bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiphoton, time‐of‐flight three‐dimensional radionuclide imaging
Authors: Powell M. D.
Published: March 2003

A method for applying time‐of‐flight (TOF) information to the three‐dimensional (3‐D) localization of multiple‐photon‐emitting radionuclides is presented. The same principle used for TOF imaging of positron annihilation photons is suggested for use with nuclides that emit two photons (gamma or x rays) in near coincidence. Two photons originating from the same decay event are detected. The locations at which they are detected become the foci of a hyperbola described by the difference in their path length. If one detector is collimated a line is established by the photon it detects, which intersects the hyperbola at the origin of the event. Should TOF imaging become a reality, this technique would extend its usefulness to nuclides that do not emit positrons.
Interface dosimetry for I‐125 sources
Authors: Ling C. Clifton , Yorke Ellen D.
Published: March 2003

Use of I‐125 in breast implants may be therapeutically beneficial due to the 40% higher dose to the tumor relative to the normal adipose tissue. The dose distribution in the interface between adipose and nonadipose tissues is studied with mathematical models and radiobiological measurements. The dose transition zone is narrow, about 10μ, which is roughly the diameter of a cell. The dimension of the transition zone is weakly sensitive to the geometry of the interface. In most geometries, cells on either side of the interface receive doses that are significantly different (25%–35%). Radiobiological measurements are consistent with calculated results, providing a check on the theoretical model.
Measurement of diagnostic x‐ray spectra using a silicon photodiode
Authors: Aoki Kiyoshi , Koyama Masaaki
Published: March 2003

Diagnostic x‐ray spectra (66–103 kV, 1–2 mA) were measured without pinhole collimator or liquid nitrogen cooling by using a silicon p–i–n photodiode. An x‐ray count rate was low (≊400 photons/s) at distances of 2–3 m away from an x‐ray target, because the i layer of the photodiode was very small (1.5 mm2 in area and 75 μm in thickness). The junction reverse current of the photodiode was 50–70 pA at room temperature and energy resolution for 59.5‐keV γ rays was 2.0 keV (full width at half‐maximum), which was limited by the electric noise of a preamplifier. Measured spectra were corrected for detector distortion. The corrected spectra were similar to those determined with a Ge detector. The i‐layer thickness of the photodiode was an important parameter for the correction. In particular, when the thickness was thin, the fraction of photoelectron escape became large; this fraction was estimated analytically for various i‐layer thicknesses. The angular dependence of the full‐energy peak efficiency was measured; for the photons whose energies were above 17.8 keV the angular dependence was isotropic except for angles greater than 60° off the photodiode axis. This method is therefore applicable to the measurement of scattered x rays.
The use of storage phosphors for portal imaging in radiation therapy: Therapists' perception of image quality
Authors: Gur David , Deutsch Melvin , Fuhrman Carl R. , Clayton Patricia A. , Weiser John C. , Rosenthal Marc S. , Bukovitz Andrew G.
Published: March 2003

Two drawbacks in quality of portal radiographs in radiation therapy are their low contrast and low spatial resolution. These are due to the low differential absorption of body tissues at therapeutic energies and to a relatively large radiation source. We used an experimental, high‐contrast sensitivity storage phosphor imaging system (Eastman Kodak Co.) to produce portal images. The system consists of a storage phosphor detector, a high‐contrast sensitivity laser scanner (12 bit), an image processing module, and a laser printer (12 bit). Patients undergoing radiation therapy treatments had both a conventional portal image and a storage phosphor image taken. Both were displayed side‐by‐side and were evaluated independently by three radiotherapists according to quality of information to verify the treatment field. Each of the three radiotherapists rated the storage phosphor images to be better (p<0.001) than the conventional images. However, rated improvements of low‐contrast storage phosphor images of the pelvis and abdomen (40) were significantly lower than those of high‐contrast (head, neck, and chest) images (53).
Dose delivery error detection by a computer‐controlled linear accelerator
Authors: Loyd M. , Chow H. , Laxton J. , Rosen I. , Lane R.
Published: March 2003

A commercial dual photon energy, computer‐controlled linear accelerator has a complex collimation and beam delivery system. For accurate dose delivery, six separate motorized elements must be properly positioned for a given beam selection. Experimentally instituted misalignments of the primary electron scattering foils, the primary collimator, the flattening filter, the scattering foil carousel, and the backscatter shield or shutter produced significant dose delivery errors. The ability of the Mylar window monitor chamber to detect these errors was examined for x‐ray beams. The fault detection system failed to interrupt dose delivery in a number of situations where the error in dose per monitor unit delivered ranged from −6% to +270% of the calibrated value.
Macroscopic dosimetry for radioimmunotherapy: Nonuniform activity distributions in solid tumors
Authors: Howell Roger W. , Rao Dandamudi V. , Sastry Kandula S. R.
Published: March 2003

In the context of radioimmunotherapy of cancer, there is a need for continued improvement of dosimetry of radionuclides localized in tumors. Current methods assume uniform distribution of radionuclides in the tumor despite experimental evidence indicating nonuniformity. We have developed a model in which nonuniform distribution of radioactivity in the tumor is taken into account. Spherically symmetric radionuclide distributions, depending linearly and exponentially on the radial position, are considered. Dose rate profiles in the tumor are calculated for potentially useful beta‐emitting radionuclides, including 32P, 67Cu, 90Y, 111Ag, 131I, and 188Re, and for 193mPt, an emitter of conversion electrons and low‐energy Auger electrons. For the radionuclide distributions investigated, high‐energy beta emitters, such as 90Y, are most effective in treating large tumors (diameter, d≳1 cm), whereas for small tumors (d∼1 mm), medium energy beta emitters such as 67Cu are better suited. Very small tumors (d<1 mm), and micrometastases are best handled with low‐energy electron emitters such as 193mPt.
Comparison of absorbed dose in bone substitute material and water using ionization measurements
Authors: Prasad Satish C. , Bassano Daniel A.
Published: March 2003

Absorbed dose in a liquid substitute for bone has been compared with absorbed dose in water for 9‐, 12‐, and 15‐MeV electron beams using ionization chamber measurements. The ionization readings were converted to dose using collisional mass stopping power ratios. The collisional mass stopping powers for the liquid substitute of bone were calculated using the Monte Carlo Code PEGS4. The results of our study show that there is an increase in dose in the liquid bone substitute compared to water at shallow depths. The maximum increase in dose was 5%, 4%, and 2% at depths of 1.2, 1.5, and 2.0 cm, respectively, for 9‐, 12‐, and 15‐MeV electrons. The density of the liquid bone substitute was 1360 kg/m3 and the effective atomic number was 11.
Measurement of dose distributions using film in therapeutic electron beams
Authors: Shiu Almon S. , Otte Victor A. , Hogstrom Kenneth R.
Published: March 2003

The feasibility of using film dosimetry data as the input data for patient treatment planning was evaluated. The central‐axis depth dose and the off‐axis ratios obtained from film measurements in a solid phantom were compared with those of ion‐chamber measurements in water. Two techniques were used to generate isodose distributions. The first technique used only the film data, i.e., the central‐axis depth dose and the off‐axis ratios used for the reconstruction were determined from the film optical density (corrected for film nonlinearity). In the second technique, the central‐axis depth dose measured by an ion chamber in a water phantom was combined with the off‐axis ratios measured using film in the “solid water” phantom. The resulting isodose distributions from both techniques were compared with the ion‐chamber measurements in water for 7‐, 12‐, and 18‐MeV electrons, and the second technique showed better agreement with the ion‐chamber measurements than did the first technique. The differences were within a clinically acceptable range.
The use of variable grid spacing to accelerate dose calculations
Authors: Niemierko Andrzej , Goitein Michael
Published: March 2003

Planning radiation therapy using three‐dimensional patient data is a very time consuming process with current hardware and software. When calculating a three‐dimensional dose distribution, the standard technique is to cover the volume of interest with a uniformly spaced matrix of points at which the dose is calculated. It is obvious that the dose is usually quite slowly varying in a large proportion of the region of interest; namely, in those regions which are either well inside or well outside the geometrical boundaries of the field. We have developed an algorithm which allows us to reduce the number of calculation points, and hence the time of calculation of the entire dose distribution, manyfold. We use a nonuniform grid of calculated points, based on the fact that the only regions which are troublesome for accurate dose interpolation are those in which large values of the second derivative of the dose as a function of position occur. We demonstrate that, at most grid points, the dose can be determined without decreasing accuracy below acceptable limits by simple linear interpolation between grid points much further apart than is usual in conventional techniques. We investigated our algorithm for one‐, two‐, and three‐dimensional examples and for Co‐60, 25‐MV photon, and 160‐MV proton beams. In situations for which an accuracy of about 1% in dose and 1.6 mm in position was desired, we found gain factors for the number of points needing direct calculation of approximately 3 (one‐dimension), 6 to 10 (two‐dimensions) and 16 (three‐dimensions).
Variable versus fixed modulation of proton beams for treatments in the cranium
Authors: Urie M. M. , Goitein M.
Published: March 2003

Dose distributions in the cranium with fixed and with variably modulated proton beams were compared. The variable modulation was designed to tailor the proximal high‐dose region of each field to the target volume surface whereas the fixed modulation beams had a constant modulation determined by the greatest extent of the target. Dose‐volume histograms of normal tissues were compared, as were the estimated complication probabilities. Twelve patients with chordomas or chondrosarcomas of the base of skull who had been treated to approximately 70 cobalt Gray equivalent (CGE) were evaluated. Dose distributions of the actual treatments were compared to those which would have been delivered had the proton beams been variably modulated; two patients for whom x‐ray plans were available were also evaluated. The greatest difference in dose between the variable and fixed modulation proton beams, averaged over all the patients, was 13.8 CGE (8.0–18.0 CGE range). Much of this reduction occurred in the brain, particularly the temporal lobes. In those temporal lobes receiving significant doses, variable modulation reduced the volume receiving more than 54 CGE by 3.0 cc; all temporal lobes had at least a 5 CGE difference to some portion, half had more than 10 CGE and three more than 15 CGE difference to some portion. The optic structures, brainstem and spinal cord received from 1 to 3 CGE less dose with the variability modulated beams. Eight of the parotid glands received more than 20 CGE to more than half their volume with the fixed modulation beams; in these, variable modulation reduced the mean dose by 5.3 CGE. The reduction in integral dose with variable as compared to fixed modulation was in the range 3 to 12%; this gain was considerably less than the gain for uniformly modulated proton beams over x‐rays in the two patients for whom x‐ray plans were available.
A technique for measuring regional bone mineral density in human lumbar vertebral bodies
Authors: Cody Dianna Dickie , Flynn Michael J. , Vickers David S.
Published: March 2003

A method for measuring the regional bone mineral density (rBMD) in human lumbar vertebral bodies using a series of contiguous computed tomography images, each 1 mm thick, is fully described. The technique has a sample volume of 0.004 cm3, a sample spacing of 0.8 × 0.8 × 1.0 mm, and results in a bone marrow radiation dose of 1.59 to 2.75 rads (0.016–0.028 Gy). The use of physical density (mg/cm3) is introduced and the measurement noise (0.7–1.3%), accuracy (2.7%), and serial precision (0.2%) have been evaluated in vitro using appropriate phantoms. The corresponding percentage errors for accuracy and precision relative to K2HPO4 concentration were 6.9% and 2.0%, respectively. A multiple region density measurement is described and evaluated.
Lead shielding thickness for dose reduction of 5 MeV electrons
Authors: Prasad S. Guru , Parthasaradhi K. , Lee Y. , Garces R.
Published: March 2003

The relative percent intensity reduction by lead (Pb) of 5 MeV electrons produced by Siemens Mevatron 77/74 for 5 cm diameter, 10×10, 15×15, and 20×20 cm2 cones both with and without buildup is measured. The thickness of lead (Pb) required to attenuate the intensity of the primary electron beam to 95% and 98% depends upon the cone size and upon the depth in phantom at which transmission measurements are made.
A proposal for the calibration of plane‐parallel ion chambers by accredited dosimetry calibration laboratories
Authors: Attix F. H.
Published: March 2003

A procedure is described by which the AAPM‐accredited dosimetry calibration laboratories could offer calibrations of plane‐parallel ionization chambers for radiotherapy dosimetry applications, by comparison with a cylindrical ion chamber in a phantom irradiated by 60Co γ rays. Ngas can thus be determined for the plane‐parallel chamber under uniform conditions of photon scatter, and without the need for an electron fluence correction.
Single‐exposure dual‐energy computed radiography
Authors: Stewart Brent K. , Huang H. K.
Published: March 2003

This paper focuses on analysis and development of a single‐exposure dual‐energy digital radiographic method using computed radiography (Fuji FCR‐101 storage phosphor system). A detector sandwich consisting of storage phosphor imaging plates and an interdetector filter is used. The goal of this process is to provide a simple dual‐energy method using typical plane‐projection radiographic equipment and techniques. This approach exploits the transparency of the storage phosphor plates, using radiographic information that would be otherwise lost, to provide energy selective information essentially as a by‐product of the radiographic examination. In order to effectively make use of the large dynamic range of the storage phosphor imaging plates (10 000:1), a computed radiography image reading mode of fixed analog‐to‐digital converter gain and variable photomultiplier sensitivity provides image data which can be related to relative incident exposure for export to the decomposition algorithm. Scatter rejection requirements necessitated crossed 12:1 grids for a field size of 36×36 cm. Optimal technique parameters obtained from computer simulation through minimization of the aluminum and Plexiglas equivalent image uncertainty under conditions of constant absorbed dose resulted as: 100 kVp using a 0.15‐mm‐thick tin (Sn) interdetector filter for the lung field. This yields a surface exposure of 23 mR and a surface absorbed dose of 0.26 mGy for a 23‐cm‐thick chest. Clinical application in evaluation of the solitary pulmonary nodule is discussed, along with an image set demonstrating this application.
A dose‐per‐pulse monitor for a dual‐mode medical accelerator
Authors: Galbraith Duncan M. , Martell E. S. , Fueurstake T. , Norrlinger B. , Schwendener H. , Rawlinson J. Alan
Published: March 2003

On a radiotherapy accelerator, the dose monitoring system is the last level of protection between the patient and the extremely high dose rate which all accelerators are capable of producing. The risk of losing this level of protection is substantially reduced if two or more dose monitoring systems are used which are mechanically and electrically independent in design. This paper describes the installation of an independent radiation monitor in a dual‐mode, computer‐controlled accelerator with a moveable monitor chamber. The added device is fixed in the beam path, is capable of monitoring each beam pulse, and is capable of terminating irradiation within the pulse repetition period if any measured pulse is unacceptably high.
Effects of x‐ray beam equalization on mammographic imaging
Authors: Lam Kwok Leung , Chan Heang‐Ping
Published: March 2003

We employed an elastic reservoir filled with tissue‐equivalent fluid as an x‐ray beam filter for exposure equalization. The filter is designed to be placed between the upper compression plate and the breast. During compression, it will conform to the shape of the peripheral region of the breast and fill the space between the upper and lower compression plates, thereby reducing the variation in x‐ray penetration between the thick and thin regions of the breast and compressing the dynamic range of the breast image. Results of a phantom study showed that the visibility of low‐contrast features was improved in the peripheral region of the equalized breast phantom image. Experimental measurements indicated that the filter reduced scattered radiation in the peripheral region. This result was confirmed by a Monte Carlo simulation study. The radiation contrast, with the effect of beam hardening included, was also improved when equalization was employed.
Signal‐to‐noise efficiency in magnetic resonance imaging
Authors: Parker Dennis L. , Gullberg Grant T.
Published: March 2003

The dependence of signal‐to‐noise ratio (SNR) on the analog filter, the sampling rate and the number and dimensions of voxels is derived for magnetic resonance imaging (MRI). It is shown that the object signal‐to‐noise ratio scales directly with the voxel volume and the square root of the number of voxels. Defining an efficiency figure of merit as the SNR divided by the square root of the imaging time, it is shown that efficiency is always improved when imaging with the lowest possible resolution (largest voxel dimensions) consistent with viewing the desired anatomical detail. The results directly imply the relative efficiency of 3‐D (volume), 2‐D (plane), 1‐D (line) and 0‐D (point) imaging techniques. It is shown that spatial averaging is an inefficient method of noise reduction in MRI. As long as voxel size is maintained constant, one can image as many pixels in the readout direction as desired with no loss in SNR; that is, the number of pixels in the readout direction has no effect on the image SNR. Further, multiple sampling of each phase encoding value (to improve SNR) has no advantage over increasing the number of pixels in the phase encoding direction while leaving the voxel size constant. Some experimental observations are given.
Wiener noise power spectra of radiological television systems using a digital oscilloscope
Authors: Rowlands J. A. , DeCrescenzo G.
Published: March 2003

Measurement of spatial noise power spectra from television based radiographic and fluoroscopic systems is essential to the understanding of their operation and optimization. However, conventional methods require acquisition and processing of large numbers of complete images, thus confining such measurements to special applications where accessible frame buffers already exist or elaborately equipped laboratories. We have developed a method which only requires storage of single TV lines or point scans. A digital oscilloscope captures these point scans and a laboratory microcomputer facilitates manipulation of the data to separate out different components of the noise power spectra. The x‐ray dependent component of the noise power spectrum so produced is not the ordinary Wiener spectrum. However, it is shown that reconstruction of the full Wiener spectrum from this is possible subject only to the requirement that the x‐ray noise spectrum at the output of the imaging system is circularly symmetric.
An application of multivariate moment‐generating functions to the analysis of signal and noise propagation in radiographic screen–film systems
Authors: Van Metter Richard , Rabbani Majid
Published: March 2003

Previous studies [J. Opt. Soc. Am. 4A 895–901 (1987)] have shown the utility of multivariate moment‐generating functions for analyzing the influence of stochastic amplifying and scattering mechanisms on the transfer of signal and noise through multistage imaging systems. Recently, these studies were extended to include cases in which the amplification or scattering parameters are themselves stochastic variables [J. Opt. Soc. Am. 6A, 1156–1164 (1989)]. In this paper we consider a special case in which amplification is followed by scattering such that the same random variable which characterizes the parameters of each amplification process also characterizes the parameters of the subsequent scattering of the amplified output events. In radiographic imaging, this can be used to describe the physics of the depth dependence of emission efficiency and light scatter in x‐ray intensifying screens, which was originally treated by Lubberts [J. Opt. Soc. Am. 58, 1475–1483 (1968)]. In this work Lubberts' original results are rederived in a more general form. They are then illustrated in terms of a diffusion model [Appl. Opt. 12, 1865–1870 (1973)] for light scatter within the intensifying screen.
Characterization of fragmented heavy‐ion beams using a three‐stage telescope detector: Detector configuration and instrumentation
Authors: Llacer J. , Schmidt J. B. , Tobias C. A.
Published: March 2003

Accelerated heavy‐ion beams used in biological and medical research are often utilized in conjunction with absorbers which lead to the fragmentation of the beam. The BERKLET, initially a two‐stage solid‐state telescope detector, was designed to make rapid, on‐line energy and linear energy transfer (LET) measurements of individual particles in a heavy‐ion beam, thus allowing characterization of fragmented beams. From data collected with the BERKLET, one is able to determine a number of important parameters. These include: residual energy and LET histograms for the full beam and for the individual Z components, relative number of particles with a given Z, and dose and track average LET's for the full beam and for the individual Z's. Improvements to the BERKLET design and changes in data analysis are discussed and contrasted with the results of an earlier BERKLET configuration. The most notable improvements are the addition of a thin scintillation detector for improved LET measurement, a tenfold improvement in the dynamic range of the event discriminator, reported here as 1:2000, and dual high‐and low‐gain amplification of the LET signals, permitting the identification of particles with Z's ranging from 12 down to 1.
A quality assurance program for monitor unit calculators
Authors: Dahl Robert A. , McCullough Edwin C. , Mellenberg David E.
Published: March 2003

A quality assurance protocol has been developed to assess the relative accuracy of computerized monitor unit calculations. Results from this testing reveal that errors (defined as the difference between the computer results and those obtained using a standard formula with manual calculation) in monitor unit calculators are possible when they are tested over a wide range of clinically relevant field sizes (including blocked fields), source‐to‐surface distances, and depths. It is suggested that computerized monitor unit calculators be checked against hand calculations and that this be permanently documented both at the time of initial implementation and following any subsequent changes in the program.
Effect of various noise sources on the detective quantum efficiency of phosphor screens
Authors: Nishikawa Robert M. , Yaffe Martin J.
Published: March 2003

We have examined the effect of screen‐structure, optical‐detector, and secondary‐quantum noise sources on detective quantum efficiency, DQE(f). This was done by using experimental measurements of screen‐structure and optical‐detector noise in combination with a theoretical model which predicts x‐ray quantum and secondary‐quantum noise for different optical and physical properties of a phosphor screen. The reduction in DQE(f) from noise sources other than x‐ray quantum noise depends on the noise power spectra (NPS) of these other sources relative to the x‐ray quantum NPS. Even though x‐ray quantum noise may be the dominant noise source at low spatial frequencies, it decreases relatively rapidly with increasing frequency so that other noise sources, which may be small at low frequencies, dominate. Our model predicts that DQE(f) can be increased, at spatial frequencies <4 mm−1, by changing the optical properties of the screen even though modulation transfer function MTF(f) may decrease. Furthermore, if screen and optical‐detector noise decrease with increasing frequency and secondary‐quantum noise sufficiently small, then DQE(f) will also be improved at frequencies >4 mm−1.
Model of the spatial‐frequency‐dependent detective quantum efficiency of phosphor screens
Authors: Nishikawa Robert M. , Yaffe Martin J.
Published: March 2003

We have developed a theoretical model to predict the modulation transfer function (MTF), the shape of the x‐ray quantum noise power spectrum (NPS), and the spatial‐frequency‐dependent detective quantum efficiency (DQE) of an x‐ray phosphor screen. The transfer of energy through the screen is modelled as a series of cascaded stochastic processes assuming that the screen consists of many thin phosphor layers. In this way, the model is able to account for the possibility of secondary‐quantum noise and the difference in shape between MTF2 and the x‐ray quantum NPS. Modelling a Kodak Min‐R screen we were able to predict both the number of light quanta emitted per absorbed x‐ray and MTF(f) to better than ±5%, and the scintillation efficiency to within 10% of experimentally measured values. The shape of the x‐ray quantum NPS is predicted to within ±5% for spatial frequencies less than about 6 mm‐1 and to within ±20% for higher frequencies.
A dose computation model for 241Am vaginal applicators including the source‐to‐source shielding effects
Authors: Nath Ravinder , Park Chang H. , King Christopher R. , Muench Philip
Published: March 2003

A dose computation model has been developed for the determination of dose distributions around vaginal plaque applicators containing encapsulated 241Am sources. Encapsulated sources of 241Am emit primarily 60‐keV photons which have a half‐value layer thickness of 1/8 mm of lead. This makes possible highly effective in vivo shielding of normal tissues at risk, by placing thin lead shields at appropriate places on the applicator. However, self‐absorption of photons in the source material itself is intense, requiring bulky sources of about 1 cm diameter. These sources also produce considerable source‐to‐source shielding which must be taken into account in dose calculations. Our dose computation model for a single source employs three‐dimensional integration of dose contributions from volume elements of the source including the effects of absorption and scattering of photons in the source material, titanium encapsulation, and water. An empirical correction to Berger's data on buildup factors of point, isotropic sources is made to account for the effects of anisotropic photon emission by cylindrical 241Am sources. The second part of our dose computation model takes into account source‐to‐source shielding effects on both primary and scattered photons for the vaginal plaque geometry. The results of the model have been verified for accuracy by comparisons with extensive dosimetry measurements using lithium fluoride thermoluminescent dosimeters.
Effects of gold and silver backings on the dose rate around an 125I seed
Authors: Cygler Joanna , Szanto Janos , Soubra Mazen , Rogers David W. O.
Published: March 2003

Measurements of the effect of either gold or silver backing on the dose rate around an 125I seed were performed using a Therados RFA7 dosimetry system and a small diode detector which was 2.5 mm in diameter and 0.06 mm thick. It was found that the presence of the gold or silver backing modifies the diode response on the side of the 125I seed away from the backing. The effect depends on the backing material and the distance from the seed. There is a small increase close to the gold backing but a decrease further away. This decrease at distances greater than 10 mm from the seed is uniformly 10%, the same as found when the seed is backed by air. There is an increase of up to 25% observed with silver backing the seed and this increase remains significant more than 30 mm from the seed. When the response increases, the results are hard to interpret quantitatively because of variations in the diode response per unit dose with photon energy and extreme sensitivity to geometric changes. Nonetheless, except for the increase at close distances with the gold, the results are in agreement with EGS4 Monte Carlo photon transport simulations which are for a simplified geometry and account for x‐ray fluorescence from the K‐shell. Furthermore, the increase in the gold‐backed case is qualitatively explained by Williamson's Monte Carlo calculations which take into account the L‐shell fluorescent x‐rays from gold.
Algorithm for dosimetry of multiarc linear‐accelerator stereotactic radiosurgery
Authors: Luxton Gary , Jozsef Gabor , Astrahan Melvin A.
Published: March 2003

Treatment planning for multiarc radiosurgery is an inherently complex three‐dimensional dosimetry problem. Characteristics of small‐field x‐ray beams suggest that major simplification of the dose computation algorithm is possible without significant loss of accuracy compared to calculations based on large‐field algorithms. The simplification makes it practical to efficiently implement accurate multiplanar dosimetry calculations on a desktop computer. An algorithm is described that is based on data from fixed‐beam tissue‐maximum‐ratio (TMR) and profile measurements at isocenter. The profile for each fixed beam is scaled geometrically according to distance from the x‐ray source. Beam broadening due to scatter is taken into account by a simple formula that interpolates the full width at half maximum (FWHM) between profiles at isocenter at different depths in phantom. TMR and profile data for two representative small‐field collimators (10‐ and 25‐mm projected diameter) were obtained by TLD and film measurements in a phantom. The accuracy of the calculational method and the associated computer program were verified by TLD and film measurements of noncoplanar multiarc irradiations from these collimators on a 4‐MV linear accelerator. Comparison of film measurements in two orthogonal planes showed close agreement with calculations in the shape of the dose distribution. Maximal separation of measured and calculated 90%, 80%, and 50% isodose curves was ≤0.5 mm for all planes and collimators. All TLD and film measurements of dose to isocenter agreed with calculations to within 2%.
A convolution algorithm for brachytherapy dose computations in heterogeneous geometries
Authors: Williamson Jeffrey F. , Baker Randall S. , Li Zuofeng
Published: March 2003

Currently‐available brachytherapy dose computation algorithms ignore heterogeneities such as tissue–air interfaces, shielded gynecological colpostats, and tissue‐composition variations in 125I implants despite dose computation errors as large as 40%. To calculate dose in the presence of tissue and applicator heterogeneities, a computer code has been developed that describes scatter dose as a 3‐D spatial integral which convolves primary photon fluence with a dose‐spread array. The dose‐spread array describes the distribution of dose due to multiple scattering about a single primary interaction site and is precomputed by the Monte Carlo method. To correct for heterogeneities traversed by the primary photons, the dose‐spread array is renormalized to reflect the density and composition of the element, and the distance to the point of interest is scaled by the pathlength of the intervening medium. Convolution calculations for 125I and 137Cs point sources in the presence of finite phantoms, air voids and high‐density shields have been compared to the corresponding Monte Carlo calculations. The convolution code absolute and relative dose rate predictions are shown to agree with Monte Carlo calculations within 3%. Direct evaluation of the 3‐D spatial convolution integral using 1‐D adaptive integration reveals efficiency gains of 20–50 relative to Monte Carlo photon‐transport calculations.
Improvement of precision in spatial localization of radio‐opaque markers using the two‐film technique
Authors: Lam Kwok L. , Ten Haken Randall K.
Published: March 2003

Radio‐opaque markers implanted inside or placed on the skin of patients can be used to detect set‐up errors and patient motion. The effects of imaging geometry accuracy for standard radiotherapy equipment on the precision of calculating the positions of radio‐opaque spherical markers using two orthogonal radiographic film projections is investigated. Inaccuracies in the imaging geometry are computed from the manually digitized positions of the marker images on each film pair. Actual marker locations are calculated with a precision limited only by the variance in manual digitization by incorporating those imaging geometry inaccuracies into their computation. Results of a phantom study using a grid of markers in a plastic block indicate that submillimeter precision can be obtained for the spatial coordinates of individual markers, and that the precision is not sensitive to the small inaccuracies in imaging geometry present within the mechanical tolerances of modern radiotherapy treatment machines and simulators.
The lens coupling efficiency in megavoltage imaging
Authors: Swindell William
Published: March 2003

In TV‐based megavoltage imaging systems it is useful to be able to estimate the geometrical efficiency g2 of the camera lens. It is shown that the appropriate expression is g2=(16n2)−1 ×[F(1+1/m)]−2, where n is the refractive index of the scintillator, F is the F‐number of the lens and m is the optical magnification. This expression yields estimates for g2 that are 5 to 10 times smaller than the estimates that have appeared previously in the literature.
Exact determination of the magnification factor for target lesions in a stereotactic frame
Authors: Wu Andrew , Flickinger J. C. , Kalend A. M. , Maitz A. H. , Lunsford L. D.
Published: March 2003

An exact formula used for determination of the magnification of the target using the conventional stereotactic biplane radiographic imaging has been derived for the stereotactic frame.
Characteristic parameters of 6–21 MeV electron beams from a 21 MeV linear accelerator
Authors: Al‐Ghazi M. S. A. L. , Lingman D. , Arjune B. , Gilbert L. D. , Thekkumthala J.
Published: March 2003

Dosimetry measurements have been carried out for the electron beams produced by a linear accelerator at energies 6, 8, 10, 14, 18, and 21 MeV. Characteristic parameters of the central axis dose distributions were derived and compared to corresponding values of electron beams from other accelerators in clinical use where such a comparison is appropriate. A comprehensive set of dosimetric parameters is provided for electron beam treatment planning. The data include central axis depth dose, range–energy parameters, beam penumbra and uniformity.
Noise, resolution, and sensitivity considerations in the design of a single‐slice emission–transmission computed tomographic system
Authors: Liew Soo Chin , Hasegawa Bruce H.
Published: March 2003

A prototype Emission–Transmission Computed Tomography (ETCT) system is being developed that will acquire single‐slice x‐ray transmission CT images simultaneously with single photon emission computed tomography (SPECT) images. This system will permit the correlation of anatomical information from x‐ray CT with functional information from SPECT images. The patient‐specific attenuation map derived from the x‐ray CT images can be used to perform attenuation correction of the SPECT images, so that accurate quantitative information can be obtained. The fan‐beam scanning geometry and the use of a segmented HPGe detector array impose special constraints on the design of the collimator for the system. Based on a signal detection model, an efficiency‐resolution figure of merit (ERFM) as a function of the collimator geometric efficiency, system resolution width, and object diameter is defined. The ERFM is proportional to the square of the detection signal‐to‐noise ratio. The collimator design parameters can then be optimized by optimizing the ERFM for an anticipated object diameter. The collimator point‐spread function, geometric efficiency, and resolution are calculated. The collimator optimized for the detection of a 1‐cm object will have a single‐slice point source efficiency of 1.2×10−4, and a FWHM of 6.5 mm at the center of the reconstruction circle, at 12 cm from the collimator face. The minimum object contrast which will give a detection SNR of 5 is 74%, for a total accumulated count per slice of 2×106.
Sensitivity of helium beam‐modulator design to uncertainties in biological data
Authors: Petti P. L. , Lyman J. T. , Castro J. R.
Published: March 2003

The goal in designing beam‐modulating devices for heavy charged‐particle therapy is to achieve uniform biological effects across the spread‐peak region of the beam. To accomplish this, the linear‐quadratic model for cell survival has been used to describe the biological response of the target cells to charged‐particle radiation. In this paper, the sensitivity of the beam‐modulator design in the high‐dose region to the values of the linear‐quadratic variables α and β has been investigated for a 215‐MeV/u helium beam, and implications for higher LET beams are discussed. The major conclusions of this work are that, for helium over the LET range of 2 to 16 keV/μ, uncertainties in measuring α and β for a given cell type which are of the order 20% or less have a negligible effect on the beam‐modulator design (i.e., on the slope of the spread Bragg peak); uncertainties less than or equal to 10% in the dose‐averaged LET at each depth are unimportant; and, if the linear‐quadratic variables for the tumor differ from those used in the beam‐modulator design by a constant factor between about 0.5 and 3, then the resultant nonuniformity in the photon‐equivalent dose delivered to the tumor is within ±2.5%. It is also shown that for any ion, if the nominal values of α or β used by the beam‐modulator design program differ from their actual values by a constant factor, then the maximum errors possible in the beam‐modulator design may be characterized by two limiting depth‐dose curves such that the ratio of the dose at the proximal end of the spread Bragg curve to the dose at the distal end of the spread peak is given by αdistal/αprox for the steepest curve, and (βdistal/βprox)1/2 for the flattest curve.
Measurement of accelerator bremsstrahlung spectra with a high‐efficiency Ge detector
Authors: Landry Danny J. , Anderson David W.
Published: March 2003

A Compton spectrometer with a high‐efficiency Ge detector was used to measure photon spectra from clinical accelerators. The response‐function matrix for the detector system was determined using ten radioactive sources that emitted gamma rays of single energy ranging from 279–1525 keV. The system was tested by measuring the spectrum from a 60Co teletherapy machine. The energy distribution of tungsten target bremsstrahlung produced by 2.2, 2.5, 15, 20, and 25 MeV electrons in two clinical accelerators were determined. Theoretical thin‐target calculations overestimate the number of photons in the high‐energy regions of the spectra, as was expected. However, theoretical thick‐target calculations underestimate the numbers of high‐energy photons.
Tissue segmentation for three‐dimensional display of human spines
Authors: Amartur S. C. , Wielopolski P. A. , Kormos D. W. , Modic M. T. , Clampitt M.
Published: March 2003

This paper presents a morphological tissue segmentation technique for the three‐dimensional visualization of the human spine. Magnetic resonance images of the human spine were obtained using motion‐compensated fast imaging with steady‐state free precession (FISP) pulse sequences. The images acquired with these sequences exhibit very high contrast between the bright cerebrospinal fluid (CSF) and the dark spinal cord and nerve roots. This phenomenon has enabled us to apply mathematical morphological processing techniques to extract the spinal nerve roots within the CSF space. The nerve roots were extracted from a set of slices and displayed in a three‐dimensional format.
Comparison of NaI(Tl), CdTe, and HgI2 surgical probes: Effect of scatter compensation on probe performance
Authors: Kwo D. P. , Barber H. B. , Barrett H. H. , Hickernell T. S. , Woolfenden J. M.
Published: March 2003

Spatial variation in the background source distribution makes tumor detection difficult for single‐detector probes. Using a single energy window that brackets the photopeak helps discriminate against background events dominated by Compton scattering. Another approach is to use the information provided by an additional window in the Compton region. The performances of NaI(Tl), CdTe, and HgI2 surgical probes have been compared under realistic simulations of a tumor‐staging procedure using optimal single‐sided energy windows and a two‐window scatter‐subtraction technique. Results showed that despite the differences in energy resolution of the detectors, the performances of the probes in a variable background were similar when optimal single energy windows were used. When the background variations were large, using information provided by a second window improved probe performance.
Geometrical aspects of scatter‐to‐primary ratio and primary dose
Authors: Nizin Paul S.
Published: March 2003

Using an approximation of Bjarngard and Petti [Phys. Med. Biol. 33, 21–32 (1988)] that the scatter‐to‐primary ratio SPR(r,d) is a linear function of the geometrical parameter z=rd/(r+d) (where r is the radius of photon beam and d is depth in phantom), a method was developed to determine primary dose directly from depth‐dose data, measured within the range of fields with lateral electronic equilibrium. This method was evaluated using tabulated TAR data for Co‐60 γ radiation in water. The results for primary dose agree within ∼1% of those from other studies. It was shown that this method is applicable within the energy range of therapeutic x rays. Primary dose was determined in the case of 6‐ and 18‐MV x rays using proposed method.
Comparison of NaI(Tl), CdTe, and HgI2 surgical probes: Physical characterization
Authors: Barber H. B. , Barrett H. H. , Hickernell T. S. , Kwo D. P. , Woolfenden J. M. , Entine G. , Ortale Baccash C.
Published: March 2003

The physical properties of three surgical probes containing different radiation detectors are compared: a NaI(Tl) scintillator with a flexible, fiber‐optic light guide, and two semiconductor detectors that operate at room temperature, CdTe and HgI2. Also compared are spectra, energy resolutions, and counting efficiencies measured at a variety of gamma‐ray energies between 30 and 1000 keV. The energy resolution of the NaI probe is substantially poorer than that of either semiconductor probe due in part to light losses in coupling the scintillator to the fiber optics. The semiconductor probes have complex spectral response due to charge‐carrier trapping and K x‐ray escape, and not all photoelectric interactions in these detectors contribute to the useful part of the photopeak. Above 120 keV the counting efficiency for the NaI probe is an order of magnitude higher than for the CdTe and HgI2 probes. Both energy resolution and counting efficiency are slightly better for the HgI2 probe than for the CdTe probe.
Energy constancy checking for electron beams using a wedge‐shaped solid phantom combined with a beam profile scanner
Authors: Rosenow Ulf F. , Islam Mohammad K. , Gaballa Hani , Rashid Harunor
Published: March 2003

An energy constancy checking method is presented which involves a specially designed wedge‐shaped solid phantom in combination with a multiple channel ionization chamber array known as the Thebes device. Once the phantom/beam scanner combination is set up, measurements for all electron energies can be made and evaluated without re‐entering the treatment room. This is also valid for the readjustment of beam energies which are found to deviate from required settings. The immediate presentation of the measurements is in the form of crossplots which resemble depth dose profiles. The evaluation of the measured data can be performed using a hand‐held calculator, but processing of the measured signals through a PC‐type computer is advisable. The method is insensitive to usual fluctuations in beam flatness. The sensitivity and reproducibility of the method are more than adequate. The method may also be used in modified form for photon beams.
NMR spin–lattice relaxation in tissues with high concentration of paramagnetic contrast media: Evaluation of water exchange rates in intact rat muscle
Authors: Sobol W. T. , Jackels S. C. , Cothran R. L. , Hinson W. H.
Published: March 2003

High concentrations of GdDTPA in extracellular water of tissue cause the intrinsic relaxation rate of water to become greater than the coupling rate between intracellular and extracellular water compartments. The fast exchange limit is no longer valid and distinctly nonexponential spin–lattice recovery is observed. T1 relaxation recovery was characterized by a double exponential curve when striated rat muscle was immersed in a highly concentrated (110 Mm) isotonic solution of GdDTPA. When the water exchange rate through cell membranes in intact muscle tissue was calculated (using a two‐compartment exchange model) and compared to similar data determined from T2 Carr–Purcell–Meiboom–Gill experiments, a significant discrepancy was found.
On the calibration of plane‐parallel ion chambers using 60Co beams
Authors: Andreo P. , Lindborg L. , Medin J.
Published: March 2003

Monte Carlo simulations of the response of cylindrical ionization chambers situated at 5 cm depth in a cubic water phantom are performed for electron and photon down to 10 keV. (AIP)
The effects of a universal wedge and beam obliquity upon the central axis dose buildup for 6‐MV x rays
Authors: McParland Brian J.
Published: March 2003

In a number of clinical situations, the dose to the skin and the superficial tissues is of concern. Both beam obliquity and a beam modifier will modify the dose delivered to these regions due to changes in the scattering geometry, scattered photon and secondary electron production, and changes in the energy spectrum of a polyenergetic beam. Some linear accelerators use a single universal wedge mounted within the treatment head. Because such a wedge is at an extended distance from the patient, its contribution to the beam contaminants incident to the skin will be limited. Measurements of the ionization in the buildup region have been performed in a polystyrene phantom irradiated with a 6‐MV x‐ray beam from a linear accelerator equipped with a universal wedge. The variation of the buildup dose with obliquity, universal wedging, and distance from the source has been measured for angles of incidence between 0° and 60° and for effective wedge angles between 0° and 45°. The results indicate that the percentage buildup has a much stronger dependence upon the angle of incidence than upon the effective wedge angle. For distances approaching the treatment head, it is shown that the universal wedge generates secondary electrons that elevate the surface dose, but that this contribution decreases with distance.