bluuebunny/crossref_metadata_2025 huggingface dataset:
Dosimetry of Sr‐90 ophthalmic applicators
Authors: Reft Chester S. , Kuchnir Franca T. , Rosenberg Ivan , Myrianthopoulos Leon C.
Published: March 2003

Sr‐90 ophthalmic applicators are commonly used for the treatment of superficial eye disorders. Although a variety of dosimetric devices such as film, thermoluminescent dosimeters (TLD's), ion chambers, and radiochromic foils have been used to measure the peak dose at the applicator surface, there is no internationally agreed upon calibration procedure. Recently, large discrepancies among calibrations of the same applicator at three institutions have been reported. Here we describe a technique to obtain the peak dose rate at the applicator surface using LiF TLD's. The technique can be used for the calibration of flat as well as curved surface applicators. Results for two flat and three concave applicators are presented. Our measurement of the surface dose rate for one of the flat applicators is compared with those obtained by four other institutions, each using different dosimetric devices.
X‐ray spectral reconstruction from attenuation data using neural networks
Authors: Boone J. M.
Published: March 2003

An artificial neural network using input data derived from attenuation measurements was trained to generate spectral profiles (relative number of photons versus energy). Once the relative spectral distribution is reconstructed, absolute spectra (number of photons per unit exposure versus energy) can be calculated. A neural network was trained on spectra generated mathematically using the Birch–Marshall model, combined with attenuation data, calculated from the spectra by numerical integration. Whereas attenuation data can be calculated in a straightforward manner from the x‐ray spectra, the reverse is not true. Several neural networks were successfully taught to reconstruct the spectra, given the attenuation data. The networks were tested using kV/inherent filtration combinations that were not in the training set, and the performance of the reconstruction was excellent. Noise in the attenuation data was simulated to test the effects of noise propagation in the reconstruction. The effects of network architecture and data averaging on noise propagation were investigated. Experimentally determined spectral data compiled by Fewell were also used to train a neural network, and the results of the reconstruction were also found to be excellent.
10‐MV x‐ray primary and scatter dose calculation using convolutions
Authors: Iwasaki Akira
Published: March 2003

Three‐dimensional (3‐D) forward and backward primary dose spread functions in water were developed for 10‐MV x rays. Three‐dimensional forward and backward primary dose spread functions in a heterogeneous medium were constructed from the ones in water using the density scaling theorem. Each of the forward and backward primary dose components were calculated using a method of convolving the primary water collision kerma distribution with the forward or backward primary dose spread function. Scatter dose was separated into forward and backward components. Each scatter dose component was calculated using a differential scatter method, a kind of convolution method, where the primary water collision kerma distribution was convolved with a differential scatter‐maximum ratio or differential backscatter factor equation. From the dose calculation and measurement results obtained for water phantoms containing a cork or aluminum slab, it was found that the 3‐D forward and backward primary dose functions were effective especially in regions where there was a loss of longitudinal and/or lateral electronic equilibrium.
A magnetic resonance imaging method of flow by successive excitation of a moving slice
Authors: Kwiat D. , Einav S. , Elad D.
Published: March 2003

A new magnetic resonance imaging of flow by a method of successive excitation of a moving slice is presented here. The method, which in general belongs to the class of time‐of‐flight methods, is based upon selectively changing the frequencies in a sequence of rf pulses in such a way that a selected slice moves through the imaged volume in a preselected direction and velocity, and is repeatedly excited. It is claimed that most of the measured signal is contributed by only those parts of the imaged substance that move with the selected slice. The activation procedure of the flowing spins, which is presented here, enhances the imaging of the flowing matter and reduces the signal from static matter. A specially tailored selective sequence of a rf α‐pulses has been developed. During this sequence, a set of rephasing 180° selective rf pulses is applied. At the end of this sequence an additional 180° rf pulse is applied to eliminate redundant signals from the static matter in the last activated slice.
Development of an anthropomorphic breast phantom
Authors: Caldwell Curtis B. , Yaffe Martin J.
Published: March 2003

A new technique for producing anthropomorphic radiologic phantoms is described. Information from a patient radiograph is used to design the phantom. Optical densities from a digitized radiograph are converted to thicknesses of a phantom material, with corrections for geometry, x‐ray spectrum, and scatter. Numerically controlled machining techniques are used to transcribe the anatomic details into the phantom material. High spatial frequencies are reproduced separately from low spatial frequencies using photochemical techniques in order to replicate finer details in the phantom. The result is a phantom consisting of a number of overlays. Radiographs of a breast phantom produced using the technique are shown, energy equivalence to attenuation of breast tissue is assessed, and methods of incorporating fine detail and quantitative features in the phantom are suggested.
Basic theory, design, and preliminary evaluation of a laser scanner for shape sensing below‐the‐knee amputees
Authors: Lancaster Jack L. , Walsh Nicolas , Faulkner Virgil
Published: March 2003

A scanner which produces surface profiles of residual limbs from below‐the‐knee amputees was constructed and evaluated. The scanner uses a turntable mounted laser to define a surface profile line, a charge‐coupled‐device video camera to view the profile, and a custom designed video processing board to extract profile data. A microcomputer is used to control data acquisition and provides the rapid processing capability necessary to resolve scan profile data into a three‐dimensional numerical description of the surface in a timely and cost effective manner. Flexibility of use as well as positional accuracy and reproducibility indicate that the laser scanner is well suited to the task of surface scanning of residual limbs for below‐the‐knee amputees. Additionally, the laser profile scanning technique can be applied to other regions of the body where a mathematical description of the surface is useful, such as in radiotherapy treatment planning.
An accurate method for direct dual‐energy calibration and decomposition
Authors: Cardinal H. Neale , Fenster Aaron
Published: March 2003

We propose the use of conic and cubic surface equations (surfaces of second and third order) to directly approximate the dual‐energy equations (the integral equations for the dual‐energy log‐signal functions, i.e., the negative logarithms of the relative detector signals, considered as functions of the basis‐material component thicknesses of the object) and especially their inverses. These types of surface equations require a minimum number of calibration points, and their solutions are smooth, monotonic functions with the correct linear asymptotic behavior. The accuracy of this method is investigated and compared to that of conventional polynomial approximations, both for simulated and real calibration data, taken from two split‐detector systems. These systems provide a more stringent test of our method than comparable dual‐kVp systems, due to the greater nonlinearity of their log‐signal and inverse functions. For these systems, we show that direct approximation of the inverse dual‐energy equations using the simple eight‐term rational form of the conic surface equation provides an extremely fast decomposition algorithm, which is accurate, robust in the presence of noise, and which can be calibrated with as few as 9 calibration points, or robustly calibrated, with a built‐in accuracy check, using only 16 calibration points. Also, we show that extreme accuracy of approximation (to within less than 10−6 in log‐signal and 1 μm in material thickness) is theoretically attainable using the eighteen‐term form of the cubic surface equation, which has a closed‐form analytic solution. Finally, we consider the effects of noise on calibration accuracy, and derive simple formulas which relate the true and apparent root‐mean‐square (rms) accuracies. These formulas then allow the comparison of the true rms calibration accuracies of various surface approximations, considered as functions of the total calibration heat loading of the x‐ray tube.
Transillumination imaging performance: A time‐of‐flight imaging system
Authors: Hebden Jeremy C. , Kruger Robert A.
Published: March 2003

A series of Monte Carlo simulations have been performed in order to investigate the feasibility of constructing a time‐of‐flight transillumination imaging system that could be used as an effective tool for screening for breast disease. The conceptual design of such a system is described, and simulated images are presented that demonstrate its likely performance. It is found that, whereas the spatial resolution achievable with such a system is only dependent upon its temporal resolution, the scattering characteristics of the tissue being imaged will strongly affect the ultimate imaging performance of such a system.
Magnetically induced surface charges on samples in magnetic resonance imaging radio‐frequency coils: Effect on electric fields, sample losses, and coil resonance
Authors: Harpen Michael D.
Published: March 2003

We present a solution to Maxwell's equations which describe the inductive interaction between an irregularly shaped sample in a magnetic resonance imaging rf coil. It is demonstrated that the inductive interaction between the coil and sample can induce charge densities on the sample surface and that these charge densities lead to observable changes in coil Q, resonant frequency offset and sample power dissipation.
Optimized tissue compensators
Authors: Robinson Donald M. , Scrimger John W.
Published: March 2003

The fabrication of retracted missing tissue compensators which account only for missing primary attenuation produces compensation which is less than optimum. Experimental results can be accurately reproduced with theoretical calculations based on primary and first‐order scattered radiation. This method of analysis may be extended to predict the shape of a compensator which will produce the desired dose distribution at a given depth in phantom. An optimized compensator is constructed based on these theoretical considerations and the analysis of its performance is presented.
Optimal design of radiation compensators
Authors: Djordjevich A. , Bonham D. J. , Hussein E. M. A. , Andrew J. W. , Hale M. E.
Published: March 2003

A method to achieve the optimal design of radiation therapy compensating filters in multibeam situations is proposed. Dose distribution is optimized such that it approaches the prescribed value in the target zone, is minimized everywhere else, and does not exceed designated limits at vulnerable regions. Field shapes, sizes, and weights, together with filter thicknesses that take into account missing tissues and body heterogeneities, are calculated as a result of this process. The dose distribution, after the application of the calculated filters and beams, is also predicted. The method is rigorous, and its accuracy depends only upon the accuracy of the applied dose calculation method. Examples are discussed and verified using data obtained from Theraplan 300L treatment planning system.
Nonlinear magnetic stereotaxis: Three‐dimensional, invivo remote magnetic manipulation of a small object in canine brain
Authors: Grady M. S. , Howard M. A. , Molloy J. A. , Ritter R. C. , Quate E. G. , Gillies G. T.
Published: March 2003

In a series of in vivo experiments on five adult canines, a small cylindrical permanent magnet (∼5‐mm diameter×5 mm long) was magnetically moved under fluoroscopic guidance from an occipital‐lobe burr hole to a predetermined destination within the brain and then removed. On three of the animals, dorsal and temporal skull markers were used to establish a coordinate system against which the motions of the seed were referenced. These procedures were sufficiently accurate to permit the guided motion of the seed along nonlinear paths within the brain, including traversal of the midline through the corpus callosum. For removal, the seed could be steered either to a frontal lobe location for extraction through an auxiliary burr hole, or back to the same burr hole through which it had been inserted. This article discusses the way in which stereotactic motions were obtained, the performance limits of the instrumentation and the precision of motion achieved.
An algorithm for generation of implant plans for high‐dose‐rate irradiators
Authors: Renner W. D. , O'Connor T. P. , Bermudez N. M.
Published: March 2003

An algorithm is described for generating a treatment plan with minimal input from the user for a remote high‐dose‐rate afterloading irradiator. The algorithm generates a plan after locating all catheters involved and an area of interest on each catheter, and two additional numbers are specified: a radial distance and a target dose. The treatment volume becomes the locus of all points that are within the specified radial distance from any point within the area of interest on any catheter (except for the end points). For a single catheter, the volume may be alternately outlined on an x‐ray film of the implant. The routine uses a linear programming formulism to compute which dwell positions are to be used, as well as the dwell time at each position, to irradiate the treatment volume to the target dose while minimizing the total volume integrated dose to the patient.
Transillumination imaging performance: Spatial resolution simulation studies
Authors: Hebden Jeremy C. , Kruger Robert A.
Published: March 2003

Monte Carlo calculations were used to simulate the propagation of visible and near‐infrared light through homogenous tissue in order to quantitate the potential spatial resolution performance for transillumination imaging. Specifically, the relative effectiveness of coaxial collimation and time of flight (TOF) detection for improving spatial resolution was investigated. The results demonstrate that significant improvements in spatial resolution can be achieved through these techniques, with TOF methods offering superior performance for a given level of detected signal intensity.
A clinical evaluation of contrast–detail analysis for ultrasound images
Authors: Lopez Hector , Loew Murray H. , Butler Priscilla F. , Hill Michael C. , Allman Robert M.
Published: March 2003

We report on the reproducibility of human observers' vanishing detection thresholds for visual targets in contrast–detail (C/D) analysis of ultrasound B‐mode images. The images used in this study contain visual targets which are circular cross sections of constant‐contrast conical structures in the C/D phantom. The vanishing threshold diameters for these targets vary as a function of the perceived size of the imaged target, target‐to‐background contrast, image noise content, and reproducibility of the decision levels of human observers for repeated observations. Our study indicates that the determination of absolute vanishing threshold diameter values for several targets of different contrast by human observers yields a high degree of error that is not predicted by existing theoretical assumptions based on a static threshold detector. We find that systematic error is introduced by the observers during the course of the experiment and that the levels of sensitivity of the observers differ widely at all times, and increase the amount of total observer error. These results suggest that, due to the large total observer error, C/D analysis may be impractical in a clinical environment, unless there is access to a team of observers specifically and extensively trained in this task. We suggest that a computer‐based observer may be more reliable for the objective performance of contrast–detail analysis as a method for evaluating ultrasound image quality and comparison of imaging systems.
Validity of microsphere model in cerebral blood flow measurement using N‐isopropyl‐p‐(I‐123) iodoamphetamine
Authors: Murase Kenya , Tanada Shuji , Mogami Hiroshi , Kawamura Masashi , Miyagawa Masao , Yamada Masafumi , Higashino Hiroshi , Iio Atsushi , Hamamoto Ken
Published: March 2003

A microsphere model is sometimes used when calculating cerebral blood flow (CBF) using N‐isopropyl‐p‐[I‐123]iodoamphetamine (IMP), and is based on the assumption that there is essentially no washout of IMP. The validity of a microsphere model was investigated by comparison with the values of CBF obtained by means of a model which takes into consideration the diffusion of IMP from brain tissue to blood (nonmicrosphere model). When calculating CBF by the latter model, the look‐up table method was used with expression of the double integral in the model equation by the recursion relations, a method which is useful for obtaining pixel‐by‐pixel values. The average rate constants for diffusion from brain to blood of gray and white matter were 0.021 and 0.016 min−1, respectively. The values of CBF obtained by applying a microsphere model to the data acquired from 0 to 3.2 min after IMP injection were overestimated by ∼23% compared with those values obtained using a nonmicrosphere model. This is considered to be due to the effect of the IMP activity in the vascular space. Values obtained using the data acquired from 3.2 to 6.4 min were underestimated by ∼15 %. When the values of CBF obtained by a microsphere model were interpolated, they became nearly equal to those obtained using a nonmicrosphere model at about 4 to 5 min after injection. This is suggested to be the reason why the underestimation due to diffusion from brain to blood is cancelled out by the overestimation due to the IMP in the vascular space. Our preliminary results suggest that it is necessary to take the diffusion of IMP from brain tissue to blood into account for the quantification of CBF using IMP.
Radiation quality investigations around a neutron therapy beam, using a high‐pressure ionization chamber
Authors: Makrigiorgos G. M. , Smathers J. B. , Myers L. T. , Benedict S. H.
Published: March 2003

The mean quality factor (Q̄) and the dose mean linear energy transfer [LET (L̄D)] a few centimeters outside a neutron therapy beam have been measured using a high‐pressure ionization chamber. Measurements have been carried out in air, 5 and 100 cm outside the treatment field edge on the patient plane for p(46)‐Be neutrons and are compared with the results obtained at the same points with a low‐pressure proportional counter (Rossi counter) and a Bonner spheres system. General agreement is found between these independent methods. The dose equivalent at 5 and 100 cm distances outside the field edge is 2% and 0.4% of the dose equivalent at the isocenter. Measurements were also performed at the same points as a function of proton energy on the Be target. Proton energies of 25–46 MeV on Be were applied. The results indicate increase in the mean quality factor at proton energies below 30 MeV.
Dynamic wedge field techniques through computer‐controlled collimator motion and dose delivery
Authors: Leavitt Dennis D. , Martin Melissa , Moeller John H. , Lee Wing Lok
Published: March 2003

Clinical treatment planning situations arise which require different wedge angles within segments of a single therapeutic x‐ray field. Idealized wedge‐shaped dose distributions, including combination of several wedge segments of different angle within a single field, are generated and delivered through computer control of asymmetric collimator motion and dose per field segment. A dual‐pass technique is introduced to provide improved adherence to the prescribed isodose distribution. Dynamic wedge distributions are verified by film densitometry and ionization chamber measurement. These results suggest the potential importance of this technique as an added clinical radiotherapy tool.
Determination of primary dose in 60Co gamma beam using a small attenuator
Authors: Nizin P. S. , Kase K. R.
Published: March 2003

A measurement technique previously proposed for determining dose from primary radiation has been tested using 60 Co gamma rays. It is shown that the dose from primary radiation is reliably determined for field sizes of 10×10 and 20×20 cm2 at depths of 0.5, 5, and 10 cm in water. With further development this technique may be useful for verifying dose from primary radiation that may be calculated using a variety of methods.
Characterization of fragmented heavy‐ion beams using a three‐stage telescope detector: Measurements of 670‐MeV/amu 20Ne beams
Authors: Llacer J. , Schmidt J. B. , Tobias C. A.
Published: March 2003

Measurements of a 670‐MeV/amu 20Ne beam at the Lawrence Berkeley Laboratory Bevalac heavy‐ion accelerator with various thicknesses of water absorber were obtained with the BERKLET. The BERKLET, a simple three‐stage solid‐state telescope detector, has been described previously. This instrument measures the linear energy transfer (LET) and residual energy of particles, allows the identification of the particle's charge, and provides a means of obtaining LET and energy statistics for the beam, separated by particle charge. The track and dose averaged LET dependence on the amount of water absorber was determined for each species of fragment in the beam. Large numbers of low‐LET particles in the fragmented beam were detected. The results of the analysis are presented followed by a discussion of the effects of multiple scattering and secondary fragmentation on the measurements. A brief discussion of the implications of the BERKLET measurements for radiobiology is also presented.
The multiple Coulomb scattering of very heavy charged particles
Authors: Wong Mervyn , Schimmerling Walter , Phillips Mark H. , Ludewigt Bernhard A. , Landis Donald A. , Walton John T. , Curtis Stanley B.
Published: March 2003

An experiment was performed at the Lawrence Berkeley Laboratory BEVALAC to measure the multiple Coulomb scattering of 650‐MeV/A uranium nuclei in 0.19 radiation lengths of a Cu target. Differential distributions in the projected multiple scattering angle were measured in the vertical and horizontal planes using silicon position‐sensitive detectors to determine particle trajectories before and after target scattering. The results were compared with the multiple Coulomb scattering theories of Fermi and Molière, and with a modification of the Fermi theory, using a Monte Carlo simulation. These theories were in excellent agreement with experiment at the 2σ level. The best quantitative agreement is obtained with the Gaussian distribution predicted by the modified Fermi theory.
Rapid measurement of T1 with spatially selective pre‐inversion pulses
Authors: McVeigh Elliot , Yang Andrew , Zerhouni Elias
Published: March 2003

A method is described for rapidly obtaining a multipoint estimate of T1 from a sample that is homogeneous over a few millimeters. An image of the longitudinal recovery curve is produced through the application of successive “pre‐inversion” slices that are perpendicular to the imaging slice. These pre‐inversion pulses are analogous to pre‐saturation pulses, but they are much thinner and the tip angle is 180°. The baseline for the recovery is measured from sections of the sample that have not been perturbed by the slice selective pre‐inversion pulses. The existence of the baseline value and the lack of slice profile effects allows a quick T1 estimate (QT1) to be made with a simple linear regression algorithm. The QT1 values are found to correlate very well with T1 values measured with the scanner in “spectrometer” mode, for volumes as small as 5×5×5 mm. Possible applications are T1 estimates in homogeneous samples and tissues, and scouting the T1 range of a tissue to be measured with higher resolution volume localization techniques.
The influence of grid size on accuracy in radiotherapy dose plotting
Authors: Smith C. W. , Morrey D. , Gray K.
Published: March 2003

The recent article by Niemierko and Goitein [Med. Phys. 16, 239–247 (1989)] illustrates well the errors which are occurring in plotting isodose lines. We wish to augment their analysis with similar work done at Cardiff a few years ago; to indicate some practical treatment outcomes they have omitted; to propose even more stringent requirements of the grid size used; and thus to further alert users and software manufacturers to this problem.
Dual‐energy mammography: A detector analysis
Authors: Boone John M. , Shaber Gary S. , Tecotzky Melvin
Published: March 2003

Dual‐energy mammography acquisition scenarios employing single‐shot techniques are examined using computer simulation. A figure of merit of the signal‐to‐noise ratio squared over the glandular dose was chosen for the optimization task due to its exposure independence. Doses were evaluated using Monte Carlo techniques. The effects of kilovoltage, prepatient filtration, front detector thickness, mid‐detector filtration thickness and composition were studied. Of the six detector pairs studied (Y2O2S/Gd2O2S, SrFBr/BaFBr, Y2O2S/LaOBr, Y2O2S/CaWO4, Y2O2S/YTaO4, and Y2O2S/LuTaO4), Y2O2S/Gd2O2S and SrFBr/BaFBr were found to be the best combinations. The effects of scatter and signal quantization were also examined. An alternative display technique whereby the tissue‐subtracted (i.e., calcium) image is colorized and overlaid onto the conventional mammogram is introduced.
Anthropomorphic radiation therapy phantoms: A quantitative assessment of tissue substitutes
Authors: Kleck Jeffrey H. , Smathers James B. , Holly F. Eugene , Myers Lee T.
Published: March 2003

Compositional analyses of samples of soft tissue and lung substitutes used in the construction of anthropomorphic radiotherapy phantoms were performed for products from Alderson Research Laboratories Inc., Kyoto Scientific Specimens Company/Capintec Inc., Humanoid Systems Inc., Radiology Support Devices Inc., and The Phantom Laboratory Inc. To assess quality control in the manufacture process, the results of these compositional analyses were compared with individual manufacturer's design specifications and electron densities measured by computed tomography analysis of actual phantom slices. A comparison of the manufacturers design specifications against reference tissue parameters from Tissue Substitutes in Radiation Dosimetry and Measurement (International Commission on Radiation Units and Measurements) indicates marked differences in the basic design of commercially available anthropomorphic phantoms.
Effect of tissue inhomogeneity on dose distribution of point sources of low‐energy electrons
Authors: Kwok Cheuk S. , Bialobzyski Philip J. , Yu Siu K. , Prestwich William V.
Published: March 2003

Perturbation in dose distributions of point sources of low‐energy electrons at planar interfaces of cortical bone (CB) and red marrow (RM) was investigated experimentally and by Monte Carlo codes EGS and the TIGER series. Ultrathin LiF thermoluminescent dosimeters were used to measure the dose distributions of point sources of 204Tl and 147Pm in RM. When the point sources were at 12 mg/cm2 from a planar interface of CB and RM equivalent plastics, dose enhancement ratios in RM averaged over the region 0–12 mg/cm2 from the interface were measured to be 1.08±0.03 (SE) and 1.03±0.03 (SE) for 204Tl and 147Pm, respectively. The Monte Carlo codes predicted 1.05±0.02 and 1.01±0.02 for the two nuclides, respectively. However, EGS gave consistently 3% higher dose in the dose scoring region than the TIGER series when point sources of monoenergetic electrons up to 0.75 MeV energy were considered in the homogeneous RM situation or in the CB and RM heterogeneous situation. By means of the TIGER series, it was demonstrated that aluminum, which is normally assumed to be equivalent to CB in radiation dosimetry, leads to an overestimation of backscattering of low‐energy electrons in soft tissue at a CB–soft‐tissue interface by as much as a factor of 2.
On methods of inhomogeneity corrections for photon transport
Authors: Wong John W. , Purdy James A.
Published: March 2003

Eight methods of photon inhomogeneity correction were examined for their photon transport approximations. The methods were categorized according to the different approaches used to model scatter photon dose contribution. They were the ratio of TAR (RTAR) and the modified Batho power law which utilized only the 1‐D density information along the primary photon path; the equivalent TAR (ETAR) and the FFT convolution methods which incorporated the 3‐D density information of the medium for empirical scatter dose calculation; the differential SAR (DSAR), the delta volume (DV), dose spread array (DSA), and differential pencil beam (DPB) methods which employed explicit 3‐D scatter ray‐trace calculation. Cobalt‐60 measurements in horizontal slab phantoms were used to allow simpler data analysis. RTAR consistently overestimated lung corrections by ∼10%. The scatter ray‐trace approach was not always better as the DSAR calculations were inferior to those using the Batho method. The ray‐tracing DV, DPB, and DSA methods agreed with measurements mostly to within 2%, at the expense of long computation time. The nonscatter ray‐tracing ETAR and FFT convolution calculations were only slightly inferior in the same geometries. These methods improve on the current 1‐D methods and should be seriously considered for fast optimization purposes in practical 3‐D treatment planning.
Line and slice selection for moving spins
Authors: Aaron R. , Shiffman C. A.
Published: March 2003

The Bloch equations are solved numerically for spins moving at constant velocity for the case of 90° slice selection, and for subsequent 180° spin‐echo line selection. The calculations use the comoving, rotating reference frame which is shown to be valid for general accelerated motions. Magnetization profiles and phase shifts are presented for slice selection for both plug and laminar flow. Line selection is studied for the case of plug flow, where a “local vector sum” is introduced to demonstrate effective localization of activated spins. In addition to local magnetization densities, the spatially integrated signals seen by a nonselective antenna are calculated. The relation between net phase shift and flow rate is examined and deviations from the predictions of simple analytic models are discussed.
Forward‐directed bremsstrahlung of 10‐ to 30‐MeV electrons incident on thick targets of Al and Pb
Authors: Faddegon Bruce A. , Ross C. K. , Rogers D. W. O.
Published: March 2003

Bremsstrahlung spectra from thick targets of Al and Pb have been measured absolutely (photons per incident electron) along the beam axis for electrons of 10‐, 15‐, 20‐, 25‐, and 30‐MeV incident energy. The spectra have a 220‐keV low‐energy cutoff. The targets were cylinders with nominal thicknesses of 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20‐cm diameter by 25‐cm‐long cylindrical NaI detector. Measured spectra were corrected for pile‐up, background, detector response, detector efficiency, attenuation in materials between the target and detector and the collimator effect. Spectra were calculated using the EGS4 Monte Carlo system for simulating the radiation transport. The simulation model included the small amount of material upstream of the target. This material contributed about 40% of the spectrum, but its presence or absence had little effect on the calculated bremsstrahlung yield. The shapes of the measured and calculated spectra were in excellent agreement. The ratio of the total number of photons in each measured spectrum to those in the corresponding calculated spectrum varied from 0.97±0.06 to 1.12±0.06, depending largely on the atomic number of the target. Absolute spectral measurements in the literature agreed with our calculations of spectral shape but showed a range of ±30% in the number of photons per incident electron relative to the calculated values, which is contrary to our result.
A study of quality of bremsstrahlung spectra reconstructed from transmission measurements
Authors: Piermattei A. , Arcovito G. , Azario L. , Bacci C. , Bianciardi L. , De Sapio E. , Giacco C.
Published: March 2003

A numerical method for the reconstruction of bremsstrahlung spectra has been applied to the analysis of simulated data. The method is found to be reliable in reconstructing x‐ray spectra of maximum energy up to 10 MeV. Measurements of aluminum transmission data carried out for five linear accelerators have also been analyzed by this method to determine the fractional energy fluence. The values of the Spencer–Attix water/air stopping power ratio, , reported as a function of the ratios , are in good agreement with computed results. Quality index values were correlated to mean incident energies of x‐ray beams used in radiotherapy. The experimental setup has also provided information regarding the softening effect on the off‐axis beam, which can be used in clinical computer dosimetry to correct conventional zero field size tissue maximum ratio.
Assessment of 125I clinical dose specification from recent dose rate evaluations
Authors: Piermattei A. , Arcovito G. , Azario L.
Published: March 2003

Measurements and theoretical evluations of dose distributions due to photon emission of the iodine seeds manufactured by 3M Company, are reported.(AIP)
Extending the concept of primary and scatter separation to the condition of electronic disequilibrium
Authors: Woo M. K. , Cunningham J. R. , Jezioranski J. J.
Published: March 2003

A major deficiency of current photon calculation methods that are based on the concept of primary and scatter separation is their inability to handle the condition of electronic disequilibrium. This deficiency is examined and it is shown that the limitation is not inherent in the algorithms themselves but is, at least in part, in the data which the algorithms use. A new concept of primary and scatter separation is developed to cover the condition of electronic disequilibrium. This new concept requires little change to the existing algorithms and only additional data are required, which are generated using Monte Carlo calculation methods. The new concept is tested using programs in the Theratronics Theraplan treatment‐planning system, and two calculation examples illustrate the ability to model electron transport and also the improvement over the existing algorithms. Close analogy of the extended concept with the convolution/superposition method of dose calculation is also indicated.
A time‐delay integration charge‐coupled device camera for slot‐scanned digital radiography
Authors: Holdsworth D. W. , Gerson R. K. , Fenster A.
Published: March 2003

We have developed a low‐noise digital camera based on a 512×96 element CCD operating in the time‐delay integration mode. This camera has been combined with an x‐ray image intensifier to record radiographic images produced by a scanning slot beam of radiation. This results in the rejection of a large fraction of scattered radiation, without a significant increase in x‐ray tube heat loading or image acquisition time. Here we describe the design of our CCD camera and the results of our investigations of camera resolution, linearity, noise, and quantum efficiency. We have found that both the resolution limit (50 mm−1) and the dynamic range (2100) of this novel camera are greater than reported values for conventional video cameras. Applications of this system in digital angiography and mammography are discussed.
Positional radiotherapy beam dosimetry using a laser heated thermoluminescent plate
Authors: Kearfott Kimberlee J. , Grupen‐Shemansky Melissa E.
Published: March 2003

A 4‐W, laser‐based system for the positional readout of thermoluminescent (TL) plates was utilized to obtain dose profiles for a radiotherapy beam. Experimentally obtained glow curves for LiF resulting from the rapid heating rates characteristic of laser heating agreed well with results predicted using a first‐order kinetic model. Excellent system linearity was obtained for sensitivity to . The useful dynamic range for this tissue‐equivalent system could extend over 8 orders of magnitude. A minimum detectable signal of 1.2 R was estimated for the presently configured system, whose spatial resolution is limited by the laser beam diameter, 1.7 mm. Depth dose profiles for the radiotherapy beam obtained using the prototype laser TL system with a specially designed composite 2.2 cm×3.3 cm×0.254 mm LiF plate agreed to within 14% of ionization chamber measured doses.
Theoretical and experimental evaluation of a temperature controller for scanned focused ultrasound hyperthermia
Authors: Lin Win‐Li , Roemer R. B. , Hynynen K.
Published: March 2003

Maintenance of the controlled temperatures at their target levels in the face of disturbances, a uniform temperature distribution within the treatment region, an acceptable temperature rise outside that volume, a fast temperature rise, and stability are desirable characteristics of an optimal hyperthermia treatment control system. This paper presents a proportional‐integral‐derivative plus bang–bang (power on at either a maximum value or at zero) feedback control system designed to meet the above requirements for a scanned focused ultrasound hyperthermia system. Treatment simulations and analytical results for a first‐order approximation of a tumor show that the controller is stable for a wide range of gains and sampling times. It was also found that there is an optimal controller gain which minimizes the peak temperature overshoot and the settling time when a step function input is applied to the system. Both the simulation results and experimental animal results show that the controlled region can be rapidly heated to the target temperature with a small overshoot and maintained at that level in the face of disturbances. The effects of temperature fluctuations due to both the periodic changes caused by the scanning and due to measurement noise can be reduced by the use of an auto regressive moving average approach. In vitro dog kidney model and in vivo dog thigh experiments show that the controller works well in practice, and verify that it can compensate for spatial and temporal blood perfusion variations. As shown in both these experiments and in simulations the controller can be used for controlling a single temperature or multiple temperature points simultaneously, thus allowing relatively uniform temperature fields to be created.
The development of intracavitary ultrasonic applicators for hyperthermia: A design and experimental study
Authors: Diederich C. J. , Hynynen K.
Published: March 2003

This study investigated the design concepts and development of a multielement intracavitary ultrasound applicator for use in hyperthermia. A necessary condition imposed on these applicators is that each transducer element be separately powered and produce collimated beams. This way, the power deposition within the target volume can be controlled by varying the power to each element. Theoretical computer simulations (acoustic and thermal) and bench experiments were used to determine the constraints on the transducer element size and the spacing between them. These have shown that the length of the cylindrical segments (or subsections of) must be greater than ∼10λ for proper collimation and that the spacing between them must be less than ∼1.5 mm for uniform heating. With these design principles in mind, applicators were constructed using sections of cylindrical transducers (wall‐thickness resonance). These were surrounded by temperature‐controlled circulating water which was enclosed by a latex membrane. This allowed for acoustic coupling and additional control over the depth of the maximum temperature from the cavity wall. This depth could be varied between the cavity surface and up to 1.5 cm for circulating water temperatures between 5 and 42 °C, respectively. These applicators were tested in vivo and were able to induce controlled transrectal heating, at depths of 2–3 cm, in the canine rectum and prostate gland.
Optimization of 125I ophthalmic plaque brachytherapy
Authors: Astrahan Melvin A. , Luxton Gary , Jozsef Gabor , Liggett Peter E. , Petrovich Zbigniew
Published: March 2003

Episcleral plaques containing 125I sources are often used in the treatment of ocular melanoma. Within four years post‐treatment, however, the majority of patients experience some visual loss due to radiation retinopathy. The high incidence of late complications suggests that careful treatment optimization may lead to improved outcome. The goal of optimization would be to reduce the magnitude of vision‐limiting complications without compromising tumor control. We have developed a three‐dimensional computer model for ophthalmic plaque therapy which permits us to explore the potential of various optimization strategies. One simple strategy which shows promise is to maximize the ratio of dose to the tumor apex (T) compared to dose to the macula (M). By modifying the parameters of source location, activity distribution, source orientation, and shielding we find that the calculated T:M ratio can be varied by a factor of 2 for a common plaque design and posterior tumor location. Margins and dose to the tumor volume remain essentially unchanged.
Ultrasound focal lesion detectability phantoms
Authors: Madsen Ernest L. , Zagzebski James A. , Macdonald Micheal C. , Frank Gary R.
Published: March 2003

Two phantoms for assessing the performance of ultrasound scanners regarding detectability of small focal lesions are described. The spherical simulated lesions in the phantoms have diameters of 2.4, 3.0, and 4.0 mm and backscatter coefficients which are 16, 9, and 6 dB below that of the surrounding tissue‐mimicking material, the latter simulating normal tissue such as liver. Random positioning and relatively large numbers of lesions deal with the statistical problem related to mistaking a normal textural fluctuation, characteristic of ultrasound images of parenchymal tissue, to be a small lesion. All lesions have ultrasonic properties that are the same as those of the surrounding material except for the backscatter coefficient; therefore, artifacts resulting from attenuation, reverberation, and/or refraction are absent, and detectability of deeper lesions is not influenced by the presence of more proximal ones. A method is outlined for using the phantoms to determine bounding proximal and distal depths of resolution, or detectability, of focal lesions of a given object contrast and size. Tests for observer dependence involving five subjects resulted in standard deviations in the distal and proximal depths of less than 0.5 cm. A repeat of the observer dependence tests 18 months later showed little change in the results. In addition, results obtained by two experienced observers are also given for a variety of specific scanners, scanning heads, and scanning parameters.
Dose‐volume considerations with linear programming optimization
Authors: Morrill Steven M. , Lane Richard G. , Wong Jorge A. , Rosen Isaac I.
Published: March 2003

A method of incorporating dose‐volume considerations within the framework of conventional linear programming is presented. This method is suitable for the optimization of beam weights and angles using a conformal treatment philosophy (i.e., tailoring the high‐dose region to the target volume only). Dose‐volume constraints are introduced using the concept that volumes of normal tissue nearer the target volume will be allowed higher dose constraints than volumes of normal tissue distal to the target volume. Each involved normal structure is divided into high‐dose and low‐dose volumes. These two volume partitions are represented by constraint points with either high‐dose or low‐dose constraints, respectively. Optimized treatment plans for three clinical sites demonstrate that this technique meets or surpasses the original dose‐volume constraints for a conformal‐type treatment plan using straightforward linear programming in a time frame that is comparable to other linear programming problems.
Results of photon absorbed‐dose measurements using the AAPM TG‐21 protocol for accelerating potentials up to 26 MV
Authors: Hazle John D. , Kirby Thomas H. , Gastorf Robert J. , Hanson William F.
Published: March 2003

The AAPM Task Group 21 protocol for the calibration of high‐energy photon and electron beams was produced to accomplish essentially two goals: (1) incorporate the latest physical data available for calculating absorbed dose from ionization measurements and (2) to eliminate inconsistencies in absorbed dose measurements made with various ion chamber and phantom combinations. The ability of the protocol was assessed to consistently determine x‐ray absorbed dose from measurements made with four Farmer‐type chambers and one parallel‐plate chamber in water, polystyrene, and acrylic phantoms. The measurements were performed using seven high‐energy x‐ray beams from 60Co to 26‐MV nominal accelerating potential. The absorbed dose to water calculated from measurements made with the various chamber and phantom combinations were found to be consistent. The doses calculated for the two most common phantom materials, water and polystyrene, were found to be in excellent agreement. This resolved a 1.6% discrepancy in the absorbed dose determined from the two phantoms using the SCRAD protocol. The doses for acrylic phantoms were found to be approximately 1.2%, low for nominal accelerating potentials less than 8.8 MV. For accelerating potentials of 8.8 MV or greater the agreement was considerably better. The mean dose determined for the parallel‐plate chamber from measurements in polystyrene was found to be within 0.7% of the mean dose determined using Farmer‐type ion chambers in all phantom materials.
Replacement correction (Prepl) for ion chamber dosimetry
Authors: Khan Faiz M.
Published: March 2003

The use of replacement correction when depth dose distribution is measured with a cylindrical ion chamber is discussed. A review of relevant data shows that the displacement of the effective point of measurement is an appropriate method to make replacement correction for the entire depth ionization curve.
Tests of an electron monitor for routine quality control measurements of electron energies
Authors: Ramsay E. B. , Reinstein L. E. , Meek A. G.
Published: March 2003

The depth dose for electrons is sensitive to energy and the AAPM Task Group 24 has recommended that tests be performed at monthly intervals to assure electron beam energy constancy by verifying the depth for the 80% dose to within ±3 mm. Typically, this is accomplished by using a two‐depth dose ratio technique. Recently, a new device, the Geske monitor, has been introduced that is designed for verifying energy constancy in a single reading. The monitor consists of nine parallel plate detectors that alternate with 5‐mm‐thick absorbers made of an aluminum alloy. An evaluation of the clinical usefulness of this monitor for the electron beams available on a Varian Clinac 20 has been undertaken with respect to energy discrimination. Beam energy changes of 3 mm of the 80% dose give rise to measurable output changes ranging from 1.7% for 20‐MeV electron beams to 15% for 6‐MeV electron beams.