Showing papers in "Medical Physics in 1983"
TL;DR: A protocol is given to provide radiological physicists with an accurate means of measuring radiation doses on phantoms, and a new quantity, the cavity‐gas calibration factor, is introduced.
Abstract: A protocol is given to provide radiological physicists with an accurate means of measuring radiation doses on phantoms. Plastics, as well as water, are considered as phantoms. A new quantity, the cavity‐gas calibration factor, is introduced.(AIP)
783 citations
TL;DR: A Monte Carlo computer model has been used to predict the distribution of absorbed dose in homogeneous tissues of different absorption/scattering ratios, for illumination both by external light beams and via implanted optical fibers.
Abstract: A Monte Carlo computer model has been developed to study the propagation of light in tissues. Light attenuation is assumed to result from absorption and isotropic scattering. The model has been used to predict the distribution of absorbed dose in homogeneous tissues of different absorption/scattering ratios, for illumination both by external light beams and via implanted optical fibers. The photon flux into optical fibers placed in the tissue as detectors has also been investigated. The results are interpreted in relation to the use of visible light irradiation for photo radiation therapy.
688 citations
TL;DR: It is demonstrated here that the detailed time dependence of the resulting trajectory of sample points determines the relative weight and accuracy with which image information at each spatial frequency is measured, establishing theoretical limitations on image quality achievable with a given imaging method.
Abstract: The fundamental operations of nuclear magnetic resonance (NMR) imaging can be formulated, for a large number of methods, as sampling the object distribution in the Fourier spatial-frequency domain, followed by processing the digitized data (often simply by Fourier transformation) to produce a digital image. In these methods, which include reconstruction from projections, Fourier imaging, spin-warp imaging, and echo-planar imaging, controllable gradient fields determine the points in the spatial-frequency domain which are sampled at any given time during the acquisition of data (the free induction decay, or FID). The detailed time dependence of the resulting trajectory of sample points (the k trajectory) determines the relative weight and accuracy with which image information at each spatial frequency is measured, establishing theoretical limitations on image quality achievable with a given imaging method. We demonstrate here that these considerations may be used to compare the theoretical capabilities of NMR imaging methods, and to derive new imaging methods with optimal theoretical imaging properties.
429 citations
TL;DR: It is found, as expected, that variations in local temperature appear as variations in the corresponding T1 image, and T1 is found to vary linearly with 1/T over the range of 0 degrees C to about 40 degrees C for blood.
Abstract: This paper represents a preliminary study of the effects of regional temperature distribution in two-dimensional nuclear magnetic resonance (NMR) T1 imaging. It is found, as expected, that variations in local temperature appear as variations in the corresponding T1 image. The potential use of NMR T1 imaging in temperature measurements is evaluated in the case of water and blood samples. Using containers where the temperature could be either known or directly controlled with reasonable accuracy, images are obtained with samples having at least two regions at different temperatures. As expected, T1 is found to vary linearly with 1/T over the range of 0 degrees C to about 40 degrees C for blood. The potential use of T1 imaging in hyperthermia applications is also discussed.
306 citations
TL;DR: In this paper, the authors proposed a new type of gamma camera for single photon emission computed tomography, which utilizes electronic collimation which is obtained from a sequential interaction of gamma radiation with a dual position-and-energy sensitive detection system.
Abstract: The detection and imaging characteristics of a new type of gamma camera for single photon emission computed tomography have been investigated. Unlike conventional gamma cameras which use mechanical collimation, the new gamma camera utilizes electronic collimation which is obtained from a sequential interaction of gamma radiation with a dual position-and-energy sensitive detection system. Coincident counting between the two detectors provides localization of activity upon a multitude of conical surfaces throughout the object, wherefrom the three-dimensional activity distribution can be reconstructed. Not only does electronic collimation provide simultaneous multiple views of the object, but a large gain in sensitivity is also indicated over a conventionally collimated gamma camera under conditions of similar spatial resolution. Detector optimization studies have been performed to design a prototype system comprising a 33 X 33 array of high-purity germanium detectors coupled to an uncollimated conventional scintillation camera. The cumulative signal-to-noise ratio in projection images obtained with this system is expected to be about a factor of 4 higher (sensitivity about a factor of 15 higher) than that obtained in a corresponding projection image with a conventional gamma camera for imaging a uniformly distributed Tc-99m source in a 20-cm-diam X 20-cm-tall cylinder. A similar gain is expected in the tomographic images.
292 citations
TL;DR: Experimental data are presented which are consistent with the general model alone and which demonstrate the limits of applicability of previous models.
Abstract: A model for the dependence of normal tissue radiation dose response functions on volume variations and dose inhomogeneities is derived using probability theory. Power law volume correction factors and the complication probability factor are shown to be special cases arising from approximations applied to this model. Both require the assumption of small probabilities of complication. Power law volume corrections are shown to require a homogeneous dose distribution. The general model is tissue specific and can be used to calculate probabilities of complication for individual organs or isoprobability doses for radiation injury. The model is applicable to both homogeneous and inhomogeneous dose distributions and has been used in computer determination of optimal treatment parameters. Experimental data are presented which are consistent with the general model alone and which demonstrate the limits of applicability of previous models.
269 citations
244 citations
TL;DR: A program for planning treatments of the eye with charged particle beams that includes the three-dimensional definition of the tumor volume and of normal structures, and the provision of arbitrary viewpoints including a "beam's eye" point of view.
Abstract: We describe a program for planning treatments of the eye with charged particle beams. Many of its features apply to treatment planning programs in general. These include the three‐dimensional definition of the tumor volume and of normal structures, the possibility of delivering the treatment beam from any direction in space, the provision of arbitrary viewpoints including a ‘‘beam’s eye’’ point of view, the design and fabrication of the field shaping aperture, the specification of the necessary beam quality (range and modulation), the calculation and display of isodose contours on any plane in space and on relevant surfaces, the identification of the extent to which normal structures are included or excluded from the beam, and the simulation of the desired alignment film with which patient alignment can be verified.
215 citations
TL;DR: Coherent scatter is often ignored in diagnostic radiology because its cross section is relatively small, and because it is assumed to be indistinguishable from primary radiation, but single-scatter calculations show that coherently scattered photons diverge sufficiently from the primary ray to degrade image contrast.
Abstract: Coherent scatter is often ignored in diagnostic radiology because its cross section is relatively small, and because it is assumed to be indistinguishable from primary radiation Single-scatter calculations, however, show that coherently scattered photons diverge sufficiently from the primary ray to degrade image contrast, and that they account for a significant fraction of the total scattered energy fluence at the image receptor Grids and large air gaps are less effective in reducing coherent single scatter than incoherent and multiple scatter For radiography of the abdomen, coherent first scatter comprises 10% of total scatter and 26% of the primary fluence before a grid, and on the order of 22% and 75%, respectively, behind a grid Coherent first scatter comprises a higher fraction of the total amount of scatter for lower energy examinations such as mammography
176 citations
TL;DR: There may be a therapeutic advantage from loading tumors with iodine and treating them with low-energy photons, according to the results of this study.
Abstract: The loading of tissue with iodine can result in the enhancement of the radiation dose absorbed from low-energy x-ray or gamma ray sources. We have explored the potential of this phenomenon for radiation therapy. We have demonstrated the effect of iodine concentration and radiation quality on this dose enhancement in lymphocytes, we have calculated the effect of such enhancement on depth dose distributions in the brain, we have estimated the iodine content in two human brain tumors during computerized tomography (CT) scans, we have studied the dispersion of the iodine contrast media after direct injection into rabbit tumors, and we have demonstrated that the combination of x-ray and contrast media injection is far more effective than either agent alone in causing the regression of mouse tumors. These results suggest that there may be a therapeutic advantage from loading tumors with iodine and treating them with low-energy photons.
167 citations
TL;DR: In this article, a two-stage reconstruction procedure was proposed for reconstructing images from data acquired with a new type of gamma camera based upon an electronic method of collimating gamma radiation.
Abstract: Iterative algorithms have been investigated for reconstructing images from data acquired with a new type of gamma camera based upon an electronic method of collimating gamma radiation. The camera is composed of two detection systems which record a sequential interaction of the emitted gamma radiation. Coincident counting in accordance with Compton scattering kinematics leads to a localization of activity upon a multitude of conical surfaces throughout the object. A two-stage reconstruction procedure in which conical line projection images as seen by each position sensing element of the first detector are reconstructed in the first stage, and tomographic images are reconstructed in the second stage, has been developed. Computer simulation studies of both stages and first-stage reconstruction studies with preliminary experimental data are reported. Experimental data were obtained with one detection element of a prototype germanium detector. A microcomputer based circuit was developed to record coincident counts between the germanium detector and an uncollimated conventional scintillation camera. Point sources of Tc-99m and Cs-137 were used to perform preliminary measurements of sensitivity and point spread function characteristics of electronic collimation.
TL;DR: The peripheral dose, defined as the dose outside of therapeutic radiation beams, has been investigated for 60Co, 4-, 6-, and 10-MV x-ray machines, which allows the identification of a simple basic data set which can characterize the PD for any particular machine.
Abstract: The peripheral dose (PD), defined as the dose outside of therapeutic radiation beams, has been investigated for 60Co, 4-, 6-, and 10-MV x-ray machines. The measurements have been carried out down to dose levels of about 0.1% of the peak dose in the beam, since that dose level may be of clinical importance in some situations. The PD measurements for the various machines are qualitatively similar, which allows the identification of a simple basic data set which can characterize the PD for any particular machine. The PD has been separated into two components: in-phantom scatter dose and transmission (leakage) dose. Knowledge of the two components is important clinically when shielding is considered.
TL;DR: A new implementation of the Wiener restoration filter that uses as its optimality criterion the minimization of the mean-square error between the undistorted image of the object and the filtered image to improve the quality of digital nuclear medicine images.
Abstract: To improve the quality of digital nuclear medicine images, we have developed a new implementation of the Wiener restoration filter The Wiener filter uses as its optimality criterion the minimization of the mean‐square error between the undistorted image of the object and the filtered image In order to form this filter, the object and noise power spectrums are needed The noise power spectrum for the count‐dependent Poisson noise of nuclear medicine images is shown to have a constant average magnitude equal to the total count in the image The object power spectrum is taken to be the image power spectrum minus the total count, except in the noise dominated region of the image power spectrum where a least‐squares‐fitted exponential is used Processing time is kept to a clinically acceptable time frame through use of an array processor Pronounced noise suppression and detail enhancement are noted with use of this filter with clinical images
TL;DR: A new system has been implemented which allows rapid switching between viewing of fluorescence, and viewing of the same area under white light illumination as in conventional bronchoscopy, and together with the video analyzer should reduce indeterminate results.
Abstract: A fluorescence bronchoscope system has been developed for imaging lung tumors by fluorescence of a previously injected, tumor-specific agent hematoporphyrin derivative. Carcinoma in situ has been localized, but there are too many false positives and negatives. A new system has been implemented which allows rapid switching between viewing of fluorescence, and viewing of the same area under white light illumination as in conventional bronchoscopy. The excitation source is a violet krypton ion laser coupled to a fused quartz fiber light conductor, with a diverging microlens to spread the light uniformly. A third-generation, microchannel plate image intensifier amplifies the weak fluorescence for viewing and video display, recording, and analysis. A movable mirror and periscope bypasses the intensifier for normal color viewing and video display and recording, with the laser shutter closed and the white light shutter open. This facilitates accurate localization, comparison of the color and fluorescence images, and precise sampling during biopsy. The improved system should reduce the false positive rate due to biopsy sampling error, and together with the video analyzer should reduce indeterminate results.
TL;DR: The detector array has been improved and the scanner is demonstrated to have better than 3-mm spatial resolution and better than 1% relative electron density discrimination.
Abstract: An earlier paper [Simpson et al., Med. Phys. 9, 574 (1982)] described a computed tomography (CT) scanner that was constructed by adding a detector array to a 4-MV isocentric linear accelerator. Since the previous article, the detector array has been improved and we now demonstrate better than 3-mm spatial resolution and better than 1% relative electron density discrimination. A series of pictures from volunteer patients is included. Normal anatomy is visualized with bone, muscle, fat, and air being clearly delineated.
TL;DR: The contribution made by contaminating electrons present in a clinical photon beam to the buildup dose in a polystyrene phantom has been calculated and compared to measurements and a Monte Carlo technique was employed.
Abstract: The contribution made by contaminating electrons present in a clinical photon beam to the buildup dose in a polystyrene phantom has been calculated and compared to measurements. A Monte Carlo technique was employed. The calculation was divided into two parts. First, the accelerator treatment head was simulated in detail using the EGS--PEGS electromagnetic shower code. Then, information obtained from these calculations was used to compute dose curves in a polystyrene phantom. Two cases were considered, one in which both electrons and photons were incident upon the phantom, and another in which electrons were eliminated from the incident beam. Results of these calculations agree with recent experimental findings. A decrease in buildup dose as well as a shift in d/sub max/ was observed when electrons were eliminated from the beam.
TL;DR: A detailed Monte Carlo approach has been employed to investigate the sources of electron contamination for the 25-MV photon beam generated by Varian's Clinac-35 and found that for small SSDs, the dominant sources of contamination were the flattening filter and the beam monitor chamber which accounted for 70% of the unwanted electrons.
Abstract: A detailed Monte Carlo approach has been employed to investigate the sources of electron contamination for the 25-MV photon beam generated by Varian's Clinac-35 Three sources of contamination were examined: (a) the flattening filter and beam monitor chamber, (b) the fixed primary collimators downstream from the monitor chamber and the adjustable photon jaws, and (c) the air volume separating the treatment head from the observation point Five source-to-surface distances (SSDs) were considered for a single field size, 28 cm in diameter at 80 cm SSD It was found that for small SSDs (80--100 cm), the dominant sources of electron contamination were the flattening filter and the beam monitor chamber which accounted for 70% of the unwanted electrons Thirteen percent of the remaining electrons originated in the downstream primary collimators and the photon jaws, and 17% were produced in air At larger SSDs, the fraction of unwanted electrons originating in air increased At 400 cm SSD, 61% of the contaminating electrons present in the beam were produced in air, 34% originated in the flattening filter and beam monitor chamber, and 5% were due to interactions in the fixed collimators downstream from the monitor chamber and the adjustable photon jaws These calculatedmore » results are substantiated by recent experiments« less
TL;DR: The measured and theoretically predicted values of detective quantum efficiency (DQE) for a CsI x-ray image intensifier are compared and the mechanism for energy broadening within the XRII is shown to be predominantly the limited number of electrons and not light absorption.
Abstract: The measured and theoretically predicted values of detective quantum efficiency (DQE) for a CsI x-ray image intensifier are compared for nine monoenergetic beams of x rays. The agreement between measurement and theory of better than +/- 5% indicates that we have a sound understanding of the physical parameters controlling the DQE. It is shown that the fraction of K-fluorescent x rays escaping from the input phosphor is independent of incident energy. The number of electrons released within the x-ray image intensifier (XRII) by an incident x ray has been measured. The mechanism for energy broadening within the XRII is shown to be predominantly the limited number of electrons and not light absorption.
TL;DR: A postmortem study was conducted to assess the feasibility of measuring bone lead concentrations noninvasively in vivo, and characterized L x rays induced with an external source of 125I in the superficial tibial cortex of the intact legs of six adults who had no history of occupational exposure to lead.
Abstract: A postmortem study was conducted to assess the feasibility of measuring bone lead concentrations noninvasively in vivo. Characteristic L x rays were induced with an external source of /sup 125/I in the superficial tibial cortex of the intact legs of six adults who had no history of occupational exposure to lead. Tibial lead concentrations in the same bones subsequently determined by flameless atomic absorption spectroscopy varied from 15 to 35 ..mu..g Pb/g wet weight. The upper limit for the modern normal range of lead in the bone is about 25 mg Pb/g wet tissue. The linear correlation coefficient (r) between the measurements made with x-ray fluorescence and lead concentration by absorption spectroscopy was 0.90. Radiation doses of 10 mGy (1 rad) to 1 cm/sup 2/ of skin, with associated doses to the marrow of adjacent bone of about 0.6 mGy (60 mrad), yielded net lead fluorescence signals ranging from one to seven times the standard deviation of background.
TL;DR: The apparatus is capable of studying two scanning geometries that uses either a fan of radiation swept laterally over the film in 5s or a spot of radiation scanned over two dimensions in a raster pattern in 8.8s, which offers good scatter rejection and regional manipulation of film exposure.
Abstract: An experimental scanning apparatus for chest radiography is described which offers good scatter rejection and regional manipulation of film exposure. The apparatus is capable of studying two scanning geometries that uses either a fan of radiation swept laterally over the film in 5 s or a spot of radiation scanned over two dimensions in a raster pattern in 8.8 s. The manipulation of tube output during the scan is achieved by pulse width modulation with film exposure monitored by a fluorescencedetector placed behind the film cassette. Measurements of the scatter rejection properties of the system show that it is superior to that of a 10:1 grid when used with 120‐kVp radiation. Phantom images with both geometries demonstrate the capability of the system to improve uniformity of film exposure and soft tissue contrast throughout the mediastinal, diaphragmatic, and retrocardiac areas. The relative characteristics of the one‐ versus two‐dimension scan geometries are given. Clinical raster images formed with the 8.8‐s scan time show signs of motion unsharpness which becomes insignificant when the scan time is reduced to 4.5 s.
TL;DR: This paper describes experimental measurements of the dose within and near large, low-density inhomogeneities simulating lungs and also behind air channels (where similar effects occur).
Abstract: Methods of correcting for tissue inhomogeneities which consider only changes in the photon fluence are of limited usefulness for 10-MV x rays. Although there is normally transient electronic equilibrium on the central axis beyond the depth of maximum buildup in soft tissue, when the beam enters a low-density material the beam profile is degraded and there is a loss of lateral electron equilibrium which reduces the dose both within and beyond the inhomogeneity. This paper describes experimental measurements of the dose within and near large, low-density inhomogeneities simulating lungs and also behind air channels (where similar effects occur). Methods of calculating correction factors are discussed.
TL;DR: The method has been applied in determining the resolution of an EMI-7070 scanner and it is shown that the assumption of uniformity of the system resolution across the CT image is fulfilled within the accuracy of the present method.
Abstract: A new approach to the measurement of the spatial resolution of a computed tomography (CT) scanner system is presented. The method is based on a direct least-squares fit of an analytical expression to a set of data obtained from a CT image of the interface between two materials. The implementation of the method in connection with the G. E. RTPLAN computer configuration is described. The method has been applied in determining the resolution of an EMI-7070 scanner and it is shown that the assumption of uniformity of the system resolution across the CT image is fulfilled within the accuracy of the present method. The reproducibility of the method has been estimated from a series of spatial resolution determinations performed on ten images taken with identical scan parameters. The standard deviation of this series was 3.2%.
TL;DR: Using a Monte Carlo computer technique, tables of parameters have been derived which are of use in the dosimetry of brachytherapy nuclides in a variety of tissues and organs and relationships linking the composition of a tissue with the dose received at any location around an implanted source are derived.
Abstract: Using a Monte Carlo computer technique, tables of parameters have been derived which are of use in the dosimetry of brachytherapy nuclides in a variety of tissues and organs. From the results, it has been possible to derive relationships linking the composition of a tissue with the dose received at any location around an implanted source. Other factors, such as the relative importance of scattered radiation, spectral degradation, and integral dose, are also discussed. In particular, attention is drawn to possible dosimetric problems arising from the clinical use of 1 2 5I.
TL;DR: Theoretical predictions and experimental results indicate that the temperature rise during the procedure may give rise tohyperthermal cell kill, and the extent of the regions with hyperthermal bioeffects in terms of tissue parameters as optical absorption and scattering, thermal conductivity, specific heat, blood flow, and optical dose parameters as Optical power and exposure time.
Abstract: This report discusses the optical and thermal distribution during photoradiation therapy of malignant tumors. Emphasis is put on the therapeutic procedure with the light dose delivered through an inserted optical fiber. Theoretical predictions and experimental results indicate that the temperature rise during the procedure may give rise to hyperthermal cell kill. The report discusses the extent of the regions with hyperthermal bioeffects in terms of tissue parameters as optical absorption and scattering,thermal conductivity,specific heat,blood flow, and optical dose parameters as optical power and exposure time.
TL;DR: This work has developed an approximate method that satisfies both constraints and treats much of the synergistic effect of multiple inhomogeneities correctly and agrees very well with measurements.
Abstract: The effects of small cavities on dose in water and the dose in a homogeneous nonunit density medium illustrate that inhomogeneities do not act independently in photon dose perturbation, and serve as two constraints which should be satisfied by approximate methods of computed tomography (CT) pixel-based dose calculations. Current methods at best satisfy only one of the two constraints and show inadequacies in some intermediate geometries. We have developed an approximate method that satisfies both these constraints and treats much of the synergistic effect of multiple inhomogeneities correctly. The method calculates primary and first-scatter doses by first-order ray tracing with the first-scatter contribution augmented by a component of second scatter that behaves like first scatter. Multiple-scatter dose perturbation values extracted from small cavity experiments are used in a function which approximates the small residual multiple-scatter dose. For a wide range of geometries tested, our method agrees very well with measurements. The average deviation is less than 2% with a maximum of 3%. In comparison, calculations based on existing methods can have errors larger than 10%.
TL;DR: The rotational total skin electron irradiation significantly reduces the patient treatment and setup time and solves the problem of beam matching, when compared to standard multiple-beam techniques.
Abstract: A technique for rotational total skin electron irradiation is presented in which the patient stands on a slowly rotating platform (SSD = 285 cm) in a large uniform linear accelerator electron field (Eo = 3.5 MeV). The beam is scattered by the transmission ionization chamber and by a special lead/aluminum scattering filter, and then degraded by a sheet of Lucite. A Farmer chamber is used as a patient dose monitor and a method for absolute dose calibration is presented. The field is uniform to within +/- 5% for dimensions of 180 X 40 cm2. The surface dose for rotational therapy is equal to 45% of the maximum dose in a stationary beam. The rotating beam exhibits a dose maximum on the surface, falls to 80% at 0.5 cm and has an x-ray contamination of approximately 4%. The surface dose rate is about 25 cGy/min for the rotating beam. The rotational beam percentage depth dose distributions, calculated using stationary beam information, agree well with measured data. The stationary beam exhibits a dose maximum at 4 mm in tissue, a surface dose of 93%, 80% dose at a depth of 1 cm, a practical range of 1.75 cm, and an x-ray contamination of 2.5%. The rotational total skin electron irradiation significantly reduces the patient treatment and setup time and solves the problem of beam matching, when compared to standard multiple-beam techniques.
TL;DR: It is shown that such a system operates as a spatial filter which attenuates film contrast for structures of spatial frequency less than the inverse scanning beam width and by manipulating the software parameters of the feedback network, it is possible to alter this filter and produce radiographs with low spatial frequency enhancement, attenuation, or contrast inversion.
Abstract: Conventional chest radiography is limited by the presence of scattered radiation and the small useful exposure range of radiographicfilm. A computer‐assisted scanning system to minimize these two effects is outlined. The system uses a small beam of radiation swept over the patient’s chest in a raster pattern to expose a conventional film cassette, while a slit collimator scanning between the patient and the film serves to reject scatteredphotons. A microcomputer measures beam attenuation by the patient with a detector placed behind the film which in turn automatically adjusts the x‐ray tube output to minimize excursions in film exposure as the beam scans. A formalism which relates the patient transmission and film exposure distribution is developed and a system transfer function is given. It is shown that such a system operates as a spatial filter which attenuates film contrast for structures of spatial frequency less than the inverse scanning beam width. By manipulating the software parameters of the feedback network, it is possible to alter this filter and produce radiographs with low spatial frequency enhancement, attenuation, or contrast inversion.
TL;DR: A technique for the automatic reconstruction of the spatial coordinates of seeds in an interstitial implant has been developed that leads to an accurate and consistent identification and reconstruction of seeds with 90% of them within 2 mm of their actual location.
Abstract: A technique for the automatic reconstruction of the spatial coordinates of seeds in an interstitial implant has been developed. Seed coordinates from three isocentric radiographs, an anterior--posterior film, and an orthogonal pair of films taken at gantry angles of +- 45/sup 0/ from the anterior--posterior direction are digitized and recorded in a random sequence. From this, the three-dimensional coordinates of the seeds in the implant are computed by matching the values of common coordinates along the axis of rotation. This rotate and match technique leads to an accurate and consistent identification and reconstruction of seeds with 90% of them within 2 mm of their actual location. These new computer routines have been added to an existing treatment planning system (AECL TP-11 treatment planning system, version 5A). We present our algorithm and technique along with clinical examples.
TL;DR: A computer program has been written to reconstruct the three-dimensional distribution of radioactive seed implants, which requires the use of three rotated films with random input of the seeds.
Abstract: A computer program has been written to reconstruct the three-dimensional distribution of radioactive seed implants. The technique requires the use of three rotated films with random input of the seeds. This algorithm differs from previous ones in the constraints that are applied, since no correlation is made between distances on the three films from the plane of rotation through the isocenter nor is there a correlation between the distances from the axis of rotation. Instead, projections from the x-ray source to the seed images on each of the three films are compared to see which combinations of seeds intersect at a point and therefore correspond to real seeds. This program has been written in Fortran IV for the PDP VAX 11/780 and 11/34 computers. Changes have been made to the TP11 program "QSEED" on the PDP 11/34 to accommodate the coordinate data from this program to produce the requisite isodose curves.
TL;DR: The scattered spectra show large changes as a function of angle, in some cases actually becoming harder than the exit unscattered beam, and the significance of this behavior relative to the imaging of targets in mammography examinations is discussed.
Abstract: We have studied the angular distribution of scattered radiation at low energies for angles between 2° and 15° from the outward normal to the exit surface of several phantoms, with 1° resolution. A cryogenically cooled germanium detector was used to measure the spectra of the scattered radiation. The differential scattering fluences, or numbers of photons per unit solid angle per unit surface area, exhibit distinct peaks at angles in the vicinity of 5°, with the angular position being only slightly energy dependent but very material dependent. The scattered spectra show large changes as a function of angle, in some cases actually becoming harder than the exit unscattered beam. The significance of this behavior relative to the imaging of targets in mammography examinations is discussed.