bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of pseudorandom numbers
Authors: Ehrhardt James C.
Published: March 2003

Computational studies requiring the generation of pseudorandom numbers are becoming increasingly common. These include Monte Carlo methodologies and studies which require the addition of “random” noise to more structured data. Although well‐established random number generators exist, many of these are not suitable for implementation on micro‐ or minicomputer systems. An algorithm for an additive generator that can be used on any computer system is described. The performance of this algorithm as implemented on a microcomputer system is discussed. Extensive testing of the statistical behavior of the resulting number sequence was performed. The algorithm appears to be an appropriate one for a variety of applications.
Absorbed dose calorimetry: Bridge nulling and voltage analysis techniques
Authors: Kubo Hideo , Brown Dennis E.
Published: March 2003

A theoretical analysis is made of the conventional calorimeter approach utilizing a two‐thermistor Wheatstone bridge—an approach which requires balancing the bridge by adjusting variable resistances. However, with the current availability of computers and digital precision measuring instruments, the conventional calorimeter approach can now be readily replaced by an automated, nonmechanical, electrical systems approach called differential calorimetry. The analytical theory outlining such an approach is also presented.
Using ion chambers with wedge‐shaped absorbers for electron energy measurements
Authors: Johnsen Stanley W.
Published: March 2003

A parallel‐plate ionization chamber placed behind a wedge‐shaped absorber and exposed to a radiotherapeutic electron beam exhibits an energy‐dependent response. Such a device designed to produce a linear energy dependence has been built and tested. The device yields results during exposure, and the difficulties of film handling, associated with similar procedures, have been eliminated. The precision of the device makes it suitable for use as a routine means of monitoring energy for quality assurance purposes.
Relaxation rates of paramagnetic solutions: Evaluation by nuclear magnetic resonance imaging
Authors: Bucciolini M. , Ciraolo L. , Renzi R.
Published: March 2003

Methods for accurate calculation of nuclear magnetic resonance relaxation times are presented. Data from inversion recovery sequences with different delay times were fitted to derive T1 information on water solutions of paramagnetic ions. T2 values of the same samples were obtained by fitting signals from a multiecho sequence. According to the theory, for the field strength used (0.5 T), T1's and T2's are nearly equal and the enhancement of relaxation rates is found to be directly proportional to molar concentration. Tests performed suggest the opportunity of using the same approach in studies of tissues.
Investigation of basic imaging properties in digital radiography. 9. Effect of displayed grey levels on signal detection
Authors: Giger Maryellen Lissak , Ohara Ken , Doi Kunio
Published: March 2003

Results of an 18‐alternative forced‐choice experiment have shown that observers were capable of detecting a signal with a contrast of 1 in terms of 10‐bit data which were displayed on a CRT monitor with an 8‐bit video generator and a window width setting of 1024. We investigated the conditions under which 10‐bit signal data can be detected when displayed using an 8‐bit video generator. Results show that the 10‐bit digital quantum noise, which was approximately Gaussian distributed, can act as a carrier of the signal data, thus allowing a signal having a contrast of a fraction of a displayed grey level to be detected. We demonstrate the relationship between the rms value of the digital noise (obtainable with a clinical digital subtraction angiography system under various exposure levels), the number of bits available in the display video generator, and the “transmitted” signal contrast displayed on the CRT monitor.
In vivo measurement of lithium in the body by a neutron activation analysis technique
Authors: Glaros D. , LoMonte A. F. , Ellis K. J. , Yasumura S. , Stoenner R. W. , Cohn S. H.
Published: March 2003

A technique has been developed for in vivo determination of lithium content in the head (and potentially in the whole body and in selected organs) of patients undergoing lithium therapy. It is based on the measurement of tritium induced by the 6Li(n,α)T nuclear reaction after neutron irradiation of the body. The fraction of tritium exhaled in the expired air in the form of HT is collected, separated from the other gases, and counted in a high‐sensitivity beta counter. The feasibility of the technique was demonstrated by measurements of lithium in the head of a sheep and in the whole body of rats, following the administration of  6LiCl (enriched 6Li isotope, 95.46% abundancy). The precision of the technique is acceptable for clinical applications based on a maximum propagated error of 8.4%. The sensitivity is 1 count/d (from T activity) per 10 mSv (total dose) and 1 μg of 6Li. This indicates that studies on patients under 6Li treatment can be performed successfully with a radiation dose to the head of 1–2 mSv.
The dosimetry of 125I seed eye plaques
Authors: Weaver Keith A.
Published: March 2003

The ability of a brachytherapy treatment‐planning computer program to calculate accurately the dose from 125I seeds at distances relevant to eye plaque therapy was investigated. Thermoluminescent dosimetry measurements were made in a plastic phantom at depths of 0.5, 0.97, and 1.5 cm, and results were corrected for finite dosimeter size and phantom effects. Doses were calculated at the same depths with an 125I seed linear source model that accounted for dose anisotropy. Measurements and calculations were found to agree within their mutual uncertainties. The presence of a gold plaque was found to reduce the dose at all measured depths by 8%.
A laser‐based multiformat camera for medical imaging
Authors: Sanders John N. , Cattell Charles L. , Bender Norman E. , Tesic Mike M. , Sones Richard A.
Published: March 2003

The spatial resolution and contrast resolution required of a multiformat camera (MFC) for medical imaging are discussed. A typical cathode‐ray tube (CRT) MFC and a prototype laser MFC are compared based on the following measured quantities: line spread function and associated contrast transfer function, noise characteristics, intensity transfer function (dynamic range), large‐area contrast, and film irradiance. The laser MFC is found to provide significantly better performance than the CRT MFC in all of these areas.
Accuracy of blood transit time estimates using temporal filtering in digital radiology
Authors: Wyman D. R. , Wilson B. C.
Published: March 2003

The application of recursive filtering in digital radiology can yield incorrect blood flow estimates if blood flow is calculated using a transit time, defined as the difference in times to peak contrast over the length of a vessel segment. The transit time error is calculated here, from computer simulations, as a function of filter design, image noise and contrast medium dispersion over the vessel segment. An error reduction procedure based on prefiltering is suggested and tested by simulation. Results indicate that the transit time error can be substantial and the reduction procedure effective, depending on the filter design and system noise level.
Broad beam attenuation of cobalt‐60 gamma rays and 6‐, 18‐, and 25‐MV x rays by lead
Authors: Van Dyk J.
Published: March 2003

The measurement of photon attenuation by absorbing materials in radiation beams is usually performed using narrow beam geometry. However, radiotherapy treatments rarely simulate narrow beam conditions. The effect of broad beams on the attenuation of cobalt‐60 gamma rays and 6‐, 18‐, and 25‐MV x rays by lead is considered both theoretically and experimentally. The theoretical calculations are based on first‐scatter considerations and indicate a dependence on treatment geometry (i.e., maximum scattering angle). Measured attenuation coefficients vary by as much as 16%, comparing narrow and broad beam data for typical treatment conditions. Broad beam attenuation coefficients, calculated from the measured zero‐area values agree with the measured results to within 2% on average with a maximum deviation of 5%. A very simple empirical procedure is proposed to determine attenuation coefficients for a wide range of field sizes. This agrees with the measured data to within 0.5% on average and 4% at worst. With irregularly shaped absorbers, a sector integration technique is considered in combination with a first‐scatter calculation. For a long rectangular field, the agreement between measured and calculated attenuation coefficients is better than 1.5% for all energies. The question of accuracy in attenuation coefficients needed to give an accuracy of 3% in relative transmission is addressed and shows a dependence on absorber thickness. For 75% transmission, the required accuracy in attenuation coefficient is 10%, whereas for 25% transmission, it should be improved to 2%.
The effect of scattered radiation in 60Co beams on wall correction factors for ionization chambers
Authors: McEwan A. C. , Smyth V. G.
Published: March 2003

The Awall correction factors used in dosimetry protocols are obtained by Monte Carlo methods that assume 60Co beams equivalent to 1.25‐MeV radiation. Real 60Co beams contain lower energy components; in addition, Awall is defined differently by different authors. In the present work, Awall as defined by Rogers, Bielajew, and Nahum (1985) has been calculated by Monte Carlo methods at several energies, for a graphite cavity chamber, using the revised stopping‐power data of Seltzer and Berger (1982). It is concluded that the true value of Awall for chambers designed for use in 60Co beams will not move more than 0.1% below the value calculated for 1.25‐MeV photons.
Investigation of basic imaging properties in digital radiography. 5. Characteristic curves of II–TV digital systems
Authors: Fujita Hiroshi , Doi Kunio , Giger Maryellen Lissak , Chan Heang‐Ping
Published: March 2003

A simple method was devised to determine the characteristic curve of image intensifier (II)–TV digital imaging systems, which relates the output pixel value to the input relative x‐ray intensity. To provide a wide range of x‐ray intensities incident on the II, we used an aluminum stepwedge consisting of nine steps with thickness increments of 6.3 mm, together with an 0.81‐mm‐thick copper plate. The x‐ray field was narrowly collimated to the area occupied by the stepwedge in order to reduce the effect of veiling glare. The relative x‐ray intensities transmitted through each step of the stepwedge were determined by using screen–film systems. The gradient curve of the system was derived from the slope of the characteristic curve. Results obtained with a Siemens Digitron 2 system showed that its characteristic and gradient curves depended upon the matrix size used, but did not change with the II field size. The validity of the characteristic curve was demonstrated by measurement of iodine attenuation curves obtained with the II–TV digital system at different exposure levels.
Calculation of the small‐angle distribution of scattered photons in diagnostic radiology using a Monte Carlo collision density estimator
Authors: Persliden Jan , Carlsson Gudrun Alm
Published: March 2003

Calculations of various physical quantities pertaining to scattered photons in diagnostic radiology are conveniently carried out using the Monte Carlo technique. Some quantities, e.g., the small‐angle distribution of scattered photons transmitted through the patient, are difficult to obtain with sufficient precision using straightforward simulation of physical experiments. By mixing the simulation of random trajectories with analytical calculations, the efficiency of deriving values for a particular field quantity may be drastically improved. This work describes a Monte Carlo collision density estimator that increases the efficiency of calculating the small‐angle distribution of transmitted scattered photons by a factor of more than 50. Examples of such distributions outside laterally infinite water slabs are given for x rays generated at 40–70 kV and for various slab thicknesses (10–200 mm). Comparison with experimental results from the literature shows that cross sections for coherent scattering which take diffraction phenomena in liquid water into account must be used to get accurate results. A discrepancy between the experimental and calculated distributions of photons transmitted at very small (<3°) angles to the normal to the slab may be interpreted in terms of experimental difficulties or insufficient accuracy in the differential scattering cross sections used in the calculations.
Planar‐pair local coils for high‐resolution magnetic resonance imaging, particularly of the temporomandibular joint
Authors: Hyde James S. , Froncisz Wojciech , Jesmanowicz Andrzej , Kneeland J. Bruce
Published: March 2003

The two‐loop–one‐gap loop–gap resonator (planar pair) has been investigated as a local receiving coil for high‐resolution proton imaging at 1.5 T (63.8 MHz). Sensitivity contours were determined on the bench by measuring the voltage induced in the coil when it is placed on the surface of a tank containing 0.1‐M NaCl, as a function of the coordinates and orientation of a radiating dipole inside the tank. Contours were obtained as a function of the diameter D of the two loops, and their separation d. For comparison, considerable data also were obtained for single‐turn circular loops as a function of loop diameter. Coil quality factors, Q's, are reported in free space and the saline tank as a function of geometric parameters. Contours also were obtained using phantoms in a whole‐body imager. Planar‐pair coils couple to slabs of approximately (2D+d)×D cross section, and are superior to circular coils for imaging anatomic structures that are somewhat elongated. A particularly important feature of planar‐pair local coils is that they are intrinsically isolated from the whole‐body coil of the imager, and no special passive or active circuits are required when the whole‐body coil is used for excitation. Planar‐pair local coils have been optimized for imaging of the temporomandibular joint, and an image is shown.
Charge storage in electron‐irradiated phantom materials
Authors: Ho Anthony K. , Paliwal Bhudatt R. , Attix F. H.
Published: March 2003

A recent article by Galbraith et al. [Med. Phys. 11, 197 (1984)] revealed the existence of dose errors due to charge storage in electron‐irradiated plastic phantoms. We have subsequently studied the same effect using similar materials, plus some others including “solid water,” which is an epoxy‐based phantom material manufactured by Radiation Measurement, Inc. Our work shows that there is minimal charge storage in solid water, as compared to polymethylmethacrylate (PMMA) and polystyrene. Since existing dosimetry protocols allow PMMA and polystyrene to be used for calibration phantoms, users should beware of the possible dosimetry errors resulting from charge storage in those plastics, and consider choosing other water‐substitute media, such as solid water, that do not display this effect.
Scatter rejection by electronic collimation
Authors: Plenkovich Dinko , Sorenson James A. , Kruger Robert A.
Published: March 2003

An electronic scanning‐slit technology is under development which involves replacing paired fore and aft slits for scatter rejection with only one fore slit. As the slit scans across the portion of the patient to be imaged, pulsed x‐ray exposures produce images of the slit in successive positions, which are digitized and stored in computer memory. Software techniques are used for tracking the slit image, and discriminating against scatter and veiling glare lying outside the slit image. Such “electronic collimation” does not require synchronization between the slit‐scanning and detector readout, which makes it much simpler than alternative methods and potentially adaptable to any digital fluorography system. The performance characteristics of a prototype unit are described and images of a low‐contrast phantom are presented.
Applications of time‐varying gradients in existing magnetic resonance imaging systems
Authors: Shenberg Itzhak , Macovski Albert
Published: March 2003

This paper describes several applications of magnetic resonance imaging (MRI) with time‐varying gradients within the framework of existing imaging systems. An alternative form of slice selection is shown where a time‐varying gradient is used, during the reception interval, to isolate the slice of interest (the reconstruction of the slice itself is treated by the conventional techniques). This system allows for precise control of the slice, thinner slices (less than 1 mm) and the ability to select multiple, closely packed slices using a single imaging sequence. All this is done by postprocessing so the slice(s) of interest can be selected after the measurements have been completed. It is also shown how time‐varying gradients can be used to generate the signals required for conventional projection‐reconstruction and two‐dimensional Fourier transform techniques (yet achieving higher resolution and using a resonant gradient system). In addition to cross‐sectional imaging, this same approach provides a simple system for projection imaging of the entire volume. All of these techniques constitute a good starting point for the exploitation of time‐varying gradients for fast, high‐resolution MRI with existing imaging systems.
Flow‐enhanced magnetic resonance imaging
Authors: Chenevert Thomas L.
Published: March 2003

A new technique for detecting blood flow in magnetic resonance imaging is proposed. This technique is tailored to enhance areas containing flow while suppressing static and nonsignal areas with the objective of optimizing the contrast of vascularized tumors. Unlike flow phase imaging, in‐plane flow directionality (parallel versus antiparallel to applied flow gradient) is removed by the proposed method to reduce phase cancellation of flow signals. This signal loss is apt to occur in instances of complicated vascularity patterns consisting of many small vessels having multiple flow directions. The new flow‐enhanced imaging method is compared to flow phase imaging by computer simulation of simple objects containing many small vessels. The results indicate that flow‐enhanced imaging yields significantly greater detectability of regions of complicated small‐vessel patterns than phase imaging.
A dynamic quality control phantom for radionuclide cardiology
Authors: Sloboda R. S. , Hardie D. I. , Schmid M. , Morris P. B. , Hughes J. , Malik M. H.
Published: March 2003

A three‐dimensional dynamic cardiac phantom suitable for quality control of equilibrium gated nuclear cardiographic procedures is described. Its flexible microprocessor based design provides an absolute standard for ejection fraction determination, ventricular volume quantitation, and physiologically gated single photon emission computed tomography. The cost of the phantom is comparable to that of commercially available phantoms suitable for ejection fraction quality control alone.
Evaluation of a prototype dual‐energy computed tomographic apparatus. II. Determination of vertebral bone mineral content
Authors: Vetter J. R. , Perman W. H. , Kalender W. A. , Mazess R. B. , Holden J. E.
Published: March 2003

A prototype dual‐energy computed tomographic (CT) scanner (Siemens Somatom DR3) with rapid kVp switching and prereconstruction processing has been used to measure vertebral bone mineral density. With this approach misregistration and beam hardening inaccuracies can be reduced considerably. Basis material images of aluminum‐ and Lucite‐equivalent density enable measurements of bone mineral density that are nearly independent of the amount of marrow fat. To simulate variable marrow fat, alcohol–water mixtures were used as media in calibration standards. A section of dried trabecular bone was also scanned immersed in varying alcohol–water mixtures. In both simulations it was shown that the dual‐energy measurement is nearly independent of marrow composition whereas the single‐energy measurement would be strongly influenced by marrow fat. Dual‐energy CT was compared to dual‐photon absorptiometry (153Gd) for the measurement of bone mineral mass of ten excised human vertebrae. There was a high degree of correlation between the two measurements (r=0.97). Dual‐energy and single‐energy CT measurements on 17 patients with suspected metabolic bone disease strongly support the conclusion that the influence of fat can lead to significant errors in single‐energy determinations of the mineral density of trabecular bone.
Accuracy of lung dose calculations for large‐field irradiation with 6‐MV x rays
Authors: El‐Khatib E. , Battista J. J.
Published: March 2003

In large‐field irradiations of the upper half‐body, there is a potential for severe respiratory complications. The incidence of severe radiation damage to lung can be reduced by limiting the lung dose and the volume of lung irradiated. It is necessary to have an accurate method of calculating the dose actually delivered to lung, and this work deals with such calculations and measurements in large (half‐body) fields of 6‐MV x rays. The results of various lung dose calculations by the Batho, the modified Batho (Lulu and Bjärngard), the simple scaled tissue–maximum ratio and the detailed equivalent tissue–air ratio methods are compared with doses measured in phantoms representing the mediastinum and adjacent lungs.
The precision of TR extrapolation in magnetic resonance image synthesis
Authors: Lee James N. , Riederer Stephen J. , Bobman Stuart A. , Johnson Jeffrey P. , Farzaneh Farhad
Published: March 2003

We present a model of noise propagation from acquired magnetic resonance (MR) images to TR ‐extrapolated synthetic images. This model assumes that images acquired at two repetition times and are used to generate synthetic images at arbitrary repetition times . The predictions of the model are compared with experimentally acquired phantom data, and show excellent agreement. The model is utilized in an analysis of two applications of MR image synthesis: scan time reduction and multiple‐image synthesis. Scan time is reduced by acquiring data at two short repetition times, and synthesizing at a longer repetition time, with less than TR. For T1=800 ms, a reduction of 20% in scan time results in a 45% reduction in signal‐to‐noise ratio SNR, when compared to direct acquisition. Reducing scan time by much more than 20% produces large noise levels in the synthetic image, and is unlikely to be useful. In multiple‐image synthesis, images are synthesized at any repetition time in the range 0 to , for contrast optimization. If =800 ms, and =2000 ms, the optimum combination of results in synthetic images whose SNR is at worst 22% less than the SNR of directly acquired images. For many values of TR, the synthetic images have SNR superior to that obtainable by direct acquisition.
An analysis of noise propagation in computed T2, pseudodensity, and synthetic spin‐echo images
Authors: MacFall James R. , Riederer Stephen J. , Wang Henry Z.
Published: March 2003

Methods are reviewed for estimating the transverse relaxation time T2 and the pseudodensity (PD) from spin‐echo measurements acquired at an arbitrary set of echo times {TE i}. Least‐squares fitting is applied to the logarithmically processed signals for the case in which the weights are proportional to the inverse of the logarithmically transformed signal variances (the minimum variance case). General formulas are derived for the estimated noise levels in the PD and T2 estimates due to the propagation of uncertainties in the original measurements. It is shown that the T2 and PD estimates are anticorrelated. Additionally, an expression is derived for the variance in a synthetic spin‐echo signal subsequently formed from the PD and T2 estimates. It is shown that under many circumstances a signal synthesized at some echo time can have a signal‐to‐noise ratio superior to that in a signal directly acquired at that time. Experimental measurements made on phantoms match the theoretical predictions to a high degree.
Microanalytical techniques for boron analysis using the 10B(n,α)7Li reaction
Authors: Fairchild Ralph G. , Gabel Detlef , Laster Brenda H. , Greenberg Dennis , Kiszenick Walter , Micca Peggy L.
Published: March 2003

In order to predict the efficacy of boronated compounds for neutron capture therapy (NCT), it is mandatory that the boron concentration in tissues be known. Various techniques for measurement of trace amounts of boron (1–100 ppm) are available, including chemical and physical procedures. Experience has shown that, with the polyhedral boranes and carboranes in particular, the usual colorimetric and spark emission spectroscopic methods are not reliable. Although these compounds may be traced with additional radiolabels, direct physical detection of boron by nondestructive methods is clearly preferable. Boron analysis via detection of the prompt‐γ ray from the 10B(n,α)7Li reaction has been shown to be a reliable technique. Two prompt‐γ facilities developed at Brookhaven National Laboratory are described. One, at the 60‐MW high flux beam reactor, uses sophisticated beam extraction techniques to enhance thermal neutron intensity and reduce fast neutron and γ contamination. The other was constructed at Brookhaven's 5‐MW medical research reactor and uses conventional shielding and electronics to provide an “on‐line” boron analysis facility adjacent to beams designed for NCT, thus satisfying one of the requisites for clinical application of this procedure. Technical restrictions attendant upon the synthesis and testing of boronated biomolecules often require the measurement of trace amounts of boron in extremely small (mg) samples. A track‐etching technique capable of detecting ng amounts of boron in mg liquid or cell samples is described. Thus it is possible to measure the boron content in small amounts (mg samples) of antibodies, or boron uptake in cells grown in tissue culture.
Penumbral measurements in water for high‐energy x rays
Authors: Dawson D. J. , Schroeder N. J. , Hoya J. D.
Published: March 2003

Ionization chambers of varying inside diameter have been used to investigate the penumbral region of 60Co, 6‐MV, and 31‐MV x‐ray beams. Measurements were made in water at varying depths up to 25 cm for a square field of side length 10 cm. The dependence of the penumbral widths on both the inside diameter of the ionization chamber and the depth in water is established along with the asymmetry of the penumbral distributions about the 50% level. A standard correction is indicated to eliminate the dependence of the measured penumbral widths on the inside diameter of the ionization chamber.
Phase and sensitivity of receiver coils in magnetic resonance imaging
Authors: McVeigh E. R. , Bronskill M. J. , Henkelman R. M.
Published: March 2003

Receiver coil response is a major cause of nonuniformities in magnetic resonance images. The spatial dependence of the sensitivity and phase of single‐saddle receiver coils has been investigated quantitatively by calculating the H1 field and comparing the results with measurements of a uniform phantom. Agreement between the measurements and calculations is excellent. A method is developed which corrects for both the nonuniform sensitivity and the phase shifts introduced by receiver coils.
Proton beam penumbra: Effects of separation between patient and beam modifying devices
Authors: Urie M. M. , Sisterson J. M. , Koehler A. M. , Goitein M. , Zoesman J.
Published: March 2003

The sharp lateral penumbra of a proton beam is often used to spare sensitive normal structures in treating clinical sites in which the target volume abuts, or even wraps around, these structures. Using Monte Carlo calculations and measurements, the factors which influence the penumbra of the proton beam at the Harvard Cyclotron Laboratory were investigated, with particular emphasis on the effects of separation between the patient and any beam modifying devices. Penumbra broadening, characterized by the distance over which the dose rises from 20% to 80% of the central dose, increases with greater amounts of scatterer introduced into the beam line. The broadening due to separation of the beam modifying devices and the patient is essentially linear with increasing air gap; the rate of increase depends on the details of these devices and on the depth of interest in the patient. For a particular portal, most of the parameters which affect the penumbra width are fixed by the patient's anatomy and the target volume. Only the thickness of the compensating bolus around the aperture edge and any air gap between the patient and the beam modifying devices can vary. Families of curves relating combinations of bolus thickness and air gap that maintain a constant penumbra width have been developed for guidelines during patient setup.
Collimated electron beams and their associated penumbra widths
Authors: Sabbas Albert M. , Jette David , Rozenfeld Martin , Pagnamenta Antonio , Lanzl Lawrence H.
Published: March 2003

The Fermi–Eyges multiple‐scattering theory for electrons is applied to calculate profiles of collimated electron beams. The dose profile below the collimator is a convolution of the intensity distribution of the electrons at the level of the collimator and the distribution arising from the propagation of a Gaussian point source from the collimator to the level of the calculation. The electrons at the level of the collimator possess an angular distribution characteristic of the configuration of the electron beam at the vacuum window. Hence, the dose profile and its associated penumbra width can be expressed in terms of the angular moments of the distribution of the electrons at the collimator. The dependence of the penumbra width on the configuration‐dependent angular spread of the electrons at the collimator accounts for differences in the size of the penumbra between two broad‐beam configurations. These differences are also seen experimentally. We have also studied the dependence of the angular moments of the electrons upon scattering foils present above the collimator and the position of the beam‐broadening device in the accelerator head.
A microdosimetric characterization of a cyclotron‐produced therapeutic neutron beam
Authors: Stafford Patrick M. , Horton John L. , Almond Peter R.
Published: March 2003

The therapeutic neutron beam of the Cyclotron Corporation's CP‐42 negative‐ion cyclotron is generated by protons of 42 MeV bombarding a thin beryllium target. Microdosimetric measurements were made for this neutron beam in a full‐scatter water phantom at nine positions inside and outside the useful beam. The lineal energy distributions and the variations of dose‐mean, frequency‐mean, and saturated lineal energy are compared for these positions. The dose fraction due to gamma rays is also calculated at each of these positions, based upon previously published techniques. A theoretical relative biologic effectiveness, based upon the dual radiation action model of Kellerer and Rossi [Curr. Top. Radiat. Res. 8, 85 (1972)] is also shown for the positions of measurement.
Coordinate transformations and calculation of the angular and depth parameters for a stereotactic system
Authors: Saw Cheng B. , Ayyangar Komanduri , Suntharalingam Nagalingam
Published: March 2003

Stereotactic systems have been used to assist in the precise implantation of radioactive sources in selected brain tumors. Use of such systems requires an algorithm that transforms spatial points in computed tomography coordinates into stereotactic frame coordinates. A simple algorithm performing the coordinate transformations, intended for inclusion in treatment‐planning software packages for interstitial brain implants, has been developed. This algorithm was formulated using the geometrical configurations of the Brown–Roberts–Wells (BRW) stereotactic system. After the transformations, the BRW angular coordinates and depth specifying the probe direction, defined from the entry point to the target point, are determined from their respective cartesian coordinates. These angular coordinates and depth on the BRW stereotactic system allow accurate neurosurgical implantations of catheters into the brain, and thereafter the insertion of radioactive sources.
A technique for computing dose volume histograms for structure combinations
Authors: Mohan Radhe , Brewster Linda J. , Barest Glenn D.
Published: March 2003

Graphical displays of three‐dimensional dose distribution data are often too complex to be easily assimilated and interpreted for the evaluation of radiation treatment plans. Histograms showing dose versus volume are convenient and useful tools for summarizing dose distribution information throughout the entire volume of a given anatomic structure. They can quickly highlight characteristics such as dose uniformity and hot and cold spots, and can be used to produce statistics including tumor control and normal tissue complication probabilities. To obtain a dose volume histogram for a given structure, it may be necessary to examine its spatial relationships with neighboring structures. They may overlap, be completely disjoint, or one may be contained within another. To resolve potential ambiguities, a procedure has been developed that assigns hierarchies to anatomical structures for the purpose of histogram calculation. The hierarchy assigned to each structure is used to determine the structure within which a given dose matrix point is considered to lie. In this manner, regions of structure intersection are assigned to one object or another, and dose volume histograms can be calculated for each structure separately. From this framework, addition and subtraction of histograms can also be performed. Details of the algorithm are presented along with an example using patient data.
Effects of finite angular steps and extent of profile data on the calculation of rotational x‐ray dose distributions
Authors: Kurup R. G. , Hogstrom K. R. , Hwang C. C.
Published: March 2003

Computer algorithms for rotational therapy beams, in most cases, perform dose calculations by summing stored fixed beam data at finite angular steps. Such an algorithm, based on the Bentley beam model, was evaluated by comparing calculations with measured data for an 18‐MV x‐ray beam. Measurements were made in a specially constructed cylindrical water phantom of 15‐cm radius using a 0.1‐cm3 ionization chamber for an arc of 180° and for a field size of 7.2×7.2 cm2 at 100‐cm source–axis distance. This study revealed that the Bentley beam model, with fixed beams summed every 10°, predicts the dose in the treatment volume, centered about the isocenter, with an accuracy of approximately 2%. However, dose at depths between the phantom surface and the treatment volume could be underestimated by as much as 10% (3% of isocenter). This was shown to be partially due to the truncated tails of the off‐axis profiles in the Bentley model, which extend only 8 mm outside the edge of the radiation field, and the large angular increment of integration (10°). Using beam profiles extending to 4 cm outside the edge of the radiation field and angular steps of 5° or less for summation of fixed beams reduced errors to less than 5%. Therefore, extended beam profiles and smaller angular steps for summing fixed beams are recommended for photon rotation calculation when increased accuracy is required.
Practical methods of electron depth‐dose measurement compared to use of the NACP design chamber in water
Authors: Ten Haken Randall K. , Fraass Benedick A. , Jost Ronald J.
Published: March 2003

Central axis relative dose versus depth measurements were performed using two different small volume thimble ionization chambers and a p‐type silicon diode in a water phantom and with two parallel‐plate ionization chambers, thermoluminescent dosimeters, and radiographic film in a popular clear polystyrene phantom. Values obtained were compared to the results of similar measurements in a water phantom performed with a plane‐parallel ionization chamber designed and optimized for use in electron beams by the Nordic Association of Clinical Physicists (NACP). The NACP chamber is expected to minimally perturb the electron fluence and be least prone to point of measurement uncertainties. Its use in a water phantom closely approximates the spirit of recent international protocols. Data were obtained for the foil scattered electron beams generated by two different accelerators for field sizes from 6 cm × 6 cm to 25 cm × 25 cm and energies between 6 and 20 MeV. Easily identifiable effective points of measurements were defined for each measurement device and standard corrections were applied to the raw data to obtain depth‐dose curves. The degree of agreement between the various techniques and the NACP‐water standard was quantitatively analyzed through comparison of the resulting depths of 50% dose and practical range. All methods were found to yield reasonable results when carefully implemented, with average differences of less than 1 mm being easily achievable. Measurements with p‐type silicon diode detectors were found to be particularly useful, as they are pointlike and appear from all practical considerations to directly represent relative dose, thus requiring little or no correction to raw readings.
Magnetic resonance imaging of stationary blood: A review
Authors: Brooks Rodney A. , Di Chiro Giovanni
Published: March 2003

The magnetic resonance imaging appearance of blood, as with other body tissues, is affected strongly by magnetic relaxation rates of the water protons. For blood containing only oxyhemoglobin, as for most tissues, the relaxation times are determined by diamagnetic effects related primarily to protein content. However blood containing either deoxyhemoglobin or methemoglobin exhibits additional paramagnetic relaxation effects, which have important consequences for magnetic resonance imaging of hematomas. First, the field inhomogeneity created by the concentration of paramagnetism in the red blood cells lowers the effective T2. This effect depends on field strength, and so is more striking at high fields, and is greater if gradient echoes are used. In fact, the observation of a difference in T2 with the two different echo methods provides an unequivocal indication of field inhomogeneity such as is produced by erythrocytes. A second paramagnetic relaxation effect is the direct interaction of protons with the electron spin of methemoglobin, which markedly lowers both T1 and T2. This effect is important in the imaging of hematomas that are at least several days old, after significant conversion of hemoglobin to the met form has taken place.
Reconstruction of blood vessels from x‐ray subtraction projections: Limited angle geometry
Authors: Kruger Robert A. , Reinecke Daniel R. , Smith Steven W. , Ning Ruola
Published: March 2003

Several algorithms have been investigated for reconstructing blood vessels from a limited number of x‐ray subtraction projections, distributed over a limited range of angles. Both computer simulations and an in vivo animal study were carried out. The best reconstruction performance was achieved using an algorithm that folded in two pieces of a priori knowledge of the vascular density distributions: (1) the object is dilute, consisting mainly of a void; and (2) the density distribution in the reconstructions is most likely to be non‐negative. Both the signal‐to‐noise ratio (SNR) and the signal to out‐of‐focus blur were quantitated. Compared to tomosynthetic reconstruction (backprojection), the amount of residual blur from out‐of‐focus planes was significantly reduced with only a small penalty in diminished SNR. The combined effect resulted in significant qualitative image improvement for real arterial distributions as demonstrated in a canine arterial imaging example.
X‐ray attenuation properties of radiographic contrast media
Authors: Lumbroso P. , Dick C. E.
Published: March 2003

A systematic study has been carried out to assess the x‐ray attenuation properties of various elements when used as radiographic contrast enhancing media. This study examines the effects of solutions of molecules with effective atomic numbers from 40 to 92 on the signal–contrast ratio of radiographic images formed with various input x‐ray spectra on suitable phantoms. A variety of x‐ray spectra were used including monoenergetic spectra, constant potential x‐ray tube spectra, and computed tomography spectra. In addition, a computer model was used to predict the effects studied. In general, the computer model is able to accurately predict the resulting signal–contrast ratio for a given combination of contrast media, input x‐ray spectra, and phantom composition. From these data and calculations, it may be possible to design new contrast media which are tailored to specific diagnostic imaging tasks.
Effective atomic numbers for low‐energy total photon interactions in human tissues
Authors: Yang N. C. , Leichner P. K. , Hawkins W. G.
Published: March 2003

A new method is introduced in which the total photon interaction cross sections per electron of human tissues are used to define effective atomic numbers for blood, bone, brain, fat, heart, kidney, liver, lung, muscle, ovary, pancreas, spleen, and water. These effective atomic numbers are equal within 4% from 10 to 200 keV in each soft tissue, whereas for bones of different chemical compositions the variation ranges from 2.86% to 5.03%. This effective atomic number definition is less energy dependent than a previous definition based on the total photon interaction cross section per atom averaged over all elements in the tissue, from which the computed effective atomic numbers varied by as much as 50% (in bone) as a function of photon energy over the energy range from 10 to 200 keV.
The forward production of high‐energy electrons from megavoltage photon beams
Authors: Biggs Peter J.
Published: March 2003

The forward production of high‐energy electrons from materials with various atomic numbers from carbon to lead has been measured for megavoltage photon beams from 4‐ to 25‐MV peak bremsstrahlung energy by placing a thin‐window parallel‐plate ionization chamber directly behind foils of the various materials. The relative forward production of electrons decreases with atomic number for energies ≤10 MV until about Z=50, after which it rises. For photon energies ≥15 MV, forward production increases with atomic number with a break point at Z∼50, beyond which the curve becomes steeper.
Dose distributions in regions containing beta sources: Plane interface in a homogeneous medium
Authors: Werner Barry L. , Das Indra J.
Published: March 2003

An analytic model to calculate dose distributions in regions containing beta sources is developed along with a solution for the dose distribution in an infinite, homogeneous medium in which there is a uniform, monenergetic, isotropic source distribution on only one side of a plane. Comparisons with published Monte Carlo calculations are made.
Dose distributions in regions containing beta sources: Small scale nonuniformities
Authors: Werner Barry L.
Published: March 2003

An analytic model for the calculation of dose in regions containing slightly nonuniform distributions of beta sources, and a solution for the dose distribution in an infinite, homogeneous medium with a uniform, monoenergetic source distribution and a sinusoidal perturbation of the source are presented.
Dose distributions around cylindrical 241Am sources for a clinical intracavitary applicator
Authors: Nath Ravinder , Gray Laurence , Park Chang H.
Published: March 2003

Encapsulated, cylindrical sources containing 2, 5, and 8 Ci of 241Am have been designed and fabricated for intracavitary irradiation of uterine cancers. Exposure rates in air and dose rates in water around these sources have been measured using an ionization chamber and a lithium fluoride thermoluminescent dosimetry system. Dose rates in water at a distance of 2.5 cm from the source center along a direction transverse to the source axis were found to be 10.4, 24.3, and 23.3 cGy/h for the 2‐, 5‐, and 8‐Ci sources, respectively, using an ionization chamber. Under the same conditions, the thermoluminescent dosimetry system yielded the values of 10.3, 23.1, and 22.3 cGy/h. It was observed that the ratio of dose‐to‐water and exposure in air is sensitive to the scattering geometry and source geometry in the case of 241Am photons. This ratio was found to increase substantially as conditions of full scattering were approached. A three‐dimensional integration model was employed for the determination of dose distributions around these sources. Results of this dose computation model have been compared against the measured data and were found to be in good agreement with each other. Average deviations of calculated data from measured data were in the range of 0.2 to 0.5 cGy/h and larger deviations were observed in the paraxial region, where the effects of oblique filtration are more severe. Computed dose rate tables were employed as reference source data in a modified version of a commercial treatment planning system in order to develop a facility for clinical dosimetry of 241Am intracavitary applications.
A new calibration phantom for quantitative computed tomography
Authors: Kalender Willi A. , Suess Christoph
Published: March 2003

We report on a new calibration phantom for quantitative computed tomography which has been improved with respect to reference materials and geometrical setup. Instead of liquid calibration solutions, we use polyethylene‐based water‐ and bone‐equivalent plastics. The size of the phantom is considerably reduced by using only two samples. This design guarantees long‐term stability and it offers advantages with respect to radiation geometry.
The development of a xenon/computed tomography cerebral blood flow quality assurance phantom
Authors: Good Walter F. , Gur David , Herron John M. , Kennedy William H.
Published: March 2003

A simple, easy to use, quality assurance and performance test phantom was developed for the xenon/computed tomography (CT) cerebral blood flow method. The phantom combines an inhalation system which allows for the simulation of xenon buildup or washout in the arterial blood as well as a multisection translatable cylinder in which several sections can be scanned during a preselected protocol to simulate the CT enhancement in brain tissue during a study. The phantom and scanning protocol are described and their use is demonstrated. The results compare favorably to the theoretically expected fast, intermediate, and slow “flow” values designed into the phantom.
Patient dose measurements in photon fields by means of silicon semiconductor detectors
Authors: Rikner Göran , Grusell Erik
Published: March 2003

Semiconductor detectors based on p‐type silicon and designed for in vivo measurement of entrance dose at the reference point from photon radiation fields, are described. To estimate the absorbed dose at the reference point from measurements with a thin detector, field‐size dependent correction factors must be applied to the reading, as the shape of the dose buildup curve varies with field size. To decrease or avoid field‐size dependent correction factors, the detector can be covered with a buildup cap. The presence of such a detector will cause perturbation of the radiation field. Therefore, the design of a detector, irrespective of its type, intended for patient dosimetry involves a compromise between minimizing the radiation field perturbation and minimizing field‐size dependent correction factors. Detectors with three different buildup caps were designed to cover the energy range from cobalt‐60 to 16‐MV x rays. The three different detector types were investigated with respect to their signal dependence on field size, field perturbation, and directional dependence. A summary of radiation damage effects on sensitivity, and of sensitivity variation with temperature is also presented.