bluuebunny/crossref_metadata_2025 huggingface dataset:
The geometric modulation transfer function of a transmission imaging system that uses a SPECT scintillation camera and parallel hole collimation
Authors: Kemp Brad J. , Prato Frank S. , Nicholson Richard L.
Published: March 2003

An analytic expression has been derived to calculate the geometric modulation transfer function of a transmission imaging system that uses parallel hole collimation for both the source and the SPECT camera. This expression describes the resolution of the transmission imaging system and replaces the need to use computer intensive Monte Carlo simulations for the system design. The geometric modulation transfer function, denoted as MTFg(ρ), where ρ is the spatial frequency vector, is represented as MTFg(ρ)=|[A2sc(sscρ)**(sccρ)] D(ρ)|, where ** denotes two‐dimensional convolution; Asc (ρ) and Acc(ρ) are the Fourier transforms (FT) of the aperture functions for the parallel hole source collimator (SC) and the camera collimator (CC) holes, respectively; D(ρ) is the FT of the camera response; and ssc and scc are scaling constants that depend on the respective collimator dimensions, the system dimensions, the object distance above camera collimator and whether MTFg(ρ) is calculated for the object or image plane. The theoretical MTFg(ρ) was verified with Monte Carlo simulations and experimental results. The formalism shows that the system resolution is characterized by the camera resolution and a combination of the resolutions of the source and camera collimators. This expression can be used to optimize the design of transmission imaging systems to be used in nuclear medicine.
Determination of differential scatter–air ratios (dSAR) for three‐dimensional scatter integration
Authors: Sontag Marc R. , Ray Sujit K.
Published: March 2003

Scatter dose may be calculated by summing the scatter contribution from individual volume elements. These contributions may be represented by differential scatter–air ratios (dSAR). Determination of dSAR from measured data is only approximately correct for second and higher orders of scatter and yields values often limited to one significant figure. Monte Carlo calculation, on the other hand, is time intensive, requires some knowledge of the beam's x‐ray spectrum, and mastering the complexities of a program such as EGS4. Total scatter dose at a point may be determined by measuring depth dose or tissue–air ratios and partitioning the dose into its primary and scatter components. Scatter may be represented by scatter–air ratios, which can be characterized by the sum of first, second, and higher orders of scatter. The first scatter dose may be computed exactly by summing the first scatter contribution from individual elements, determined from the first principle. Separation of dSAR into primary attenuation and depth‐independent terms allows the latter to be precomputed once for a given energy and stored in tabular form. Second scatter may be treated in a similar manner. The higher orders of scatter are computed by subtracting the sum of calculated first and second scatter doses from the total scatter dose. Elements close to and approximately 1 cm above the point of calculation contribute most heavily to the first scatter dose. Compared to the first scatter dose, the second scatter dose contribution is lower, particularly for elements close to the point of calculation. First scatter dose constitutes most of the scatter dose for small field sizes and shallow depths. For larger field sizes and greater depths, second and higher orders of scatter dose become more significant. As beam energy increases, the first scatter dose comprises a greater fraction of total scatter dose.
A novel comprehensive quality control instrument for medical accelerators
Authors: Wielopolski L. , Waldman I. , Zedek I. J. , Bush G. A.
Published: March 2003

A comprehensive quality control instrument for calibration of medical accelerators that use photon, electron, or proton beams in teleradiotherapy is described. It employs a fluorescence screen, mounted on a central stage with four degrees of freedom, monitored by a CCD camera. A single set‐up of the instrument enables one to perform mechanical, light laser, and radiation tests, at arbitrary angles of the accelerator gantry. The new device provides for quantitative evaluation of the tests performed and provides for documentation of the test results in real time. The device provides significant time savings with concurrent improvement in accuracy for tests performed during installation and acceptance processes, and the implementation of quality control procedures for medical accelerators.
Experimental validation of Monte Carlo dose calculations about a high‐intensity Ir‐192 source for pulsed dose‐rate brachytherapy
Authors: Valicenti R. K. , Kirov A. S. , Meigooni A. S. , Mishra V. , Das R. K. , Williamson J. F.
Published: March 2003

Despite widespread use of high‐intensity Ir‐192 remotely afterloaded sources, no published measured or calculated dose‐rate tables for currently used source designs are available. For a pulsed dose‐rate Ir‐192 source, both transverse axis (0.5–10 cm) and two‐dimensional polar dose‐rate profiles (1.5, 3, and 5 cm) were measured with thermoluminescent dosimetry in a solid water phantom. Dose rates were normalized to measured air‐kerma strength, and the source geometry was verified by pinhole autoradiography and transmission radiography. At each measurement point, dose rates were calculated by a Monte Carlo photon transport (MCPT) code, which realistically modeled the experimental phantom, source, and detector geometry. Agreement between MCPT absolute dose‐rate calculations and measurements averaged 3% and was less than 5%, demonstrating that Monte Carlo simulation is an accurate and powerful tool for two‐dimensional dosimetric characterization of high activity Ir‐192 sources.
Measurements of the effect of storage at various temperatures on the T1 of ex vivo tissues
Authors: Lewa Czeslaw J. , de Certaines Jacques D.
Published: March 2003

The detailed measurements of the effects of storage at various temperatures on the nuclear magnetic resonance (NMR) spin‐lattice relaxation time T1 are reported. On the basis of the results, a method to monitor tissue transformation in hyperthermia of biological systems using magnetic resonance imaging (MRI) techniques is discussed. The method was found on proton relaxation time spin‐lattice T1 variation measurements. Parameter F described as tissue structure transformation and/or protein denaturation rate index is discussed.
An algorithm for improving the accuracy of discrete ROI integrals
Authors: Hauser D. L. , Wayner P. C. , Taylor D. L.
Published: March 2003

Many problems in the analysis of medical digital images, e.g., digitized x‐ray radiograph, computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET), require a detailed and precise analysis of user chosen regions of interest (ROIs). Examples of their use include calculating integrals of area, volume, mass, structural moments, and statistical measures for either organs, tumors, or the musculoskeletal system. Among various ROI scan conversion schemes, binary approximate scan conversion is usually preferred due to its efficiency. In this paper, geometric area error is tabulated for typical scan conversion techniques, including whole pixel (WP) approximation and subpixel (SP) approximation methods, and compared to exact pixel (EP) coverage methods for medical ROIs. A new efficient and general EP method for scan conversion of these ROIs is presented. The algorithm traverses the boundary of the polygon while simultaneously scan converting the ROI, and calculates the fractional area of each pixel covered at the perimeter. The resultant geometric area is substantially more accurate than the SP or WP methods, without a significant loss of speed. The numerical results for a ROI with a large ratio of boundary to polygon area demonstrated that the geometric error for a SP method was 40% of the total polygon area, and 150% of the total polygon area for a WP method. The new algorithm could “exactly” calculate the pixel coverage area, in addition to being four times faster than the widely used EP method of Catmull. Efficient and accurate calculation of ROI integrals is essential for comparative analysis.
Performance evaluation of Annular Arrays in practice: The measurement of phase and amplitude patterns of radio‐frequency deep body applicators
Authors: Schneider C. J. , Kuijer J. P. A. , Colussi L. C. , Schepp C. J. , Van Dijk J. D. P.
Published: March 2003

An approach to a solution of two major problems in operating Annular Phased Arrays in deep body hyperthermia is presented: an E‐field sensor capable of measuring phase and amplitude at 70 MHz and the concept of a power transmission factor to determine the effective amplitude of each applicator. In the four‐waveguide Phased Array operating at 70 MHz, which is in clinical use at the department of Radiotherapy of the Academic Medical Center (AMC), the incident fields of the waveguides were scanned in phase and amplitude over the complete aperture midplane, inside an elliptical and a square phantom filled with saline. As a check on the application of the superposition principle, superpositions of the incident fields were compared with the electric field in the measured interference set‐ups. With all four applicators radiating at equal amplitude and in phase, the maximum difference over the complete midplane of the phantom between superimposed and measured interference scans was 20% and 10° in the elliptical phantom, and 20% and 30° in the square phantom. After having determined nominal amplitude and phase patterns by a vector probe, any interference set‐up can be superimposed from measurement of the actual incident field of each applicator. Therefore, the availability of a vector sensor as described here will contribute to solve a problem of hyperthermia quality assurance: the performance evaluation of Phased Arrays.
Accounting for the variation in collision kerma‐to‐terma ratio in polyenergetic photon beam convolution
Authors: Hoban Peter W.
Published: March 2003

In photon beam convolution, the distribution of energy deposition about a primary photon interaction site due to charged particles set in motion at that site is represented by the primary kernel. Energy deposited due to scattered photons, bremsstrahlung, and annihilation photons is represented by the scatter kernel. As the energy deposited in each kernel voxel is normalized to the energy imparted at the interaction site, it is known as a fractional energy distribution. In terma‐based convolution, where kernels are normalized to total energy imparted at the interaction site and are convolved with the terma in the dose calculation process, the sum of fractional energies contained in the primary kernel is equal to the ratio of collision kerma (Kc) to terma (T) corresponding to the energy spectrum used to generate the kernel. Since the ratio of collision kerma to terma increases with depth as the beam hardens, the integral fractional energy in a primary kernel formed for the spectrum at the surface is less than the ratio Kc/T at depth. This causes primary dose to be increasingly underestimated with depth and scatter dose to be increasingly overestimated. Single polyenergetic convolution (using polyenergetic primary and scatter kernels formed using a polyenergetic primary photon spectrum) is thus not as rigorous as if a separate convolution is performed for each energy component. The ratio of true primary dose to single polyenergetic primary dose increases almost linearly with depth and is almost equal to the Kc/T ratio. Primary and scatter dose are calculated correctly if a single polyenergetic convolution is performed in terms of Kc (for primary) and T−Kc (for scatter), where the kernels are weighted sums of monoenergetic kernels normalized to Kc and T−Kc. With this method, it is ensured that total primary energy deposited due to primary photon interactions in a unit mass at a point is equal to Kc at that point.
A comparison of beam‐hardening artifacts in x‐ray computerized tomography with gadolinium and iodine contrast agents
Authors: Ruth Christopher , Joseph Peter M.
Published: March 2003

Lanthanide‐based compounds such as Gd‐DTPA are currently used as contrast agents in MRI. Recent experiments using CT and transmission radiography show that Gd agents can increase image contrast by up to a factor of 2 relative to more commonly used iodinated agents on an equi‐molar basis. It has also been suggested that beam hardening artifacts may be reduced with Gd. This hypothesis was experimentally tested on three different CT scanners using a circular water equivalent phantom with contrast filled tube inserts. It was found that the artifacts were a factor of 1.3–1.8 more pronounced with the iodinated contrast compared with Gd‐DTPA. A theoretical model which uses an experimentally derived photon energy spectrum is proposed which relates the strength of beam hardening artifacts to the variance (over the energy spectrum) of the attenuation coefficient of the contrast agent. This allows easy assessment of the relative magnitudes of the artifact for different contrast agents.
A K edge filter technique for optimization of the coherent‐to‐Compton scatter ratio method
Authors: Harding G. , Armstrong R. , McDaid S. , Cooper M. J.
Published: March 2003

The ratio method involves forming the ratio of the elastic to inelastic x‐ray scatter signals from a localized region of a scattering medium to determine its mean atomic number. An analysis is presented of two major error sources influencing the ratio method: firstly statistical (photon) noise and secondly multiple scattering and self‐attenuation of the primary and scatter radiations in the medium. It is shown that a forward scattering geometry minimizes errors of both types for substances composed of elements with low and medium atomic number. However, owing to the small energy separation (∼100 eV) of coherent and Compton scatter for this geometry, they cannot be distinguished directly with semiconductor (e.g., Ge) detectors. A novel K edge filter technique is described which permits separation of the elastic and Compton signals in the forward‐scatter geometry. The feasibility of this method is demonstrated by experimental results obtained with Ta fluorescence radiation provided by a fluorescent x‐ray source filtered with an Er foil. The extension of this technique to the “in vivo” measurement of low momentum transfer inelastic scattering from biological tissues, possibly providing useful diagnostic information, is briefly discussed.
A direct approach for the determination of absorbed dose from electron beams using non‐water phantoms
Authors: Lu X‐Q. , Chin L. M.
Published: March 2003

Non‐water solid phantoms are often used in the determination of absorbed dose to water for electron beams. Protocols have been established and widely accepted. In these procedures, several assumptions in addition to the Spencer–Attix conditions are required, and several correction factors are needed. A direct approach, in which the conversion is carried out in a single step using a modified Spencer–Attix formula, is studied in this paper. The approach is consistent with the protocols for water phantom, and the conversion factors can be calculated using Monte Carlo simulation. The behavior of the conversion factors is described by comparing the results from the AAPM protocol and experiment data for three electron energies (6, 12, and 16 MeV). This study demonstrates that for beam calibration at dmax, the results from the new approach agree with those from the protocol with a maximum discrepancy of 1% for PMMA and 1.3% for polystyrene. For the depth dose measurement from near the surface to R80, the agreement is within 1.5% for PMMA, 2.5% for polystyrene, and 2.8% for electron solid water. It also demonstrates that for electron solid water, the new approach provides better agreement with experiment data for the beam calibration at dmax.
Double‐film technique for the determination of the homogeneity of line sources
Authors: Cuypers Johan P. , de Vos Robert J.
Published: March 2003

A new technique is presented for determining the homogeneity of the linear activity. The method uses two x‐ray films that are firmly pressed against opposing sides of a needle containing a radioactive line source. The homogeneity of the linear activity of 68 iridium‐192 wires has been measured. About 80% of the measured linear activities showed deviations of no more than 5% from the mean; about 20% of the measured linear activities showed deviations of 5%–10% from the mean. Individual wires may contain deviations of 5% or more extended over several centimeters, which causes the dose in the prescription point to deviate 5% from the desired value. Four wires (8%) were rejected for this reason.
Portal dosimetry using a liquid ion chamber matrix: Dose response studies
Authors: Zhu Yunping , Jiang Xun‐Qing , Van Dyk Jake
Published: March 2003

Current intensive investigations of electronic portal imaging devices (EPIDs) have prompted their potential application to portal dosimetry. In this paper, the progress made in using a commercial liquid ion chamber matrix EPID for portal dosimetry is discussed. The pixel value of the liquid ion chamber element was calibrated against dose by exposing the imager to 6‐MV x‐ray beams of various intensities obtained with various thicknesses of lead attenuators and a range of source to detector distances. Absolute dose values were determined using an ion chamber on the central axis at the depth of maximum dose in a solid water phantom. The pixel values of the matrix were determined for various field sizes in order to evaluate the dependence of pixel value on dose at those field sizes. It was confirmed that the pixel value was proportional to the square root of the dose rate and was nearly independent of the field size. The 2D pixel values were converted to 2D dose maps in the water phantom after applying a correction for the effect of horns in the flood calibration field. The flood calibration field was used to obtain the relative sensitivity of each pixel. Good agreement was observed (normally better than 1% in relative standard deviation) between the converted dose distribution obtained from the pixel matrix and the direct dose measurement using an ion chamber scanned in a water phantom in regions of shallow dose gradient. For application to on‐line portal dosimetry, both the short‐ and long‐term stability of this EPID system were found to be within 1% relative standard deviation. This system, together with an accurate portal dose calculation procedure, can be used for on‐line radiotherapy dose verification.
A mathematical model of radiation field edge localization
Authors: Wang Hui , Fallone B. Gino
Published: March 2003

A mathematical model of the average edge slope across the radiation field border in a portal image was investigated in order to improve the precision of edge localization in a previously reported algorithm for automatic extraction of the radiation field from double‐ and single‐exposure portals. The model involves a global rather than a local approach to edge localization, and employs a hyperbolic function with four parameters to characterize the behavior of the radiation field penumbra. The location of the radiation field edge is determined from one of these parameters. This model was tested on a group of portal images acquired with different conditions in our clinic. Evaluation results of this model and improvements in the performance of our portal image segmentation algorithm will be presented.
An MR compatible flow simulator for intravascular pressure simulation
Authors: Plewes D. B. , Urchuk S. N. , Kim S. , Soutar I.
Published: March 2003

An MR compatible flow simulator is described which generates physiologically realistic pressure and flow waveforms. The simulator is based on a servomotor‐driven gear pump which produces pulsatile flow by modulation of the servomotor rotation rate. Operation of the simulator is under the control of a personal computer, which executes an iterative feedback loop to minimize errors between measured and desired pressure waveforms. The simulator is totally automatic, requiring only a few minutes of iteration to generate the desired pressure waveform. Accurate sinusoidal waveforms with frequencies up to 10 Hz have been generated using the simulator, with high‐frequency contamination of the measured waveform at least 80 dB below the fundamental frequency. Aortic waveforms have been produced with realistic flow rates and pressure variations. The pump assembly is mechanically straightforward and can operate at an 8‐m distance from the flow phantom to allow the device to be isolated from the MR magnet room.
Lateral electron equilibrium and electron contamination in measurements of head‐scatter factors using miniphantoms and brass caps
Authors: Li X. Allen , Soubra M. , Szanto J. , Gerig L. H.
Published: March 2003

The head‐scatter factor (Sh) can be measured with a narrow miniphantom or a metal cap provided it is completely covered by the photon beam and its lateral size is thick enough to prevent electron contamination contributions. The effects of lateral electron equilibrium (LEE) and electron contamination on the Sh values were studied. The EGS4 Monte Carlo technique was used to calculate the minimum beam radii (rLEE) required to achieve complete LEE for photon beams ranging from 60Co to 24 MV. The measurement shows that the error introduced to the Sh value due to lateral electron disequilibrium is negligible. The radii of the miniphantoms or the sidewall thicknesses of the caps can be reduced below rLEE provided they are thick enough to prevent the effect of electron contamination.
Analysis of central‐axis doses for high‐energy x rays
Authors: Bjärngard Bengt E. , Vadash Paul
Published: March 2003

The purpose of this study was to improve on the analytical expressions used to describe central‐axis doses for high‐energy x‐ray beams, in particular, the component due to phantom‐scattered photons. The beams were characterized by quantities related to the physical processes, namely, transmission, head‐scatter, and phantom‐scatter factors, which were described separately with mathematical functions. Transmission in water was measured in a narrow beam and head scatter with a small phantom in air. The phantom‐scatter factors, i.e., the ratios between total and primary dose, were deduced from measured central‐axis doses per monitor unit. Based on previous work, it was assumed that this scatter factor is proportional to the depth d if the ratio between the depth and the field size s is constant. The proportionality constant was examined as a function of this ratio d/s and the effective linear attenuation coefficient μ. Two quality‐dependent parameters were extracted. One expresses the probability of scatter and was numerically close to μ. The other, which has not previously been studied, reflects the directional distribution of the scattered photons and was also found to be a linear function of μ. Thus the scatter factors can be estimated if μ is known. Central‐axis doses were described by these formulas with 2.5% maximum error at 6 MV, 0.8% at 25 MV. To achieve this result, only a few measurements were needed for selected d and s, which indicates that the model used for the scatter factor is realistic. When the method was applied to 10‐MV and 15‐MV x‐ray beam data measured by another institution, about ±2% accuracy resulted.
Filter wheel equalization for chest radiography: A computer simulation
Authors: Boone John M. , Duryea J. , Steiner Robert M.
Published: March 2003

A chest radiographic equalization system using lung‐shaped templates mounted on filter wheels is under development. Using this technique, 25 lung templates for each lung are available on two computer controlled wheels which are located in close proximity to the x‐ray tube. The large magnification factor (≳10×) of the templates assures low‐frequency equalization due to the blurring of the focal spot. A low‐dose image is acquired without templates using a (generic) digital receptor, the image is analyzed, and the left and right lung fields are automatically identified using software developed for this purpose. The most appropriate left and right lung templates are independently selected and are positioned into the field of view at the proper location under computer control. Once the templates are positioned, acquisition of the equalized radiographic image onto film commences at clinical exposure levels. The templates reduce the exposure to the lung fields by attenuating a fraction of the incident x‐ray fluence so that the exposure to the mediastinum and diaphragm areas can be increased without overexposing the lungs. A data base of 824 digitized chest radiographs was used to determine the shape of the specific lung templates, for both left and right lung fields. A second independent data base of 208 images was used to test the performance of the templates using computer simulations. The template shape characteristics derived from the clinical image data base are demonstrated. The detected exposure in the lung fields on conventional chest radiographs was found to be, on average, three times the detected exposure behind the diaphragm and mediastinum. The simulated filter wheel equalization technique yielded detected exposure levels that were approximately equal in both the lung fields and the rest of the image. In addition to illustrating that the filter‐wheel equalization technique may be feasible for chest radiography, the simulations also provided important information for the mechanical construction of a filter wheel equalization system.
Reconstruction of 12 MV bremsstrahlung spectra from measured transmission data by direct resolution of the numeric system AF=T
Authors: Catala A. , Francois P. , Bonnet J. , Scouarnec Ch.
Published: March 2003

An investigation of x‐ray spectral reconstruction from transmission data by direct resolution of the matrix system A*F=T (using spectral algebra formalism) has been previously presented. The resolution has been done with simulated spectrum. In this paper, the method on a real case of a 12 MV photon beam was tested. A special study of the setup has been made to estimate and reduce the experimental errors that could alter the results. In order to convert F(E) (a fraction of the signal due to a photon of energy E) into photon fluence Φ(E), the chamber energy response R(E) has been studied and an approximated analytical function for its representation was proposed. Spectra reconstructed from different transmission data using different attenuators, buildup caps, and ionization chambers have been compared to verify the uniqueness of the reconstructed spectra. To test the validity of the results, dosimetric values, such as Depth Dose Data have been calculated, from our spectrum, using a specific code developed by Kosunen et al. The results show a good agreement between the measured and calculated data.
Calculation of monitor units for a linear accelerator with asymmetric jaws
Authors: Rosenberg I. , Chu J. C. H. , Saxena V.
Published: March 2003

A simple approach was developed that calculates the output factors and tissue maximum ratio of an asymmetric field at any point within the open field, and specifically both at the central axis (when the jaws do not shadow it) and at the effective center of the open field, using the existing tables for symmetric fields and the multidepth profile information for the largest available field size (either open or with a wedge present). Day's method was adapted to calculate the effective values of the usual field‐size‐dependent parameters. This approach makes these parameters also dependent on the location of the calculation point relative to the field edges in an asymmetric field. This algorithm was tested by comparing its predictions with measurements of asymmetric and half blocked fields, with and without wedges, in a water phantom at different depths and off‐axis distances. The agreement between calculated and measured dose rate is within 1%–3% even in highly asymmetric fields for both 6‐ and 18‐MV photons.
The development of PdNi thermoseeds for interstitial hyperthermia
Authors: Meijer J. G. , van Wieringen N. , Koedooder C. , Nieuwenhuys G. J. , van Dijk J. D. P.
Published: March 2003

Magnetic induction heating of thermoseed implants can be used to produce highly localized hyperthermia in deep‐seated tumors. Automatic temperature control throughout the tumor can be achieved by the self‐regulating character of ferromagnetic seeds, which corrects for local variations in heat loss due to blood perfusion. An increased sharpness of the ferromagnetic transition at the Curie temperature, Tc, improves the performance of self‐regulating control. This was realized for palladium–nickel alloys by a “cold working” procedure preceded and followed by annealing. Palladium–nickel seeds with a predetermined Tc were produced, showing a sharp decrease at Tc of the magnetic susceptibility and the heat production.
A verification of the Monte Carlo code MCNP for thick target bremsstrahlung calculations
Authors: DeMarco J. J. , Solberg T. D. , Wallace R. E. , Smathers J. B.
Published: March 2003

The bremsstrahlung spectra from thick targets of Be, Al, and Pb are calculated using the Monte Carlo code MCNP (Monte Carlo N‐particle). The current version of MCNP (v.4A) incorporates a coupled electron‐photon transport scheme that allows the user to estimate the photon fluence produced from primary electron interactions. The simulation parameters are based on bremsstrahlung measurements of 15 MeV electrons incident on thick targets of Be, Al, and Pb at various angles between 0° and 90°. The integrated yield and mean energy of each bremsstrahlung spectrum is calculated for the three targets at these angles. For angles less than 60° the integrated yield calculated by MCNP4A is within 6% of measured values for the three targets. Furthermore, predicted mean energy is within 7% of the values derived from measurement for all angles tested. Also compared are the performances of two MCNP4A fluence tallies; a next‐event estimator (detector tally) and a track length estimator (cell tally). Timing studies indicate the detector tally will perform the integrated yield calculations to a precision of 1% approximately 10 to 50 times faster than the conventional cell tally for an emission angle of 0°.
General ion recombination for ionization chambers used under irradiation conditions relevant for diagnostic radiology
Authors: Geleijns Jacob , Broerse Johan J. , Zweers Dirk , Zoetelief Johannes
Published: March 2003

General ion recombination has been studied under irradiation conditions relevant for diagnostic radiology and with four different ionization chambers. When the exposure time is appreciably shorter than the ion transit time, the exposure can be designated as pulsed irradiation. On the contrary, for relatively long irradiation times, the term continuous irradiation can be applied. Recombination was estimated by measuring the collected charge at various collecting potentials of the ionization chamber. This is a well‐known method in radiotherapy, but unfortunately it cannot be used in diagnostic radiology with typical exposure meters, since they do not offer the option of varying the collecting potential. For exposures with diagnostic x‐ray units, an alternative approach is to vary the exposure or exposure rate over a wide range at a constant collecting potential. Experimental and theoretical estimates of ion recombination did not yield similar values. This might be due to several causes, such as differences between the actual and the nominal dimensions and volumes of the ionization chambers, due to errors and uncertainties in the physical parameters used in the theoretical models or due to deviations of the shape of the ionization chambers from the perfect cylindrical or parallel plate geometry. For better accuracy, corrections for recombination losses should therefore be based on experimental verification rather than on theoretical models.
Radiological properties of a prototype multi‐rod collimator for producing irregular fields in photon radiation therapy
Authors: Maughan Richard L. , Powers William E. , Blosser Gabe F. , Blosser Emanuel J. , Blosser Henry G.
Published: March 2003

A prototype multi‐rod collimator for producing irregular fields in photon radiation therapy has been designed and built. The mechanical details of the design and operation of the multi‐rod collimator are discussed. Beam profiles for an approximately 10×10 cm2 field have been measured at various depths in phantom, and compared with profiles obtained using the secondary collimator jaws alone and with cast metal blocks. The ability of the collimator to produce irregular fields is demonstrated with reference to some commonly encountered therapy fields and the ability to produce central blocks and island blocks is discussed. Isodose curves for selected irregular fields are presented.
Calculated Bragg curves for ionization chambers of different shapes
Authors: Bichsel Hans
Published: March 2003

Bragg curves for proton beams are different in shape if measured with different detectors, such as silicon diodes, extrapolation, and thimble chambers. Calculations with an analytic algorithm are presented which show that such differences are caused, at least in part, by the shapes of the detectors. Similar differences must be expected for all beams of heavy charged particles. Dose conversion factors representing the conversion of a measured dose into dose in water are discussed.
The influence of scatter on the design of optimized intensity modulations
Authors: Chen Zhe , Wang Xiaohong , Bortfeld T. , Mohan Radhe , Reinstein Lawrence
Published: March 2003

In contrast to the traditional treatment planning process where beam parameters are adjusted in order to produce a desired dose distribution, the inverse treatment planning technique starts from the desired dose distribution and employs mathematical inversion to derive a set of optimal intensity profiles for beams with predefined directions. Crucial to the success of the inverse technique is the mathematical inversion process that yields the optimized intensity modulation. This paper systematically investigates the influence of phantom scatter in the process of designing the optimized intensity modulation and assesses its clinical consequences through the resultant treatment plans. Optimized intensity modulations and the resultant dose distributions were calculated for four prostate patients and for a simple cubic phantom under two conditions: one includes scatter and the other ignores scatter in designing the optimized intensity modulation. The effects of scatter were assessed by comparing the isodose distributions, the dose–volume histograms of the target and of the critical organs, and the tumor control probabilities and the normal tissue complication probabilities of two set prostate treatment plans. The comparison reveals that the dose homogeneity to the target is significantly improved when scatter is included in the optimization process. For prostate patients, the dose to the volume enclosed by the outer rectal contour is concomitantly reduced. Our results suggest that it is important to include the scatter in the iterative design of the optimized intensity modulations.
Dynamic universal wedge
Authors: Shackford Hobart , Bjärngard Bengt E. , Vadash Paul
Published: March 2003

A computer‐controlled equivalent of the universal wedge was designed by moving one collimator with step‐wise constant velocity to produce the same primary‐dose profile as a 20×20‐cm2 conventional 58‐deg wedge. It was used for smaller fields without changing the profile and combined with open beams to alter the wedge angle. Output factors in air and wedge factors in air and water were determined experimentally for the dynamic wedge and compared to predictions based on the assumption that the primary dose is proportional to the number of monitor units that the point of interest is in the open portion of the beam. This model was found to be accurate within about 2% and the deviations caused by head‐scatter changes and collimator transmission after the moving collimator has passed. Measurements of the wedge factor in water indicated that the scatter factors for large wedged beams slightly exceeded those for open beams.
Quantitative verification of 192Ir PDR and HDR source structure by pin‐hole autoradiography
Authors: Kirov A. S. , Meigooni A. S. , Zhu Y. , Valicenti R. K. , Williamson J. F.
Published: March 2003

For precise localization of the center and determination of the dimensions of the radioactive material within the capsule of brachytherapy sources, we have developed a method based on simultaneous pin‐hole autoradiography of two sources. We constructed a variable magnification pin‐hole camera consisting of two telescopically fitted Plexiglas cylinders which can accommodate two radioactive sources on the plate covering the top cylinder. The 192Ir pulsed and high dose‐rate sources were studied and an 192Ir seed was used as a reference source. The magnification factor was determined from the dimensions of the 192Ir seed image, which was geometrically well defined by a separate transmission radiography experiment. The observed position for the center of radioactivity in the PDR and the HDR source capsules are in agreement with the vendor specifications. The distance from the tip of the PDR capsule to its center of radioactivity was found in this way to be 0.79±0.21 mm, which agrees with the position (0.85 mm) of the pellet situated closer to the tip as specified by the vendor. Quantitative verification of the internal source structure using this method enhances the accuracy with which the dose distribution near brachytherapy sources can be predicted by three‐dimensional Monte Carlo calculations.
Head phantoms for neutron capture therapy
Authors: Harling O. K. , Roberts K. A. , Moulin D. J. , Rogus R. D.
Published: March 2003

A water‐filled head phantom that is designed for use in boron neutron capture therapy is described. The shape of this ellipsoidal phantom, based on the Snyder head model, and its composition are designed to simulate the neutron slowing down properties of the human skull and brain. Small ion chambers or activation foils can be placed in many locations within the phantom volume. This permits accurate three‐dimensional mapping of all relevant dose components and use of these dose contours for beam development as well as for benchmarking of computer‐based patient treatment codes.
Characterization of vertebral strength using digital radiographic analysis of bone structure
Authors: Caldwell C. B. , Willett K. , Cuncins A. V. , Hearn T. C.
Published: March 2003

Bone mineral densitometry (BMD) is useful in predicting fracture risk, but, unfortunately, there is a significant degree of overlap in BMD measurements of patients who have a high risk of fracture and patients with a low risk of fracture. In this study, a method of characterizing trabecular bone structure in digitized radiographs of vertebrae is proposed and assessed. A significant correlation between bone “structure” and the compressive strength of vertebral bodies was found. The utility of the parameter for distinguishing between “weak” and “strong” bone samples was assessed using receiver operating characteristic (ROC) analysis. Using this analysis, the structural parameter produced an area under the ROC of 0.88±0.05, while a bone density measure produced an area of 0.79±0.07. The results suggest that the addition of a measure of bone structure to the conventional measures of bone density may prove useful in predicting the quality of bone when considering surgical or medical intervention for osteoporotic conditions.
BEAM: A Monte Carlo code to simulate radiotherapy treatment units
Authors: Rogers D. W. O. , Faddegon B. A. , Ding G. X. , Ma C.‐M. , We J. , Mackie T. R.
Published: March 2003

This paper describes BEAM, a general purpose Monte Carlo code to simulate the radiation beams from radiotherapy units including high‐energy electron and photon beams, 60Co beams and ortho‐voltage units. The code handles a variety of elementary geometric entities which the user puts together as needed (jaws, applicators, stacked cones, mirrors, etc.), thus allowing simulation of a wide variety of accelerators. The code is not restricted to cylindrical symmetry. It incorporates a variety of powerful variance reduction techniques such as range rejection, bremsstrahlung splitting and forcing photon interactions. The code allows direct calculation of charge in the monitor ion chamber. It has the capability of keeping track of each particle's history and using this information to score separate dose components (e.g., to determine the dose from electrons scattering off the applicator). The paper presents a variety of calculated results to demonstrate the code's capabilities. The calculated dose distributions in a water phantom irradiated by electron beams from the NRC 35 MeV research accelerator, a Varian Clinac 2100C, a Philips SL75‐20, an AECL Therac 20 and a Scanditronix MM50 are all shown to be in good agreement with measurements at the 2 to 3% level. Eighteen electron spectra from four different commercial accelerators are presented and various aspects of the electron beams from a Clinac 2100C are discussed. Timing requirements and selection of parameters for the Monte Carlo calculations are discussed. Dedication: This paper is dedicated to the memory of our friend and colleague, Jiansu Wei, who made a significant contribution to this project before he passed away on March 15, 1993.
Instrumentation and dosimeter‐size artifacts in quantitative thermoluminescence dosimetry of low‐dose fields
Authors: Meigooni Ali S. , Mishra Vivek , Panth Harish , Williamson Jeffrey
Published: March 2003

Thermoluminescence dosimetry is extensively used for quantitative dose measurements in various irradiation fields such as dosimetry of brachytherapy sources. In this application, small doses on the order of 0.5 cGy must be accurately measured, which requires careful control of instrumentation, energy‐dependence, and nonlinearity of detector response. Several investigators have observed the presence of some undesirable signals when the thermoluminescent dosimeters (TLDs) were read without any nitrogen gas flow in the TLD reader. Others have indicated that the “prereadout” annealing technique is the same as the “preirradiation” technique for doses above 10 cGy, but they have not extended their study to lower doses. The goal of this study is to investigate dependence of sensitivity and linearity of the TLD response to the flow of nitrogen gas in the TLD reader at low dose level, annealing technique, and TLD size. The effect of nitrogen flow sensitivity and linearity of two different sizes of lithium fluoride TLD‐100 chips has been studied. Our data indicate a large standard deviation of TLD sensitivity, up to a factor of 2, when TLDs were read without nitrogen gas flow in the TLD reader. In addition, a large deviation from linearity was observed for doses below 5 cGy. When the reading‐chamber was purged with nitrogen gas, dispersion of the responses of the TLDs that were exposed to the same dose fell to within 5%. At precision levels of 2% and 5%, the low dose limits are 1 cGy and 0.5 cGy, respectively, for large chips and 15 cGy and 1 cGy for small chips, if TLDs are read with nitrogen gas flow in the TLD reader. The full results of our investigation are presented.
Calculation of dose in asymmetric photon fields
Authors: Gibbons John P. , Khan Faiz M.
Published: March 2003

A method is introduced to calculate monitor units to points off axis. Extensive data are presented comparing this method with measured values of dose per monitor unit on the central ray of asymmetric fields produced by a variety of linear accelerators. The technique demonstrates improvement over existing methods that use large‐field profile data. The method is found to be both simple and accurate: Agreement within ±2% is obtained using parameters readily available within the clinic.
Physical characteristics of a clinical d(48.5)+Be neutron therapy beam produced by a superconducting cyclotron
Authors: Maughan Richard L. , Yudelev Mark
Published: March 2003

The Harper Hospital and Wayne State University fast neutron therapy facility is the only one in the world to use a compact superconducting cyclotron and multirod collimator. Neutrons are produced by the interaction of the 48.5‐MeV deuteron beam with a thick internal beryllium target and the compact accelerator is gantry mounted to allow full 360° rotation of the neutron beam about the therapy couch. The deuteron beam strikes the beryllium target at a glancing angle. A flattening filter is used to flatten the asymmetric neutron beam which results from this geometry. Details of the flattening filter design and construction are discussed. The physical characteristics of the resulting neutron therapy beam were measured. The central axis depth‐dose values are approximately equivalent to those of a 4‐MV photon beam. The dose buildup curve reaches its maximum value at a depth of 9 mm in a water phantom and the surface dose is approximately 42%. The beam penumbra produced by the multirod collimator has been measured in terms of the distance between the 20% and 80% isodose lines. The penumbra width for a 10×10‐cm2 field at a depth of 10 cm in a water phantom is 1.65±0.1 cm, and is comparable to that achieved with other high‐energy neutron beams. The long‐term stability of the dose monitoring system has been measured and found to be satisfactory. The physical characteristics of the neutron beam are comparable with those of other modern fast neutron therapy facilities.
Electron mass scattering powers: Monte Carlo and analytical calculations
Authors: Li X. Allen , Rogers D. W. O.
Published: March 2003

Values of electron mass scattering power, T/ρ, for various materials have been calculated by using the EGS4 Monte Carlo system and by integration of the Molière multiple‐scattering distribution. The energy range covered is 0.5–100 MeV. Monte Carlo calculations test the concept of T/ρ “experimentally” and assess the contribution to electron mass scattering power from effects such as Moller scatter and energy‐loss straggling. The Monte Carlo results agree within 2% with the analytical results calculated from Molière multiple‐scattering theory at energies less than 20 MeV for high‐Z materials and for energies less than 50 MeV for low‐Z materials. At higher energies the Monte Carlo calculations include the effects of bremsstrahlung production which can significantly increase values of T/ρ. For low‐Z materials and electron energies less than 60 MeV, the Monte Carlo calculated T/ρ values are generally 22% higher than those given by ICRU Report 35, while those for high‐Z materials and energies less than 25 MeV are found to be consistent (within 1%) with ICRU Report 35. The effects of Moller scatter, which significantly affect T/ρ for low‐Z materials, as well as bremsstrahlung effects, are included in the present Monte Carlo calculations. If the tabulated T/ρ data of ICRU Report 35 are modified to include the Moller scatter effect, then for energies less than 60 MeV they are generally 6% less than the present Monte Carlo data for low‐Z materials as well as for copper. It is shown that T/ρ is a well‐defined constant over an appropriate range of slab thickness except when bremsstrahlung effects are significant. It is found that T/ρ is proportional to E−n, where n is in the range of 1.5–2.0 for the energies considered here. The Monte Carlo calculations are shown to agree well with various relevant experimental measurements. Accurate T/ρ data, which should include the effect of Moller scatter, are necessary in electron‐beam treatment planning, especially for a small field size. The choice of the depth step in the implementation of pencil‐beam codes should not violate the slab‐thickness limits for T/ρ data.