bluuebunny/crossref_metadata_2025 huggingface dataset:
An automatic seed identification technique for interstitial implants using three isocentric radiographs
Authors: Rosenthal M. S. , Nath Ravinder
Published: August 2003

A technique for the automatic reconstruction of the spatial coordinates of seeds in an interstitial implant has been developed. Seed coordinates from three isocentric radiographs, an anterior–posterior film, and an orthogonal pair of films taken at gantry angles of ±45° from the anterior–posterior direction are digitized and recorded in a random sequence. From this, the three‐dimensional coordinates of the seeds in the implant are computed by matching the values of common coordinates along the axis of rotation. This rotate and match technique leads to an accurate and consistent identification and reconstruction of seeds with 90% of them within 2 mm of their actual location. These new computer routines have been added to an existing treatment planning system (AECL TP‐11 treatment planning system, version 5A). We present our algorithm and technique along with clinical examples.
Nonstandard deviations
Authors: Goitein Michael
Published: August 2003

An estimate of uncertainty without a statement as to whether the estimate is a standard deviation or related quantity or an indication of the associated confidence level is meaningless. Nevertheless, the necessary qualification is often omitted. While such omissions are probably most often inadvertent, they may occur because the conventional single standard deviation of a normal distribution is inappropriate. Some unusual uncertainty intervals are therefore suggested. These include noninteger multiples of the standard deviation such as 1.5 standard deviations, asymmetric error bounds, and identification of the possibility of long tails or truncated bounds to the probability density function.
Beam scanning for heavy charged particle radiotherapy
Authors: Goitein Michael , Chen George T. Y.
Published: August 2003

An important aspect of heavy charged particle radiotherapy is its ability to localize dose to the target volume. Current techniques generally employ beam delivery schemes which use range modulated beams in which the Bragg peak is spread out over a range of depths. The range modulation is constant over the entire beam cross section. This paper examines the potential gain from the use of variable range modulation over the beam cross section which can be achieved through beam scanning. An analytic technique has been developed to calculate a gain factor, related to integral dose, for a number of different charged particle beams. The dependence of the gain factor on tumor depth, diameter, and size has been examined. These analytic results have been compared with computer calculations of treatment plans simulating beam scanning and fixed modulation delivery techniques for actual clinical cases. The rationale for beam scanning, and its potential impact on tumor control is discussed.
Prospectively gated cardiac computed tomography
Authors: Moore S. C. , Judy P. F. , Garnic J. D. , Kambic G. X. , Bonk F. , Cochran G. , Margosian P. , McCroskey W. , Foote F.
Published: August 2003

A fourth‐generation scanner has been modified to perform prospectively gated cardiac computed tomography (CT). A computer program monitors the electrocardiogram (ECG) and predicts when to initiate the next scan in a gated series in order to acquire all projection data for a desired phase of the heart cycle. The system has been tested with dogs and has produced cross‐sectional images of all phases of the cardiac cycle. Eight to ten scans per series were sufficient to obtain reproducible images of each transverse section in the end‐diastolic and end‐systolic phases. The radiation dose to the skin was ∼1.4 cGy per scan. The prospectively gated system is more than twice as efficient as a retrospectively gated system in obtaining complete angular projection data for a 10% heart cycle window. A temporal smoothing technique to suppress reconstruction artifacts due to sorting inconsistent projection data was developed and evaluated. Image noise was reduced by averaging together any overlapping projection data. Prospectively gated cardiac CT has also been used to demonstrate that the error in attenuation measured with a single nongated CT scan through the heart can be as large as 50–60 CT numbers outside the heart in the lung field.
A comparison of high‐energy accelerator depth dose data
Authors: Gastorf Robert J. , Hanson William F. , Kirby Thomas H. , Shalek Robert J. , Berkley Lawrence W. , Chu Connel
Published: August 2003

Accurate depth dose information is necessary for the use of high‐energy radiotherapy photon beam units. It would be useful, therefore, to have one set of published data available for each different type unit manufactured to which physicists can compare their measured data. Pertinent questions are raised regarding the similarity between accelerators and their central axis depth dose characteristics, the availability of adequate published central axis depth dose data, and the minimum amount of data needed to determine the applicability of published data to a particular machine. Data taken by the Radiological Physics Center (RPC) for 4–10 MV units are analyzed and compared with published data in an attempt to answer these questions.
Determination of the half‐life of I‐125 encapsulated clinical seeds using a Si(Li) detector
Authors: Kubo Hideo
Published: August 2003

The half‐life of two I‐125 encapsulated clinical seeds was measured with a Si(Li) semiconductor detector. Two measurements of each seed yielded an average half‐life of 59.56±0.17 days. The results are discussed in terms of the influence of possible I‐126 contamination and leakage of I‐125 activity through encapsulation. The present results are compared with a frequently cited value of 60.25±0.06 days. The difference between the two values was found to be statistically significant at a 95% confidence level.
A note on the measurement of focal spot sizes utilizing star test patterns at low magnification factors
Authors: Bakus Abnoeal D. , Lin Pei‐Jan Paul
Published: August 2003

The validity of focal spot size measurement utilizing the star test pattern at magnification factors less than 1.4× has been confirmed by the use of a 0.4° vertex angle star test pattern and a traveling microscope. It has been found that the apparent nonlinearity of the star test pattern method in the measurement of focal spot sizes at low magnification factors is introduced by the observers as a systematic error due to the difficulty in determining the blur diameter within reasonable accuracy.
Thermal energy deposition from a single‐loop rf whole‐body applicator
Authors: Zwicker R. D. , Sternick E. S.
Published: August 2003

Whole‐body hyperthermia at moderate temperatures (40.0±0.5 °C) is currently being investigated to assess its effectiveness as an adjuvant to other cancer therapies. At our institution, the whole‐body heating is achieved by a combination of hot air circulation and rf inductive heating at 27.1 MHz. This paper reports the results of a study of the thermal energy deposition pattern associated with the rf applicator. The applicator consists of a single loop of nearly elliptical shape with dimensions 20×46 cm. The loop is embedded in a mattress and is fed by lead wires entering the side of the mattress. Temperature mapping in a heated gelatin phantom placed just above the rf loop showed maximum heating near the lead wires and a cool region above the center of the loop. Calculations of the magnetically induced and capacitive contributions to the electric field near the applicator indicate that the heating near the lead wires is largely capacitive in nature. This feature of the rf heating is of interest because capacitive fields result in preferential heating of fatty tissues. Further calculations showed that the capacitive heating contribution falls away much more rapidly than the induced field contribution with increasing distance from the plane of the rf loop.
Coherent scatter in diagnostic radiology
Authors: Johns P. C. , Yaffe M. J.
Published: August 2003

Coherent scatter is often ignored in diagnostic radiology because its cross section is relatively small, and because it is assumed to be indistinguishable from primary radiation. Single‐scatter calculations, however, show that coherently scattered photons diverge sufficiently from the primary ray to degrade image contrast, and that they account for a significant fraction of the total scattered energy fluence at the image receptor. Grids and large air gaps are less effective in reducing coherent single scatter than incoherent and multiple scatter. For radiography of the abdomen, coherent first scatter comprises 10% of total scatter and 26% of the primary fluence before a grid, and on the order of 22% and 7.5%, respectively, behind a grid. Coherent first scatter comprises a higher fraction of the total amount of scatter for lower energy examinations such as mammography.
A protocol for the determination of absorbed dose from high‐energy photon and electron beams
Authors:   Task Group 21, Radiation Therapy Committee, AAPM
Published: August 2003

A protocol is given to provide radiological physicists with an accurate means of measuring radiation doses on phantoms. Plastics, as well as water, are considered as phantoms. A new quantity, the cavity‐gas calibration factor, is introduced.(AIP)
Selective iodine imaging using lanthanum K fluorescence
Authors: Rutt Brian K. , Cunningham Ian A. , Fenster Aaron
Published: August 2003

Time‐dependent subtraction techniques have tremendously improved the visualization of iodinated vessels. These techniques suffer from motion artifacts, however, since the mask image is acquired several seconds prior to the contrast image. A K‐edge subtraction technique for selectively imaging iodine using the characteristic x rays from a lanthanum secondary target is presented. The Kα lines of lanthanum closely straddle the K edge of iodine. A prepatient iodine filter will alter the ratio of Kα1 to Kα2 intensities; by subtracting two images made with different Kα1/Kα2ratios, iodinated structures can be isolated. Since the method requires no mask image, motion artifacts are reduced. Preliminary images acquired with an image intensifier/photodiode array detector system are shown. Theoretical performance of this method is compared to other iodine‐selective imaging techniques on the basis of exposure and dose. The feasibility of using lanthanum K fluorescence for rapid clinical iodine‐selective imaging is discussed.
The effects of motion on quantitative vessel measurements
Authors: Parker D. L. , Clayton P. D. , Gustafson D. E.
Published: August 2003

Optimal visualization of moving structures such as the heart and coronary arteries using digital radiographic imaging systems is a difficult problem that can involve tradeoffs between temporal, spatial, and density resolution. The motion dependence of four angiographic measures of vessel dimensions is given and demonstrated experimentally. Although the densitometric cross‐sectional area is shown to be independent of motion, densitometric thickness (contrast) decreases and the apparent width (distance between edges) and densitometric width both increase with motion. Knowledge of the velocity allows the apparent width but not the densitometric width to be corrected. It is also shown that current limitations which result in tradeoffs between kVp and x‐ray pulse duration seriously compromise the advantages of using small focal spots in coronary artery imaging.
An MTF method immune to aliasing
Authors: Droege Ronald T. , Rzeszotarski Mark S.
Published: August 2003

The variance derived from the images of cyclic bar patterns can be used to determine the modulation transfer function (MTF) of an imaging system. Unlike most MTF methods, it is applicable even when the imaging system undersamples the test object and generates aliasing errors. The validity of the variance method in the presence of aliasing is established theoretically and by computer simulation.
Calculated buildup curves for photons with energies up to 60Co
Authors: Rogers D. W. O. , Bielajew A. F.
Published: August 2003

This report investigates the buildup region of the depth‐dose curve for broad parallel beams of 60Co and lower energy photons incident on slabs of tissue as defined by the International Commission on Radiation Units and Measurements (ICRU) and other materials. The EGS Monte Carlo system was used to simulate the transport of photons and electrons in the materials. Calculated buildup regions of the depth‐dose curves are presented for photons incident on ICRU tissue. For 60Co beams, the primary and scatter dose components are presented, both for the buildup region and at depth in the phantom. Effects of beam radius were investigated and found to have only a small effect in the buildup region of the central axis depth‐dose curve. The calculated effects of beam radius are larger at depth and in good agreement with experiment. Effects of scatter and attenuation in a small miniphantom of water were investigated as a function of photon energy and were found to cancel within 0.8%. The mean distance of electron transport was calculated for 60Co beams incident on carbon, water, polystyrene, aluminum, and lead.
Neutron energies in medical electron accelerator rooms
Authors: LaRiviere Philip D.
Published: August 2003

Two experimental methods are described for obtaining average energies of neutron spectra produced by an electron accelerator in a medical treatment room. In the first method, the dose equivalent is measured by means of indium activation foils in an Andersson–Braun moderator. Fluences are determined by the usual activation foil techniques. These data lead to the conversion factor, neutrons/(cm2 s) per mrem/h, which uniquely defines the effective energy of the unknown spectrum. In the second method, the fast neutron broad‐beam attenuation in polyethylene is obtained from measurement of the thermalized fluence versus depth curve, whose slope in the exponential region parallels that of the fast neutron spectrum. It was found for a 24‐MV linear accelerator that the effective energy varied from 0.28 MeV at the isocenter under closed collimators to 0.016–0.024 MeV in the entrance maze. These results compare favorably with other measurements and calculations. It is concluded that the dose equivalent can be measured to within ±50% with an activation foil in an Andersson–Braun moderator and that the characteristic energy obtained by attenuation at the isocenter closely approximates the average energy of the neutron spectrum escaping the treatment head.
Dose measurements and calculations of small radiation fields for 9‐MV x rays
Authors: Arcovito G. , Piermattei A. , D'Abramo G. , Bassi F. Andreasi
Published: August 2003

Measurements of dose distribution for square fields with sizes ranging from 1×1 to 30×30 cm for a 9‐MV x‐ray beam from a Neptune 10 linear accelerator, manufactured by CGR, are reported. Special attention was paid to field sizes smaller than 4×4 cm, used in radiosurgery techniques. To express the dose‐monitor units relationship, total, collimator, and phantom scatter correction factors were obtained by experimental measurements. A strong dependence of these factors on the smallest field sizes (<4×4 cm) was shown. Measurements of the maximum depth dose dmax, plotted as a function of field size, showed a maximum at about 5×5 cm, in good agreement with previous results. dmax was also measured for the smallest fields, demonstrating that the contaminating electron component of the x‐ray beam was not responsible for the dmax shift. Analysis of the penumbra width of cross dose distributions, as a function of field sizes, allowed us to postulate that the dmax shift could be due to the phantom scattered photons, which in turn were generated by the collimator scattered photons. Newly derived tissue‐maximum ratio and scatter‐maximum ratio data were used for dose profile calculations of 2×2, 4×4, and 10×10 cm field sizes. The agreement between experimental and calculated data was found to be ±2% within the geometrical edges of the fields and ±6% outside of them. A dose profile from the isocenter of a 2×2 cm square field moving through a 360° rotation arc was obtained and compared with that from the center of a 125I shielded source, as measured by Ling. Advantages and problems relating to the use of x‐ray beams from linear accelerators in radiosurgery are discussed.
Replacement correction factors for photon and electron dose measurements
Authors: Bjärngard Bengt E. , Kase Kenneth R.
Published: August 2003

A “gradient correction factor” and an “electron fluence correction factor” are used in the AAPM Task Group 21 dosimetry protocol, the product of the two being the “replacement correction factor.” The separation of two physical processes, implied by the concepts, is theoretically unfounded. The proof of this statement rests on Fano's theorem, the conditions for which constitute a case without variations in fluence or its directional and energy distributions. Deviations from these conditions represent gradients in the radiation field, affect the response of a measurement cavity, and do so by perturbing the fluence of electrons that deposit energy in the cavity. Thus, in beams of both electrons and photons, corrections are required when fluence variations exist in the vicinity of the cavity. The rationale for an electron fluence correction based on “in‐scattering” and “obliquity” is weak, since the effects only occur when gradients are present. The choice of values for the two correction factors at the depth of maximum dose are particularly inconsistent with the actual characteristics of the radiation field.
Photodynamic and photohyperthermic response of malignant tumors
Authors: Svaasand Lars O.
Published: August 2003

This work presents an analytical dosimetry model for photodynamic therapy of malignant tumors. A hyperthermic contribution originating from the absorption of optical irradiation in the tissue may be equally taken into account. The model has been used to analyze tumor response during photodynamic therapy of an in vivo murine tumor. This tumor model was C3H/Tif mammary adenocarcinoma in C3D2F1/Bom mouse strain.
Determination of electron beam output factors for a 20‐MeV linear accelerator
Authors: Mills Michael D. , Hogstrom Kenneth R. , Fields Robert S.
Published: August 2003

The demands of a busy clinic require that basic machine calculations be performed as accurately, rapidly, and simply as possible. For the electron beam of the Therac 20 Saturne linear accelerator, a method suitable for a programmable calculator is described to predict the dose output from the measurement of selected fields. One‐dimensional output factors were measured; these are defined as output factors of rectangular fields where one side is always equal to the side of the square reference field. The output of an arbitrary rectangular field X, Y is given by the product of the output factors OF(X,Y)=OF(X,10)×OF(10,Y), where 10 is the side of the square reference field. The measured data indicate that the output of very large rectangular and square fields is underestimated using this method for the lower energies. A correction factor of the form CF=C×[(X−10)(Y−10)/‖(X−10)(Y−10)‖1/2] results in agreement with measured data to within 1.5% for all energies.
Analytic evaluation of depths of dose calculation points for external beam radiation therapy treatment planning
Authors: Starkschall George
Published: August 2003

A method has been developed to calculate the depth of a point in a patient contour analytically. In this method, the patient contour is approximated by a Fourier series comprising 5–10 terms. The depth of a calculation point is the distance between the calculation point and the intersection of the ray line with the contour line. Depth calculations using this method are compared with calculations using a more conventional method in which the intersection is found by representing the contour as a set of straight‐line segments between pairs of contour points. This method gives depth values comparable in accuracy to the more conventional method and computed approximately twice as fast.
Energy and angular distributions of photons from medical linear accelerators
Authors: Mohan Radhe , Chui Chen , Lidofsky Leon
Published: August 2003

For accurate three‐dimensional treatment planning, new models of dose calculations are being developed which require the knowledge of the energy spectra and angular distributions of the photons incident on the surface of the patient. Knowledge of the spectra is also useful in other applications, including the design of filters and beam modifying devices and determination of factors to convert ionization chamber measurements to dose. We have used Monte Carlo code (egs) to compute photon spectra for a number of different linear accelerators. Both the target and the flattening filter have been accurately modeled. We find the mean photon energy to have a value lower than the generally perceived value of one‐third the maximum energy. As expected, the spectra become softer as the distance from the central axis increases. Verification of the spectra is performed by computing dose distributions and half‐value layers in water using the calculated spectra and comparing the results with measured data. We also examined the angular distributions of photons incident on the surface of the phantom. In currently used models of dose computations, it is assumed that the angular distribution of photons with respect to fan lines emanating from the source is negligible. Although the angular spread of photons with respect to the incident direction has been found to be small, its contribution to the diffuseness of the beam boundaries is significant.
Variations in response to radiation of a nylon‐walled ionization chamber induced by humidity changes
Authors: Mijnheer B. J.
Published: August 2003

The volume of nylon‐walled ionization chambers varies with relative air humidity due to the hygroscopic properties of nylon. A 6% increase in exposure calibration factor of a commonly employed 0.6 cm3 nylon‐walled ionization chamber was observed when the relative air humidity decreased from 98% to 11%. The increase as well as the decrease in volume shows an initial fast change during the first day followed by an exponential change with half‐lives of about 1.2 days and 2.4 days, for water uptake and water loss, respectively.
Compensating filter design using radiographic stereo shift information
Authors: Gerbi Bruce J.
Published: August 2003

This paper describes a method of designing 3‐dimensional compensating filters for radiation therapy using photon beams. A radiopaque grid is placed on the patient surface and stereo shift radiographs are taken of the treatment area. With the aid of a computer, tissue deficit information is calculated. Isothickness lines are plotted for the different missing tissue thicknesses and lead sheets with proper magnification are cut from these plots and assembled into the final tissue compensator.
Quantitative evaluation of left ventricular function using computed tomography
Authors: Guthaner Diana F. , Nassi Menahem , Bradley Beverly , Gould Elizabeth B. , Mai Christopher H. , Schmidt Karen E.
Published: August 2003

Computed tomography (CT) provides a noninvasive technique with high resolution cross‐sectional tomographic images which allow volume measurements of an object, independent of its geometric configuration. A phantom of known volume with controllable periodic motion was used to validate the CT method of volume determination. A good correlation (P<0.05) was achieved. Missing angle reconstruction algorithms for gating were applied to estimate left ventricular volumes and ejection fraction in an experimental animal, and the results compared with a standard angiographic method. Left ventricular volumes correlated poorly, whereas the ejection fraction obtained correlated well (r=0.9). The discrepancies may be attributed in part to the CT method in which difficulties were recognized in defining the left ventricular borders at the base of the heart and partial volume effect, and in part to inaccuracies in the standard angiographic method. Once validated, this method has been applied to the animal model in the form of a pilot study.
Electron beam central axis depth dose measurements
Authors: Kirby Thomas H. , Gastorf Robert J. , Hanson William F. , Berkley Lawrence W. , Gagnon William F. , Hazle John D. , Shalek Robert J.
Published: August 2003

Central axis depth dose measurements were made by the Radiological Physics Center on over 70 electron‐producing machines used in radiation therapy. These data were consistent for each machine model and nominal energy. However, the data show that depth dose relations can vary significantly among different machine models for electron beams having the same nominal energy. Analysis shows that both the method used to achieve beam flatness and the mean incident electron energy determine the central axis depth dose curve past the depth of maximum dose. A linear relation of depth dose versus mean incident electron energy is used to predict depth dose to within 2 mm for most electron beams used clinically at depths greater than d95.
Can the AAPM Task Group 21 protocol lead to optimum ion chamber designs?
Authors: Jayaraman S. , Rozenfeld M. , Lanzl L. H. , Chung‐Bin A.
Published: August 2003

The recently published AAPM Task Group 21 protocol for high‐energy dosimetry is complicated in that it requires the physicist to obtain the values of about a dozen different physical variables by looking them up in tables or graphs. This should be compared with the procedure of earlier protocols using the concept of a single multiplier Cλ. We have investigated how the physical principles outlined in the improved AAPM protocol could be utilized for the redesign of the therapy‐level ion chambers in such a way that one can reduce the number of factors that need to be looked up in tables or graphs for the calibration of high‐energy teletherapy photon beams. In our analysis presented in this paper we found that one such design could be for an ion chamber having a wall of acrylic or Bakelite of a thickness not exceeding 0.1 g/cm2 and having an inner diameter of 6 mm, and used in conjunction with a cobalt‐60 buildup cap of thickness 0.35 g/cm2 made of acrylic, Bakelite, or Tufnol. If a chamber of such a design is used in a water phantom, the dosimetry practically reduces to the simplicity of the former protocols of depending on a single value of energy‐dependent multiplier to be obtained from a table. With the above design parameters, it becomes possible to eliminate the explicit need to incorporate the factors Pwall, Prepl, Awall, βwall, and the variable α, representing the fraction of ionization due to electrons from the wall material of the chamber.
A new digital tomosynthesis method with less artifacts for angiography
Authors: Haaker Paul , Klotz Erhard , Koppe Reiner , Linde Rolf , Möller Holger
Published: August 2003

A new nonlinear reconstruction method for tomosynthesis is described. This method is suited for “dilute” objects, i.e., objects in which most of the voxels have negligibly small absorption. Images of blood vessels filled with contrast material approximate this condition if the background is subtracted. The technique has been tested experimentally using a wire phantom and a prepared human heart. The results show significantly less artifacts than the well‐known backprojection. It is possible to get diagnostic image quality with a few projections. The reconstruction algorithm can be realized with dedicated real‐time hardware.
A simplified approach for modulation transfer function determinations in computed tomography
Authors: Nickoloff Edward L. , Riley Richard
Published: August 2003

In order to determine the modulation transfer functions (MTF's) for x‐ray computed tomography (CT) scanners, a measurement must be performed to obtain either the point spread function (PSF) or the line spread function (LSF). Thereafter, the usual procedure is to interpolate between the measured points and to determine the Fourier transforms numerically in order to obtain the MTF. Since this must usually be done many times to evaluate various reconstruction kernels and scan modalities, the process is tedious. Fortunately, it can be greatly simplified by utilizing a mathematical function to describe the PSF or LSF. Measured data for five CT scanners indicates that the PSF can usually be described by a Gaussian function. Hence, the MTF can be written in a generalized form eliminating the necessity of performing Fourier transformations each time. The MTF is determined directly from a single performance characteristic related to the full width at half maximum. The accuracy of the approach is compared with detailed MTF calculations for five CT scanners and it is shown to agree favorably with this data.
Investigation of basic imaging properties in digital radiography. 3. Effect of pixel size on SNR and threshold contrast
Authors: Giger Maryellen Lissak , Doi Kunio
Published: August 2003

The effect of pixel size on the signal‐to‐noise ratio (SNR) and threshold detection of low‐contrast radiologic patterns was investigated theoretically for digital radiographic systems. The SNR based on the perceived statistical decision theory model, together with the internal noise of the human eye–brain system, was calculated by using two‐dimensional displayed digital signal spectra and noise Wiener spectra. Threshold contrasts were predicted from the calculated SNR for various combinations of object size and shape, pixel size, resolution, and noise. Predicted threshold contrasts agreed well with those determined experimentally in an observer performance study. The threshold contrast of small objects increased substantially as the pixel size increased beyond 0.2 mm. For pixel sizes of 0.1 and 0.2 mm, however, the threshold contrasts were similar. Since a digital system is not shift invariant, a range of threshold contrast results for a small object and a large pixel, depending on the alignment of the object position relative to the sampling coordinates.
Physical measurements of heavy metal filter performance
Authors: Burgess A. E.
Published: August 2003

Contrast, relative tube loading, and relative entrance exposure have been measured for a number of heavy metal filters (Gd, Ho, Yb, and W) at several filter thicknesses, tube voltages (70–100 kV), and phantom thicknesses (1.5–2.5 cm of aluminum). The rare‐earth filters consistently gave higher contrast (relative to water) for air, CaCl2, iodine, and oil than the standard 2 mm of added aluminum, at a given tube voltage. It was found that the best filter choices gave a constant product of relative exposure and relative tube loading.
Scatter integration with right triangular fields
Authors: Siddon Robert L. , Dewyngaert J. Keith , Bjärngard Bengt E.
Published: August 2003

The concept of the equivalent field is used extensively in radiotherapy dose calculation algorithms. The rationale for using equivalent fields is to allow dose calculations for a wide variety of field shapes, while maintaining dose calculational data for only a few, very regularly shaped fields. A common example is the table of equivalent squares of rectangular fields presented by Day in the British Journal of Radiology. Recently, in searching for fast dose calculation algorithms for irregular fields, we introduced the concept of the equivalent square of a right triangular field. It is shown that an arbitrary irregular field of N vertices may be decomposed into 2N right triangular fields, each with a precalculated equivalent square. The scatter at the point of calculation due to the irregular field is then obtained as a sum of the scatter contributions from the equivalent squares. The scatter integration with right triangles is compared with scatter integration using program irreg.
A method for matching NBS x‐ray beam qualities with a half‐ or full‐wave rectified generator
Authors: Goetsch S. J. , Kamande J. , Attix F. H.
Published: August 2003

A method has been devised to enable users of half‐ and full‐wave rectified x‐ray sets to match the revised National Bureau of Standards (NBS) x‐ray beam qualities obtained with a constant potential generator. The method provides a rapid technique for determining the added filter necessary to attain the same first half‐value layers and nearly the same homogeneity coefficients as NBS. Beam qualities from 100 to 250 kVcp have been duplicated and used to perform blind calibrations at four beam qualities on each of two ion chambers owned by the National Bureau of Standards.
Fast calculation of the exact radiological path for a three‐dimensional CT array
Authors: Siddon Robert L.
Published: August 2003

Ready availability has prompted the use of computed tomography (CT) data in various applications in radiation therapy. For example, some radiation treatment planning systems now utilize CT data in heterogeneous dose calculation algorithms. In radiotherapy imaging applications, CT data are projected onto specified planes, thus producing “radiographs,” which are compared with simulator radiographs to assist in proper patient positioning and delineation of target volumes. All these applications share the common geometric problem of evaluating the radiological path through the CT array. Due to the complexity of the three‐dimensional geometry and the enormous amount of CT data, the exact evaluation of the radiological path has proven to be a time consuming and difficult problem. This paper identifies the inefficient aspect of the traditional exact evaluation of the radiological path as that of treating the CT data as individual voxels. Rather than individual voxels, a new exact algorithm is presented that considers the CT data as consisting of the intersection volumes of three orthogonal sets of equally spaced, parallel planes. For a three‐dimensional CT array of N3 voxels, the new exact algorithm scales with 3N, the number of planes, rather than N3, the number of voxels. Coded in fortran‐77 on a VAX 11/780 with a floating point option, the algorithm requires approximately 5 ms to calculate an average radiological path in a 1003 voxel array.
Direct measurement of electron contamination in cobalt beams using a charge detector
Authors: Galbraith Duncan M. , Rawlinson J. Alan
Published: August 2003

A method is described in some detail for measuring the magnitude and penetration of the electron contamination in photon beams using a pancake charge detector. It is shown that the response of the detector to a photon beam can be separated from the component due to the electron contamination. In the present work, the detector is used to measure the electron fluence in a 60Co photon beam. This fluence is subsequently converted to dose by comparison with the fluence and dose measured from a pure electron beam (90Sr). This study proves, within experimental error, that the observed changes in the buildup region, with the collimator opening for both filtered and unfiltered 60Co beams, are due to electron, rather than photon, contamination.
Removal of image intensifier veiling glare by mathematical deconvolution techniques
Authors: Seibert J. A. , Nalcioglu O. , Roeck W.
Published: August 2003

X‐ray images acquired with an image intensifier detector system suffer from veiling glare, a low‐frequency degradation described by a point spread function (PSF). The PSF has two experimentally determined parameters unique to a given image intensifier. This information is utilized to deconvolve the degradation from digitally acquired images. Results demonstrate a significant increase in contrast ratio of high‐contrast objects after deconvolution and image restoration.
Neutron sources in a 24‐MV medical linear accelerator
Authors: LaRiviere Philip D.
Published: August 2003

During the assembly of the prototype Clinac 2500, a unique opportunity arose to measure the cumulative total neutron yield of the machine as the major subassemblies were added, section by section. Differentiation of the results led to an inventory of the relative strength of the several neutron sources identified. The method utilized the fact that the scattered neutron fluence in a cavity in concrete is nearly uniform throughout the space and proportional to the fast neutron source strength. When normalized to the neutron output of the guide with bend magnet vacuum chamber, the relative neutron source strengths found were target, 1.9; primary collimator, 2.4; steel filter, 0.6; tungsten filter, 1.5; and jaw system, 0.8 open and approximately 2.0 closed. There is reasonable agreement between these measurements and published estimates of the yields from the major components of the treatment head.
Calculation of radiation doses for nonuniformly distributed β and γ radionuclides in soft tissue
Authors: Kwok Cheuk S. , Prestwich William V. , Wilson Brian C.
Published: August 2003

In cancer radioimmunotherapy nonuniform distributions of radionuclides in geometrically irregular tumor volumes will be encountered. If the tumor is surrounded by a homogeneous layer of soft tissue, the radiation dose distribution in the tumor due to a beta‐emitting nuclide can be calculated by convolution of the appropriate Loevinger's point source dose function, J, with the nuclide concentration distribution. Computer programs using fast Fourier transform techniques have been developed to evaluate the three‐dimensional spatial convolution accurately and efficiently. Equivalent techniques have also been applied to gamma‐emitting nuclides by replacing J in the convolution by the photon point isotropic specific absorbed fraction. Illustractive results are presented for the radiation dose distribution in a 1‐ml target volume with activity distribution which is (i) uniform, (ii) limited by diffusion into the target volume, or (iii) limited by diffusion and shedding of radionuclide in the peripheral layers of the target volume. Calculations for the three cases are presented for 32P,131I, and 15‐, 30‐, and 100‐keV photon emitters.
Photonuclear target systems for producing clinically useful quantities of 11C using an electron linear accelerator
Authors: Piltingsrud Harley V. , Robbins Philip J.
Published: August 2003

Described in this paper are what we believe to be the first practical photonuclear target systems for production of 11C containing CO and CO2 using bremsstrahlung produced from an electron linear accelerator similar to certain radiotherapy accelerators. This is a continuation of work reported earlier concerning a similar target system presently being used for production of 15O–O2. The 11C producing systems utilized liquid carbon dioxide, liquid cyclohexane, and liquid glacial acetic acid target materials. The carbon dioxide and glacial acetic acid target materials produced principally a 11C–CO product material. The cyclohexane target material produced a 11C–hydrocarbon product which was then oxidized to CO2. Target activity yields for these systems, normalized to a 20‐cm‐long by 10‐cm‐diam target chamber irradiated in a bremsstrahlung field produced by a 26‐MeV, 100‐μA electron beam, were 1.9×108 Bq (5 mCi) at 7.4×108 Bq g−1 (20 mCi g−1) for carbon dioxide, 1.4×108 Bq (3.8 mCi) at 3.7×1010 Bq g−1 (1 Ci g−1) for cyclohexane, and 7.4×108 Bq (20 mCi) at 3.7×1010 Bq g−1 (1 Ci g−1) for glacial acetic acid.
Software reliability and algorithm validation for medical imaging: Performance of common edge detection methods in nuclear medicine
Authors: Cahill Patrick T. , Knowles R. James R.
Published: August 2003

Ten common edge detection algorithms plus the NNA (nearest‐neighbor algorithm) developed by the authors were evaluated under the conditions of low ID (information density) and low spatial resolution commonly found in nuclear images. Both phantoms and clinical images were used, and isolated regions as well as overlapping regions with variable backgrounds were analyzed at various ID's. Threshold criteria were also adaptively varied as a function of local ID. Performance was quantitated in terms of (a) accuracy of area determinations, (b) receiver operating characteristic operating points, and (c) shape preservation. With adaptive thresholding at high ID's, several of the methods performed well on isolated regions and adequately on overlapping regions, but only the NNA performed consistently at low ID's as well as at high ID's.
Calculation of the uncertainty in the dose delivered during radiation therapy
Authors: Goitein Michael
Published: August 2003

There is, inevitably, uncertainty in our knowledge of the dose at any point within an irradiated patient. A technique is presented for estimating this uncertainty by performing three parallel calculations, one using nominal values and the others extreme values of the parameters upon which the dose depends. Such calculations can be made with almost any algorithm for calculating dose. They result in an estimate, at some specified confidence level which is determined by the data used, of the range of dose likely at any point. Such calculations should help therapists to avert over‐ or underdosage which might not be evident in conventional calculations of the nominal dose.
Characteristics of the photon beam from a new 25‐MV linear accelerator
Authors: Aldrich John E. , Andrew John W. , Michaels Howard B. , O'Brien Peter F.
Published: August 2003

The Therac 25 is a relatively compact therapy machine, the heart of which is a double‐pass electron linear accelerator. The electron beam is injected into the accelerator at the treatment head end of the machine and is accelerated back down the arm to an energy of 13 MeV. At this end of the machine a magnet system reflects the beam back into the structure where it gains up to an additional 12 MeV of energy. After leaving the linear accelerator the beam is bent by an achromatic head magnet through 270° to the treatment head. The machine produces eight electron beams and a 25‐MV photon beam. In this work only the parameters of the photon beam are addressed based on measurements at the first two clinical sites. Percentage depth doses, tissue phantom ratios, and beam symmetry and stability are presented and discussed.
Analysis of utilization‐related parameters on a 6‐MV linear accelerator
Authors: Dawson D. J. , Gribble M. A.
Published: August 2003

An analysis of unscheduled downtime and of patient treatment data for a 6‐MV linear accelerator is presented for a 7‐yr period. Following the initial year of clinical use, the unscheduled downtime has averaged 2.4%, with a corresponding loss of 1.3% in patient treatments. The quality assurance, preventive maintenance, and repairs are handled in‐house, with an investment of approximately 500 man‐hours annually.
Theoretical optimization of dual‐energy x‐ray imaging with application to mammography
Authors: Johns Paul C. , Yaffe Martin J.
Published: August 2003

Detection of a target object in a radiological image is often impeded by an obscuring background “clutter” resulting from the contrast between various materials in the neighborhood of the target. Dual‐energy techniques can reduce or remove this clutter. In order for the target to be detectable in the image after dual‐energy processing, the signal‐to‐noise ratio (SNR), defined as the difference between the target and the background divided by the photon noise in the difference, must exceed some threshold. A given SNR may be obtained for a wide range of the energies of the two x‐ray beams and the ratio of their fluences. A theoretical model is developed which permits the choice of beams to be optimized with respect to some critical parameter—in this case, patient dose. The analysis is applied to the detection of calcifications in mammography. For an ideal imaging system, we predict that the optimum beam energies are 19 and 68 keV. A dose of 0.42 cGy is required to obtain an SNR of 5 for detection of a 0.02‐cm cubic calcification in the resulting clutter‐free image. This can be reduced to 0.16 cGy if the higher energy image is smoothed, prior to dual‐energy processing, such that its variance is reduced to one‐fourth of its unsmoothed value.
Measured performance characteristics of a solid‐state linear detector array
Authors: Sones Richard A. , Morgan Douglas R. , Tesic Mike M. , Mattson Rodney A.
Published: August 2003

Experimental studies of a solid‐state linear detector array developed for a prototype scanning slit digital chest radiographic unit have been completed. The detector consists of a strip of scintillating material, optically coupled to a linear silicon photodiode array. Measured performance characteristics of the detector, such as sensitivity, modulation transfer function, and detective quantum efficiency, are presented for several different scintillators. Results indicate that direct x‐ray absorption events in the silicon photodiode can degrade detective quantum efficiency. Results also indicate that the inexpensive preamplifier circuits used in the digital chest prototype contribute negligible noise at diagnostic x‐ray photon fluence rates.
A photon dose distribution model employing convolution calculations
Authors: Boyer Arthur , Mok Ed
Published: August 2003

A three‐dimensional photon beam calculation is described which models the primary, first‐scatter, and multiple‐scatter dose components from first principles. Three key features of the model are (1) a multiple‐scatter calculation based on diffusion theory, (2) the demonstration of the modulation transfer function of the radiation dose transport process, and (3) the use of the finite fast Fourier transform to perform the required convolutions. The results of calculations for cobalt‐60 in a homogeneous phantom are used to verify the accuracy of the model.
Exposure contribution from Ti K x rays produced in the titanium capsule of the clinical I‐125 seed
Authors: Kubo Hideo
Published: August 2003

The iodine‐125 radioactive nuclide produces three intrinsic photons, i.e., the 27.4‐keV tellurium (Te) Kα and 31.0‐keV Kβ x rays, and 35.5‐keV gamma rays. In addition, a silver‐wire‐type seed is known to have silver (Ag) Kα and Kβ x rays as a result of interactions between silver atoms and the intrinsic photons. Since the radioactive iodine‐125 is completely sealed with a 0.05‐mm‐thick titanium capsule, fluorescence x rays from titanium atoms are also expected. This study reports (1) abundance of Ti K x rays at 4.5 keV, (2) contribution to exposure from these photons amounting to approximtely 19% of the total exposure at 1‐cm source‐to‐detector distance (SDD), (3) exposure uncertainties of 23% at 1‐cm SDD when Ti K x rays are excluded, and (4) negligible dose contribution to tissue from these photons.
MR image artifacts from periodic motion
Authors: Wood Michael L. , Henkelman R. Mark
Published: August 2003

Artifacts due to periodic motion during the acquisition of magnetic resonance (MR) images have been studied. A mechanical device was constructed to oscillate a small sample along any line within a 0.15‐T Technicare imager. Two‐ and three‐dimensional images were obtained using various frequencies and amplitudes of oscillation. Computer simulations of these experiments yielded images which agreed with the experiments. We demonstrated that movement influences MR images locally through blurring, and also generates ghost artifacts along the phase‐encoding directions of the Fourier transform imaging technique.
The effect of free electrons on ionization chamber saturation curves
Authors: Bielajew A. F.
Published: August 2003

The effect of the collection of free electrons—those which have not formed negative ions in the gas of a parallel‐plate ionization chamber—is considered for the case of continuous radiation. A theory and its solution, valid for the entire voltage range, is given. The solution is based upon the saturation theory of Townsend, and free electrons are included in a perturbative fashion. The collection of free electrons becomes important at exposure rates above 430 R/min. Since the electron contribution involves an extra dependence on exposure rate, some of the wide variation in the cavity gas parameter (m value) reported in the literature may be due to the presence of free electrons.
The Physics of Radiation Therapy by F. M. Khan
Authors: Paliwal Bhudatt R.
Published: August 2003

(Accepted for publication 8 July 1985)