Showing papers in "Medical Physics in 1995"
1,882 citations
TL;DR: BEAM, a general purpose Monte Carlo code to simulate the radiation beams from radiotherapy units including high-energy electron and photon beams, 60Co beams and orthovoltage units, is described.
Abstract: This paper describes BEAM, a general purpose Monte Carlo code to simulate the radiation beams from radiotherapy units including high-energy electron and photon beams, 60Co beams and orthovoltage units. The code handles a variety of elementary geometric entities which the user puts together as needed (jaws, applicators, stacked cones, mirrors, etc.), thus allowing simulation of a wide variety of accelerators. The code is not restricted to cylindrical symmetry. It incorporates a variety of powerful variance reduction techniques such as range rejection, bremsstrahlung splitting and forcing photon interactions. The code allows direct calculation of charge in the monitor ion chamber. It has the capability of keeping track of each particle's history and using this information to score separate dose components (e.g., to determine the dose from electrons scattering off the applicator). The paper presents a variety of calculated results to demonstrate the code's capabilities. The calculated dose distributions in a water phantom irradiated by electron beams from the NRC 35 MeV research accelerator, a Varian Clinac 2100C, a Philips SL75-20, an AECL Therac 20 and a Scanditronix MM50 are all shown to be in good agreement with measurements at the 2 to 3% level. Eighteen electron spectra from four different commercial accelerators are presented and various aspects of the electron beams from a Clinac 2100C are discussed. Timing requirements and selection of parameters for the Monte Carlo calculations are discussed.
1,444 citations
TL;DR: It was found that the regional change in cerebral blood volume in the primary motor area overlaps the global change around the motor cortex.
Abstract: The effect of motor activity on the left fronto‐central region of the human brain was analyzedspatially and temporally by using noninvasive near‐infrared light (NIR) topography. The changes in oxygenation states caused by motor activity were measured using intensity‐modulated NIR spectroscopy at ten measurement positions on the head surface. The subject randomly performed unilateral finger opposition for 30 s as motor stimulation. When the subject performed contralateral (right) finger movement, significant increases in both oxygenated hemoglobin (oxy‐Hb) and total hemoglobin (total‐Hb) and decreases in deoxygenated hemoglobin (deoxy‐Hb) were observed in a particular area. By mapping the static topograms of the changes of each Hb and comparing them with an anatomical image of MRI, it was found that the particular area was located on the motor cortex along the central sulcus. By mapping the dynamic topograms of the changes of total‐Hb, which reflect the cerebral blood volume, and analyzing the spatiotemporal hemodynamic changes associated with the brain activity, it was found that the regional change in cerebral blood volume in the primary motor area overlaps the global change around the motor cortex. These results demonstrate that NIR topography can be used to effectively observe the human brain activity.
706 citations
TL;DR: The new proton therapy facility is being assembled at the Paul Scherrer Institute (PSI) and the status of the facility and first experimental results are introduced for later reference.
Abstract: The new proton therapy facility is being assembled at the Paul Scherrer Institute (PSI). The beam delivered by the PSI sector cyclotron can be split and brought into a new hall where it is degraded from 590 MeV down to an energy in the range of 85-270 MeV. A new beam line following the degrader is used to clean the low-energetic beam in phase space and momentum band. The analyzed beam is then injected into a compact isocentric gantry, where it is applied to the patient using a new dynamic treatment modality, the so-called spot-scanning technique. This technique will permit full three-dimensional conformation of the dose to the target volume to be realized in a routine way without the need for individualized patient hardware like collimators and compensators. By combining the scanning of the focused pencil beam within the beam optics of the gantry and by mounting the patient table eccentrically on the gantry, the diameter of the rotating structure has been reduced to only 4 m. In the article the degrees of freedom available on the gantry to apply the beam to the patient (with two rotations for head treatments) are also discussed. The devices for the positioning of the patient on the gantry (x rays and proton radiography) and outside the treatment room (the patient transporter system and the modified mechanics of the computer tomograph unit) are briefly presented. The status of the facility and first experimental results are introduced for later reference.
651 citations
TL;DR: Initial experimental results suggest that this approach produces "reasonable" reconstructions for absorbers distributed within a narrow plane embedded within a highly scattering medium.
Abstract: The theoretical underpinnings of photoacoustic ultrasound (PAUS) reconstruction tomography are presented. A formal relationship between PAUS signals and the heterogeneous distribution of optical absorption within the object being investigated is developed. Based on this theory, a reconstruction approach, analogous to that used in x‐ray computed tomography, is suggested. Initial experimental results suggest that this approach produces ‘‘reasonable’’ reconstructions for absorbers distributed within a narrow plane embedded within a highly scattering medium.
642 citations
TL;DR: It is demonstrated 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.
Abstract: 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.
642 citations
TL;DR: Data and techniques that allow the medical physicist to estimate the radiation dose the fetus will receive and to reduce this dose with appropriate shielding are presented and clinical examples show that proper shielding can reduce the radiation doses to the fetus by 50%.
Abstract: Approximately 4000 women per year in the United States require radiotherapy during pregnancy. This report presents data and techniques that allow the medical physicist to estimate the radiation dose the fetus will receive and to reduce this dose with appropriate shielding. Out-of-beam data are presented for a variety of photon beams, including cobalt-60 gamma rays and x rays from 4 to 18 MV. Designs for simple and inexpensive to more complex and expensive types of shielding equipment are described. Clinical examples show that proper shielding can reduce the radiation dose to the fetus by 50%. In addition, a review of the biological aspects of irradiation enables estimates of the risks of lethality, growth retardation, mental retardation, malformation, sterility, cancer induction, and genetic defects to the fetus.
366 citations
TL;DR: 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.
Abstract: 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.
360 citations
TL;DR: Results using simulated data suggest that qualitative images can be produced that readily highlight the location of absorption and scattering heterogeneities within a circular background region of close to 4 cm in diameter over a range of contrast levels, suggesting that absolute optical imaging involving simultaneous recovery of both absorption and scatter profiles in multicentimeter tissues geometries may prove to be extremely difficult.
Abstract: A finite element reconstruction algorithm for optical data based on a diffusion equation approximation is presented. A frequency domain approach is adopted and a unified formulation for three combinations of boundary observables and conditions is described. A multidetector, multisource measurement and excitation strategy is simulated, which includes a distributed model of the light source that illustrates the flexibility of the methodology to modeling adaptations. Simultaneous reconstruction of both absorption and scattering coefficients for a tissue-like medium is achieved for all three boundary data types. The algorithm is found to be computationally practical, and can be implemented without major difficulties in a workstation computing environment. Results using simulated data suggest that qualitative images can be produced that readily highlight the location of absorption and scattering heterogeneities within a circular background region of close to 4 cm in diameter over a range of contrast levels. Absorption images appear to more closely identify the true size of the heterogeneity; however, both the absorption and scattering reconstructions have difficulty with sharp transitions at increasing depth. Quantitatively, the reconstructions are not accurate, suggesting that absolute optical imaging involving simultaneous recovery of both absorption and scattering profiles in multicentimeter tissues geometries may prove to be extremely difficult.
344 citations
TL;DR: The results showed that the flat-panel detector for digital radiology can potentially satisfy the detector design requirements for radiography (e.g., chest radiography and mammography) and is not quantum noise limited below the mean exposure rate typically used in fluoroscopy.
Abstract: 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.
297 citations
TL;DR: High sensitivity of phase-contrast radiography is demonstrated with a rat cerebellar specimen without staining it with a contrast medium and the layer structure of the cerebellum can be observed in the obtained image while there is no clear structure in the corresponding absorption-cont contrast image.
Abstract: Phase‐contrast radiography using an x‐ray interferometer is presented for observing organic matter. High sensitivity of phase‐contrast radiography is demonstrated with a rat cerebellar specimen without staining it with a contrast medium. The layer structure of the cerebellum can be observed in the obtained image while there is no clear structure in the corresponding absorption‐contrast image. Quantitative image analysis is made possible by converting an x‐ray interference pattern to an x‐ray phase shift distribution. The lipid distribution in the cerebellum is discussed by evaluating x‐ray phase shifts before and after lipid removal.
TL;DR: The forced choice method can be superior for experiments using synthetic images, under some conditions, and the rating scale ROC method is preferable when the limiting constraint is the number of images and at the same time it is not feasible to use the forced choice with more than four alternatives.
Abstract: The receiver operating characteristic (ROC) method has been successfully used in medical imaging for 20 years. It has been so successful that many people think of it as an end in itself rather than just one of several ways to assess human observer performance of image based decision tasks. Studies of human and ideal observer decision performance are designed to estimate a measure of observer performance (e.g., efficiency, d' or da). The experimenter would like to obtain accurate and precise estimates using a relatively small number of images or decision trials because of a variety of constraints. One purpose of this paper is to introduce medical physicists to another effective psychophysical measurement technique, the forced choice method. The second purpose is to present a comparison of the forced choice and ROC methods, with particular attention to sampling statistics considerations. In brief, the rating scale ROC method is preferable when the limiting constraint is the number of images and at the same time it is not feasible to use the forced choice with more than four alternatives. The forced choice method can be superior for experiments using synthetic images, under some conditions.
TL;DR: The Monte Carlo method is used to calculate complete two-dimensional dose-rate distributions about the most widely used high dose- rate source design, as well as the Nucletron pulsed dose-rates (PDR) 192Ir source.
Abstract: Despite the large number of single-stepping source pulsed and high dose-rate (HDR) remote after-loading devices in clinical use, the published literature contain little data characterizing dose-rate distributions around the high-intensity (4 x 10(3)-4 x 10(4) microGy m2h-1) 192Ir sources currently used in these devices. We have used the Monte Carlo method to calculate complete two-dimensional dose-rate distributions about the most widely used high dose-rate source design, as well as the Nucletron pulsed dose-rate (PDR) 192Ir source. A Monte Carlo photon transport code, incorporating the detailed internal geometry of the source, was used to calculate the dose rate per unit air-kerma strength in water medium on the transverse bisecting axis over the 0.15-12 cm distance range. In addition, polar dose profiles were calculated at distances ranging from 0.25 to 5 cm. The PDR and HDR dose-rate distributions are tabulated using the formalism endorsed by the Interstitial Collaborative Working Group and the AAPM Task Group 43, and includes dose-rate constant, radial dose function, anisotropy function, geometry function, and anisotropy factors. The dose-rate constants, lambda, of the MicroSelectron/HDR and PDR sources were found to be 1.115 and 1.128 cGy h-1 per unit air-kerma strength, respectively, in good agreement with previously published data for low dose-rate interstitial 192Ir sources. Oblique filtration by the high-density iridium metal core resulted in deviations from anisotropy as large as 35%-55% near the longitudinal axis of the source. Dose-rate distributions are also presented in Cartesian ("away" and "along") coordinates.
TL;DR: It is 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.
Abstract: 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.
TL;DR: The results indicated that, over a wide range of true-positive (TP) cluster detection rate, the CNN classifier could reduce the number of false-positive clusters per image by more than 70%.
Abstract: We are developing a computer program for automated detection of clustered microcalcifications on mammograms. In this study, we investigated the effectiveness of a signal classifier based on a convolution neural network(CNN) approach for improvement of the accuracy of the detection program. Fifty‐two mammograms with clustered microcalcifications were selected from patient files. The clusters on the mammograms were ranked by experienced mammographers and divided into an obvious group, an average group, and a subtle group. The average and subtle groups were combined and randomly divided into two sets, each of which was used as training or test set alternately. The obvious group served as an additional independent test set. Regions of interest (ROIs) containing potential individual microcalcifications were first located on each mammogram by the automated detection program. The ROIs from one set of the mammograms were used to train CNNs of different configurations with a back‐propagation method. The generalization capability of the trained CNNs was then examined by their accuracy of classifying the ROIs from the other set and from the obvious group. The classification accuracy of the CNNs for the ROIs was evaluated by receiver operating characteristic (ROC) analysis. It was found that CNNs of many different configurations can reach approximately the same performance level, with the area under the ROC curve (A z ) of 0.9. We incorporated a trained CNN into the detection program and evaluated the improvement of the detection accuracy by the CNN using free response ROC analysis. Our results indicated that, over a wide range of true‐positive (TP) cluster detection rate, the CNN classifier could reduce the number of false‐positive (FP) clusters per image by more than 70%. For the obvious cases, at a TP rate of 100%, the FP rate reduced from 0.35 cluster per image to 0.1 cluster per image. For the average and subtle cases, the detection accuracy improved from a TP rate of 87% at an FP rate of four clusters per image to a TP rate of 90% at an FP rate of 1.5 clusters per image.
TL;DR: For diagnostic x rays, the inherent spatial resolution of an amorphous selenium (a-Se) plate is much better than that of CsI layers used in x-ray image intensifiers.
Abstract: This is a theoretical study of the inherent spatial resolution of the latent image on the surface of an amorphous selenium (a‐Se) plate used for diagnostic x‐ray imaging. The following effects are considered: (A) ranges of primary photoelectrons; (B) reabsorption of K fluorescence; (C) reabsorption of Compton scatteredphotons; (D) diffusion; (E) the geometric effect due to oblique incidence of x rays; (F) electrostaticeffect; and (G) the space charge effect. The modulation transfer function of a‐Se in the diagnostic x‐ray energy range has been estimated. In conclusion, (A) and (E) are the main factors limiting the resolution, and for diagnostic x rays, the inherent spatial resolution of a‐Se plates is much better than that of CsI layers used in x‐ray image intensifiers.
TL;DR: It is found that the precision in range can be improved by a detailed analysis of the calibration data obtained from tissue-substitute measurements, by a factor of 2.5.
Abstract: Proton radiography is investigated for its use as a quality control tool in proton therapy. Images were produced both with range and range uncertainty information of protons passing through phantoms (Alderson phantom and a sheep's head). With the range images the correct positioning of the patient with respect to the beam could be verified. The range uncertainty images were used to quantitatively detect range variations of protons passing through inhomogeneities in the patient. These measurements can be used to indicate critical situations during proton therapy or to determine the safety margin around the tumor volume. With the range information the precision of different calibrations of computer tomography Hounsfield values to relative proton stopping power, used for proton treatment planning, was determined. It is found that the precision in range can be improved by a detailed analysis of the calibration data obtained from tissue-substitute measurements, by a factor of 2.5. The resulting range errors are in the order of the positioning precision (approximately 1 mm).
TL;DR: Improved algorithms that calculate the hemoglobin saturation by including background-tissue absorption and blood vessel distribution are discussed and some apparent absorption coefficients measured in vivo in animals and humans are presented.
Abstract: We report the development of a heterogeneous resin-tube model to study the influence of blood vessels on the apparent absorption of the system, mu a(sys), using a time-resolved technique. The experimental results show that mu a(sys) depends on the absorption inside the tubes, mu a(tube), tube diameters, and tube-to-sample volume ratios. A mathematical expression relating mu a(sys) and mu a(tube) is derived based on the experimental results and is verified by time-resolved Monte Carlo simulations for heterogeneous models. This analytical formula predicts that the apparent absorption coefficient measured on a biological organ is a volume-weighted sum of the absorption coefficients of different absorbing components. We present some apparent absorption coefficients measured in vivo in animals and humans and discuss improved algorithms that calculate the hemoglobin saturation by including background-tissue absorption and blood vessel distribution.
TL;DR: A tutorial of MTF, NPS, and NEQ concepts for digital systems is presented, along with a complete theoretical treatment of the complications from undersampling.
Abstract: The proper understanding of modulation transfer function (MTF), noise power spectra (NPS), and noise equivalent quanta (NEQ) in digital systems is significantly hampered when the systems are undersampled. Undersampling leads to three significant complications: (1) MTF and NPS do not behave as transfer amplitude and variance, respectively, of a single sinusoid, (2) the response of a digital system to a delta function is not spatially invariant and therefore does not fulfill certain technical requirements of classical analysis, and (3) NEQ loses its common meaning as maximum available SNR2 (signal-to-noise) at a particular frequency. These three complications cause the comparisons of MTF and NEQ between undersampled digital systems to depend on the frequency content of the images being evaluated. A tutorial of MTF, NPS, and NEQ concepts for digital systems is presented, along with a complete theoretical treatment of the above-mentioned complications from undersampling.
TL;DR: It is found that texture features at large pixel distances are important for the classification task and a linear discriminant classifier using the multiresolution texture features can effectively classify masses from normal tissue on mammograms.
Abstract: We investigated the feasibility of using multiresolution texture analysis for differentiation of masses from normal breast tissue on mammograms. The wavelet transform was used to decompose regions of interest (ROIs) on digitized mammograms into several scales. Multiresolution texture features were calculated from the spatial gray level dependence matrices of (1) the original images at variable distances between the pixel pairs, (2) the wavelet coefficients at different scales, and (3) the wavelet coefficients up to certain scale and then at variable distances between the pixel pairs. In this study, 168 ROIs containing biopsy-proven masses and 504 ROIs containing normal parenchyma were used as the data set. The mass ROIs were randomly and equally divided into training and test groups along with corresponding normal ROIs from the same film. Stepwise linear discriminant analysis was used to select optimal features from the multiresolution texture feature space to maximize the separation of mass and normal tissue for all ROIs. We found that texture features at large pixel distances are important for the classification task. The wavelet transform can effectively condense the image information into its coefficients. With texture features based on the wavelet coefficients and variable distances, the area Az under the receiver operating characteristic curve reached 0.89 and 0.86 for the training and test groups, respectively. The results demonstrate that a linear discriminant classifier using the multiresolution texture features can effectively classify masses from normal tissue on mammograms.
TL;DR: This overview indicates areas in which consensus on methodology for physical image-quality measurement has been established and concepts such as "noise equivalent quanta" and "detective quantum efficiency" (DQE) have been found useful for normalizing physical measurements on an absolute scale.
Abstract: Consensus has been developing over the past few decades on a number of measurements required for the laboratory assessment of medical imaging modalities. Nevertheless, understanding of the connection between these measurements and human observer performance in a broad range of tasks remains far from complete. Focusing primarily on projection radiography to provide concrete examples, this overview indicates areas in which consensus on methodology for physical image-quality measurement has been established. Concepts such as "noise equivalent quanta" (NEQ) and "detective quantum efficiency" (DQE) have been found useful for normalizing physical measurements on an absolute scale and for relating those measurements to the decision performance of a hypothetical "ideal observer" that effectively performs decision tasks from the image data. The connection between ideal observer performance and human performance, as determined by receiver operating characteristic (ROC) analysis, remains to be understood for many clinically relevant tasks.
TL;DR: The paper reviews some of the basic interactions of protons with matter that are relevant to radiation treatment and summarizes current knowledge pertaining to the response of biological systems to proton beam radiation.
Abstract: Beams of protons have been used for radiation treatment for approximately 40 years. This paper provides a short summary of activities from institutions that have contributed to the field over this period. The paper reviews some of the basic interactions of protons with matter that are relevant to radiation treatment and summarizes current knowledge pertaining to the response of biological systems to proton beam radiation. The technical means of producing and delivering proton treatment beams are discussed along with examples of the implementation of these methods at treatment facilities around the world. Finally, the clinical dosimetry of proton beams is summarized and sample data are presented.
TL;DR: The beam profiles were found to be more sensitive indicators of the electron beam energy than the depth dose curves, which indicates that in regions up to within 1 cm of the field edge, the measured and computed doses generally agree to within 2%-3%.
Abstract: A generic Monte Carlo model of a photon therapy machine is described. The model, known as McRad, is based on EGS4 and has been in use since 1991. Its primary function has been the characterization of the incident photon fluence for use by dose calculation algorithms. The accuracy of McRad is examined by comparing the dose distributions in a water phantom generated using only the Monte Carlo data with measured dose distributions for two machines in our clinic; a 6 MV Varian Clinac 600C and the 15 MV beam from a Clinac 2100C. The Monte Carlo generated dose distributions are computed using a dose calculation algorithm based on the use of differential pencil beam kernels. It was found that the match to measured data could be improved if the model is tuned by adjusting the energy of the electron beam incident on the target. The beam profiles were found to be more sensitive indicators of the electron beam energy than the depth dose curves. Beyond the depths reached by contaminant electrons, the computed and measured depth dose curves agree to better than 1%. The comparison of beam profiles indicate that in regions up to within 1 cm of the field edge, the measured and computed doses generally agree to within 2%–3%.
TL;DR: The Spencer-Attix water/air restricted mass collision stopping-power ratio is calculated in realistic electron beams in the energy range from 5-50 MeV for a variety of clinical accelerators including the Varian Clinac 2100C, the Philips SL75-20, the Siemens KD2, the AECL Therac 20, and the Scanditronix Medical Microtron 50.
Abstract: The Spencer-Attix water/air restricted mass collision stopping-power ratio is calculated in realistic electron beams in the energy range from 5-50 MeV for a variety of clinical accelerators including the Varian Clinac 2100C, the Philips SL75-20, the Siemens KD2, the AECL Therac 20, and the Scanditronix Medical Microtron 50. The realistic clinical beams are obtained from full Monte Carlo simulations of the clinical linear accelerators using the code BEAM. The stopping-power ratios calculated using clinical beams are compared with those determined according to the AAPM and the IAEA protocols which were calculated by using monoenergetic parallel beams. Using the energy-range relationship of Rogers and Bielajew [Med. Phys. 13, 687-694 (1986)] leads to the most consistent picture in which the stopping-power ratios at dmax derived from mono-energetic calculations underestimate the stopping-power ratios calculated with the realistic beam by 0.3% at 5 MeV and up to 1.4% at 20 MeV. The stopping-power ratios at dmax determined according to the AAPM TG-21 protocol (1983) are shown to overestimate the realistic stopping-power ratios by up to 0.6% for a 5-MeV beam and underestimate them by up to 1.2% for a 20-MeV beam. Those determined according to the IAEA (1987) protocol overestimate the realistic stopping-power ratios by up to 0.3% for a 5-MeV beam and underestimate them by up to a 1.1% for a 20-MeV beam at reference depth. The causes of the differences in the stopping-power ratios between the realistic clinical mono-energetic beams are analyzed quantitatively. The changes in the stopping-power ratios at dmax are mainly due to the energy spread of the electron beam and the contaminant photons in the clinical beams. The effect of the angular spread of electrons is rather small except at the surface. Data are presented which give the corrected stopping-power ratios at dmax or reference depth starting from those determined according to protocols for any energy of clinical electron beams with scattering foils. For scanned clinical electron beams the correction to stopping-power ratios determined according to protocols is found to be less than 0.5% at dmax or reference depth for all beam energies studied. We quantify the differences in the stopping-power ratios determined using the depth of 50% ionization level and the depth of 50% dose level. The differences are very small except for very-high-energy beams (50 MeV) where they can be up to 0.8%.
TL;DR: A completely automated algorithm is presented which is capable of identifying both the right- and left-lung fields on digitized chest radiographic images, and is compared to several other simple segmentation techniques.
Abstract: A completely automated algorithm is presented which is capable of identifying both the right‐ and left‐lung fields on digitized chest radiographic images. The algorithm is tested on a sample of 802 chest images against lung fields drawn by a human observer. The average accuracies are found to be 0.957±0.003 and 0.960±0.003 for right‐ and left‐lung regions, respectively. To put them into perspective, the results are compared to several other simple segmentation techniques. These include a comparison of two sets of lung fields drawn by the human observer at different times which yielded accuracies of 0.967±0.005 and 0.967±0.004 for right‐ and left‐lung regions, respectively.
TL;DR: The EGS4 Monte Carlo technique was used to calculate the minimum beam radii (rLEE) required to achieve complete LEE for photon beams ranging from 60Co to 24 MV and shows that the error introduced to the Sh value due to lateral electron disequilibrium is negligible.
Abstract: The head-scatter factor (Sh) can be measured with a narrow miniphantom or a metal cap provided it is completely covered by the photon beam and its lateral size is thick enough to prevent electron contamination contributions. The effects of lateral electron equilibrium (LEE) and electron contamination on the Sh values were studied. The EGS4 Monte Carlo technique was used to calculate the minimum beam radii (rLEE) required to achieve complete LEE for photon beams ranging from 60Co to 24 MV. The measurement shows that the error introduced to the Sh value due to lateral electron disequilibrium is negligible. The radii of the miniphantoms or the sidewall thicknesses of the caps can be reduced below rLEE provided they are thick enough to prevent the effect of electron contamination.
TL;DR: An extrafocal source model has been developed to explain the dependence of head scatter and beam penumbra on field size and will be particularly useful for fields defined by independent jaw and multileaf collimation systems.
Abstract: An extrafocal source model has been developed to explain the dependence of head scatter and beam penumbra on field size. In this model, the x‐ray source of a medicallinear accelerator is described by two components: a small but intense focal component; and a broadly distributed extrafocal component of low intensity. The extrafocal component is so large that it can be ‘‘eclipsed’’ by the field‐defining collimators. Extrafocal radiation was found to account for 12% of the energy fluence on the central axis of the 6 MV x‐ray beam from a Varian Clinac 2100c accelerator. Head scatter factors were calculated ‘‘in‐air’’ for symmetric, asymmetric, and half‐blocked fields. Calculations agreed with measured values to better than 0.5%, on average. However, head scatter factors for asymmetric fields were underestimated by 1.2% when one of the field dimensions was reduced to 4 cm (the minimum jaw setting that was tested). The extrafocal source model was combined with a convolution/superposition dose calculation algorithm to calculate dose‐per‐monitor‐unit calibration (output) factors and beam dose profiles in water. These dose calculations predict the degradation of the field edge as a function of field size, and calculate output factors to within 0.5%, on average. In the most extreme case of a 4 cm field width, output factors were underestimated by 2%. Dose profiles are predicted without the aid of an empirical fit to measured beam penumbra data. The extrafocal source model will be particularly useful for fields defined by independent jaw and multileaf collimation systems.
TL;DR: The algorithm described provides a fully automated method for the segmentation of individual microcalcifications in an area of the mammogram known to contain a cluster.
Abstract: This paper presents a computerized method for the automated segmentation of individual microcalcifications in a region of interest (ROI) known to contain a cluster in digital mammograms.Mammographic parenchyma can be accurately modeled with the fractal approach, but not areas with microcalcifications. The digitized image is divided into 16×16‐pixel overlapping windows and those accurately modeled by the fractal model are eliminated. The next steps include local thresholding of the ROIs using an iterative method, the elimination of some of the artifacts and identification of the clustered microcalcifications using a clustering algorithm. The evaluation was performed on 81 simulated clusters superimposed on normal mammographic backgrounds and on a representative database of 408 real mammograms. Microcalcification locations were identified by two radiologists independently. These locations were compared to those found by the computer algorithm. An average of 59% of the simulated microcalcifications and 69% of the microcalcifications common to both radiologists were detected. The algorithm described provides a fully automated method for the segmentation of individual microcalcifications in an area of the mammogram known to contain a cluster.
TL;DR: 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 and electron beams, showing an energy dependence of the dose ratios.
Abstract: 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 +/- 03% 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
TL;DR: A kinetic model of FMISO cellular bioreduction has been developed to relate cellular oxygen concentration to the cellular FMISO reaction rate constant, kappa A, and this model was able to accurately determineKappa A for a variety of computer-generated time-activity values, including those for hypothetical heterogeneous tissue regions and poorly perfused tissue regions.
Abstract: [F‐18]fluoromisonidazole (FMISO), a positron‐emitting nitroimidazole, binds preferentially to hypoxic cells. It has been used to image hypoxia in human tumors with positron emission tomography(PET). In order to quantify tumor oxygenation status from these PET data, a kinetic model of FMISO cellular bioreduction has been developed to relate cellular oxygen concentration to the cellular FMISO reaction rate constant, κ A . Also, a compartmental model of FMISO transport and metabolism has been developed to compute the volume average κ A in tissue regions from [F‐18]FMISO PET time‐activity data. This compartmental model was characterized using Monte Carlo simulations and [F‐18]FMISO PET time‐activity data. The model performed well in Monte Carlo simulations; performance was enhanced by fixing three of the seven model parameters at physiologically reasonable values. The four parameters optimized were blood flow rate, κ A , and two partial volume/spillover correction factors. The model was able to accurately determine κ A for a variety of computer‐generated time‐activity curves, including those for hypothetical heterogeneous tissue regions and poorly perfused tissue regions. The model was also able to fit [H‐3]FMISO time‐activity data from 36B‐10 rat tumors as well as [F‐18]FMISO PET time‐activity data from a human patient with a base of the tongue squamous cell carcinoma. The κ A values in muscle ROIs were comparable to those in well‐oxygenated cell monolayers while κ A values in tumor ROIs were greater, suggesting the presence of hypoxic cells in the tumor.