bluuebunny/crossref_metadata_2025 huggingface dataset:
A general model of microcirculatory blood flow effects in gradient sensitized MRI
Authors: Kennan Richard P. , Gao Jia‐Hong , Zhong Jianhui , Gore John C.
Published: March 2003

A general expression is derived for the NMR signal from a fluid undergoing random directional flow such as encountered within the microcirculation. The dependence of the echo amplitude on flow velocity, sample morphology, and experimental parameters are described in terms of a temporal velocity autocorrelation function. The width of the correlation function determines whether the flow can properly be described as diffusive. Comparison is made between the velocity autocorrelation method outlined here and the IVIM model for tissue perfusion. Conditions for the validity of the latter approach for extracting physiologic information from apparent diffusion measurements are discussed. The approach outlined leads to a more robust measure of microcirculatory blood velocity from NMR measurements.
Comparison of the low‐contrast detectability of a screen‐film system and third generation computed radiography
Authors: Cook L. T. , Insana M. F. , McFadden M. A. , Hall T. J. , Cox G. G.
Published: March 2003

Contrast‐detail (CD) analysis was used to compare the low‐contrast detectability of computed radiography (CR) and screen‐film (SF) as applied to the task of adult chest radiography. A phantom was constructed and imaged using the same exposure factors throughout all experiments. Within‐observer variance, between‐observer variance, and image sample variance were calculated and used to estimate the standard error for each experiment. The results of these CD experiments agreed with the predictions of the Rose model. Observers performed equally well for low‐contrast target detection using CR and SF.
Implementation of a modified birdcage resonator for 19F/1H MRI at low fields (0.14 T)
Authors: Samaratunga R. C. , Pratt R. G. , Zhu Yong , Massoth R. J. , Thomas S. R.
Published: March 2003

Fluorine‐19 nuclear magnetic resonance of perfluorinated blood substitute materials provides a method for determination of oxygen tension (pO2) in vivo. Use of a double resonant 19F/1H radio frequency coil allows convenient correlation between the high resolution anatomic presentation of proton images and the fluorine distribution. However, quantitative 19F measurements require an RF coil with good H1 field homogeneity over the image volume and a high quality factor (Q) to minimize errors caused by the low signal‐to‐noise levels available in in vivo imaging and image nonuniformities introduced by the large chemical shift of fluorocarbons. The birdcage coil design provides a high Q structure with optimum H1 field uniformity and fill factor. However, at low resonance frequencies, the inherently low inductance of the birdcage geometry requires the use of a large number of chip capacitors giving rise to unwieldy coil fabrication and increased cost. This communication describes a modification to the birdcage design that reduces the chip capacitor requirement by at least a factor of 4 for a given dimension, yet retains the essential characteristics of the birdcage design. The modified structure was tuned for double resonance at 5.7/6.0 MHz for 19F/1H magnetic resonance imaging at 0.14 T. For a coil with a length to diameter ratio of 1.67, an H1 uniformity of ±2% for the 19F resonance was obtained over a cylindrical region with radius ∼0.6r (r=radius of coil) and length ∼1.8r within the coil.
Input/output characteristics of a matrix ion‐chamber electronic portal imaging device
Authors: Yin Fang‐Fang , Schell M. C. , Rubin Philip
Published: March 2003

The input/output characteristics of a matrix liquid ion‐chamber electronic portal imaging device (EPID) are investigated to elucidate the imaging properties of EPIDs. The radiation input to the detector, represented by dose rate, and the pixel value output from the device are related by a characteristic curve. Various incident radiation intensities are obtained by changing the source‐to‐detector distance (SDD). For each incident radiation intensity, an electronic portal image is obtained using a field size of 5×5 cm2. The output pixel value of the EPID is represented by the average pixel value of a region of interest of 9×9 pixels centered at a selected point. The effects of various accelerator settings, such as the repetition‐rate setting and photon energy, gantry angle, field size, SDD, and acquisition mode of the EPID on characteristic curves are investigated at the central axis. The off‐axis response of the detector is also examined. The derivative of the pixel value with respect to the input dose rate is used to analyze the detector contrast. Results indicate that the output pixel value is not a linear function of the incident radiation intensity. The detector contrast is comparable between photon energies of 10 and 6 MV and increases at low dose rates. The response of the imaging device varies substantially with acquisition mode, but is less sensitive to the SDD used for calibration. Characteristic curves are consistent for different gantry angles at the central axis and with the off‐axis locations when the same gantry angle is used for imaging and calibration, but vary with off‐axis locations when the gantry angle is not at the calibration direction. Characteristic curves are also found to vary with different field sizes, but are similar in shape.
Optimization of spectral shape in digital mammography: Dependence on anode material, breast thickness, and lesion type
Authors: Fahrig Rebecca , Yaffe Martin J.
Published: March 2003

It has been proposed that breast cancer detection can be improved through the use of digital mammography. It is hypothesized that the choice of proper shape of the x‐ray spectrum incident upon the breast can yield an improved image signal‐to‐noise ratio (SNR) for a given dose. To test this hypothesis, an energy transport model incorporating measured breast tissue attenuation coefficients and published exposure‐to‐dose conversion values was developed to describe the image acquisition process. The choice of applied kilovoltage and filter for Mo and W target x‐ray sources has been optimized with respect to SNR and absorbed dose for detectors based on a Gd2O2S scintillating screen under the conditions of perfect coupling of light between the screen and a solid state photodetector. For the W spectra, the optimum filter–kVp combinations could provide 41%, 13%, and 42% improvements in SNR for 2‐cm, 6‐cm and 8‐cm breasts, respectively, over the conventional Mo filtration, for a practical imaging time of 1.0 s. W and Mo spectra produce similar SNR values for a given filter thickness except for the 4‐cm breast. Given the limitations of current technology, however, the W spectra produce the optimum SNRs in a shorter imaging time for breast thicknesses greater than and less than 4 cm. The maximum SNR for imaging both infiltrating ductal carcinoma and calcifications is provided by the same filter–kVp combination, allowing optimization based on breast thickness and composition only. The model can now be used to compare and improve upon novel detector designs.
A Monte Carlo study of accelerator head scatter
Authors: Chaney E. L. , Cullip T. J. , Gabriel T. A.
Published: March 2003

The production of off‐focus x rays in the head of a 6 MV linac has been investigated using the egs4 Monte Carlo code. The purpose of the study was to identify the sources of off‐focus radiation and the relative contribution for each source. Even though a particular energy and linac were modeled, the broad conclusions are expected to be general since the effects of head scatter are similar for most conventional head designs, regardless of manufacturer, energy, and model. The head components that were modeled include the exit window of the accelerating structure, target, beam stopper, flattening filter, monitor chamber, primary and secondary collimators, and air. Monoenergetic 6 MeV electrons were followed through the exit window, target, and beam stopper until all energy was expended. Primary‐ and higher‐order x rays produced throughout the head were followed until they were either absorbed or passed through a plane at the isocenter. Sites of off‐focus radiation were found to be diffusely distributed throughout the head, with the most intense sources being the primary collimator, flattening filter, and beam stopper. Data analysis shows that the collimator effect is determined primarily by the volume of the extended head‐scatter source that is exposed to the point of measurement through the collimating system. The results of this study provide a rationale for developing extended source models to calculate the collimator factor for fields defined by arbitrary collimation. An additional advantage is an improvement in the agreement between measured and calculated isodose distributions.
Quantitative bremsstrahlung imaging of yttrium‐90 using a Wiener filter
Authors: Shen Sui , DeNardo Gerald L. , DeNardo Sally J.
Published: March 2003

Yttrium‐90 (90Y), a pure beta emitter, is an attractive radionuclide for radioimmunotherapy of cancer. Therapeutic management requires quantitative imaging to measure the pharmacokinetics of the radionuclide in the patient for radiation dosimetric calculations. The bremsstrahlung emissions can be utilized to acquire an image of beta sources using a gamma camera. Quantitation of 90Y by bremsstrahlung imaging is difficult because of poor image quality that results from septal penetration and scatter secondary to the broad bremsstrahlung energies. In this work, quantitative methods for bremsstrahlung imaging of 90Y sources that involved the use of (a) a Wiener filter to deconvolve the septal penetration and scatter while suppressing image noise, and (b) the geometric mean of the conjugate view (GM) and effective point source (EPS) methods to quantify activities were investigated. An abdominal phantom was prepared with 90Y activities in the liver, spleen, tumors, and background volumes that were similar to those observed in patient studies. A twofold improvement in resolution recovery for full width at tenth maximum of the line spread function at 11 cm depth in water was achieved using Wiener restoration. Definition of the organ and tumor edges was greatly enhanced and cross talk between adjacent sources was suppressed after Wiener restoration. These improvements in image quality led to more accurate estimation of organ and tumor activities. Using the optimum attenuation correction method for GM and EPS quantitation of filtered bremsstrahlung images, estimates of individual activities (≤17% error) and cumulated activities (≤8% error) in all of the sources were accurate except for a tumor of 2 cm diameter. The results of this study provide the basis for a method to quantify beta source distribution and demonstrate the potential use of bremsstrahlung imaging in clinical settings.
Spatial variation of section sensitivity profile in spiral computed tomography
Authors: Wang Ge , Vannier Michael W.
Published: March 2003

The section sensitivity profile (SSP) is widely used to describe the longitudinal image resolution in computed tomography (CT). Due to asymmetry in spiral CT interpolation, the SSP depends on the transverse position of the associated longitudinal line. Spatial variation in the spiral CT SSP has not been investigated previously. To determine the SSP variation over the scan field, the SSP for the half‐scan interpolation method in terms of its transverse position was formulated. Based on the SSP formula, mathematical analysis and numerical simulation were performed. It was shown that the SSP associated with a transverse slice is antisymmetric with respect to the abscissa axis (which connects the slice center and the source location in the slice). Moment analysis was conducted on the SSP up to order three. The area under the SSP is equal to that under the longitudinal detector response function. As a result, with a normalized SSP the total mass of an object approximately remains after reconstruction. The mean of the SSP is equal to zero, although the SSP is asymmetric. The standard deviation and skewness of the SSP were numerically simulated and plotted for typical imaging parameters. It was demonstrated that for a 50° fan angle (which is extended by two extreme rays in a fan beam) the relative change in the slice thickness is no more than 10%, while the skewness of the SSP is within about 0.15. The smaller the fan angle, the less the SSP spatial variation. Therefore, in spiral CT the SSP along the longitudinal axis can be used as a representative of the SSP family.
Thick phosphor screens for on‐line portal imaging
Authors: Wowk B. , Shalev S.
Published: March 2003

Gd2O2S phosphor screens between 250 and 1000 mg/cm2 thick were evaluated for use in megavoltage imaging systems. The phosphor layers were placed on brass plates ranging from 1 to 5 mm thick, each with and without an optical back reflector (white paint). Light output and spatial resolution were measured at 6‐ and 23‐MV x‐ray energies. Light output was found to increase linearly with phosphor thickness up to 500 mg/cm2, reaching a plateau at 1000 mg/cm2. Spatial resolution [modulation transfer function (MTF)] decreased exponentially with phosphor thickness up to 750 mg/cm2, where a minimum was reached. The variation of MTF with phosphor thickness was found to obey a simple empirical relation.
Inclusion of energy straggling in a numerical method for electron dose calculation
Authors: Papiez L. , McLellan J. , Sandison G. A. , Sawchuk S. , Lu X. , Battista J. J.
Published: March 2003

Energy straggling along electron trajectories has been incorporated into a numerical algorithm for electron beam dose calculations. Landau's theory is used to predict, at any point in the absorber, the broadening of the primary electron energy spectrum due to energy loss straggling. Numerical calculations have been performed for electron beams with energies of 10–30 MeV incident upon water in order to determine the variation of dose with depth and variation of energy spectra with pathlength. These calculations are compared with the results of Monte Carlo simulations performed with the EGS4 code. The inclusion of energy loss straggling in the numerical calculations leads to predictions of energy spectra and dose deposition that are in good agreement with modified Monte Carlo simulations in which bremsstrahlung is ignored and the energy given to knock‐on electrons is deposited at the site of their creation. Less satisfactory agreement was achieved when these calculations were compared to full Monte Carlo simulations that included the bremsstrahlung events and transported the knock‐on electrons. It is concluded that bremsstrahlung energy loss must also be included into this algorithm, if an acceptable dose computation accuracy is to be achieved for clinical applications.
Multiple Coulomb scattering and spatial resolution in proton radiography
Authors: Schneider Uwe , Pedroni Eros
Published: March 2003

A simple formula for the spatial resolution of transmission proton radiography is derived for two different methods of measuring the proton coordinates. The effect of multiple Coulomb scattering and energy loss are taken into account. Experimental measurements of the spatial resolution have been done and are compared with the calculations. The technique of measuring entrance and exit coordinates in coincidence for each single proton improves the spatial resolution by a factor of 8 compared to a single coordinate measurement.
A miniature MOSFET radiation dosimeter probe
Authors: Gladstone D. J. , Lu X. Q. , Humm J. L. , Bowman H. F. , Chin L. M.
Published: March 2003

Prototype miniature dosimeter probes have been designed, built, and characterized employing a small, radiation sensitive metal oxide semiconductor field effect transistor (MOSFET) chip to measure, in vivo, the total accumulated dose and dose rate as a function of time after internal administration of long range beta particle radiolabeled antibodies and in external high energy photon and electron beams. The MOSFET detector is mounted on a long narrow alumina substrate to facilitate electrical connection. The MOSFET, alumina substrate, and lead wires are inserted into a 16 gauge flexineedle, which, in turn, may be inserted into tissue. The radiation dosimeter probe has overall dimensions of 1.6 mm diam and 3.5 cm length. The MOSFET probe signals are read, stored, and analyzed using an automated data collection and analysis system. Initially, we have characterized the probe's response to long range beta particle emission from 90Y sources in solution and to high energy photon and electron beams from linear accelerators. Since the prototype has a finite substrate thickness, the angular dependence has been studied using beta particle emission from a 90Sr source. Temperature dependence and signal drift have been characterized and may be corrected for. Measurements made in spherical volumes containing 90Y with diameters less than the maximum electron range, to simulate anticipated geometries in animal models, agree well with Berger point kernel and EGS4 Monte Carlo calculations. The results from the prototype probes lead to design requirements for detection of shorter range beta particles used in radioimmunotherapy and lower photon energies used in brachytherapy.
Spatial distribution of bremsstrahlung in a dual electron beam used in total skin electron treatments: Errors due to ionization chamber cable irradiation
Authors: Das Indra J. , Copeland John F. , Bushe Harry S.
Published: March 2003

The spatial distribution of the bremsstrahlung dose in total skin electron therapy and large electron fields has been assumed to be symmetric along the match line; however, the measured data in the entire plane have not been available. Das et al. [“Spatial distribution of bremsstrahlung in total skin electron irradiation,” Med. Phys. 19, 810 (1992)] reported the ion‐chamber measured bremsstrahlung data in the Stanford six‐field technique and found that the bremsstrahlung dose profile was 2.5 times greater in the caudal plane (beam facing toward floor) as compared to the cephalic plane. This observation was recently quoted by Klevenhagen [Physics and Dosimetry of Therapy Electron Beams (Medical Physics, Madison, WI, 1993)]. Further study using thermoluminescent dosimeters and careful measurements using ion chambers showed that the bremsstrahlung radiation was indeed symmetric at the match line as theoretically predicted and was not a function of the beam angle or the type of floor. The earlier observations of Das et al. were found to be due to cable effects. Ionization chamber cable response in a large electron field and the associated errors in the bremsstrahlung distribution are presented. It is suggested that a cable‐free dosimeter should be used for measuring low levels of bremsstrahlung radiation in large electron fields and at extended distances.
Computerized detection of abnormal asymmetry in digital chest radiographs
Authors: Armato Samuel G. , Giger Maryellen L. , MacMahon Heber
Published: March 2003

A technique for automated detection of abnormal asymmetry in digital chest radiographs is being developed. Such a method could be used to prescreen chest radiographs to bring obviously abnormal cases to the immediate attention of a radiologist. In addition, this technique may be used to detect large‐area abnormalities which may cause other, more lesion‐specific computer algorithms to fail. Asymmetric abnormalities are detected by multiple stages of gray‐level thresholding. Lung contours are determined, and after a centroid test is used to eliminate contours external to the lungs, the areas of remaining contours are calculated. The present scheme, applied to a database of 70 chest images, yielded a sensitivity of 91% and a specificity of 80%.
A new dual‐isotope convolution cross‐talk correction method: A TL‐201/TC‐99m SPECT cardiac phantom study
Authors: Knešaurek Karin
Published: March 2003

Simultaneous dual‐isotope SPECT imaging provides a clear advantage in situations where two concurrent metabolic, anatomic, or background measurements are desired. It obviates the need for two separate imaging sessions, reduces patient motion problems, and provides exact image registration between images. However, a potential limitation of dual‐isotope SPECT imaging is contribution of scattered and primary photons from one radionuclide into the second radionuclide's photopeak energy window, referred to here as cross‐talk. Cross‐talk in both photopeak energy windows can significantly degrade image quality, resolution, and quantitation to an unacceptable level. Simple cross‐talk correction method used in dual‐radionuclide in vitro counting, even applied on a pixel‐by‐pixel basis, does not account for the differences in spatial distribution of the photopeak and cross‐talk photons. Here a new convolution cross‐talk correction method is presented. The convolution filters are derived from point response functions (PRFs) for Tc‐99m and Tl‐201 point sources. Three separate acquisitions were performed, each with two 20% wide energy windows, one centered at 140 keV and another at 70 keV. The first acquisition was done with Tc‐99m solution only, the second with Tl‐201 solution only, and the third with a mixture of Tc‐99m and Tl‐201. The nonuniform RH‐2 thorax–heart phantom was used to test a new correction technique. The main difficulty and limitation of the convolution correction approach is caused by the variation in PRF as a function of depth. Thus, average PRF should be used in the creation of an approximative filter. The results of this study show that simple cross‐talk correction provides Tl‐201 images of poor quality. However, the proposed method, based on convolution technique, improves the quality of the simultaneous Tl‐201/Tc‐99m SPECT imaging.
Dose distributions produced by a shielded vaginal cylinder using a high‐activity iridium‐192 source
Authors: Waterman Frank M. , Holcomb David E.
Published: March 2003

Algorithms commonly used to compute the dose distribution around a 192Ir source do not calculate the dose by summing the primary and scatter dose components. Thus if the scatter component is reduced significantly by the insertion of a shield into a vaginal cylinder, the algorithm may overestimate the dose delivered to the tumor volume. This problem was investigated by measuring the relative dose around a 2.5 cm diameter shielded vaginal cylinder in a water phantom by use of a 0.147 cm3 thimble ionization chamber. Measurements were made with the cylinder unshielded and with 0.8 cm thick 90°, 180°, and 270° tungsten shields inserted. A reduction in dose was observed on the unshielded side of the cylinder which increased with distance from the source and with the angle subtended by the shield. This reduction did not exceed 2% within 1 cm from the surface of the cylinder with any shield. However, the reduction in dose increased linearly to 3%, 5%, and 15% at a distance of 10 cm from the source with the 90°, 180°, and 270 ° shields, respectively. On the shielded side of the cylinder, the dose at the surface is reduced to about 13% of its value without the shield; however, this percentage increases with distance reaching a maximum 2–4 cm from the cylinder. The maximum values obtained with the 90°, 180°, and 270° shields were 23%, 17%, and 16%, respectively.
Comparison of beam‐hardening and K‐edge filters for imaging barium and iodine during fluoroscopy
Authors: Gagne Robert M. , Quinn Philip W. , Jennings Robert J.
Published: March 2003

This study investigated the dose reduction performance of several beam‐hardening and K‐edge filter materials for the imaging of barium or iodine during fluoroscopy. A computer model was developed to simulate the effect of added filtration on entrance exposure rate (Xp), integral dose rate (Di), contrast (C), signal to noise ratio (SNR), imaging performance per dose (SNR2/Di), and tube load. The model incorporated the response characteristics, in both manual and automatic control modes of operation, of fluoroscopic systems to increasing or decreasing x‐ray intensity at the input of the image intensifier. Input parameters to the computer model included choice of filter material and thickness, a barium or iodine test object, tube potential, phantom thickness, a CsI input phosphor, and a set of algorithms for controlling the fluoroscopic system. In all cases, the performance of systems with added filtration was judged with respect to a reference system operating under comparable conditions. In general, either beam‐hardening or K‐edge filters provided a significant reduction in entrance exposure and integral dose rates, but with an attendant increase in tube load. For a fluoroscopic system constrained to follow a representative automatic brightness control algorithm, added filtration provided a reduction in entrance exposure and integral dose rates for all phantom or uniformly distributed barium thicknesses. However, the imaging performance per dose, in some cases, decreased rapidly and was less than that of the reference system at large thicknesses. Only a change in the algorithm controlling the kVcp and mA operating points on the fluoroscopic system provided an imaging performance per dose greater than the reference system's at large thicknesses. The practical implementation of adding filtration to fluoroscopic systems is most simply accomplished with beam‐hardening filters rather than K‐edge filters. However, the systems with K‐edge added filtration can provide slightly better performance when used over a limited range of phantom thicknesses such as the range normally associated with pediatric patients.
Noninvasive high‐voltage measurement in mammography by crystal diffraction spectrometry
Authors: Deslattes Richard D. , Levin Jon C. , Walker Mary D. , Henins Albert
Published: March 2003

Wavelength dispersive crystal diffraction spectrometry has been applied to the measurement of the accelerating voltage on an x‐ray source in a prototype experiment in the mammographic source. The results indicate that this noninvasive approach can yield determinations of such voltages within 0.1 kV, a level of imprecision that appears adequate for high‐level standardization of such potentials.
Measurement of slice sensitivity profiles in spiral CT
Authors: Polacin Arkadiusz , Kalender Willi A. , Brink James , Vannier Michael A.
Published: March 2003

In conventional computed tomography (CT), ramps, typically thin sheets of aluminum inclined at 45°, are established tools for measurements of slice sensitivity profiles (SSP). In spiral CT, however, they yield inconsistent results for different positions along the longitudinal axis. It is explained herein how ramp profiles result as a superposition of true SSPs and artifacts, the artifacts being caused by the slice interpolation process due to the difficult interpolation condition in this particular geometry. In direct consequence, ramp tests yield spatially variant results. As an alternative tool for measuring the slice sensitivity profile, the use of a thin high‐contrast sheet held between two disks which approximate the ideal test of a delta impulse in the longitudinal direction is suggested. Resulting SSPs are shown for both methods; the delta method, in agreement with theoretical predictions, provided smooth symmetrical SSPs independent of table position in agreement with theoretical predictions. It is concluded that ramps are inadequate test objects to determine SSPs in spiral CT.
X‐ray scatter in megavoltage transmission radiography: Physical characteristics and influence on image quality
Authors: Jaffray D. A. , Battista J. J. , Fenster A. , Munro P.
Published: March 2003

The physical characteristics of x rays scattered by the patient and reaching the imaging detector, as well as their effect on verification (portal) image quality, were investigated for megavoltage (0.1–20 MeV) x‐ray beams. Monte Carlo calculations and experimental measurements were used to characterize how the scatter and primary fluences at the detector plane were influenced by scattering geometry and the energy spectrum of the incident beam. The calculated scatter fluences were differentiated according to photon energy and scattering process. Scatter fractions were measured on a medical linear accelerator (Clinac 2100c, 6 MV) for a typical imaging geometry using an ionization chamber and a silicon diode. After correction for the energy dependence of the chamber and diode, the scatter fractions generated by the Monte Carlo simulations were found to be in excellent agreement with the measured results. In order to estimate the effect of scatter on image quality, the scatter and primary signals (i.e., energy deposited) produced in five different types of portal imaging detectors (lead plate/film, storage phosphor alone, lead plate/storage phosphor, Compton recoil‐electron detector, and a copper plate/Gd2O2S phosphor) were calculated. The results show that, for a specified geometry, the scatter fraction can vary by an order of magnitude, depending on the sensitivity of the imaging detector to low‐energy (<1 MeV) scattered radiation. For a common portal imaging detector (copper plate/Gd2O2S phosphor), the scattered radiation (i) reduced contrast by much as 50% for a fixed display‐contrast system, and (ii) decreased the differential‐signal‐to‐noise ratio (DSNR) by 10%–20% for a quantum‐noise‐limited portal imaging system. For currently available TV‐camera‐based portal imaging systems, which have variable display contrast, the reduction in DSNR depends on the light collection efficiency and the noise characteristics of the TV camera. Overall, these results show that scattered radiation can reduce contrast significantly in portal films while deteriorating image quality only moderately in on‐line systems.
Dosimetry of effective wedge fields produced by an internal wedge
Authors: Boyer Arthur L. , Tung Samuel , Otte Victor , Wells Nathan , Steadham Roy
Published: March 2003

A procedure is described to calculate the monitor unit ratios required to produce effective wedge fields having a desired wedge angle by combining an internal 60° wedge with an open field. Complementary procedures are derived and demonstrated for calculating the effective wedge dose distributions with wedge angles of 15°, 30°, and 45° using the central axis depth dose data and off‐axis ratios of the open field and the 60° wedged field. Measurements at five points on and off the central axis within each field and measurements of the effective wedge factor demonstrated that the calculated wedge distributions were correctly delivered to within 2% in all cases.
Management of radiation oncology patients with implanted cardiac pacemakers: Report of AAPM Task Group No. 34
Authors: Marbach J. R. , Sontag M. R. , Van Dyk J. , Wolbarst A. B.
Published: March 2003

Contemporary cardiac pacemakers can fail from radiation damage at doses as low as 10 gray and can exhibit functional changes at doses as low as 2 gray. A review and discussion of this potential problem is presented and a protocol is offered that suggests that radiation therapy patients with implanted pacemakers be planned so as to limit accumulated dose to the pacemaker to 2 gray. Although certain levels and types of electromagnetic interference can cause pacemaker malfunction, there is evidence that this is not a serious problem around most contemporary radiation therapy equipment.
A precise investigation on the TL behavior of LiF: Mg, Cu, P (GR‐200A)
Authors: Furetta C. , Leroy C. , Lamarche F.
Published: March 2003

LiF: Mg, Cu, P is a TL material presenting unique dosimetric features. The TL sensitivity of this material was studied as a function of the annealing temperature and of the repeated cycles of annealing–irradiation–readout. A fading study was carried out over a period of 40 days with the purpose of checking the stability of the stored dosimetric information as a function of different annealing temperatures. A detailed statistical analysis of sets of data, obtained from repeated measurements on a group of ten dosimeters, is presented.
Rotary scanning equalization radiography: An efficient geometry for equalization mammography
Authors: Sabol John M. , Tam Ka Kay , Plewes D. B.
Published: March 2003

The detection of cancer in the radiographically dense breast is problematic, since the breast will produce a range of exposure that exceeds the useful dynamic range of high contrast film‐screen combinations. It has been shown previously that mammographic scanning equalization radiography (MSER) can be used to overcome the latitude limitations of film‐screen mammography. However, the tube loading of MSER is orders of magnitude greater than conventional mammography. A new rotary geometry for equalization radiography is proposed, in which the image receptor is exposed by repeated scans of a modulated slot beam, oriented at a variety of scanning angles with respect to the object. The superposition of the exposure from appropriately modulated, rotated slot beams produces an entrance exposure that will effectively equalize the film exposure. The principle advantages of this geometry is its simplicity and reduced tube loading. To determine the effectiveness and feasibility of RSER the effect of conventional, MSER, and RSER have been numerically simulated on the appearance of clinical mammograms, the relative heat loading, and the fraction of the breast imaged with high contrast are calculated. It is found that RSER produces images that are free of artefacts, and exhibit a similar degree of equalization, as found in MSER images. RSER accomplishes this with only four scanning angles, and a beam that is approximately 4 cm wide. The resulting tube loading is only three times greater than that found in conventional imaging. Numerical simulations indicate that RSER is a simple, feasible means of overcoming the latitude limitations of film‐screen mammography.
An analytical edge spread function model for computer fitting and subsequent calculation of the LSF and MTF
Authors: Boone John M. , Seibert J. Anthony
Published: March 2003

The previous work of Yin, Giger, and Doi [Med. Phys. 17, 962–966 (1990)] demonstrated that using a computerized fit of an analytic line spread function to experimentally measured data is very useful for determining the presampling modulation transfer function of an imaging system. In this report, the work of Yin et al. is extended to include an analytic expression for the edge spread function (ESF). By fitting experimentally determined edge spread function data to the analytical expression, the normalized line spread function (LSF) and the normalized modulation transfer function (MTF) can be easily calculated from four ESF fit coefficients. The extension from the line spread function to the edge spread function should be valuable in cases where slit measurements are impractical, for example, in very high resolution imaging systems where the required slit dimensions become impractically small, or in measurements of the transfer properties of scattered radiation or off‐focus radiation, where large area exposures are necessary.
A simulation of emission and transmission noise propagation in cardiac SPECT imaging with nonuniform attenuation correction
Authors: Tung Chi‐Hua , Gullberg Grant T.
Published: March 2003

Transmission computed tomography provides information needed for nonuniform attenuation correction of cardiac single photon emission computed tomography (SPECT). Nonuniform attenuation correction is accomplished using an iterative ML‐EM algorithm and a projection‐backprojection operation that incorporates attenuation factors measured from the reconstructed transmission map. The precision and accuracy of the attenuation corrected emission reconstruction is a function of emission and transmission statistics. This paper presents an error propagation analysis that uses a mathematical cardiac chest phantom to simulate various combinations of total emission counts C̄ and transmission flux Ī0 under ideal imaging conditions (without geometric response distortion and without scatter). The spatial average, spatial variance, and accuracy measures for a 4×4 pixel region in the heart are tabulated after 30 iterations of the ML‐EM algorithm. The confidence intervals for these measures were determined from 1000 realizations of reconstructions from projections randomly generated with the same transmission and emission statistics. It can be shown empirically from the simulation results that the spatial %rms uncertainty for the simulated cardiac region has a simple expression: %rms2=K1/C̄+K2/Ī20+B2 where K1 and K2 are least‐square estimates based on the simulation results, and B is the measured spatial %rms uncertainty for the simulation at infinite statistics. For a transmission incident flux of 1500 events per projection bin of 0.712 cm and typical clinical emission events totaling 1×105, the spatial %rms uncertainty is approximately 14%. At clinical transmission and emission statistics, the statistical noise in the simulated attenuation‐corrected reconstructions are dominated by the emission statistics.
Design of a high‐flux epithermal neutron beam using 235U fission plates at the Brookhaven Medical Research Reactor
Authors: Liu Hungyuan B. , Brugger Robert M. , Rorer David C. , Tichler Paul R. , Hu Jih‐Perng
Published: March 2003

Beams of epithermal neutrons are being used in the development of boron neutron capture therapy for cancer. This report describes a design study in which 235U fission plates and moderators are used to produce an epithermal neutron beam with higher intensity and better quality than the beam currently in use at the Brookhaven Medical Research Reactor (BMRR). Monte Carlo calculations are used to predict the neutron and gamma fluxes and absorbed doses produced by the proposed design. Neutron flux measurements at the present epithermal treatment facility (ETF) were made to verify and compare with the computed results where feasible. The calculations indicate that an epithermal neutron beam produced by a fission‐plate converter could have an epithermal neutron intensity of 1.2×1010 n/cm2.s and a fast neutron dose per epithermal neutron of 2.8×10−11 cGy.cm2/nepi plus being forward directed. This beam would be built into the beam shutter of the ETF at the BMRR. The feasibility of remodeling the facility is discussed.
A performance study of the Loma Linda proton medical accelerator
Authors: Coutrakon G. , Hubbard J. , Johanning J. , Maudsley G. , Slaton T. , Morton P.
Published: March 2003

More than three years have passed since Loma Linda treated the first cancer patient with the world's first proton accelerator dedicated to radiation therapy. Since that time, over 1000 patients have completed treatments and the facility currently treats more than 45 patients per day. With a typical intensity of 3×1010 protons per pulse and 27 pulses per minute, dose rates of 90–100 cGy/min are easily achieved on a 20‐cm diameter field. In most cases, patient treatment times are 2 min, much less than the patient alignment time required before each treatment. Nevertheless, there is considerable medical interest in increasing field sizes up to 40‐cm diameter while keeping dose rates high and treatment times low. In this article, beam measurements relevant to intensity studies are presented and possible accelerator modifications for upgrades are proposed. It is shown that nearly all intensity losses can be ascribed to the large momentum spread of the injected beam and occur at or near the injection energy of 2 MeV. The agreement between calculations and measurements appears quite good. In addition, optimum beam characteristics for a new injector are discussed based upon the momentum acceptance and space charge limits of the Loma Linda synchrotron.
A simple magnetic spectrometer for radiotherapy electron beams
Authors: Deasy J. O. , Almond P. R. , McEllistrem M. T. , Ross C. K.
Published: March 2003

A small, lightweight, single‐focusing magnetic spectrometer was designed, assembled, and tested for analysis of electron beams from radiotherapy electron linacs. The objective was to develop a low cost, simple device that could be easily replicated in other medical centers, and to demonstrate the practicality of individual electron counting for precise analysis of electron spectra. Two methods of spectroscopy have been developed. One method consists of counting electrons individually as a function of magnetic field setting. Electrons are deflected through 90° in the magnetic spectrometer, through an exit slit, and into a scintillation detector. A second method consists of recording the complete spectrum of electron energies from the accelerator on a strip of film at a single magnetic field setting. A critical design element is the 10‐cm long collimator for electrons entering the magnet gap, with defining apertures and scraper slits. The spectrometer's cleanliness of transmission, energy calibration, and resolution were all tested at 10 and 16 MeV using the nearly monoenergetic electron beam of the accelerator at the National Research Council of Canada (NRCC). These accelerator tests, and also Monte Carlo trajectory simulations, both show that contamination of the transmitted spectrum due to scattered or knock‐on electrons is negligible. Low‐energy characteristics were tested using a 90Sr+90Y beta‐particle source. The energy calibration of the 90° spectrometer mode was based on mapping the magnetic field and also electron trajectory computer simulations. That calibration agrees with the NRCC's own calibrated scale to 0.8% for the single‐particle counting method and to 1.3% for the film method. The energy resolution was measured to be 2% at 10 MeV, which is adequate for radiotherapy linac measurements. The acceptance half angle is 0.5° or less, depending on the aperture size, which is adequate for electron angular distribution measurements within the forward cone of the electron beam. Used with film, the spectrometer is a simple, accurate, and highly transportable device for measuring radiotherapy electron energy spectra.
Materials for thermoluminescent dose detectors and photon radiation energy detectors intended for intercomparison procedures of radiation therapy units
Authors: Kalmykov Leonid Z.
Published: March 2003

Composite thermoluminescent (TL) materials characterized by different TL sensitivity to energy of photon radiation, namely, double‐component mixtures of finely dispersed ingredients: thermoluminescent phosphor (LiF or CaSO4:Dy) and nonluminescent material (salts with various Zeff values) have been described. The TL sensitivity of such TL composites was demonstrated to depend upon Zeff values inherent to separate components and upon their concentration ratio in the mixture. Within the range of x‐ray energy values from 10 to 120 keV, the LiF‐based materials containing Li2CO3 as nonluminescent material were dosimetrically equivalent to soft tissues. The availability of powder‐like TLD materials containing 65% LiF+35% Li2CO3 can definitely serve the basis for extending the application area for the well‐known procedure adopted by IAEA/WHO for comparative testing of x‐ray therapy units with HVL<0.5 mm Cu and for starting the intercomparison of units intended for superficial x‐ray therapy. The use of the dose detectors based on TLD of 35% LiF+65% Li2CO3 composition in orthovoltage x‐ray therapy units excludes the necessity for introducing corrections for the dependence of detector's readings on energy levels. LiF‐ and CaSO4:Dy‐based TL materials containing PbSO4 as a nonluminescent material are, by a factor of 10, more sensitive to energy variations than other known thermoluminescent phosphors. With the application of TLD based on above materials into two‐detector systems intended for determining Eeff values of x‐ray radiation, the ambiguity of readings in low‐energy region has been eliminated, whereas the range of measurement appeared to be significantly expanded.
Coumarin chemical dosimeter for radiation therapy
Authors: Collins A. K. , Makrigiorgos G. M. , Svensson G. K.
Published: March 2003

Aqueous coumarin was investigated as a possible dosimeter for radiation therapy applications. Coumarin‐3‐carboxylic acid in aqueous solutions converts upon irradiation to the highly fluorescent 7‐hydroxy‐coumarin‐3‐carboxylic acid. The intensity of the fluorescence signal is linearly proportional to the number of the formed 7‐hydroxy‐coumarin‐3‐carboxylic acid molecules, which in turn is proportional to the radiation‐absorbed dose. Basic characterization of the dosimetric properties (linearity with dose, energy and dose‐rate dependence, postirradiation stability, and reproducibility) was performed. The system exhibits nearly linear behavior with dose, in the range of 0.1 to 50 Gy, is stable for at least 166 days following the irradiation, is reproducible within the same solution (±2%) and is energy independent for 6‐ to 15‐MV x‐ray energies. A reduction of 18% of the fluorescence signal was observed by changing the dose rate from 0.8 to 4 Gy/min. The origin of the dose‐rate effect was investigated and attributed to impurities, predominantly transition metal ions. Removing the impurities by recrystallization of the coumarin and using ultrahigh‐purity water reduced the dose‐rate dependency to less than 4% in the examined dose‐rate range (0.8–4 Gy/min).
A patient‐to‐computed‐tomography image registration method based on digitally reconstructed radiographs
Authors: Lemieux L. , Jagoe R. , Fish D. R. , Kitchen N. D. , Thomas D. G. T.
Published: March 2003

An automatic method for the accurate registration of computed tomography (CT) data with two camera‐calibrated radiographs is presented. The registration is based on the skull as visualized both in the plain radiographs and in radiographs digitally reconstructed from CT. A reference coordinate system is established based on the radiographic projection parameters obtained using an angiographic stereotactic localizer. The CT‐derived reconstructed radiographs are aligned iteratively at multiple resolutions until a best match is found by adjusting the position and orientation of the CT data set relative to the reference coordinate system. The results of experiments with a skull phantom performed under stereotactic control which show that reliable registration is possible with an accuracy better than 1 mm are presented. Possible applications include intraoperative patient‐to‐CT frameless registration and registration of radiographic data with frameless CT for depth electroencephalogram electrode position confirmation.