bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of performance of antiscatter grids in digital radiography: Effect on signal‐to‐noise ratio
Authors: Chan Heang‐Ping , Lam Kwok Leung , Wu Yuzheng
Published: March 2003

We developed a theoretical model which describes the improvement of signal‐to‐noise ratio (SNR) by a grid in digital radiography. The model takes into account the effects of spatial variations in the scatter‐to‐primary ratio and in the large‐area contrast over an image with structured background on quantum noise, and the effects of noise in the imaging system such as electronic noise and digitization noise. Based on the theoretical model, we analyzed the effects of these factors on the SNR when a grid is employed. We performed experimental measurements to evaluate the improvement in the SNR by a grid when quantum noise is the dominant noise source. It was found that the measured SNR improvement factor due to quantum noise agreed closely with that determined from the measured transmission values of a grid, as predicted from our theoretical model. In order to evaluate the relative performance of grids with various geometric design parameters for digital radiographic systems, we employed Monte Carlo calculations and determined the transmission values of a number of grids under various scatter conditions. The calculated SNR improvement factor, due to quantum noise, correlated well with the measured improvement of the SNR by the grids. Our model predicts that the SNR improvement factor depends strongly on the local contrast ratio and also on the scatter‐to‐primary ratio. The SNR improvement factor is higher in the underpenetrated regions than in the well‐penetrated regions of an image.
The validity of the density scaling method in primary electron transport for photon and electron beams
Authors: Woo M. K. , Cunningham J. R.
Published: March 2003

In the convolution/superposition method of photon beam dose calculations, inhomogeneities are usually handled by using some form of scaling involving the relative electron densities of the inhomogeneities. In this paper the accuracy of density scaling as applied to primary electrons generated in photon interactions is examined. Monte Carlo calculations are compared with density scaling calculations for air and cork slab inhomogeneities. For individual primary photon kernels as well as for photon interactions restricted to a thin layer, the results can differ significantly, by up to 50%, between the two calculations. However, for realistic photon beams where interactions occur throughout the whole irradiated volume, the discrepancies are much less severe. The discrepancies for the kernel calculation are attributed to the scattering characteristics of the electrons and the consequent oversimplified modeling used in the density scaling method. A technique called the kernel integration technique is developed to analyze the general effects of air and cork inhomogeneities. It is shown that the discrepancies become significant only under rather extreme conditions, such as immediately beyond the surface after a large air gap. In electron beams all the primary electrons originate from the surface of the phantom and the errors caused by simple density scaling can be much more significant. Various aspects relating to the accuracy of density scaling for air and cork slab inhomogeneities are discussed.
Calculation of three‐dimensional photon primary absorbed dose using forward and backward spread dose‐distribution functions
Authors: Iwasaki Akira
Published: March 2003

This paper describes a method of calculating three‐dimensional (3‐D) photon primary absorbed dose in a homogeneous or heterogeneous medium. The method is based on a technique of convolving a pair of forward and backward spread dose‐distribution functions with the primary water collision kerma distribution. Both spread dose‐distribution functions can be constructed by analyzing the zero‐area tissue‐maximum ratio, the primary absorbed dose near the beam exit surface, and the laterally spread dose distribution. Primary absorbed dose calculations are performed along the beam axis for 10‐MV x rays. The characteristic patterns of primary dose in heterogeneous phantoms containing an aluminum or lunglike material slab can be obtained.
Dose perturbations at interfaces in photon beams: Secondary electron transport
Authors: Werner Barry L. , Das Indra J. , Salk William N.
Published: March 2003

An improved, quantitative version of the partial fluence model [Med. Phys. 14, 585 (1987)] for the calculation of dose perturbations at media interfaces in photon beams is presented and compared with measurements made at interfaces between polystyrene and materials ranging in atomic number from aluminum to lead, for photon beams ranging in energy from 60Co to 24 MV.
Neural networks in radiology: An introduction and evaluation in a signal detection task
Authors: Boone John M. , Sigillito Vincent G. , Shaber Gary S.
Published: March 2003

Neural networks are a computer architecture, implementable in software or hardware, that allow an entirely new approach to the computerized perception of data. These so‐called connectionist models are inspired by what is known about the architecture of biological neurons, in which the “intelligence” or processing capability of the network is a result of the interconnection strengths between large arrays of nonlinear processing nodes. Neural networks are described and then are used to analyze the common radiological problem of pattern recognition on a noisy background. Classical signal detection theory is used to compare network performance against that of human observers, using computer‐generated sets of very simple “nodules.” The neural network performed with better accuracy, relative to human observer performance, in the detection of this elementary test object. Although these results may not scale up with more complex images, the favorable performance of neural networks at this level suggests that further investigation is warranted.
Estimation of geometrical parameters and collimator evaluation for cone beam tomography
Authors: Gullberg Grant T. , Tsui Benjamin M. W. , Crawford Carl R. , Ballard J. Glen , Hagius John T.
Published: March 2003

A method is presented for estimating the geometrical parameters for a cone beam detector geometry from the coordinates of the centroid of a projected point source sampled over 360°. Nonlinear expressions are derived for the coordinates of the centroids in terms of the geometrical parameters which include: the two‐dimensional coordinates of the projection of the center of rotation onto the detector image plane; the focal length; the distance from the focal point to the center of rotation; and the spatial coordinates of the point source itself. Experimental data were obtained using a rotating gamma camera with a symmetrically converging collimator. The Marquardt algorithm was used to estimate the parameters for this particular cone beam geometry. The method was able to estimate the geometrical parameters and evaluate the accuracy of the collimator construction.
The calculation of transmission through a photon beam attenuator using sector integration
Authors: Papież Ewa , Froese Gerd
Published: March 2003

A calculational scheme is presented for the prediction of the transmitted fraction (TR) through an attenuator of known material and physical dimensions, at any point in a photon beam, for a beam of any shape or size. The method considers the total TR to be composed of scatter and primary components and computes the scatter component by sector integration. The input for the calculations consists of measured narrow‐ and broad‐beam transmitted fractions through lead in air for various circular fields, thicknesses of the attenuator, and angles from the central axis, in a geometry approximating typical treatment conditions. The method has been tested for the case of a uniform half slab and a 45° wedge in a 4‐MV photon beam. It was found that the use of TR values obtained by the above method reduced the maximum absorbed dose computation error from 8% computed with a commercially available algorithm to 3%, in a typical treatment setup. This method is generally applicable to any shaped attenuator such as a wedge or compensator covering whole or part of a radiation field.
Dose distributions in radiosurgery
Authors: Pike G. Bruce , Podgorsak Ervin B. , Peters Terence M. , Pla Conrado , Olivier André , Souhami Luis
Published: March 2003

A PC‐based, three‐dimensional treatment planning system, which may be used for planning of radiosurgical treatments with the Gamma unit or with any of the radiosurgical techniques based on isocentric linear accelerators (linacs), is described and used to calculate isodose distributions for various linac‐based radiosurgical techniques ranging from the single plane rotation to a 4‐π geometry. The latter gives an isotropic dose falloff outside the target volume but cannot be used for practical radiosurgery, while the single plane rotation is simple to use but gives unacceptably shallow dose falloffs in the transverse plane. Dose falloffs for several other techniques of varying degrees of complexity are shown and discussed. Also discussed is the effect of beam energy and beam profiles on radiosurgical dose distributions.
Acceptance testing of an automated scanning water phantom
Authors: Mellenberg David E. , Dahl Robert A. , Blackwell C. Robert
Published: March 2003

The setup of an automated scanning water phantom must be concluded with a methodical functional acceptance test. This test must include mechanical, radiological, and calculational facets. Only after an acceptance test is successfully completed can data collected with a new or repaired system be trusted. An acceptance test outline was written and applied to a newly purchased Wellhofer scanning system. This testing procedure and its results are presented in this paper.
Use of the AAPM standard for digital image interchange in nuclear medicine
Authors: Cradduck T. D.
Published: March 2003

The recent paper by Maguire and Noz [Med. Phys. 16, 818–823 (1989)] provides a valuable review of the present situation regarding the interchange of digital image files. They are to be commended for the efforts they have made to promote a suitable standard for image file interchange both within individual modalities and between modalities.
Image feature analysis and computer‐aided diagnosis in digital radiography: Automated analysis of sizes of heart and lung in chest images
Authors: Nakamori Nobuyuki , Doi Kunio , Sabeti Victoria , MacMahon Heber
Published: March 2003

We are developing an automated method for determining a number of parameters related to the size and shape of the heart and of the lungs in chest radiographs. In order to obtain standard patterns of the cardiac shadow as “gold standards,” four radiologists traced their best estimates of the entire contour of the heart, including the largely invisible inferior margin, on 11 radiographs. These contours were analyzed by Fourier transform, and the results were used as a guide to obtain a shift‐variant cosine function which was applied to the prediction of the cardiac contour by fitting a limited number of detected heart boundary points. These points were obtained from analysis of edge gradients in two orthogonal directions. A simple observer study indicated that the contours of the heart shadows computed for 60 chest radiographs were generally acceptable to radiologists for estimation of the size and area of the projected heart. We also detected the rib cage and the edges of the diaphragm, which enabled us to determine the projected thoracic area. From these results, we calculated the cardiothoracic ratio and other parameters, such as the ratio of the projected heart area to the projected thoracic area.
Maximizing film contrast for scanning equalization radiography
Authors: Plewes D. B. , McFaul J. , Ivanovic M.
Published: March 2003

The results of a numerical simulation indicate that scanning equalization radiography (SER) provides the opportunity to use film/screen combinations of significantly higher contrast than currently practical in conventional chest radiography. These calculations which attempt to maximize overall image contrast while minimizing areas of over and under exposure demonstrate that optimum film gradient varies inversely with scanning beam area. Furthermore, we show that in an SER system which we have optimized for chest imaging that the peak film gamma gradient can be increased from that typical of conventional films (3.0–3.5) to approximately 7 with advantage.
A reusable perfusion supporting tissue‐mimicking material for ultrasound hyperthermia phantoms
Authors: Chin Ronny B. , Madsen Ernest L. , Zagzebski James A. , Jadvar Hossein , Wu Xue‐Kui , Frank Gary R.
Published: March 2003

A new ultrasonically and thermodynamically tissue‐mimicking material is reported. The material is well suited for use in phantoms for testing ultrasound hyperthermia systems or related predictive models. Controlled convective heat transfer effects, mimicking to some extent perfusive heat transfer in tissues, can be instituted in the material with appropriate fluid sources and sinks. The material consists of closely packed agar spheres varying in diameters from 0.3–3.6 mm. The interstitial space between spheres is filled with 10% n‐propanol solution. The material has two practical advantages over the solid‐gel‐type tissue‐mimicking materials. The first advantage is that it allows rapid return of a hyperthermia phantom to thermal equilibrium following a heating test by rapid circulation of the perfusion fluid. The second advantage is that the material is in a “liquid” form. It can be easily siphoned in and out of phantom containers of any geometric shape for different purposes without change in its physical properties. Methods for measuring ultrasonic and thermodynamic properties of the material and the results of the measurements are reported. The physical parameters measured are the intensity attenuation and absorption coefficients, the ultrasonic speed, the thermal conductivity, specific‐heat capacity and the mass density. Temperature measurements in a hyperthermia phantom made of the material are also reported.
Measurement of dose in the buildup region using fixed‐separation plane‐parallel ionization chambers
Authors: Gerbi Bruce J. , Khan Faiz M.
Published: March 2003

Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed‐separation plane‐parallel ionization chambers are most commonly used for this purpose. Recent papers have re‐emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed‐separation plane‐parallel ionization chambers. Data for Co‐60, 6‐, 10‐, 18‐, and 24‐MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane‐parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of >19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.
Measurement of dose distributions of linear energy transfer in matter irradiated by fast neutrons
Authors: Schell M. C. , Pearson D. W. , DeLuca P. M. , Haight R. C.
Published: March 2003

A detector has been developed and used to measure dose distributions versus linear energy transfer to thin gas targets in spherical geometry from fast neutron irradiation of tissue‐equivalent plastic and carbon. The detector is a hemispherical proportional counter with a Cs(T1) scintillator at the center of the hemisphere. The coincidence of the proportional counter signals constrain the measurements to charged particles traversing the radius of the hemisphere. The charged particle energy deposition distributions are directly measured for a known pathlength. The A‐150 kerma factor was measured at a neutron energy of 14.8 MeV and is in agreement with tabulated values. The carbon kerma factor measurements are less than the tabulated value at 14.8 MeV. The alpha‐particle production in carbon was measured for neutron energies from 14.1 to 14.8 MeV and is compared with existing data.
Electron beam characteristics on a Philips SL25
Authors: Palta Jatinder R. , Daftari Inder K. , Ayyangar K. M. , Suntharalingam N.
Published: March 2003

Dosimetry measurements at nominal electron energies of 4, 6, 8, 10, 12, 15, 17, 20, and 22 MeV were made for different sized, open‐sided applicators on two Philips SL25 linear accelerators. Measurements include beam flatness, percentage depth dose, surface dose, isodose curves, field size dependence, output at extended distances, virtual source position, and required low melting point alloy thickness for field shaping. These measurements are presented to document the characteristics of electron beams with a new type of applicator design on this series of Philips accelerators.
Image feature analysis and computer‐aided diagnosis in digital radiography: Effect of digital parameters on the accuracy of computerized analysis of interstitial disease in digital chest radiographs
Authors: Katsuragawa Shigehiko , Doi Kunio , Nakamori Nobuyuki , MacMahon Heber
Published: March 2003

We are developing a computerized method for measurement of lung texture in digital chest radiographs for detection and characterization of interstitial disease. Physical texture measures are obtained from analysis of the power spectrum of the lung texture. We have investigated the effect of digital parameters such as pixel size, regions of interest size, the number of quantization levels, and the peak frequency of the visual system response, as well as the effect of the unsharp masking technique on the performance of this computerized method. We calculated the texture measures by changing digital parameters for 100 normal lungs and 100 abnormal lungs in our database. Receiver operating characteristic (ROC) curves were employed for evaluation of the performance of this computerized method for distinguishing between normal and abnormal lungs. We used the area under the ROC curve to compare the detection accuracy for interstitial infiltrates. We believe that the results of this study may be useful as a guide in the design of computerized schemes for lung texture analysis in digital chest radiographs.
Testing of a medical linear accelerator's computer‐control system
Authors: Weinhous Martin S. , Purdy James A. , Granda Conrad O.
Published: March 2003

In August of 1987, the Radiation Oncology Center at the Mallinckrodt Institute of Radiology became the first academic‐medical‐center user of Varian's computer‐controlled therapy accelerator, a prototype version of the 2100C. Installation was accomplished by the retrofit of a computer system to our two‐year‐old Clinac 1800. Being well aware of the malfunctions that occurred in other computer‐controlled accelerators, we took extraordinary measures to assure proper operation of the new, computerized, system. Our acceptance procedure included tests of (i) mechanical systems (isocentricity, digital readouts, etc.); (ii) radiation parameters (flatness, symmetry, output, etc.); (iii) manual safety systems (emergency off switches, etc.); (iv) the computer console systems (communications integrity, state integrity, etc.); and (v) the interlock systems (some 50 electrical, mechanical, and/or computer‐controlled interlocks). As items (i), (ii), and (iii) do not differ significantly from the tests for a non‐computer‐controlled machine, they will not be discussed here. Rather, this report will concentrate on the methods that were devised to test the computer‐control and interlock systems.
Characteristics of photon beams from Philips SL25 linear accelerators
Authors: Palta Jatinder R. , Ayyangar Komanduri , Daftari Inder , Suntharalingam Nagalingam
Published: March 2003

The Philips SL25 accelerator is a multimodality machine offering asymmetric collimator jaws and a new type of beam bending and transport system. It produces photon beams, nominally at 6 and 25 MV, and a scattered electron beam with nine selectable energies between 4 and 22 MeV. Dosimetric characteristics for the 6‐ and 25‐MV photon beams are presented with respect to field flatness, surface and depth dose characteristics, isodose distribution, field size factors for both open and wedged fields, and narrow beam transmission data in different materials.
Evaluation of analog contrast enhancement and digital unsharp masking in low‐contrast portal images
Authors: Weiser John C. , Gur David , Gennari Rose C. , Deutsch Melvin
Published: March 2003

An experimental high‐contrast sensitivity storage phosphor imaging system was used to produce double‐exposure localization portal images of abdominal and pelvic treatment fields. The images were contrast enhanced by using an analog windowing technique and edge enhanced with a digital unsharp masking routine. A laser printer was used to print the storage phosphor images onto film. Conventional images were obtained by placing film in the cassette with the storage phosphor plates prior to exposure. Four radiation oncologists rated the storage phosphor and conventional films for perceptibility of anatomical detail needed to verify the placement of the treatment field. Contrast enhancement alone did not result in a significant improvement in perceptibility over unprocessed conventional film (p>0.20). However, the combination of contrast and edge enhancement did result in a significant improvement over conventional film (p<0.05).
A method for obtaining an approximate Wiener filter
Authors: Madsen Mark T.
Published: March 2003

The Wiener restoration filter yields the minimum mean‐square error between the restored image and the true object function. However, it has found limited use because, in its usual formulation, it requires information about the object power spectrum which is generally unknown. In this paper, it is shown that the Wiener filter can be derived from the noise‐free image power spectrum, and a method is presented for estimating this from the observed data. From this estimate an approximate Wiener filter was calculated. The method was tested on three sets of simulated data which included a constant background, rectangular defects, and Gaussian defects at varying contrast and noise levels. The performance of the approximate Wiener filter was compared both to the true Wiener filter and to the standard 1‐2‐1 three‐point smooth. The results confirmed that the approximate Wiener filter adapted to the information content of the observed data and closely matched the performance of the true Wiener filter. The approximate Wiener filter outperformed the three‐point smooth in all cases, especially at low contrast and high noise levels. The approximate Wiener filter can be calculated without operator intervention and requires little additional computation time over conventional Wiener filter techniques.
On the technique of extrapolation to obtain wall correction factors for ion chambers irradiated by photon beams
Authors: Bielajew Alex F.
Published: March 2003

Wall correction factors, which correct ion chamber response for photon attenuation and scatter, can differ by as much as 1.0% for spherical chambers depending on whether they are obtained experimentally by extrapolation measurements or by Monte Carlo simulation. This difference is not explained by experimental or calculational statistics which lie in the range 0.05%–0.2%. In this paper it is demonstrated that linear extrapolation of experimental data for spherical chambers is inappropriate, owing to the curvature of the chamber walls. A simple nonlinear theory is constructed that resolves the difference. The Monte Carlo calculations and the nonlinear theory are compared with extrapolation measurements for the NIST (formerly NBS) spherical chambers. It is concluded that wall correction factors should be obtained by Monte Carlo calculation for spherical chambers and that linear‐extrapolation techniques should be regarded with suspicion for all chambers.
An in‐line optical image translator with applications in x‐ray videography
Authors: Picot Paul A. , Cardinal H. Neale , Fenster Aaron
Published: March 2003

Many applications in radiography require, or would benefit from, the ability to translate, i.e. move, an optical image in the detector plane. In this paper, we describe the design and characterization of a prism‐based optical image translator for insertion into existing XRII‐video imaging systems. A pair of prisms rotatable about the optical axis form a very compact in‐line optical image translator for installation in the parallel light path between an x‐ray image intensifier and its video camera. Rotation of the prisms translates the XRII optical image on the camera target. With the addition of x‐ray and light collimators to limit the image to a single video line, x‐ray streak images may be acquired. By rotating an object in the x‐ray beam during a streak, a complete computed tomography (CT) data set may be acquired. This image translator can translate an image anywhere in the focal plane of a 50‐mm‐output lens within a 40‐mm‐diam circle. The prisms have an aperture of 50 mm, permitting an optical speed of F/2 with a 50‐mm output lens. The design is insensitive to angular alignment errors. This image translator is achromatic, since the spectral width of the output phosphors of image intensifiers is sufficient to introduce blurring in a nonachromatic design. A prism‐based image translator introduces image distortion, since the prisms do not operate at minimum deviation. The distortion is less than 4% over all parts of a typical detector area, and less than 1% in the central region of the image. The prisms were found not to degrade the modulation‐transfer function of an optical system.
Light equalization radiography
Authors: Kruger Robert A. , Reinecke Daniel R. , Power Robert L.
Published: March 2003

An electro‐optical, photographic dodging technique, called light equalization radiography (LER), has been developed. The use of LER extends the dynamic range of radiographic film by enhancing the film contrast in the “toe” region of a radiographic film. Contrast recorded above some predetermined optical density is unaffected. The use of LER permits high‐contrast films to be used in radiographic exams where low‐contrast, “latitude” film types have been used traditionally, e.g., in chest radiography.
Validity of transition‐zone dosimetry at high atomic number interfaces in megavoltage photon beams
Authors: Das Indra J. , Kase Kenneth R. , Meigooni Ali S. , Khan Faiz M. , Werner Barry L.
Published: March 2003

Measurement of dose or dose perturbation factors at high atomic number interfaces are usually performed with a thin‐window parallel‐plate ion chamber. In a transition region, under nonequilibrium conditions, accuracy of ion chamber readings for the dose measurements has often been questioned. This paper critically analyzes the factors (stopping power ratio and charge collection) for the dose measurements at interfaces. Monte Carlo simulations were performed to investigate the secondary electron spectrum produced by photon beams and to calculate the stopping power ratios at the point of measurement. The validity of dose measurements was studied for the photon beams in the range of Co‐60 gamma rays to 24‐MV x rays at bone and lead interfaces with polystyrene, using thermoluminescent dosimeters, extrapolation chamber and several types of commercially available parallel‐plate ion chambers. It is observed that for energies >10 MV most parallel‐plate chambers can be used to measure dose accurately. At lower energies, however significant differences between measured doses with different detectors were noticed. It is suggested that at high‐Z interfaces and lower energies, the dose measurements should be performed with ultrathin‐window parallel‐plate ion chambers or extrapolation chambers.
Film dosimetry analyses on the effect of gold shielding for Iodine‐125 eye plaque therapy for choroidal melanoma
Authors: Wu Andrew , Krasin Frank
Published: March 2003

One of the methods currently being used to treat choroidal melanoma employs an episcleral plaque containing I‐125 radioactive seeds. However, comprehensive dosimetry studies on the plaque are scarce and controversial. For this work, we use film to study the dosimetry outside the lip of the gold shield of the eye plaque. This lip around the gold shield was made to protect the critical structures behind and adjacent to the lesion. Since the changes of energy spectrum of I‐125 in tissue are negligible, film dosimetry seems to be a logical choice because of high spatial resolution required around the lip of the gold plaque. For this study, we first established an H and D curve with dose expressed in a unit of specific dose rate constant. This avoids absolute dose measurements. All film density measurements are made with a 1‐mm aperture scan, normalized to the density at the prescription point for tumor of 3–5‐mm apical height, i.e., 5 mm from the interior surface of sclera, and converted to percentage isodose curves. With a gold shield, it is found that when the plaque is placed against the optical nerve, the optical disk and macula, located at 2 mm outside the lip, on the exterior surface of sclera, may receive 85% of the prescription dose for a 12‐mm plaque and 58% for a 16‐mm plaque. For tumors of 8‐mm apical height, the optical nerve would receive more than the prescription dose.
A study of effective attenuation coefficient for calculating tissue compensator thickness
Authors: Bagne Farideh R. , Samsami Nina , Hoke Steven W. , Bronn Donald G.
Published: March 2003

Dose uniformity throughout the treatment volume is essential to precision radiation therapy. Tissue compensators are often used as a means to eliminate dose nonuniformity resulting from surface contour irregularities. This paper evaluates the accuracy of using an effective attenuation coefficient for calculating the thickness of missing tissue. This coefficient is found to vary strongly with thickness of missing tissue when the initial depth is situated in the buildup region. The use of a single attenuation coefficient produces errors as high as 54% in the calculated compensator thickness when 10‐MV x rays are used. At depths greater than the depth of maximum dose, the attenuation coefficient remains a function of field size, not the initial depth.
EGS4 Monte Carlo determination of the beta dose kernel in water
Authors: Simpkin Douglas J. , Mackie T. Rockwell
Published: March 2003

The EGS4 Monte Carlo code has been used to simulate the emission and energy deposition in H2 O about point sources of monoenergetic electrons and radionuclides of potential use in radioimmunotherapy. The radiations studied were 0.05, 0.1, 0.5, 1.0, 2.0, and 3.0 MeV monoenergetic electrons and 32P, 67Cu, 90Y, 105Rh, 131I, 153Sm, 186Re, and 188Re β particles and conversion and Auger electrons. Ten batches of 10 000 electrons (or 10 000 radionuclide decays) each were started isotropically at a point in an infinite homogeneous H2 O phantom. The parameter–reduced electron–step transport algorithm (PRESTA) version of the EGS4 Monte Carlo code was used to follow these and their progeny, scoring the energy deposited in thin spherical shells. The scaled dose kernels are calculated and compared to kernels available in the literature. These previously published kernels either completely ignore secondary electrons or are based on a Monte Carlo code which improperly sampled the Landau energy straggling distribution.
New quantitative and qualitative approaches to the inverse problem of electrocardiology: Their theoretical relationship and experimental consistency
Authors: Greensite Fred , Huiskamp Geertjan , van Oosterom Adriaan
Published: March 2003

In addition to formidable theoretical obstacles that a proposed solution to the inverse electrocardiology problem must overcome, there are great practical difficulties in establishing its accuracy in actual clinical application. However, the recent appearance of two fundamentally independent treatments of the inverse problem raises the possibility that they may be used in tandem to help establish their individual accuracy. Thus, if the two methods give incompatible results in application then one of the methods must be inaccurate. Conversely, if the two methods give compatible results then the accuracy of both methods is supported (for the particular quantities measured) subject only to the validity of the assumptions common to both methods. We have compared results from the application of a quantitative “integral equations based” method with that of a qualitative “differential topology inspired” approach in three healthy volunteers. The output examined consists of measurements of the times of appearance of epicardial sources (depolarization wavefront breakthroughs) and sinks of the ventricular surface activation map. The extent of agreement on source/sink times between the methods was consistent with the resolution limits imposed by noise and discrete sampling on derivatives of the electrocardiogram. When events defined by the integral method occurring within 2 ms of each other are grouped together (and their times averaged), the two methods agreed on source/sink times to within 3 ms except in two instances where they differed by 5 ms. The measurements made by the two methods were found to be highly correlated (R=0.95). While the quantitative method alone rests on a variety of modeling and procedural assumptions, the only assumption common to both methods is the uniform dipole layer hypothesis. Thus, subject to this single assumption, one may infer the accuracy of the quantitative method in healthy individuals for epicardial source/sink times. On the other hand, coupling with the far more detailed quantitative method allows further useful characterization of the output of the qualitative method. In particular, this study provides convincing evidence that the major deflections of the spatial velocity electrocardiogram are coupled to particular epicardial sources and sinks, as has been previously conjectured on theoretical grounds. This raises the possibility of bedside evaluation of these epicardial events.
Quantitative radiographic imaging using a photostimulable phosphor system
Authors: Floyd Carey E. , Chotas Harrell G. , Dobbins James T. , Ravin Carl E.
Published: March 2003

We have evaluated a photostimulable phosphor x‐ray imaging system [Philips Computed Radiography (PCR) system] for use in quantification of x‐ray exposure in diagnostic radiography. An exponential function was fitted to data yielding quantitative x‐ray exposure values as a function of digital pixel values. We investigated several factors that affect the accuracy of exposure measurement using the PCR including repeatability, background noise as a function of time delay between plate erasure and use, sensitivity variation between different plates, nonuniformity of sensitivity within a plate, decay of the latent image between time of exposure and readout (observed as a change in sensitivity), and the accuracy with which the (exponential) calibration function yields exposure values as a function of digital pixel values. The calibration was performed over the exposure range from 5.1×10−9 to 2.0×10−5 C/kg (0.02–75 mR). The accuracy of exposure measurements made with a single imaging plate is between 1.6% and 4.2%. If measurements from several plates are involved, the uncertainty in the final measurement will be between 5% and 5.9%.
Determination of build‐up region over‐response corrections for a Markus‐type chamber
Authors: Mellenberg David E.
Published: March 2003

Over‐response corrections for a widely used parallel plate ionization chamber were determined using contemporaneous measurement of build up for 4, 6, 10 and 18‐MV photon beams utilizing a commercially available extrapolation chamber (PTW model 23392). The resultant over‐response corrections were essentially independent of field size (5×5 to 30×30 cm) and were less for increased depth (from the surface) and higher x‐ray energy. The over‐response of the parallel plate chamber (Markus‐type, PTW model 329) was 13.8, 10.7, 6.2 and 4.7 percent (absolute) at the surface for 4, 6, 10 and 18‐MV x‐ray energies. At only 2 mm depth, the over‐response of the Markus chamber under investigation decreased to 50% of surface value over‐response. These corrections, so derived, are applied to Markus parallel plate chamber build‐up measurements by simple subtraction of the derived corrections. For example, a 6‐MV percentage depth dose of 37.5% measured at the surface with the Markus chamber is reduced by 10.7% so to agree with the surface dose indicated by the PTW extrapolation chamber for this x‐ray field, that is, 26.8%.
Dosimetry on transverse axes of 125I and 192Ir interstitial brachytherapy sources
Authors: Nath Ravinder , Meigooni Ali S. , Meli Jerome A.
Published: March 2003

Dose rates along the transverse axes of 125I model 6702, 125I model 6711 and 192Ir 0.2‐mm steel sources for interstitial brachytherapy have been measured in a solid‐water phantom for distances up to 10 cm using LiF thermoluminescent dosimeters (TLDs). Specific dose rate constants, the dose rates in water per unit source strength 1 cm along the perpendicular bisector of the source, are determined to be 0.90±0.03, 0.85±0.03, and 1.09±0.03 cGy h−1 U−1 for 125I model 6702, 125I model 6711 and192 Ir 0.2‐mm steel sources, respectively (1 U=unit of air kerma strength =1 μGy m2 h−1=1 cGy cm2 h−1). In older and obsolete units of source strength (i.e., mCi apparent), these are 1.14±0.03, 1.08±0.03, and 4.59±0.15 cGy h−1 mCi−1 (apparent). Currently accepted values of specific dose rate constant for 125I sources are up to 20% higher than our measured values which are in good agreement with the results of our Monte Carlo simulations. But for 192Ir there is good agreement between our measured value of the specific dose rate constant and currently accepted values. The radial dose function for 125I model 6702 is found to be consistently larger than that for 125I model 6711, with an increasing difference as the distance from the source increases. Our measured values for the radial dose function for 125I sources are in good agreement with the results of our Monte Carlo simulation as well as the measured values of Schell et al. [Int. J. Radiat. Oncol. Biol. Phys. 13, 795–799 (1987)] for model 6702 and Ling et al. [Int. J. Radiat. Oncol. Biol. Phys. 9, 1747–1752 (1983)] for model 6711. However, some of the recently reported Monte Carlo values of the radial dose function for 125I sources are significantly larger than measured values; up to 18% at a distance of 5 cm. Our measured radial dose function for the 192Ir seed is in good agreement with our Monte Carlo calculated values, and with both our earlier data for the high activity 192Ir source of a remote afterloader and recommended values by Meisberger et al. [Radiol. 90, 953–957 (1968)].
Dose rate determination for 125I seeds
Authors: Chiu‐Tsao Sou‐Tung , Anderson Lowell L. , O'Brien Keran , Sanna Robert
Published: March 2003

Dose rates in water have been determined for the two types of 125I seed currently used in brachytherapy. The need for such determinations became evident when water/air ratios measured with a silicon diode were found to be lower than expected. Extensive measurements using lithium fluoride thermoluminescent dosimeters (TLD's) have been performed in a solid water phantom, at distances from 0.1 to 10 cm from the seed center and at angular increments of 10°, 15°, or 30° within a plane through the seed axis. Dose calibration of the TLD's was accomplished by irradiation in air with 125I seeds of the same type and of strengths traceable to a calibration at the National Institute of Standards and Technology (NIST). Relative calibration of TLD's was monitored by irradiation, in an oven‐type x‐ray machine, of control dosimeters simultaneously and all dosimeters intercurrently with the 125I irradiations. Values obtained for the dose rate constant, i.e., dose rate per unit air‐kerma strength at 1 cm on the transverse axis, were 0.853 and 0.932 cGy h−1 U−1 (1.08 and 1.18 cGy h−1 mCi−1) for the 6711 and 6702 seeds, respectively. Measured data were supplemented with Monte Carlo‐calculated relative dose rate data generated using the MORSE code. These calculations used 100 energy groups from 10 to 35.4 keV and involved energy collection bins ranging from 0.025 to 1.2 cm on an edge. Normalized at 1 cm, transverse axis calculated data are not significantly different from measured data (ours or cited literature) at distances either <2.5 or >8 cm. Normalized at different distances along the transverse axis, our off‐axis calculated and measured distributions agree closely at all angles but differ from literature measured distributions at small (≤1 cm) distances and, for small angles, increasingly at larger distances (≥5 cm).
Dose distributions in regions containing beta sources: Uniform spherical source regions in homogeneous media
Authors: Werner Barry L. , Rahman Mahfuzur , Salk William N. , Kwok Cheuk S.
Published: March 2003

The energy‐averaged transport model for the calculation of dose rate distributions is applied to uniform, spherical source distributions in homogeneous media for radii smaller than the electron range. The model agrees well with Monte Carlo based calculations for source distributions with radii greater than half the continuous slowing down approximation range. The dose rate distributions can be written in the medical internal radiation dose (MIRD) formalism.
Dose distributions in regions containing beta sources: Irregularly shaped source distributions in homogeneous media
Authors: Werner Barry L.
Published: March 2003

Methods are introduced by which dose rate distributions due to nonuniform, irregularly shaped distributions of beta emitters can be calculated using dose rate distributions for uniform, spherical source distributions. The dose rate distributions can be written in the MIRD formalism.
Calculation of water/air stopping‐power ratios using EGS4 with explicit treatment of electron‐positron differences
Authors: Malamut C. , Rogers D. W. O. , Bielajew A. F.
Published: March 2003

Using the EGS4 Monte Carlo simulation program, a general purpose code has been written to calculate Bragg–Gray and Spencer–Attix stopping‐power ratios for use in radiation dosimetry. The stopping‐power ratios can be calculated in any material in any region in a general cylindrical geometry with a large number of source geometries possible. The calculations take into account for the first time the differences between the stopping powers and the inelastic scattering of positrons and electrons. The results show that previous calculations ignoring these effects were accurate. The present results agree, typically within 0.1%, with the Spencer–Attix water‐to‐air stopping‐power ratios for broad parallel beams of electrons given in the AAPM and IAEA protocols except at the surface where the present calculations follow the buildup of secondary electrons in more detail and see a 2% reduction in the stopping‐power ratios.
A comparison of ionization‐chamber and water‐calorimeter dosimetry for high‐energy x rays
Authors: Schulz R. J. , Huq M. Saiful , Venkataramanan N. , Motakabbir Kazi A.
Published: March 2003

The temperature‐regulated, flexible, water calorimeter developed in the authors' laboratory was shown previously to yield a dose‐to‐water from 4‐MV x rays that is in very close agreement with ionization measurements made in accordance with the AAPM dosimetry protocol. The range of beam energies for this type of comparison has been increased to include 60Co, and 4‐, 6‐ and 25‐MV x rays. The grand mean of the ratios of doses obtained from the calorimeter and ionization chamber, the Cal/Ion ratio, for the four beam energies studied is 1.001±0.001. As no significant trend with beam energy was detected, it is concluded that the calorimeter and ionization chamber yield equally accurate results. Because the calibration of the calorimeter depends solely upon the accuracy with which water temperatures in the range 2–10 °C can be measured, and dose is given by the product of the specific heat of water and the temperature change produced by irradiation, the water calorimeter has the potential to place radiation dosimetry on a much firmer foundation than presently exists.
Comparison of dose calibration by ionometric and chemical dosimetry for 6‐ and 45‐MV x‐ray beams
Authors: Zwicker R. D. , Wu Andrew , Sternick E. S.
Published: March 2003

This work reports the results of an experiment carried out to investigate the compatibility of TG‐21 ion chamber calibration with Fricke dosimetry near the extremes of the clinically available megavoltage photon energy range. Doses determined from ion chamber measurements and from chemical dosimetry are compared for a 6‐MV linear accelerator x‐ray beam and 45‐MV betatron beam. The Fricke dosimetry was carried out by the National Institute of Standards and Technology. Agreement within 1% between the two techniques was obtained at both photon energies using a currently accepted G‐value.
Central axis depth dose curve for electron beams
Authors: Strydom W. J.
Published: March 2003

In this article an analytical equation for electron depth dose is proposed for electron energies from 6–20 MeV. The equation contains four parameters and it fits the build‐up region, fall‐off region as well as the bremsstrahlung background region. The calculated values from this equation fit within 1,5% of the measured data in the build‐up region and in the fall‐off region within 0,5 mm for the energy range 5–10 MeV and within 1 mm for the range 12–20 MeV. This equation can be applied beyond the practical range.
Scattering effects on the dosimetry of iridium‐192
Authors: Serago Christopher F. , Houdek Pavel V. , Pisciotta Vincent , Schwade James G. , Abitbol Andre A. , Lewin Alan A. , Poole Darrell O. , Marcial‐Vega Victor
Published: March 2003

Dosimetry calculations for iridium‐192 sources generally assume that a sufficient medium surrounds both the iridium source(s) and the point of calculation so that full scattering conditions exist. In several clinical applications the iridium sources may be anatomically located so that the full scattering requirement is not satisfied. To assess the magnitude of this problem, relative measurements were made with a small ionization chamber in phantoms near air and lung‐equivalent interfaces. Dose reduction caused by decreasing the volume of scattering material near these interfaces was then evaluated for a few clinical applications. The results show that reductions on the order of 8% may be expected at the interface with minimal dose reduction within the volume of the implant itself. In addition, the results indicate the verification of source strength of iridium sources in phantom require phantom dimensions determined by the source‐chamber separation distance.