bluuebunny/crossref_metadata_2025 huggingface dataset:
Contrast‐detail analysis of image degradation due to lossy compression
Authors: Cook L. T. , Insana M. F. , McFadden M. A. , Hall T. J. , Cox G. G.
Published: March 2003

A contrast‐detail (CD) experiment was performed to study the effect of lossy compression on computed radiographic (CR) images. Digital CR images of a phantom were compressed by quantizing the full‐frame discrete cosine transform and Huffman encoding the result. Since low‐contrast detectability is directly linked to an important radiological task, namely, the detection of noncalcified pulmonary nodules in adult chest radiographs, the goal of the study was to quantify any loss in low‐contrast detectability due to compression. Compression ratios varied significantly among compressed images, despite the use of fixed compression parameters; detectability could be specified by a single parameter of a CD curve; there was no significant reduction in detectability for an average compression ratio of 11:1; and, there was a statistically significant degradation in detectability for an average compression ratio of 125:1.
Analytical representation of head scatter factors for shaped photon beams using a two‐component x‐ray source model
Authors: Yu M. K. , Sloboda R.
Published: March 2003

An analytic representation proposed for the relative intensity distribution of the extra‐focal source in a two‐component x‐ray source model serves as the basis for calculation. From this representation, a closed‐form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra‐focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multi‐leaf collimators (MLCs). For 6‐ and 15‐MV photon beams from Varian 600C and 2300CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.
Spatial and temporal analysis of human motor activity using noninvasive NIR topography
Authors: Maki Atsushi , Yamashita Yuichi , Ito Yoshitoshi , Watanabe Eiju , Mayanagi Yoshiaki , Koizumi Hideaki
Published: March 2003

The effect of motor activity on the left fronto‐central region of the human brain was analyzed spatially 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.
A scintillating fiber beam‐energy monitor for electron beam therapy
Authors: Aoyama Takahiko , Maekoshi Hisashi , Tsuzaka Masatoshi , Koyama Shuji
Published: March 2003

A new type of beam‐energy monitor is described, which has been developed to check the energies of electron beams from radiotherapy accelerators. The monitor consists of a pair of scintillating fibers and photodiode read‐out circuits that measure the energy dependence of electron transmission through a wedge‐shaped absorber. A linear energy response and 1% accuracy for energy constancy measurements are attained with the monitor. The monitor having advantages of simple mechanical and electronic constructions, small size, and low cost is suitable for practical use as a portable device.
Ultrasound contrast‐detail analysis: A comparison of low‐contrast detectability among scanhead designs
Authors: Hall Timothy J. , Insana Michael F. , Harrison Linda A. , Soller Nannette M. , Schlehr Kimberly J.
Published: March 2003

Contrast–detail (CD) analysis was used to compare the low‐contrast detection capabilities of expert observers using different array‐type scanhead technologies. Five expert observers viewed five different contrast targets to obtain CD curves for each scanhead. Differences in CD curves are interpreted in terms of the image contrast, resolution, and noise. It was found that differences in low‐contrast detectability were due to differences in beam properties. Clinical images obtained during patient examinations are used to show how some clinically relevant tasks are distributed in their contrast and size.
A modeling approach for quantifying tumor hypoxia with [F‐18]fluoromisonidazole PET time‐activity data
Authors: Casciari J. J. , Graham M. M. , Rasey J. S.
Published: March 2003

[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, κA. Also, a compartmental model of FMISO transport and metabolism has been developed to compute the volume average κA 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, κA, and two partial volume/spillover correction factors. The model was able to accurately determine κA 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 κA values in muscle ROIs were comparable to those in well‐oxygenated cell monolayers while κA values in tumor ROIs were greater, suggesting the presence of hypoxic cells in the tumor.
Simultaneous optimization of dynamic multileaf collimation and scanning patterns or compensation filters using a generalized pencil beam algorithm
Authors: Gustafsson Anders , Lind Bengt K. , Svensson Roger , Brahme Anders
Published: March 2003

A very flexible iterative method for simultaneous optimization of dynamic multileaf collimation, scanning patterns and compensation filters has been developed. The algorithm can account for and optimize almost all the degrees of freedom available in a modern radiation therapy clinic. The method has been implemented for three dimensional treatment planning. The algorithm has been tested for a number of cases where both traditional wedge filters and block collimators, and modern equipment such as scanned beams and multileaf collimators are available. It is shown that the algorithm can improve heavily on traditional uniform dose plans with respect to the probability of achieving tumor control without causing severe complications (P+) simply by finding the optimal beam weights and block collimator settings. By allowing more complex equipment to deliver the dose and by accounting for their increased flexibility during the optimization, the dose plan can be substantially improved with respect to the applied objective functions. It is demonstrated that flexible lateral collimation combined with compensators or scanned beams in most cases allow close to optimal dose delivery. Here both the calculation time and the amount of primary computer memory needed has been reduced by performing the dose calculations in a cone beam coordinate system allowing the use of approximately spatially invariant energy deposition kernels. A typical calculation time for optimization of a two‐field technique in a three dimensional volume is about 20 s per iteration step on a Hewlett‐Packard 735 workstation. A well converged solution is normally obtained within about 50–100 iterations or within 15–30 min.
Toward consensus on quantitative assessment of medical imaging systems
Authors: Metz Charles E. , Wagner Robert F. , Doi Kunio , Brown David G. , Nishikawa Robert M. , Myers Kyle J.
Published: March 2003

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.
The 200‐MeV proton therapy project at the Paul Scherrer Institute: Conceptual design and practical realization
Authors: Pedroni Eros , Bacher Reinhard , Blattmann Hans , Böhringer Terence , Coray Adolf , Lomax Antony , Lin Shixiong , Munkel Gudrun , Scheib Stefan , Schneider Uwe , Tourovsky Alexander
Published: March 2003

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.
Modeling transmission and scatter for photon beam attenuators
Authors: Ahnesjö Anders , Weber Lars , Nilsson Per
Published: March 2003

The development of treatment planning methods in radiation therapy requires dose calculation methods that are both accurate and general enough to provide a dose per unit monitor setting for a broad variety of fields and beam modifiers. The purpose of this work was to develop models for calculation of scatter and transmission for photon beam attenuators such as compensating filters, wedges, and block trays. The attenuation of the beam is calculated using a spectrum of the beam, and a correction factor based on attenuation measurements. Small angle coherent scatter and electron binding effects on scattering cross sections are considered by use of a correction factor. Quality changes in beam penetrability and energy fluence to dose conversion are modeled by use of the calculated primary beam spectrum after passage through the attenuator. The beam spectra are derived by the depth dose effective method, i.e., by minimizing the difference between measured and calculated depth dose distributions, where the calculated distributions are derived by superposing data from a database for monoenergetic photons. The attenuator scatter is integrated over the area viewed from the calculation point of view using first scatter theory. Calculations are simplified by replacing the energy and angular‐dependent cross‐section formulas with the forward scatter constant r20 and a set of parametrized correction functions. The set of corrections include functions for the Compton energy loss, scatter attenuation, and secondary bremsstrahlung production. The effect of charged particle contamination is bypassed by avoiding use of dmax for absolute dose calibrations. The results of the model are compared with scatter measurements in air for copper and lead filters and with dose to a water phantom for lead filters for 4 and 18 MV. For attenuated beams, downstream of the buildup region, the calculated results agree with measurements on the 1.5% level. The accuracy was slightly less in situations where the scatter component is very large, as for very large fields with very short filter to detector distances. The implementation of the model into treatment planning systems is discussed.
Verification of water equivalence of FeMRI gels using Monte Carlo simulation
Authors: Chan Maria F. , Ayyangar Komanduri
Published: March 2003

Dosimetric characteristics of the FeMRI gels for various energy photon and electron beams were investigated using the Integrated Tiger Series (ITS) Monte Carlo codes. The FeMRI gels have been reported as being tissue‐substitutes which can be shaped to any desired contour for three‐dimensional (3D) mapping of dose distribution using magnetic resonance (MR) scanning. This study indicated that the central axis depth dose, off‐axis depth dose, transverse profile at the depth of 50% of maximum dose, and energy deposition for photons and electrons in the gels are close to those in water. From these comparisons, it is concluded that FeMRI gels are suitable substitutes for water for the purposes of dose measurements of both photons and electrons used in radiotherapy.
Mechanical and dosimetric quality control for computer controlled radiotherapy treatment equipment
Authors: Thompson Antoinette V. , Lam Kwok L. , Balter James M. , McShan Daniel L. , Martel Mary K. , Weaver Tamar A. , Fraass Benedick A. , Ten Haken Randall K.
Published: March 2003

Modern computer controlled radiotherapy treatment equipment offers the possibility of delivering complex, multiple field treatments with minimal operator intervention, thus making multiple field conformal therapy practical. Conventional quality control programs are inadequate for this new technology, so new quality control procedures are needed. A reasonably fast, sensitive, and complete daily quality control program has been developed in our clinic that includes nearly automated mechanical as well as dosimetric tests. Automated delivery of these quality control fields is performed by the control system of the MM50 racetrack microtron, directed by the CCRS sequence processor [D. L. McShan and B. A. Fraass, Proceedings of the XIth International Conference on the use of computers in Radiation Therapy, 20–24 March 1994, Manchester, U.K. (North Western Medical Physics Department, Manchester, U.K., 1994), pp. 210–211], which controls the treatment process. The mechanical tests involve multiple irradiations of a single film to check the accuracy and reproducibility of the computer controlled setup of gantry and collimator angles, table orientation, collimator jaws, and multileaf collimator shape. The dosimetric tests, which involve multiple irradiations of an array of ionization chambers in a commercial dose detector (Keithly model 90100 TrackerTM System) rigidly attached to the head of the treatment gantry, check the output and symmetry of the treatment unit as a function of gantry and collimator angle and other parameters. For each of the dosimetric tests, readings from the five ionization chambers are automatically read out, stored, and analyzed by the computer, along with the geometric parameters of the treatment unit for that beam. The present work describes in detail these quality control tests that have been implemented in our clinic to help confirm the mechanical and dosimetric consistency of an MM50 racetrack microtron system [Masterson et al., Int. J. Radiat. Oncol. Biol. Phys. 28, 1219–1227 (1994)].
Evaluation of the dosimetric characteristics of a diamond detector for photon beam measurements
Authors: Rustgi Surendra N.
Published: March 2003

The dosimetric properties of a new diamond detector for the measurement of relative dose in photon beams have been investigated and compared to those of a silicon p‐type photon diode and a 0.14 cm3 ionization chamber. The mass energy absorption ratio of carbon to water is nearly constant over a wide energy range making the diamond detector nearly tissue equivalent. The directional dependence of the radiation response of the diamond detector for cobalt 60, 6 MV and 18 MV photon beams was more uniform than that of the diode. As the incident photon beam moves from 0° (parallel to the detector stem) to a direction transverse to the detector stem (90°), the diamond detector sensitivity remains nearly uniform whereas the diode sensitivity diminishes by approximately 15%–22%. The spatial resolution of the diamond detector, as measured by penumbra width, is slightly less than that of the diode detector but clearly superior to that of the 0.14 cm3 ionization chamber. The tissue maximum ratio measurements for small size photon fields (diameter ≤4 cm) with the diamond, diode, and a Markus parallel plate chamber are in excellent agreement. The diamond detector with high radiation sensitivity and spatial resolution is an excellent choice as a detector in photon fields with high dose gradients such as brachytherapy and radiosurgery.
Image feature analysis for computer‐aided diagnosis: Accurate determination of ribcage boundary in chest radiographs
Authors: Xu Xin‐Wei , Doi Kunio
Published: March 2003

A computerized method for accurate determination of the ribcage boundary in chest images has been developed for use in computer‐aided diagnosis (CAD) schemes for automated detection of abnormalities such as the pulmonary lung nodules, pneumothorax, interstitial disease, cardiomegaly, and interval changes in clinical chest images. With our method, the average position of the top of the lung in the chest image is determined first. Top lung edges and ribcage edges are determined within search ROIs, which are selected over top lung cages and ribcages. Three polynomial functions are applied separately to yield smooth curves for top lung edges and right and left ribcage edges. The complete ribcage boundary is then obtained by smoothly connecting three curves. A total of 1000 radiographs were digitized to 1k×1k matrix size and a 10‐bit gray scale with a laser scanner and analyzed by our method. The subjective evaluation indicated that our method produced moderately to highly accurate results in approximately 96% of the 1000 cases examined.
Mean mass energy absorption coefficient ratios for megavoltage x‐ray beams
Authors: Furhang Eli E. , Chui Chen‐Shou , Lovelock Michael
Published: March 2003

Mean mass energy absorption coefficient ratios of acrylic, polystyrene, and water to air, were calculated using Monte Carlo generated energy spectra. The energy spectra were calculated for 4‐ to 50‐MV x‐ray beams, from machines using flattening filters and scanning beams. The validity of these spectra was verified by comparing the measured ionization ratios with the calculated values. The agreement was found to be within 1.9%. For beams of energy below 6 MV, our estimates of the mean mass energy absorption coefficient ratios agree well with those recommended by the TG‐21 protocol. For higher energy beams, the discrepancy increases to about 3%. It was found that the discrepancy is attributable to the different spectra used in these calculations.
A new sigmoidal function describing the small field dose profile data from a linear accelerator
Authors: Lam Chan F.
Published: March 2003

Dose profile data from small circular fields have been used in treatment dose planning for stereotactic radiosurgery. Generally, a two‐dimensional interpolation of the measured beam profiles from circular collimators is used to calculate the dose at any axial depth and radial distance from the central axis. Instead, the dose profile data can be transformed into a sigmoidal form. A new three parameter sigmoidal function was developed to fit the transformed (sigmoidal) dose profile data. The values of the three estimated parameters were found to follow either linearly or exponentially as a function of axial depth. Thus, instead of linear interpolation, these formulas can be used to calculate dose at any axial depth and radial distance from the central axis for circular collimators of various diameters. This new sigmoidal function provides another formula to describe dose profile data from circular collimator of small fields.
The nonlinear partial volume effect and computed tomography densitometry of foam and lung
Authors: Kemerink Gerrit J. , Lamers Rob J. S. , Thelissen Guillaume R. P. , van Engelshoven Jos M. A.
Published: March 2003

A quantitative study was performed to assess the magnitude of the nonlinear partial volume effect (NLPVE) in computed tomography (CT) densitometry of polyethene foam and lung. This effect arises in materials having density variations on the scale of the sampling area of an individual CT‐detector element. It causes a systematic underestimation of the density determined with CT. Foam samples and a resected lung of a goat were imaged with high resolution (20 lp/mm) using a mammography system, and the observed optical density variation in the images was converted into a distribution of pathlengths that x rays penetrate within the solid component of the cellular material. The obtained pathlength distribution was used to calculate the transmission, as seen by a single detector in computed tomography. Comparison with the transmission through homogeneous material of the same thickness gave an estimate of the NLPVE. For the foams studied, the CT‐determined density was found to be too low by approximately 0.3%–0.5% due to this effect. Although these density errors are small, in calibrations of a CT scanner they may be of significance. For lung the underestimation of the density was less than 0.1%. These experimentally derived, NLPVE related CT‐density errors are 32%–84% of those calculated from a simple model of a cellular solid.
Effects of induced charge in the kinestatic charge detector
Authors: Wagenaar Douglas J. , Terwilliger Robert A.
Published: March 2003

The principle of the kinestatic charge detector (KCD) for digital radiography depends on the synchronization of the scan velocity of a parallel plate drift chamber with the cation drift velocity. Compared with line‐beam scanners, this motion‐compensated imaging technique makes better use of the x‐ray tube output. A Frisch grid traditionally has been used within the KCD to minimize unwanted signal contributions from both cations and negative charge carriers during irradiation. In this work the charge induction process in a parallel plate geometry was investigated for the special case of the KCD. In the limit of infinite plates, the cathode charge density due to both cations and negative charge carriers increases quadratically in time for a kinestatically scanned narrow slit. In the KCD the cathode is segmented into an array of narrow electrodes, each aligned with the incident x‐ray beam. Our conformal mapping computation determined that the shape of the induced charge signal depends critically on Δx/w, the ratio of electrode width to drift gap. Our conclusion introduces the possibility of eliminating the Frisch grid from the KCD design because the value of Δx/w required for transverse sampling in the KCD is sufficiently low as to allow “self‐gridding” to take effect.
Comparison of receiver operating characteristic and forced choice observer performance measurement methods
Authors: Burgess Arthur E.
Published: March 2003

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.
Calculation of stopping‐power ratios using realistic clinical electron beams
Authors: Ding G. X. , Rogers D. W. O. , Mackie T. R.
Published: March 2003

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%.
Backscatter dose perturbation in kilovoltage photon beams at high atomic number interfaces
Authors: Das Indra J. , Chopra Kashmiri L.
Published: March 2003

Dose perturbations at tissue interfaces have been of significant concern since the beginning of this century. However, comprehensive studies related to the backscatter perturbation in kilovoltage beams are still limited. The dose perturbation depends on various parameters, including beam energy, field size, and the thickness, width, position, and atomic number, Z, of the inhomogeneity creating the interface with soft tissue. Using a thin window parallel plate ion chamber having relatively flat response at low energies, the dose perturbation was measured as backscatter dose perturbation factor, BSDF, at various interfaces in kilovoltage x‐ray beams. The BSDF is defined as the ratio of doses with and without an interface for identical setup conditions. Results indicate that the BSDF is strongly dependent on beam energy, like the backscatter factor. Contrary to its behavior in megavoltage beams, BSDF in kilovoltage beams does depend on the field size, suggesting a contribution from scattered photons and fluorescent radiation, originating in the high‐Z material. The thickness of the high‐Z medium is not critical, since a fraction of a millimeter is sufficient to provide full backscatter. The interface effect with wide inhomogeneity has two distinct regions: the high dose region (BSDF>1.0), which is very localized and disappears within a fraction of a millimeter, and the low dose region (BSDF<1.0), which is observed up to 10 cm. The dependence of BSDF is neither a quadratic function of Z nor a cube root of beam energy, indicating that the interface effect is complex and not predominantly due to photoelectron transport.
Source localization for template implants with particular reference to stepping‐source afterloaders
Authors: Weaver Keith A. , Pickett Barby , Roberts L. W. , Stuart Alex
Published: March 2003

Source localization from radiographs can be very difficult for template‐guided implants if the needle images overlap. At UCSF several techniques to make this task easier have been developed. The techniques include selection of an optimum simulator gantry angle, use of different types of dummies, and differential dummy loading. In addition, several modifications have been made in our brachytherapy planning computer program to facilitate source entry. As a result of these improvements, source localization is now accomplished in much less time with improved accuracy.
A displacement model for thermoluminescent dosimetry in radioimmunotherapy
Authors: Woo Milton K. , Chen Zhengguo
Published: March 2003

A model is developed to enable dose determination for thermoluminescent dosimeters immersed in a radioactive solution such as used in radioimmunotherapy. For low energy beta emitters used in such therapy the size of the dosimeter results in a much lower light output than when irradiated with an external Cobalt‐60 (60Co) beam to the same dose as delivered to the medium. The model takes the size of the dosimeter into account and hence allows calculation of the dose in the actual medium. The application to different dosimeter sizes as well as different radionuclide energies is also illustrated. Finally, the model can be extended to dose calculation in a mixed gamma and beta irradiation geometry.
Optimization of automatic portal image analysis
Authors: Gilhuijs K. G. A. , Touw A. , van Herk M. , Vijlbrief R. E.
Published: March 2003

The purpose of this study is to quantify and optimize the performance of an automatic portal image analysis procedure under clinical conditions and to compare the performance with that of human operators. A new method, based on analysis of variance, is introduced to quantify the clincial performance of portal image analysis tool in terms of systematic and random variations. The automatic portal image analysis procedure is based on chamfer matching. Two image enhancement techniques have been investigated in the automatic procedure: morphological top‐hat (MTH) transformation and multiscale medial axis (MMA) transformation. The performance of these enhancements was quantified and optimized as a function of filter size using images obtained from clinical treatment. All images used for this study were obtained from pelvic treatment fields by means of an electronic portal imaging device. The random variations in the alignment of AP fields are typically 0.5 mm and 0.5° (1 SD) for both the human operators and the optimized automatic analysis procedure. Random variations in the alignment of lateral pelvic fields are typically twice as large for all operators. MMA enhancement yields smaller random variations than MTH enhancement for lateral fields, but the differences are marginal for AP fields. The optimized automatic analysis procedure has a success rate ranging from 99% for AP large fields to 96% for lateral fields and 85% for AP boost fields. The accuracy of the method is comparable with the accuracy of the human operators for most investigated fields. For lateral boost fields and simultaneous boost fields, the random variations of the automatic analysis are typically two times larger than the variations of the human operators. Automatic analysis is 4 to 20 times faster than human operators yielding a large reduction in work load.
Photon dose calculation incorporating explicit electron transport
Authors: Yu C. X. , Mackie T. R. , Wong J. W.
Published: March 2003

Significant advances have been made in recent years to improve photon dose calculation. However, accurate prediction of dose perturbation effects near the interfaces of different media, where charged particle equilibrium is not established, remain unsolved. Furthermore, changes in atomic number, which affect the multiple Coulomb scattering of the secondary electrons, are not accounted for by current photon dose calculation algorithms. As local interface effects are mainly due to the perturbation of secondary electrons, a photon–electron cascade model is proposed which incorporates explicit electron transport in the calculation of the primary photon dose component in heterogeneous media. The primary photon beam is treated as the source of many electron pencil beams. The latter are transported using the Fermi–Eyges theory. The scattered photon dose contribution is calculated with the dose spread array [T. R. Mackie, J. W. Scrimger, and J. J. Battista, Med. Phys. 12, 188–196 (1985)] approach. Comparisons of the calculation with Monte Carlo simulation and TLD measurements show good agreement for positions near the polystyrene–aluminum interfaces.
Confirmation of target localization and dosimetry for 3D conformal radiotherapy treatment planning by MR imaging of a ferrous sulfate gel head phantom
Authors: Chan Maria F. , Ayyangar Komanduri M.
Published: March 2003

A detailed methodology has been developed to verify the three‐dimensional (3D) radiation dose mapping of conformal therapy radiation treatment planning by FeMRI dosimetry. A phantom that consisted of a human skull filled with a 1‐mM solution of ferrous sulfate and 0.1‐N sulfuric acid gelled with 7.5% (by weight) gelatin was employed. With a spherical target volume in the head phantom, five noncoplanar conformal beams were designed through the use of a 3D treatment planning system developed in‐house. The phantom was irradiated with a 6‐MV linear accelerator to a total dose of 25 Gy delivered to the periphery of the target volume. The phantom and a set of calibration vials were scanned simultaneously in a GE 1.5T MR imager with six different multiscan inversion‐recovery pulse sequences. The values of T1 were evaluated on a pixel‐by‐pixel basis through the use of custom‐built software on a UNIX workstation and were converted to dose using calibration data. Comparisons of dose distributions between those measured by FeMRI and those calculated by 3D treatment planning show good agreement.
Influence of shape on the accuracy of grid‐based volume computations
Authors: van't Veld A. A. , Bruinvis I. A. D.
Published: March 2003

The influence of the shape of a region of interest (ROI) on the uncertainty in the sampled volume of the ROI is investigated for computations with regular Cartesian grids. Both mathematically defined volumes and clinically relevant ROIs were studied. The sampling uncertainty is shown to depend on the compactness of the ROI and on effects of grid matching and translational symmetry. In clinical ROIs without translational symmetry the estimate of the sampling uncertainty is improved up to a factor of 2.3 by taking the compactness of the ROI into account. In a spherical ROI grid‐matching effects were demonstrated by means of Fourier transforms. In this type of ROI, grid‐matching effects decrease as well as increase the sampling uncertainty up to a factor of 1.6. Translational symmetry is shown to cause a decrease in the sampling uncertainty convergence power from for spherical ROIs, to for stringlike or for pancakelike cylinders. For clinical ROIs with translational symmetry, similar decreases were found. With the theory derived and these symmetry effects taken into account the experimental uncertainty of volume computation can be estimated for most clinical ROIs within a factor of 2.5. Special care should be taken in grid sampling of volumes inside isodose surfaces of rectangular field techniques. For the volume of a prostate an uncertainty level of 1% or 5% is obtained with less than 1050 or 80 grid points, respectively, while for such an isodose surface up to 16 000 or 500 grid points are required for the same uncertainty levels.
A Monte Carlo model of photon beams used in radiation therapy
Authors: Lovelock D. M. J. , Chui C. S. , Mohan R.
Published: March 2003

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%.
Measurement of a photon penumbra‐generating kernel for a convolution‐adapted ratio‐TAR algorithm for 3D treatment planning
Authors: Low Daniel A. , Zhu Xiao‐Rong , Harms William B. , Purdy James A.
Published: March 2003

A method has been developed to measure a photon penumbra‐generating kernel using dosimetry equipment available in most radiation therapy departments. The kernel is used in a convolution‐adapted ratio‐TAR algorithm in our three‐dimensional treatment planning system. The kernel is assumed to be invariant with respect to off‐axis position, axially symmetric, and is divided into short‐ and long‐range components, with a different measurement technique for each. The data required to obtain the short‐range component are measured by scanning across a split‐field geometry incident on a water phantom. The derivative of the measured profile is proportional to one‐dimensional projections across the kernel. Because the kernel is axially symmetric, only one profile measurement is required for each depth. A CT reconstruction technique is used to extract the radial dependence of the kernel from the strip integrals. Electronic noise in the acquisition system yields significant uncertainties in the kernel shape for distances beyond 3 cm. The long‐range portion of the kernel is obtained by examining tissue–air ratios (TARs). The derivative of the TAR at the center of a circular field is proportional to the kernel value at the distance corresponding to the radius of the field. The kernel measurement method was tested by comparing measured and calculated square‐field profiles at a variety of depths. Agreement was within 1% within the field boundary and 3% outside the field boundary for all depths.
A simple method of obtaining off‐axis half‐value layers for megavoltage photon beams
Authors: Barish Robert J.
Published: March 2003

We describe a simplified technique for the acquisition of off‐axis beam‐quality data (i.e., half‐value layers in water) for medical linear accelerators. A measurement protocol is presented in which an ordinary beam‐scanning water phantom is used to accurately position an ionization chamber at appropriate measuring positions. Attenuation coefficients are calculated by performing a regression on the measured data. The technique uses the smallest fields for which lateral electron equilibrium can be established.
Accounting for primary electron scatter in x‐ray beam convolution calculations
Authors: Keall Paul , Hoban Peter
Published: March 2003

Fermi–Eyges electron‐scattering theory has been incorporated into the primary dose calculation for external x‐ray beam radiotherapy using the convolution method. Incorporating scattering theory into the convolution technique accounts for the density distribution between the interaction and deposition sites, whereas conventional convolution methods only consider the average density between these two points. As the lateral spread of electrons ejected from an interaction site depends on the density distribution, the energy deposition (and hence dose distribution) is predicted more accurately if scattering is accounted for. This new method gives depth dose curves which show better agreement with Monte Carlo calculations in a (slab inhomogeneity) lung phantom than a conventional convolution method, especially at high energies and small field sizes where lateral electronic disequilibrium exists at the central axis. For a 5×5‐cm2 18‐MV beam incident on the lung phantom, a reduction in the maximum error between the convolution and Monte Carlo depth dose curves from 5% to 2.5% is obtained when scattering theory is used in the primary dose calculation. Incorporating scattering theory into the convolution calculation increases the computation time of the primary dose by a factor of 3.
Calorimetric determination of the absorbed dose‐to‐water beam quality correction factor kQ for high‐energy photon beams
Authors: Vatnitsky Stanislav M. , Siebers Jeffrey V. , Miller Daniel W.
Published: March 2003

A method has been developed for measuring photon beam quality correction factors kQ using direct transfer with a water calorimeter. kQ values were measured for beam qualities varying between 6 and 23 MV for a Capintec PR‐06 and a PTW W30001 cylindrical ionization chamber. Measured values were intercompared with published sets of computed kQ values and agreement was found to be within measurement uncertainty (1%, one standard deviation).
Magnetic‐resonance imaging techniques for detection of elasticity variation
Authors: Fowlkes J. B. , Emelianov S. Y. , Pipe J. G. , Skovoroda A. R. , Carson P. L. , Adler R. S. , Sarvazyan A. P.
Published: March 2003

The relative success of manual palpation in the detection of breast cancer would suggest that a method for remote palpation resulting in a measurement of tissue elasticity could provide a diagnostic tool for detecting cancerous lesions deeper within the breast. This presumption is based in part on the excellent contrast between neoplastic and normal tissue due to the large (orders of magnitude) relative variation in the shear elastic modulus. By comparison, the bulk deformational modulus maintains the same value to within 20% for most soft tissues. A specific method of magnetic‐resonance imaging (MRI) which measures tissue displacements has been used in experiments with a phantom containing regions of increased Young's modulus as a demonstration. The spatial modulation of magnetization technique uses the displacement of a spatial grid pattern caused by spin saturation to track regional motion. Mathematical reconstruction of the distribution of elastic moduli is shown for select examples. Any modality, e.g., MRI, ultrasound, etc., which can detect local tissue motion with sufficient spatial resolution can be used and therefore the results presented here should give an indication of the utility of such motion tracking techniques to future measurement of tissue elasticity.
Slit camera focal spot measurement errors in mammography
Authors: Tang S. , Barnes G. T. , Tanner R. L.
Published: March 2003

Mammography x‐ray tube focal spot sizes are routinely measured during acceptance testing and annual performance audits. The National Electrical Manufacturers Association (NEMA) recommends the slit camera for this purpose. Investigated were the effect of slit rotational misalignment, tilt misalignment, image film density, film and screen‐film image receptors, microscope magnification and reticule accuracy, and observer variation on slit camera focal spot measurements. Our results indicate that small rotational misalignment (<5°) and tilt misalignment (<3°) introduce insignificant error. Measured focal spot size increased slightly with image optical density, indicating that for consistent results the image optical density variations should be minimized. Also desirable for accurate field measurements is a high power microscope (25–50×) and a reticule with divisions of ≤0.02 mm. Screen‐film imaging consistently resulted in a slightly smaller measured focal spot size than direct film. The greatest source of error was due to observer variation. Of interest is that reader variability showed a consistent pattern and variation between two measurements by the same observer was much smaller than between observer variation, suggesting that standardized criteria should be established and a method of reader training developed. The length of the focal spot is defined at a reference axis angle specified by the mammography unit manufacturer. Presented is a tabulation of the focal spot geometry and reference axis angles for the majority of mammography units currently and recently marketed in North America.