bluuebunny/crossref_metadata_2025 huggingface dataset:
A signal processing model of diagnostic x‐ray scatter
Authors: Smith Steven W. , Kruger Robert A.
Published: March 2003

A model of scatter is developed from a signal processing approach. The scattering process is viewed as a nonlinear filter (NLF), which transforms a two‐dimensional signal representing phantom thickness into a two‐dimensional signal of scattered radiation. The NLF point spread function (PSF) is derived from a single scattering model, using the principles of Compton scattering and x‐ray attenuation. The PSF is characterized by three approximations: a constant geometric shape, a volume that depends on the phantom thickness, and a width that depends on the phantom‐to‐detector distance. This leads to a closed form expression for the scatter‐to‐primary ratio as a function of phantom thickness, field size, photon energy, source‐to‐phantom distance, and phantom‐to‐detector distance. The NLF model is compared with previously reported measurements using constant thickness phantoms, and discrepancies are discussed. The good agreement found between the NLF model and measured data shows that the functional dependence of scatter on the above parameters, previously only explained in terms of empirical models or Monte Carlo simulations, can be incorporated into a signal processing model.
Experimental verification of a technique for predicting scattered radiation transfer: Application to low photon energies
Authors: Bernstein H. , Muntz E. P.
Published: March 2003

It is shown that predictions can be made of scatter‐to‐primary ratios for a variety of mammographic configurations. The different configurations can be produced by changes in source–detector distance; source–phantom distance; air gap; photon energy; phantom composition, thickness, and cross‐sectional area. A detailed analysis of the effect of a grid is also considered. The predictions are not computationally intensive. Experimental verifications of the predictions for varying phantom cross‐sectional dimensions, source–detector distance, and air gaps with and without a grid have been carried out. Variations with phantom thickness were not considered in this paper. Detailed comparisons between experiment and theory indicate that scatter‐to‐primary ratios were predicted within one standard deviation and the coefficient of variation is within 3% for most of the data points with the worst case coefficient of variation about 6%. Measurements of grid secondary transmission have also been compared to theoretical predictions and agreement is within the coefficient of variation of about ±15%.
Diagnostic effects of edge sharpening filtration and magnification on digitally subtracted renal images
Authors: Kimme‐Smith Carolyn , Gomes Antoinette S. , Cochran Sachiko T. , Barbaric Zoran L. , Lois Juan F.
Published: March 2003

The improved appearance of digital radiographs filtered to improve local contrast and sharpen edges has not increased acceptance of these images by radiologists. Furthermore, many radiologists assert that correct diagnosis is not improved with these filtered images. This study was designed to test this assertion for digital subtraction angiograms (DSA) of renal images. Four experiments are described. First, phantom studies identified filters and their parameters thought likely to be acceptable and useful in diagnosing renal images formed by DSA. Second, these filters and parameters were then tested on medical images to assess their acceptance by radiologists. Third, display modes of windowing, positive/negative presentation, and magnification were varied for filtered and unfiltered images to assess preferences of radiologists. Fourth, filtered and unfiltered magnified images were used to test improved diagnosis. In the final experiment, 148 images from 33 renal studies (15 normal, 18 abnormal) were magnified, gray level windowed, and filtered. Diagnosis was not improved by the two edge sharpening filters tested.
Theoretical model of phase separation of erythrocytes, platelets, and plasma at branches
Authors: Perkkiö J. , Hokkanen J. , Keskinen R.
Published: March 2003

Analytical expressions for the changes of the erythrocyte and platelet concentration are calculated as blood flows through a single bifurcation. The effect of the separation surface, which divides the blood into the two daughter vessels, upon the concentration changes is studied. Numerical fitting to the experimental data shows that both the erythrocyte and platelet distributions across the vessel are nonuniform. The maximum of the erythrocyte concentration is in the core, and of the platelet concentration, in the vicinity of the wall. A separation surface that is convex towards the axis of the vessel yields better fit to the experimental data than a concave one. Together with other studies, this suggests that flow‐divider geometry depends on flow conditions and the vessel diameters. The results can be used for estimation of the distribution of erythrocytes and platelets in the microvasculature.
Source–surface distance and source–axis distance beam data acquisition
Authors: Barish Robert J. , Barish Susan V.
Published: March 2003

Radiation beam characteristics are usually obtained by scanning in a water phantom. Central axis depth dose data, beam profiles, and isodose curves are derived from these measurements. Certain treatment planning systems require that beam profiles be scanned isocentrically rather than at a constant source–surface distance. We have looked at profiles taken both ways, with appropriate field‐size correction, and find no significant difference between them. This should simplify data acquisition in many cases.
The photon sensitivity of a moderated activation neutron detector
Authors: McGinley Patton H.
Published: March 2003

A method is described for estimating the overresponse produced by neutrons generated in the hydrocarbon moderator of a neutron dosimetry system by photon reactions. It is shown that photon interactions in the moderator produce an apparent neutron fluence per unit absorbed dose of 1.40×106 n/cm2/Gy‐x for a flattened 33‐MV x‐ray beam and 2.66×106 n/cm2/Gy‐x for the same beam without the flattening filter in place. The photon response of the moderated activation neutron detector was found to decrease rapidly with end point energy of the x‐ray beam in the range 33 to 18.7 MeV.
An improved nuclear magnetic resonance diffusion coefficient imaging method using an optimized pulse sequence
Authors: Ahn C. B. , Lee S. Y. , Nalcioglu O. , Cho Z. H.
Published: March 2003

Two‐dimensional diffusion coefficient maps (images) of a carefully controlled diffusion phantom have been measured by a new diffusion imaging sequence using a 0.6‐T whole‐body nuclear magnetic resonance (NMR) scanner having a gradient field strength of 2.5 mT/m. The free induction decay (FID) data for the diffusion coefficient images was collected by varying the duration of the readout gradient in the conventional two‐dimensional Fourier imaging sequence. The experimental results obtained by the proposed NMR diffusion measurement technique indicate a close agreement with other previous measurements. The selection of optimum spin‐echo time for maximum signal‐to‐noise ratio (SNR) in diffusion imaging is studied and also experimentally confirmed. Finally, a preclinical study with human volunteers has been performed and results are presented.
A new pulse sequence for determining T1 and T2 simultaneously
Authors: Graumann R. , Fischer H. , Oppelt A.
Published: March 2003

Determination of the relaxation times T1 and T2 which are important for tissue characterization generally requires the use of different pulse sequences in magnetic resonance imaging. In this study, a new pulse sequence which facilitates simultaneous determination of the T1 and T2 times is presented. Determination takes place in this case pixel by pixel from the measured images. The measuring time corresponds in this case approximately to that of a normal spin‐echo sequence with long repetition time and two data acquisitions. The functional dependence of the accuracy of the T1 and T2 determination upon external errors, e.g., angle of rotation errors, is discussed. The tissue contrast behavior of the individual echoes is shown and its dependence on pulse parameters is explained.
Improved “nonisolated‐sensor” solid polystyrene calorimeter
Authors: Zeitz Louis , Ulin Kenneth , Caley Richard
Published: March 2003

A “nonisolated‐sensor” solid polystyrene calorimeter is described which permits absorbed dose measurements with precision of less than 0.3% (standard error of the mean). The accuracy for obtaining absolute absorbed dose was estimated by comparisons with cavity ionization measurements. The calculation of absorbed dose with ionization chambers was carried out based upon the TG‐21 AAPM dosimetry protocol. Measurements in a 60Co γ‐ray field with three different polystyrene parallel‐plate ion chambers in a polystyrene phantom did not differ by more than 1.5% from that obtained with the polystyrene calorimeter. Measurements taken over a period of 247 days are compared with the expected values on the basis of the decay of 60Co. The calorimeter system, with its capability of acquiring, printing, storing, plotting, and analyzing the data by computer, is described.
Errors in wedged field isodose distributions
Authors: Amols H. I. , Lagueux B.
Published: March 2003

A combination of wedged field and beam rotation can result in treatment planning errors. A simple formula is given which describes this effect.
Investigation of basic imaging properties in digital radiography. 8. Detection of simulated low‐contrast objects in digital subtraction angiographic images
Authors: Ohara Ken , Chan Heang‐Ping , Doi Kunio , Giger Maryellen Lissak , Fujita Hiroshi
Published: March 2003

We investigated the effects of imaging and display conditions on the detectability of low‐contrast objects in digital subtraction angiographic (DSA) images. The test images were produced by superimposition of low‐contrast objects on a uniform noisy background obtained with a DSA system. We employed 18‐alternative forced‐choice (18‐AFC) experiments and predictions based on statistical decision theory to study the dependence of the threshold contrasts of the test objects on the object size, incident x‐ray exposure, display window width, and display medium. The results indicated that the threshold contrast decreased with increasing object size, and that the detectability of an object of a given size increased with increasing incident x‐ray exposure and decreasing width of the display window. We found that the signal‐to‐noise ratio (SNR) obtained from the perceived statistical decision theory model, which includes the observer's internal noise, can accurately predict the detectability of low‐contrast objects in DSA images. The threshold SNR corresponding to 50% correct detection in the 18‐AFC experiments had a constant value of 3.8, in agreement with results reported previously for screen–film systems. The theoretical model will be useful for prediction of the performance of a DSA system based on its physical characteristics, and for evaluation of the tradeoff between patient exposure and diagnostic accuracy for a given DSA unit.
Scattered photons produced by beam‐modifying filters
Authors: Huang Pin‐Hua , Chin Lee M. , Bjärngard Bengt E.
Published: March 2003

When a beam‐modifying filter such as a wedge or a compensator is placed in an x‐ray beam, scattered photons are generated in the filter material. The magnitude of the dose contribution from these photons for a 4‐MV x‐ray beam was measured. At a distance of 30 cm from the filter, a copper sheet of 1‐cm thickness produced a dose contribution on the centerline of about 6% of the transmitted primary dose in a 20×20 cm2 field. At the edges of the beam, this contribution was only about one‐half that on the centerline. The presence of these scattered photons leads to only minor dose errors in clinical applications if simple procedures are followed to account for their contribution.
Differential pencil beam dose computation model for photons
Authors: Mohan R. , Chui C. , Lidofsky L.
Published: March 2003

Differential pencil beam (DPB) is defined as the dose distribution relative to the position of the first collision, per unit collision density, for a monoenergetic pencil beam of photons in an infinite homogeneous medium of unit density. We have generated DPB dose distribution tables for a number of photon energies in water using the Monte Carlo method. The three‐dimensional (3D) nature of the transport of photons and electrons is automatically incorporated in DPB dose distributions. Dose is computed by evaluating 3D integrals of DPB dose. The DPB dose computation model has been applied to calculate dose distributions for 60Co and accelerator beams. Calculations for the latter are performed using energy spectra generated with the Monte Carlo program. To predict dose distributions near the beam boundaries defined by the collimation system as well as blocks, we utilize the angular distribution of incident photons. Inhomogeneities are taken into account (1) by attenuating the primary photon fluence exponentially utilizing the average total linear attenuation coefficient of intervening tissue, (2) by multiplying photon fluence by the linear attenuation coefficient to yield the number of collisions in the scattering volume, and (3) by scaling the path between the scattering volume element and the computation point by an effective density.
Investigation of basic imaging properties in digital radiography. 7. Noise Wiener spectra of II–TV digital imaging systems
Authors: Giger Maryellen Lissak , Doi Kunio , Fujita Hiroshi
Published: March 2003

We used Wiener spectral analysis in order to investigate the different noise sources and the effects of various parameters such as pixel size, image intensifier (II) field size, and exposure level on the noise in an II–TV digital system. The digital Wiener spectra in terms of relative x‐ray intensity were determined directly from the digital noise data in terms of pixel values, by use of the characteristic curve of the imaging system. From averaged, subtracted, and/or combination images, the amount of structure mottle relative to the amount of quantum mottle was estimated. We found that a substantial amount of structure mottle was included in our II–TV digital subtraction angiography system, whereas the electronic noise of the TV system was quite small relative to the quantum and structure mottle. The effects of time jitter on the noise in single‐frame images (consisting of multiple video frames) and in subtracted and averaged images were also investigated.
Toward an automated analysis system for nuclear magnetic resonance imaging. I. Efficient pulse sequences for simultaneous T1–T2 imaging
Authors: O'Donnell M. , Gore J. C. , Adams W. J.
Published: March 2003

Simultaneous T1 and T2 images have been obtained using multiple‐echo self‐normalizing sequences. Differential T1 and T2 discrimination for two of these sequences, double saturation recovery and inversion–saturation recovery, have been explored in detail. The inversion–saturation recovery sequence exhibits T1 discrimination comparable to, and in many cases better than, that of optimal T1 weighted conventional pulse sequences, but is poor for T2 imaging. The double saturation recovery sequence yields comparable tissue discrimination for T1 and T2 imaging, but is poor compared to optimal T1 weighted and T2 weighted conventional pulse sequences. Simultaneous T1 and T2 images represent quantitative maps of intrinsic tissue properties, and hence form a natural basis for an automated image analysis and tissue classification system.
Lead as surface bolus for high‐energy photon and electron therapy
Authors: Moyer Robert F. , King Gerald A. , Hauser John F.
Published: March 2003

Dose distributions were evaluated under thin sheet lead used as surface bolus for 4‐ and 10‐MV photons and 6‐ and 9‐MeV electrons using a parallel‐plate ion chamber and film. A narrow, significantly low dose region (−17%) was noted for 4‐MV photons, whereas a 6% increase in dose was present for 10‐MV photons. The dose was elevated 15%–22% near the surface of electron fields with lead bolus, but depth dose relationships were similar to soft‐tissue‐equivalent (STE) bolus. Investigation of partial‐field bolus (2‐cm‐diam circle) documented reduced doses due to lack of lateral electron equilibrium for 10‐MV photons, which was less evident using lead, and large edge effects (up to 30%) for electrons using either lead or STE bolus. Dose distributions on sloped surfaces with electron fields were similar for lead and STE; both require thickness adjustment to achieve a desired effective thickness normal to the surface. Lead bolus has been used successfully in clinical practice for photons.
Encoding patient contours using Fourier descriptors for computer treatment planning
Authors: Mok Edward C. , Boyer Arthur L.
Published: March 2003

Frequently it is desirable to digitize patient's external and internal contours from computer‐assisted tomography (CAT) scan images and to use them for computer treatment planning. After the contours are digitized, each contour could contain over 1000 points. It is a common practice to reduce the number of contour points by interpolation methods in order to use them in a treatment planning program, and in order to save storage space. This paper describes an alternative method for encoding contours. The x–y coordinates of each contour point are represented as a complex number, x+jy. The discrete Fourier transform (DFT) of the array of complex numbers is then computed. Only the 50 lowest frequency components of the DFT are retained. Each contour is then represented by these 50 complex numbers, known as Fourier descriptors. The original contour is restored by performing the inverse Fourier transform. All the frequency components higher than 50 are assumed to be zero during the inverse Fourier transform. The algorithm is described in detail.
Determinations of Ngas and Prepl factors from commercially available parallel‐plate chambers: AAPM Task Group 21 protocol
Authors: Kubo H. , Kent L. J. , Krithivas G.
Published: March 2003

Using a Farmer chamber as a reference dosimeter, we have measured the Ngas (cavity‐gas calibration factor) and Prepl (replacement correction factor) values for four parallel‐plate chambers: (1) a Holt chamber, (2) a Capintec chamber, (3) a Markus chamber, and (4) an SHM chamber. Ngas determinations were done primarily using 60Co photons and nominal 18‐MeV electrons for Ngas determinations. For Prepl determinations, we used nominal 18‐, 15‐, 12‐, 9‐, 6‐, and 4‐MeV electrons.
Microradiography with an x‐ray image magnifier: Application to dental hard tissue
Authors: Kuriyama Masao , Dobbyn Ronald C. , Takagi Shozo , Chow Laurence C.
Published: March 2003

The superior spatial resolution obtained with parallel‐beam microradiography over conventional contact microradiography has allowed us to image microstructural features of dental hard tissue not previously reported. Our efforts to extend these techniques to provide a real‐time capability for viewing in situ demineralization and remineralization effects, at and below the 1‐μm level, have resulted in an instrument with several novel and unique features. Using a synchrotron radiation source of x rays and diffraction image magnification, we are now able to change magnification at will (x‐ray zoom lens). In addition, the energy range over which the instrument operates gives one considerable flexibility in optimizing image contrast. The techniques of parallel‐beam microradiography, and diffraction image magnification are applicable to problems in many other areas of science. Using examples within dental research, the uniqueness and versatility of these new techniques are discussed.
The production of secondary electrons in an electron beam
Authors: Werner Barry L. , Das Indra J. , Khan Faiz M.
Published: March 2003

Convenient methods for calculating the ratio of restricted to unrestricted collision stopping power in water over a wide range of initial and cutoff energies, the rate of production of secondary electrons, and the primary electron dose distribution in an electron beam, are presented.
Rotating laminar emission camera with Ge‐detector: Further developments
Authors: Mauderli W. , Fitzgerald L. T.
Published: March 2003

A germanium (Ge) camera with laminar collimator–detector geometry, ROLEC, simulating a 195×195 mm2 detection area, has been constructed and tested. The detector consists of five separate Ge blocks grooved into distinct electrical channels. Results show that ROLEC measuring times have to be about 3× greater than Anger camera measuring times to achieve a 5% signal‐to‐noise ratio with each device. Both spatial and energy resolution of the ROLEC exceed that of the Anger camera but sensitivity variations along the detector strips pose a significant problem in development of a clinically useful device.
Vector implementation of Chang's attenuation correction method for single photon emission computed tomography
Authors: Sloboda Ronald S.
Published: March 2003

A vector version of Chang's attenuation correction method for single photon emission computed tomography accommodating arbitrary convex body contours is presented and its implementation on a general purpose array processor described. For a constant linear attenuation coefficient μ=0.10 cm−1 and contour shapes similar to those encountered in clinical work, correction factors calculated with the vector algorithm are the same to within a few percent as those obtained using the original Chang method. Typical execution speed of the new algorithm is 0.01 s/correction factor for a 64 point contour.
Image restoration from nonuniform static field influence in modified echo‐planar imaging
Authors: Sekihara Kensuke , Kohno Hideki
Published: March 2003

Experimental results of images restored from the influence of nonuniform static fields in modified echo‐planar imaging are presented. The same restoration technique is used as that proposed for Fourier imaging. Experimental results show that both density variation and geometrical distortion can almost completely be eliminated in the restored image. Image restoration can relax the stringent requirement on the amplitude of the time‐modulated gradient needed in modified echo‐planar imaging.
Dosimetric evaluation in heterogeneous tissue of anterior electron beam irradiation for treatment of retinoblastoma
Authors: Kirsner Steven M. , Hogstrom Kenneth R. , Kurup Rajendra G. , Moyers Michael F.
Published: March 2003

A dosimetric study of anterior electron beam irradiation for treatment of retinoblastoma was performed to evaluate the influence of tissue heterogeneities on the dose distribution within the eye and the accuracy of the dose calculated by a pencil beam algorithm. Film measurements were made in a variety of polystyrene phantoms and in a removable polystyrene eye incorporated into a tissue substitute phantom constructed from a human skull. Measurements in polystyrene phantoms were used to demonstrate the algorithm's ability to predict the effect of a lens block placed in the beam, as well as the eye's irregular surface shape. The eye phantom was used to measure dose distributions within the eye in both the sagittal and transverse planes in order to test the algorithm's ability to predict the dose distribution when bony heterogeneities are present. Results show (1) that previous treatment planning conclusions based on flat, uniform phantoms for central‐axis depth dose are adequate; (2) that a three‐dimensional heterogeneity correction is required for accurate dose calculations; and (3) that if only a two‐dimensional heterogeneity correction is used in calculating the dose, it is more accurate for the sagittal than the transverse plane.
Replacement correction factors for electron measurements with a parallel‐plate chamber
Authors: Casson Harvey , Kiley James P.
Published: March 2003

The AAPM Task Group dosimetry protocol is ambiguous regarding the replacement correction factors Prepl to be applied to electron measurements with parallel‐plate chambers. By intercomparison with a cylindrical chamber whose Prepl values at dmax may be calculated from Task Group 21, the Prepl values for a PTW/Markus parallel‐plate chamber have been determined in the range of mean incident energies of 5–11 MeV. The Prepl values for this chamber are found to differ significantly from unity, if one assumes that the cylindrical chamber values are valid.
Magnetic resonance imaging system stability: Temporal variability in signal intensity, signal‐to‐noise, T1, and T2 measurements on a 0.15‐T resistive system
Authors: Slone Richard M. , Fitzsimmons Jeffrey R.
Published: March 2003

Signal intensity (SI) variability was evaluated on a 0.15‐T resistive instrument using a phantom and was found to increase with time, repetition time (TR), and echo time (TE), ranging from 0.24% standard deviation (SD) over 34 min for 500/30 (TR/TE) images, to 2.1% SD over 5 days for 2000/30 images. Signal‐to‐noise (S/N) variability increased with time and TE but not TR and ranged from 4.2% SD over 34 min to 7.1% SD over 5 days in 500/30 images. Variability in T1 and T2 measurements on phantoms ranged from 1.8% to 4.8% SD for T1 and 3.6% to 6.5% SD for T2 in the biological range over 5 h. High reproducibility of SI, T1, and T2 measurements was demonstrated over a 6‐week period in normal muscle measurements.
A comparison of air‐cavity inhomogeneity effects for cobalt‐60, 6‐, and 10‐MV x‐ray beams
Authors: Beach J. L. , Mendiondo M. S. , Mendiondo O. A.
Published: March 2003

The inclusion of air‐filled spaces in treatment fields creates a potential dosimetric problem due to the loss of charged particle equilibrium near the air–tissue interface. We have used a simulated larynx phantom and a small buildup/extrapolation chamber to compare the magnitude and spatial extent of underdosing and overdosing at the distal surface for two linear accelerators (10‐ and 6‐MV x rays) and a cobalt‐60 machine. Surface doses were compared to doses measured in a similar but homogeneous phantom to give observed/expected ratios (O/E), which were greater than 1.0 for large field sizes and less than 1.0 for small field sizes on all machines. The minimum field sizes which produce no surface underdosing for a simulated 2‐cm‐diam larynx are roughly 7×7 cm for 10‐MV x rays, 6×6 cm for 6‐MV x rays, and 5×5 cm for cobalt‐60. In addition, the depth over which underdosing occurs is seen to increase with increasing energy.
Determination of electron beam mean incident energy from d50 (ionization) values
Authors: Ten Haken Randall K. , Fraass Benedick A.
Published: March 2003

Depth‐ionization measurements have been obtained with an air‐filled Nordic Association of Clinical Physicists (NACP) design parallel‐plate ionization chamber in a water phantom for ten foil‐scattered electron beams from two different machines with nominal energies between 6 and 20 MeV and field sizes from 6×6 to 25×25 cm2. Depths of 50% ionization and practical range have been determined from least‐squares fits to both the raw data and values corrected to parallel‐beam geometry using measured virtual source distances. Depths of 50% dose have also been obtained from fits to divergence‐corrected depth‐dose measurments performed under identical conditions using, a p‐type silicon diode detector. Utilizing accepted conversion factors between mean incident energy (Ē0) and depth of 50% dose for parallel incident beams, and taking advantage of the fact that p‐type silicon diode detector readings are nearly directly indicative of relative dose, conversion factors between Ē0 and depth of 50% ionization for divergence‐corrected and raw, uncorrected finite source‐surface distance depth‐ionization data are empirically determined. Those values, obtained using the results of both ETRAN and EGS4 dose calculations as base lines, are compared to values currently recommended for use in clinical dosimetry.
Ferrokinetic studies with stable isotopes as tracers
Authors: Cantone M. C. , Molho N. , Pirola L. , Hansen Ch. , Roth P. , Werner E.
Published: March 2003

A methodology for ferrokinetic studies based on the administration of iron stable isotopes was developed. Fractional plasma clearance and intestinal iron absorption in rabbits were determined using the double tracer technique. Three rabbits were given 58Fe solution intravenously and about 40 min later 57Fe solution orally. Blood samples were drawn at different times following administration. The analysis of the single iron isotopes content in plasma samples was made by proton nuclear activation. The results were compared with those obtained from the administration, to the same rabbits, of the radioactive isotopes 55Fe and 59Fe. The agreement was found to be satisfactory.
Dosimetry of 24‐MV x rays from a linear accelerator
Authors: Krithivas G. , Rao S. N.
Published: March 2003

Dosimetric characteristics of a 24‐MV photon beam produced by a Varian Clinac 2500 linear accelerator are presented. Particular attention is paid to measurements and applications relating to depth of maximum dose and scatter correction factors. New experimental methods were adopted to investigate scatter factors for field sizes ranging from 5×5 to 40×40 cm. For the largest field investigated, a 3% phantom scatter factor relative to a 10×10 cm field was determined. From the study, clinically useful scatter–phantom ratios were generated. The study also demonstrated that the estimated correction factors for scatter in a medium caused negligible changes (≃0.1%) on the percent depth‐dose values derived from the measured tissue–phantom ratio (TPR) data. The paper also quantitatively compares results obtained for dosimetric parameters like scatter output, tissue–phantom ratio, and percent depth‐dose with those obtained from similar machines used for radiotherapy. Measured data relating to these parameters are expressed by taking a reference depth of 8 g/cm2 in a medium for normalization purposes and its importance discussed. Additional measurements presented in this work include beam quality and collimator effect on dose rate. Phantoms of various sizes and materials and a variety of detectors were used throughout the investigation.
Scanned‐projection digital mammography
Authors: Nishikawa Robert M. , Mawdsley Gordon E. , Fenster Aaron , Yaffe Martin J.
Published: March 2003

The effectiveness of film–screen mammography is limited by tradeoffs between latitude and contrast, film granularity, and the need to increase dose when antiscatter methods are used. We are currently developing a scanned‐projection digital mammography (SPDM) system to overcome these limitations. The system consists of a pair of scanning slits, a high‐resolution x‐ray image intensifier tube, a linear photodiode array, and a digital display. The detective quantum efficiency of the SPDM system at spatial frequencies up to 3 cycles/mm is similar to that of mammographic film–screen combinations, but is lower at high frequencies. For low‐contrast objects as small as 0.1 mm in diameter, the signal‐to‐noise ratio is currently equal to that of optimally exposed mammographic film–screen images for equal dose to the breast and superior for regions which would be underexposed or overexposed on film. This is achieved by the use of a low‐noise detector system, geometric magnification, and scatter elimination. Images of a contrast‐detail phantom and excised breast tissue illustrate the superior contrast sensitivity of SPDM.
Practical application of the differential Batho method for inhomogeneity correction on kerma in a photon beam
Authors: Yuen K. , Kornelsen R. O.
Published: March 2003

The Batho equation gives a satisfactory method to correct the dose for points in the electronic equilibrium region for a uniform slab of inhomogeneity in a photon beam. In spite of the many investigations, we believe no simple and adequate method has been found for routine clinical dose calculations which require dose correction of a small‐volume inhomogeneity in an arbitrary location. In the present report, we combine the values of the two calculation types of the differential Batho method, which we have developed previously, to give a new calculated value for the scatter perturbation due to an annulus of inhomogeneity. The coefficients in the combination, which we derived from a detailed analysis of the scatter perturbation, are simple geometrical ratios. The new calculated values are in good agreement with measured values. We believe this application of the differential Batho method can provide a practical and accurate method of correcting for inhomogeneities of any size and shape in clinical dose calculations.
Total inhomogeneity correction including chemical shifts and susceptibility by view angle tilting
Authors: Cho Z. H. , Kim D. J. , Kim Y. K.
Published: March 2003

A correction technique of the total magnetic field inhomogeneity effects including the localized object induced inhomogeneities such as chemical shift and susceptibility is developed and its usefulness is experimentally demonstrated. With this new and simple technique all the inhomogeneity induced artifacts can be corrected simultaneously. The basic idea of this method is to add a compensation gradient of the same amplitude as the selection gradient in simultaneity with the reading gradient in such a way that the view angle is tilted. Thereby all the inhomogeneity induced geometrical shifts and hence the intensity changes are corrected, since the addition of the compensation which is independent from the field inhomogeneities including both chemical shifts and susceptibility. This technique has been theoretically examined and its usefulness is demonstrated by experiments.
Comparison of measured and calculated dose distributions around an iridium‐192 wire
Authors: Gillin M. T. , Lopez F. , Kline R. W. , Grimm D. F. , Niroomand‐Rad A.
Published: March 2003

The relative dose distribution around a 5.0‐cm‐long piece of 192Ir wire has been measured using LiF chips. Measurements were made at distances of 0.25 to 5.0 cm away from the source and distances of 0.0 to 4.0 cm along the source. In addition, measurements were also made at several distances along the axis of the source. Attention was paid to the errors associated with these measurements. A comparison was made between a commercial software program, isodos, an analytical solution to the Sievert integral, and the measurements. Good agreement was obtained at distances along and away from the source. Major disagreements were found at points along the source axis.
A comparison of solid phantoms with water for dosimetry of 125I brachytherapy sources
Authors: Meigooni Ali S. , Meli Jerome A. , Nath Ravinder
Published: March 2003

Dosimetry of brachytherapy sources is critically dependent on precise measurement of the source–detector distance. A solid phantom can be precisely machined and hence distances can be accurately determined. In this work LiF thermoluminescent chips are used for absolute dose rate measurements in solid water, polymethylmethacrylate (PMMA), and polystyrene. These media are examined for their suitability in the dosimetry of 125I by comparing depth doses in each phantom. Measurements and Monte Carlo calculations show that solid water is equivalent to water for the dosimetry of 125I seeds, however, polystyrene and PMMA are not equivalent to water. Also, photon energy spectra for several depths in each phantom material have been calculated and are used to determine average photon energy and mass energy absorption coefficients as a function of depth.
Leakage radiation from electron applicators on a medical accelerator
Authors: Pennington Edward C. , Jani Shirish K. , Wen B‐Chen
Published: March 2003

The leakage characteristics of electron applicators on our Clinac 2500 linear accelerator have been measured. The leakage radiation in the patient plane and at the surface of the electron applicators has been measured for applicator sizes from 6 cm×6 cm to 25 cm×25 cm and beam energies from 6 to 22 MeV. For certain applicator/energy combinations the leakage radiation was significant. The leakage radiation, relative to the central axis dose, was found to be up to 7% in the patient plane and up to 39% at the applicator surface. Reducing the collimator setting or adding lead at select locations on the applicator surface was effective in reducing the magnitude of the radiation leakage.
Evaluation of a photon and an electron beam of a 6‐MV linear accelerator
Authors: Sharma Subhash C. , Modur Prabakar , Basavatia Ram
Published: March 2003

The first Mitsubishi medical linear accelerator in the United States was commissioned in April 1985. This unit EXL‐8 (marketed by Mitsubishi International Corporation) produces 8‐MeV electron beams in addition to 6‐MV x rays. It is a 100‐cm source–axis distance isocentric machine. Acceptance testing and performance evaluation of this accelerator were completed. Our measurements included beam characteristics and dosimetry parameters for both modalities. Central axis % depth dose (% DD), tissue‐maximum ratio, field size output factors, wedge factors, etc., for this Linac 6‐MV beam are reported. Characteristics of the 8‐MeV electron beam, namely % DD data, isodose curves, and cone ratios for various electron applicators are presented.
Phantom evaluation of angiographer performance using low frame rate acquisition fluoroscopy
Authors: Fritz Steven L. , Mirvis Stuart E. , Osher Pais S. , Roys Steven
Published: March 2003

The authors have investigated the effect of 5 frames/s television fluoroscopy on the time required to selectively catheterize five test arteries in an angiographic phantom. Here, 5 frames/s acquisition was accomplished by sampling frames from a 30 frames/s video signal. Sampled frames were stored in a video memory which provided continuous display to the fluoroscopist between samples. The test phantom was a plastic model of an aorta with branching vessels immersed in an isodense suspension of barium in water. For four of the five vessels there was no significant difference in time required for catheter placement between 30 frames/s and 5 frames/s.
A depth dependence determination of the wedge transmission factor for 4–10 MV photon beams
Authors: McCullough E. C. , Gortney Janet , Blackwell C. Robert
Published: March 2003

The depth dependence (up to 25 cm) of the in‐phantom wedge transmission factor (WTF) has been determined for three medical linear accelerator x‐ray beams with energies of 4, 6, and 10 MV containing 15°–60° (nominal) brass wedges. All measurements were made with a cylindrical ionization chamber in water, for a field size of 10×10 cm2 with a source–skin distance of 80 or 100 cm. We conclude that, for the accelerators studied, the WTF factor at depth is less than 2% different from that determined at dmax (for the nominal wedge angles and photon energies studied) unless the depth of interest is greater than 10 cm. Up to the maximum depth studied (25 cm) the relative wedge factor—that is, wedge factor at depth compared to that determined at dmax —was about equal to or less than 1.02 for the 15° and 30° wedges and any of the photon beam energies studied. For the seldom utilized combination of a nominal wedge angle in excess of 45° with a depth greater than 10 cm, the WTF at depth can differ from the WTF determined at dmax, by up to 5%. Since the wedge transmission factor is reflective of relative percent dose data, our results also indicate that it is in error to use open field percent depth doses for certain combinations of wedge angle, photon energy, and depth.
Tissue‐mimicking gelatin–agar gels for use in magnetic resonance imaging phantoms
Authors: Blechinger J. C. , Madsen E. L. , Frank G. R.
Published: March 2003

A new variety of tissue‐mimicking materials for use in 1H nuclear magnetic resonance (NMR) phantoms has been developed and extensively tested, principally at 10 MHz and at room temperature. The materials can be formed with a broad range of T1 's and T2 's representative of soft tissues. They are mixtures of various percentages of agar, animal hide gelatin, water, and glycerol. Small concentrations of formaldehyde and n‐propanol prevent melting through 100 °C and prevent bacterial invasion. The materials are easily produced. A thorough description of compositions and production procedures is given. T1 's exhibit about a 5%/ °C rise in temperature. T2 's exhibit less than a 1% rise/ °C. Long‐term (12 months) stability is exhibited both for NMR properties and for absence of fluid extrusion. Preliminary results indicate that T1 's depend on the Larmor frequency in a similar way to that observed in soft tissues.
Modifying constrained least‐squares restoration for application to single photon emission computed tomography projection images
Authors: Penney Bill C. , King Michael A. , Schwinger Ronald B. , Baker Stephen P. , Doherty Paul W.
Published: March 2003

Image restoration methods have been shown to increase the contrast of nuclear medicine images by decreasing the effects of scatter and septal penetration. Image restoration can also reduce the high‐frequency noise in the image. This study applies constrained least‐squares (CLS) restoration to the projection images of single photon emission computed tomography (SPECT). In a previous study, it was noted that CLS restoration has the potential advantage of automatically adapting to the blurred object. This potential is confirmed using planar images. CLS restoration is then modified to improve its performance when applied to SPECT projection image sets. The modification was necessary because the Poisson noise in low count SPECT images causes considerable variation in the CLS filter. On phantom studies, count‐dependent Metz restoration was slightly better than the modified CLS restoration method, according to measures of contrast and noise. However, CLS restoration was generally judged as yielding the best results when applied to clinical studies, apparently because of its ability to adapt to the image being restored.
An improved electron multiple‐scattering distribution for Monte Carlo transport simulation
Authors: Al‐Beteri A. A. , Raeside D. E.
Published: March 2003

An improved electron multiple‐scattering distribution is presented in the form of a composite function which combines three expressions valid over different scattering angle regions: a modified relativistic Mott single‐scattering term for large angle scattering, a modified Moliere Gaussian term for small angle scattering, and an exponential term for the intermediate angle scattering region. The exponential term has two adjustable parameters which make possible the smooth transition from the large to the small scattering angle regions. The proposed distribution exhibits better agreement with experiment than other multiple‐scattering distributions commonly used in Monte Carlo electron transport codes, is amenable to direct sampling over a continuous range of electron energies and step sizes, can be used for elements of any atomic number, and is particularly suitable for use on small memory computers.
Theoretical optimization of a split septaless xenon ionization detector for dual‐energy chest radiography
Authors: Cardinal H. Neale , Fenster Aaron
Published: March 2003

It is proposed that digital scanned projection radiography of the chest be performed by using an energy‐sensitive septaless xenon ionization detector (SXID) to obtain dual‐energy images. The proposed detector is composed of a front region, sensitive to low‐energy x rays, and a rear region, sensitive to high‐energy x rays, separated by a suitable filter layer. We have developed a simple, precise theoretical formulation for dual‐energy optimization, and applied it to the split SXID. We describe the variation of optimum detector performance with source kilovoltage and filtration (material and thickness), and hence heat loading, under conditions of constant exposure and constant dose. We estimate dose as the average absorbed dose to an equivalent water layer of suitable thickness, assuming slab geometry, so that the calculation is as simple as that for exposure.
A comparison of the speeds of three convolution algorithms
Authors: Boyer Arthur L. , Wackwitz Ronald , Mok Edward C.
Published: March 2003

The speeds of three computer algorithms suitable for use in three‐dimensional radiotherapy planning codes were compared. Two of the algorithms are based on ray‐tracing methods, the first algorithm uses a fast ray‐tracing procedure directly and the second employs a table lookup procedure; the table was originally calculated by ray tracing. The third algorithm was a convolution procedure using the fast Fourier transform. Benchmark programs were written to compare the fundamental running speeds of the three algorithms operating on three‐dimensional arrays of various sizes. The convolution procedure employing the three‐dimensional fast Fourier transform had the shortest running times on a VAX/750 (Digital Equipment Corp.) computer. We concluded that this algorithm holds significant potential for practical three‐dimensional dose calculations.
Best estimate of luminal cross‐sectional area of coronary arteries from angiograms
Authors: Lee P. L. , Selzer R. H.
Published: March 2003

We have reexamined the problem of estimating the luminal area of an elliptically shaped coronary artery cross section from two or more radiographic diameter measurements. The expected error is found to be much smaller than the maximum potential error. In the case of two orthogonal views, closed form expressions have been derived for calculating the area and the uncertainty. Assuming that the underlying ellipse has limited ellipticity (major/minor axis ratio less than five), it is shown that the average uncertainty in the area is less than 14%. When more than two views are available, we suggest using a least‐squares fit method to extract all available information from the data.
A survey of the information gained from planning treatment with a computer
Authors: Doppke Karen P. , Goitein Michael
Published: March 2003

We designed a questionnaire to be filled out by the physicist at the time of planning treatment with the aid of a computer in order to assess what was learned during the course of the planning session. We analyzed the results to gauge the impact of the 70 treatment planning sessions conducted during the one month period in our department in which questionnaires were completed (about 72% of all external beam plans made in that period). In 65 instances (93%) an initial judgment as to how treatment would be delivered had already been made and in 56 of these cases the patient's treatment had already been simulated by the time the computer calculation was made. Changes in the intended plan were made in eight of the 65 cases (12%). In 38% of the cases, more than one plan was developed and, when this was the case, on average records of 2.6 plans were made. Those performing the plans were asked to judge their value to the overall treatment of the patient, and the results were as follows: essential–19 cases (27%); very helpful–22 cases (31%); and somewhat helpful–29 cases (41%).
The optimization of pencil beam widths for use in an electron pencil beam algorithm
Authors: McParland Brian J. , Cunningham John R. , Woo Milton K.
Published: March 2003

Pencil beam algorithms for the calculation of electron beam dose distributions have come into widespread use. These algorithms, however, have generally exhibited difficulties in reproducing dose distributions for small field dimensions or, more specifically, for those conditions in which lateral scatter equilibrium does not exist. The work described here has determined that this difficulty can arise from the manner in which the width of the pencil beam is calculated. A unique approach for determining the pencil beam widths required to accurately reproduce small field dose distributions in a homogeneous phantom is described and compared with measurements and the results of other calculations. This method has also been extended to calculate electron beam dose distributions in heterogeneous media and the results of this work are presented. Suggestions for further improvements are discussed.