bluuebunny/crossref_metadata_2025 huggingface dataset:
Cylindrical chamber dimensions and the corresponding values of Awall and Ngas/(NXAion)
Authors: Gastorf Robert , Humphries Leroy , Rozenfeld Martin
Published: March 2003

Cylindrical ionization chamber descriptions and physical dimensions that are necessary for the calculation of Awall and Ngas are presented. Also included are the corresponding calculated values of Awall and Ngas/(NXAion).
Pulse sequence generated oblique magnetic resonance imaging: Applications to cardiac imaging
Authors: Bernstein M. A. , Perman William H. , Besozzi Myrwood C. , Thomasson David M.
Published: March 2003

A pulse sequence procedure for producing oblique magnetic resonance images is described. Using this procedure we present a new, accurate method to obtain true short‐axis views and true long‐axis views (both parallel and perpendicular to the septal plane) of the heart. The method is accurate regardless of the orientation of patient's heart. The method does not require the patient to be rotated, nor otherwise moved, and does not require any additional hardware. The method is experimentally verified with both human and phantom studies. The phantom study indicates accuracy of approximately 1° with a commerical scanner that reports angular measurements to a precision of 1°. Application of the short‐axis views to measurement of left ventricular volume, and possible advantages of Gauss–Legendre integration for this measurement are discussed. Finally, multiphase oblique cardiac images are presented.
Dose comparisons for mammographic systems
Authors: Speiser R. C. , Zanrosso E. M. , Jeromin L. S. , Carlson R. A.
Published: March 2003

Dose estimates were made (1) for Kodak Min‐R screens combined with Kodak Min‐R film and Kodak Ortho M film, both with and without a 5:1 Bucky grid; (2) for standard xeroradiographic techniques in negative development mode; and (3) for the new, higher sensitivity xeroradiographic process of the Xerox 175 System. The estimates were derived from exposure versus depth measurements in phantoms made of BR12 breast simulation material using thermoluminescent detectors. A molybdenum target source with molybdenum filtration, at a half‐value layer of 0.37‐mm Al, was used for the screen–film measurements. All xeroradiographic measurements were made with a tungsten target source with aluminum filtration at half‐value layers of 1.5 to 1.56 mm Al. Mean glandular dose estimates for the Min‐R screen/Ortho M film combination with Bucky grid and for the new xeroradiographic process were found to be similar. Dose reduction with the new xeroradiographic system was achieved through a more sensitive photoreceptor and more sensitive development, which also improved the unique imaging characteristics of xeroradiography.
A high‐resolution digital dosimetric system for spatial characterization of radiation fields using a thermoluminescent CaF2:Dy crystal
Authors: Atari Nader A. , Svensson Göran K.
Published: March 2003

A high‐resolution digital dosimetric system has been developed for the spatial characterization of radiation fields. The system comprises the following: 0.5‐mm‐thick, 25‐mm‐diam CaF2:Dy thermoluminescent crystal; intensified charge coupled device video camera; video cassette recorder; and a computerized image processing subsystem. The optically flat single crystal is used as a radiation imaging device and the subsequent thermally stimulated phosphorescence is viewed by the intensified camera for further processing and analysis. Parameters governing the performance characteristics of the system were measured. A spatial resolution limit of 31±2 μm (1σ) corresponding to 16±1 line pair/mm measured at the 4% level of the modulation transfer function has been achieved. The full width at half maximum of the line spread function measured independently by the slit method or derived from the edge response function was found to be 69± 4 μm (1σ). The high resolving power, speed of readout, good precision, wide dynamic range, and the large image storage capacity make the system suitable for the digital mapping of the relative distribution of absorbed doses for various small radiation fields and the edges of larger fields.
MgO:Fe as an absolute radiation dosimeter
Authors: Schultheiss Timothy E.
Published: March 2003

Iron impurities can take the 1+, 2+, or 3+ valence state in substitutional lattice sites in MgO. Virtually no Fe3+ remains after annealing in a reducing atmosphere. Upon irradiation with x rays or electrons, holes are trapped at Fe2+ sites. The resulting Fe3+ ion absorbs ultraviolet photons at 285 nm. Monitoring the radiation‐induced growth of the Fe3+ optical absorption band provides the basis of using MgO:Fe as a solid‐state dosimeter analogous to the ferrous sulfate chemical dosimeter.
A technique for routine calibration of thermoluminescent dosimeters in the diagnostic energy range
Authors: Morgan Tommie J. , Bencomo Jose A. , Marsh Lee , Finn David , Hidalgo‐Salvatierra Oscar
Published: March 2003

Four different techniques for calibrating lithium fluoride thermoluminescent dosimeters were compared at two beam qualities in the diagnostic energy range. The techniques included in‐air, saturation backscatter, and Bureau of Radiological Health (BRH) test stand geometry with and without an ionization chamber. The results indicate that there are no statistical differences between any of the calibration techniques evaluated. Hence, the technique using the BRH test stand with an ionization chamber is a simple and reproducible calibration procedure which may be used in a routine diagnostic radiology quality assurance program.
Estimated spectrum of a 4‐MV therapeutic beam
Authors: Ahuja Sain D. , Steward Palmer G. , Roy Tapankumar S. , Slessinger Eric D.
Published: March 2003

We have made a comparative investigation of the estimated spectra obtained by the Laplace transform analysis of the transmitted exposure data measured in an absorption study of a 4‐MV x‐ray beam. Four transform pair models currently used with this method have been evaluated. It has been determined that the Archer–Wagner model provides a valid representation of the measured transmission data and yields an estimated spectrum which most closely resembles a Monte Carlo spectrum calculated for a 4‐MV therapeutic x‐ray beam available from a typical medical accelerator.
Measurement and calculation of neutron leakage from a medical electron accelerator
Authors: Uwamino Y. , Nakamura T. , Ohkubo T. , Hara A.
Published: March 2003

The leakage neutron spectra and dose equivalent were systematically measured in the irradiation field, treatment room, maze, and outside the shielding door at the microtron medical electron accelerator facility of the National Cancer Center, Tokyo. For these measurements, we used two types of multimoderator neutron spectrometers (Bonner spheres containing indium activation detectors and 3He detector), an aluminum activation detector, and a commercially available neutron rem counter. The measured results were compared with the combined calculation of the one‐dimensional ANISN and two‐dimensional DOT3.5 discrete ordinates transport codes. The calculation was performed by using a measured source spectrum in the irradiation field and by computer modeling of the maze entrance. The calculation indicated good agreement in spectral shape and agreement with experiment within a factor of 2 in absolute dose‐equivalent values. This transport calculation was systematically repeated for different geometrical and material parameters, and simple analytical formulas and their parameters applicable for shielding design of a medical electron accelerator facility were obtained in general form.
Thermographically determined specific absorption rate patterns of 434‐MHz applicators
Authors: Chou Chung‐Kwang , Guy Arthur W. , McDougall John A. , Dong Art , Luk Kenneth H.
Published: March 2003

The specific absorption rate (SAR) patterns of two 434‐MHz hyperthermia applicators, models TCA 434‐1 (9×20 cm) and TCA 434‐2 (13×25 cm), were evaluated thermographically using a phantom model. The phantom model consisted of a 2‐cm‐thick layer of fat and a 10‐cm depth of muscle contained in a 30×30 cm base Plexiglas box. The model was bisected in the middle. Polyester screens at the interface allowed the synthetic gel to make electrical contact between the two halves of the muscle tissue. Octyl alcohol was applied to the fat interface to ensure continuity of dielectric properties. Thermograms were taken for both applicators over the following areas of the exposed model: (1) fat surface, (2) internal surface with E‐field parallel to interface, and (3) internal surface with E‐field perpendicular to interface. SAR's were calculated from the temperature rise (8 °C maximum), net input power (550–650 W), exposure time (15–60 s), and specific heat of the muscle (0.86 kcal/kg °C). A factor of 0.42 needs to be multiplied to correct for the specific heat of fat. High localized SAR's along the broad sides of the applicators were seen when the applicators were in direct contact with the phantom. With the use of a 0.8‐cm polystyrene foam spacing, the SAR's within the aperture of the applicators were relatively uniform. The patterns of the two applicators were quite similar. However, the TCA 434‐1 applicator is smaller and more applicable for clinical conditions.
Stopping‐power and mass energy‐absorption coefficient ratios for Solid Water
Authors: Ho Anthony K. , Paliwal Bhudatt R.
Published: March 2003

The AAPM Task Group 21 protocol provides tables of ratios of average restricted stopping powers and ratios of mean energy‐absorption coefficients for different materials. These values were based on the work of Cunningham and Schulz. We have calculated these quantities for Solid Water (manufactured by RMI), using the same x‐ray spectra and method as that used by Cunningham and Schulz. These values should be useful to people who are using Solid Water for high‐energy photon calibration.
Output factors and dose calculations for blocked x‐ray fields
Authors: Meli Jerome A.
Published: March 2003

Output factors for blocked fields have been measured in a polystyrene phantom for four collimator field sizes and two blocking schemes using 6‐MV x rays. For all measurements the phantom surface was at the calibration source–surface distance (SSD) because, as is shown, the calculation of dose to any point in a phantom at an arbitrary SSD can be expressed in terms of the output factor for the field size at the calibration distance. It is found that output factors are a function of both the surface field size of the blocked field and the collimator field size. Specifically, the output factor for a blocked field is less than that for the collimator field size used but greater than that for an unblocked field of the same surface field size formed by collimator settings only. A method is proposed for utilizing these data to calculate the output factor for any collimator and blocked field size. The validity of the method is checked by using it to calculate dose to a point in a phantom and comparing this to the measured dose.
Off‐center ratios for three‐dimensional dose calculations
Authors: Chui Chen‐Shou , Mohan Radhe
Published: March 2003

A new method is proposed for computing the off‐center ratios (OCR's) in three‐dimensional dose calculations. For an open field, the OCR at a point is computed as the product of the primary OCR (POCR) and the boundary factors (BF's). The POCR describes the beam profile for an infinite field, that is, without the effect of the collimators. It is defined as the ratio of the dose at a point off the central ray to the dose at the point on the central ray at the same depth for an infinite field. The POCR is a function of radial distance from the beam central ray and depth. The BF describes the shape of the beam in the neighborhood of the field boundary defined by the collimators. It is defined as the ratio of the OCR at a point for a finite field to the OCR at the same point for an infinite field. The BF is a function of distance from the field boundary, depth, and field size. For a wedged field, we assume that the boundary factors remain the same as for open fields but the POCR's are altered. The changes in beam profiles are described by a factor called the wedge profile factor (WPF), defined as the ratio of the dose at a point for the largest wedged field to the dose at the same point for an open field of the same field size. The WPF is a function of lateral distance from the beam central plane and depth. Calculated OCR's using this new method are in agreement with the measured data along both the transverse and the diagonal directions of the field.
Fluence perturbation in photon beams under nonequilibrium conditions
Authors: Nilsson Bo , Montelius Anders
Published: March 2003

The perturbation effect in parallel‐plate ionization chambers used for buildup measurements has been investigated. The fluence perturbation due to electrons emitted through the side walls are thoroughly investigated by measurements using film and extrapolation chambers and by calculations. The electron fluence varies both with side wall material and chamber geometry. In order to obtain a small perturbation effect, the chamber should have a large guard width compared to the electrode separation and the side walls should have as large an angle as possible with the central axis. The side wall should be of the same material as the rest of the chamber. The perturbation effect is also dependent on the electron contamination of the beam and angular distribution of the electron fluence. It is thus not possible to correct the perturbation effect in one parallel‐plate chamber with fixed plate separation with correction factors obtained with extrapolation chambers of other dimensions. In order to make accurate surface dose measurements extrapolation chambers are therefore strongly recommended in favor of fixed parallel‐plate chambers.
Correlation of microdosimetric measurements with relative biological effectiveness from clinical experience for two neutron therapy beams
Authors: Stinchcomb Thomas G. , Kuchnir Franca T. , Myrianthopoulos Leon C. , Horton John L. , Roberts William K.
Published: March 2003

Microdosimetric measurements were made for the neutron therapy beams at the University of Chicago and at the Cleveland Clinic with the same geometry and phantom material using the same tissue‐equivalent spherical proportional counter and standard techniques. The energy deposition spectra (dose distributions in lineal energy) are compared for these beams and for their scattered components (direct beam blocked). The model of dual radiation action (DRA) of Kellerer and Rossi is employed to interpret these data in terms of biological effectiveness over this limited range of radiation qualities. The site‐diameter parameter of the DRA theory is determined for the Cleveland beam by setting the biological effectiveness (relative to 60Co gamma radiation) equal to the relative biological effectiveness value deduced from radiobiology experiments and clinical experience. The resulting value of this site‐diameter parameter is then used to predict the biological effectiveness of the Chicago beam. The prediction agrees with the value deduced from radiobiology and clinical experience. The biological effectiveness of the scattered components of both beams is also estimated using the model.
Dosimetric considerations of d(15)+Be and p(26)+Be neutron beams from an isocentric cyclotron facility
Authors: Nair Ravi P. , Al‐Siari Ahmed , Skaggs Lester S.
Published: March 2003

To select the optimum therapeutic neutron beam available from a CS30 medical cyclotron (manufactured by the Cyclotron Corporation, Berkeley, California), central axis depth dose data and output dose rates were compared for the bombardment of beryllium with either the proton or deuteron beams available from the machine. The effect on these parameters of filtering the beams with either pure polyethylene, polyethylene loaded with 5% boron, or polyethylene loaded with 10% lithium was studied. A 4‐cm, 10% lithiated filter used with a 26‐MeV proton beam was selected for therapeutic use. Buildup curves, beam profiles at several transverse planes for different field sizes, and comparison of beam profiles with 60Co are given.
The propagation of relativistic heavy ions in multielement beam lines
Authors: Schimmerling Walter , Rapkin Marwin , Wong Mervyn , Howard Jerry
Published: March 2003

We describe calculations of the energy loss, range, stopping power, multiple scattering, and other related properties of a high‐energy heavy‐ion beam at any one of a set of beam line elements. A beam line element (e.g., any beam modification, detection, or control device) is characterized by its thickness, areal density, aperture, and function. The loss of multiply scattered particles to any finite‐aperture detector is calculated in the small‐angle approximation, and the position of the Bragg peak, as given by particles stopping in the second of two ionization chambers used for Bragg curve measurements, is estimated. A general purpose computer program, propagate, has been written to allow addition, deletion, and modification of the beam line elements used in the calculation and to provide a convenient means of repeating such calculations for arbitrary beam lines. Calculations and experimental measurements are compared and found to be in satisfactory agreement.
Theoretical analysis of regional blood flow studies
Authors: Perkkiö J. , Keskinen R. , Heikkonen J. , Mäntylä M.
Published: March 2003

Regional blood flow measurements were analyzed using a two‐compartment model. The distribution of injected tracer between blood and tissue was assumed to be in equilibrium only at the beginning of the experiment. Equations for the determination of the partition coefficient of injected radionuclides from in vivo measurements were derived. These equations can also be used for estimation of the relative blood content of a tumor or an organ. The results were applied to experiments, where three diffusible radionuclides were injected simultaneously into patients with diagnosed tumors.
Thermodynamic considerations of acrylic cement implant at the site of giant cell tumors of the bone
Authors: Krishnan Engikolai C. , Nelson Charles , Neff James R.
Published: March 2003

A discussion of the thermodynamic aspects of a relatively new treatment method for giant cell tumors of the bone is presented in this paper. The advantages of implanting methylmethacrylate acrylic bone cement into a curetted tumor site are briefly discussed and placed in perspective relative to more prevalent surgical treatments. As the bone cement self‐heats while curing, the possibility of heat necrosis in the bone exists. However, the damage due to heat may be beneficial in reducing the rate of tumor recurrence. A thermodynamic consideration of the treatment situation appears to be warranted. After a general introduction and a brief literature review, the theoretical thermodynamic equations are developed. Once the basic equations for the heat transfer from the cement or the bone are derived, there is then a discussion of the various characteristics of bone and methylmethacrylate crucial to the analysis, such as, thermal conductivity, specific heat, density, and heat generation parameters. Finally, in order to reduce the theory to a form which may be used practically, the equations derived are written in terms of finite‐difference equations, which approximate them numerically. Different equations are written for each type of heat transfer condition encountered in the cement–bone system as spacial variances in material and geometry occur. The equations derived may be used to model the system allowing one to predict the time‐dependent temperature distribution in bone during the curing of acrylic cement. Using computer techniques to reduce the equations obtained from this analysis, and knowing the temperature at which adjacent cells die, a zone of necrosis may be mapped surrounding the acrylic impact.
Magnetic resonance fast Fourier imaging
Authors: Cuppen J. J. M. , Groen J. P. , Konijn J.
Published: March 2003

A new method for fast magnetic resonance imaging is presented. It provides a more rapid data acquisition than two‐dimensional Fourier imaging (2DFI) by a factor which may be chosen depending on the required signal‐to‐noise ratio of the image. In addition to the readout gradient of 2DFI, the present method employs an oscillating modulation gradient. In this way, a curved alternating trajectory in k space is sampled after each spin excitation. For a p‐times accelerated data acquisition, the trajectory consists of p periods, where p is of the order of 2 to 8 for low‐frequency gradient modulation but can be chosen higher if certain hardware requirements are met. Adequate sampling density in k space is obtained by scanning shifted trajectories after subsequent spin excitations. The method can be combined with volume imaging (3DFI) and multiple slice 2DFI. It was implemented on a standard Philips Gyroscan system without any hardware modifications. Results obtained for an acceleration factor p=4 are shown.
Characterization of the point spread function and modulation transfer function of scattered radiation using a digital imaging system
Authors: Boone John M. , Arnold Ben A. , Seibert J. Anthony
Published: March 2003

A digital radiographic system was used to measure the distribution of scattered x radiation from uniform slabs of Lucite at various thicknesses. Using collimation and air gap techniques, [primary+scatter] images and primary images were digitally acquired, and subtracted to obtain scatter images. The scatter distributions measured using small circular apertures were computer fit to an analytical function, representing the circular aperture function convolved with a modified Gaussian point spread function (PSF). On the basis of goodness of fit criterion, the proposed Gaussian function is a very good model for the scatter PSF. The measured scatter PSF's are reported for various Lucite thicknesses. Using the PSF's, the modulation transfer functions are calculated, and this spatial frequency information may have value in analytical scatter removal techniques, grid design, and air gap optimization.
Buildup/surface dose and exit dose measurements for a 6‐MV linear accelerator
Authors: Purdy James A.
Published: March 2003

The central axis dose distribution in the buildup region for the Varian Clinac 6/100 6‐MV x‐ray beam was measured in a polystyrene phantom using a fixed volume (0.5 cm3) parallel‐plate ionization chamber (2.4‐mm plate separation). Results for the surface dose measurements ranged from approximately 8% of the maximum dose for a 5×5 cm field, up to 36% for a 40×40 cm field, 100‐cm source–skin distance. The effect of a 0.6‐cm‐thick polycarbonate blocking tray and metal filters on the surface and buildup dose is also reported. In addition, ionization measurements were made to document the dose perturbations caused by the absence of backscattering material at the exit surface of a polystyrene phantom. Exit dose measurements showed a 15% reduction in dose with essentially no scattering material beyond the measurement point. Near full scatter condition could be restored by placing 5–10 mm (depending on field size) of unit density material directly behind the ion chamber's distal surface.
Performance characteristics of a widely used orthovoltage x‐ray therapy machine
Authors: Scrimger J. W. , Connors S. G.
Published: March 2003

Experimental results sufficient to provide a data base for the Philips RT 250 x‐ray machine are presented. A representative selection of operating conditions covering the working range of the machine from 75 to 250 kVp, were used. Beam quality, depth dose data, field flatness, and filtration characteristics were studied, and selected results are reported.
Toward an automated analysis system for nuclear magnetic resonance imaging. II. Initial segmentation algorithm
Authors: O'Donnell M. , Gore J. C. , Adams W. J.
Published: March 2003

Image segmentation algorithms based on hierarchical clustering have been developed for analysis of T1 and T2 nuclear magnetic resonance images. Application of these algorithms to simultaneous T1–T2 images of healthy volunteers extracted fundamental tissue types in the brain. These algorithms also were used both to identify the extent of the region of involvement of a subject with a history of a grade 3 astrocytoma of the right frontal lobe of the brain, and to characterize the tissue within the region of involvement. These results suggest that a simple segmentation algorithm can produce reasonable clustering of tissue types within the brain.
Scatter correction algorithm for digitally acquired radiographs: Theory and results
Authors: Boone John M.
Published: March 2003

A scatter correction algorithm for digitally acquired radiographs (SCADAR) is presented. SCADAR requires the acquisition of two digital images, taken at different object‐to‐detector distances. These two images are digitally magnification compensated, and subtracted. The primary component in the resulting difference image Δ is mathematically eliminated, and hence the Δ image is used as a measure of the local contribution of scattered radiation. A gray scale transformation is used to transform the Δ image to a scattered component image, which is then smoothed by Fourier filtering, using a matched filter to increase the signal‐to‐noise ratio. The smoothed scatter image is then subtracted from its corresponding original image, resulting in the corrected SCADAR image. Implementation of SCADAR can result in a large increase in image contrast, and a significant reduction in shading due to scattered radiation effects. The mathematical derivation of the algorithm is developed, and experimental verification is given for some of the principles used. Using experimental images acquired from scattering phantoms, the results of SCADAR on various image parameters such as contrast and detail signal‐to‐noise ratio are discussed.
Energy resolution in a high‐pressure gas scintillation proportional chamber
Authors: Fahey Frederic H. , Zimmerman Robert E. , Judy Philip F. , Lanza Richard C.
Published: March 2003

A high‐pressure gas scintillation proportional chamber has been designed and constructed to image x and gamma rays for medical applications. The chamber contains 4 atm of pure xenon. Ultraviolet light emitted from excited xenon atoms within the detector is collected by a hexagonal array of seven UV‐sensitive photomultiplier tubes, which in turn are separated from the pressurized gas by 1‐cm‐thick fused‐silica windows. A model was used to predict the energy resolution of the device as a function of fill‐gas pressure, voltage within the detector, and light‐collection efficiency. The energy resolution improved with increasing scintillation region voltage from 17% full width at half maximum (FWHM) at 1.9 kV to 10% FWHM at 3.0 kV for 59.5‐keV photons; once above 1.5 kV, there was no improvement with increasing drift voltage. The addition of the signals from the peripheral phototubes to that of the center phototube did not substantially improve the energy resolution of the device. This was because the noise that was present yielded a high correlation between the phototubes; when this noise was incorporated into the model, the energy resolution of the multiphototube system was accurately estimated. The energy resolution of the gas scintillation proportional chamber was found to be superior to the sodium iodide Anger camera at 59.5 keV by a factor of 2. Further improvement can be obtained by increasing the scintillation region voltage and by increasing the light‐collection efficiency by moving the scintillation region closer to the phototubes.
Monte Carlo calculations of scatter dose for small field sizes in a 60Co beam
Authors: Kijewski Peter K. , Bjärngard Bengt E. , Petti Paula L.
Published: March 2003

Photon dose calculations are frequently based on the separation of primary and scatter dose components. For small field sizes it is difficult to make this separation from measured data. Such problems lend themselves to solution by calculational methods, which permit rigorous definition of primary and scatter dose. Using Monte Carlo methods, we verify an equation derived from O'Connor's scaling theorem relating scatter–primary ratios at small depths and field sizes to those at larger depths and fields. This equation may prove to be useful when separating primary and scatter dose components, and consequently in dose calculations involving irregular fields or inhomogeneities.
Approximations to saturation curves in gas‐filled parallel‐plate ionization chambers
Authors: Heese R. N. , Podgorsak E. B. , Fallone B. G.
Published: March 2003

Saturation curves were measured for air and nitrogen in the exposure rate range from ∼1 to ∼500 R/min. Because of the differences between the ion–ion versus electron–ion recombination processes, the air and nitrogen saturation curves differ considerably even when measured under identical chamber and exposure rate conditions. A comparison between our experimental data and various approximations to saturation curves in air, developed by others, is given. It is then shown that the air saturation curve can be approximated by a sum of three simple exponential functions.
Variation of the count‐dependent Metz filter with imaging system modulation transfer function
Authors: King Michael A. , Schwinger Ronald B. , Penney Bill C.
Published: March 2003

A systematic investigation was conducted of how a number of parameters which alter the system modulation transfer function (MTF) influence the count‐dependent Metz filter. Since restoration filters are most effective at those frequencies where the object power spectrum dominates that of the noise, it was observed that parameters which significantly degrade the MTF at low spatial frequencies strongly influence the formation of the Metz filter. Thus the radionuclide imaged and the depth of the source in a scattering medium had the most influence. This is because they alter the relative amount of scattered radiation being imaged. For low‐energy photon emitters, the collimator employed and the distance from the collimator were found to have less of an influence but still to be significant. These cause alterations in the MTF which are more gradual, and hence are most pronounced at mid to high spatial frequencies. As long as adequate spatial sampling is employed, the Metz filter was determined to be independent of the exact size of the sampling bin width, to a first approximation. For planar and single photon emission computed tomographic (SPECT) imaging, it is shown that two‐dimensional filtering with the Metz filter optimized for the imaging conditions is able to deconvolve scatter and other causes of spatial resolution loss while diminishing noise, all in a balanced manner.
Optical factors affecting the detective quantum efficiency of radiographic screens
Authors: Drangova M. , Rowlands J. A.
Published: March 2003

Parameters related to the detective quantum efficiency (DQE) of several representative screens of different thicknesses, phosphor grain sizes, and optical properties were measured by the scintillation spectrum method, using monoenergetic x rays produced from x‐ray fluorescence. The experimental results, including those for spectral shape and average light energies (EA) emitted, are compared with conventional theories of the operation of screens. It was hoped that this would vindicate the theory of the effect of optical properties and so permit the simple calculation of all parameters related to DQE from standard x‐ray attenuation tables. Rather more substantial energy‐dependent deviations of EA are found than was previously realized, which preliminary analysis suggests are due to both optical effects and photoelectron escape. We conclude that although DQE for a single energy can be calculated by simplified methods to within ±10%, the effective DQE when polyenergetic beams are used is much less accurately estimated and requires a fuller theoretical treatment.
Development of a tissue‐equivalent phantom for diaphanography
Authors: Linford J. , Shalev S. , Bews J. , Brown R. , Schipper H.
Published: March 2003

A phantom is proposed for quality control and calibration of instruments used for transillumination of the breast for early detection of cancer. A container is filled with a material having optical properties very similar to those of breast tissue, and internal objects are viewed by transmitted light. The phantom can be used to optimize operating conditions for the visualization of small or deep‐seated lesions.
Monte Carlo dosimetry for 125I and 60Co in eye plaque therapy
Authors: Chiu‐Tsao Sou‐Tung , O'Brien Keran , Sanna Robert , Tsao Hung‐Sheng , Vialotti Charles , Chang Yu‐Shang , Rotman Marvin , Packer Samuel
Published: March 2003

Monte Carlo calculations of radiation dosimetry using morse code are performed for 125I and 60Co point sources in a cylindrical head phantom that simulates the geometry of eye plaque therapy for choroidal melanoma. We obtain the dose variation in the eye at submillimeter intervals over distances as close as 1 mm and up to 2.5 cm from the source. The calculations for 125I are performed for the phantom media of water, protein, and a homogenized protein–water mixture simulating the composition of the eye. Relative dose functions for 125I for these phantom media are fitted to second‐degree polynomials. Agreement is found with published results. The relative dose function for 60Co at eye position in the water head phantom is fitted to a third‐degree polynomial and compared with that for 60Co at the center of a large water sphere. A boundary effect due to the head phantom–air interface on the dose distribution for 60Co is demonstrated. The dose falloff with distance is faster for the eye geometry compared with the bulk geometry. We also show that the relative dose distributions within the tumor are comparable for 125I and 60Co by comparing their relative dose functions. This result is consistent with the success of clinical trials of large melanoma treatments with 125I plaques.
Evaluation of a prototype dual‐energy computed tomographic apparatus. I. Phantom studies
Authors: Kalender W. A. , Perman W. H. , Vetter J. R. , Klotz E.
Published: March 2003

We report the evaluation of a prototype dual‐energy implementation using rapid kVp switching on a clinical computed tomographic scanner. The method employs prereconstruction basis material decomposition of the dual‐energy projection data. Each dual‐energy scan can be processed into conventional single‐kVp images, basis material density images, and monoenergetic images. Phantom studies were carried out to qualitatively and quantitatively evaluate and validate the approach.
Area weighted convolutional interpolation for data reprojection in single photon emission computed tomography
Authors: Schwinger Ronald B. , Cool Steven L. , King Michael A.
Published: March 2003

A data reprojection algorithm has been developed for use in single photon emission computed tomography on an array processor equipped computer system. The algorithm makes use of an accurate representation of pixel activity (uniform square pixel model of intensity distribution), and is rapidly performed due to the efficient handling of an array‐based algorithm and the fast Fourier transform on parallel processing hardware. The algorithm consists of using a pixel driven nearest‐neighbor projection operation to an array of subdivided projection bins. The subdivided project bin array is then convolved with the angle‐dependent projection of the area of a uniform square pixel and compressed to original bin size. The new algorithm has thus been named the area weighted convolution (AWC) method of interpolation. When compared to nearest‐neighbor and linear interpolation algorithms, the new AWC algorithm was found to be more accurate, having an accuracy approaching that of the line length algorithm. It also yielded an easier and more efficient implementation on parallel hardware than line length or linear interpolation, with faster execution times than either.
A system for quantitatively assessing certain performance parameters of gamma camera systems from reflective print and film transparency final images
Authors: Piltingsrud H. V. , Hoops R. G.
Published: March 2003

Past efforts at routine evaluation of gamma camera systems performance, using final product images, primarily resulted in qualitative assessments. This paper describes a system for quantitatively determining certain performance parameters of gamma camera systems using paper‐backed print and transparent film test images of the same type used for viewing by the nuclear medicine physician. By use of appropriate transmission and line source phantoms, the gamma camera system's spatial resolution, distortion, and flood field uniformity as a function of position in the image field, are determined using a semiautomated image scanning and data reduction system. This system consists of a high‐resolution scanning microdensitometer/reflectometer, interfaced to a computer, along with necessary programs to locate bar and line grid intersection points in the image. Also included are algorithms for calculating the various image parameters desired. This system may provide an analytical method for quantitatively assessing certain performance parameters of gamma camera systems, using the final‐product images from the cameras.
A helium‐ion beam for stereotactic radiosurgery of central nervous system disorders
Authors: Lyman John T. , Kanstein Leal , Yeater Fred , Fabrikant Jacob I. , Frankel Kenneth A.
Published: March 2003

A new radiation beamline configuration for stereotactic heavy‐ion Bragg peak radiosurgery of patients with intracranial deep arteriovenous malformations, including carotid‐cavernous fistulas, has been developed using the 230 MeV/u helium ion beam at the 184‐in. Synchrocyclotron at the University of California, Berkeley. The modified beam has five characteristics: (1) uniform field between 10 and 40 mm in diameter; (2) variable depth of penetration between 40 and 140 mm; (3) stopping region for primary ions that can be broadened up to 40 mm; (4) sharply defined lateral and distal borders; and (5) dose rate greater than 2 Gy/min. It is adapted to the ISAH (irradiation stereotactic apparatus for humans) at the 184‐in. Synchrocyclotron, with effective stereotactic localization of defined volumes within the brain, and is designed to reach all intracranial targets. It has proven suitable for all patients with intracranial vascular disorders treated with stereotactic radiosurgery at our laboratory.
Effects of scattered radiation and veiling glare in dual‐energy tissue–bone imaging: A theoretical analysis
Authors: Shaw Chorng‐Gang , Plewes Donald B.
Published: March 2003

Dual‐energy subtraction imaging allows tissue and bone structures to be separated from each other and attenuating thicknesses measured. Potential applications include chest imaging, bone mineral measurement, angiography, and mammography. However, intrinsic to most x‐ray detectors is the acceptance of scattered radiation as part of the image signal. Added to that is the veiling glare component when an image intensifier is used. Together, they result in erroneous transmission measurement and degrade the accuracy of energy subtraction processing. In this paper, the effects of scattered radiation and veiling glare on energy subtraction images are examined theoretically. A model is derived and used to compute the effects on the thickness signals, image contrast, and image noise as a function of the scatter glare to primary ratios. The ratios were measured on a point‐by‐point basis for a Rando chest phantom. For 96% of the image field studied, the thickness signals may be subject to an error ranging from 0 to −22.5 cm for tissue and 0 to 5.2 cm for bone. The image contrast in the tissue image may be reduced by a factor ranging from 1 to 59. The percentage of the uncanceled bone signals ranges from −52% to 52%. The contrast‐to‐noise ratio may be reduced by a factor ranging from 1 to 18.
Uniformity in dosimetry protocols for therapeutic applications of fast neutron beams
Authors: Mijnheer B. J. , Wootton P. , Williams J. R. , Eenmaa J. , Parnell C. J.
Published: March 2003

In this supplement to both the American and the European protocols for clinical neutron dosimetry, new recommendations are given with respect to the basic physical parameters and experimental techniques employed. For neutron dosimetry, the air kerma or exposure calibration in a photon beam is the most suitable method for the calibration of tissue‐equivalent ionization chambers until calibration in a standard neutron field becomes available. More recent data are recommended for the physical parameters required for the photon calibration as well as for the measurements in the neutron beam. Water is recommended as the reference phantom material due to its similarity in absorption and scattering properties to muscle. The resulting overall change in absorbed dose calculated according to this supplement, compared with the original protocols, will be smaller than about ±2% due to differences in the basic physical parameters. An additional change of several percent occurs at depth in a phantom as a result of the difference between water and the muscle‐equivalent liquid formerly recommended as the reference phantom material.
Measurements in high‐intensity beams from medical linear accelerators
Authors: O'Brien P. F. , Barnett R. B. , Michaels H. B. , Siwek R. A.
Published: March 2003

There is a potential to produce very high doses and dose rates on dual mode (electron and x‐ray capability) medical electron accelerators [C. J. Karzmark, Br. J. Radiol. 40, 697 (1967)]. Measurements were taken on two accelerators to determine the size of the doses which were possible. The experiments demonstrate the limitations of many of the commonly used dosimeters when they are used outside of the dose and dose rate ranges normally encountered in a radiotherapy department. Dose rates from 110 to 165 cGy/pulse were measured using ionization chamber, ferrous sulphate and thermoluminescent dosimetry, and water calorimetry.
Radiation absorbed dose to bladder walls from positron emitters in the bladder content
Authors: Powell Gregory F. , Chen Chin‐Tu
Published: March 2003

A method to calculate absorbed doses at depths in the walls of a static spherical bladder from a positron emitter in the bladder content has been developed. The beta ray dose component is calculated for a spherical model by employing the solutions to the integration of Loevinger and Bochkarev point source functions over line segments and a line segment source array technique. The gamma ray dose is determined using the specific gamma ray constant. As an example, absorbed radiation doses to the bladder walls from F‐18 in the bladder content are presented for static spherical bladder models having radii of 2.0 and 3.5 cm, respectively. Experiments with ultra‐thin thermoluminescent dosimeters (TLD's) were performed to verify the results of the calculations. Good agreement between TLD measurements and calculations was obtained.
Radiofrequency penetration and absorption in the human body: Limitations to high‐field whole‐body nuclear magnetic resonance imaging
Authors: Röschmann P.
Published: March 2003

This study presents experimental results about the effective depth of penetration and about the radiofrequency (rf) power absorption in humans as a function of frequency. The frequency range investigated covers 10 up to 220 MHz. For the main part, the results were derived from bench measurements of the quality factor Q, and of the resonance frequency shift due to the loading of the coil. Different types of head‐, body‐, and surface coils were investigated loaded with volunteers or metallic phantoms. For spin‐echo imaging at 2 T (85 MHz), the local specific absorption rate (SAR) was found to be ≊0.05 W/kg using a π pulse of 1‐ms duration and pulse repetition time TR =1 s. Measurements of the quality factor Q as a function of frequency show that the SAR depends upon the frequency f according to ∼f 2.15. The effective depth of rf penetration as derived drops from 17 cm at 85 MHz to 7 cm at 220 MHz. Head imaging with B1 penetrating from practically all sides into the object should be possible up to 220 MHz (5 T) with SAR values staying within the local limit of 2 W/kg as set by the FDA. Whole‐body imaging of large subjects as well as surface coil imaging is depth limited above 100‐MHz frequency. Perturbation methods are applied in order to separate the total rf power deposition in the patient into dielectric and magnetic contributions. The observed effects due to interactions of rf magnetic fields with biological tissue contradict predictions based on homogeneous tissue models. A refined tissue model with regions of high electrical conductivity, subdivided by quasi‐insulating adipose layers, provides a rationale for a better understanding of the underlying processes. At frequencies below 100 MHz, the rf power deposition in patients is apparently more evenly distributed over the exposed body volume than currently assumed.
Oblique magnetic resonance imaging of the bronchi
Authors: Bernstein M. A. , Perman William H. , Unger June M. , Besozzi Myrwood C.
Published: March 2003

The trachea and main bronchi of a supine patient in a magnetic resonance (MR) scanner are not contained in a single standard coronal plane, but instead intersect this coronal plane at some angle, usually 20°–35°. We have developed a new MR imaging protocol to determine the oblique imaging plane which best contains the trachea and main bronchi. The resulting oblique images simplify anatomical identification, and allow the user to select additional oblique planes which cut any desired portion of main bronchus in true cross section. Accurate lumen shapes and areas may then be extracted from these cross‐sectional images. The method does not require the patient to be moved or rotated, and does not require hardware modification. We demonstrate the clinical application of the protocol with both a normal volunteer and a patient with an endobronchial tumor. The use of gradient echo pulse sequences together with this protocol to distinguish between vessels and bronchi is presented. We provide phantom verification to demonstrate the quantitative accuracy of the method to provide lumen areas.
Vessel imaging using dual‐energy tomosynthesis
Authors: Liu Jing , Nishimura Dwight , Macovski Albert
Published: March 2003

A method has been developed that combines dual‐energy subtraction and tomosynthesis for vessel imaging in intravenous angiography. This paper describes the procedure for doing tomosynthesis on a fan‐beam rotational‐motion system and gives the point responses of the imaging system. Phantom studies show that dual‐energy tomosynthesis improves the visualization of desired vessels lying on a selected plane. The results may be feasible for some applications such as clinical diagnosis of coronary artery diseases.
Dose distributions in dynamic stereotactic radiosurgery
Authors: Pike Bruce , Podgorsak Ervin B. , Peters Terence M. , Pla Conrado
Published: March 2003

A treatment planning technique for calculation of dose distributions in dynamic stereotactic “radiosurgery” with a 10‐MV isocentrically mounted linear accelerator is presented. The treatment planning for dynamic radiosurgery is a three‐dimensional problem, since during treatment both the gantry and the couch rotate simultaneously, the gantry from 30° to 330° and the couch from 75° to −75°. The patient surface and anatomical information is obtained from a family of computed tomography or magnetic resonance scans, and a stereotactic frame is used for target localization, treatment setup, and patient immobilization during the treatment. The dose calculational algorithm follows the gantry and couch rotation in an incremental fashion, and relies on measured stationary beam central axis percentage depth doses and dose profiles to calculate the normalized tissue‐maximum‐ratio distributions over a matrix of points defined on one of three orthogonal planes (transverse, sagittal, or coronal). The dose calculation algorithm is discussed in detail and calculated dose distributions for single plane and dynamic radiosurgery compared with measured data.