bluuebunny/crossref_metadata_2025 huggingface dataset:
A model to generate beam profiles of the Varian Clinac 4 for three‐dimensional dose calculation: Open fields
Authors: Lam Kam‐Shing , Lam Wing‐Chee
Published: August 2003

For the Clinac 4, open field profiles measured in the principal plane have higher intensity “horns” than those in off‐axis planes. The maximum deviation occurs at 1‐cm depth, where in the worst case of large field sizes and off‐axis distances the deviation can be as high as 16% in the region near the horns. A model is proposed to generate open field beam profiles in off‐axis planes, based on measured profiles in the transverse principal plane and in the largest field size diagonal plane of the machine. Within the central 90% portion of the field, the maximum deviation of the generated profiles from the measured ones at the same off‐axis distance varies from about 3% at 1‐cm depth to about 2% at 13‐cm depth and then increases to less than 5% at 25‐cm depth, even for very large field sizes and off‐axis distances. Very little additional computer time and data storage are required for this procedure. Using these profiles, the Milan and Bentley method can be extended to three‐dimensional treatment planning with good accuracy.
Over‐response of tissue‐equivalent ionization chambers at low energies: Effect of high‐Z surface contamination
Authors: Hedrick W. R. , Hykes D. L. , Starchman D. E.
Published: August 2003

Inner wall surface contamination of an ionization chamber by high‐Z materials can produce an over‐response to low‐energy photons which is greatly underestimated by the percentage composition of the contaminant in the entire chamber. Experience with a chamber contaminated in such a manner is presented. Examples are included which illustrate the magnitude of possible clinical effects from low‐energy units which have been calibrated with a chamber contaminated with a thin layer of relatively high‐Z material.
Transition metal–chelate complexes as relaxation modifiers in nuclear magnetic resonance
Authors: Brown Mark A. , Johnson G. Allan
Published: August 2003

Studies are reported of relaxation modifiers for use in nuclear magnetic resonance (NMR) imaging systems. Chelate complexes of transition metal salts are under investigation to determine their ability to reduce the spin–lattice relaxation time (T1) of the nucleus under observation and to reduce the toxicity of the metal ion. The ethylenediaminetetraacetate (EDTA) complexes of FeCl3, GdCl3, and MnCl2 are not as effective as the respective salts in reducing T1 of water protons at 90 MHz. For Mn, this diminution in ability is offset by a significant reduction in toxicity. Explanations for this loss of effectiveness are discussed.
A photoacoustical radiation dosimeter
Authors: Mascarenhas S. , Vargas H. , Cesar C. L.
Published: August 2003

A new type of radiation dosimeter using the photoacoustical effect is described. The photoacoustical radiation dosimeter (PARD) is capable of directly measuring the energy absorbed in the detecting element. For a completely absorbing element, the energy fluence rate in the radiation beam is measured. It is thus a calorimetric dosimeter. Since the energy sensor can be calibrated with another form of energy, it has the potential for being an absolute radiation dosemeter. Measurements were made using 50 to 100 kVp x rays with exposure rates at the detector of 2.6×10−6 C/kg/s (10 mR/s) to 5×10−5 C/kg/s (200 mR/s). The minimum measurable exposure rate at 90 kVp is 5×10−7 C/kg/s (2 mR/s). For a given x‐ray spectrum, the PARD has a linear response with radiation intensity and an inverse response with chopping frequency. With appropriate design parameters, we believe the PARD may be used to measure any photon energy or any type of radiation particle.
Mailed TL dosimeters for monitoring the output from diagnostic x‐ray equipment
Authors: Servomaa Antti , Toivonen Matti , Kiuru Aaro
Published: August 2003

Mailing thermoluminescent (TL) dosimeters for exposure by users of diagnostic x‐ray equipment is a simple and inexpensive way of monitoring output from x‐ray units. The readings given by TL dosimeters were originally compared with ionization chamber measurements and shown to give reliable and compatible results. Mailed TL dosimeters alone were later used in a university hospital to check the functioning of exposure timers in thorax and Bucky stands. A thorough survey of the outputs from all x‐ray units in use throughout the country can be performed within a few months using this technique.
Testing coincidence of x‐ray and light beam
Authors: Freeman Donald K.
Published: August 2003

Recent government legislation in many countries around the world calls for specific alignment accuracy between x‐ray and light beams for diagnostic x‐ray machines. This technical report describes an alignment model that can be used to check the coincidence of both the x‐ray field and its central ray with the light field and its cross hairs.
A differential method for inhomogeneity correction on dose in a photon beam
Authors: Yuen K. , McParland B. J. , Kornelsen R. O.
Published: August 2003

For a uniform slab of inhomogeneity in a supervoltage beam, correction factors can be calculated from the Batho equation. In this report, we present a method for calculating the effect of an annular inhomogeneity, concentric about the beam axis, upon the dose at a point on the axis and below the annulus. A derivation of the equation required in the calculation for supervoltage radiation is given. Results from measurements made in 60Co beams for polystyrene foam, cedar, and aluminum annuli, all having 3.0×3.0 cm2 in cross section but with different inside diameters, are compared with correction values calculated by the method. For situations where the annulus is just submerged in the phantom, measured and calculated values are in good agreement. For a general situation, two calculation types are proposed and the data show that in general the measured scatter perturbation lies between the calculated values of the two types. Application of our technique predicts a sign reversal in the scatter perturbation due to an inhomogeneity. This reversal has previously been observed and reported and is also demonstrated in our measurements.
A unified description of NMR imaging, data‐collection strategies, and reconstruction
Authors: King Kevin F. , Moran Paul R.
Published: August 2003

In nuclear magnetic resonance (NMR) imaging by the zeugmatographic methods, there is a common and unified theoretical description. All forms of two‐dimensional and three‐dimensional imaging involve NMR data which trace various geometric representations, in reciprocal transform space, of the subject's spatially blurred “effective” spin density. The effective density is proportional to the physical density modulated spatially by the several factors of receiver coil (B/I) ratio, rf pulse excitation terms, T1‐relaxation terms, and T2‐relaxation terms. These factors depend upon the rf pulse sequence and field‐gradient modulation sequence, and they may be calculated according to some model or directly measured. From this viewpoint, all different imaging modes appear as variations in data‐collection and image‐reconstruction strategies. The results are used here to describe slice‐oriented polar and Cartesian strategies, three‐dimensional Cartesian and two forms of spherical strategies, and multiecho strategies of the “planar‐echo” type.
Temperature distributions induced in pig tissues by a water‐cooled disk electrode rf system
Authors: Prionas S. D. , Hahn G. M.
Published: August 2003

Thermal dosimetry is one of the most challenging and necessary aspects of therapeutic hyperthermia. Treatment optimization and assessment of efficacy of the therapy can only be achieved by a quantification of the thermal distributions in normal and malignant tissue induced by specific configurations of heat sources. Radial and axial temperature distributions induced in normal porcine tissues by a pair of disk‐shaped direct contact electrodes operating at 0.5 MHz are presented in this communication. Temperature increments induced in the skin, adipose tissue, and skeletal muscle tissue are correlated with the magnitude of the incident power density for various sizes of circular electrodes. The observed thermal distributions very likely relate to the heterogeneous thermal and electrical properties of these tissues.
Improved lung dose calculation using tissue–maximum ratios in the Batho correction
Authors: El‐Khatib E. , Battista J. J.
Published: August 2003

We have reexamined the Batho power law for computing the dose within and beyond lung irradiated with small and large fields of cobalt‐60 and 6‐MV x rays. Using slab phantoms consisting of two materials, agreement between calculated and measured doses was within 2% inside lung for 6‐MV x irradiation, but much poorer (9%) for cobalt‐60 irradiation. For cobalt‐60 irradiation, tissue–air ratios (TARs) were used initially in the Batho equation, while for 6‐MV x rays, tissue–maximum ratios (TMRs) were used. When we substituted TMR values instead of TAR values for cobalt‐60, we found marked improvement by nearly 5% in the accuracy of dose calculated within lung. This was confirmed by numerical comparison of the Batho expression with an analytic solution of the primary and first‐scattered radiation. We therefore encourage the use of TMRs for cobalt‐60 radiation, expecially for larger radiation fields, and provide measured data tables for field sizes up to 50×50 cm2, and depths up to 30 cm. In addition to unifying the dosimetry for all megavoltage irradiation, this approach improves the accuracy of doses calculated within lung.
An improved electron energy‐loss straggling algorithm for Monte Carlo transport codes
Authors: Weinhous Martin S. , Nath Ravinder
Published: August 2003

The commonly used Blunck and Leisegang electron energy‐loss distribution falls off too rapidly with increasing energy loss. Also, for large thicknesses and/or low‐Z media, where their distribution should approach Landau's, it normalizes to 0.92 rather than 1.0, it overestimates the number of very small energy‐loss events, and its peak is shifted from λ=−0.225 to 0.1. Because of these shortcomings, calculations made using this distribution yield a mean straggled energy loss which is lower than the value predicted by the continuous slowing down approximation (CSDA). An improved version of the Blunck–Leisegang distribution, which exhibits better normalization and falloff, has been developed. Further, an algorithm was created which (depending on the CSDA energy loss, Z, A, electron energy, and transport step size) samples the electron's straggled energy loss from the more accurate of the available distribution functions.
On‐line characterization of heavy‐ion beams with semiconductor detectors
Authors: Llacer J. , Tobias C. A. , Holley W. R. , Kanai T.
Published: August 2003

Heavy‐ion beams used in biomedical studies suffer a substantial amount of nuclear reactions (fragmentation) as they traverse matter. Since it has been demonstrated that dose and linear energy transfer (LET) are not a sufficient description of a beam for the purpose of understanding its biological effects, it is necessary to be able to separate the components of a complex beam so that their individual effects can be analyzed. A simple and small assembly consisting of a thin silicon LET detector, in time coincidence with a thick germanium residual energy detector has been used in measurements of the components of Ne‐20 and Si‐28 high‐energy ion beams. The detector system can be placed at any experimental area without difficulty and it can carry out a beam analysis in a few minutes, making it very appropriate for fast on‐line measurements and verification of beam characteristics. LET values measured by the silicon detector agree well with results of the Bethe stopping‐power calculations, and the dose measured for the beam components can be used to obtain Bragg curves that are in good agreement with those obtained by ionization chamber measurements on the same beams. The numbers and LET distribution of primaries and fragments at different positions of the Bragg curves, as well as fractional dose contributed by the different components are determined directly from the experimental data. Particle velocity distributions can be obtained for the higher Z fragments. Limitations and advantages of the simple measurement technique are discussed.
The use of lead‐lined cones to improve the beam characteristics of a 50‐kVp contact therapy x‐ray beam
Authors: Sontag Marc R. , Bloch Peter
Published: August 2003

Measurements were made to calibrate a Philips RT‐50 contact therapy unit using an EG&G extrapolation chamber. The measured half‐value layer of 0.27 mm in aluminum agrees closely with published data. Exposure rates in air for 4‐cm source‐to‐surface distance (SSD) cones fall off faster than can be explained by the inverse square law alone. Lining the cone with lead results in exposure rates that follow the inverse square law at all distances from the cone end. Depth dose measurements made for 2‐ and 4‐cm SSD cones produced the anomalous result of greater depth dose for the shorter SSD. This is due to a low‐energy component arising from the end of the 4‐cm SSD cone, producing an artificially high surface dose. Lining the cone with lead reduces the component and significantly increases the depth dose. The low‐energy component is believed to be 6–8 keV characteristic x rays that arise from the stainless‐steel cone. Lining the cone with lead either absorbs these x rays or prevents their production.
Flywheel energy storage for x‐ray machines
Authors: Siedband Melvin P. , Showers Donald K.
Published: August 2003

X‐ray image quality for stop‐motion exposures is greatly affected by the system power capability. High power levels are required for adequate resolution, which often precludes the use of mobile x‐ray systems for stop‐motion exposures. Currently available mobile systems use (1) 90‐V nickel–cadmium batteries capable of 120 A, (2) a power line of 220 V ac, 60 Hz capable of about 100 A, or (3) a capacitor discharge unit using 1.0‐μF capacitors and limited to 17‐mAs equivalent output (compared to three‐phase systems at 100 kVp). In each case, instantaneous power is usually limited to 10 kW. An alternative means which now appears to be a practical power source for mobile x‐ray systems is the flywheel energy storage system. A 5‐kg flywheel has been constructed which runs at 10 000 rpm and stores 25 000 J while drawing only a few hundred watts to bring the system up to speed. When coupled to an aircraft alternator, pulsed power levels of 25 kW have been achieved. The aircraft alternator also has the advantage of high‐frequency output which has permitted the use of smaller high‐voltage transformers. This system permits the generation of powerful x rays using low‐power sources, such as single automobile batteries, common 115‐V outlets, or electrical sources of poor regulation such as found in third world countries.
On the steady‐state drift conditions of a water calorimeter in clinical megavoltage photon and electron beams
Authors: Kubo Hideo , Brown Dennis E.
Published: August 2003

We have investigated the characteristic behavior of a water calorimeter in a clinical environment where the temperature is not strictly controlled. Since the ambient temperature plays a crucial role in the successful operation of a calorimeter, its effects were closely monitored. The results were compared with theoretical predictions based on a simple model of heat transfer by conduction. They were found useful in analyzing the radiation dose measurements. In particular, a relationship between the absorbed dose and the pre‐ and postirradiation thermal drifts was examined. Finally, some suggestions are made concerning the desirable conditions under which a water calorimeter may be operated.
Dose in the buildup region outside the primary beam
Authors: Horton J. L.
Published: August 2003

The dose received in the buildup region outside the primary beam has been measured for 60Co gamma rays and 6‐ and 18‐MV x rays. The variation of this dose with field size and depth was evaluated. The effect of blocking trays and the importance of their design was determined.
The use of the ferrous sulphate dosimeter for intercomparison of absorbed dose from electron beams
Authors: Chen W. L. , Chang S. C.
Published: August 2003

An intercomparison of absorbed dose from high‐energy electron beams using Fricke dosimetry at four radiotherapy centers in the north of Taiwan is described. Two water phantoms of 30×30×30 cm3 and a number of Fricke dosimeters were used at each center. The dosimeters were irradiated to an absorbed dose of approximately 45 Gy (4500 rad) at different electron energies. The measurements, calibration procedures, and the factors affecting the response of the dosimeters are discussed. The results of the intercomparison of the four centers are given in detail.
Improvements in the equation for calculating the contribution to osmotic potential of the separate solutes of water solutions
Authors: Cochrane T. T.
Published: August 2003

A minor conceptual improvement, the elimination of an approximation and its revision in relativistic terms, has resulted in a twofold increase in the predictability of the equation for calculating the contribution to osmotic potential of the separate solutes of water solutions. The molecular model inherent to its formulation would provide unifying principles for visualizing water movements associated with solute concentration gradients in tissues.
Relative dose efficiencies of antiscatter grids and air gaps in pediatric radiography
Authors: McDaniel David L. , Cohen Gerald , Wagner Louis K. , Robinson Lawrence H.
Published: August 2003

The relative dose efficiencies (RDE) of various antiscatter grids and air gaps were determined for conditions simulating those found in pediatric radiography, using phantoms representing a newborn child, a 5‐yr‐old and a 10‐yr‐old child. Our data indicate than an air gap is best for the newborn, due to the low levels of scatter. The 8:1 fiber grid or 15.2‐cm air gap without a grid can improve dose efficiency (DE) for the 5‐yr‐old child by 20%–25% relative to the 3.3‐cm air gap and no‐grid technique, while for the 10‐yr‐old child, DE can be improved by 40% with an 8:1 fiber grid.
Fluorometer for endoscopic diagnosis of tumors
Authors: Profio A. E. , Doiron D. R. , Sarnaik J.
Published: August 2003

A filter fluorometer suitable for endoscopic applications has been developed for detection and characterization of superficial tumors by the fluorescence of a previously injected, tumor‐specific agent, hematoporphyrin derivative. Fluorescence is excited by violet light conducted through a fiberoptic lightguide in the endoscope, and the fluorescence emission together with reflected violet are collected by another fiberoptic lightguide. The red fluorescence and violet are separated by a dichroic mirror and filter and detected in photomultiplier tubes. The ratio of the fluorescence signal to the reflected violet signal is proportional to the ratio of the fluorescence yield to the violet reflectivity but is insensitive to variations in distance, angle, and violet power. The instrument may be useful for localizing small tumors, and for quantitative measurements of the amount of hematoporphyrin derivative in the tumor, a requirement for accurate dosimetry in photoradiation therapy.
Application of the library least‐squares analysis to whole‐body counter spectra derived from an array of detectors
Authors: Wielopolski L. , Cohn S. H.
Published: August 2003

The library least‐squares method was applied to the analysis of gamma‐ray spectra obtained from an array of 54 NaI(Tl) detectors in a whole‐body counter. The analysis of spectra which were obtained over a period of 8 yr demonstrates the applicability of the method despite inherent variations encountered in large counting systems. The elements of interest analyzed were total‐body K, Ca, Na, Cl, and P. Least‐squares fits obtained with library standards derived from distributed sources were better than those obtained from library standards derived from localized sources.
Electron contamination due to Lucite in a 45‐MV photon beam
Authors: Zwicker R. D. , Wu Andrew , Curran B. H. , Sternick E. S.
Published: August 2003

The presence of the Lucite blocking tray in a 45‐MV betatron beam leads to a considerable loss in the skin‐sparing properties of this unit. Measurements conducted with a parallel‐plate chamber show that the surface dose is approximately doubled (up to 58% of the maximum absorbed dose) and the depth of maximum dose is reduced by as much as 1.5 cm by the Lucite. These results suggest that separate dosimetry data should be kept for blocked and unblocked fields. The penetration of the electron contamination to the open field depth of maximum dose can lead to erroneous results if attenuation measurements are made at that depth. Studies of the effect of the lead sheet and leaded acrylic on the surface dose showed that under normal circumstances the presence of high‐Z materials increases the surface dose, hence these materials cannot in general be used as electron filters on very high energy x‐ray units. A Luciteless blocking tray with universal triangular corner blocks has been designed for use in special cases requiring a reduced surface dose.
The implementation of the AAPM Task Group 21 protocol by the Radiological Physics Center and its implications
Authors: Gastorf Robert J. , Hanson William F. , Shalek Robert J. , Berkley Lawrence W.
Published: August 2003

The Radiation Therapy Committee of the American Association of Physicists in Medicine appointed Task Group 21 to write a new protocol for the calibration of high‐energy photon and electron therapy beams. This protocol updates the physical parameters used in the calculations and is intended to account for differences in ionization chamber design and some differences between phantom materials that were not considered in previous protocols. This paper discusses how the Radiological Physics Center (RPC) intends to implement the new protocol, the changes required in the RPC calibration techniques, and the magnitude of the change in the RPC calculations of absorbed dose resulting from the implementation of the new protocol. Although the change in the RPC absorbed‐dose calculations will be only 0%–2% over the range of photon and electron energies of interest, some institutions using specific dosimetry systems may find their absorbed‐dose calculations changing by 4% or more.
Shortening the calculation time of photon dose distributions in an inhomogeneous medium
Authors: Boyer Arthur L.
Published: August 2003

A procedure is described for calculating photon dose distributions using convolutions of radiation interaction kernels with calculated fluence distributions. The advantages and disadvantages of the concept are discussed.
Portal film quality: A multiple institutional study
Authors: Reinstein L. E. , Durham M. , Tefft M. , Yu A. , Glicksman A. S. , Eaton W.
Published: August 2003

The variation in quality of the several thousand portal films submitted to the Quality Assurance Review Center (QARC) has been substantial. To ascertain the nature and severity of this problem, a detailed study of “whole brain” portal films which were taken at 23 different radiotherapy departments for patients entered on three national leukemia studies was performed. Each film was analyzed in two ways: (a) independent subjective evaluation by four experienced radiotherapists and (b) measurement of objective parameters. Scores from 416 evaluations together with measured parameters were stored in a data base system for easy statistical manipulation. The dependence of perceived film quality on these objective parameters has been correlated and is the subject of this report.
Rapid in vivo localization of a lost brachytherapy seed
Authors: Woolfenden James M. , Barber H. Bradford , Oleson James R. , Wild Walter J.
Published: August 2003

We adapted a small sodium iodide scintillation detector for rapid, in vivo localization of a lost brachytherapy seed. Using the detector, we were able to locate and remove an Ir‐192 seed that was not visible to the unaided eye.
An improved optical backpointer for isocentric radiotherapy machines
Authors: Aldrich John E.
Published: August 2003

A simple optical backpointer has been designed as a retrofit for isocentric radiotherapy machines. Two lasers are used to project two fan beams of light which intersect in line with the central ray of the radiation beam. The system is mechanically rugged, and the projected cross is visible even in room light.
The use of multivariate regression analysis in contrast‐detail studies of CT scanners
Authors: Trajtenberg Manuel
Published: August 2003

Previous studies of the imaging performance of computed tomography (CT) scanners, and other imaging modalities, have failed to apply appropriate statistical methods to data analysis, thus impairing the accuracy and significance of results. Given that imaging performance involves a number of interrelated variables and an element of randomness, its empirical assessment requires multivariate regression analysis. This method is used here to analyze anew a set of contrast‐detail data from a previous study on CT scanners. The main issues considered are the specification of the proper functional form linking perceptibility, dose and contrast, the estimation of the contrast and dose coefficients, and of scanner‐specific constants to be used in computing indices of imaging quality. One of the main empirical findings is that the dose coefficient of the CT scanners studied is significantly less than that predicted by the theoretical model: 1/5 instead of 1/3. This result suggests that actual dose used in routine clinical studies could be reduced substantially without impairing much the quality of the images. On the other hand, the coefficient of contrast does correspond to its predicted value, i.e., 2/3. The methodology used here is not limited to the contrast‐detail framework, but is applicable to, and indeed essential in, empirical studies of the performance of any imaging modality.
The collection efficiency of an ionization chamber in a pulsed and magnetically swept electron beam: Limits of validity of the two‐voltage technique
Authors: Conere T. J. , Boag J. W.
Published: August 2003

The two‐voltage method of determining the collection efficiency of an ionization chamber exposed to a pulsed and magnetically swept electron beam depends upon the form of the saturation curve agreeing closely with the formula based on ionic conduction and negligible space charge. The possible disturbing effects of charge transport by free electrons and of overlapping pulses are discussed and the conditions to be satisfied for the reliable use of the method are defined and demonstrated experimentally.
Analysis of variations in contrast‐detail experiments
Authors: Cohen Gerald , McDaniel David L. , Wagner Louis K.
Published: August 2003

Three sources of variability in a contrast‐detail (CD) experiment have been quantitated: within‐observer variance, between‐observer variance, and sample variance. It is concluded that (1) it is more efficient to increase the numbers of replicated images and observers than to increase the number of readings; (2) sampling and between‐observer variations are approximately equal; (3) one can expect approximately 10% standard errors in the measured value of threshold detail or threshold contrast in a CD experiment which employs four observers, four replicate image samples, and one reading per observer.
Dual‐film cassette technique for studying the effect of radiographic image quality on diagnostic accuracy
Authors: Higashida Yoshiharu , Doi Kunio , Lehr James L. , MacMahon Heber
Published: August 2003

In order to investigate the relationship between diagnostic accuracy and radiographic image quality, we have developed a dual‐film cassette which produces two radiographs simultaneously with a single exposure. One of these radiographs is of standard quality; the second is of lower quality because of degraded spatial resolution and a significantly lower exposure. We have studied the basic physical properties of the standard and low‐dose screen–film systems which we use in the dual‐film cassette by comparing their beam hardening, scatter fractions, contrasts, modulation transfer functions (MTFs), and Wiener spectra. The x‐ray spectrum incident on the low‐dose system contained more high‐energy photons than that incident on the standard system, and the scatter fractions for the low‐dose system were slightly less than or comparable to those for the standard system. While the radiographic contrast produced by the two systems were generally comparable, the standard system had a slightly higher contrast than the low‐dose system in some cases. The MTF of the low‐dose system was considerably lower than that of the standard system, and the low‐dose system had a noise level considerably greater than did the standard system. Phantom images and clinical radiographs indicated that, for the low‐dose system, the image quality degraded significantly.
The effect of various dissolved gases on the heat defect of water
Authors: Ross C. K. , Klassen N. V. , Smith G. D.
Published: August 2003

Recent measurements of the absorbed dose to air‐saturated water, made using water calorimetry and assuming a zero heat defect for irradiated water, gave results 2%–5% higher than those determined by more conventional means. According to the current radiation chemical model for air‐saturated water, the dose measured by water calorimetry assuming a zero heat defect should actually be 2% too low because of the endothermicity of the radiolysis processes in water. In order to examine possible sources for this discrepancy, we have constructed a small calorimeter (holding 100 ml of water) with which to measure the temperature rise in irradiated water saturated with various gases. The gases used were air, oxygen, argon, nitrogen, and hydrogen/oxygen mixtures. Irradiations were carried out with 20‐MV x rays at a dose rate of 0.41 Gy/s. Our results are consistent with model calculations, except for some differences for accumulated doses of less than 100 Gy. The discrepancies we find at low doses and the discrepancies observed by others using water calorimeters may arise from impurities in the water.
A constrained least‐squares optimization method for external beam radiation therapy treatment planning
Authors: Starkschall George
Published: August 2003

A method has been developed to optimize the selection of beam weights for external beam radiation therapy treatment planning. This method uses a matrix transformation method to achieve a least‐squares fit to doses at preselected points in a target volume. This method is more general than the conventional least‐squares method in that it allows one to incorporate as constraints maximum allowable doses delivered to critical organs, as well as the requirement that all beam weights be non‐negative. A computer algorithm DOSEOPT has been written which takes a matrix of doses to various points from beams whose field sizes and orientations have been predetermined, and performs the constrained least‐squares fit to desired doses in the target volume and maximum doses to critical organs. Implementation of this algorithm and its application to several cases are discussed.
Pseudonoise coded ultrasonic holographic imaging
Authors: Chen Victor C. , Pao Yoh‐Han
Published: August 2003

In this paper, a pseudonoise (PN) coded ultrasonic holographic imaging system design is studied with computer simulation. A system utilizing PN coded phase‐modulated ultrasonic signals is capable of yielding both high lateral and high range resolution. It is found that the time for data acquisition and image reconstruction is much less than that required for some comparable methods. The PN coded imaging system also has excellent noise immunity, and this is of great practical importance. Some results for a simulated three‐dimensional imaging geometry are shown to demonstrate the characteristics of PN coded ultrasound systems.
Detective quantum efficiency of x‐ray image intensifiers: Comparison of scintillation spectrum and rms methods
Authors: Rowlands J. A. , Taylor K. W.
Published: August 2003

For monoenergetic x rays, the detective quantum efficiency (DQE) of an x‐ray image intensifier (XRII) obtained by measuring the ratio of signal to root‐mean‐square (rms) noise is compared with the DQE obtained by the scintillation spectrum method. To obtain agreement between the two methods, the rms noise must be measured at a very low frequency because of the temporal response RXRII(f) of the XRII. This takes a long time and the measurement of the DQE from the scintillation spectrum is much quicker. Measurement of RXRII(f) shows that it is dependent on x‐ray flux rate. RXRII(f) measured at the same flux rate as DQE permits correction of DQE measurements to zero frequency. In order to estimate DQE for polyenergetic beams, we need to know as a function of energy not only the DQE but the amount of energy deposited per absorbed x ray. The latter is also obtained from the rms measurement.
Compton scatter densitometry with polychromatic sources
Authors: Duke Peter R. , Hanson James A.
Published: August 2003

The use of x‐ray tubes as radiation sources for Compton scatter densitometry (CSD) is evaluated theoretically. Bias correction factors (bcf's) which correct for differential attenuation due to Compton shift and polychromaticity, and differential scattering cross section due to polychromaticity, are defined to facilitate comparison of monochromatic and x‐ray tube CSD devices. Computer modeling indicates that a 100‐kVp x‐ray tube spectrum hardened with 0.64 mm of Pb filtration yields bcf's 0.008 to 0.34% higher than a 70‐keV monochromatic source for various samples of nylon, and human muscle and bone. This difference is insignificant because multiple scatter errors are typically 1000% higher than errors corrected by bcf's. The high equivalent activities of x‐ray tubes (10 to 80 TBq) and their small focal spots (1.0 to 2.0 mm) allow the long‐standing CSD problems of marginal statistical precision, long counting times, and low spatial resolution to be overcome.
Compton scatter with polychromatic sources for lung densitometry
Authors: Hanson James A. , Moore William E. , Figley Melvin M. , Duke Peter R.
Published: August 2003

A mobile lung densitometer using conventional x‐ray tubes, NaI detectors, and principles of two‐source, two‐detector Compton scattered densitometry, is described. The device is capable of one to two per second density measurements from a 45‐cm3 volume with a precision of 5%. The expected in vivo accuracy (2%–3%) is determined by using an anthropomorphic phantom with replaceable lung inserts. The unintentional detection of multiple‐scattered x rays results in a small density‐dependent error. This error is predictable and relatively insensitive to differences in surrounding absorbers such as the chest wall. With this device, dynamic in vivo densitometry of the lung in the clinical laboratory and intensive care unit will be possible.
Radionuclide selection and model absorbed dose calculations for radiolabeled tumor associated antibodies
Authors: Wessels Barry W. , Rogus Ronald D.
Published: August 2003

An absorbed dose calculation comparison has been computed for radiolabeled tumor associated antibodies distributed over a standard geometry and tumor location. Half‐life data, maximum specific activities, and relative organ doses of nine radionuclides, Cu‐67, Br‐77, Br‐82, Y‐90, Tc‐99m, In‐111, I‐131, Re‐186, and At‐211, have been compiled in which the radionuclides were assumed to be coupled with antibody. These nuclides were chosen on the basis of physical characteristics that warranted their inclusion as either imaging or therapy radiolabels. Radionuclide biodistribution data based on current available estimates for antibody uptake and clearance in humans has been adopted. Re‐186 and Y‐90 have been determined to be among the best therapy radiolabels since they possess sufficiently long half lives necessary for tumor localization, little or no gamma radiation, intermediate beta energy, stable daughter products, and have a reasonable chance to form a stable chelate with an antibody system.
Grid controlled x‐ray tube switching time: Implications for rapid exposure control
Authors: Plewes D. B. , Vogelstein E.
Published: August 2003

The transient response properties of a conventional grid controlled x‐ray source and the implications for rapid exposure control are studied. The use of a cathode cable with a length of only 1 m to connect the x‐ray tube and the grid control electronics results in a switching time of 19 μs. The phototiming artifacts from the delayed radiation delivered during this transition time can be severe for short pulses. An analog correction technique that anticipates the magnitude of the delayed radiation is used to overcome this limitation and allows accurate phototiming over a wide range of pulse widths ranging down to 20 μs.
Dosimetry of small fields for Therac 20 electron beams
Authors: Sharma Subhash C. , Wilson David L. , Jose Baby
Published: August 2003

The Therac 20 medical linear accelerator produces electron beams of 6, 9, 13, 17, and 20 MeV. We measured depth dose, isodose curves, and output factors for small electron fields using an ionization chamber, film, and thermoluminescent dosimeters. Tables and graphs were generated from these measurements for accurate treatment planning of various blocked and open fields.
The magnetic field dependence of water proton T1 in aqueous solutions: Implications for magnetic imaging contrast media
Authors: Bryant Robert G. , Polnaszek Carl , Kennedy Scott , Hetzler Joanne , Hickerson David
Published: August 2003

The magnetic field dependence of the water proton nuclear magnetic relaxation induced by a nitroxide radical both free and covalently bound to serum albumin has been measured from 0.23 mT to 0.7 T. The field dependence of the water proton 1/T1 shows only a minor dependence on the degree of radical immobilization, though the electron spin resonance (ESR) spectrum is very sensitive to the correlation time of the radical. The results and theory are described in more detail elsewhere (Journal of Chemcial Physics, in press). The observed relaxation behavior arises because the long electron relaxation time of the nitroxide forces the correlation time for the magnetic interaction driving the water proton relaxation to become the correlation time for the relative translational motion of the water and the nitroxide. The relative insensitivity of the water proton relaxation to nitroxide immobilization should not change if the nitroxide is used as a contrast reagent for magnetic imaging in vivo.
Determination of Aion and Pion in the new AAPM radiotherapy dosimetry protocol
Authors: Attix F. H.
Published: August 2003

Simple equations are given by which accurate values of Aion and Pion can be obtained for use in applying the AAPM Task Group 21 dosimetry protocol.
Dosimetric characterization of the 18‐MV photon beam from the Siemens Mevatron 77 linear accelerator
Authors: Palta Jatinder R. , Meyer Jeffrey A. , Hogstrom Kenneth R.
Published: August 2003

A comprehensive set of dosimetric measurements has been made on the Mevatron 77.80.67 18‐MV photon beam. Percentage depth dose, dose in the buildup region, field size dependence of output, transmission through lead, tray attenuation, and isodose curves for the open and wedged fields were measured using an ionization chamber in water and polystyrene phantoms. These dosimetric measurements sufficiently characterized the beam to permit clinical use. The depth dose at 10‐cm depth for a 10×10 cm2 field at 100‐cm source‐to‐skin distance (SSD) is 80.9%, which meets design specifications. Central axis depth‐dose data were fitted to within 0.5% by a set of polynomial equations utilizing a two‐dimensional linear regression analysis. Tissue–maximum ratios calculated from depth‐dose data agree with measured data to within 2%. Output differences as large as 2.5% were measured for rectangular fields depending on which collimator jaws defined the long dimension of the field. The field size dependence of output was fit to within ±0.1% by a linear regression. The half‐value thickness of the beam was measured to be 13 mm of lead.
A simple digitizer technique for determining the effective center of spoke shots
Authors: Biggs Peter J.
Published: August 2003

A method is described for computing the effective center of an x‐ray spoke shot pattern and determining its separation from a given reference point, such as the mechanical isocenter. This method is as quick as, but more accurate than, the standard technique of using a ruler.
A xenon ionization detector for scanned projection radiography:95‐channel prototype evaluation
Authors: Drost D. J. , Fenster A.
Published: August 2003

We have designed, constructed, and tested a 95‐channel prototype xenon ionization detector for use in scanned projection radiography (SPR). This detector has higher spatial resolution, is more dose efficient, and is easier to construct than computed tomography (CT) xenon ionization detectors. It consists of two parallel plates separated by a 2‐mm gap filled with xenon gas at 2 MPa (20 atm). One plate is a high‐voltage electrode while the other is a circuit board etched to form an array of metal collector strips focused on the x‐ray source. The resulting detector channels are 0.5 mm wide and 6 cm long. In this paper we present results from measurements of system noise and detector channel calibration. We compared the detector system to a screen/film system and found that it allows the detection of structures with 0.17% radiographic contrast compared to 2% contrast required for detection with screen/film when tested by imaging a 10‐cm‐thick Lucite phantom with a 10×10−6 C/kg exposure. From images of resolution test patterns, the limiting resolution of the detector is 2.0 1p/mm at 1.6 magnification. Images of reduction mammoplasty tissue samples, obtained with 1/17 the exposure of screen/film images, had the same low‐contrast sensitivity but contained less high‐contrast detail than the film images.
Tc‐99m attenuation coefficients in water‐filled phantoms determined with gamma cameras
Authors: Harris C. Craig , Greer Kim L. , Jaszczak Ronald J. , Floyd Carey E. , Fearnow Edgar C. , Coleman R. Edward
Published: August 2003

Quantitative imaging with gamma cameras requires compensation for attenuation of source photons. Some methods of compensation make use of a constant or average estimated attenuation coefficient μ. A value for μ of 0.15 cm−1 for 140.5‐keV photons in water or tissue is commonly used. This value, however, neglects scattered photons which are detected within the energy window in gamma camera imaging. Values for μ of 0.12 cm−1 used in attenuation compensation of Tc‐99m single‐photon emission computed tomography scans of uniform cylindrical sources have been shown to give improved results compared with use of μ=0.15 cm−1. In this study, gamma cameras and a multichannel pulse‐height analyzer were used to determine effective values of μ for photons in water as a function of energy window. Two cylindrical water‐filled phantoms, circular and elliptical, were used with a point source of Tc‐99m at depths up to 18 cm. Energy data were integrated over the top half of the photopeak, and over 10%, 20%, and 30% windows centered on the photopeak. Attenuation curves were exponential for all photopeak windows with values of μ of 0.12±0.014 cm−1 for all windows up to 20% and 0.1 cm−1 for a 30% window. This study suggests that a value of μ of 0.11–0.12 cm−1 is, in fact, appropriate for use in attenuation compensations where an average is required.
Compensator filters made with compact Moire camera and computer
Authors: Mok E. C. , Boyer Arthur L.
Published: August 2003

This paper describes a light, easy to use Moire camera and a computer program written to implement it as part of a clinical compensating filters design system.
Vrx: A verify–record system for radiotherapy
Authors: Dickof Peter , Morris Phil B. , Getz Darrell
Published: August 2003

A system of computer programs has been created to allow the entry of radiotherapy treatment details as defined by the physician, the verification of the machine parameters at every treatment, and the recording of the entire course of treatment. Various utility programs are available to simplify the use and maintenance of the system. The majority of the code is written in FORTRAN‐77, the remainder in MACRO‐11. The system has been implemented on a PDP 11/60 minicomputer for use with a Mevatron linear accelerator, the implementation required minor hardware changes to the accelerator.
Interlaced versus progressive readout of television cameras for digital radiographic acquisitions
Authors: Seibert J. Anthony , Barr David H. , Borger David J. , Hines Horace H. , Bogren Hugo G.
Published: August 2003

A comparison was made of two methods for reading the TV camera target for digital radiographic acquisition, interlaced and progressive readout. Experiments were performed with a plumbicon TV camera to measure x‐ray exposure requirements and resolution capabilities after readout for the two modes. Real time digital (30 frames per second, 2562 matrix) and single‐frame electrocardiogram gated images of the contrast‐filled coronary arteries in a dog were acquired with both formats. Results demonstrate a reduction in patient exposure of up to four times for single‐frame progressive readout images over interlaced acquisition for static images of comparable quality. Imaging of rapid motion objects (e.g., coronary arteries) with interlaced TV scanning results in “scalloping” of edges and resolution degradation. The short pulse width capability of progressive readout eliminates motion blurring and preserves object detail. Progressive readout is the preferred mode of TV camera readout for most digital radiographic applications due to more flexible x‐ray techniques, superior images, and total utilization of the input x‐ray flux for lower patient exposure.
A statistical analysis of ionometrically measured 6‐MV x‐ray beam parameters
Authors: Dawson D. J. , Gribble M. A.
Published: August 2003

A statistical analysis has been made of ionometrically measured parameters related to the central‐axis output, the beam symmetry, the beam flatness, and the depth dose for the 6‐MV x‐ray beam from the Therac‐6 linear accelerator. The means and coefficients of variation of the measurements are determined on a monthly basis for each parameter. Mean coefficients of variation less than ± 1% have been determined and are useful in establishing the optimal limits of acceptance of these parameters. The results of this study indicate that the long‐term stability of the radiation parameters associated with this unit can be maintained within acceptable limits with a consistent quality‐assurance program.
On the selection of stopping‐power and mass energy‐absorption coefficient ratios for high‐energy x‐ray dosimetry
Authors: Cunningham John R. , Schulz Robert J.
Published: August 2003

A method for the selection of average stopping‐power and energy‐ absorption coefficient ratios has been developed. The quality of the x‐ray beam is characterized by the ratio of ionization chamber readings at depths of 20 and 10 cm in water . For convenience, a relationship is established between experimental and the nominal accelerating potential (MV) of the accelerator. Experimental are related to and in a three‐step process. First, using experimental and theoretical spectra in the range 60Co to 45 MV, were calculated for primary and first‐scatter photons, and a graph of experimental versus calculated for these same spectra was constructed. Second, and were calculated for a large number of primary spectra [for most of which experimental were not available] and a graph constructed that related these quantities and calculated as above for this group of spectra. Third, using the graphs from the preceding steps, graphs relating the calculated and with experimental were constructed. Data are presented for water, polystyrene, acrylic, graphite, A‐150, C‐552, Bakelite, and nylon for beams with nominal accelerating potentials in the range 2–45 MV.