimedes’ volume displacement method with the values measured by the scattering method. The precision was determined to be 3% by measuring the bone mineral density of a calibration phantom intermittently over a ten‐month period by the scattering method.
Quantitative assessment of bone mineral by photon scattering: Calibration considerations
Authors: Leichter I. , Karellas A. , Shukla S. S. , Looper J. L. , Craven J. D. , Greenfield M. A.
Published: August 2003

The ratio of the coherent‐to‐Compton photons scattered from bone can be used to measure its mineral density. Conversion of this ratio (R) to bone mineral density (BMD) requires calibration using bone simulating phantoms. The widely used aqueous solution of K2HPO4 proved unsatisfactory for calibration purposes when using the coherent‐to‐Compton technique. These solutions differ markedly in their scatter spectra and composition from trabecular bone. In this study a new and more realistic series of phantoms is proposed which simulates well the trabecular bone of the calcaneum. These phantoms are made of bone ash suspended in white petrolatum in varying concentrations. A calibration curve has been established using these phantoms with a range of BMD values of 0 to 347 mg/cm3. The scatter spectra, and range of R values and BMD of these phantoms are in very good agreement with those of real trabecular bone. A measuring device has been built for the determination of the BMD of the calcaneum by using the established calibration curve.
Physical treatment planning of total‐body irradiation: Patient translation and beam‐zone method
Authors: Quast Ulrich
Published: August 2003

In the treatment of acute leukemias, high doses of chemotherapy and total‐body irradiation (TBI) are used prior to bone marrow transplantation. However, the doses needed equal or exceed the tolerance of vital organs at risk. Thus precision and reliability of treatment are required. A new concept of TBI is introduced, combining a patient translation whole‐body irradiation technique with the beam‐zone method for individual treatment planning. The patient rests first in the prone, then in the supine position on a specially constructed flat couch and is moved horizontally with a preset constant velocity, completely through a vertical 60Co beam, twice for PA/AP TBI. This technique enlarges the useful beam size, improves the photon fluence uniformity, and reduces the depth‐dose inhomogeneity. In translation irradiation, the product of velocity and integrated dose is the significant measurement. This velocity–dose product can be described sufficiently as a function of the local thickness and the local effective field size. Using the beam‐zone method, calculation and checking of the dose at reference points in homogeneous, inhomogeneous, or shielded regions is simplified and reduces to taking dose measurements at the center of equivalent square phantoms. Since April 1983, about 20 leukemia patients received TBI by this individually planned translation‐irradiation technique. A dose homogeneity within ±7% is achieved in the entire body.
Signal strength on a 0.15‐T magnetic resonance imager
Authors: Hardy P. A. , Bronskill M. J. , Henkelman R. M.
Published: August 2003

Signal strength in magnetic resonance (MR) images made on a 0.15‐T imager was calculated for spin‐echo (SE) and inversion‐recovery with echo (ISE) pulse sequences and was compared to measurements of signal strength for aqueous MnCl2 solutions whose T1 and T2 relaxation times encompassed the range of values commonly found for tissues. Although measured signal strength generally agreed with calculated signal strength (correlation coefficient=0.996 for both SE and ISE), significant reductions in measured strength (>15%) were sometimes observed. Diffusion alone could not account for this discrepancy. Further experiments showed that imaging gradients reduced the effective T2 estimated from the imager. In addition, SE pulse sequences with short repetition times exhibited a reduction in signal strength.
Noise and filtration in magnetic resonance imaging
Authors: McVeigh E. R. , Henkelman R. M. , Bronskill M. J.
Published: August 2003

Noise in two‐dimensional Fourier transform magnetic resonance images has been investigated using noise power spectra and measurements of standard deviation. The measured effects of averaging, spatial filtering, temporal filtering, and sampling have been compared with theoretical calculations. The noise of unfiltered images is found to be white, as expected, and the choice of the temporal filter and sampling interval affects the noise in a manner predicted by sampling theory. The shapes of the imager's spatial frequency filters are extracted using noise power spectra.
Enhancement of electron beam dose distributions by longitudinal magnetic fields: Monte Carlo simulations and magnet system optimization
Authors: Weinhous Martin S. , Nath Ravinder , Schulz R. J.
Published: August 2003

A Monte Carlo electron–photon transport code was developed in order to determine the effects of static, longitudinal, magnetic fields on dose distributions produced by high‐energy electron beams, and to optimize the design of a superconducting magnet system. As a result of these simulations, a 20‐cm‐i.d., 30‐cm‐o.d., 15‐cm‐tall, single‐coil, magnet system was designed that could be incorporated into a mobile treatment table for use with a standard radiation therapy accelerator. Operating at a current density of 18 kA/cm2, the magnet would produce field strengths of 1–4 T in the phantom and 0.01 T at the accelerator exit window. Magnetically enhanced dose distributions, calculated for 20‐ and 30‐MeV electron beams, show a pronounced Bragg peak, steeper gradients to the sides and rear, and a roughly fourfold increase in the peak dose to entrance dose ratios relative to those similarly calculated without a magnetic field. These magnetically enhanced dose distributions have the potential for sparing intervening tissue when high‐energy electrons are used for the treatment of deep‐seated tumors.
Application of a Laplace transform pair model for high‐energy x‐ray spectral reconstruction
Authors: Archer Benjamin R. , Almond Peter R. , Wagner Louis K.
Published: August 2003

A Laplace transform pair model, previously shown to accurately reconstruct x‐ray spectra at diagnostic energies, has been applied to megavoltage energy beams. The inverse Laplace transforms of 2‐, 6‐, and 25‐MV attenuation curves were evaluated to determine the energy spectra of these beams. The 2‐MV data indicate that the model can reliably reconstruct spectra in the low megavoltage range. Experimental limitations in acquiring the 6‐MV transmission data demonstrate the sensitivity of the model to systematic experimental error. The 25‐MV data result in a physically realistic approximation of the present spectrum.
The influence of phantom size on output, peak scatter factor, and percentage depth dose in large‐field photon irradiation
Authors: Podgorsak E. B. , Pla C. , Evans M. D. C. , Pla M.
Published: August 2003

Machine outputs, peak scatter factors, and central axis percentage depth dose distributions were measured for various phantom sizes in large radiation fields produced at extended distances by cobalt, 6‐MV, and 10‐MV photon beams. The results can be applied to practical total body irradiation procedures which usually involve treatment volumes smaller than the actual field sizes in order to provide a uniform total body exposure to radiation. Our study addresses the question of the appropriate phantom dimension to be used in the calibration of photon beams employed in total body irradiations. The measurements show that the machine outputs are only slightly dependent on phantom size; the percentage depth dose distributions, however, are strongly dependent on the phantom size, suggesting that machine data for total body irradiations should be measured in phantoms whose dimensions approximate the patient during the total body irradiation. Peak scatter factors measured in large‐field/small‐phantom configurations link up well with the published small‐field/large‐phantom data. The finite patient thickness lowers the dose to points close to the beam exit surface by a few percent, when compared to dose measured at the same depths in infinitely thick phantoms. The surface doses in large radiation fields are essentially independent of phantom cross sections and range from 40% for the 10‐MV beam, to 65% for the 6‐MV beam and 80% for the cobalt beam.
Isodose plotting for pen plotters
Authors: Rosen Isaac I.
Published: August 2003

A general algorithm for treatment plan isodose line plotting is described which is particularly useful for pen plotters. Unlike other methods of plotting isodose lines, this algorithm is designed specifically to reduce pen motion, thereby reducing plotting time and wear on the transport mechanism. Points with the desired dose value are extracted from the dose matrix and stored, sorted into continuous contours, and then plotted. This algorithm has been implemented on DEC PDP‐11/60 and VAX‐11/780 computers for use with two models of Houston Instrument pen plotters, two models of Tektronix vector graphics terminals, a DEC VT125 raster graphics terminal, and a DEC VS11 color raster graphics terminal. Its execution time is similar to simpler direct‐plotting methods.
Scatter rejection by air gaps: An empirical model
Authors: Sorenson James A. , Floch Jacqueline
Published: August 2003

Scatter rejection by air gaps was analyzed in the context of a model in which scattered radiation was treated as if it originated from an “effective scatter point source” (ESPS), located between the focal spot of the x‐ray tube and the exit surface of the phantom or patient. Excellent agreement was found between the ESPS model and scatter measurements performed on phantoms for a variety of experimental conditions. Values for Xs (distance from phantom exit surface to effective scatter point source) were consistently in the range 15–20 cm. Applications of the model for predictions of scatter rejection by air gaps and comparisons to other scatter rejection methods are presented.
Minimum entrance exposure estimates for information recording in diagnostic radiology
Authors: Brodie Ivor , Gutcheck Robert A.
Published: August 2003

Based upon previously derived expressions, a procedure is given for estimating the minimum entrance exposure necessary to record a desired information set for use in any diagnostic radiological situation. By separating the different variables contributing to the final image and making suitable approximations, a simple relationship of four separable components is obtained. Some typical examples are given to illustrate the relative importance of each component, and to obtain numerical estimates of exposure values. These examples show that the theoretical entrance exposure range for most diagnostic situations, where a realistic information set is desired, lies between 2.5×10−3 and 1.2×10−2 mC/kg (10–50 mR). It is also concluded that a 75‐kVp x‐ray source, filtered to remove the low‐energy photons and with a mean photon energy of 40 keV, should be sufficiently close to optimum in most diagnostic situations to record the desired information in the above entrance exposure range.
Detection of a leak in a “sealed” monitor chamber
Authors: Boese H. R. , Cormack D. V.
Published: August 2003

A suspected leak in a “sealed” monitor chamber in the Siemens Mevatron 74 linear accelerator is investigated experimentally and results are given.
Spectral transmittance and contrast in breast diaphanography
Authors: Ertefai Sima , Profio A. Edward
Published: August 2003

Diaphanography is an imaging technique using transillumination with visible and near‐infrared radiation, and a video camera, to diagnose breast disease, including cancer. Originally based on luminance contrast only, there is now interest in false‐color multispectral imaging in selected spectral bands to improve tissue differentiation. Some success has been achieved, but the scientific basis for the results was unknown. This research is concerned with measurements of the diffuse transmittance of breast tissues as a function of wavelength in the 600–1060 nm range (and calculations of contrast with a one‐dimensional diffusion theory model). Carcinoma and glandular tissues were found to have similar spectral transmittances with an increase in transmittance between 750 and 900 nm, and an absorption window around 960 nm. Adipose tissue showed a distinct transmittance minimum at 930 nm. In vivo measurement of an intact normal breast showed a minimum at about 825 nm, as yet unexplained. The transmittance data and reflectance data were used to derive the scattering and absorption coefficients. The diffusion length was also determined from radiance versus depth measurements. These coefficients were used for calculations of contrast in a one‐dimensional slab model, with and without a layer of cancerous tissue positioned between two slabs of normal tissue.
Depth dose characteristics of elongated fields for electron beams from a 20‐MeV accelerator
Authors: Sharma Subhash C. , Wilson David L.
Published: August 2003

In a Therac‐20 linear accelerator, 6–20 MeV electron beams are normally produced by shaping a scanned electron beam through primary x‐ray collimators and secondary electron trimmers. The collimator settings range continuously from 2 to 30 cm. Depth dose and field flatness parameters were measured for small elongated fields of the various electron energies. Depth dose of narrow fields defined either by the machine's collimator or lead cutouts agreed with data predicted from small square fields using the “square‐root” method.
AAPM Task Group 21 protocol: Dosimetric evaluation
Authors: Paul J. M. , Koch Richard F. , Philip P. C.
Published: August 2003

The AAPM Task Group 21 protocol is the latest amongst the protocols introduced in recent years covering the dosimetry of high‐energy x‐ray and electron beams used in radiotherapy. We have found that the Cλ value for 15‐MV x rays requires 1.6% upward adjustment, whereas the CE values for electrons in the range 7–18 MeV require downward adjustment of 1%–3% compared to our previous values. The adjustment values given in this paper pertain to the specific measuring systems and high‐energy beams considered here and would not necessarily be applicable to other systems. The dose to medium, calculated for different modalities, was verified by thermoluminescent dosimetry and the results were found to be consistent. These values of Cλ and CE have also been compared with those derived using other protocols, particularly the Nordic Association of Clinical Physics 80 and 81, and the discrepancies discussed.
Models in radiotherapy: Definition of decision criteria
Authors: Schultheiss Timothy E. , Orton Colin G.
Published: August 2003

A method is presented by which dose distributions in radiotherapy may be judged. This method, based on statistical decision theory, combines the calculated probabilities of radiation induced complications (including failure to control disease) with the therapist's judgment of the morbidity of each complication to yield a single value representing the clinical utility of the dose distribution. Using this figure of merit, alternative dose distributions can be compared on a clinical basis. If the morbidities associated with each injury are satisfactorily estimated, and the dose response parameters are adjusted to match clinical experience, this method can be used to evaluate novel treatment techniques prior to their implementation. Automatic optimization algorithms can be used to maximize the figure of merit as a function of the physical treatment parameters so as to provide a statistically optimal treatment.
Measurement of signal intensities in the presence of noise in MR images
Authors: Henkelman R. Mark
Published: August 2003

Power spectrum or magnitude images are frequently presented in magnetic resonance imaging. In such images, measurement of signal intensity at low signal levels is compounded with the noise. This report describes how to extract true intensity measurements in the presence of noise.
The effect of thickness of the waterproofing sheath on the calibration of photon and electron beams
Authors: Gillin Michael T. , Kline Robert W. , Niroomand‐Rad Azam , Grimm Daniel F.
Published: August 2003

The TG‐21 protocol recommends using a thin sheath for waterproofing an ion chamber used in the calibration of photon and electron beams. A thickness of 0.5 mm is suggested for a material having a composition and density close to that of water. This work investigates the effect on the calibration of photon beams ranging from Co60 to 25 MV, and electron beams ranging from nominal energies of 7–18 MeV, for changes in the thickness of the waterproofing sheath from 0.5 to 5.5 mm. For photon beams, a maximum change of 1.2% was found for the 25‐MV x‐ray beam. For electron beams, a maximum change of 0.5% was found for 10‐MeV electrons. It is concluded that the thickness of the waterproofing sheath is not a very sensitive variable, assuming the thickness is between 0.5 and 2.0 mm.
Acceptance testing computerized radiation therapy treatment planning systems: Direct utilization of CT scan data
Authors: McCullough Edwin C. , Holmes Timothy W.
Published: August 2003

The availability of computerized radiation therapy treatment planning systems that utilize computed tomography (CT) scan data requires testing additional to that routinely needed for non‐CT systems. These additional items include dimensioning verification, establishing CT number‐to‐tissue property conversions, verifying the accuracy of heterogeneity corrected dose predictions and autocontouring. One testing protocol is presented and sample results from an Atomic Energy of Canada Theraplan L system are presented.
An inexpensive kVp penetrameter
Authors: Ghilardi Netto Thomaz , Cameron John R.
Published: August 2003

In this report we describe an improvement upon the design by Stanton and Lightfoot for a simple photographic null method to determine the kVp of a diagnostic region x‐ray source. The new version has two primary advantages over the Ardran and Crooks design in that it is both simpler and less expensive to produce.
Temperature–pressure compensation for a linear accelerator electron beam dosimeter
Authors: Hrejsa Allen F. , Soen Jerry , Jankowiak Paul
Published: August 2003

Routine weekly calibration of a Siemens Mevatron 20 linear accelerator with 3‐, 5‐, 7‐, 10‐, 12‐, 15‐, and 18‐MeV electron energies demonstrated fluctuations in dose/monitor unit for the electron beam on the order of 3%–6%. Evaluations and study of the problem demonstrated that the electron chamber, which is open to atmosphere, was undergoing significant temperature changes during the course of a treatment day. The inability of the chamber to compensate for these changes in temperature and pressure led to the addition of a compensating circuit by the manufacturer. The results of the addition of this circuit were evaluated for several extended periods throughout the year, and it was found that the changes in dose/monitor were reduced to approximately ±0.5%.
Nuclear resonance scattering measurement of human iron stores
Authors: Wielopolski L. , Ancona R. C. , Mossey R. T. , Vaswani A. N. , Cohn S. H.
Published: August 2003

Hepatic iron stores were measured noninvasively in 31 patients (thalassemia, hemodialysis, hemosiderosis, refractory anemia) with suspected iron overload, employing a nuclear resonance scattering (NRS) technique. The thalassemia patients were undergoing desferrioxamine chelation therapy during the NRS measurements. The hemodialysis patients were measured before chelation therapy. Iron levels measured by NRS were in general agreement with those determined in liver biopsies by atomic absorption spectroscopy. In addition, NRS measurements from the thorax of some of these patients suggest that this method may also prove useful for clinical assessment of cardiac iron.
Daily check instrument for photon and electron beam quality assurance of medical linacs
Authors: Lutz Wendell R. , Maddox B. Jerry , Kase Kenneth R.
Published: August 2003

An instrument for daily beam checks of medical accelerators is described. Using three silicon diodes, the instrument provides information about calibration, beam symmetry, flatness, and energy variation for both photon and electron beams. Operating characteristics of the instrument and the electronic circuit design permit simple and reliable measurements.
Nuclear magnetic resonance of convective dispersive flow
Authors: Harpen Michael D.
Published: August 2003

Presented is a solution to a differential equation governing the transverse magnetization of a fluid whose flow is governed by the process of convective dispersion. The solution describes the case of flow parallel to an applied magnetic field gradient with boundary conditions and velocity profiles consistent with those likely to be encountered in nuclear magnetic resonance imaging of arterial or venous blood flow.
Calorimetric determination of the cavity‐gas calibration factor Ngas
Authors: Schulz R. J. , Weinhous Martin S.
Published: August 2003

In the recently published AAPM protocol for the dosimetry of high‐energy photons and electrons, the response of an ionization chamber is defined as the dose to the gas in the chamber per unit electrometer reading, Ngas. Using a graphite calorimeter, Ngas has been determined for a Farmer‐type ionization chamber using 4‐ and 25‐MV x rays. The procedure was to measure the dose to graphite using the calorimeter, and then obtain the response of the chamber at the same depth in a graphite phantom. Equation (9) of the AAPM protocol was then used to calculate Ngas. The values of Ngas determined with the calorimeter are within 1% of Ngas calculated according to the AAPM protocol, using the 60Co exposure‐calibration factor.
In vivo 31‐P NMR of photoactivated hematoporphyrin derivative in cat brain
Authors: Chopp Michael , Helpern J. A. , Frinak Stanley , Hetzel Fred W. , Ewing James R. , Welch K. M. A.
Published: August 2003

In vivo 31‐P nuclear magnetic resonance (NMR) spectroscopy was performed on cat brains injected with hematoporphyrin derivative (HpD). A 2‐cm‐diam region of the right parietal lobe was photoactivated with red light. The 31‐P NMR spectra of the photoactivated hemisphere revealed increased inorganic phosphate and decreased phosphocreatine and adenosine triphosphate (ATP) levels, compared to spectra obtained from the control hemisphere. In the absence of drug, no difference in spectra was observed between the photoradiated and control lobes. Our studies suggest that in vivo 31‐P NMR spectroscopy may be used to monitor the effects of phototherapy on tissue high‐energy phosphate metabolism.
Dental enamel as an in vivo radiation dosimeter
Authors: Pass B. , Aldrich J. E.
Published: August 2003

The determination of the radiation exposure history of the population has become increasingly important in the study of the effects of low‐level radiation. The present work was started to try to obtain an in vivo dosimeter that could give an indication of radiation exposure. Dental enamel is the only living tissue which retains indefinitely its radiation history, and electron spin resonance measurements have shown that the radiation signal can be resolved down to about 10 cGy. Measurements on samples from the general population give radiation exposure estimates that are reasonable, and one measurement on a patient who had radiotherapy to the mouth area showed a good correlation with tumor dose. We believe that this is an important new indicator of radiation dose and taken together with exposure histories should provide important data for epidemiological studies as well as accidental exposures.
Second‐order multiple‐scattering theory for charged‐particle teletherapy beams
Authors: Jette David
Published: August 2003

The Fermi‐Eyges theory of multiple scattering, applicable to charged‐particle teletherapy beams, has been generalized to second order in small quantities representing deviations from the particles’ initial lines of travel and direction. This second‐order multiple‐scattering theory provides increased accuracy, and in particular it takes into account the skewness of the particles’ paths, in calculating dose. Only the assumptions and certain results of the second‐order theory are presented in this communication, for the immediate use of other investigators. An application to rectangular fields shows that the second‐order theory predicts one component of the buildup of electron central axis depth dose, which the (first‐order) Fermi–Eyges theory cannot do. (The other major component of buildup, which is not yet incorporated into this theoretical work, is due to high‐energy secondary electrons.) Various approximate calculations of electron central axis depth dose are compared with the second‐order calculation.
Investigation of basic imaging properties in digital radiography. 4. Effect of unsharp masking on the detectability of simple patterns
Authors: Loo Leh‐Nien D. , Doi Kunio , Metz Charles E.
Published: August 2003

The signal‐to‐noise ratios (SNRs) of simple radiologic patterns processed by the unsharp‐masking technique were calculated on the basis of a statistical decision theory model that includes both the observer's visual transfer function and a noise component internal to his eye–brain system. For a variety of processing parameters, the contrast‐detail diagrams predicted from this SNR agreed qualitatively with experimental results obtained in observer performance studies. Unsharp masking with a large mask and a large weighting factor improved the detection of square objects to a level comparable with that achieved by the overall contrast enhancement technique using a factor of 4. However, unsharp masking with a small mask and a large weighting factor can substantially degrade the detectability of these objects. The potential practical utility of the unsharp‐masking technique is discussed.
Effects of plastic protective caps on the calibration of therapy beams in water
Authors: Hanson William F. , Tinoco J. Antonio Dominguez
Published: August 2003

Plastic 60Co buildup caps have been widely used to protect ionization chambers when calibrating high‐energy x‐ray and electron beams in water, and have been used consistently by the Radiological Physics Center. Recent calibration protocols base their calculations on a theory that assumes that no protective cap is used during calibration in phantom. The change in ionization within the chamber due to the presence of a protective cap has been investigated for acrylic and polystyrene caps of various wall thicknesses, using60Co and x‐ray beams from 6–25 MV and electron beams from 7–18 MeV. The change has been shown to be small, no more than 0.5% for x rays and 0.7% for electrons using acrylic 60Co caps. The change for polystyrene is seen to be as much as twice that for acrylic. Empirical correction factors to compensate for this effect have been determined. A basis in theory for photons is suggested by an extension of the theory in recent protocols. The effect for electrons is explained only qualitatively.
Comparison of two independent exposure measurement techniques for clinical I‐125 seeds
Authors: Kubo Hideo
Published: August 2003

This investigation contains the results of exposure measurements on two types of clinical I‐125 seeds: a model 6702 no‐marker seed, and a model 6711 silver wire seed. Measurements were made locally with a parallel‐plate mammography chamber, traceable to the National Bureau of Standards (NBS) calibration. The same seeds were also measured at NBS using a reentrant chamber that was developed and calibrated there. The mammography chamber measurements showed an approximately 5% higher exposure rate for the no‐marker seed than the reentrant chamber measurements. On the silver wire seed, good agreement was obtained between the two techniques. It was found that exposure rate variations in the plane perpendicular to the long axis of the seed (or seed axis) were substantial, due in part to the nonsymmetric distribution of radioactive material in the seed.
Characteristics of electron beams from a new 25‐MeV linear accelerator
Authors: O'Brien P. , Michaels H. B. , Aldrich J. E. , Andrew J. W.
Published: August 2003

Clinically useful electron fields are produced on the Atomic Energy of Canada, Limited Therac 25 linear accelerator by computer‐controlled scanning of the electron beam. Measurements were made to determine the properties of these electron fields. Central axis percentage depth dose and bremsstrahlung background were compared for these fields and for the fields from selected machines that use scattering foils. Dose calibrations were made in both water and polystyrene using the American Association of Physicists in Medicine Task Group 21 protocol. Measurements were made to determine the relative output factors, virtual source position, and the attenuation of the electron fields by lead.
A parametric model of the spectral periodicity of stimulus frequency otoacoustic emissions
Authors: Lineton Ben , Lutman Mark E.
Published: August 2003

A model for estimating the spectral period of stimulus frequency otoacoustic emissions (SFOAEs) is presented. The model characterizes the frequency spectrum of an SFOAE in terms of four parameters which can be directly related to cochlear mechanical quantities featuring in the theory of SFOAE generation proposed by Zweig and Shera [J. Acoust. Soc. Am. 98, 2018–2047 (1995)]. The results of applying the parametric model to SFOAEs generated by cochlear models suggest that it gives a sensitive measure of spectral period. It is concluded that the parametric model may be a useful tool for detecting small changes in cochlear function using SFOAE measurements.
The mammalian cochlear map is optimally warped
Authors: LePage Eric L.
Published: August 2003

The form of the mammalian cochlear frequency-position map has been well described by Greenwood and empirical values found for its coefficients for a number of species. The apical portion of the mammalian map is spatially compressed relative to the base, and this nonuniformity in the representation of frequency is evidently consistent across species. However, an evolutionary reason for this consistency, encompassing critical band behavior with respect to position, is conspicuously missing. Likewise, the length of the cochlea in any mammal, including echolocating species, is related to body size, but attempts to explain the length in terms of frequency limits, range, or resolution have no general explanation. New insight stems from a hypothesis in which the map curvature may be appreciated as an adaptation for optimal frequency resolution over the auditory range. It is demonstrated numerically that the mammalian curve may be considered a member of a family of curves which vary in their degree of warp. The “warp factor” found to be common across mammals is an optimal trade-off between four conflicting constraints: (1) enhancing high-frequency resolution; (2) setting a lower bound on loss of existing low-frequency resolution; (3) minimizing map nonuniformity; and (4) keeping the whole map smooth, thereby avoiding reflections.
Comparison between ocean-acoustic fluctuations in parabolic-equation simulations and estimates from integral approximations
Authors: Flatté Stanley M. , Vera Michael D.
Published: August 2003

Line-integral approximations to the acoustic path integral have been used to estimate the magnitude of the fluctuations in an acoustic signal traveling through an ocean filled with internal waves. These approximations for the root-mean-square (rms) fluctuation and the bias of travel time, rms fluctuation in a vertical arrival angle, and the spreading of the acoustic pulse are compared here to estimates from simulations that use the parabolic equation (PE). PE propagations at 250 Hz with a maximum range of 1000 km were performed. The model environment consisted of one of two sound-speed profiles perturbed by internal waves conforming to the Garrett–Munk (GM) spectral model with strengths of 0.5, 1, and 2 times the GM reference energy level. Integral-approximation (IA) estimates of rms travel-time fluctuations were within statistical uncertainty at 1000 km for the SLICE89 profile, and in disagreement by between 20% and 60% for the Canonical profile. Bias estimates were accurate for the first few hundred kilometers of propagation, but became a strong function of time front ID beyond, with some agreeing with the PE results and others very much larger. The IA structure functions of travel time with depth are predicted to be quadratic with the form θv2c0−2(δz)2, where δz is vertical separation, c0 is a reference sound speed, and θv is the rms fluctuation in an arrival angle. At 1000 km, the PE results were close to quadratic at small δz, with values of θv in disagreement with those of the integral approximation by factors of order 2. Pulse spreads in the PE results were much smaller than predicted by the IA estimates. Results imply that acoustic tomography of internal waves at ranges up to 1000 km can use the IA estimate of travel-time variance with reasonable reliability.
Estimation of broadband power levels radiated from turbulent boundary layer-driven ribbed plates having dissimilar sections
Authors: Rumerman M. L.
Published: August 2003

Previous papers developed an approach for estimating the vibration-related broadband acoustic power radiated by a ribbed steel plate in water driven by turbulent boundary layer pressures. This was extended to apply to plates composed of identical ribbed sections joined at infinite impedance supports. In this paper the basic model is modified so that it can be applied to plates composed of dissimilar sections joined at clamped supports. The response is formulated directly in terms of power and the effects of the supports are accounted for by power scattering coefficients, which are calculated through a Wiener–Hopf analysis. Sample calculations are shown to agree with results of other formulations. Only frequencies below bending coincidence are considered.
Thermal modeling and performance analysis of a thermoacoustic refrigerator
Authors: Holmberg David G. , Chen G. S. , Lin H. T. , Wo Andrew M.
Published: August 2003

A heat-driven thermoacoustic refrigerator has been designed and tested. A detailed thermal model of the device is presented. Energy balances within the system are discussed using external, heat exchanger, and stack control volumes in order to clarify the relationships of work and heat fluxes below and above onset. Thermal modeling is discussed as a tool for performance analysis as well as for determining system heat losses and finding input heat flows required by a thermoacoustic code. A method of using the control volume balance equations to find stack work and device efficiencies is presented. Experimental measurements are compared to DELTAE thermoacoustic modeling predictions. Modeling results show that viscous losses within the system have a significant impact on the device performance as well as on the ability of DELTAE to accurately predict performance. Modeling has led to an understanding of system performance and highlighted loss sources that are areas for improvement in a redesign.
Localization of notches with Lamb waves
Authors: Benz Rüdiger , Niethammer Marc , Hurlebaus Stefan , Jacobs Laurence J.
Published: August 2003

A time–frequency representation (TFR) is used to analyze the interaction of a multimode and dispersive Lamb wave with a notch, and then serves as the basis for a correlation technique to locate the notch. The experimental procedure uses a laser source and a dual-probe laser interferometer to generate and detect Lamb waves in a notched plate. The high fidelity, broad-bandwidth, point-like and noncontact nature of laser ultrasonics are critical to the success of this study, making it possible to experimentally measure transient Lamb waves without any frequency biases. A specific TFR, the reassigned spectrogram, is used to resolve the dispersion curves of the individual modes of the plate, and then the slowness-frequency representation (SFR) of the plate is calculated from this reassigned spectrogram. By considering the notch to be an additional (second) source, the reflected and transmitted contributions of each Lamb mode are automatically identified using the SFRs. These results are then used to develop a quantitative understanding of the interaction of an incident Lamb wave with a notch, helping to identify mode conversion. Finally, two complementary, automated localization techniques are developed based on this understanding of scattering of Lamb waves.
Acoustic analysis of monophthong and diphthong production in acquired severe to profound hearing loss
Authors: Palethorpe Sallyanne , Watson Catherine I. , Barker Rosalind
Published: August 2003

The effect of diminished auditory feedback on monophthong and diphthong production was examined in postlingually deafened Australian-English speaking adults. The participants were 4 female and 3 male speakers with severe to profound hearing loss, who were compared to 11 age- and accent-matched normally hearing speakers. The test materials were 5 repetitions of hVd words containing 18 vowels. Acoustic measures that were studied included F1, F2, discrete cosine transform coefficients (DCTs), and vowel duration information. The durational analyses revealed increased total vowel durations with a maintenance of the tense/lax vowel distinctions in the deafened speakers. The deafened speakers preserved a differentiated vowel space, although there were some gender-specific differences seen. For example, there was a retraction of F2 in the front vowels for the female speakers that did not occur in the males. However, all deafened speakers showed a close correspondence between the monophthong and diphthong formant movements that did occur. Gaussian classification highlighted vowel confusions resulting from changes in the deafened vowel space. The results support the view that postlingually deafened speakers maintain reasonably good speech intelligibility, in part by employing production strategies designed to bolster auditory feedback.