bluuebunny/crossref_metadata_2025 huggingface dataset:
Neutron measurements around medical electron accelerators by active and passive detection techniques
Authors: Tosi G. , Torresin A. , Agosteo S. , Para A. Foglio , Sangiust V. , Zeni L. , Silari M.
Published: March 2003

Passive and active detection techniques have been employed in order to measure neutron fluence rates and corresponding exposure rates around medical electron accelerators operating at energies well above the neutron binding energies of the structural materials. In these conditions from the treatment head, in the direct photon flux and from the shielded region, a fast neutron flux emerges which is partly absorbed and partly scattered by the walls, eventually establishing a nearly uniform thermal and epithermal flux in the room. Both direct and scattered flux contribute to the dose to the patient. A smaller neutron dose rate can also be found outside the treatment room, where the therapy staff works. Passive detectors, of moderation type, have been employed in the treatment room and 3He active detectors in the external zones. For the treatment room the activation data were compared with results of Monte Carlo simulation of the neutron transport in the room. Technical features of the two measures are briefly presented and results obtained around three different types of accelerators are reported. At the higher beam energies, i.e., 25 MV, a neutron dose of 0.36 Sv was estimated in the treatment field in addition to a therapeutic x‐ray dose of 50 Gy. At lower energies or out of the treatment field the neutron dose drops significantly. In the external zones the dose rates everywhere are below the recommended limits and normally very low, the highest values being recorded in positions very close to the access door of the treatment room.
Electron dose calculation using multiple‐scattering theory: Evaluation of a new model for inhomogeneities
Authors: Jette David , Walker Susan
Published: March 2003

In this article, the fifth in a series on the calculation of electron dose using multiple‐scattering theory, the predictions of a new model for dealing with localized inhomogeneities will be examined (Med. Phys. 18, 123–132, 1991). That model is in the form of a perturbation series, and for a thick half‐slab configuration explicit formulas are worked out for the dose directly deposited by the primary electrons, for three reasonable cutoffs of the series. The predictions of this model with EGS4 Monte Carlo calculations for the half‐slab configuration are compared, and they are found to be quite accurate in the region under the edge of the half‐slab. On the other hand, the “Hogstrom algorithm,” which is currently the most advanced method in routine clinical use, is found to give poor accuracy for this configuration.
Thermal and scatter effects on the radiation sensitivity of well chambers used for high dose rate Ir‐192 calibrations
Authors: Podgorsak Matthew B. , DeWerd Larry A. , Thomadsen Bruce R. , Paliwal Bhudatt R.
Published: March 2003

High dose rate (HDR) iridium sources must be calibrated regularly because of the short half‐life of Ir‐192. High dose rate sources can now be calibrated using a new well‐type chamber that allows easy, reproducible source calibrations. The chamber includes a styrofoam insulator that surrounds the source in the well. A study of the radiation sensitivity of the well chamber exposed to an HDR Ir‐192 source at two different activities (300 and 230 GBq) revealed that the sensitivity of the chamber varies by as much as 1.1% as the chamber is moved toward a scattering surface. Second, with the styrofoam insulator removed, the air temperature within the ion collecting volume increased during exposure, causing a gradual decrease in chamber sensitivity of 0.15% in 30 min. This temperature increase was caused by heat transfer from radiation emitted by the Ir‐192 source, and diminished as the source decayed. However, with the styrofoam insulator around the central aluminum tube in the well, the source cannot heat the collecting volume and thus thermal equilibrium between the ion collecting volume and its environment is maintained throughout an exposure. The radiation sensitivity of the commercial well chamber was found to be constant for exposure times of 30 min.
Current status of physical measurements of the skeleton
Authors: Greenfield Moses A.
Published: March 2003

An overview is presented of some of the major methods of measuring the skeleton in the past 30 years. These include single photon absorptiometry (SPA), dual photon absorptiometry (DPA), quantitative computed tomography (QCT), and recently dual energy radiographic absorptiometry (DRA), also called DEXA (dual energy x‐ray absorptiometry). In addition to these methods, all attempting to measure bone mineral density, regional and total body calcium have been determined by in vivo neutron activation analysis (IVNAA). An attempt to determine bone quality as contrasted with bone quantity has been made using ultrasound, with measurements of speed of sound and of attenuation as useful parameters to characterize bone tissue. While the various methods for measuring bone density have been most useful, no one method includes all the features required to be entirely satisfactory: excellent precision and accuracy and the ability to measure volumetric density in gm/cm3. Least successful has been the ability to predict fracture risk, an essential goal in helping the patient.
Mammography screen–film selection: Individual facility testing technique
Authors: Kimme‐Smith Carolyn , Bassett Lawrence , Gold Richard H. , Parkinson Brett
Published: March 2003

Variations in tube output, film processing, and radiologist's preferences affect the screen–film combination that is appropriate for any particular mammographic facility. A technique to test a variety of screen–film combinations for screening mammography is described. Films are selected for testing because of their sensitometric characteristics. Dose and clinical reliability are established with phantoms before the screen–film combinations are used to image consecutive patients having bilateral examinations. The mammograms selected for evaluation are those with similar optical density ranges, and which also may be compared to available previous mammograms or which have unusual mammographic findings. All radiologists reading mammograms at a facility independently score the selected cases. Scores of “unacceptable,” “acceptable,” or “outstanding” are assigned to four basic imaging characteristics: sharpness, contrast, visibility of skin line, and noise. Interobserver variations by this method require normalization, unlike ROC analysis which is not applicable for this data because of the absence of proved pathologic diagnoses. The testing of 5 films and two screens using 42 patient examinations required 2 h of time from each radiologist. It took 7 h of the physicist's time to pretest the 5 films, select the 42 acceptable examinations for testing by the radiologists, and summarize the data.
A study of interface effects in 60Co beams using a thin‐walled parallel plate ionization chamber
Authors: Nilsson B. , Montelius A. , Andreo P.
Published: March 2003

A large plane‐parallel ionization chamber has been constructed to investigate interface effects in 60Co beam. The designed geometry yields negligible perturbation from the side walls, as opposed to the large effects existing in commercially available plane‐parallel chambers. The chamber has been used to investigate interface phenomena in transition zones using a wide range of elements (Z=4‐82) as front‐ and back‐scattering media and a clinically relevant 60Co γ‐ray field size. The effects of varying the chamber height discretely (0.5–11 mm) and increasing the wall thickness (1–9 mg/cm2) have been investigated. The variation of the measured ionization with the experimental setup (air gap between backscatter material and chamber wall, measurements at dmax and at 5‐cm depth, varying the material both in front of and behind the chamber, etc.) has also been investigated. The simple geometry of the ion chamber has been found optimum for benchmark studies of Monte Carlo calculations. The ion chamber is suited for investigating experimentally the effects of varying transport parameters used in Monte Carlo simulations. The results presented show that the complex physical mechanisms governing 60Co interface dosimetry still make Monte Carlo condensed‐history (macroscopic) techniques uncertain. It has been found that the EGS4 Monte Carlo system, together with the user code DOSRZ V4.0 and the PRESTA algorithm, yields good agreement with experiments for low and medium Z (main interest in dosimetry and radiotherapy), but may underestimate up to 10% the backscatter from high‐Z materials even when transport parameters are optimized.
The enhancement of radiotherapy verification images by an automated edge detection technique
Authors: Leszczynski Konrad W. , Shalev Shlomo , Cosby N. Scott
Published: March 2003

Adaptive histogram equalization techniques are known to be effective for the enhancement of contrast in portal images acquired during radiotherapy treatments. A significant drawback is the loss of definition on the edges of the treatment field. Analysis of this problem shows that it can be remedied by separating the treatment field from the background prior to the enhancement, and using only the pixels within the field boundary in the enhancement procedure. An edge extraction algorithm has been developed for delineating the treatment field in portal images, and consists of four modules that are applied to the original portal image in sequence. In the first step, edges are enhanced with a derivative of Gaussian operator that assures high response to the field edges relative to anatomical or other edges in the image. Pixels for which the response of the edge operator was the strongest are subsequently connected by an edge following algorithm to produce a raw contour of the field. In the last two steps the contour is refined by converting it into straight line segments and appending to the contour any parts of the field edge that might have been missed out during the initial edge following. The final contour encloses exclusively those pixels that belong to the treatment field, and the adaptive histogram equalization is applied selectively to this region. The combination of edge detection and selective enhancement was shown to produce images of superior contrast on the patient's anatomical features as well as accurate definition of treatment field edges.
Verification data for electron beam dose algorithms
Authors: Shiu Almon S. , Tung Samuel , Hogstrom Kenneth R. , Wong John W. , Gerber Russell L. , Harms William B. , Purdy James A. , Haken Randall K. Ten , McShan Daniel L. , Fraass Benedick A.
Published: March 2003

The Collaborative Working Group (CWG) of the National Cancer Institute (NCI) electron beam treatment planning contract has performed a set of 14 experiments that measured dose distributions for 28 unique beam‐phantom configurations that simulated various patient anatomic structures and beam geometries. Multiple dose distributions were measured with film or diode detectors for each configuration, resulting in 78, 2‐D planar dose distributions and one, 1‐D depth‐dose distribution. Measurements were made for 9‐ and 20‐MeV electron beams, using primarily 6×6‐ and 15×15‐cm applicators at several SSDs. Dose distributions were measured for shaped fields, irregular surfaces, and inhomogeneities (1‐D, 2‐D, and 3‐D), which were designed to simulate many clinical electron treatments. The data were corrected for asymmetries, and normalized in an absolute manner. This set of measured data can be used for verification of electron beam dose algorithms and is available to others for that purpose.
Volume‐based geometric modeling for radiation transport calculations
Authors: Li Zuofeng , Williamson Jeffrey F.
Published: March 2003

Accurate theoretical characterization of radiation fields is a valuable tool in the design of complex systems, such as linac heads and intracavitary applicators, and for generation of basic dose calculation data that is inaccessible to experimental measurement. Both Monte Carlo and deterministic solutions to such problems require a system for accurately modeling complex 3‐D geometries that supports ray tracing, point and segment classification, and 2‐D graphical representation. Previous combinatorial approaches to solid modeling, which involve describing complex structures as set‐theoretic combinations of simple objects, are limited in their ease of use and place unrealistic constraints on the geometric relations between objects such as excluding common boundaries. A new approach to volume‐based solid modeling has been developed which is based upon topologically consistent definitions of boundary, interior, and exterior of a region. From these definitions, FORTRAN union, intersection, and difference routines have been developed that allow involuted and deeply nested structures to be described as set‐theoretic combinations of ellipsoids, elliptic cylinders, prisms, cones, and planes that accommodate shared boundaries. Line segments between adjacent intersections on a trajectory are assigned to the appropriate region by a novel sorting algorithm that generalizes upon Siddon's approach. Two 2‐D graphic display tools are developed to help the debugging of a given geometric model. In this paper, the mathematical basis of our system is described, it is contrasted to other approaches, and examples are discussed.
Accelerator‐based epithermal neutron beam design for neutron capture therapy
Authors: Yanch J. C. , Zhou X‐L. , Shefer R. E. , Klinkowstein R. E.
Published: March 2003

Recent interest in the production of epithermal neutrons for use in boron neutron capture therapy (BNCT) has prompted an investigation into the feasibility of generating such neutrons with a high current proton accelerator. Energetic protons (2.5 MeV) on a 7Li target produce a spectrum of neutrons with maximum energy of roughly 800 keV. A number of combinations of D2O moderator, lead reflector, 6Li thermal neutron filtration, and D2O/ 6Li shielding will result in a useful epithermal flux of 1.6×108 n/s at the patient position. The neutron beam is capable of delivering 3000 RBE‐cGy to a tumor at a depth of 7.5 cm in a total treatment time of 60–93 min (depending on RBE values used and based on a 24‐cm diameter×19‐cm length D2O moderator). Treatment of deeper tumors with therapeutic advantage would also be possible. Maximum advantage depths (RBE weighted) of 8.2–9.2 (again depending on RBE values and precise moderator configuration) are obtained in a right‐circular cylindrical phantom composed of brain‐equivalent material with an advantage ratio of 4.7–6.3. A tandem cascade accelerator (TCA), designed and constructed at Science Research Laboratory (SRL) in Somerville MA, can provide the required proton beam parameters for BNCT of deep‐seated tumors. An optimized configuration of materials required to shift the accelerator neutron spectrum down to therapeutically useful energies has been designed using Monte Carlo simulation in the Whitaker College Biomedical Imaging and Computation Laboratory at MIT. Actual construction of the moderator/reflector assembly is currently underway.
A digital‐radiographic technique for invitro tissue thickness measurement using iodine displacement
Authors: Holdsworth D. W. , Dunmore P. J. , Roach Margot R. , Fenster A.
Published: March 2003

A novel digital radiographic method has been developed for measuring the thickness of a tissue sample by iodine displacement. This simple, accurate method is useful both in medical research and in the comparison of pathological and clinical findings. Radiographic measurements of tissue samples in air suffer from the limitation that the quantity measured depends on the product of both thickness and radiographic attenuation coefficient. This technique allows one to obtain thickness measurements from a digital radiograph of a tissue sample suspended in a bath of radio‐opaque contrast agent. The attenuation of the iodinated contrast agent is much higher than that of tissue or calcium. Thus the resulting image is determined largely by the contour of the surrounding bath, and is only slightly influenced by the composition of the tissue. This technique improves both the accuracy and precision of radiographic thickness determination. In this paper the iodine displacement technique is described and the accuracy and precision of thickness measurements in appropriate phantoms are quantified. This technique has been used with both image intensifier and screen‐film based imaging systems to obtain thickness maps of calcified human aorta, with precision better than 4% and spatial resolution of 2.5 mm−1.
Evaluation of video gray‐scale display
Authors: Nawfel Richard D. , Chan Karen H. , Wagenaar Douglas J. , Judy Philip F.
Published: March 2003

Setting up and maintaining video display monitors properly will help to reduce display variation and improve overall presentation of the radiological image. Display monitor gray‐scale characteristics were examined using the SMPTE test pattern. This test pattern may be used as a standard for adjusting brightness and contrast. The controls should be adjusted to display the full dynamic range so that the 5% and 95% signal levels in the pattern are visible. Measured luminance on a laboratory workstation used for radiological perceptual experiments, and on the Siemens CT gray‐scale monitor was determined to range from 0.17 to 76.0 nit, and 0.17 to 24.66 nit, respectively. These were compared with the range of approximately 17 to 514 nit for a typical film–viewbox combination. Characteristic curves were determined for both monitors, and CRT gammas were 3.34 and 2.48 for the perceptual workstation and CT console, respectively. The display gamma was determined from fitting luminance data to a log–log plot of luminance versus input gray level. The usefulness of the SMPTE test pattern for visual presentation as well as photometric measurement is demonstrated.
A comparison of semiempirical models for generating tungsten target x‐ray spectra
Authors: Bissonnette Jean‐Pierre , Schreiner L. John
Published: March 2003

Models for the computer generation of tungsten target x‐ray spectra proposed by Birch and Marshall [Phys. Med. Biol. 24, 505–517 (1979)] and recently by Tucker, Barnes, and Chakraborty [Med. Phys. 18 211–218 (1991)] are compared. Some basic differences in the equations for the number of bremsstrahlung photons of different energies in the spectra are discussed. The models are compared in terms of their ability to characterize x‐ray spectra from constant potential clinical units using three parameter equivalent spectra (EQSPEC) determined from the fit of model generated transmission curves through aluminum to measured data. The Kramers model for x‐ray generation is included for completeness. It is shown that two of the models generate very similar x‐ray spectra from given transmission curves although the fitting parameters in the EQSPEC characterization differ.
A fast and accurate algorithm for volume determination in MRI
Authors: Peck D. J. , Windham J. P. , Soltanian‐Zadeh H. , Roebuck J. R.
Published: March 2003

The determination of volumes in clinical MRI studies are prohibitive because of the time required to compute an accurate volume. Techniques that speed up the calculation are prone to large errors which make most impractical for an accurate diagnosis. A linear filter, called the eigenimage filter, has been developed that separates a desired feature from other features which interfere with its observation in an image. Using the images produced by this technique (eigenimages), the amount of operator interaction required to calculate volumes are significantly reduced. The technique also has the ability to correct for partial volume averaging effects and as a result a more accurate volume can be determined. The technique was applied to a computer simulation and two phantom studies. The time required to calculate the volume was less than 1 min per slice and the errors in accuracy and reproducibility were less than 2% for all studies.
Differential‐pencil‐beam dose calculations for charged particles
Authors: Petti Paula L.
Published: March 2003

The use of a convolution or differential‐pencil‐beam (DPB) algorithm has been studied for charged‐particle dose calculations as a means of more accurately modeling the effects of multiple scattering. Such effects are not reflected in current charged‐particle dose calculations since these calculations rely on depth‐dose data measured in homogeneous water‐equivalent phantoms and use ray‐tracing techniques to calculate the water‐equivalent pathlength from patient CT data. In this study, isodose plots were generated from three‐dimensional dose calculations using Monte Carlo, DPB, and standard ray‐tracing methods for a 4‐cm modulated 150‐MeV proton beam incident on both homogeneous and heterogeneous phantoms. To simulate therapy conditions with charged particles, these studies included cases where compensating boluses were introduced to modify the particle range across the treatment field. Results indicate that multiple‐scattering effects, including increased penumbral width as a function of beam penetration and the “smearing” of isodose distributions downstream from complex heterogeneities, are well modeled by the DPB algorithm. The DPB algorithm may also be used to obtain more useful estimates of the dose uncertainty in regions near the end of the beam's range downstream from complex heterogeneities than can be derived from standard ray‐tracing calculations.
Highly stable solid‐state x‐ray detector array
Authors: Takahashi Tetsuhiko , Nakagawa Manabu , Yoshida Minoru , Takeuchi Hiroshi
Published: March 2003

A detailed analysis of the effects of temperature changes, over time and between array elements, on the generation of circular artifacts in images produced by x‐ray computed tomography was reported. We give formulas for calculating—according to the x‐ray energy, detector sensitivity, and observed image quality (contrast and spatial resolution)—the maximum offset temperature coefficient and maximum gain temperature coefficient that will allow circular‐artifact‐free imaging. A temperature‐controlled and insulated solid‐state x‐ray detector array, consisting of Gd2O2S:Pr,Ce,F ceramic scintillators coupled to crystal Si pin‐photodiodes and designed to meet the requirements for these coefficients, produced high‐resolution artifacts‐free CT images of a phantom head.
Some noise properties of 2DFT MR images from asymmetrically sampled data
Authors: Hua Jianmin , Hurst Gregory C. , Duerk Jeffrey L.
Published: March 2003

This report describes noise statistics in 2DFT MR images, expanding the earlier work of Henkelman and others to include variably asymmetric sampling and conjugate synthesis reconstruction. The effects of low‐order polynomial and Fourier phase correction used with conjugate synthesis are also explicitly considered. This analysis shows that complex images obtained by conjugate synthesis have an elliptical noise distribution, with the smaller axis corresponding to the imaginary image channel. Derivations and simulations predict a ratio of mean to standard deviation in the background of magnitude images varying from the known value of (approximately 1.91) for full symmetry to (approximately 1.32) at fully asymmetric or half‐echo sampling; these predictions are validated over a range of asymmetry by experimental measurements. These results are important for predicting and interpreting image noise when using asymmetric sampling.
Stereotactic radiosurgery: Dose‐volume analysis of linear accelerator techniques
Authors: Serago Christopher F. , Houdek Pavel V. , Bauer‐Kirpes Bernhard , Lewin Alan A. , Abitbol Andre A. , Gonzalez‐Arias Sergio , Marcial‐Vega Victor A. , Schwade James G.
Published: March 2003

Stereotactic radiosurgery of the brain may be accomplished with a linear accelerator by performing several noncoplanar arcs of a highly collimated beam focused at a point. The shape of the radiation distribution produced by this technique is affected by the beam energy, field size, and the number and size of the arcs. The influence of these parameters on the resulting radiation distributions was analyzed by computing dose volume histograms for a typical brain. Dose volume functions were computed for: (a) the energy range of 4–24 MV x rays; (b) target sizes of 1–4 cm; and (c) 1–11 arcs and dynamic rotation. The dose volume histograms were found to be dependent on the number of arcs for target sizes of 1–4 cm. However, these differences were minimal for techniques with 4 arcs or more. The influence of beam energy on the dose volume histogram was also found to be minimal.
Image information transfer properties of x‐ray intensifying screens in the energy range from 17 to 320 keV
Authors: Ginzburg A. , Dick C. E.
Published: March 2003

The image information transfer properties of a number of x‐ray fluorescent screens have been measured for x‐ray energies from 17 to 320 keV. The detective quantum efficiency of the screens at each x‐ray energy has been determined by separate measurements of the x‐ray absorption efficiency and the statistical factor associated with the emission of optical photons upon absorption of an incident x ray. Data have been recorded for both rare‐earth phosphor screens and calcium tungstate screens. The value of the statistical factor for optical photon emission tends toward a constant value as the incident energy increases. Comparisons of the image information transfer properties are presented for several screens, which have been measured over a ten year interval. The utility of the screens for high‐energy radiography is discussed.
Low‐energy imaging with high‐energy bremsstrahlung beams: Analysis and scatter reduction
Authors: Mah Dennis W. , Galbraith Duncan M. , Rawlinson J. Alan
Published: March 2003

The contrast and zero spatial frequency signal‐to‐noise ratio produced by a method for radiation therapy portal imaging known as low‐energy imaging with high‐energy bremsstrahlung beams have been mathematically analyzed. The analysis makes extensive use of Monte Carlo techniques and incorporates the detector, the spectrum, phantom, and geometry. The analysis is validated through comparison with measured data including subject contrast measurements and the attenuation of the beam with lead. Scatter reduction is found to be potentially the most effective method to improve contrast and SNR for a film based system. A large fraction of the scatter detected is of a much higher energy than that found in diagnostic radiology. Hence, traditional antiscatter grids, such as those used in diagnostic radiology, are ineffective. The analysis and theory from the literature are applied to design a new grid which is more appropriate for this application. The grid produces a modest improvement according to a contrast‐detail study.
Automatic on‐line inspection of patient setup in radiation therapy using digital portal images
Authors: Gilhuijs K. G. A. , van Herk M.
Published: March 2003

A new method is presented for inspection of patient setup in radiation therapy by automatic comparison of the patient position relative to the beam position in portal and simulator images. Quantification of patient‐setup errors in terms of translation, rotation, and magnification is achieved by chamfer matching, a robust technique to match drawings and images, which is applied to both anatomy outlines and field edges. Applied to field edges, chamfer matching detects and visualizes deviations in field shape. Applied to anatomy outlines, the matching procedure quantifies and visualizes deviations in patient position relative to the radiation field. To test the method and to judge its feasibility, its behavior for four hundred different patient‐setup deviations, which were simulated in four clinical images, was examined. These images show a top view of the pelvic region. The performance was measured in terms of accuracy and success rate for numerous cost functions and distance codings associated with the chamfer matching procedure. An average accuracy of 1.8 mm was found, a success rate of 90%, and an average overall computation time of 3 s on a 486 microcomputer. The whole analysis procedure is fast enough to allow on‐line application.
Thermoluminescent characteristics of LiF:Mg,Ti from three manufacturers
Authors: Fairbanks E. J. , DeWerd L. A.
Published: March 2003

LiF:Mg,Ti thermoluminescent dosimeters (TLD's) were obtained from three manufacturers with the specification that they all lie within ±5% of the mean. All were chips of size 3 mm×3 mm×1 mm. The three sample groups were compared on the basis of their energy response, linearity of the thermoluminescent (TL) response with increasing exposure, sensitivity, and reproducibility of TL response. All TLD groups showed similar characteristics in energy response and linearity. This is to be expected since the base material and dopants were the same. However, the sensitivity varied among the three groups by as much as 40% and reproducibility measurements gave standard deviations ranging from 7% to 14% among the groups. Annealing procedures were identical in all cases, as were irradiation and readout conditions.
MIRD formulation
Authors: Watson Evelyn E. , Stabin Michael G. , Siegel Jeffry A.
Published: March 2003

Equations are given for calculating absorbed doses in nuclear medicine. (AIP)
Pharmacokinetic modeling
Authors: Strand Sven‐Erik , Zanzonico Pat , Johnson Timothy K.
Published: March 2003

For radiation dosimetry calculations of radiolabeled monoclonal antibodies, (MAb), pharmacokinetics are critical. Specifically, pharmacokinetic modeling is a useful component of estimation of cumulated activity in various source organs in the body. It is thus important to formulate general methods of pharmacokinetic modeling and of pharmacokinetic data reduction, leading to cumulated activities. In this paper different types of models are characterized as “empirical,” “analytical,” and “compartmental” pharmacokinetic models. There remains a pressing need for quantitative studies in man for a proper understanding of the pharmacokinetics of MAb. Pharmacokinetic modeling of radiolabeled MAb in vivo has relied on relatively limited studies in man and complementary detailed measurements in animals. In either case, any model chosen for analysis of such data is inevitably based on measurements of limited accuracy and precision as well as assumptions regarding human physiology. Very few macroscopic compartmental pharmacokinetic models for MAb, have been tested over a range of conditions to determine their predictive ability. Extracorporeal immunoadsorption represents one approach for drastically altering the biokinetics of antibody distribution, and may serve to validate a given pharmacokinetic model. In addition to macroscopic modeling, the microscopic evaluation of the time‐dependent distribution of radiolabeled MAb in tissues is of utmost importance for a proper understanding of the kinetics and radiobiologic effect. Many tumors do not exhibit homogeneous uptake. A mathematical understanding of that distribution is thus essential for accurate tumor dosimetry estimates. This review summarizes methodologies for pharmacokinetic modeling, critically reviews specific pharmacokinetic models and demonstrates the capability of modeling for predictive calculations of altered pharmacokinetics, emphasizing its use in dosimetric calculations.
Microdosimetric concepts in radioimmunotherapy
Authors: Humm J. L. , Roeske J. C. , Fisher D. R. , Chen G. T. Y.
Published: March 2003

In microdosimetry particular emphasis is placed on the stochastic fluctuation of dose in small target volumes such as individual cell nuclei or chromatin fiber, and their relevance to radiobiologic toxicity. Thus microdosimetry is intimately associated with models of radiation action. There are three principal areas where microdosimetry has been applied: (1) radiation protection, (2) high LET radiotherapy, e.g., neutron therapy, and (3) incorporated radionuclides, and in this latter category the importance of microdosimetry to the radiobiology of radiolabeled antibodies is becoming increasingly recognized. The objective of microsdosimetry is the complete characterization of energy deposition within all target volumes throughout the tissue of interest. The importance and relevance of this pursuit will depend upon the properties of the radionuclide emissions and the spatial distribution of the radionuclide relative to the target volumes. If the distribution of internal emitters within both malignant and normal tissue is uniform, the application of microdosimetry to radioimmunotherapy (RIT) is limited to α‐emitters and Auger emitters. Under such circumstances the traditional MIRD formalism for the evaluation of tumor and tissue doses from the commonly used β‐emitters is entirely adequate. This, however, is rarely the case. When the distribution of radiolabeled antibody is nonuniform, techniques of dose averaging over volumes greater in size than the individual target volumes can become inadequate predictors of the biological effect. The concepts, methods, and realm of applicability of microdosimetry within the field of radioimmunotherapy are emphasized in this paper.
Display equalization: A new display method for portal images
Authors: Moseley J. , Munro P.
Published: March 2003

A method of displaying portal images which permits the user to optimize the display contrast (windowing and leveling) without clipping parts of the image has been developed. The method finds the average signal (brightness) in small regions of the image, generally 1/64th of the total image area. The difference between the average signal in each small region and the global average over the entire image is subtracted from the original image. This eliminates changes in average signal that occur over large spatial dimensions, such as those due to varying patient thickness, without obscuring changes in signal that occur over small spatial dimensions, such as those due to anatomic structures. The technique is conceptually simple, does not introduce any visually distracting noise into the image, and is not computationally intensive. The display method has been shown to work well with images acquired from a TV camera‐based imaging system used in radiation therapy.
Evaluations of two solid water parallel‐plate chambers in high‐energy photon and electron beams
Authors: Kubo Hideo
Published: March 2003

Using high‐energy photon and electron beams, the response of two solid water parallel‐plate chambers is evaluated against a PTW (Physicallsch‐Technische Werkstatten) Farmer‐type chamber. These two chambers are (1) a Memorial “Holt” chamber which was further modified by this author to take advantage of electrically conductive solid water, and (2) a thin window parallel‐plate chamber designed by Attix. The evaluations are made for (1) stability and reproducibility, (2) polarity effect, (3) ion recombination correction, (4) field size dependence on output, (5) dose rate dependence on output, and (6) absorbed dose comparison in high‐energy photon and electron beams using the American Association of Physicists in Medicine Task Group 21 (TG21) protocol.
Three‐dimensional verification of patient placement during radiotherapy using portal images
Authors: Bijhold J.
Published: March 2003

The use of a single portal image for the three‐dimensional verification of patient placement during radiotherapy has been investigated. In this study, a deviation in the patient positioning is quantified by a three‐dimensional translation and rotation of the patient. The parameter values are obtained by fitting the projections of anatomical match points in the simulator and the portal image, using the three‐dimensional gantry coordinates of these points in the intended treatment setup. Two methods for finding an analytical solution for the fitting problem are presented. One method yields a solution from the affine transformation of the portal image (shift, rotation, magnification, and stretching in a specified direction) that fits the projections of the match points on the simulator image. The second and more accurate method yields a solution that expresses the estimated parameter values and their covariances as functions of image and three‐dimensional gantry coordinates of the selected matchpoints. The robustness and sensitivity of the solution is implicitly given by these expressions. The applications of these methods are illustrated by experiments with a human pelvic bone.
Anisotropy functions for 103Pd, 125I, and 192Ir interstitial brachytherapy sources
Authors: Nath Ravinder , Meigooni Ali S. , Muench Philip , Melillo Anthony
Published: March 2003

Anisotropy of dose distributions around 103Pd, 125I, and 192Ir sources for interstitial brachytherapy was examined. Dose rates around 125I models 6702 and 6711 and 192Ir sources were measured using lithium fluoride thermoluminescence dosimeters (LiF TLDs) in a water‐equivalent, solid phantom. From these measured data for 125I and 192Ir and the previously published measured data for 103Pd, isodose rate contours were determined using a bivariate interpolation and smooth surface fitting algorithm. The anisotropy functions, F(r,θ), as defined by the Interstitial Collaborative Working Group (ICWG) for each source, were determined. Also, 4π‐averaged anisotropy factors, φan(r), for use in point source approximation, have been calculated at radial distances varying from 1–10 cm for 103Pd, 125I, and 192Ir sources. The anisotropy factors had average values of 0.90, 0.93, 0.95, and 0.98 for 103Pd, 125I model 6711, 125I model 6702, and 192Ir, respectively. The anisotropy factors determined from dose measurements in phantom are observed to be closer to unity than from those determined previously from in‐air measurements. This can be attributed to the smoothing of two‐dimensional dose distributions due to the presence of more scattered photons in the phantom measurements compared to in‐air measurements. Because in‐phantom measurements simulate more closely the brachytherapy patient, data from these experiments are recommended for a more accurate determination of dose distributions around clinical brachytherapy implants. In this work, we present a complete set of source data for two‐dimensional dosimetry following the ICWG formalism.
Analysis of projection geometry for few‐view reconstruction of sparse objects
Authors: Henri C. J. , Collins D. L. , Peters T. M.
Published: March 2003

In this paper certain projections are examined as to why they are better than others when used to reconstruct sparse objects from a small number of projections. At the heart of this discussion is the notion of “consistency,” which is defined as the agreement between the object's 3‐D structure and its appearance in each image. It is hypothesized that after two or more projections have been obtained, it is possible to predict how well a subsequent view will perform in terms of resolving ambiguities in the object reconstructed from only the first few views. The prediction is based on a step where views of the partial reconstruction are simulated and the use of consistency to estimate the effectiveness of a given projection is exploited. Here some freedom is presumed to acquire arbitrary as opposed to predetermined views of the object. The principles underlying this approach are outlined, and experiments are performed to illustrate its use in reconstructing a realistic 3‐D model. Reflecting an interest in reconstructing cerebral vasculature from angiographic projections, the experiments employ simulations based on a 3‐D wire‐frame model derived from an internal carotid arteriogram. It is found that for such an object, the predictions can be improved significantly by introducing a correction to account for the degree to which the object possesses some symmetry in shape. For objects sufficiently sparse, this correction is less important. It is concluded that when the number of projections is limited, it may be possible to favorably affect the reconstruction process in this manner.
The effect of strong longitudinal magnetic fields on dose deposition from electron and photon beams
Authors: Bielajew Alex F.
Published: March 2003

The use of strong, uniform, longitudinal magnetic fields for external electron and photon beam irradiation is considered. Using the EGS4 Monte Carlo code modified to account for the presence of magnetic fields, dramatic narrowing of penumbra for photon and electron irradiations is demonstrated. In the vicinity of heterogeneities, “hot” and “cold” spots due to multiple scattering in electron beams are reduced substantially. However, in the presence of strong magnetic fields, the effect of inhomogeneities can be observed far from the location of the inhomogeneity due to reduced “washout” caused by lateral multiple scattering. The enhanced “Bragg peak,” proposed or calculated by other authors, is not observed on the central axis of broad beams, owing to lateral equilibrium. It is proven that for broad parallel beams, the central axis depth‐dose curve is independent of the strength of the external longitudinal magnetic field, as long as it is uniform. However, strong longitudinal magnetic fields can induce enhancements by redirection of the electron fields coming from point sources. Strong uniform longitudinal magnetic fields provide a way of controlling the spreading of electron beams due to multiple scattering, making the electron beams more “geometrical” in character, simplifying dose‐deposition patterns, possibly allowing electron beams to be used in new ways for radiotherapy. Photon therapy also benefits from strong uniform longitudinal magnetic fields since the penumbra or other lateral disequilibrium effects associated with lateral electron transport can be eliminated.
An automated image‐registration technique based on multiple structure matching
Authors: Petti Paula L. , Kessler Marc L. , Fleming Terri , Pitluck Samuel
Published: March 2003

A new image‐registration technique that matches multiple structures on complementary imaging data sets (e.g., CT and MRI) has been developed and tested with both phantom and patient data. The algorithm assumes a rigid‐body transformation and is suitable for correlating structures within the cranium or at the skull base. The basic premise of the new technique is that an optimum transformation is achieved when the relative volume lying outside of the intersection between a structure and its transformed counterpart is a minimum. This relative volume is calculated numerically using a random sampling approach, and a binary searching algorithm was used to step through the nine‐dimensional parameter space consisting of three rotation angles, three scaling factors and three components of a translation vector. For the nine tests using phantom data, the automated structure‐matching technique was able to predict the correct rotation angles to within ±1°. The expected clinical performance of the new technique was assessed by comparing results obtained with the new method to those obtained using other techniques for 12 patients who were treated with charged particles at Lawrence Berkeley Laboratory (LBL) and who had image‐registration studies performed as part of their treatment plan. For 9 of the 12 patients considered, the new structure‐matching technique produced a significantly better registration than the older methods, as measured by the resultant average relative volume lying outside of the intersection between any structure and its transformed counterpart. For the other three patients, results were not significantly different for the new structure‐matching method and the older techniques.
Laser‐induced thermoelastic deformation: A three‐dimensional solution and its application to the ablation of biological tissue
Authors: Albagli Douglas , Dark Marta , von Rosenberg Charles , Perelman Lev , Itzkan Irving , Feld Michael S.
Published: March 2003

Under certain conditions, laser light incident on a target material can induce an explosive removal of some material, a process called laser ablation. The photomechanical model of laser ablation asserts that this process is initiated when the laser‐induced stresses exceed the strength of the material in question. Although one‐dimensional calculations have shown that short pulsed lasers can create significant transient tensile stresses in target materials, the stresses last for only a few nanoseconds and the spatial location of the peak stresses is not consistent with experimental observations of material failure in biological tissues. Using the theory of elasticity, analytical expressions have been derived for the thermoelastic stresses and deformations in an axially symmetric three‐dimensional solid body caused by the absorption of laser light. The full three‐dimensional solution includes three stresses, radial, circumferential and shear, which are necessarily absent in the simple one‐dimensional solution. These stresses have long‐lived components that exist for eight orders of magnitude longer in time than the acoustic transients, an important point when the details of dynamic fracture are considered. Many important qualitative features are revealed including the spatial location of the peak stresses, which is more consistent with experimental observations of failure.
On the primary barrier in diagnostic x‐ray shielding
Authors: Dixon Robert L.
Published: March 2003

The traditional approach to shielding design for diagnostic x‐ray facilities has been to designate as primary barriers the floor and those walls on which the useful beam may impinge, and to ignore the attenuation provided by the patient, grid, cassette, cassette holder, and x‐ray table in computing the required thickness of these barriers. The degree of attenuation provided by the aforementioned materials has been measured on three‐phase x‐ray equipment including a variety of modern x‐ray tables, grids, and cassettes. The primary beam is shown to be attenuated by more than two orders of magnitude at 100 kVp by the x‐ray tables tested prior to impinging on the floor (ignoring patient attenuation). If patient attenuation is included, the attenuation is more than three orders of magnitude. Transmission factors as well as lead and concrete equivalencies for the various attenuating materials have been determined and included in tabular form. Example calculations for a heavy work load show that only a modest amount of concrete is required in the floor as a primary barrier if attenuation by these materials is taken into consideration (less than 2.5 cm at 80 kVp and less than 4 cm at 100 kVp) and the required secondary barrier may be thicker than the primary barrier. As verification, the actual exposure just below the table bucky was measured over a period of 3 weeks in two radiographic rooms using film dosimetry, and was found to be less than 0.015 mSv week−1 (1.5 mR/week) for a heavy work load of 24 patients per day, thus no primary beam shielding would have been required in the floor for these x‐ray rooms.
Basic optothermal diffusion theory for interstitial laser photocoagulation
Authors: Wyman Douglas R. , Whelan William M.
Published: March 2003

A theoretical basis for interstitial laser photocoagulation (ILP) practiced with point‐emitting fiber tips has been established by solving the bioheat transfer equation, using basic Green's function methods, for steady and instantaneous point sources of both optical energy and direct heat. Three combination optical and thermal parameters have been identified that strongly influence temperature distributions during ILP. These are defined here as optothermal heat capacities and an optothermal diffusion length, all of which characterize how a thermal diffusion temperature profile is flattened and reduced when optical diffusion is added. Relevance and limitations of this theory for practical ILP are discussed. A useful result is a mathematical verification of previous empirical observations that point optical sources heat tissues less than point heat sources of the same power. A comparison of normalized theoretical temperature transients with published measurements suggests that in normal liver, blood perfusion cooling may exceed thermal conduction by a factor of 5.6±1.7.