bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements and calculations of the absorbed dose distribution around a 60Co source
Authors: Tiourina T. B. , Dries W. J. F. , van der Linden P. M.
Published: March 2003

The data from Meisberger et al. [Radiology 90, 953–957 (1968)] are often used as a basis for dose calculations in brachytherapy. In order to describe the absorbed dose in water around a brachytherapy point source, Meisberger provided a polynomial fit for different isotopes taking into account the effect of attenuation and scattering. The validity of the Meisberger coefficients is restricted to distances up to 10 cm from the source, which is regarded to be satisfactory for most brachytherapy applications. However, for more distant organs it may lead to errors in calculated absorbed dose. For this reason dose measurements have been performed in air and in water around a high activity 60Co source used in high dose rate brachytherapy. Measurements were carried out to distances of 20 cm, using ionization chambers. These data show that at a distance of about 15 cm the amount of scattered radiation virtually equals the amount of primary radiation. This emphasizes the contribution of scattered radiation to the dose in healthy tissue far from the target volume, even with relatively high energy photon radiation of 60Co. It is also shown that the Meisberger data as well as the approach of Van Kleffens and Star [Int. J. Radiat. Oncol. Phys. 5, 557–563 (1979)] lead to significant errors in absorbed dose between distances of 10 and 20 cm from the source. In addition to these measurements, the Monte Carlo code has been used to calculate separately primary dose and scattered dose from a cobalt point source. The calculated results agree with the experimental data within 1% for a most distant dose scoring region.
A study of the effect of cone shielding in intraoperative radiotherapy
Authors: Papanikolaou Nikos , Paliwal Bhudatt
Published: March 2003

The primary goal of intraoperative radiation therapy is to irradiate the intraoperatively determined tumor target volume with a single fraction of tumoroidal dose while minimizing the dose to all adjacent healthy tissues. To reduce dose outside the treatment volume, lead sheets are often used to cover the external surface of the cone tip thus providing a shielding for the tissues outside the field. In this paper, the effect of the shielding on the depth dose distributions and dose profiles at different depths is studied based on experimental data. The results were also compared against an EGS4 Monte Carlo code for the same geometry as the measurements. The cones varied in size having diameters of 5 cm, 7 cm, and 9 cm, and the electron energies ranged from 6 MeV to 22 MeV. The depth dose curves and dose profiles (at two different depths in the phantom) were measured and computed with and without the lead shielding for the various combinations of cone sizes and electron energies using a water phantom to simulate the patient. It was found that the presence of lead increases on average across the treatment area the dose to the tumor from 2% up to 5%, while the dose outside the cone was reduced by as much as 75%. Both measurements and calculations were found to be in agreement.
The influence of heavy water on boron requirements for neutron capture therapy
Authors: Wallace S. A. , Mathur J. N. , Allen B. J.
Published: March 2003

Neutron penetration in tissue is a major limitation of thermal NCT, as such much work has centered upon the epithermal neutron beam in an effort to improve this situation. Further gains in neutron flux penetration, and thus therapeutic ratios, are possible if natural water is replaced with heavy water prior to therapy. Applying MCNP to a heterogeneous ellipsoidal skull/brain model, advantage depth and therapeutic depth parameters are studied as a function of heavy water replacement for a range of tumor to blood boron ratios. Both thermal (0.025 eV) and epithermal (2–7 keV) ideal neutron beams are analyzed. Using 10B ratios in the range of documented human uptake, the thermal advantage depth improved by approximately 0.7 cm for 20% D2O replacement, however, the therapeutic depth increased by less than half this value. For the epithermal beam, both the advantage depth and the therapeutic depth increased by over 1 cm. Effects of heavy water replacement on 10B requirements to therapeutically treat the midline of the brain are also evaluated.
A statistically tailored neural network approach to tomographic image reconstruction
Authors: Kerr John P. , Bartlett Eric B.
Published: March 2003

In previous work it has been shown that a standard backpropagation neural network can be trained to reconstruct sections of single photon emission computed tomography (SPECT) images based on the planar image projections as inputs. In this study, it is demonstrated that an artificial neural network (ANN) trained on a series of simulated SPECT images or trained on a set of rudimentary geometric images can learn the planar data‐to‐tomographic image relationship for 64×64 tomograms. As a result, a properly trained ANN can produce accurate, novel image reconstructions but without the high computational cost inherent in some traditional reconstruction techniques. We also present a method of deriving activation functions for a backpropagation ANN that make it readily trainable for cardiac SPECT image reconstruction. The activation functions are derived from the estimated probability density functions (p.d.f.s) of the ANN training set data. The performance of the statistically tailored ANNs are compared with the performance of standard sigmoidal backpropagation ANNs, both in terms of their trainability and generalization ability. The results presented demonstrate that statistically tailored ANNs are significantly better than standard sigmoidal ANNs at reconstructing novel tomographic images based on a simulated SPECT image training set or a rudimentary geometric image training set. Neural network based image reconstruction has two potential advantages over conventional reconstruction methods. The first advantage is that ANNs can rapidly reconstruction tomograms. Secondly, the quality of the reconstructions produced are directly correlated to the quality of the images used to train the ANN.
Beam characteristics of a new generation 50 MeV racetrack microtron
Authors: Masterson M. E. , Chui C. S. , Febo R. , Hung J. D. , Fuks Z. , Mohan R. , Ling C. C. , Kutcher G. J. , Bjork S. , Enstrom J.
Published: March 2003

The first of a new generation of microtron accelerators has been installed and tested. It is currently in use for multisegment conformal radiotherapy at our institution. The unit produces x rays and electrons from 10 to 50 MeV in 5 MeV increments. It incorporates a 64 leaf, doubly focused multileaf collimator (MLC), which can be used to shape x‐ray and electron beams. Both x‐ray and electron beams are produced by magnetically scanning the electron beams from the accelerator. The new generation unit incorporates a purging magnet to sweep away any primary or secondary electrons that pass through the target(s). In this paper, the beam characteristics of the accelerator that were studied during acceptance testing are described. Representative examples of depth doses, beam profiles, output factors, and elementary beam distributions are presented and discussed, in comparison with the earlier generation of microtron accelerators and with other radiotherapy machines.
X‐ray source and the output factor
Authors: Zhu Timothy C. , Bjärngard Bengt E. , Shackford Hobart
Published: March 2003

When the collimator setting of a linear accelerator is made sufficiently small, the output factor in air, R, is greatly reduced because the collimators obstruct the periphery of the x‐ray source. This has been utilized to examine the size of the source by varying the width y of a narrow field and determining how R(y) varies. The source diameters in the two principal directions were clearly influenced by the design of the accelerators. The x‐ray sources of two accelerators with bending magnets were found to be noncircular while that of a linear accelerator without a magnet showed circular symmetry. The position of the source relative to the axis of collimator rotation was determined by measuring R for offset narrow fields. For one of the accelerators, the source was initially moving and off the central axis by about 2 mm for the first five monitor units. The results correlated well with sharpness in portal–film images. The technique can serve to evaluate the major source characteristics in acceptance testing and quality control.
Beam hardening errors in post‐processing dual energy quantitative computed tomography
Authors: Goodsitt Mitchell M.
Published: March 2003

A computer simulation study was performed to assess the errors due to x‐ray beam hardening in the fat and bone estimates of a post‐processing dual‐energy quantitative computed tomography technique. The “central” calibration method was employed in which calibration standards are inserted within a torso phantom of a size similar to that of the “patient.” Although beam hardening errors are reduced with this method, they still occur as a result of mismatches between the torso phantom and patient body sizes. Two mismatch situations were investigated. In one, a single torso phantom was used for all subject sizes (i.e., one‐size‐fits‐all). In the other, closest matches were made from a set of three different sized torso phantoms (small, medium, and large). In all cases, the compositions of the calibration standards that were inserted into the torso phantoms consisted of bone, fat (glycerol trioleate), and an average fat‐free red marrow. Fifteen patient sizes were simulated ranging from 20 to 34 cm in diameter. There were 21 patients of each size. The vertebrae in these subjects contained known amounts of bone mixed in marrows of composition determined from chemical analyses of cadaver marrow samples. Vertebrae consisting of mixtures of the calibration standard materials were also studied. The computed effective x‐ray beam energies at the vertebra location for the various subject sizes ranged from 54.3 to 56.4 keV at 80 kVp and from 74.4 to 78.8 keV at 140 kVp. Maximum root mean square errors due to beam hardening were 1.2 mg/ml bone and 0.007 mass fraction fat for one‐size fits‐all central calibration, and 0.31 mg/ml bone and 0.002 mass fraction fat for closest match central calibration. These errors are two to four times smaller than the maximum beam hardening errors obtained when the calibration standards and vertebrae are made of identical materials. This study indicates that for a realistic situation in which the compositions of the calibration standards and patient's marrow differ to some extent, the contribution of beam hardening to the overall error in fat and bone estimates is small. Furthermore, these errors can be minimized through the use of a central calibration technique in which a closest match to the patient's body size is made from a set of three different sized torso phantoms.
Registration error quantification of a surface‐based multimodality image fusion system
Authors: Hemler Paul F. , Napel Sandy , Sumanaweera Thilaka S. , Pichumani Ramani , van den Elsen Petra A. , Martin Dave , Drace John , Adler John R. , Perkash Inder
Published: March 2003

This paper presents a new reference data set and associated quantification methodology to asses the accuracy of registration of computerized tomography (CT) and magnetic‐resonance (MR) images. Also described is a new semiautomatic surface‐based system for registering and visualizing CT and MR images. The registration error of the system was determined using a reference data set that was obtained from a cadaver in which rigid fiducial tubes were inserted prior to imaging. Registration error was measured as the distance between an analytic expression for each fiducial tube in one image set and transformed samples of the corresponding tube obtained from the other. Registration was accomplished by first identifying surfaces of similar anatomic structures in each image set. A transformation that best registered these structures was determined using a nonlinear optimization procedure. Even though the root‐mean‐square (rms) distance at the registered surfaces was similar to that reported by other groups, it was found that rms distances for the tubes were significantly larger than the final rms distances between the registered surfaces. It was also found that minimizing rms distance at the surface did not minimize rms distance for the tubes.
Digital portal image registration by sequential anatomical matchpoint and image correlations for real‐time continuous field alignment verification
Authors: McParland Brian J. , Kumaradas J. Carl
Published: March 2003

Detection of radiotherapy field misalignments with electronic portal imaging devices requires the precise initial registration of the digital portal image with a reference image indicating the prescribed field alignment. Moreover, for real‐time continuous detection this registration must be performed rapidly—arguably within 250 ms. The quality of this registration is sensitive to the ability of the user to accurately identify corresponding anatomical landmarks in the image pair. To improve the accuracy of the registration and, ultimately, that of the field misalignment measurement, we have developed a sequential digital portal image registration method using both user‐identified anatomical matchpoints and image information. A first pass generates registration parameters from user‐provided matchpoint coordinates and explicitly accounts for the uncertainty in matchpoint identification. The second pass uses both the initial registration parameters and image information to further improve the registration quality by maximizing cross correlations between segments of the image pair. As this registration method does not use massive matrix/vector computations common to other algorithms, it is inherently faster and well‐suited for real‐time field placement error detection. On a platform representative of those controlling many commercial electronic portal imaging devices (486 CPU), this algorithm registers portal images in times of less than 6 ms per matchpoint with errors of less than 2% in magnification, 0.5° in in‐plane rotation, and less than 1 pixel dimension in in‐plane translation. As the algorithm assumes a rigid‐body geometry, it is sensitive to out‐of‐plane rotations. A quantitative analysis of this algorithm is presented, indicates its accuracy, and describes its sensitivity to out‐of‐plane rotations.
Monte Carlo studies of x‐ray energy absorption and quantum noise in megavoltage transmission radiography
Authors: Jaffray D. A. , Battista J. J. , Fenster A. , Munro P.
Published: March 2003

The subject contrast of bony anatomy in megavoltage medical radiographs is very low, making detection of bony landmarks difficult if additional noise sources are introduced into the images. One source of noise, which is inherent to the x‐ray detection process, is x‐ray energy absorption noise. X‐ray energy absorption noise results from variations in the amount of energy deposited in the imaging detector per interacting x ray. These variations increase the noise content of the image. In this study, EGS4 Monte Carlo simulations of x‐ray interactions in metal plate phosphor screen detectors have been performed to determine the distribution of energy absorption events within the phosphor screen. From these “absorbed energy distributions (AEDs)”, the x‐ray energy absorption noise and the quantum absorption efficiency of the detector are determined. These calculations are performed for a range of detector thicknesses (0.1–4 mm) and x‐ray energies (0.1–10 MeV). A number of conclusions can be drawn from these investigations. (i) The x‐ray absorption noise reduces the detective quantum efficiency (DQE) of metal plate/phosphor screen detectors by as much as 50% at energies used in megavoltage imaging (1–10 MeV). (ii) It is important to include secondary particle (electron) transport in estimating the quantum absorption efficiency of these detectors. For instance, the quantum efficiency of a typical portal detector is approximately 2%, even though 4%–5% of the incident photons are attenuated. (iii) The metal “conversion” plate commonly used in megavoltage imaging enhances the DQE of the phosphor screen by increasing the quantum absorption efficiency and reducing the magnitude of the x‐ray absorption noise.
Dose distributions and dose rate constants for new ytterbium‐169 brachytherapy seeds
Authors: MacPherson M. S. , Battista J. J.
Published: March 2003

Relative dose distributions in the vicinities of two new prototypes of ytterbium‐169 brachytherapy sources have been measured using lithium fluoride thermoluminescent dosimeters (TLD) placed in a solid water phantom. The type 6 seed consists of four Yb2O3 spheres contained in a 0.075‐mm thick titanium tube, with overall dimensions of 4.5 mm in length and 0.8 mm in diameter. The type 8 seed contains a cylindrical segment of Yb wire, 2.5 mm long, sealed in a 0.075‐mm thick titanium tube 4 mm in length and 0.5 mm in diameter. The relative dose distributions, measured with TLD, were compared with those predicted by Monte Carlo simulations (MCNP code). Experimental and theoretical dose distributions were generally in agreement within 5% of the local dose value. Source strength was determined from activity measurements using a high purity germanium (HPGe) spectrometer. With source strengths established, thermoluminescence was then used to measure the dose rate constant, Λ0, for the type 8 seed. The measured Λ0 of 1.34±0.10 cGy U−1 for the type 8 seed agrees, within statistical uncertainty, with the value of 1.25±0.05 cGy U−1 predicted through Monte Carlo simulation. Comparisons are made with experimental data reported by other investigators.
Investigations of large vessel cooling during interstitial laser heating
Authors: Whelan W. M. , Wyman D. R. , Wilson B. C.
Published: March 2003

Interstitial laser heating of tissues is influenced by blood flow in the treatment region. Temperature gradients around large blood vessels may result in local underheating of tissues. A three‐dimensional, time‐dependent finite difference model of interstitial laser heating around large vessels is presented. A thermal conduction model was developed using a transport theory approximation for the energy distribution from an optical line source. Calculated transient temperature profiles and temperature reductions around 0.144 and 0.400 cm diam vessels show qualitative agreement with those measured in a series of tissue phantom studies. Experiments and calculations for a large vessel located ∼1.0 cm from the optical source indicate that temperature reductions are less than 1 °C at distances greater than ∼1.0 cm from the vessel surface. The model also indicates that significant reductions in the extent of a thermal coagulation boundary can occur if a large vessel is situated inside the normal coagulation zone.
Control of the necrosed tissue volume during noninvasive ultrasound surgery using a 16‐element phased array
Authors: Fan Xiaobing , Hynynen Kullervo
Published: March 2003

Focused high‐power ultrasound beams are well suited for noninvasive local destruction of deep target volumes. In order to avoid cavitation and to utilize only thermal tissue damage, high frequencies (1–5 MHz) are used in ultrasonic surgery. However, the focal spots generated by sharply focused transducers become so small that only small tumors can be treated in a reasonable time. Phased array ultrasound transducers can be employed to electronically scan a focal spot or to produce multiple foci in the desired region to increase the treated volume. In this article, theoretical and experimental studies of spherically curved square‐element phased arrays for use in ultrasonic surgery were performed. The simulation results were compared with experimental results from a 16‐element array. It was shown that the phased array could control the necrosed tissue volume by using closely spaced multiple foci. The phased array can also be used to enlarge a necrosed tissue volume in only one direction at a time, i.e., lateral or longitudinal. The spherically curved 16 square‐element phased array can produce useful results by varying the phase and amplitude setting. Four focal points can be easily generated with a distance of two or four wavelengths between the two closest peaks. The maximum necrosed tissue volume generated by the array can be up to sixteen times the volume induced by a similar spherical transducer. Therefore the treatment time could be reduced compared with single transducer treatment.
Magnetic resonance imaging k‐space segmentation using phase‐encoding groups: The accuracy of quantitative measurements of pulsatile flow
Authors: Li Hongfang , Clarke Geoffrey D. , NessAiver Moriel , Liu Haiying , Peshock Ronald
Published: March 2003

The use of phase‐encode grouping (PEG) allows acquisition of a complete cardiac cine in a single breath hold, eliminating respiratory artifacts and improving edge definition. One approach to quantitative magnetic resonance (MR) flow measurements in pulmonary, coronary, and renal arteries uses phase velocity mapping with a PEG, single breath‐hold, cine technique. Phantom studies were designed to evaluate the accuracy of MR phase velocity mapping (MRV) with PEG in continuous and pulsatile flow measurements in small vessels. Experiments were performed on a 1.5‐T MRI system and results were compared with those obtained by timed volume measurements and a transit‐time ultrasound flowmeter. Flow velocity, pulsatile flow wave form, pulsing rate, and PEG size were varied. For continuous flow, MRV measured velocities correlated well with those calculated from timed volume measurements (r>0.998) under mean spatial velocities ranging from 20–100 cm s−1. The temporal mean velocity difference between MRV and ultrasound flowmeter measurements was less than ±2.9 cm s−1 under pulsatile flow conditions with PEG sizes of 5 or less, however, the mean velocity difference was >7 cm s−1 for PEG sizes of 8 or more. Accurate MR velocity measurement with PEG in small vessels under conditions of pulsatile flow is possible if an appropriate PEG size is selected.
Quantitation of SPECT performance: Report of Task Group 4, Nuclear Medicine Committee
Authors: Graham L. Stephen , Fahey Frederic H. , Madsen Mark T. , van Aswegen Andries , Yester Michael V.
Published: March 2003

A comprehensive performance testing program is an essential ingredient of high‐quality single‐photon emission computed tomography (SPECT). Many of the procedures previously published are complicated, time consuming, or require a special testing environment. This Task Group developed a protocol for evaluating SPECT imaging systems that was simple, practical, required minimal test equipment, and could be performed in a few hours using processing software available on all nuclear medicine computers. It was designed to test rotational stability of uniformity and sensitivity, tomographic spatial resolution, uniformity and contrast, and the accuracy of attenuation correction. It can be performed in less than three hours and requires only a Co‐57 flood source, a line source, and a tomographic cylindrical phantom. The protocol was used 51 times on 42 different cameras (seven vendors) by four different individuals. The results were used to establish acceptable ranges for the measured parameters. The variation between vendors was relatively small and appeared to reflect slight differences in basic camera performance, collimation, and reconstruction software. Individuals can use the tabulated values to evaluate the performance of individual systems.
Optimization of pencil beam widths for electron‐beam dose calculations
Authors: Antolak John A. , Mah Ernest , Scrimger John W.
Published: March 2003

The pencil beam method of calculating dose distributions for electron‐beam radiotherapy has been very useful, however, several limitations in the approach have been recognized. One such limitation is the lack of a mechanism to model range straggling of electrons. For stationary electron‐beam calculations, range straggling is incorporated incompletely in the planar‐fluence‐to‐dose conversion factor, which uses measured percentage depth dose curves to force the calculated percentage depth dose to reproduce the measurement. When calculating the dose distribution for an arced beam using a pencil beam algorithm, insufficient modeling of the pencil beams leads to larger errors than when using a stationary beam algorithm. The calculated depth of maximum dose is systematically overestimated by the pencil beam calculations. We will show that the lack of a way to account for range straggling in the arc‐electron pencil beam calculation is primarily responsible for this discrepancy. Methods of incorporating range straggling into the electron pencil beam dose calculation have been presented before, but no data have been shown to support their use for heterogeneous phantoms (patients). This paper presents a similar range‐straggling modification, as well as data to show that this model can predict pencil beam width to within 20% for heterogeneous slab phantoms. For stationary electron‐beam calculations, the calculated isodose lines follows the measured isodose lines to within 1 mm down to the 10% dose level. Incorporating this modification into the arc‐electron pencil beam calculation improves the agreement between the depths of the calculated and measured maximum doses to within 2 mm; however, the magnitude of the maximum dose is as much as 3%–4% lower for the calculated arc‐electron beams. This study discusses reasons for these discrepancies and presents possible improvements.
How water equivalent are water‐equivalent solid materials for output calibration of photon and electron beams?
Authors: Tello V. M. , Tailor R. C. , Hanson W. F.
Published: March 2003

The water equivalency of five “water‐equivalent” solid phantom materials was evaluated in terms of output calibration and energy characterization over a range of energies for both photon (Co‐60 to 24 MV) and electron (6–20 MeV) beams. Evaluations compared absorbed doses calculated from ionization measurements using the same dosimeter in the solid phantom materials and in natural water (H2O). Ionization measurements were taken at various calibration depths. The Radiological Physics Center's standard dosimetry system, a Farmer‐type ion chamber in a water phantom, was used. Complying with the TG‐21 calibration protocol, absorbed doses were calculated using eight measurement and calculational techniques for photons and five for electrons. Results of repeat measurements taken over a period of 2 1/2 years were reproducible to within a ±0.3% spread. Results showed that various combinations of measurement techniques and solid phantom materials caused a spread of 3%–4% in the calculation of dose relative to the dose determined from measurements in water for all beam energies on both modalities. An energy dependence of the dose ratios was observed for both photons and electrons.
Noise reduction by frame averaging: A numerical simulation for portal imaging systems
Authors: Swindell W. , Mosleh‐Shirazi M. A.
Published: March 2003

We have studied the usefulness of both pre‐ and post‐ADC frame summing for the purpose of reducing the effect of quantum noise and digitization noise in portal imaging systems. The study is based on the fluorescent‐screen video‐camera type of system. The study predicts the not‐surprising result that provided the noise level at the ADC input is sufficiently large, the overall SNR can be increased by a factor of , where M1 and M2 are the number of frames summed before and after the ADC. The study also predicts, somewhat unexpectedly, that there is an operating region in which increasing M1 actually decreases the SNR in the final image. To avoid this region M1 must be less than approximately 6×22B (1+δ̄−1)1/2/(iaccf), where B is the number of ADC bits, δ̄ is the mean number of optical photons detected by the video camera per detected x‐ray photon, iacc is the open‐field number of detected x‐ray photons per accelerator pulse per pixel, and f is the patient transmission factor. An equivalent statement is that the rms noise at the input to the ADC, σin, must exceed approximately 0.4q where q is the quantization interval of the ADC. It is possible that some systems operate in or close to this region. A second feature of this anomalous behavior is that the final image is not necessarily improved by increasing the number M2 of post‐ADC‐summed frames. For example, when σin/q=0.2, there is no improvement in the overall rms error for M2>32. It is also shown that the standard deviation of the final image is not a suitable indicator of output image quality and that its use can give rise to meaningless results.
Analysis of spiculation in the computerized classification of mammographic masses
Authors: Huo Zhimin , Giger Maryellen L. , Vyborny Carl J. , Bick Ulrich , Lu Ping , Wolverton Dulcy E. , Schmidt Robert A.
Published: March 2003

Spiculation is a primary sign of malignancy for masses detected by mammography. In this study, we developed a technique that analyzes patterns and quantifies the degree of spiculation present. Our current approach involves (1) automatic lesion extraction using region growing and (2) feature extraction using radial edge‐gradient analysis. Two spiculation measures are obtained from an analysis of radial edge gradients. These measures are evaluated in four different neighborhoods about the extracted mammographic mass. The performance of each of the two measures of spiculation was tested on a database of 95 mammographic masses using ROC analysis that evaluates their individual ability to determine the likelihood of malignancy of a mass. The dependence of the performance of these measures on the choice of neighborhood was analyzed. We have found that it is only necessary to accurately extract an approximate outline of a mass lesion for the purposes of this analysis since the choice of a neighborhood that accommodates the thin spicules at the margin allows for the assessment of margin spiculation with the radial edge‐gradient analysis technique. The two measures performed at their highest level when the surrounding periphery of the extracted region is used for feature extraction, yielding Az values of 0.83 and 0.85, respectively, for the determination of malignancy. These are similar to that achieved when a radiologist's ratings of spiculation (Az=0.85) are used alone. The maximum value of one of the two spiculation measures (FWHM) from the four neighborhoods yielded an Az of 0.88 in the classification of mammographic mass lesions.
DQE(f) of four generations of computed radiography acquisition devices
Authors: Dobbins James T. , Ergun David L. , Rutz Lois , Hinshaw Dean A. , Blume Hartwig , Clark Dwayne C.
Published: March 2003

Measurements were made of the MTF(f), NPS(f), and DQE(f) of four generations of computed radiography (CR) imaging plates and three generations of CR image readers. The MTF generally showed only a minor change between generations of plates and readers, but the DQE(f) has improved substantially from a very early plate/reader combination to a more recent one. The DQE in the more recent plate/reader combination is 1.3× greater at low frequencies and about 3× greater at high frequencies than the much earlier versions. Thus there has been substantial improvement in the imaging performance obtainable with CR since some of the early observer studies which indicated poorer performance with CR than with screen–film.
X‐ray imaging using amorphous selenium: Feasibility of a flat panel self‐scanned detector for digital radiology
Authors: Zhao Wei , Rowlands J. A.
Published: March 2003

We investigate a concept for making a large area, flat‐panel detector for digital radiology. It employs an x‐ray sensitive photoconductor to convert incident x‐radiation to a charge image which is then electronically read out with a large area integrated circuit. The large area integrated circuit, also called an active matrix, consists of a two‐dimensional array of thin film transistors (TFTs). The potential advantages of the flat‐panel detector for digital radiography include: instantaneous digital radiographs without operator intervention; compact size approaching that of a screen‐film cassette and thus compatibility with existing x‐ray equipment; high quantum efficiency combined with high resolution. Its potential advantages over the x‐ray image intensifier (XRII)/video systems for fluoroscopy include: compactness; geometric accuracy; high resolution, and absence of veiling glare. The feasibility of the detector for digital radiology was investigated using the properties of a particular photoconductor (amorphous selenium) and active matrix array (with cadmium selenide TFTs). The results showed that it can potentially satisfy the detector design requirements for radiography (e.g., chest radiography and mammography). For fluoroscopy, the images can be obtained in real‐time but the detector is not quantum noise limited below the mean exposure rate typically used in fluoroscopy. Possible improvements in x‐ray sensitivity and noise performance for the application in fluoroscopy are discussed.
Radiation protection requirements for medical x‐ray film
Authors: Suleiman Orhan H. , Conway Burton J. , Fewell Thomas R. , Slayton Robert J. , Rueter Fred G. , Gray Joel
Published: March 2003

Previous darkroom shielding requirements for medical x‐ray film assumed that the film should not be exposed to diagnostic x‐ray radiation levels greater than 2 μGy (0.2 mR) for the life of the film. Modern medical x‐ray films are much less sensitive to ionizing radiation, with most films showing at least an order of magnitude less sensitivity than previously assumed. Conversely, these same films when loaded in cassettes using modern intensifying screens exhibit an order of magnitude greater sensitivity when these cassettes are exposed to ionizing radiation. These data suggest that protection of modern medical x‐ray film, stored in a darkroom, may require less shielding than previously assumed. Conversely, film loaded in a cassette will require greater shielding.
Determination of the thermal neutron flux in a fast neutron beam by use of a boron‐coated ionization chamber
Authors: Lüdemann L. , Matzen T. , Matzke M. , Schmidt R. , Scobel W.
Published: March 2003

The thermal neutron distribution in slow and fast neutron beams is usually determined using the foil activation method. In this work a small magnesium walled ionization chamber, in which the inner surface of the wall has been coated with 10B to increase the sensitivity for thermal neutrons, is used to estimate the thermal neutron component of the beam. After calibration and determination of the directional response in a thermal neutron beam a comparison with foil activation at different depths in water was performed to investigate the reliability of the ionization measurements. The chamber was used in a computer controlled water phantom to measure the depth and lateral distribution of the thermal neutron dose. With this arrangement two‐dimensional scans of the thermal neutrons could be performed quickly and with high accuracy.
T1 mapping from spin echo and stimulated echoes
Authors: Franconi F. , Seguin F. , Sonier C. B. , Le Pape A. , Akoka S.
Published: March 2003

We present an imaging method to obtain a map of the spin‐lattice relaxation time. Images were acquired with the same spatial resolution and in the same time as for a regular spin‐echo acquisition. The sequence was based on the simultaneous acquisition of a spin echo and several stimulated echoes with the same intensity except for T1 weighting which increases with the interval between the excitation pulse and the readout pulse. T1 values obtained on phantoms were compared to those from the inversion‐recovery method and show the accuracy (2%) and the precision (5%) of the method. T1 images of the brain of a healthy volunteer are presented and demonstrate the ability of the method to obtainT 1 mapping in vivo in 12 min and without susceptibility artifacts. In vivo and in vitro results were compared to those obtained by a TOMROP sequence in the same acquisition time.
The finite element method for the propagation of light in scattering media: Boundary and source conditions
Authors: Schweiger M. , Arridge S. R. , Hiraoka M. , Delpy D. T.
Published: March 2003

This paper extends our work on applying the Finite Element Method (FEM) to the propagation of light in tissue. We address herein the topics of boundary conditions and source specification for this method. We demonstrate that a variety of boundary conditions stipulated on the Radiative Transfer Equation can be implemented in a FEM approach, as well as the specification of a light source by a Neumann condition rather than an isotropic point source. We compare results for a number of different combinations of boundary and source conditions under FEM, as well as the corresponding cases in a Monte Carlo model.
Measurements of capillary x‐ray optics with potential for use in mammographic imaging
Authors: Abreu C. C. , Kruger D. G. , MacDonald C. A. , Mistretta C. A. , Peppler W. W. , Xiao Q. F.
Published: March 2003

Capillary optic arrays are bundles of hollow glass capillaries which guide x rays in a manner similar to the way fiber optics guide light. Focused postpatient capillary optic arrays have the potential to significantly improve both contrast and resolution of mammographic images compared to conventional antiscatter grids. Contrast can be improved by the nearly total scatter rejection of the optic. Effective resolution can be improved by geometric magnification without increased focal spot blurring. The best results were found for borosilicate glasses, with transmissions in excess of 60% for 22‐cm‐long fibers. To evaluate the scatter rejection properties, the transmission of off‐axis radiation was measured. Transmission drops to <1% at an angular displacement of 2.7 mrad. Transmission of a bulk capillary array dropped to near zero if the source was at an angle of 2.5 mrad. This implies excellent scatter rejection capabilities. To evaluate whether unchanneled photons might still reach the detector, absorption measurements were also performed on fibers and arrays. Absorption was found to be adequate for scatter rejection. All of the data agreed well with numerical simulations. Performance calculations for two potential optics geometries gave promising results.
Evaluation of digitally reconstructed radiographs (DRRs) used for clinical radiotherapy: A phantom study
Authors: McGee Kiaran P. , Das Indra J. , Sims Colin
Published: March 2003

Digitally reconstructed radiographs produced from a commercial CT simulator have been evaluated using an in‐house test phantom. The phantom consists of a polystyrene cubic block of dimension 15 cm. It contains four test patterns to measure contrast detail, modulation transfer function, ray line divergence accuracy, and spatial distortion. A total of six CT data sets, which vary by CT slice thickness and separation between slices, as well as CT slice reconstruction area have been used to analyze digitally reconstructed radiographs produced by the system. Results show that contrast detail is independent of slice thickness and separation but dependent upon slice reconstruction area for small object diameters (≤1.5 mm). Half field (24‐cm diameter) reconstruction images provide lower threshold contrasts than full field (48‐cm diameter) scans. The modulation transfer function for each data set was calculated and the spatial frequency at which the modulation transfer function is 50% (f50) of the maximum indicates that high contrast resolution depends on slice thickness for both the full and half field reconstructions. For full field scans, f50 values were 0.19, 0.10, and 0.10 line pairs/mm for the 2 mm\2 mm (slice thickness\separation), 5 mm\3 mm, and 5 mm\5 mm data sets, respectively. Similarly, half field f50 values were 0.19, 0.10, and 0.10 line pairs/mm for these same three thickness\separation data sets, respectively. The error in the ray tracing component of the digitally reconstructed radiograph algorithm for source to skin distances between 60 and 200 cm was 1.0 mm while the spatial linearity error was ≤2.5 mm. Errors in CT simulator collimator and table rotations were calculated by measuring the angle between a grid pattern inlayed onto two orthogonal faces of the phantom and a graphical grid superimposed onto the digitally reconstructed radiograph by the CT simulator software. Measured angular differences were ≤1.0°. The largest error in shifting the CT simulator field isocenter was 2.2 mm and occurred on the 5‐mm slice thickness and separation CT data sets.
Notes on the legacy of the Röntgen rays
Authors: Rosenow Ulf F.
Published: March 2003

The discovery of the Röntgen rays and the events connected to it are extraordinary in many respects. Röntgen never disclosed the full details of the experiment which led to the discovery on November 8, 1895. He observed the x‐rays by chance. Neither he nor any other scientist had an idea that such radiation might exist. However, it needed a Röntgen to make the discovery, an experimenter of his superior capabilities. His achievement was the culmination point of the development of physics as an experimental science in the 19th Century. This development of physics is described in this report in some detail, together with the institutional structure of university physics in Germany and the status of technical achievements at the time of Röntgen. Röntgen was suspicious, if not disdainful, of theoretical physics which slowly had gained in importance and in institutional representation. Thus, Röntgen's famous discovery, possible only to a mind not prejudiced by theoretical considerations or expectations, happened at a point in the history of physics when predominantly theoretical concepts introduced the paradigm change from “classical” to “modern” physics: Maxwell's electromagnetic theory, Planck's quantum hypothesis, and Einstein's relativity principle and explanation of the photo‐electric effect. The tremendous speed by which the news of the “new kind of rays” spread around the world and the sensation this news caused launched an intensity of research on x‐rays unprecedented in other areas of research and is reflected in a description of the publication history. The traditional working style of the Institute Director Röntgen, the structure of his Institute, his lack of interest in theory, the burden of his sudden fame and other factors made it practically impossible for him to compete with the rapid development and to contribute substantially to the research on x‐rays. Even the award of the first Nobel Prize in physics in 1901 did not stimulate him to undertake new research activities. He even succeeded in avoiding presentation of his Nobel Lecture. He took the role of the interested observer, sometimes diverted by priority disputes, mainly with Lenard, or by sensational press reports, and retreated increasingly into final solitude, possibly his answer also to the abundance of honors heaped upon him. The impact of his discovery was, however, enormous. In physics it gave impulses to the discovery of radioactivity, of the identification of the electron, and the development of the model of the atom; and in medicine in its immediate applications in diagnosis and therapy.
Multiwindow scatter correction techniques in single‐photon imaging
Authors: Haynor David R. , Kaplan Mitchell S. , Miyaoka Robert S. , Lewellen Thomas K.
Published: March 2003

We studied the performance of linear scatter correction methods for single‐photon imaging with Tc‐99m and Tl‐201, using a numerical model of the Rollo phantom and measurements with a gamma camera modified to record position and energy information in list mode form. We compared the performance of these methods to per‐image optimized linear methods and to locally adaptive linear methods, and developed estimates of the limits on accuracy of scatter correction imposed by the presence of Poisson noise. For both Tc‐99m and Tl‐201 imaging at a fixed depth, particularly at low count rates, the performance of dual‐window methods, or of adaptive methods, is near the best possible for linear methods. Smoothing of the scatter estimate results in minor improvement for Tl‐201. Substantial gaps between the performance of any of these linear methods and the limits imposed by Poisson noise remain and are due primarily to bias, with the gap for Tl‐201 being larger than that for Tc‐99m.
Effects of scatter generated by beam‐modifying absorbers in megavoltage photon beams
Authors: Islam Mohammad K. , Van Dyk Jake
Published: March 2003

Transmission through a beam‐modifying absorber consists of attenuated primary beam and scattered radiation generated by the absorber. The primary component of the transmitted beam is characterized by the narrow beam attenuation coefficient which depends upon the energy of the beam and type of the absorber. In addition to beam energy and absorber material, the scatter component also depends on field size, thickness and shape of the absorber, location of the absorber with respect to the source, and the point of calculation. Based upon Compton first‐scatter, a method has been developed to calculate effective broad beam transmission through any arbitrarily shaped absorber with variable thickness for any points on and off the central axis. The method requires predetermined narrow beam attenuation coefficients as a function of thickness. Transmission calculations for various absorbers such as wedges and attenuators were performed for cobalt‐60 and 6‐MV beams and were compared with measured data. For a cobalt‐60 beam, the measured transmission fraction through a 1.33‐cm‐thick absorber (alloy, consisting of 55% bismuth and 45% lead) for a field size of 24×24 cm2 is 17% higher than the calculated value using a narrow beam attenuation coefficient. Also, for the same absorber, measured central axis transmission is as much as 3.6% higher compared to off‐axis locations. The measured transmission fraction through a 1.33‐cm absorber was found to differ by as much as 13% and 14% for Cobalt‐60 and 6 MV, respectively, as the chamber‐to‐source distance was varied from 70 to 110 cm. The agreement between calculated and measured values is within 0.5% for both energies whereas conventional narrow beam calculations would have yielded errors of 18% and 19%, respectively. Similar agreement was obtained when comparing calculated and measured wedge factors as a function of field size, with the maximum deviation being 0.7%. Measured scattered doses, due to an attenuator covering part of a beam, show a maximum for a thickness of approximately one mean‐free path. This is also predicted by calculations with an agreement of 0.3%.
White polystyrene as a substitute for water in high energy photon dosimetry
Authors: Christ Gunter
Published: March 2003

White polystyrene (C8H8 with 2 wt.% to TiO2) has been compared to water using photon beams with nominal accelerating potential of 4 MV, 6 MV, 15 MV and 25 MV. The absolute reading and hence the measured absorbed dose is found to be up to 3% lower in white polystyrene which can be taken into account by using a correction factor. From these data differences in relative measurements (TMR or percentage depth dose) can be calculated showing differences of less than 0.7% normalized to maximum dose. Since the correction factor shows a dependence on photon energy and field size the use of a common scaling factor is not possible. The effect on photon dosimetry is discusssed.