bluuebunny/crossref_metadata_2025 huggingface dataset:
Automatic shimming for localized spectroscopy
Authors: Holz D. , Jensen D. , Proksa R. , Tochtrop M. , Vollmann W.
Published: March 2003

Localized in vivo nuclear magnetic resonance studies often require a high spectral resolution not achievable with the basic shim of a whole‐body magnetic resonance magnet. Therefore, the magnetic field homogeneity needs to be optimized in the selected volume of interest within a reasonable time. For this purpose, a method of automatic shimming was developed and tested on phantoms and volunteers. The volume selection is performed by means of a surface coil or by using a localization method which generates a stimulated echo from the volume of interest. The optimization procedure uses the time integral over the magnitude of the free induction decay or echo signal as homogeneity criterion. A complete shimming process generally requires only 80 transients. Test experiments were conducted on various volume sizes ranging from 2×2×2 cm3 to 15×15×15 cm3 inside a large phantom. The resulting linewidth in small volumes at the magnet center compared well with the natural linewidth determined by means of the Carr–Purcell–Meiboom–Gill sequence. As expected, shimming in selected volumes at off center positions led to somewhat broader lines. Results obtained on volunteers demonstrate the practical value of this rapid, automatic shimming method for in vivo studies.
Routine quality control tests for film–screen mammographic systems with automatic exposure control
Authors: Geise Richard A. , Morin Richard L. , Wasserman Neil F.
Published: March 2003

Quality control of the contrast and density of mammograms is of extreme importance not only because of patient dose considerations but also because of the need to monitor changes in the breast over extended periods of time. A phantom and test technique has been developed and used at two institutions for monitoring the ability of mammographic generators and phototiming systems to provide consistent contrast and density. The phantom consists of a solid acrylic block and an embedded aluminum step wedge designed specially for low kVp use. Optical densities of various portions of the phantom are used to determine constancy of density and contrast. By minimizing fluctuations due to processing and film handling, normal variations were reduced enough to determine changes in contrast and density due to generator and phototimer changes equivalent to those monitored in processor quality control programs. The data have been correlated with changes in processor function. Changes in density and contrast values have also been related to phototimer malfunction and reduced image quality.
The efficiency of clinical microwave applicators measured by a calorimetric method
Authors: Anhalt D. , Hynynen K.
Published: March 2003

When inducing localized hyperthermia for superficial cancer therapy with microwaves there has often been question about the total power output from the applicator. Although specific absorption rates and thermograms are used to obtain localized power distributions and heating patterns, these provide, at best, only an approximation of the total power applied to tissues or phantoms. In this paper a calorimetric technique for obtaining total microwave output power from applicators is described. An experimental apparatus was constructed and it was found to be accurate to approximately ±5 W. The power output from four clinical microwave applicators as a function of applied electric power was measured and the efficiency was found to be 40% in average. Along with enhancing quality assurance, the areas of hyperthermia research which may benefit the most from this calorimetric technique are computer modeling and patient treatment planning.
An explanation of the dose inhomogeneity caused by bolus in an electron portal
Authors: Asp Lloyd W.
Published: March 2003

When an electron beam is incident upon a stepped surface, such as that which exists when a slab of bolus is present with it's square edge within the portal, the dose distribution is strikingly perturbed. An easily demonstrable explanation is given which utilizes the customary, flat‐incident surface electron beam dose distribution.
Acquisition and display of radiation dose distributions using microcomputer technology
Authors: Antolak John , Mah Ernest , Battista Jerry J. , Scrimger John W.
Published: March 2003

In the commissioning or quality assurance of a medical linear accelerator or a computerized radiotherapy planning system, the traditional approach usually consists of acquiring and comparing one‐dimensional dose profiles. This methodology is tedious and incomplete since only a portion of the radiation field can realistically be sampled. We have developed an automated measurement system which allows efficient measurement and display of complete two‐dimensional dose distributions. The general purpose microcomputer used (IBM PC/XT compatible) can be interfaced economically to any water phantom dosimetry system equipped with a three axis scan controller, and can also communicate data to the treatment planning system. This allows for direct comparison of measured with computed dose distributions, thus revealing discrepancies in the dose computation algorithms used. In this paper, we describe the interface between the microcomputer, a conventional water dosimetry system (Therados RFA‐3), and a treatment planning computer. We report our early experience with acquiring dose distributions and show sample comparisons with computed results for megavoltage electron beams incident on homogeneous and heterogeneous systems.
Monte Carlo simulation of the scattered radiation distribution in diagnostic radiology
Authors: Boone John M. , Seibert J. Anthony
Published: March 2003

Monte Carlo techniques were employed to evaluate the point spread function (PSF) of scattered radiation in diagnostic radiology. The Monte Carlo procedure is described and shown to compare well with Monte Carlo scatter analysis of other authors. The intensity and distribution of the PSF are described independently. The effects of object thickness, air gap, and beam spectra are examined. An analytic derivation of the scatter PSF is presented in a companion article, and the Monte Carlo results discussed herein are used for comparison.
Correction for scattered radiation and other background signals in dual‐energy computed tomography material thickness measurements
Authors: Vetter J. R. , Holden J. E.
Published: March 2003

Both beam hardening and the detection of scattered radiation cause nonlinearity errors in x‐ray computed tomography (CT), leading to artifacts and CT number inaccuracies. Dual‐energy measurements can be used to correct beam hardening effects to a high degree of accuracy. However, in the imaging of thick body sections the transmitted intensity of the primary beam is low, making scatter the most significant cause of CT number inaccuracy. Furthermore, the scatter‐to‐primary ratio is energy dependent, causing a shift in the apparent effective atomic number of the absorbing material. We have measured scatter under a variety of conditions on a third generation CT scanner with dual‐energy capability in order to determine its effect on the accuracy of quantitative measurements. The effects of off focus radition, detector cross‐talk, and detector dark current were also accounted for in the analysis of the measured scatter data. Our results indicate that on this well collimated system, an accurate correction for scatter can be made based on the detected intensity in projections.
Dual‐energy x‐ray projection imaging: Two sampling schemes for the correction of scattered radiation
Authors: Wagner Frederick C. , Macovski Albert , Nishimura Dwight C.
Published: March 2003

In addition to the familiar problems of reduced contrast and signal‐to‐noise ratio (SNR) in the single energy case, dual‐energy subtractions in the presence of scattered radiation suffer further degradations from: (1) artifacts due to nonuniform subtraction of scatter, and (2) a serious deterioration of the signal of interest. To determine the expected performance of scatter correcting schemes, we simulated energy subtractions performed in the presence of scatter. We discuss scatter's detrimental effects on contrast and SNR in these simulations and the expected improvements from scatter corrections to within 5% to 10%. We introduce two sampling schemes for the correction of scatter. Each scheme requires two measurements, and each involves placing an x‐ray opaque sampling grid between the source and the object. In the first method, the grid is an array of lead disks present only during one measurement. Using these samples we generate an estimate of the scatter field and then subtract it from the second measurement yielding a scatter corrected image. In the second method, the grid is an array of lead strips present during both measurements but displaced between measurements by one‐half of a strip spacing to completely sample the image. From the two measurements we generate an image to be corrected, an estimate of the scatter field, and a scatter corrected image. In phantom studies implemented on a digital fluoroscopy system, we observed for single energy images of blood vessel phantoms improved contrast and field uniformity. For scatter corrected selective material cancellations in human phantoms we observed improved contrast and significant reduction in artifacts. In both cases we observed no significant loss in SNR. These results facilitate the implementation of efficient large area detectors for dual‐energy imaging.
Computer aided design and verification of megavoltage tissue compensators for oblique beams
Authors: Faddegon Bruce A. , Pfalzner Paul
Published: March 2003

A computer based radiotherapy dose compensator system is presented. A plane of uniform dose (pud) is chosen which passes through the central axis at a reference depth in the patient. The pud is oriented at an arbitrary angle. Tissues thicknesses are the ray line distances from the patient surface to the pud. Relative dose estimates on the pud are derived from tissue‐air ratio, tissue‐maximum ratio, or tissue‐phantom ratio tables interpolated for field size and tissue thickness. An inverse square law correction is also applied. Compensator thicknesses are calculated from measured effective attenuation coefficients to attenuate the beam by a factor proportional to the estimated dose on the pud. Water phantom measurements for 60Co for both oblique incidence and wedge geometries demonstrate dose uniformity to better than ±4%. Dose measurement with radiographic films sandwiched between the two halves of a wax phantom verify adequate compensation for neck irradiation by 60Co opposed lateral beams and 6‐MV x‐ray inferiorly angled lateral beams. Compensator factors used for absolute dose determination are accurate to better than ±3%. The dose modeling routine, QCBEAM, is found to be sufficiently accurate for routine compensator verification for these geometries.
Unexpectedly large dose rate dependent output from a linear accelerator
Authors: Cheng Pocheng , Kubo Hideo
Published: March 2003

During our routine calibration of a Varian Clinac‐20 linear accelerator, the absorbed dose for a fixed monitor unit (mu) was found to decrease with increasing dose rate. Between dose rates of 100 and 500 mu/min, there was up to 20% difference in absorbed dose for a 20‐MeV electron beam. The cause of this problem was a failure in the electronics circuit of an integrating board. This paper presents our analysis of the problem and suggests a possible means of isolating such a failure to warn technologists, physicists, and engineers.
Contamination of ionization chambers by talcum powder
Authors: Hanson William F. , Arnold Donald J. , Shalek Robert J. , Humphries Leroy J.
Published: March 2003

This is a word of caution to anyone using ionization chambers protected by thin rubber sheaths in water. Four Farmer‐type ionization chambers contaminated with talcum powder were received for calibration by the Accredited Dosimetry Calibration Laboratory at the University of Texas M. D. Anderson Cancer Center. The chambers show a marked energy dependence (5% to 20%) to soft orthovoltage x rays. The response of the contaminated chambers is compared with the chambers’ response before contamination and after cleaning. Techniques for identifying contaminated chambers and suggestions for cleaning them are presented.
Nuclear magnetic resonance signal from flowing nuclei in rapid imaging using gradient echoes
Authors: Gao Jia‐Hong , Holland Scott K. , Gore John C.
Published: March 2003

A theoretical description of the nuclear magnetic resonance (NMR) signal from flowing nuclei has been developed for rapid imaging sequences that use small flip angles and gradient refocused echoes. Both laminar and plug flow models have been considered and formulas derived relating mean image signal intensity to flip angle, pulse sequence repetition interval (TR), and flow velocity. It is shown that the rate of approach to steady‐state conditions determines the degree of flow enhancement. Experimental measurements have been performed on a flow phantom in a whole‐body NMR imaging system operating at 0.15 T using the spoiled FLASH sequence with different radiofrequency pulse flip angles and flow rates. There is excellent agreement between the experimental results and the theoretical predictions up to the onset of turbulence.
Optical image processing with liquid‐crystal display for image intensifier/television systems
Authors: Lam Kwok Leung , Chan Heang‐Ping , Doi Kunio
Published: March 2003

We have studied the effect of real‐time optical image processing (OIP) in an image intensifier/television (II–TV) radiographic imaging system by using a liquid‐crystal display (LCD) placed between the II and the TV camera. The LCD compresses the dynamic range of the transmitted image by modulating the spatial distribution of the light intensity of the image from the output phosphor of the II. The degree of dynamic‐range compression can be designed so that the dependence of the signal‐to‐noise ratio (SNR) of the LCD–TV system on x‐ray intensity matches that of the quantum noise. We measured the physical properties of an LCD and evaluated its capability for OIP. Our experimental results demonstrate that it is feasible to use an LCD to compress the dynamic range and to improve the SNR of the image. The advantages of implementing OIP with an LCD in image acquisition systems in which a TV camera is used are discussed.
Improvement of spatial resolution properties of image intensifier–TV digital systems with a multiple‐narrow‐slit beam imaging technique
Authors: Kume Yuichiro , Doi Kunio
Published: March 2003

Multiple‐slit beam imaging technique with an image intensifier (II)–TV digital system has been developed to remove scatter and veiling glare while high x‐ray beam utilization is maintained. Although the contrast and signal‐to‐noise ratio (SNR) are improved with this technique, the overall image quality obtainable with an II–TV digital system is still limited due to low spatial resolution, which is mainly caused by the large pixel size, i.e., by the small matrix size used. In order to overcome the limitation of the pixel size, we have developed a new method of improving the resolution properties of the II–TV digital system by use of a multiple‐slit assembly (MSA) having a narrow slit width. When the slit width of the MSA is narrower than the pixel size of the II–TV digital system, two signals from a given slit due to different MSA placements may be detected by the same pixel in different image frames, and the detected signals of the slit images are mapped to a large matrix. In this way, the spatial resolution in the direction perpendicular to the slit openings can be improved along with the increased contrast and SNR as the scatter and veiling glare can be removed. Experimental results are presented, and the effect of an anisotropic resolution property on the overall image quality is discussed.
Application of Fermi scattering theory to a magnetically scanned electron linear accelerator
Authors: Sandison George A. , Huda Walter
Published: March 2003

This paper uses a solution to the Fermi electron transport equation for an isotropic point source to characterize the magnetically scanned broad electron beams from the Sagittaire Therac 40 accelerator in the air space above patients. Thick lead collimation is shown to be adequately modeled by an infinitely thin absorbing plate when used to predict penumbra shape. A relationship between broad beam penumbra width and the value of the root‐mean‐square spatial Gaussian spread σ(z) of an elementary pencil beam is derived. This relationship is applicable for any rectangular field size. Measurement of the variation in broad beam penumbra width with source–surface distance (SSD) for a 7‐MeV beam locates the isotropic source to be coincident with the exit window of the accelerator and indicates that the scattering effect of the monitor chamber may be considered negligibly small. Using this source location accurate predictions of beam profile shape for any clinically used beam energy, SSD, or field size are made in the presence of lead trimmer collimation. Field penumbra beyond the photon collimation system is formed in each lateral direction by two lead blocks whose faces are aligned along a diverging ray emanating from the source. The photon collimator closest to the source restricts the field size causing a variation of both fluence and the mean square angle spread of the electrons across the plane at the level of the lower collimator. This variation is accounted for by introducing an empirical perturbation factor into the mathematical formalism. An interesting feature of this perturbation factor is that it is field size dependent and its effect on penumbra width may be scaled for both beam energy and SSD to accurately predict beam profile shape.
Radiation contamination and leakage assessment of intraoperative electron applicators
Authors: Bagne Farideh R. , Samsami Nina , Dobelbower R. R.
Published: March 2003

In intraoperative radiation therapy, it is critical to reduce the radiation contamination outside the useful field by as much as physically feasible. Additionally, a uniform dose is clinically desirable across the tumor volume. A study of the Medical College of Ohio applicators indicates that the radiation contamination outside the field can be as high as 18% of the central axis dose. The effects of the photon collimator setting on the magnitude and energy of the radiation contamination are discussed and means are presented for reducing this unwanted radiation. The dose nonuniformity across the field is found to be virtually independent of the photon collimator setting and is shown to be mostly due to the transparent applicator wall. The clinical significance of the findings is discussed.
Influence of magnetic resonance imaging on human body core and intravascular temperature
Authors: Vogl T. , Krimmel K. , Fuchs A. , Lissner J.
Published: March 2003

Deep and superficial body temperatures were measured by in vitro and in vivo experiments, using a fluoroptic procedure and a variety of magnetic and electromagnetic fields, in the course of magnetic resonance imaging (0.35 T; 1.5 T). In vitro experiments were performed to select and check the appropriate temperature method. Temperature measurements in the human body were carried out centrally (esophageal and rectal measurements). In vivo experiments in 30 volunteers showed no significant changes (p=0.05) in central or peripheral temperatures resulting from the application of static or dynamic fields or radiofrequency.
A method for comparing beam‐hardening filter materials for diagnostic radiology
Authors: Jennings Robert J.
Published: March 2003

The necessity for using adequate beam filtration in diagnostic radiology is well known. Although aluminum is the most widely used filter material for diagnostic x‐ray applications, the possibility that other materials might have superior properties has prompted a number of studies that have attempted to determine both the type and the amount of filtration most appropriate for a given situation. This paper describes a method based on precise matching of spectral shape that permits the absolute ranking of beam‐hardening materials. Matching of spectral shape ensures equality of such parameters as image contrast and patient dose. Spectrally equivalent filters can then be ranked on the basis of the transmission of one relative to another. Following the development of the theory behind the method and an algorithm for implementing it, the method is applied to the evaluation of a variety of materials for use as filters in diagnostic radiology. Experimental verification of a few of the calculated results is also described. Both calculated and experimental results show that normal aluminum filters are about 10% less efficient than filters of materials such as copper, brass, or iron. Since the approach followed here was the basis for several early investigations of filtration for orthovoltage therapy, a brief comparison of results from these early reports with results calculated using the method developed here is also presented.
The use of a bar code scanner to improve the utility and flexibility of record and verify systems used in radiation therapy
Authors: Miller R. W. , van de Geijn J.
Published: March 2003

Record and verify systems used in radiation therapy serve a useful purpose in verification of machine parameters for each radiation field and monitoring the treatment as it is administered. There are, however, limitations as to the completeness of this monitoring. These restrictions are primarily due to design limitations of accelerators, which provide only a limited number of hardwired signals for use by such systems. The extent of the signals provided varies among manufacturers. As a result, some commonly used treatment accessories, such as blocking trays, may not be recognized by these systems. Additionally, current commercial record and verify systems cannot be expanded to accommodate institution‐specific, customized treatment accessories or devices for positioning or immobilization of patients. This paper describes a complementary approach to providing device detection using a bar code scanner to read coded labels mounted on treatment accessories and download the data into the record and verify system for processing. A microcomputer‐based system employing a portable bar code scanner was developed to evaluate the potential of this concept. Implications of adding bar code scanners to record and verify systems are discussed.
Evaluation of a beam‐spot camera for megavoltage x rays
Authors: Lutz Wendell R. , Maleki Nasser , Bjärngard Bengt E.
Published: March 2003

A beam‐spot camera for measurements of x‐ray focal spots of accelerators has been evaluated. The device consists of closely packed 0.25‐mm‐thick lead and 0.25‐mm‐thick cardboard strips. It is placed on radiographic film with the lamellae parallel to the beam axis and an exposure made. The images were scanned with a microdensitometer. The results indicate that the broadening of the source intensity profile at half maximum is of the order 1 mm, which permits the use of the beam‐spot camera for acceptance testing and quality control. Longer tails in the density profile limit the quantitative information that can be extracted from the images.
Field size dependence of wedge factors
Authors: Palta Jatinder R. , Daftari Inder , Suntharalingam N.
Published: March 2003

The radiation output in the presence of wedge filters is characterized by the wedge transmission factor and open beam field size factors. Conventionally, the wedge factor for high‐energy photons is measured in a water phantom at depth of maximum dose for a reference field size. Experimental measurements on different wedges indicate that the wedge factors are a function of field size. An analysis of these data show that this is primarily caused by the change in scattered radiation from the treatment head in the presence of wedge filters. The change in phantom scatter and radiation backscattered to the monitor chamber are minimal. For 4‐ or 6‐MV x rays with a 60° wedge, the use of a single wedge factor measured for 10 cm×10 cm field introduces errors of up to 3.5%, for a 16‐cm‐wide field. For a 20‐cm‐wide field with this wedge, the error is 7%. Thinner wedges exhibit less differences.
Distance and depth measurements of radioactive objects utilizing rotating slant‐hole collimators
Authors: Lancaster Jack L. , Kopp David T. , Lasher John C. , Blumhardt Ralph
Published: March 2003

It is desirable to have an accurate method to determine the depth of various organs which are commonly imaged in nuclear medicine. The method presented here utilizes the geometrical characteristics of the rotating slant‐hole collimator to calculate distances and depth. The accuracy of the calculated distance was evaluated for point sources in air separated by 11 cm. The reproducibility of the calculated distance was evaluated as a function of the extent of collimator rotation between observations for angular differences of 45°, 90°, 135°, and 180°. The theory of the method is discussed and the relative error in distance calculation analyzed mathematically as a function of (1) collimator rotation angle error, (2) collimator slant angle error, and (3) position calculation error. Our findings indicate this method to be accurate to within 2% with a reproducibility range of 3%–5% for point sources.
The three parameter equivalent spectra as an index of beam quality
Authors: Boone John M.
Published: March 2003

A parametric spectral model based on the work of Birch and Marshall is used to characterize the x‐ray spectra of a specific x‐ray system. Using least‐squares comparison between measured and calculated attenuation data, an equivalent spectrum (EQSPEC) is iteratively found which very closely matches the measured attenuation characteristics of the x‐ray system. The resulting parametric spectrum is a function of the anode angle (θ), equivalent kilovoltage (kVeq), and the equivalent aluminum filtration (Aleq), and these three parameters can serve as very concise yet very accurate indices of beam quality. The utility of the EQSPEC for characterization and reporting of x‐ray spectra (and thus beam quality) may have numerous applications in diagnostic imaging procedures where spectral quality is an important consideration.
Image feature analysis and computer‐aided diagnosis in digital radiography: Detection and characterization of interstitial lung disease in digital chest radiographs
Authors: Katsuragawa Shigehiko , Doi Kunio , MacMahon Heber
Published: March 2003

We are developing an automated method for determining physical measures of lung textures in digital chest radiographs in order to detect and characterize interstitial lung disease. With this method, the underlying background density variations caused by the gross lung and chest wall anatomy are corrected for in order to isolate the fluctuating patterns of the underlying lung texture for subsequent computer analysis. The power spectrum of lung texture, which is obtained from the two‐dimensional Fourier transform, is filtered by the visual system response of the human observer. The magnitude and coarseness (or fineness) of the lung textures are then quantified by the root‐mean‐square (rms) variation and the first moment of the power spectrum, respectively. Preliminary results indicate that the rms variations and/or the first moments of the texture of abnormal lungs with various interstitial diseases are clearly different from those of normal lungs. Our results suggest strongly that quantitative texture measures calculated from digital chest images may be useful to radiologists in their assessment of interstitial disease.
Two algorithms for the three‐dimensional reconstruction of tomograms
Authors: Cline H. E. , Lorensen W. E. , Ludke S. , Crawford C. R. , Teeter B. C.
Published: March 2003

Three‐dimensional (3‐D) surface reconstructions provide a method to view complex anatomy contained in a set of computed tomography (CT), magnetic resonance imaging (MRI), or single photon emission computed tomography tomograms. Existing methods of 3‐D display generate images based on the distance from an imaginary observation point to a patch on the surface and on the surface normal of the patch. We believe that the normalized gradient of the original values in the CT or MRI tomograms provides a better estimate for the surface normal and hence results in higher quality 3‐D images. Then two algorithms that generate 3‐D surface models are presented. The new methods use polygon and point primitives to interface with computer‐aided design equipment. Finally, several 3‐D images of both bony and soft tissue show the skull, spine, internal air cavities of the head and abdomen, and the abdominal aorta in detail.
Dose distributions in regions containing beta sources: Large spherical source regions in a homogeneous medium
Authors: Werner Barry L. , Kwok Cheuk S. , Das Indra J.
Published: March 2003

The energy averaged Boltzmann equation model is applied to the determination of dose distributions in infinite, homogeneous media with uniform, monoenergetic, isotropic source distributions in spherical regions of radius larger than the electron range. The generalization to the case of spherically symmetric source distributions is made. Comparisons with dose distributions calculated by the integration of dose point kernels derived from Monte Carlo calculations are presented.
The generalized geometry of eye plaque therapy
Authors: Kepka Alan G. , Johnson Patricia M. , Kline Robert W.
Published: March 2003

A calculation is described that enables the rapid assessment of dose rate at various points of interest within the eye (lens, optic nerve, etc.) for the treatment of choroidal melanoma by plaque therapy. 125I seeds are used as the radiation source. The location of the plaque and its associated seeds relative to the eye (in a Cartesian coordinate system) is determined from the description of the tumor, as drawn and dimensioned on a fundus‐view diagram by the ophthalmologist. This requires a computer to numerically solve an equation, which is derived in the framework of spherical geometry. Further results of this calculation yield data files that serve as the input to a conventional brachytherapy treatment planning program. This enables the visualization of the dose distribution within a plane that contains the major axis of the tumor in order to assess the adequacy of the treated volume.
Investigation of basic imaging properties in digital radiography. 12. Effect of matrix configuration on spatial resolution
Authors: Fujita Hiroshi , Giger Maryellen Lissak , Doi Kunio
Published: March 2003

We investigated the effects of three matrix configurations (a conventional 512×512 matrix, a double‐sampling matrix, and a high‐resolution 1024×1024 matrix) on the resolution properties of an image intensifier (II)–TV digital radiographic imaging system. Expressions for the optical transfer function were derived theoretically for each of the three matrix configurations, and the corresponding digital modulation transfer functions (MTF's) were calculated using the measured MTF's of the presampling analog components and the parameters of the digital system. In addition, digital images of a star pattern phantom and a hand phantom were obtained with the II–TV system for each matrix type. From our results, we conclude that, in order to increase the resolution of an II–TV digital imaging system, it is necessary not only to increase the matrix size, but also to improve the resolution capabilities of analog components such as the II and/or TV camera. In addition, if the system cannot be upgraded to a 1024×1024 matrix configuration, a double‐sampling technique with an 512×512 matrix may be employed.
Gypsum mixtures for compensator construction
Authors: Weeks K. J. , Fraass B. A. , Hutchins K. M.
Published: March 2003

The characteristics and properties of a new material used for the fabrication of compensators are presented. This material is a special, refined gypsum. It requires a factor of 3 less water to prepare than ordinary gypsums and as a result the attenuation properties are stable over time. The material may be used by itself or mixed with fine metal particles to increase the attenuation per unit thickness. Gypsum, gypsum+steel, and gypsum+iron were investigated. The results of attenuation measurements in narrow‐ and broad‐beam geometries appropriate to design of clinical dose modifying compensators are presented. Practical and technical details associated with the use of these materials are given. These compounds are found to be easy to use, versatile, reliable, environmentally safe, and inexpensive. In addition, an example of their use for dose compensation is given.
Measurement of the spatial Wiener spectrum of nonstorage imaging devices
Authors: Drangova M. , Rowlands J. A.
Published: March 2003

All previous methods for measuring image noise spectra require a noise realization, a static image, typified as a photograph which can be scanned to create the Wiener spectrum. We wished to analyze the spatial noise power spectrum at the output phosphor of a continuously irradiated imaging device, an x‐ray image intensifier (XRII), which is incapable of image storage and thus the image is continually changing as a function of both time and space. Our new method utilizes a pair of slits to measure the relative Wiener spectrum of the temporally changing components of the image (i.e., x‐ray quantum and XRII gain noises). By measuring the modulation transfer function and the Wiener spectrum of the same XRII on the same apparatus it was possible to demonstrate the spatial frequency dependence of the detective quantum efficiency. Adaptations of the method should permit the measurement of Wiener spectra of fluoroscopic television systems directly from the TV monitor.
Computer simulation of image intensifier‐based computed tomography detector: Vascular application
Authors: Ning Ruola , Kruger Robert A.
Published: March 2003

This present study reports the results of a computer simulation whose aim was to predict the low‐contrast imaging performance of which a conventional x‐ray image intensifier with charge coupled device (CCD) camera would be capable if incorporated into a computed tomography (CT) volume imager. A vascular imaging task was modeled in our simulation. The effects of detector noise, x‐ray exposure levels, analog‐to‐digital conversion (ADC) precision and residual levels of detected x‐ray scatter were considered. The results of this simulation indicate that the low‐contrast imaging performance of an image intensifier‐based CT system was most limited by the CCD detector readout noise. Given this limitation the detection of greater than about 100 000 detected photons/pixel/projection gave marginal improvement in low‐contrast resolution. At these exposures 12 bit ADC precision resulted in little additional image noise. The effects of detecting scattered x rays are twofold; decreasing the signal‐to‐noise ratio associated with our modeled artery and introducing a cupping artifact. Based on the results from the simulation, it appears that an image intensifier‐based CT system is a feasible concept from a noise viewpoint, if the anticipated imaging task is intravenous angiography.
Detection of small radiation sources: The effect of mode of count‐rate presentation
Authors: Borgstrom Mark C. , Barber H. Bradford , Woolfenden James M. , Seeley George W. , Warren John L.
Published: March 2003

We used an observer‐performance study to compare four different modes of presenting count‐rate data from a radiation detector. Observers searched for small, concealed radioactive sources using a hand‐held radiation‐detector probe. The modes of count‐rate presentation were a ratemeter, howler, multichannel scaler, and HRM III. The HRM III calculates the statistical significance of the difference between current and previous count rates and presents the result as an audio signal. We tested six observers with each mode of count‐rate presentation, calculated receiver operating characteristic (ROC) curves, and used the areas under the ROC curves to compare the different modes. Observer performance was better with the multichannel scaler and HRM III than with either the ratemeter or the howler. The results suggest that observer performance with devices that use ratemeters or howlers can be enhanced by improving the mode of count‐rate presentation.
An improved technique for aligning the light field with the radiation fields on radiotherapy accelerators
Authors: Johnsen Stanley W. , Hamm Robert W.
Published: March 2003

An improved scheme has been developed for measuring the coincidence of the light field and radiation fields of a radiotherapy accelerator. Scans of the light and radiation fields are made sequentially using a photodiode on standard field plotting apparatus. By superimposing the two plots, differences between the two fields’ boundaries can be accurately determined. The technique is simple and avoids uncertainties inherent in common techniques employing film.
Focal spot size measurements with pinhole and slit for microfocus mammography units
Authors: Kimme‐Smith Carolyn , Bassett Lawrence W. , Gold Richard H.
Published: March 2003

Many mammography units now have microfocal spots ranging in size from 0.09 to 0.4 mm for magnification radiography. On site measurements of these focal spots are not possible without invasive procedures if National Equipment Manufacturers Association (NEMA) specifications are to be followed. This paper describes a method to perform such measurements expeditiously (1.5 h) and noninvasively. We describe how this method differs from NEMA test specifications, and report the test results of 22 units from ten manufacturers. Six of the focal spots were larger than the manufacturer's specifications which were based on current NEMA standards. Emphasis is placed on the need for verifiable standards for mammography x‐ray systems.
Dosimetric aspects of the therapeutic photon beams from a dual‐energy linear accelerator
Authors: Al‐Ghazi M. S. A. L. , Arjune B. , Fiedler J. A. , Sharma P. D.
Published: March 2003

Parameters of the photon beams (6 and 20 MV) from a dual‐energy linear accelerator (Mevatron‐KD, Siemens Medical Laboratories, CA) are presented. The depth dose characteristics of the photon beams are dmax of 1.8 and 3.8 cm and percentage depth dose of 68% and 80% at 10‐cm depth and 100‐cm source–surface distance for a field size of 10×10 cm2 for 6 and 20 MV, respectively. The 6 and 20 MV beams were found to correspond to nominal accelerating potentials of 4.7 and 17 MV, respectively. The stability of output is within ±1% and flatness and symmetry are within ±3%. These figures compare favorably with the manufacturer's specifications.
Extraction of pencil beam kernels by the deconvolution method
Authors: Chui Chen‐Shou , Mohan Radhe
Published: March 2003

A method has been developed to extract pencil beam kernels from measured broad beam profiles. In theory, the convolution of a symmetric kernel with a step function will yield a function that is symmetric about the inflection point. Conversely, by deconvolution, the kernel may be extracted from a measured distribution. In practice, however, due to the uncertainties and errors associated with the measurements and due to the singularities produced in the fast Fourier transforms employed in the deconvolution process, the kernels thus obtained and the dose distributions calculated therefrom, often exhibit erratic fluctuations. We propose a method that transforms measured profiles to new, modified distributions so that they satisfy the theoretical symmetry condition. The resultant kernel from the deconvolution is then free of fluctuations. We applied this method to compute photon and electron dose distributions at various depths in water and electron fluence distributions in air. The agreement between measured and computed profiles is within 1% in dose or 1 mm in distance in high dose gradient regions.
Monte Carlo simulation of diagnostic x‐ray scatter
Authors: Papin Patrick J. , Rielly Patrick S.
Published: March 2003

A Monte Carlo method was developed and implemented to simulate x‐ray photon transport. Simulations consisted of a pencil beam of monoenergetic photons with energies from 50 to 110 keV incident on water and aluminum slabs. The dependence of scatter fraction and multiple scattering on x‐ray energy, scatterer thickness, and material is reported in both number and energy fluence. The average energy of scattered photons reaching the detector plane is also reported. Comparisons are made to previous x‐ray scatter computations.
Data predictability for compression of digital fluorography images
Authors: Niv Ron , Shimoni Yair
Published: March 2003

Images obtained by digital fluorography were checked for compressability. These images include images of coronary vessels and images of peripheral vessels. These images have a very low signal‐to‐noise ratio compared to the optical images usually used for developing compression methods. Configurational entropy was used to represent the information content of these images. Reversible prediction algorithms were extensively checked in a search for minimal residual information, enabling more efficient reversible compression. Optimal results were obtained for algorithms based on two or three neighboring pixels and a semiempirical rule, based on the noise level, was found which decides on the best approach. It was found that raw data images are more predictable than subtracted images although the latter are visually preferred.
Energy deposition patterns in an amputated human lower leg heated with a miniannular phased array
Authors: Charny Caleb K. , Guerquin‐Kern Jean‐Luc , Levin Ronald L.
Published: March 2003

The energy deposition patterns in both alcohol‐fixed and unfixed amputated human lower legs produced by a miniannular phased array (MAPA) applicator have been determined. The nontumor bearing portions of four human legs, amputated for therapeutic purposes, were heated within the MAPA. Experimental measurements of the time rate of temperature rise at many locations inside the leg (between 125 and 150) were transformed to specific absorption rate (SAR) values at each point. A simple model was developed which predicts the axial variations in SAR inside the heated limb based upon quantitative details of the leg's geometry obtained from computerized tomography scans. The axial location of the region of maximum energy deposition was predicted by the model with a precision of approximately 1 to 2 cm. Significant time rate of temperature rise was measured inside the cortical portion of the tibia, while the temperature rise in the cancellous (marrow) portion of the tibia was negligible. The alcohol fixation process appears to have no significant effect on the energy deposition patterns within the various leg tissues.
Fiberglass limb phantoms: Fabrication and use for quantitative scintigraphy
Authors: Bading James R. , DiResta Gene R. , Corbally Martin T. , Conlon Kevin C.
Published: March 2003

Quantitative radionuclide scintigraphy often requires empirical calibration factors derived from phantoms which simulate the radioactivity distribution, tissue geometry and tissue composition of the region of interest. This paper describes a method in which casts made with fiberglass tape are used to form realistic, water‐fillable phantoms of the limbs. Phantoms were constructed for the hind legs of the dog and rabbit, species frequently used in developing new radioscintigraphic techniques. Leg bones removed from euthanized animals were mounted anatomically within the casts. The dimensions of the phantom cavities were determined by x‐ray computed tomography. A procedure was developed for orienting the phantoms to match the hind leg geometry of a given experimental setup. Use of the phantoms for image activity calibration is illustrated for a geometric‐mean counting technique used to determine 99mTc activity densities in soft‐tissue regions of the dog thigh. Generalization of the calibration technique to planar and tomographic imaging is straightforward. Insitu measurements of 99mTc activity density obtained by external counting were compared with in vitro radioassays of excised tissue. For 22 tissue samples obtained from four dogs, the in situ and in vitro data were linearly correlated (r=0.98, p≪0.001) over a 50‐fold range of activity density. The mean and standard deviation of the observed percent discrepancies [% discrepancy=100(in situ−in vitro)/in vitro] were (7.8±2.9) and (13.7±2.1), respectively. A supplementary investigation indicated that the discrepancies resulted from a combination of various, individually small sources of error. In conclusion, durable phantoms with cavities closely simulating the irregular outer contours of the limbs can readily be fabricated with fiberglass tape in the laboratory setting. Use of such phantoms permits measurement of regional activities in the limbs with an average error of <15%.
The exposure rate constant for a silver wire 125I seed
Authors: Hashemi Alex M. , Mills Michael D. , Hogstrom Kenneth R. , Almond Peter R.
Published: March 2003

The physical characteristics of the newer silver wire 125I seed were measured with a scintillation spectrometer to compare them with those of the original gold sphere 125I seed. The exposure rate constant was determined by converting the count rate from a scintillation spectrometer into the photon‐fluence rate incident upon the detector, then calculating the exposure rate from the photon‐fluence rate. The exposure rate constant measured perpendicular to the long axis of the seed is 1.361 R cm2/mCi h (1.192 cGy cm2/mCi h)±3.7%, a value that compares favorably with the theoretical exposure rate constant of 1.354 R cm2/mCi h (1.186 cGy cm2/mCi h) calculated from the 125I emissions data. A value of 1.309 R cm2/mCi h was previously reported for the gold sphere 125I seed using the same technique. The angular intensity distribution and anisotropy factor of the silver wire 125I seed are shown to be very similar to those of the gold sphere 125I seed, leading to the conclusion that the clinical application of the two types of 125I seeds need not change.
Calibration and use of the Wisconsin kVp Test Cassette
Authors: Ranallo Frank N. , Goetsch Steven J. , DeWerd Larry A. , Liss Margaret M. , Borras Cari , White Cheryl , Barton John , Mian Tariq , Malik Salmi , Wolff Michele
Published: March 2003

The Ardran–Crooks kVp test cassette is widely used in diagnostic radiology to provide a rapid, simple, noninvasive measurement of x‐ray tube potential. A modified version of this cassette called the Wisconsin kVp Test Cassette was introduced commercially in the U. S. in 1972. Since then, the method of calibration of these cassettes has changed significantly. Wisconsin kVp Test Cassettes calibrated by the manufacturer prior to August 1982 may yield underestimates of kVp measurements, particularly when using the 90–110 and 110–130 kVp regions with single‐phase units. In August 1982 significant improvements in the calibration methods were implemented. The resultant change in calibration is demonstrated by data from the Centers for Radiological Physics. Present calibration methods are believed to be accurate within the greater of ±2 kVp or 2% of actual peak tube potential. Proper use of the cassette is necessary to achieve this level of accuracy.
A method of measuring the primary dose component in high‐energy photon beams
Authors: Nizin Paul , Kase Kenneth
Published: March 2003

We present an approach for deriving the primary component of dose from measurements in a megavoltage gamma or x‐ray beam. The theoretical development of the approach is discussed showing that primary dose can be obtained from four measurements of ionization in narrow beam geometry and two measurements of ionization in a large beam in a phantom. The uncertainty in the final result is analyzed and is about an order of magnitude greater than the uncertainty in an individual measurement.