bluuebunny/crossref_metadata_2025 huggingface dataset:
A generalized formulation of diffusion effects in μm resolution nuclear magnetic resonance imaging
Authors: Ahn C. B. , Cho Z. H.
Published: March 2003

A generalized formulation of the diffusion related nuclear magnetic resonance (NMR) signal is derived from a random walk model. Previous analyses performed in the NMR spectroscopy were the formulations of the diffusion related signal amplitude at a specific time, such as the spin echo formation time. They are, in general, not applicable to continuous time domain analyses. In this paper, we have extended the theory to the two‐dimensional imaging case and derived an analytical formula useful for the computation of the diffusion affected signal as a function of continuous time for a time variant gradient. This formulation will be useful in NMR imaging, especially in NMR microscopy where the diffusion associated signal attenuation is serious due to the strong gradient fields (100–1000 G/cm), and at the same time data are acquired continuously for the acquisition period. In addition to the loss of the resolution and signal‐to‐noise ratio due to the random phase fluctuation by diffusion, the variation of the intensity during the data acquisition period introduces a line broadening whose full width at half‐maximum is found to be much larger than the bandwidth‐limited resolution or diffusion related intrinsic resolution. This line spreading effect is integrated in a computer simulation and is evaluated as an integral part of the overall diffusion effects in μm resolution NMR imaging or NMR microscopy.
A new photoconductor imaging system for digital radiography
Authors: de Monts Humbert , Beaumont François
Published: March 2003

Amorphous selenium is a material often used in the x‐ray imaging system. The main application is in xeroradiography where the structure of the sensor is a layer of selenium on a conductive substrate. The signal is a charge density on the surface which is revealed by a toner or by electrostatic probe for digitalization. In the system described here, the sensor structure is different for the sensor is covered by an electrode, a thin layer of metal, which gives another interface. The reading system needs the scanning of a light beam and the resolution power depends on the size of the beam. It is easier to scan a light beam than electrostatic probes so a more compact system can be realized. In the process, there are two phases: the storage and the reading. The time spent between the two phases reduces the quality of the image, and an in situ reading system, integrated to the radiographic machine will be, for this reason, more efficient. Also, the sensor needs good memory effect. One has investigated different sensors based on a structure of a thin photoconductive layer between two electrodes to find a memory effect. We have already seen this phenomena in the Bi12 SiO20 (B. Richard, “Contribution à l'étude d'un procédé d'imagerie radiologique utilisant le photoconducteur Bo12 SiO20,” Ph.D. thesis, Paris, 1987). In amorphous selenium with some dopants and some type of metallic contact, the memory effect is important enough to realize a system. With 2×2 cm samples, a complete x‐ray digital imaging system has been built. The feasability and quality of such a system has been proved. The performances of this sensor and the results we have obtained, encouraged us to design a 20×20 cm system. In this paper we show the way it can be achieved and its performance.
The peanut orbit: A modified elliptical orbit for single‐photon emission computed tomography imaging
Authors: Esser Peter D. , Jakimcius Andy , Foley Lisa
Published: March 2003

The front collimator surface of a conventional single‐photon emission computed tomography (SPECT) detector system transcribing an elliptical orbit to approximate body contour could, under certain circumstances, penetrate the ellipse and make patient contact. The problem is associated both with the large front surface dimensions of tomographic cameras and the need to maintain the camera perpendicular to a radius through the axis of rotation. To aid in the development of an improved body contour orbit, software has been developed to simulate SPECT imaging systems. A major feature of the algorithms is a spatially calibrated graphic representation of the pallet, patient and orbit of the camera head. Based on computer simulations performed with this software, a modified elliptical orbit has been proposed for patient contour SPECT scanning on two different types of tomographic acquisition systems: a dual camera, rotating ring system and a single camera, multimotion stand system. This peanut orbit has the advantage of minimizing collimator patient distance, as does the elliptical orbit, but also compensates for camera motion that could potentially produce patient contact. Versions of the peanut orbit algorithms have been installed and are operational on clinical systems similar to those that were simulated. The ring‐based gantry system (with cameras mounted on cantilevered arms) requires additional software to correct for translational shift in the field of view as a function of radial distance from the center of the system. This is done during uniformity correction. Standard unmodified backprojection software is used for reconstruction. The peanut orbit may represent an improved approach to body contour imaging in rotational SPECT.
Investigation on the reduction of electron contamination with a 6‐MV x‐ray beam
Authors: Parthasaradhi K. , Prasad S. Guru , Rao B. M. , Lee Y. , Ruparel R. , Garces R.
Published: March 2003

Experimental investigations have been carried out on the reduction of electron contamination of a 6‐MV x‐ray beam of Clinac model 1800 for square field sizes 5×5 to 30×30 cm2 in steps of 5 cm and for rectangular field sizes 19×7 and 7×19 cm2. The electron contamination of both the open beam and the beam with the tray can be effectively reduced by placing a lead foil filter immediately below the blocking tray. Measurements at 100‐cm source‐skin distance with filter in place showed a reduction in dose in the buildup region and also a displacement of the location of Dmax to greater depths, even for small field sizes such as 10×10 cm2.
Determination of accurate dosimetric parameters for beveled intraoperative electron beam applicators
Authors: Dahl Robert A. , McCullough Edwin C.
Published: March 2003

Intraoperative electron beam therapy requires accurate dose maximum/monitor unit (Dmax / MU) values for both flat and beveled ended applicators (cones). Measurement of Dmax /MU values with either solid or nontilting scanning water systems may give rise to inaccuracies due to the difficulty in locating an accurate position for dmax, since the ionization chamber usually cannot be scanned along the central axis of the beveled intraoperative applicator. A linear one‐dimensional scanner (which permits ionization measurements to be made as a function of water depth) has been modified to provide scanning along a line up to 30° from the perpendicular to the phantom surface. This modification has proven helpful in improving the accuracy of certain dosimetric parameters (e.g., Dmax /MU) of beveled applicators. For example, we found inaccuracies which arose when we measured Dmax /MU of beveled intraoperative radiation therapy cones in either solid or other scanning water systems were greatly reduced, especially for the lower electron beam energies (e.g., 6 and 9 MeV).
Equations for Ngas and Nair in terms of NX and NK
Authors: Attix F. H.
Published: March 2003

Task Group 21 of the American Association of Physicists in Medicine defined the cavity‐gas calibration factor Ngas for ionization chambers in a protocol for radiotherapy dosimetry published in this journal [Med. Phys. 10, 741 (1983)]. Later, Schulz et al., [Med. Phys. 13, 755 (1986)] published a letter of clarification that proposed a revised equation for Ngas in which the effect of atmospheric humidity was specifically dealt with. The present report points out errors in both of those presentations of the Ngas equation, and derives formulations for cases where the standardization laboratory does or does not correct for ambient humidity during chamber calibration. In the case where humidity is corrected for, the resulting quantity is more properly called “Nair.” The Ngas equation is stated in terms of the air kerma calibration factor NK as well as the more familiar exposure calibration factor NX.
Arterial flow characterization with a photodiode array based imaging system
Authors: Cunningham I. A. , Yamada S. , Hobbs B. B. , Fenster A.
Published: March 2003

An x‐ray imaging system is described that can be used for obtaining arterial blood flow information. The system consists of a linear photodiode array image detector, simple optical and mechanical components, and a data acquisition microcomputer that connect to a conventional x‐ray image intensifier based fluorography system. Flow information is obtained by detecting the movement of a small, locally injected bolus of radio‐opaque contrast agent. This is done by determining the bolus mass, integrated over the cross‐sectional area, at each of 1024 positions along the length of the artery with a sampling rate of up to 200 samples per s. It is shown in a phantom study that the peak flow velocity can be measured with an accuracy of ±5% by detecting the bolus arrival times at each of the 1024 positions. The mean velocity is obtained with similar accuracy using a cross‐correlation technique and a modified form of the Stewart–Hamilton principle. In addition, it is shown that the separation and reattachment points resulting from flow separation near a stenosis can be determined from the bolus clearance times. The locations of these points are consistent with theoretical values for the cosine shaped symmetric 89% stenosis used in this study.
Image feature analysis and computer‐aided diagnosis in digital radiography: Classification of normal and abnormal lungs with interstitial disease in chest images
Authors: Katsuragawa Shigehiko , Doi Kunio , MacMahon Heber
Published: March 2003

In order to detect and characterize interstitial disease in the lungs, we are developing an automated method for the determination of physical texture measures, which assess the magnitude and coarseness (or fineness) of lung texture in digital chest radiographs. This method is based on an analysis of the power spectrum of lung texture. We now describe an automated classification method for distinction between normal and abnormal lungs with interstitial disease, in which we employ these texture measures and their data base. This computerized method includes three independent tests, one for a definitely abnormal focal pattern, one for a relatively localized abnormal pattern, and one for a diffuse abnormal pattern. The performance of this computerized classification scheme is compared with that of radiologists by means of receiver operating characteristic (ROC) analysis. Our results indicate that this computerized method can be a valuable aid to radiologists in their assessment of interstitial infiltrates.
A fast low‐noise line scan x‐ray detector
Authors: Glüer Claus‐C. , Dix Wolf‐R. , Engelke Klaus , Graeff Walter , Kupper Wolfram , Stellmaschek Karl‐H.
Published: March 2003

A fast, low‐noise line scan detector (NIKOS) for digital radiography has been developed. It consists of an input x‐ray phosphor screen that is coupled to a modified Reticon photodiode array by means of fiber optics with incorporated image intensifier. In its current version the detector can be operated with a maximum 500 Hz image acquisition rate for interlaced readout of two lines of 128 pixels each. Using a Gd2O2S:Tb x‐ray input phosphor, an afterglow of 25% in the first subsequent readout was observed. We also conducted afterglow measurements on several other powder and single‐crystal phosphors and the photodiode array. Using CdWO4, the afterglow of the detector is limited by the lag of the photodiode array of 4.5%. By modifying the readout electronics the noise of the photodiode array was reduced to below 1 Graylevel, corresponding to a signal‐to‐noise ratio of 5200. The detective quantum efficiency (DQE) of the detector ranged from 0.18 to 0.4 for typical signal levels. The sensitivity was 10% saturation per 1.9 mR entrance dose. The modular design of the NIKOS detector allows for individual selection of each component to optimize performance for a given application.
Effect of intervertebral changes of the spinal trabecular and cortical mineral content on the precision requirements in longitudinal single and dual energy computed tomography examinations
Authors: Sandor Tamas , Weissman Barbara , Brown Edward
Published: March 2003

Statistical considerations related to the measuring and analyzing procedures in computed tomography (CT) scans indicate that determination of a 2.5% change in spinal mineral content (SMC) which is assumed necessary in clinical practice, requires a 0.9% combined precision in setting the location and direction of the scan plane through the vertebral body, and defining the region of interest. In single energy examinations the short‐term precision of CT scanners was found to be sufficiently high to achieve this goal, but the variability due to subjective finding of scan direction requires the automation of slice selection and additional averaging of data from more than one scan slice. The precision of calcium images was much lower, and one conclusion of this study is that dual energy technology is not suitable for longitudinal examinations, where small changes in SMC have to be determined. The SMC of the vertebrae T12, Ll, L2, and L3 as measured in 41 subjects was compared and a significant decrease in trabecular bone in single energy examinations and increase in cortical bone was found from T12 to L3. This suggests that the comparison of SMC data should be done over time for each vertebra separately, in order to preserve diagnostic information.
Radial dose distribution of 192Ir and 137Cs seed sources
Authors: Thomason C. , Higgins P.
Published: March 2003

The radial dose distributions in water around 192Ir seed sources with both platinum and stainless steel encapsulation have been measured using LiF thermoluminescent dosimeters (TLD) for distances of 1 to 12 cm along the perpendicular bisector of the source to determine the effect of source encapsulation. Similar measurements also have been made around a 137Cs seed source of comparable dimensions. The data were fit to a third order polynomial to obtain an empirical equation for the radial dose factor which then can be used in dosimetry. The coefficients of this equation for each of the three sources are given. The radial dose factor of the stainless steel encapsulated 192Ir and that of the platinum encapsulated 192Ir agree to within 2%. The radial dose distributions measured here for 192Ir with either type of encapsulation and for 137Cs are indistinguishable from those of other authors when considering uncertainties involved. For clinical dosimetry based on isotropic point or line source models, any of these equations may be used without significantly affecting accuracy.
Preliminary experimental investigation of invivo magnetic manipulation: Results and potential application in hyperthermia
Authors: Grady M. S. , Howard M. A , Molloy J. A. , Ritter R. C. , Quate E. G. , Gillies G. T.
Published: March 2003

The first in vivo experiments in support of a new technique for delivering stereotaxic hyperthermia have been conducted at the Experimental Surgery Facility of the University of Virginia's Medical Center. We call this technique the “Video Tumor Fighter.” In each of twelve trials a single, small permanent magnet or train of small permanent magnets was implanted on the brain surface of adult canine models. In three of the trials, this “seed” (typically 6‐mm diameter ×6‐mm long) was moved by magnetic manipulation to different locations within the brain. In two other trials, the seed moved along the interface between the brain and the inner vault of the skull. The noncontact magnetic manipulation was accomplished by coupling the permanently magnetized seed to the large dc magnetic field gradient created by a water‐cooled coil surrounding the animal's head. The seed's motions were monitored with x‐ray fluoroscopy; its rate of movement was found to be ∼0.8 mm s−1. The forces required to produce these motions were on the order of 0.07 N. We document here the instrumentation used in these trials, describe the experimental procedures employed, and discuss the technical aspects of the results.
Output calibration in solid water for high energy photon beams
Authors: Reft Chester S.
Published: March 2003

The AAPM Protocol recommends the use of water, polystyrene or acrylic media for measuring the output of high energy photon beams. It provides the appropriate restricted mass stopping powers and mass energy absorption coefficients for converting the dose to these media to dose to water. A water‐equivalent solid has been developed for dosimetric applications [C. Constantinou, F. Attix, and B. Paliwal, Med. Phys. 9, 436 (1982)]. Calculated values for the restricted mass stopping powers and mass energy absorption coefficients have been published for this material. [A. Ho and B. Paliwal, Med. Phys. 13, 403 (1986)]. The accuracy of these calculations was investigated by making output measurements, following the Protocol, with a Farmer type chamber in four materials for Co‐60, 4, 6, 10, 18, and 24 MV photon beams. The results show that the scaled dose to water for the different media agree to better than 1%, and the analysis supports the methodology of the Protocol for obtaining the dose to water from the different media.
Constrained reconstruction: A superresolution, optimal signal‐to‐noise alternative to the Fourier transform in magnetic resonance imaging
Authors: Haacke E. Mark , Liang Zhi‐Pei , Izen Steven H.
Published: March 2003

Many problems in physics involve imaging objects with high spatial frequency content in a limited amount of time. The limitation of available experimental data leads to the infamous problem of diffraction limited data which manifests itself by causing ringing in the image. This ringing is due to the interference phenomena in optics and is known as the Gibbs phenomenon in engineering. Present tehniques to cope with this problem include filtering and regularization schemes based on minimum norm or maximum entropy constraints. In this paper, a new technique based on object modeling and estimation is developed to achieve superresolution reconstruction from partial Fourier transform data. The nonlinear parameters of the object model are obtained using the singular value decomposition (SVD)‐based all‐pole model framework, and the linear parameters are determined using a standard least squares estimation method. This technique is capable, in principle, of unlimited resolution and is robust with respect to Gaussian white noise perturbation to the measured data and with respect to systematic modeling errors. Reconstruction results from simulated data and real magnetic resonance data are presented to illustrate the performance of the proposed method.
Comments on: “Absorbed dose comparison among commercial ionization chambers in polystyrene and acrylic phantoms”
Authors: Spokas John J.
Published: March 2003

In the measurements performed by Kubo and Cheng (Ref. 1) of absorbed dose in phantoms using several commercially available ionization chambers, the 0.5 cc air‐equivalent Exradin chamber is reported as having a sub‐standard performance. It is contended that the Exradin performed as well as others. (AIP)
Effects of axial spatial resolution and sampling on object detectability and contrast for multiplanar positron emission tomography
Authors: Kearfott K. J. , Kluksdahl E. M.
Published: March 2003

A multiplanar positron emission tomography (PET) system is simulated using Gaussian curves to model the axial point spread functions (PSFs) of the planes to study the effects of resolution and sampling. Poor spatial resolution or insufficient sampling may cause deleterious data losses or artifacts in the reconstructed image. For a multiplanar PET system with an axial full width at half‐maximum (FWHM) of 6 mm and a 12 to 13 mm ring separation, a ripple in sensitivity of 9% is observed. A 1 mm object placed at the central direct plane results in detection of 59% of the signal in that plane. The theoretical observed contrast of a 3 mm object positioned at the center plane is 25% of the true contrast and decreases to 24% when the object is positioned between the central direct and cross planes. A PET system with an axial FWHM of 12 mm and a ring separation of 5–6 mm has a uniform sensitivity. A 1 mm object placed at the central direct plane detects 14% of the object signal in that plane. The theoretical observed contrast for a 3 mm object is 13% of true contrast when the object is positioned between the central direct and cross planes. It should be noted that all dimensions refer to the z direction through the center of the gantry in the simulated multiplanar system. The uniform sensitivity due to wider axial FWHMs decreases the amount of data loss for inter‐ring gaps; however, the blurring associated with wider FWHMs decreases observed contrast. This tradeoff between narrow or wide FWHMs and small or large inter‐ring gaps must be considered carefully in the design of a multiplanar PET system.
Image formats: Five years after the AAPM standard for digital image interchange
Authors: Maguire G. Q. , Noz M. E.
Published: March 2003

The publication of AAPM Report No. 10 was the first attempt to standardize image formats in the medical imaging community. Since then, three other groups have formed (CART—the Scandinavian collaboration for Computer Assisted Radiation Therapy treatment planning; ACR–NEMA, a collaboration whose purpose is to formulate a standard digital interface to medical imaging equipment; and COST B2 Nuclear Medicine Project a European collaboration whose purpose is to define a format for digital image exchange in Nuclear Medicine). The AAPM format uses key‐value pairs in plain text to keep track of all information associated with a particular image. The radiation oncology community in the U.S. has been defining key‐value pairs for use with CT, nuclear medicine and magnetic resonance (MR) images. The COST B2 Nuclear Medicine Project has also adopted this format and together with the Australian/New Zealand Society of Nuclear Medicine Technical Standards Sub‐Committee which has also adopted this format, has defined an initial set of key‐value pairs for Nuclear Medicine images. Additionally, both ACR–NEMA and CART have been defining fields for use with the same types of images. The CART collaboration has introduced a database which is available electronically, but is maintained by a group of individuals. ACR–NEMA operates through committee meetings. The COST B2 Nuclear Medicine Project operates through electronic (and postal where necessary) mail. To insure a consistent set of field names in such a rapidly developing arena requires the use of a server rather than a committee. Via a server a person would inquire if a particular field had been defined. If so, the defined name would be returned. If not, the person would be given the opportunity to define the field. The next inquiry would return the previously defined field. As new modalities are added to the imaging repetoire, it would be easier and faster to ensure the consistency and adequacy of the database; e.g., in the present version of its standard, the ACR–NEMA fields are adequate for CT but there are very few fields suitable for describing the parameters associated with nuclear medicine and MR images.
Addendum: Output calibration in solid water for high energy photon beams [Med. Phys. 16, 299 (1989)]
Authors: Reft Chester S.
Published: March 2003

The identification of water‐equivalent material used in the title publication is given to be model No. 451 manufactured by Radiation Materials, Inc. (RMI). New data is presented for solid water model No. 457.(AIP)
Characterization of transmitted motion in fetal lung: Quantitative analysis
Authors: Adler Ronald , Rubin Jonathan M. , Bland Peyton , Carson Paul
Published: March 2003

A simple two‐dimensional analytic model is evaluated for transmitted cardiac motion in fetal lung. The model treats the latter as being an incompressible viscoelastic medium. The mean radial deformation in an elastic medium is demonstrated to depend on a length parameter , where μ, ρ, and ω corespond to elastic shear modulus, mass density, and frequency of cardiac motion, respectively. Digitized M‐mode images are demonstrated as a feasible method to measure such deformations in vivo. Data for two patients are presented to illustrate the technique.
On the measurement of half‐value layer in film–screen mammography
Authors: Wagner Louis K. , Archer Benjamin R. , Cerra Frank
Published: March 2003

Sources of variations in the measurement of half‐value layer (HVL) in film–screen mammography are evaluated. Two principal sources are the energy responses of ionization chambers and the elemental compositions of the 1100 aluminum‐alloy attenuation materials. Methods in measuring the thickness of attenuation materials are also an important potential source of error. These factors as well as the influence of geometry of measurement, position of measurement in the field, and precision of radiation output are examined. Recommendations for consistency in measurement are provided.
Effects of nonmineral tissues on measurement of bone mineral content by dual‐photon absorptiometry
Authors: Sorenson James A.
Published: March 2003

Computer simulations were performed to investigate the effects of nonmineral tissue (overlying soft tissue and nonmineral components of bone) on the measurement of bone mineral content by dual‐photon absorptiometry (DPA). Selected experimental studies were performed to test the predictions of the simulations. The results indicate that variations in the composition of nonmineral tissue can significantly affect measurement precision and accuracy at the levels achievable with modern x‐ray based absorptiometry systems (0.5%). Thus biological variables may limit further improvements in measurement precision and accuracy. Biological variables also can affect intercalibration of systems that use different radiation sources and/or calibration techniques.
Some possible effects of a catheter on the arterial wall
Authors: Karahalios George T.
Published: March 2003

When a catheter is inserted through an artery, the wall shear stress increases and the secondary flow exhibits an increased number of instantaneous vortices. These effects become more evident when the annular gap between the catheter and the artery becomes more narrow. Accordingly it is suggested that these factors may affect the preferential distribution of the atheromatous plaques at the sites of the wall with an increased permeability.
Improving precision and safety in the use of beam modifying devices in radiation therapy
Authors: Miller R. W. , van de Geijn J. , Fraass B. A.
Published: March 2003

Reliable and safe implementation of beam modifying devices such as wedges and block trays requires careful design and construction. Inappropriate design may pose problems ranging from user‐hostile operation to hard‐to‐track, but significant variations in actual position in a beam. This may cause variation in actual wedge output factors, or variation in the position of a block tray. In case of simple mechanical failure or personnel mistake, design related mechanical conditions may result in injury to either a patient or a staff member. This paper is based on experience with linear accelerators from one manufacturer, but similar conditions are likely to exist with other radiation machines. A simple technical modification is offered which improves both accuracy and reproducibility in the placement of wedge‐type filters. For our machines the solution also provides improved safety in the use of both wedge trays and block trays.
Improved quantification of radionuclide uptake using deconvolution and windowed subtraction techniques for scatter compensation in single photon emission computed tomography
Authors: Yanch J. C. , Flower M. A. , Webb S.
Published: March 2003

A comparison of two methods of scatter compensation in single photon emission computed tomography (SPECT) imaging is made on the basis of improvements in quantification. The methods, scatter‐window subtraction and constrained deconvolution of an average point source response function (PSRF), are described; the theoretical basis of each method is also briefly assessed. Improvements in relative quantification offered by each method are measured by examining ratios of counts in hot cylinders to counts in a slightly radioactive background. The cylinder/background concentration ratio was varied by a factor of five; the sizes of the cylinders remained constant. Keeping the diameter of the cylinders constant allowed for an assessment of the effect of concentration differences, without the conflicting effect of variation in the size of the hot source. Results showed that while both scatter‐window subtraction and constrained deconvolution offer quantification improvements in the final image, the method of prereconstruction deconvolution of a planar PSRF from each planar projection is substantially more successful than either scatter‐window subtraction or other methods of implementing the deconvolution procedure.
Levels of leakage radiation from electron collimators of a linear accelerator
Authors: Das K. R. , Cramb J. A. , Millar R. M. , Kenny M. B. , Patterson W. R. , Ackerly T. L. , Perkins A. E. , Chongkitivitya K.
Published: March 2003

The leakage radiation from electron applicators used with our linear accelerator has been measured. For the applicators 6×6 to 25×25 cm size, the leakage was measured in the plane of the patient and on the sides of the applicators with the available electron energies of 6,9,12,15 and 18 MeV. The levels were significant. The highest leakage on the side was for the combination of 6×6‐cm applicator and 9‐MeV electrons (32%) and in the plane of the patient for 25×25‐cm applicator with 18 MeV (10%) relative to the peak dose. Adding lead 1–2 mm, at appropriate locations inside the applicators has reduced the leakages to acceptable levels without affecting the beam parameters.
Measurement of the presampling modulation transfer function of film digitizers using a curve fitting technique
Authors: Yin Fang‐Fang , Giger Maryellen L. , Doi Kunio
Published: March 2003

A curve fitting technique combined with an angulated slit image has been developed for the measurement of the presampling modulation transfer function (MTF) of film digitizers. The noisy line spread functions (LSFs) acquired from an angulated slit image are fitted using a combination of two functions by means of a nonlinear least‐square fitting technique. The parameters in the model function for each LSF are obtained by minimizing the residual root mean square (RMS), and then averaged over all the LSF fittings. The resulting analytical function is representative of the continuous presampled LSF. We have found that a combination of Gaussian and exponential functions provides a good fit to the LSFs obtained with film digitizers. The corresponding analytical Fourier transformation of the model function yields the presampling MTF, without Nyquist frequency limitation. Measurements of spatial resolution properties using this method were performed for two laser scanners and an optical drum scanner.
Logistic representation of the sensitometric response of screen–film systems: Empirical validation
Authors: Willis Charles E. , Bencomo José A.
Published: March 2003

An empirical logistic model that linearizes the sensitometric response data of screen–film systems over the entire dynamic range of exposures is presented. This linearization is evident when the net optical densities are scaled as fractions of the net saturation density and plotted on commercial logit, probit, or double‐log paper, except under conditions of reciprocity law failure. Weighted linear regression analysis shows that the intercept, but not the slope, depends on the screen–film system used. Previous work indicates that the slope is also independent of development time and photon energy. The model is verified through an analysis of tabulated sensitometric data published by the Center for Devices and Radiological Health.
Digital TV tomography: Description and physical assessment
Authors: Takahashi Mutsumasa , Yoshioka Senya , Bussaka Hiromasa , Higashida Yoshiharu , Kamiya Masami , Tsuneoka Masayuki
Published: March 2003

A digital TV tomography system, capable of retrospective reconstruction of multiple digital tomographic images, has been developed and its basic physical characteristics have been evaluated. The multiple tomographic images were formed through restrospective reconstruction of digital data acquired on a linear tomographic x‐ray unit and an image intensifier–television system. Digital data were obtained with a series of 30 pulsed exposures during linear motion of the system. A distortion correction algorithm for the convex surface of the image intensifier was developed in order to reduce image distortion. A phantom study showed that the square‐wave response at the fulcrum plane was slightly inferior to that in conventional tomography. There was also a slight decrease in the square‐wave response away from the fulcrum plane and upon application of a correction algorithm, as compared with the response of the original reconstructed image at the fulcrum plane. The exposure dose for a single image was approximately half that in conventional tomography. Because of many advantages, including low exposure, short examination time, digital image manipulation, and applicability to picture archiving and communication systems, this is likely to become an important method in radiology when further technical refinements have been made.
Harmonic structure of cylindrical imaging coils
Authors: Harpen Michael D.
Published: March 2003

Presented is a derivation of an exact closed‐form expression for the spectra of resonant frequencies for magnetic resonance imaging birdcage coils in the limit of long coils with many elements. Results of the analysis are compared with observation in a variety of birdcage coils where it is shown that for even relatively short coils the progression of resonant frequencies are of the form ωm=ω1m±1/2+low pass; −high pass, with m integer and ω1 the primary resonance.
Problems associated with simulated light sensitometry for low‐crossover medical x‐ray films
Authors: Haus Arthur G. , Dickerson Robert E.
Published: March 2003

Over the past ten years the evolution of medical x‐ray films has been toward films with reduced intensifying‐screen light crossover in order to reduce blur and obtain higher spatial resolution. For films with very low crossover, misleading and incorrect sensitometric data may be obtained for film contrast evaluation and processor control if a simulated light sensitometer with a single‐sided, light‐exposing device is used. Screen light exposures were made using an inverse square, intensity‐scale sensitometer. Simulated light exposures were made using a widely used single‐sided, simulated‐light sensitometer commonly used for film processor quality control, and a new simulated‐light sensitometer capable of producing either single‐ or double‐sided sensitometric exposures. The films used included one single‐emulsion film and three double‐emulsion medical x‐ray films with light‐crossover values ranging from ∼3% to 30%. Sensitometric data showed a significant distortion (bump) in the characteristic curve for the 3% light‐crossover film exposed with the single‐sided, simulated‐light sensitometer.
Small‐field electron dosimetry for the Philips SL25 linear accelerator
Authors: Rashid H. , Islam M. K. , Gaballa H. , Rosenow U. F. , Ting J. Y.
Published: March 2003

Electron‐beam characteristics of a Philips SL25 linear accelerator have been studied. Central‐axis percentage depth doses, cross‐beam profiles and beam output factors of 6‐, 10‐, and 20‐MeV beams, selected from the available energy range of 4 to 22 MeV, are reported in this paper. The main thrust of this work is to determine the systematic variation of beam characteristics, especially the output factor, with standard cone sizes and cerrobend beam‐shaping cutouts down to a field size of 2×2 cm Output factors for the standard cones (open field) are energy dependent in a complex manner, increasing with the cone size for the 6‐MeV beam whereas decreasing for 10‐ and 20‐MeV beams. The output factor falls below unity at lower energies (6 and 10 MeV) for fields with at least one side smaller than 6 cm, and stays nearly constant for the 20‐MeV beam. Measured output factors of small fields are least squares fitted by a second‐order polynomial function. Output factors for small rectangular fields have been derived from the one‐dimensional and square‐root formulas, and the equivalent‐square method. Only the one‐dimensional formula predicts the measured output factors of highly elongated fields to within ±1% experimental uncertainties. Different cones with the same size electron cutout show a varied dose response, primarily due to variation in scattered electron contamination from the cones.
Therapy imaging: A signal‐to‐noise analysis of a fluoroscopic imaging system for radiotherapy localization
Authors: Munro P. , Rawlinson J. A. , Fenster A.
Published: March 2003

We have been developing a digital fluoroscopic imaging system to replace the portal films that are currently used to verify patient positioning during radiotherapy treatments. Our system differs from previously reported devices in the construction of the detector and in the operation of the TV camera. The signal, noise, and signal‐to‐noise properties of this system have been determined by measuring the modulation transfer function [MTF(f)], the noise power spectra [NPS(f)], and by calculating the detective quantum efficiency [DQE(f)] of the system. The results show: (i) that the spatial resolution of the system is determined largely by the lens of the TV camera and by frame grabber; and (ii) that the noise in the system is dominated by the secondary light quanta, due to the poor light collection efficiency of the optical chain. Despite these physical limitations, a contrast‐detail study shows that the fluoroscopic system is better at detecting large, low contrast objects than portal films. Therefore the system is already a reasonable alternative to portal films and modifications to the metal plate/phosphor detector, lens, TV camera, and frame grabber should improve the performance of the system further.
Doses on the central axes of narrow 6‐MV x‐ray beams
Authors: Bjärngard B. E. , Tsai J.‐S. , Rice R. K.
Published: March 2003

The absorbed doses on the central axes of narrow beams (radii 0.07–2.5 cm) of 6‐MV x rays have been studied by experiments and Monte Carlo simulations. The measurements were made in a geometry used for irradiation of intracranial lesions. For radii <1.0 cm the dose on the central axis is progressively reduced due to electron disequilibrium. This leads to measurement artifacts when the detector is too large, as was readily observed with ionization chambers. Radiographic and radiochromic films were used with densitometric evaluation to provide the resolution necessary to measure absorbed doses for the narrowest beams. The contribution by phantom‐scattered photons is significant even at small field sizes, and scatter factors were determined from the experimental results. Photons scattered by the auxiliary collimator did not add appreciably to the dose on the central axis. The data were used to characterize the dose‐to‐kerma ratio as a function of beam radius. Differences between experimental results and those from Monte Carlo calculations were observed.
Dose distribution of 125I sources in different tissues
Authors: Huang David Y. C. , Schell Michael C. , Weaver Keith A. , Ling Clifton C.
Published: March 2003

The low‐energy photons of 125I deposit energy in tissues primarily by the photoelectric effect, which is strongly dependent on the atomic number Z. Thus dose distributions of 125I sources in media of different Z are not the same. LiF and CaF2 thermoluminescent dosimeters were used for relative dose distribution measurements in materials that are equivalent to muscle, breast, and bone. The experimental data are in good agreement with published results for muscle, and in reasonable agreement with Monte Carlo calculations for all the tissues tested. These measured data provide the basis for dose distribution calculations when 125I seeds are implanted in or near inhomogeneous tissues.
Hyperthermia system combined with a magnetic resonance imaging unit
Authors: Delannoy J. , LeBihan D. , Hoult D. I. , Levin R. L.
Published: March 2003

Magnetic resonance imaging (MRI) has recently been proposed as a method to monitor, noninvasively, temperature, blood flow, and cell metabolism during oncologic hyperthermia (HT). To heat and “image” simultaneously, it is necessary to combine a HT device and a MRI unit. As a demonstrative example of the problems associated with implementing such a system, a mini‐annular phased array hyperthermia applicator was combined with a 0.5‐T whole body MRI unit. With the aid of filters, baluns, and switches, the HT applicator and the MRI unit were made compatible. The overall system was tested using a muscle‐equivalent, cylindrically shaped polyacrylamide gel phantom. No interference between the HT device and the MRI unit was observed. Noninvasive temperature images, with a resolution better than 1 °C/cm, were obtained from images of molecular diffusion recorded before and during heating.
Computerized detection of pulmonary nodules in digital chest images: Use of morphological filters in reducing false‐positive detections
Authors: Giger Maryellen Lissak , Ahn Nicholas , Doi Kunio , MacMahon Heber , Metz Charles E.
Published: March 2003

Currently, radiologists can fail to detect lung nodules in up to 30% of actually positive cases. If a computerized scheme could alert the radiologist to locations of suspected nodules, then potentially the number of missed nodules could be reduced. We are developing such a computerized scheme that involves a difference‐image approach and various feature‐extraction techniques. In this paper, we describe our use of digital morphological processing in the reduction of computer‐identified false‐positive detections. A feature‐extraction technique, which includes the sequential application of nonlinear filters of erosion and dilation, is employed to reduce the camouflaging effect of ribs and vessels on nodule detection. This additional feature‐extraction technique reduced the true‐positive rate of the computerized scheme by 13% and the false‐positive rate by 50%. In a comparison of the scheme with and without the additional feature‐extraction technique, inclusion of the additional technique increased the detection sensitivity by about half at the level of three to four false‐positive detections per chest image.
Collimator scatter in modeling radiation beam profiles
Authors: Rosen Isaac I. , Loyd Michael D. , Lane Richard G.
Published: March 2003

In computer dose calculations using scatter–air ratio sector summation algorithms, the primary dose from the target to points away from the central axis of a beam is computed using an exponential intensity model of the source and a transmission parameter for the collimator. This model works well inside the beam and near edges but is inaccurate outside the beam at distances of more than 1–2 cm from beam edges. We have modified the standard beam profile model to include a dose contribution representing photon radiation scattered from the collimators. Collimator edges are treated mathematically as line sources and an adjustable parameter is introduced which represents the activity per unit length of the collimator edges. Dose from the collimator edges is assumed to decrease purely geometrically as the inverse of the square of the distance and no modification is made for tissue attenuation. With these assumptions, the total collimator scatter dose to a point is most accurately computed by a line integral over the edges of the beam outline. This modification fits naturally into the standard scatter–air ratio sector summation computer algorithm but adds significantly to dose computation time. Some approximations eliminate the line integration and lead to a collimator scatter term which is proportional to field perimeter and independent of off‐axis distance. The modified dose model was tested by comparing measured dose profiles with computed ones using x‐ray beams from Philips (6 and 15 MV) and Varian (4 and 6 MV) accelerators. There was significant improvement in fit compared to the standard beam model for points outside the radiation beam.
A patient‐equivalent attenuation phantom for estimating patient exposures from automatic exposure controlled x‐ray examinations of the abdomen and lumbo–sacral spine
Authors: Conway B. J. , Duff J. E. , Fewell T. R. , Jennings R. J. , Rothenberg L. N. , Fleischman R. C.
Published: March 2003

The Joint Commission on Accreditation of Healthcare Organizations requires diagnostic radiology facilities to know the approximate amount of radiation received by an average patient during radiographic examinations at the facility. Automatic exposure controlled (AEC) techniques are used for many of these exams, and a standard patient‐equivalent phantom is necessary when estimating patient exposure on such systems. This is of particular importance if exposures are to be compared among AEC systems with different entrance x‐ray spectra. We have developed a phantom, LucAl Abdomen, to facilitate determining the average patient exposure from AEC anteroposterior (AP) abdomen and lumbo–sacral (LS) spine radiography. The phantom is relatively lightweight, transportable, sturdy, and made of readily available inexpensive materials (Lucite and aluminum). It accurately simulates the primary and scatter transmission through the soft tissue and L‐4 spinal regions of a patient‐equivalent anthropomorphic phantom for x‐ray spectra typically used in abdomen/LS spine radiography. A clinical evaluation to verify the patient‐equivalence of three commercial anthropomorphic phantoms (Humanoid, Rando, 3‐M) and two acrylic/aluminum phantoms (ANSI and LucAl Abdomen) has been conducted. The design and development of the LucAl Abdomen phantom and the evaluation of all phantoms is described.
Tissue compensators with use of vinyl lead sheets for head and neck portals on 4‐MV x rays
Authors: Jani Shirish K. , Pennington Edward C.
Published: March 2003

Tissue compensators made with a high atomic number material are compact and retain the skin‐sparing effect of megavoltage beams. In this note, we have described the use of a commercially available vinyl lead as a material for tissue compensators for use with 4‐MV beams on a Clinac‐4/80. The dosimetric data obtained included the sheet's lead equivalence, thickness ratios, surface dose and buildup region, and transmission factors. The presence of a compensator did not alter the beam's skin‐sparing effects. It was concluded that the vinyl lead sheets allowed an easy and rapid fabrication of a tissue compensator for head and neck portals of up to 15×15 cm.
Determination of the neutron and photon spectra of a clinical fast neutron beam
Authors: Moyers M. F. , Horton J. L.
Published: March 2003

A simple technique to determine the neutron and photon spectra of a clinical fast neutron beam is described. This technique involves making narrow beam attenuation measurements with a pair of ionization chambers and an iterative fitting program to analyze the data. A method is also described for determining the first‐guess neutron spectrum for input into the iterative program. The results of the analysis yield spectra suitable for use in dose calculation algorithms and dosimetry protocols. Presented here is the first‐known published photon spectrum from a clinical machine.