bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance characteristics of transmission imaging using a uniform sheet source with parallel‐hole collimation
Authors: Cao ZongJian , Tsui Benjamin M. W.
Published: March 2003

Transmission imaging is receiving increasing attention in SPECT due to the need to compensate for nonuniform attenuation in cardiac‐chest SPECT. The quality of a transmission image has an important effect on the measured attenuation distribution. To improve image quality, knowledge of the performance characteristics of a transmission imaging system is essential. The characteristics, spatial resolution, detection efficiency, photon flux, and exposure to the object, of a transmission imaging system consisting of a gamma camera and a uniform sheet source have been studied. The results demonstrate that spatial resolution of a transmission imaging system can be improved by use of a high‐resolution source collimator at the price of a moderate decrease in detection efficiency, in comparison to the uncollimated case. Also, the source collimator significantly reduces the photon flux and exposure to the object. This investigation suggests that a high‐resolution collimator be used with an intense sheet source to improve spatial resolution and reduce statistical noise with low exposure to the patient. This research further suggests that the amount of source activity is determined by the requirement of image quality, detection geometry, and allowed absorbed dose to the patient.
Clinical dosimetry for implementation of a multileaf collimator
Authors: Boyer Arthur L. , Ochran Timothy G. , Nyerick Carl E. , Waldron Timothy J. , Huntzinger Calvin J.
Published: March 2003

In order to initiate the use of a multileaf collimator (MLC) in the clinic, a set of technical procedures needs to be available sufficient to create MLC leaf settings and to deliver an accurate dose of radiation through the MLC‐shaped field. Dosimetry data for clinical use of the MLC were measured. Dosimetric characteristics included central axis percent depth dose, output factors, and penumbra. In this paper, it has been concluded that a dose control monitor unit calculation procedure that has been applied to the use of conventional secondary field‐shaping blocks can be applied to the multileaf collimator dosimetry. The multileaf collimator penumbra (20% to 80%) is only slightly wider (1–3 mm) than the penumbra of the conventional collimator jaws. Beam's‐eye‐view comparisons made between the isodose curves in fields shaped by conventional Cerrobend blocks and isodose curves in fields shaped by the multileaf collimator demonstrated that the 50% isodose line at 10‐cm depth exhibited the discrete steps of the multileaf collimator leaves, but that the 90% and 10% isodose curves of the multileaf were close to those shaped by Cerrobend blocks.
Skin dose measurements for head and neck radiotherapy
Authors: Fiorino Claudio , Cattaneo Giovanni Mauro , del Vecchio Antonella , Longobardi Barbara , Signorotto Patrizia , Calandrino Riccardo , Fossati Vittorio , Volterrani Fabio
Published: March 2003

The use of immobilization plastic masks in head and neck radiotherapy can partially eliminate skin benefits derived from the utilization of megavoltage photon beams. Filters and blocks between the patient and the accelerator can further increase the skin dose value. In this study, the increase in surface dose due to 2 and 3.2 mm of plastic material utilized for patient immobilization was measured. Then, the effect of blocking trays, wedges, and blocks on skin dose in typical conditions for head and neck irradiation was evaluated. The measurements were obtained with a NE2534 chamber (Markus type) on a perspex phantom for 6 MeV x‐rays from an accelerator.
Treatment planning for asymmetric jaws on a commercial TP system
Authors: Woo M. K. , Fung A. , O'Brien P.
Published: March 2003

In this work, the accuracy of the asymmetric jaws planning feature in a commercial treatment planning (TP) system is assessed. In the latest version of this software, the off‐axis beam quality variation is handled by a function g(d,r), which is derived from measured horizontal beam profiles at four different depths. The calculated and measured isodoses for a 6‐MV linear accelerator with asymmetric jaws agree to ±0.5% along the central axis and to within 2 mm at the beam edge. Formulas for treatment time calculations using the output data reported by the computer program are described, as well as formulas for manual calculations based on pregenerated data tables. Doses calculated based on these formulas are compared to measurement and the accuracy is ±1% and ±2% for the computer and manual calculations, respectively. It is concluded that this version of the treatment planning system as well as the treatment time calculation formulas can be used adequately for asymmetric jaw computerized and manual treatment planning.
Evaluation of the buildup effect of an 192Ir high dose‐rate brachytherapy source
Authors: Park H. C. , Almond P. R.
Published: March 2003

The buildup effect of the 192Ir radioactive source employed as a gamma emitter of the Selectron high dose‐rate (HDR) afterloader was evaluated by studying the ratio of exposure in water to exposure in air as a function of distance. In 1968, Meisberger et al. [Radiology 90, 953‐957 (1968)] calculated a third‐order polynomial fit to a selected average between measured and calculated values. The objective of this investigation, however, is to evaluate the factor for the 192Ir source configuration used in the HDR system, which was different from the source design used by Meisberger et al. This paper presents the measured ratio using an ionization chamber and the calculated ratio using a Monte Carlo simulation code. The experimental and theoretical results show only minor disagreement with the data of Meisberger et al.. However, the results show significant disagreement when they are compared to the model developed by van Kleffens and Star [Int. J. Radiat. Oncol. Biol. Phys. 5 557–563 (1979)], which may indicate the need to reevaluate the algorithm presently employed in HDR treatment planning system. The comparison with other published data will be discussed.
Dose distributions produced by shielded applicators using 241Am for intracavitary irradiation of tumors in the vagina
Authors: Muench Philip James , Nath Ravinder
Published: March 2003

Dosimetric characteristics of shielded vaginal applicators containing encapsulated 241Am sources are investigated in this work. Encapsulated 241Am sources emit primarily 60‐keV photons which are more effectively shielded by thin layers of high atomic number materials than the 662‐keV photons from 137Cs sources. With 241Am, it is possible to achieve almost unidirectional irradiation of localized vaginal tumors. The drastic decrease in irradiation volume on the contralateral side (uninvolved with tumor) is observed to decrease dose by up to 20%, even in the forward direction (unshielded side toward the tumor) of the applicator. A possible explanation for the observed effects of shields in both the forward and backward directions is the reduction of scattered photon fluence due to absorption of photons in the lead shield via photoelectric effect. Current theoretical models do not include this perturbation effect caused by shields on brachytherapy applicators.
Green's function solution to the tissue bioheat equation
Authors: Vyas Reeta , Rustgi M. L.
Published: March 2003

A Green's function solution to the tissue bioheat equation including blood flow in cylindrical geometry is obtained. Numerical results for temperature variation in the bovine muscle are reported when the tissue is exposed to neodymium–yttrium–aluminum garnett (Nd:YAG) lasers with Gaussian profile and a comparison with recent measurements is made. A strong dependence of the tissue temperature on the beam radius and pulse time is found.
A finite‐size pencil beam model for photon dose calculations in three dimensions
Authors: Bourland J. D. , Chaney E. L.
Published: March 2003

A three‐dimensional dose computation model employing a finite‐size, diverging, pencil beam has been developed and is demonstrated for Cobalt‐60 γ rays. The square cross‐section pencil beam is simulated in a semi‐infinite water phantom by convolving the pencil beam photon fluence with the Monte Carlo point dose kernel for Cobalt‐60. This finite‐size pencil beam is calculated one time and becomes a new data base with which to build larger beams by two‐dimensional superposition. The pencil beam fluence profile, angle correction for beam divergence, the Mayneord inverse square correction, radial and angular sampling rates, error propagation, and computation time have been investigated and are reported. Radial and angular sampling rates have a great effect on accuracy and their appropriate selection is important. Percent depth doses calculated by finite‐size pencil beam superposition are within 1% of values calculated by full convolution and the agreement with values from the literature is within 6%. The latter disagreement is shown to be due to a low‐energy photon component which is not modeled in other calculations. Computation time measurements show the pencil beam method to be faster than full convolution and one implementation of the differential‐scatter‐air‐ratio (dSAR) method.
Backscatter coefficient imaging using a clinical scanner
Authors: Boote Evan J. , Zagzebski James A. , Madsen Ernest L.
Published: March 2003

A clinical ultrasound scanner has been integrated with a digital data acquisition system to record echo signals for off‐line processing of quantitative acoustic backscatter images. The method used to determine backscatter coefficients accounts for experimental factors related to the beam directivity function, the transmitting and receiving electronics, and the attenuation path of the beam. After characterization and calibration of the ultrasound scanner according to the data processing requirements, the quantitative backscatter coefficient for tissue‐mimicking phantoms are within 14% of a value predicted by scattering theory. On five normal volunteers, preliminary in vivo liver images of the acoustic backscatter coefficient are obtained. Results from this study are compared to previously published in vitro results.
Image feature analysis and computer‐aided diagnosis in digital radiography: Automated detection of pneumothorax in chest images
Authors: Sanada Shigeru , Doi Kunio , MacMahon Heber
Published: March 2003

In order to aid radiologists in the diagnosis of pneumothorax from chest radiographs, an automated method for detection of subtle pneumothorax is being developed. The computerized method is based on the detection of a fine curved‐line pattern, which is a unique feature of radiographic findings of pneumothorax. Initially, regions of interest (ROIs) are determined in each upper lung area, where subtle pneumothoraces commonly appear. The pneumothorax pattern is enhanced by the selection of edge gradients within a limited range of orientations. Rib edges included in this edge‐enhanced image are removed, based on the locations of posterior ribs that are determined separately. A subtle curved line due to pneumothorax is then detected by means of the Hough transform. The detected pneumothorax pattern is marked on the chest image displayed on a CRT monitor. With the present computer method applied to 50 chest images (28 normals and 22 abnormals with pneumothorax), we were able to detect 77% of pneumothoraces, with 0.44 false‐positives per image.
Contrast‐detail curves for liver CT
Authors: Judy Philip F. , Swensson Richard G. , Nawfel Richard D. , Chan Karen H. , Seltzer Steven E.
Published: March 2003

Contrast‐detail curves were constructed for liver computed tomographic (CT) images using an objective method. Stimuli were created by superimposing disks at specified locations on sets of 92 normal liver CT images. Bright and dark disks of 9 sizes and 36 possible image contrasts were used. Sets of 92 stimuli were rendered on film at five window widths (64, 128, 256, 512, and 1024 HU). The contrast‐detail (CD) curve flattened substantially for disks larger than 7‐mm diameter, and its slope (on a log‐log plot) was less than predicted from signal‐detection theory. Manipulation of display window manipulation had little impact on this disks' visibility. The results indicate that human observers have difficulty visualizing large, low‐contrast details on liver CT scans, and suggest that narrowing the display window will have little effect on this limitation.
Region of interest fluoroscopy
Authors: Rudin Stephen , Bednarek Daniel R.
Published: March 2003

In some medical imaging applications, it is necessary to visualize only the center of the field of view with optimal quality. For example, often in interventional radiographic procedures only the region directly adjacent to the catheter tip must be well seen. A new imaging approach which reduces the number of photons exposing the patient outside a region of interest (ROI), while allowing the photon fluence to be maintained or increased in the ROI, may make more optimal use of the total integral radiation dose to the patient as well as enable increased contrast and reduced artifacts in the ROI. A demonstration is given with an angiographic phantom, for an ROI which is less than 10% of the total field of view and where the periphery receives 6% of the ROI exposure. Contrast is improved about 30% in the ROI, and yet the images are adequate in the periphery for visualizing high contrast reference features while there is a reduction in total integral patient dose. Details of the technique are discussed along with requirements for clinical implementation.
Simple calculation of the electron‐backscatter factor
Authors: Tabata Tatsuo , Ito Rinsuke
Published: March 2003

The authors have studied the dependence of the electron‐backscatter factor (EBF) on mean electron energy and on backscatterer atomic number by using the semiempirical depth‐dose code EDMULT. A plane‐parallel electron beam is assumed to be normally incident on a polystyrene slab, which is backed with a layer (backscatterer) of different materials of effectively semi‐infinite thickness. A small air cavity to measure ionization is embedded in the polystyrene slab at the boundary facing the backscatterer. The EBF is defined as the ratio of the ionization with the backscatterer to the ionization with a full polystyrene medium, and is approximated by the ratio of the doses computed at the depth of the cavity. Values of EBF obtained show trends similar to the experimental data of Klevenhagen et al. [Phys. Med. Biol. 27, 363–373 (1982)], although the former are generally lower than the latter. When the typical energy spread of clinical electron beams is taken into account, the difference between the experimental and the calculated values is reduced. The present results also show the same trend of increase of the backscatter factor with increasing energy as observed by Klevenhagen et al. in some series of measurements for the lead backscatterer at the lowest energies. This is explained by the rapid buildup of the dose with depth for electrons of low initial energies incident on the full polystyrene medium.
On the field‐size dependence of relative output from a linear accelerator
Authors: Dunscombe P. B. , Nieminen J. M.
Published: March 2003

The radiation output in air on the central axis of a linac photon beam has been modeled as the sum of two components. These are a point source representing radiation direct from the target and a distributed source representing scatter in the flattening filter and primary collimator. By fitting only two parameters, the ratio of the two components for a 20×20 field and a width parameter for the distributed source this semi‐empirical model describes the relative outputs of square, symmetric rectangular, and asymmetric rectangular fields with an average error of 0.25% for the field sizes studied.
A rapid method for electron beam energy check
Authors: Pisciotta Vincent J. , Wu Xiaodong , Fiedler Jeffrey A. , Serago Christopher F. , Calish‐Watzich Marcia L. , Schwade James G. , Houdek Pavel V.
Published: March 2003

Assessment of electron beam energy and its long term stability is part of standard quality assurance practice in radiation oncology. Conventional depth‐ionization or depth‐film density measurements are time consuming both in terms of data acquisition and analysis. A procedure is described utilizing ionization measurements at two energy specific depths. It is based on a linear relationship between electron beam energy and its practical range. Energy shifts within the range covered by the two measurement depths are easily resolved. Within a range of ±0.50 MeV (±1.30 MeV) around the established mean incident energy of 5.48 MeV (20.39 MeV), the method accuracy is better than 0.10 MeV.
Demonstration of megavoltage and diagnostic x‐ray imaging with hydrogenated amorphous silicon arrays
Authors: Antonuk Larry E. , Boudry John , Huang Weidong , McShan Daniel L. , Morton Edward J. , Yorkston John , Longo Michael J. , Street Robert A.
Published: March 2003

Flat‐panel imagers consisting of the first large area, self‐scanning, pixelated, solid‐state arrays made with hydrogenated amorphous silicon (a‐Si:H) are under development by the authors for applications in diagnostic x‐ray and megavoltage radiotherapy imaging. The arrays, designated by the acronym MASDA for multi‐element amorphous silicon detector array, consist of a two‐dimensional array of a‐Si:H photodiodes and thin‐film transistors and are used in conjunction with scintillating materials. Imagers utilizing MASDA arrays offer a variety of advantages over existing technologies. This article presents initial megavoltage and diagnostic‐quality x‐ray images taken with several such arrays including the first examples of anatomical‐phantom images. The external readout electronics and imaging techniques required to obtain such images are outlined, the construction, operation, and advantages of the arrays briefly reviewed, and the future potential of this new technology discussed.
Parametrized x‐ray absorption in diagnostic radiology from Monte Carlo calculations: Implications for x‐ray detector design
Authors: Boone John M.
Published: March 2003

The integral dose to the patient and image signal to noise ratio (SNR) are inexorably coupled in x‐ray‐based diagnostic imaging. Advancements and optimal design of imaging devices need to consider the SNR as well as patient dose. The figure of merit, FOM=(SNR)2/(integral dose), is a useful parameter in optimizing detector designs because it is independent of input exposure, and therefore eliminates exposure as a design consideration. Although numerical calculation of the SNR is relatively straightforward in most cases, the integral dose calculation is made complex due to its scatter component's high dependency on both x‐ray energy and patient thickness. Monte Carlo calculations over a range of monoenergetic x‐ray energies were used to calculate total energy absorption, and the results are parametrized using polynomial expressions. The results are shown to be applicable to any arbitrary polyenergetic spectrum. An example using the above FOM is given to illustrate the utility of the parametrized results. The parametrized results may prove useful in the computer simulations of x‐ray detector systems where the above FOM is utilized.
Improvement in specificity of ultrasonography for diagnosis of breast tumors by means of artificial intelligence
Authors: Goldberg Victor , Manduca Armando , Ewert Daniel L. , Gisvold John J. , Greenleaf James F.
Published: March 2003

A set of ultrasonograms of lesions from 200 patients between the ages of 14 and 93 years who underwent mammography followed by ultrasonographic examination and excisional biopsy has been studied with computer vision techniques to improve the ultrasonographic specificity of the diagnosis. Selected features representing the texture of the lesion were calculated and then classified by an artificial neural network. This network was biased toward correctly classifying all the malignant cases at the expense of some misclassification of the benign cases. The network diagnosed the malignant cases with 100% sensitivity and 40% specificity (compared with 0% specificity for the radiologists diagnosing the same set of cases in the breast imaging setting), and tests performed with a leave‐one‐out technique indicate that the network will generalize well to new cases. This suggests that methods based on neural network classification of texture features show promise for potentially decreasing the number of unnecessary biopsies by a significant amount in patients with sonographically identifiable lesions.
Testing mammography equipment: Evolution over a 4‐year period
Authors: Kimme‐Smith Carolyn , Bassett Lawrence W. , Gold Richard H. , Fox Sarah A. , Solberg Timothy
Published: March 2003

The results of quality control (QC) tests on 70 mammography units in Southern California from 1986 to 1990 are reported. Thirteen facilities, selected because they housed all of the mammography units in three communities involved in a National Institutes of Health research project, had their units tested twice at an interval of 1 year. Fourteen self‐selected units were also tested twice at intervals ranging from 1–3 years. Forty‐three self‐selected units in 31 additional facilities had testing only once. All 70 units underwent measurement of focal spot size or resolution, tube output, half‐value layer (HVL), automatic exposure control (AEC) accuracy, relative kVp accuracy, mean glandular dose, and imaging of several test objects. The test results for the units tested once showed no significant differences compared to those tested twice. For the latter, once the units were tested and determined to be acceptable, retesting showed differences only in overall film optical density, dose, and AEC performance.
Pretransfusion blood irradiation: Clinical rationale and dosimetric considerations
Authors: Masterson Mary Ellen , Febo Robert
Published: March 2003

The irradiation of blood before transfusion into immunosuppressed patients is an increasingly common technique used to prevent graft‐versus‐host disease. A technical procedure is described for the calibration of blood irradiators, including the determination of absolute dose rate and relative dose distribution over the blood volume. Results of dose rate measurements on commercially available irradiators indicate differences of +5% to −13% with manufacturer‐supplied calibrations and variations in the relative dose rate over the irradiation volume from 70% to 180%. The clinical implications of these findings and the need for accurate dosimetry are discussed.
Measurement of the source size of a 6‐ and 18‐MV radiotherapy linac
Authors: Loewenthal E. , Loewinger E. , Bar‐Avraham E. , Barnea G.
Published: March 2003

The quality of portal imaging is strongly affected by the source size of the radiotherapy machine. The effective source size of the dual‐energy clinac 1800 (6 and 18 MV) was measured with a 50 μm wide and 120 mm long slit formed by two tungsten‐copper alloy blocks. A series of slit images were obtained by translating the slit horizontally. The images were analyzed using a microdensitometer. The measured data was simulated using an analytical model of the source and its size was derived by a best‐fit analysis. For both energies the FWHM was found to be 1.5±0.1 mm.
Gadolinium as a neutron capture therapy agent
Authors: Shih Jing‐Luen A. , Brugger Robert M.
Published: March 2003

The clinical results of treating brain tumors with boron neutron capture therapy are very encouraging. Researchers around the world are once again making efforts to develop this therapeutic modality. Gadolinium‐157 is one of the nuclides that holds interesting properties of being a neutron capture therapy agent. It is estimated that tumor concentrations of up to 300 μg157Gd/g tumor can be achieved in brain tumors with some MRI contrast agents such as Gd‐DTPA and Gd‐DOTA, and up to 800 μg157Gd/g tumor can be established in bone tumors with Gd‐EDTMP. Monte Carlo calculations indicate that with 250 ppm of 157Gd in tumor, neutron capture therapy can deliver 2000 cGy to a tumor of 2‐cm diameter or larger with 5×1012 n/cm2 of thermal neutron fluence at the tumor. Dose measurements with films and TLDs in phantoms verified these calculations. More extended Monte Carlo calculations demonstrate that neutron capture therapy with Gd possesses comparable dose distribution to B neutron capture therapy. With 5×1012 n/cm2 thermal neutrons at the tumor, Auger electrons from the Gd produced an optical density enhancement on films that is similar to the effect caused by about 300 cGy of Gd prompt gamma dose and may further enhance the therapeutic effects.
A review of the processes by which ultrasound is generated through the interaction of ionizing radiation and irradiated materials: Some possible applications
Authors: Baily Norman A.
Published: March 2003

The production of acoustic waves following the absorption of energy deposited by ionizing radiation, with a consequent production of localized thermal spikes has been confirmed by a number of papers published in the physics literature. This paper reviews the basic theory and presents most of the supporting experimental data. Some of the experimental methods used and the results obtained are summarized. In addition to the rather straightforward and routine use of acoustic phenomena produced by ionizing radiation for the detection and measurements of such radiation, there are some special applications that appear to be especially attractive for medical physics. Some of these are unique to ionizing radiation in that the amplitude of the ultrasound wave is proportional to the energy deposited in small volumes at localized sites of these interactions, while others derive from methodologies already in use with nonionizing radiations. The detection and measurement of this ultrasonic radiation could possibly lead to methods for the study of such fundamental phenomenon as track structure, precision localization of therapeutic treatment beams, and even the possible imaging of internal anatomic structures to provide on‐line portal images.
Considerations regarding the application of capillary optics to medical radiography
Authors: Wang Yi , Mistretta Charles A. , Shefer Ruth E. , Manning Herbert L.
Published: March 2003

The generation of monoenergetic or quasimonoenergetic x‐ray spectra has been accomplished by several methods including the use of K‐edge filtration, characteristic radiation, crystal monochromators, and multilayer mirrors. In this paper some practical questions are discussed regarding the possibility of obtaining narrow‐band spectra using x‐ray reflection in glass capillary optics which have been reported recently in connection with the focusing and generation of parallel x‐ray beams. Derivation of formulas for double and triple reflection with and without additional K‐edge filtration imply that these methods are superior to the use of filtration alone. It is shown that the double reflection scheme is most sensitive to the angular divergence of the beam emanating from the capillary array used to generate the incident parallel beam. Simulations using three reflections predict output spectra which are relatively insensitive to blurring due to angular divergence. A small amount of K‐edge filtration in combination with three reflections provides considerable sharpening of the x‐ray spectrum. Aside from the spectral selectivity provided by the capillary array, the reduced divergence of the primary transmission may have advantages for scatter rejection in situations where the air gap between the patient and detector is increased by an amount consistent with the resolution requirements of the application. In parallel beam situations, the use of capillary arrays between the patient and detector may also be useful for the rejection of divergent scattered radiation.
Computerized detection of clustered microcalcifications in digital mammograms: Applications of artificial neural networks
Authors: Wu Yuzheng , Doi Kunio , Giger Maryellen L. , Nishikawa Robert M.
Published: March 2003

Artificial neural networks have been applied to the differentiation of actual “true” clusters from normal parenchymal patterns and also to the differentiation of actual clusters from false‐positive clusters as reported by a computerized scheme for the detection of microcalcifications in digital mammograms. The differentiation was carried out in both the spatial and frequency domains. The performance of the neural networks was evaluated quantitatively by means of receiver operating characteristic (ROC) analysis. It was found that the networks could distinguish clustered microcalcifications from normal nonclustered areas in the frequency domain, and that they could eliminate approximately 50% of false‐positive clusters of microcalcifications while preserving 95% of the positive clusters, when applied to the results of the automated detection scheme. A large, comprehensive training database is needed for neural networks to perform reliably in clinical situations.
A new x‐ray and light field superimposition detector
Authors: Van der Meer Frits , Storm Joop
Published: March 2003

A fast and accurate method to determine the alignment of the diagnostic x‐ray and light fields was developed. Following the exposure, the boundaries of an x‐ray field are determined by a custom‐built x‐ray field position detector and indicated by light‐emitting diodes. The alignment of the supposed and the actual x‐ray fields agree (within ±2 mm) with the corresponding results of the film‐based procedure.
Two new experimental methods of calculating scatter fraction as a function of depth in scattering media: A comparison study
Authors: Knešaurek Karin
Published: March 2003

Two new experimental methods of calculating scatter fraction (SF), as a function of depth, are presented and used in a comparison study. These methods are based on the assumptions that the total point spread function (psft) consists of geometrical (psfg) and scattering (psfs) components, and that the psft and its components are radially symmetric functions. Both methods assume that psfg is a two‐dimensional (2‐D) Gaussian function. The first proposed method is based on the serial model of the psft. According to the serial model, psfs is also a 2‐D Gaussian function. However, the second method is based on the assumption that the psfs is a monoexponential function. Thus the main difference between these two experimental methods is the way in which the psfs is treated. Published data obtained by Monte Carlo simulations compared to the new experimental results shows that for depths less than 5 cm, both new methods yield smaller values for scatter fraction than Monte Carlo simulation or the subtraction method. However, for depths from 5 to 20 cm, the results of these two new methods were very close to the values obtained by both the Monte Carlo simulation and the subtraction method. These two methods have allowed assessment of the scatter fraction with higher accuracy and reliability and reporting data concludes that the new methods are less error sensitive than the subtraction method.
Automated lesion detection and lesion quantitation in MR images using autoassociative memory
Authors: Raff U. , Newman Francis D.
Published: March 2003

Previous efforts concerning lesion extraction in radiologic images indicated that autoassociative memory models can be a valuable tool in automated lesion detection. Preliminary results are expanded to resolve the technical problems of image registration and magnification. Instead of operating on selected portions of the MR images, each entire image matrix is operated upon as image vector comprising all stacked columns of the matrix. Spin density weighted images (TR=3000 ms and TE=40 ms) of 42 normal subjects were remapped and standardized with respect to location and magnification. All image vectors were orthonormalized to span a linear manifold. Standardized abnormal image vectors were then tested by the stored autoassociative memory and the abnormalities (novelties) were extracted by application of an autocorrelation matrix to the input vector. The autocorrelation matrix is computed using image vectors from normal subjects. The lesions (multiple sclerosis and tumors) are then identified as the orthogonal component to the linear manifold spanned by the basis vectors of the normal brain scans. Lesion extraction has been achieved with the intention of quantitating and staging diseased parenchyma after automated edge detection.
Dosimetric characteristics of wedges mounted beyond the blocking tray
Authors: Ochran T. G. , Boyer A. L. , Nyerick C. E. , Otte V. A.
Published: March 2003

In a beam accessory configuration for a linear accelerator using a prototype multileaf collimator, newly designed wedges were mounted beyond the blocking tray. The isodose curves, depth of maximum dose, surface dose, and wedge transmission factors were measured for the wedges designed for this unique configuration. The same set of wedges was used for both 6‐ and 18‐MV x rays. The shape of the wedged isodose curves was essentially unchanged from those produced by the conventional wedges located above the blocking tray. The isodose curves exhibited the desired wedge angles over the range of field sizes from 5×5 to 15×40 cm. In the 10×10‐cm field, the average difference between the observed wedge angle and the desired wedge angle was 3.8°. The surface doses ranged from 18% to 35% for the wedged 10×10‐cm fields as compared with about 15% for the same open field. Dosimetrically the wedges were acceptable for clinical use.
A review of electronic portal imaging devices (EPIDs)
Authors: Boyer Arthur L. , Antonuk Larry , Fenster Aaron , Van Herk Marcel , Meertens Harm , Munro Peter , Reinstein Larry E. , Wong John
Published: March 2003

On‐line electronic portal imaging devices are beginning to come into clinical service in support of radiotherapy. A variety of technologies are being explored to provide real‐time or near real‐time images of patient anatomy within x‐ray fields during treatment on linear accelerators. The availability of these devices makes it feasible to verify treatment portals with much greater frequency and clarity than with film. This article reviews the physics of high‐energy imaging and describes the operation principles of the electronic portal imaging devices that are under development or are beginning to be used clinically.
A chamber and electrometer calibration factor as determined by each of the five AAPM accredited dosimetry calibration laboratories
Authors: Blackwell C. Robert , McCullough Edwin C.
Published: March 2003

The same calibration grade ionization chamber and electrometer were sequentially sent to all five currently AAPM Accredited Dosimetry Calibration Laboratories (ADCL). A Cesium‐137 based check device was utilized to ensure chamber and electrometer factor constancy and showed a maximum deviation of 0.32% (typically less than 0.1%) over the 227 days needed to complete the intercomparison. The chamber and electrometer calibration factor provided by each of the five ADCLs were analyzed for consistency. The maximum percent difference in reported chamber factor between all five ADCLs was 1.40%. The reported electrometer factor had a maximum discrepancy of only 0.50%. System (chamber plus electrometer) factors as provided by three of the five ADCLs had a maximum discrepancy of 1.51%.
Electron bolus design for radiotherapy treatment planning: Bolus design algorithms
Authors: Low D. A. , Starkschall G. , Bujnowski S. W. , Wang L. L. , Hogstrom K. R.
Published: March 2003

Computer algorithms to design bolus for electron beam radiotherapy treatment planning were investigated. Because of the significant electron multiple scatter, there is no unique solution to the problem of bolus design. However, using a sequence of operators, a bolus can be designed that attempts to meet three important criteria: adequate dose delivery to the target volume, avoidance of critical structures, and dose homogeneity within the target volume. Initial calculation of bolus shape was based upon creation operators forcing either the physical or the effective depths of the distal surface of the target volume to a specified value. Modification operators were then applied to the bolus to alter the shape to better meet the design criteria. Because the operators each address a single dosimetric issue, they can often adversely affect some other attribute of the dose distribution. In addition, an extension operator is used to design the bolus thickness outside the target volume. Application of these operators is therefore carried out in certain sequences and each may be used more than once in the design of a particular bolus. The effects of these operators on both the bolus and the resulting dose distribution are investigated for test geometries and patient geometries in the nose, parotid, and paraspinal region.
Optimal electron‐beam treatment planning for retinoblastoma using a new three‐dimensional Monte Carlo‐based treatment planning system
Authors: Al‐Beteri A. A. , Raeside D. E.
Published: March 2003

Electron‐beam treatment planning for retinoblastoma was investigated and an optimal treatment plan was devised for a particular case using a new three‐dimensional Monte Carlo‐based treatment planning system known to be capable of correctly predicting dose perturbations caused by body surface obliquities and tissue heterogeneities. Computed tomography (CT) data files were used to construct a three‐dimensional eye phantom representing the anatomy of a child's orbit. Dose distributions in sagittal, transverse, and coronal planes were predicted with 1‐mm resolution. Study of these distributions led to an optimal treatment plan consisting of an anterior‐lateral pair, with the anterior field being a 10‐MeV, 30‐mm‐diam circular field, centrally blocked by a 10‐mm‐diam lucite lens shield and the lateral field being a 16‐MeV, 30×25‐mm D‐shaped field. The anterior field delivers a therapeutic dose to the ora serrata, but it underdoses the posterior retinal surface behind the lens shield; the lateral field provides the necessary boost dose to the posterior retinal surface. An equally weighted combination of the two fields produces a dose distribution in which the entire retinal surface receives a therapeutic dose, with less than 10% of that dose being delivered to the lens, brain, and the contralateral orbit.
Systematic bias in basis material decomposition applied to quantitative dual‐energy x‐ray imaging
Authors: Gingold Eric L. , Hasegawa Bruce H.
Published: March 2003

Basis material decomposition represents dual‐energy x‐ray attenuation measurements in terms of the attenuation coefficients or thicknesses of two standard materials which, when combined, produce attenuation equivalent to the object being measured. In tomographic imaging, the reconstructed attenuation coefficient is calculated in terms of the attenuation coefficients of the basis materials, while in projection imaging, the thicknesses of two materials can be specified in terms of the basis materials. This analysis shows that basis material decomposition is exact in a dual‐monoenergetic system, but for broad spectra, x‐ray beam hardening introduces a bias into quantitative measurements. The error is small enough that it can be ignored when dual‐energy imaging is used primarily to enhance the contrast of one material over another. The magnitude of the error in quantitative measurements depends on the details of the specific application including the energy of the x‐ray beam, and the composition and thickness of the materials included in the object. The magnitude of the error for dual‐energy bone densitometry has been analyzed using a first‐order propagation of error analysis and the calculations verified by computer simulation. This analysis shows that the magnitude of the systematic error can be as high as 3% for 1 g/cm2 of bone mineral when aluminum and acrylic basis materials are used for the calibration. This systematic error is eliminated when the basis materials are the same as the materials that are being quantified (i.e., bone mineral and water).
Describing the signal‐transfer characteristics of asymmetrical radiographic screen‐film systems
Authors: Van Metter Richard
Published: March 2003

The measurement of modulation transfer function for radiographic screen‐film systems depends critically upon a proper linearization of the measured line spread function. This is normally done by photographic photometry (i.e., using the measured density versus log exposure relationship to transform the density line spread function into an exposure line spread function). It has been long appreciated that this procedure may fail for asymmetrical dual screen systems that use film with emulsion coated on both sides of the support. The advent of asymmetrical and near‐zero crossover films that can be used with highly asymmetric screen pairs has prompted a reinvestigation of these concerns about the definition and measurement of modulation transfer function. For such cases, it is useful to define the contrast transfer function, which is a function of exposure and spatial frequency. When normalized by its zero frequency value the contrast transfer function can serve as the “effective MTF” for low‐contrast input signals in such systems. In the limit of symmetrical systems this quantity approaches the conventionally measured MTF. The utility of this approach is demonstrated by applying it to a commercially available asymmetrical screen‐film combination.
Application of the EM algorithm to radiographic images
Authors: Brailean James C. , Little Darnell , Giger Maryellen L. , Chen Chin‐Tu , Sullivan Barry J.
Published: March 2003

The expectation maximization (EM) algorithm has received considerable attention in the area of positron emitted tomography (PET) as a restoration and reconstruction technique. In this paper, the restoration capabilities of the EM algorithm when applied to radiographic images is investigated. This application does not involve reconstruction. The performance of the EM algorithm is quantitatively evaluated using a “perceived” signal‐to‐noise ratio (SNR) as the image quality metric. This perceived SNR is based on statistical decision theory and includes both the observer's visual response function and a noise component internal to the eye–brain system. For a variety of processing parameters, the relative SNR (ratio of the processed SNR to the original SNR) is calculated and used as a metric to compare quantitatively the effects of the EM algorithm with two other image enhancement techniques: global contrast enhancement (windowing) and unsharp mask filtering. The results suggest that the EM algorithm's performance is superior when compared to unsharp mask filtering and global contrast enhancement for radiographic images which contain objects smaller than 4 mm.
On the spatial resolution of clinical thermometers
Authors: Gerig L. H. , Szanto J. , Raaphorst G. P.
Published: March 2003

In this work the spatial resolution of type T (copper‐Constantan) and type K (Chromel‐Alumel) multipoint thermocouple thermometers was investigated. For clinical use, thermocouples are usually inserted within catheters to provide easy access, reproducibility, and a sterile environment. The effect of several types of catheter on the ability of these thermometers to accurately resolve thermal gradients was studied. The influence of the number of wire pairs within the multipoint thermometer was also investigated. A mathematical model has been developed to determine the spatial resolving power of these thermometers. Results indicate that type K thermocouples are generally superior to type T and the use of catheters plays a significant role in spatial resolution. Moreover, the use of mineral oil as a thermal coupling agent between the catheter and the thermometer was also found to have some effect.
Dosimetry of ruthenium‐106 eye applicators
Authors: Davelaar Jacob , Schaling David F. , Hennen Leonard A. , Broerse Johan J.
Published: March 2003

In view of the importance of clinical applications of ruthenium‐106 beta‐ray sources for the treatment of choroidal melanoma, experimental, and theoretical approaches are presented for the dosimetry of such sources. The absolute dose and percentage depth dose of ruthenium applicators have been measured with an extrapolation ionization chamber. For a special flat applicator the absolute dose could be measured with an accuracy of ±5%, which is determined by the collection efficiency of the extrapolation chamber. The percentage depth dose of concave applicators, employed in the clinical situation, could only be measured at a distance larger than 5 mm due to their geometry and the outer dimensions of the extrapolation chamber. A computer simulation was therefore developed for the absorption and scattering of electrons, taking into account the geometry and materials of the applicator, to predict the percentage depth dose at distances smaller than 5 mm. The calculated and experimentally determined depth doses are in good agreement. With the aid of the computer simulation a depth dose determination for concave applicators can be made for clinically relevant distances <10 mm from the source surface with an absolute accuracy of ±10%.
Comments to “Chamber replacement correction in absorbed dose calibrations,” by J. E. Burns
Authors: Andreo P. , Lindborg L. , Medin J.
Published: March 2003

The ionization chamber replacement correction in absorbed dose calibration is discussed. (AIP)
Albedos for 4‐, 10‐, and 18‐MV bremsstrahlung x‐ray beams on concrete, iron, and lead—Normally incident
Authors: Lo Yuan‐Chyuan
Published: March 2003

Backscattering of radiation incident on a solid medium has been treated with some success as a reflection phenomenon. The concept of albedo has been used in the literature to deal with this problem. A Monte Carlo program has been used in this study to simulate the generation of bremsstrahlung beams from medical linear accelerators. The accuracy of the calculated spectra have been checked indirectly by calculating TPRs and comparing them with measured values. The dose albedos for 60Co and 137Cs beams normally incident on concrete have been calculated and compared with experimentally determined values in the literature. The generated energy spectra are then used to calculate the albedo factors for 4‐, 10‐, and 18‐MV bremsstrahlung x‐ray beams normally incident on concrete, iron, and lead. The number albedo decreases with increase in the atomic number of the reflecting medium. However, for high‐energy photons, the dose albedo increases with increase in the atomic number of the medium. This is an important factor that needs to be considered in the design of shielding. The calculated energy spectra can be used in other problems and the albedo factors can be applied in practical shielding design for medical linear accelerators.
Two‐radiograph reconstruction using six geometrical solution sets and least‐squares method
Authors: Tabushi Katsuyoshi , Itoh Susumu , Sakura Mizuyoshi , Kutsutani‐Nakamura Yuzuru , Iinuma Takeshi A. , Arai Tatsuo , Irifune Toraji
Published: March 2003

When two radiographic projections are available for reconstruction, it was found that six different combinations of equations could be used to obtain the geometrical solutions for the position of any point. No errors in the image coordinates read from the radiographs resulted in identical solutions for the six equations. Inaccuracies or errors present in the image coordinates generated differences among the six solutions. In this case, a least‐squares method could be used to determine the optimum position. The utility of such a least‐squares optimizing approach is presented in the context of a clinical example.