bluuebunny/crossref_metadata_2025 huggingface dataset:
Relation between tolerance dose and treatment field size in radiotherapy
Authors: Prasad Satish C.
Published: August 2003

A relation is developed between tolerance dose and field size from an analysis of available clinical data on skin‐tolerance doses for various treatment field sizes in radiotherapy. It is shown that plots of log‐dose against log‐field‐size for various fractionation schedules give isoeffect lines all having approximately the same slope. This suggests a power law relation between tolerance dose and field size. A generalized form of NSD equation, incorporating the field‐size dependence of tolerance dose, is shown to follow naturally when this new relation is combined with Ellis’ NSD relation. Certain discrepancies between the sets of data used in the present analysis are pointed out and it is suggested that more data are needed before the generalized NSD relation can be applied under varying conditions of dose, time, fractionation, and field size.
Electron paramagnetic resonance investigation of high‐spin iron (III) in cancer
Authors: Warner F. W. , Stjernholm R. , Cohn I.
Published: August 2003

The concentration of iron (III)–transferrin (IT) in whole blood and serum, along with another high‐spin (five unpaired electrons) iron complex (probably IT) accumulated by tumor tissue, was investigated by electron paramagnetic resonance (EPR) spectroscopy during the development of Murphy–Sturm rat lymphosarcoma. The observed changes in concentration (μg/ml) of IT in sera/blood were generally complementary to those from tissue and the character of the modifications suggested the existence of three distinct phases of systemic response to the implantation: (1) an initial response, evidenced by a sharp reduction in serum IT and somewhat high tissue‐IT concentration (μg/g); (2) a period in which the tumor is becoming established, indicated by relatively constant tissue IT levels and near normal serum IT; and (3) the onset of rapid cell multiplication, characterized by increased total tissue‐IT accumulation that rises to above 200% of normal available serum iron, increasing tissue‐IT concentration, and rapidly declining serum‐IT concentration. The results suggest that, in the face of an implanted tumor there are two detectable abnormal serum‐IT responses: (1) an initial change, probably due to systemic blockage of iron reutilization; and (2) extraction of IT from serum by multiplying tumor cells, which is probably a major contributor to reduced serum‐IT levels and ultimately anemia.
Nationwide evaluation of 60Co teletherapy calibrations
Authors: Dicey Bruce B. , Moe Harold J. , Tongue Thomas M.
Published: August 2003

As a result of a reported incident which resulted in the unintended overexposure of several hundred patients, teletherapy installations throughout the United States were evaluated to verify the accuracy of teletherapy equipment calibrations. Selected licensees received TLDs for exposure under specified conditions. Site visits were made to each licensee showing an apparent discrepancy greater than ± 5% and intercomparison measurements made. This paper describes results of a sampling of 373 teletherapy licensees and the success of site visits in reducing final discrepancies to less than ± 5%. TLD and site visit measurement techniques, results, comparison with data from other studies, and possible sources of uncertainty in the management of a large‐scale screening program are discussed.
Solid‐state electrophotography with Al2O3
Authors: DeWerd L. A. , Moran P. R.
Published: August 2003

Solid‐state thermocurrent and radioconductivity (electrical conductivity during irradiation) experiments are described with attention to the feasibility of using ionic solids for radiographic imaging. The radioconductivity varies with temperature giving rise to temperature windows of potential usefulness. The electrophotographic image is formed at atmospheric pressure using Al2O3, collecting the charge on mylar film. Development is by the power cloud technique. A transparency, which can be viewed as a conventional radiographic image, is easily produced. A successful transfer image, free of electric discharge artifact, was produced.
Computer algorithm for electron beam treatment planning
Authors: Khan Faiz M. , Lee Joseph M. F.
Published: August 2003

A computer algorithm has been developed for electron beam treatment planning. The method uses a limited amount of stored experimental dose distribution data, performs interpolation, and stores generated beam information on an optimized fan line‐depth line grid. Experimental verification of the computer program showed agreement within ±5% for beam generation and air gap corrections.
Photon contamination in 8–20‐MeV electron beams from a linear accelerator
Authors: Gur D. , Bukovitz A. G. , Serago C.
Published: August 2003

The amount of x‐ray contamination near the surface of a phantom irradiated with electron beams was measured directly. Measurements were done to ascertain if photon contamination in the beam contributes a higher dose to the more superficial layers of an irradiated medium than indicated by conventional methods. A 1.4‐kG magnetic field was used to deflect the electron beams generated by a Philips SL/75‐20 linear accelerator. The electron energies studied were 8, 10, 12, 14, 17, and 20 Mev. After sweeping the electron beam, a significant amount of photon contamination was measured in all cases. The characteristic qualities of the photon contamination were measured directly in a water tank. They were found to agree with those of bremsstrahlung spectra generated in a thin target with a virtual source at the location of the scattering foil.
Detectability in computed tomographic images
Authors: Hanson Kenneth M.
Published: August 2003

The detection limitations inherent in statistically limited computed tomographic (CT) images are described through the application of signal detection theory. The detectability of large‐area, low‐contrast objects is shown to be chiefly dependent upon the low‐frequency content of the noise power spectral density. For projection data containing uncorrelated noise, the resulting ramplike, low‐frequency behavior of the noise power spectrum of CT reconstructions may be conveniently characterized by the number of noise‐equivalent x‐ray quanta (NEQ) detected in the projection measurements. The NEQ for a given image may be determined either from a measurement of the noise power spectrum or from the noise granularity computed with an appropriate weighting function. A measure of the efficiency of scanner dose utilization is proposed which compares the average dose to that required by an ideal scanner to obtain the same NEQ.
A fundamental accuracy limitation on measurements of brachytherapy sources
Authors: Boyer Arthur L.
Published: August 2003

The dose rate gradients in the first few centimeters away from brachytherapy sources are extremely high. Attempts at direct measurements of the dose rates in the near vicinity of such sources are made with the smallest detectors available. Nevertheless, it is not obvious that the agreement between reported measurements and theoretical calculations can be justified when the dose rate gradients across the detector volumes employed are so great. A figure of merit is derived here which indicates how well the dose averaged over the volume of the detector corresponds to the dose at the center of the detector. This figure of merit provides a means to assess the maximum accuracy one can expect to achieve in a measurement made at a given distance from a linear source based on the dimensions of the source and the dimensions and shape of the detector.
Effect of arm orientation on bone mineral mass and bone width measured using the Cameron–Sorenson technique
Authors: Schlenker Robert A. , Kotek Thomas J.
Published: August 2003

Bone mineral content and bone width were measured at the distal and midshaft locations in the right radius and ulna with the arm in different orientations. The values of mineral content, width, and the ratio of bone mineral content divided by width, determined at each bone site were compared to establish the effect of orientation. Seven orientations were studied, two of which involved rotations of the forearm and five of which involved rotations of the upper arm about the axis of the forearm. Rotation of the forearm was found to affect the values of one or more of the measured quantities at all of the sites. Orientation of the upper arm was found to affect the values of all quantities at the midshaft radius, but not to affect the values of any of the quantities at the other sites.
Radiographic quality, tube potential, and patient dose
Authors: Ort Michael G. , Gregg Earle C. , Pillai K. M. , Rao P. S.
Published: August 2003

Measurements were made to determine if there was an optimal kVp that would maximize radiographic contrast for various materials while minimizing average patient dose. Radiographs were made of a 17‐cm‐thick plexiglass phantom (to simulate a typical abdomen) with added regions of plexiglass, hydroxyapatite, and iodine with peak kilovoltages from 40 to 150 kVp and 2 mm Al added filtration. Radiographs were also made of a 4‐cm‐thick plexiglass phantom to simulate a typical breast, with added regions of plexiglass, hydroxyapatite, and fat at a peak kilovoltage from 35 to 100 kVp and with 1 mm added filtration. For the thicker phantom it was found that the contrast per average absorbed dose for the added plexiglass, hydroxyapatite, and iodine increased by about 3, 2, and 0.4, respectively, when going from 50 to 150 kVp. Contrast was measured relative to a reference region on the phantom. Similar data for the thinner phantom showed an increase of about 35% in contrast per average absorbed dose for plexiglass and fat but a decrease by about 30% for hydroxyapatite as the kVp changed from 35 to 100 kVp.
Quantum fluctuations in radiographic screen‐film systems
Authors: Rao Gopala U. V. , Fatouros Panos P.
Published: August 2003

With the recent introduction of fast rare‐earth screens into the market, the question of noise and how it influences image quality becomes particularly important. In this paper a compact formula involving familiar and measurable quantities is derived for the specification of the noise arising from the spatial fluctuations of the x‐ray quanta absorbed in the screens. Experimental evidence in support of this concept is also given.
Variation in output factor caused by secondary blocking for 7–16 MeV electron beams
Authors: Choi Myung C. , Purdy James A. , Gerbi Bruce , Abrath Fred G. , Glasgow Glenn P.
Published: August 2003

The effect of secondary blocking on the output factor for 7–16 MeV electron beams is investigated for the two most widely used methods of field shaping, i.e.,(a) shield on surface, and (b) shield at end of cone with subsequent air gap. For these two methods differences in output on the order of 10% are observed. These effects are explained in terms of the blocking geometry and its influence on scattered radiation.
Measurement of the effect of inhomogeneities and compensating bolus in clinical pion beams
Authors: Hogstrom K. R. , Smith A. R. , Somers J. W. , Lane R. G. , Rosen I. I. , Simon S. L. , Kelsey C. A.
Published: August 2003

Measurements of the effects of Teflon and air inhomogeneities on the ionization distributions of clinical negative‐pion beams have been made at the Los Alamos Meson Physics Facility. Inhomogeneity location and pion‐beam energy vary the effect of multiple coulomb scattering on the dose distribution lying in the penumbra of the inhomogeneity. CH2 bolus adequately corrects for the effects of these inhomogeneities. Bolus misalignment less than 0.5 cm does not seem critical because of large multiple coulomb scattering of the pion beam. However, this and secondary particles emitted from pion stars prevent the pion beam from being precisely shaped with sharp edges, as demonstrated by measurements under a patient bolus.
H and D curves of screen–film systems: factors affecting their dependence on x‐ray energy
Authors: Vyborny Carl J.
Published: August 2003

The sensitometric properties of radiographic screen–film systems are investigated with regard to their dependence on x‐ray energy. Sensitometric measurements and a heuristic model for density formation are used to show that variation in the relative amount of light emitted by front and back screens may be the most important factor influencing the change in system H and D curves with x‐ray energy.
Empirical equation for tissue‐maximum ratios/scatter‐maximum ratios for indirectly ionizing radiotherapy beams
Authors: Schell M. C. , Deye J. A.
Published: August 2003

A six‐parameter equation has been fitted to the tissue‐maximum ratio/scatter maximum ration (TMR/SMR) tables for 60Co, 4‐, 6‐, and 10‐MV photon beams as well as the 35‐MeV d→Be neutron beam. The empirical equation has been shown to fit the 60Co tissue–air ratio/scatter–air ratio (TAR/SAR) data equally well. The TMR/SMR model was developed from a three‐parameter equation designed to eliminate the graphical extrapolation of zero‐field size TARs.
Extraction of information from CT scans at different energies
Authors: Millner Michael R. , McDavid William D. , Waggener Robert G. , Dennis Michael J. , Payne William H. , Sank Victor J.
Published: August 2003

The image displayed in computed tomography is a scaled representation of attenuation coefficients within the patient's body. A number of authors have presented methods by which additional information (such as electron density, effective atomic number, and extrapolated attenuation coefficients for therapy applications) can be extracted from CT scans carried out at different energies. In the present paper, the dual‐energy method described by Rutherford has been used to produce complete images of effective atomic number and electron density of a known phantom (the AAPM phantom) in order to investigate the usefulness of applying this method to current commercial scanners.
Dosimetric properties of neutrons from 21‐MeV deuteron bombardment of a deuterium gas target
Authors: Weaver K. A. , Eenmaa J. , Bichsel H. , Wootton P.
Published: August 2003

Spectra, yields, average energies, and kerma rates in tissue of neutrons from 21‐MeV deuteron bombardment of deuterium gas targets have been calculated for target thicknesses of 1, 3.5, and 5 MeV. A high pressure gas cell was constructed and was filled with 33 atm of D2 gas (equivalent to an energy loss of 3.5 MeV for 21‐MeV deuterons); dose rate, dose buildup, and depth‐dose properties of neutrons produced by the D(d,n) reaction were measured. Dosimetric properties of these neutrons are superior to those of neutrons from a thick Be target bombarded by a deuteron beam of the same energy.
Relative carcinogenic effects of different mammography techniques
Authors: Muntz E. P.
Published: August 2003

Comparisons of different mammography techniques can be significantly altered by assumptions about the relevant measures for carcinogenic effects in mammography at low doses. Using a combination of recent data available from the literature, a study has been made of the significance of various assumptions concerning both the appropriate measures for carcinogenesis in mammography and the shape of the dose–effects relationship. It is found that for techniques typical of current reduced‐dose mammography systems, the comparison of the relative carcinogenic effects of the techniques is significantly affected by both the measures that are used and the shape of the dose–effects relationship.
Choosing a therapy electron accelerator target
Authors: Hutcheon R. M. , Schriber S. O. , Funk L. W. , Sherman N. K.
Published: August 2003

Angular distributions of photon depth dose produced by 25‐MeV electrons incident on several fully stopping single‐element targets (C, Al, Cu, Mo, Ta, Pb) and two composite layered targets (Ni–Al, W–Al) were studied. Depth‐dose curves were measured using TLD‐700 (thermoluminescent dosimeter) chips embedded in lucite phantoms. Several useful therapy electron accelerator design curves were determined, including relative flattener thickness as a function of target atomic number, “effective” bremsstrahlung endpoint energy or beam “hardness” as a function of target atomic number and photon emission angle, and estimates of shielding thickness as a function of angle required to reduce the radiation outside the treatment cone to required levels.
A laser scanner for the densitometric analysis of radiographs
Authors: Revesz George , Kundel Harold L.
Published: August 2003

An instrument is described which can scan radiographs and collect densitometric readings for subsequent computer analysis. A laser beam is focused onto the film. Raster scan of the beam is provided by two orthogonal mirror galvanometers. The light, attenuated by the radiograph, is gathered by a lens in such a way that it is directed onto a small detector regardless of the scan position. Beam position signals and density readings are stored and analyzed in a minicomputer.
Digital readout of feedback control temperature for use in local current field hyperthermia
Authors: Kopecky Walter J. , Parrino Peter A.
Published: August 2003

From clinical and biological experience it is oftentimes necessary to administer hyperthermia with a minimum number of temperature monitoring tumor probes. For this reason a digital readout device has been developed to monitor the feedback temperature of a hyperthermia generator. In this way the thermometry used to monitor tumor core temperature during biological or clinical hyperthermia can be minimized. The unit and its capabilities are described.
Effects of focal spot intensity distribution and collimator width in reconstructive x‐ray tomography
Authors: Prasad Satish C.
Published: August 2003

In computed tomography the finite width of the collimator aperture implies that the image is reconstructed from scans having finite strip width. This blurs the image and limits scanner resolution. Bracewell has discussed the nature of the blurring produced and suggested a method to partially restore the image under the assumption that the x‐ray focal spot has a uniform intensity distribution. However, pinhole radiographs of x‐ray focal spots reveal intensity distribution in the scanning plane which peaks at the edges of the focal spot with a reduced intensity in the center. A model is presented which simulates this intensity distribution as the sum of two point sources of x rays separated by a uniform line source. This results in a point absorber response function which is a superposition of two rectangular distributions and the one discussed by Bracewell. The corresponding transfer function is derived which shows that the intensity distribution of the focal spot influences the high spatial frequency components of the reconstructed image more than the low frequency components.
Gamma dose measurement in a d (80)+Be neutron beam
Authors: Harrison George H. , Cox Colin R.
Published: August 2003

The γ‐ray dose component of a d (80)+Be neutron beam was measured with a 6Li‐shielded Geiger–Mueller counter in a tissue‐equivalent liquid phantom. The proportion of γ‐ray dose to total dose increased from 0.07 near the surface to 0.09 at 25 cm depth. Due to thermal neutron sensitivity, the G–M count rate increased by 25% in the absence of 6Li shielding.
Concerning hyperthermia phantom
Authors: Astrahan Melvin A.
Published: August 2003

The author describes the development of a tissue‐like hyperthermia phantom useful for point by point mapping of hyperthermia fields using transistor probes. This phantom is superior to an earlier model due to its higher melting point of 60°. (AIP)
Echocardiogram analysis in a pattern recognition framework
Authors: Chu W. K. , Raeside D. E. , Chandraratna P. A. N. , Brown R. E. , Poehlmann H.
Published: August 2003

Echocardiogram analysis is treated in a pattern recognition framework. Anterior mitral leaflet waveforms are classified for the four‐class problem consisting of the classes “normal,” “mitral stenosis,” “mitral valve prolapse,” and “idiopathic hypertrophic subaortic stenosis.” In addition, aortic root waveforms and left ventricular wall waveforms are classified for the two‐class problem consisting of the classes “normal” and “idiopathic hypertrophic subaortic stenosis.” One common method of analysis (Fourier analysis) underlies each classification scheme. Classification accuracy is sufficiently good to warrant the inference that successful automated decision‐making based on the algorithms investigated is feasible.
Electrons as the cause of the observed dmax shift with field size in high energy photon beams
Authors: Biggs Peter J. , Ling C. Clifton
Published: August 2003

For megavoltage x‐ray beams, it is well known that the percent depth‐dose increases considerably with field size in the buildup region, resulting in a significant shift in the apparent position of maximum dose, dmax. The nature of this increase has been investigated using a sweeping magnet placed just below the treatment head of a 25‐MV linac. Measurements show that for increasing magnetic fields the dose in the buildup region is continually reduced, until a point is reached beyond which no additional reduction is observed. Here the buildup curve is essentially field size independent. These results clearly show that electrons are the primary cause of dose increase with field size in the buildup region, in contrast to a recent publication claiming that scattered photons are the cause. Further measurements were made by blocking out the primary beam at the level of the jaws and measuring the depth dose of the scattered electrons originating from the jaws. The results show that a thickness of approximately 1 g cm−2, of either polystyrene or lead, reduces the dose by a factor of two, providing further evidence that the scattered component of the beam consists of low energy electrons.
Examination of the factors Ac and Aeq for cylindrical ion chambers used in cobalt‐60 beams
Authors: Holt J. G. , Fleischman R. C. , Perry D. J. , Buffa A.
Published: August 2003

The calibration of a cobalt‐60 beam in a phantom with an ion chamber, which has been calibrated with respect to exposure, requires the use of a displacement correction factor which essentially corrects the photon fluence for the attenuation and scatter when the chamber with buildup cap is removed and replaced by phantom material. To determine the displacement factor, Ac, a special set of cylindrical ionization chambers with various volumes were constructed out of polystyrene. Tissue–air ratios were measured with these chambers for cobalt‐60 gamma rays in a polystyrene phantom, and the ratio Ac/Aeq was experimentally determined. In order to calculate Ac from this ratio, Aeq was determined also. It was found that Ac depended on chamber diameter only, and not on field size or depth. A value of 0.990 for Aeq is recommended and a table of Ac for chambers of different outer diameters is included.
Computerized patient contours using the scanning arm of a compound B‐scanner
Authors: Hills John F. , Ibbott Geoffrey S. , Hendee William R.
Published: August 2003

Full utilization of the precision of newer radiation therapy devices requires patient contours drawn with greater accuracy than is possible with the conventional lead wire technique. Polaroid photographs can introduce large errors due to distortion and small image size. Techniques including electromechanical or optical devices and CT scans offer improved accuracy, but often at added expense. A method for obtaining contours has been developed which utilizes a treatment planning minicomputer (equipped with an analog‐to‐digital converter and plotter) and a commercially available ultrasound B‐scanning arm. Voltages corresponding to the X–Y position of the tip of the scanning arm are fed from the scanner to the A/D interface, smoothed, scaled, and plotted. The resulting drawing is a full scale external patient contour. The accuracy of this method is compared to alternative techniques.
Thermoluminescence in hydroxyapatite
Authors: Chapman M. R. , Miller A. G. , Stoebe T. G.
Published: August 2003

A thermoluminescent (TL) glow peak has been shown to exist at 80°C in synthetic hydroxyapatite, in amorphous calcium phosphate, and in deproteinated rat bone mineral. This TL glow peak is excited by a thermal pretreatment at elevated temperature, followed by exposure to ultraviolet (UV) light. When measured on a cylic TL‐readout UV‐exposure basis after an original thermal pretreatment, the intensity of this 85°C glow peak displays a logarithmic decay curve characteristic of a transfer thermoluminescence process: A deep trap is activated during the annealing process, and the subsequent UV exposure causes charge transfer to the 85°C glow peak, as is verified using electron spin resonance. Exposure to the atmosphere appears to decrease the TL intensity levels, presumably due to water adsorption; vacuum drying and additional thermal pretreatment reverse this effect on the TL intensities. TL measurements are applied to the comparison of bone mineral samples of varying age. Increases in overall TL intensity are demonstrated between a four‐week old normal rat group and eight‐ and fourteen‐week‐old normal rat groupings. The TL intensity is also increased by metabolic disorder which causes changes in bone mineral chemistry and crystallography.
Absorbed dose in the presence of contrast agents during pediatric cardiac catheterization
Authors: Callisen Hannes H. , Norman Amos , Adams Forrest H.
Published: August 2003

Administration of x‐ray contrast agents during heart catheterization examination increases the absorbed radiation dose in tissue. To estimate the dose absorbed by the blood of children undergoing diagnostic heart catheterization and angiocardiography, a number of measurements and calculations were conducted. First, entrance and exit exposures to the patient were measured with thermoluminescent dosimeters calibrated for the diagnostic x‐ray energy range. Second, a dose enhancement factor was calculated from mass energy absorption coefficients for various concentrations of the contrast media and at selected x‐ray energies. Third, the dose enhancement factor was estimated from survival of peripheral blood lymphocytes suspended in varying concentrations of the contrast agent during exposure to graded doses of x‐rays. Fourth, a mean absorbed dose to the patient's blood was calculated using (a) the dose enhancement factor determined above, (b) an estimate of the mean exposure in the irradiated body volume calculated from the entrance and exit exposure measurements, (c) an effective iodine concentration in the blood during the exposure time, and (d) a ratio correcting for the distribution and circulation of the blood. For eight pediatric patients monitored, absorbed doses to the blood ranged between 3 and 12 rad. These values were two to three times greater than the expected dose without administration of a contrast agent.
Concerning Li2B4O7 thermoluminescence dosimeters
Authors: Soares Christopher G. , Ehrlich Margarete
Published: August 2003

This letter reports the comparison of two commercially available types of lithium borate thermoluminesence dosimeters—one in which crystalline lithium borate was pressed into chips, and the other in which lithium borate was dispersed in a glass matrix. When irradiated with cobalt 60 gamma radiation, the response of a single sample of each type was reproducible to within 1%. However, differences between the two samples were apparent in their long term storage characteristics. (AIP)
An interesting form of high‐energy gamma‐ray–detector interaction and its effect on spectral peak centroid determination
Authors: Zamenhof Robert G.
Published: August 2003

The pulse‐height continuum produced in a radiation detector following the interaction of photons by pair‐production is discussed. The event when one annihilation photon escapes from the detector without interaction while the other escapes after one Compton scattering interaction produces a characteristic component in the pulse‐height spectrum. The significance of this component in influencing the accurate determination of the centroid of the double‐escape peak is examined and shown to be negligibly small.
Electron beam depth dose scaling by means of effective atomic number reconstructed from CT scans
Authors: Datta Ratna , Datta Sobhendranath , McDavid William D. , Waggener Robert G.
Published: August 2003

A method is developed by which the computed tomography scans of a medium carried out at a number of diagnostic energies can be utilized to obtain the insitu “Effective Atomic Number” and “Effective Density”. The electron depth dose curves in water when scaled by these factors estimate the actual electron depth dose distribution in that medium. It appears that the use of CT scans for electron beam treatment planning, in the management of cancer, is quite promising.
Computer generation of dose distributions for a fast‐neutron therapy beam
Authors: Shapiro P. , August L. S. , Theus R. B.
Published: August 2003

A system has been developed for the computer generation of dose distributions for the MANTA‐NRL fast‐neutron radiotherapy beam. This program is based on scatter–air ratio (SAR) techniques. A method has been developed to unfold the effect of the neutron‐beam profile in the derivation of SARs so that the SARs obtained are those which would be derived if the beam profile were flat. Tables of zero‐area tissue–air ratios and SARs are presented. Comparisons of calculated and measured dose distributions are shown. An empirical correction to the usual SAR methods was required to obtain agreement between calculated and measured dose distributions at source‐to‐skin distances (SSD) which are different from the SSD at which the SAR are derived.
Evaluation of a new x‐ray film with reduced crossover
Authors: Rao Gopala U. V. , Fatouros Panos P.
Published: August 2003

The improvement in MTF that can be obtained by the reduction of crossover exposures is evaluated using an experimental film supplied by the 3M Company (3M XUD). It was found that image resolution, as measured by the equivalent passband (2 <∫∞0[M (f)]2df), of 3M Trimax 8 screens when used with the experimental 3M XUD film was 2.9 c/mm as compared to 1.8 c/mm when used with a conventional double coated film such as Kodak Ortho G or 3M XD. On the other hand, system speed with 3M XUD film was only 1.6 mR−1 as compared to 3.2 mR−1with 3M XD film and 2.4 mR−1 with Kodak Ortho G film. These findings were substantiated by phantom studies as well.
X‐ray computed tomography in the presence of arbitrary symmetrical focal spot intensity distributions
Authors: Verly Jacques G.
Published: August 2003

A loss of resolution in tomographic images is due to the well‐known bimodal spatial variations in photon emission across the x‐ray focal spot combined with the finite width of the x‐ray detector. Such departures from the ideal infinitely narrow beams which are usually considered can be taken into account by replacing the customary line integrals by suitably chosen integrals over strips. The image degradation resulting from using those strip integrals as ingredients in conventional reconstruction formulas is expressible as a complicated point spread function, which can always be computed in any given situation. Restoring the reconstructed image to provide higher resolution is a nontrivial operation, which however greatly benefits from experience already gained in dealing with optical imagery degraded by the aberrations of astigmatism and curvature of field. Typical point spread functions are shown for a simple bimodal source intensity distribution of primordial importance.
An anthropomorphic torso section phantom for ultrasonic imaging
Authors: Madsen Ernest L. , Zagzebski James A. , Ghilardi‐Netto Thomaz
Published: August 2003

Using stable materials capable of mimicking any soft tissues of the human body—quantitatively with respect to speed of sound, density, and attenuation coefficient and semiquantitatively with respect to scatter—we have constructed a torso section phantom containing simulated bowel gas, kidney, kidney fat pad, liver, tumor, cyst, bony structures, and resolution fibers. Simple geometries and precise knowledge of ultrasonic properties allow artifacts to be produced and explained with great confidence. Artifacts due to refraction and reflection are particularly dramatic. Phantoms such as this should be useful as stage one phantoms for training ultrasonographers. Such phantoms should also be of use to those who are doing research in ultrasonic image processing. This might involve, for example, recognizing image artifacts and reducing their effect in the formation of the image.
How big is a pinhole?
Authors: Baird L. C.
Published: August 2003

Numerical estimates are formulated for the pinhole size required for a distortion free pinhole camera. The appropriate formulas are shown for both large and small pinholes.
Transverse axial film tomography with a therapy simulator
Authors: Kessaris Nikitas D. , Harisiadis Leon
Published: August 2003

A tomographic attachment was designed and developed that attaches to our existing simulator. The film tomograms obtained with the aid of this device exhibit very low geometrical distortion and good contrast and resolution for the objects used in this investigation. In addition, our results to date show that adjacent areas with densities differing by as little as 5% can be distinguished on film.
A method for testing scattered radiation grids
Authors: Nielsen Bengt
Published: August 2003

A simple method of evaluating contrast improvement factors and other related quantities has been developed. With this method a water or lucite phantom is imaged with a lead disk on a nonscreen film using a scattered radiation grid with unknown characteristics. The image is then compared with that obtained with the same arrangement but with a calibrated scattered radiation grid. The method described does not need sophisticated instruments and can be used in any department of diagnostic radiology.
Lipowitz metal shielding thickness for dose reduction of 6–20 MeV electrons
Authors: Purdy James A. , Choi Myung C. , Feldman Arnold
Published: August 2003

The relative dose reduction by Lipowitz metal of 6 to 20 MeV electrons from a Varian Associates Clinac–20 linear accelerator has been measured using a parallel plate thin wall ionization chamber. Metal thicknesses required for a 5% attenuation level for a 10×10 cm2 field are as follows: 6 MeV–2.3 mm, 9 MeV–4.4 mm, 12 MeV–8.5 mm, 16 MeV–18.0 mm, 20 MeV–25.0 mm.
A useful algorithm for calculating central ray TAR and TMR values
Authors: Wrede Don E. , Dawalibi Hisham
Published: August 2003

An algorithm utilizing a two‐dimensional polynomial power functions least squares fit has been applied to the calculations of TAR in the case of CO‐60 teletherapy and TMR in the case of an 8MV SL75/10 linear accelerator giving less than ±1/2% errors respectively between calculated and measured values. The TAR and TMR algorithms are used as subroutines in pocket sized and table top programmable calculators or computers and thus provide rapid calculation of treatment time or monitor units as well as given dose.
A model for calculating the effects of small inhomogeneities on electron beam dose distributions
Authors: Perry David J. , Holt J. Garrett
Published: August 2003

Charged particles penetrating a medium are repeatedly deflected from their line of motion by coulomb interactions with target nuclei. This physical feature should be included in the calculation of dose distributions when the irradiated medium contains small inhomogeneities. We use the mean path taken by particles in homogeneous media to represent their motion through inhomogeneous materials. This procedure adapts the small angle multiple scattering formalism to yield manageable, approximate solutions when a medium contains arbitrarily composed inhomogeneities. To promote the development of this procedure into an electron beam treatment planning program, we discuss the salient features which arise in connection with electron beam inhomogeneity calculations. We use a solid phantom containing an air cavity just below its surface as a particular example of a small inhomogeneity. We discuss qualitatively and quantitatively the dose distributions which result from their irradiation with an 11‐MeV electron beam.
Comparison of the cobalt‐60 photon beam dose distributions corrected by three shift‐factors for oblique incidence
Authors: Iwasaki Akira , Ishito Toshio
Published: August 2003

A new factor for shifting isodose curves for oblique Co‐60 photon beams has been deduced. Like the two other known factors, this shift factor depends on the source‐to‐surface distance (SSD), the depth dose (D) and the derivative of D with depth. A comparison of corrected dose distributions has been made by using the three shift factors for the 15×15‐cm2 field incident on a surface at 45°. In general, the corrected dose distributions given by the new shift factor show the best agreement with the measured depth dose data.
Measurements of computed tomography x‐ray fields utilizing the partial volume effect
Authors: Jucius Robert A. , Kambic George X.
Published: August 2003

Due to the x‐ray field geometry, exposure measurements on computed tomographic (CT) scanners require modification of widely used measurement techniques. Ionization chambers have geometries which do not allow measurement of narrow rotating x‐ray fields. However, by utilizing the partial volume effect, an ion chamber can be calibrated to measure narrow x‐ray fields with a spatial resolution of down to 1 mm. The number of scans required to reconstruct a complete profile depends on the resolution required, and is typically on the order of 10 to 20 scans. This is accomplished using a series of attenuating sleeves over the ionization chamber. The chamber and sleeves are calibrated against a standard chamber using large field geometries. Comparisons with TLD measurements are shown to verify the accuracy of the technique.
A linear regression analysis of the gamma dose in fast neutron beams
Authors: Almond Peter R. , Rosanky Steven L.
Published: August 2003

The dual dosimeter technique for determining both the absorbed dose of neutrons and photons in a mixed field has been applied to multiple dosimeter use. The data were analyzed by a linear regression method which yields the neutron dose from the slope and the photon dose from the intercept and an estimation of the uncertainty of the photon dose can also be obtained. Measurements were made on a high energy neutron beam and the photon dose obtained both as a function of field size and depth in a tissue equivalent phantom.
Accelerator leakage measurements
Authors: Dixon Robert L.
Published: August 2003

The effects on shielding needs of a 15 Mv Mevatron‐20 and a 10 Mv clinac 18 accelerator are discussed in the light of recent TVL measurements.(AIP)
Semi‐empirical model for depth dose distributions of megavoltage x‐ray beams
Authors: Ahuja Sain D. , Stroup Steven L. , Bolin Marion G.
Published: August 2003

The dose distribution due to absorption of photon energy fluence in a homogeneous water phantom irradiated by megavoltage x‐ray beams has been analyzed with a semiempirical model. The method generalizes an analytical formalism for the scattering component of dose within a water phantom which was developed recently for monoenergetic photon beams. Contributions to dose via Compton interaction and pair creation form the essential structure of the secondary component formula. Both the central‐axis percent depth dose and off‐central‐axis ratios can be determined for beams of different sizes, used at any value of source to surface distance. The input data include the values of linear attenuation and energy‐absorption coefficients in water at energies between 10 keV and the equivalent energy of the beam. Predicted values of the central‐axis percent depth dose and the off‐central‐axis ratios are compared with the measured data for 2, 4, 6, 8, 10, 14, 20, 35, 45, and 70 MVp x‐ray beams. For the central‐axis percent depth dose, agreement is within 3% for fields of sizes between 5×5 and 20×20 cm2, and 5% for larger fields, for beams of MVp up to 20. For higher energy beams, comparison was made only for the 10×10 cm2 fields and the discrepancies were within 3%. For the off‐central‐axis ratios, agreement between the predicted and measured values is within 5% over the umbra region but worsens in the penumbra region and geometrical shadow. This formalism requires large computer storage for generating data for all realistic beams irradiating normal‐size phantoms.
Advantage of underweight
Authors: Callisen H. H. , Cochran S. T. , Khodadoust A. A. , Norman A.
Published: August 2003

Exposure measurements made during a diagnostic radiological study revealed that heavy set patients pay a higher price in absorbed dose than lighter people.
Comparison of equivalent photon energy calibration methods in computed tomography
Authors: Judy Philip F. , Adler Gary J.
Published: August 2003

A method of specifying the equivalent photon energy as the energy that gives the maximum correlation between linear attenuation coefficient and CT value of six standard materials, including water, was compared with standard method that specified equivalent photon energy as the energy at which water's linear attenuation coefficient is equal to the detected energy fluence averaged coefficient of water. Comparisons were made for various tube potentials, thicknesses of aluminum filtration, and water phantom thicknesses. Using the experimental data, the first method predicted changes in equivalent photon energy equal to 0.3 keV kVp−1, 2.8 keV g−1 cm2, and 0.75 keV g−1 cm2, respectively, for the specified conditions; the precision was ±2.2 keV. Both methods estimated the same equivalent photon energies within 3 keV. This similarity was shown to be a result of the characteristics of water's attenuation coefficient. The effect of uncertainty in measured CT values and material density on the equivalent photon energy was estimated. The equivalent photon energy was used to predict CT values for high atomic number water solutions, 5 mg/ml. The difference between the measured CT values and the predicted was less than 10 CT number for elements of less than 60.
View sampling requirements in fan beam computed tomography
Authors: Joseph Peter M. , Schulz Raymond A.
Published: August 2003

It is known that good CT reconstructions require that many views of the scanned object be obtained, where each view consists of many rays which traverse the patient. It is known that the required number of views depends on the diameter of the region reconstructed and the spatial resolution achieved. We present a new analysis of this question, valid for reconstructing from fan beam views, which shows that larger fan angles will require more views. Computer simulations, using a fan beam reconstruction algorithm, confirmed the predicted fan angle dependence and demonstrated that streak artifacts will result if too few views are used. The effect is also demonstrated experimentally by a phantom scan on a clinical CT scanner. Implications for clinical CT scanning are discussed.
An inexpensive variable‐frequency microwave system for hyperthermia
Authors: Cunningham David E. , Frey Randall A. , Velkley Donald E.
Published: August 2003

A microwave hyperthermia system is described which uses surplus communications equipment. The most costly components of the microwave system, the signal generator and amplifier, were obtained through the Federal Surplus Supply System. The cost of the entire system, including the 2450 MHz applicator and temperature monitoring system, is under $1200. Extensive testing has demonstrated the capability of heating several cm3 mouse tumors to a temperature of 42.5 °C. Depth of heating can be varied by adjusting the frequency of the oscillator or by adjusting the output attenuator.
Performance characteristics of improved antiscatter grids
Authors: Sorenson James A. , Niklason Loren T. , Knutti David F.
Published: August 2003

The performance characteristics of prototype grids constructed with 0.1‐mm‐thick Ta strips were investigated. Significant improvement in scatter cleanup was noted in comparison to commercially manufactured grids, which are constructed with 0.04–0.05‐mm‐thick Pb strips. Highly efficient scatter rejection, approaching that of slit radiography techniques, may be possible using crossed grids constructed with 0.1‐mm Ta strips. By constructing a grid from a crossed pair of linear grids and moving these grids at right angles to each other, grid lines can be eliminated from crossed grid images.
Resolution of pinhole collimators used with gamma cameras
Authors: Goulet Pierre , Joy M. L. G.
Published: August 2003

The line spread function (LSF) of an imaging system incorporating a pinhole of 0.275 cm radius and a scintillation camera has been measured and its full width at half maximum (FWHM) compared to the values predicted by an expression based on the resolution index of Anger. In most cases, the actual resolution of the system is approximately 20% superior to that predicted by the above expression. However, good agreement is observed between our measured LSFs and those computed using a frequency domain method. Based on these computed LSFs, an empirical expression relating the FWHM of the total system to pinhole and camera parameters with an accuracy of ±3% is presented. This expression is applicable to most pinhole imaging systems in use today.
TLD reader with photon counting and dc techniques for wide range radiation dose measurement
Authors: Sankaran A. , Kannan S. , Gangadharan P.
Published: August 2003

A TLD reader incorporating the techniques of photon counting and dc methods has been developed for radiation dose measurements in the range of 10−6 Gy (10−2 Gy=1 rad) to 100 Gy for varied applications in radiation protection and for dosimetry in medical physics. It utilizes state‐of‐the‐art technology and is provided with reproducible heating programmer, glow curve output, digital dose display, check light source and data processing facilities. The reader has been designed for easy operation and maintenance. The precision of the reader is about ±6% (1 σ) for 10−5 Gy and better than ±2% (1 σ) for doses ⩾0.5 cGy. Further improvements are being effected to measure doses down to 10−7 Gy.
X‐ray beam characteristics of the Varian Clinac 6‐100 linear accelerator
Authors: Coffey Charles W. , Beach J. Larry , Thompson Donald J. , Mendiondo Marta
Published: August 2003

Beam characteristics and dosimetry measurements of the 6 MV x‐ray beam from a Varian Associates Clinac® 6‐100 linear accelerator are presented. Percentage depth dose tables are given as a function of field size for square fields. Tissue‐maximum ratios and scatter‐maximum ratios are tabulated as a function of depth and field size. Investigation of surface dose and depth of maximum dose reveal a dependence on field size. Other beam parameters measured and presented include field flatness, symmetry, SSD dependence, and penumbra. Treatment planning parameters including field size output factors and wedge factors are discussed.