bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of head phantom size on 10B and 1H[n,γ]2H dose distributions for a broad field accelerator epithermal neutron source for BNCT
Authors: Gupta N. , Niemkiewicz J. , Blue T. E. , Gahbauer R. , Qu T. X.
Published: March 2003

The effect of head phantom size on the 10B and 1H[n,γ]2H dose distributions for a broad epithermal neutron radiation field generated by an accelerator‐based epithermal neutron source for boron neutron capture therapy (BNCT) have been studied. Also two techniques for calculating the absorbed gamma dose from a measured gamma‐ray source distribution are compared: a Monte Carlo technique, which is well accepted in the BNCT community, and a Point Kernel technique. The count‐rate distribution in the central plane of three rectangular parallelopiped head water phantoms irradiated with an epithermal neutron field was measured with a boron trifluoride (BF3) detector. This epithermal neutron field was produced at the Ohio State University Van de Graaff Accelerator Facility. The 10B absorbed dose and the gamma‐ray source have the same distribution in the head phantom as the BF3 count‐rate distribution. The absorbed gamma dose from the measured source distribution was calculated using MCNP, a Monte Carlo code, and QAD‐CGGP, a Point Kernel code. The most pronounced effect of phantom size on 10B absorbed dose was on the dose rate at the depth of maximum dose, dmax. An increase in dose rate at dmax was observed with a decrease in phantom size, the dose rate in the smallest phantom being larger by a factor of 1.4 than the dose rate in the largest phantom. Also, dmax for the phantoms shifted deeper with a decrease in phantom dimensions. The shift between the largest and the smallest phantoms was 6 mm. Finally, the smaller phantoms had lower entrance 10B dose as a percent of the dose at dmax, or better skin sparing. Our calculations for the gamma dose show that a Point Kernel technique can be used to calculate the dose distribution as accurately as a Monte Carlo technique, in much shorter computation times.
An improvement in the use of plaster bandage for patient contouring
Authors: Barish Robert J.
Published: March 2003

The setting time of plaster bandage used for patient contouring can be decreased by adding potassium sulfate to the dip water. A small amount of this inexpensive chemical can have a significant effect. This improvement gives the physicist sufficient control of the process so that contours may be made from plaster as quickly as they are made with stiff wire, but with improved accuracy.
Uncertainty analysis of field placement error measurements using digital portal and simulation image correlations
Authors: McParland Brian J.
Published: March 2003

An uncertainty analysis of a field placement error measurement method using correlations between digital portal and simulation images has been performed. This analysis used the Monte Carlo method to simulate random uncertainties arising in image registration and portal field edge detection. The uncertainty analysis used data from a field placement error study to predict the dependences of the measurement method upon image matchpoint number and spatial distribution, image registration, and uncertainties arising in the detection and description of the field edge in a portal image. It was shown that the uncertainty in the field registration had the most significant effect upon the measurement uncertainty, whereas the number of matchpoints had an extremely limited effect (beyond the minimum required for the mathematical procedure used in registration). In contrast, it was shown that an accurate field placement error measurement required that the number of pixels describing a field edge used for comparisons should exceed 20, thus automatically setting a minimum field length necessary for field placement comparisons. It was also shown that the spatial uniformity of the matchpoints in an image pair had a limited effect upon the field placement error measurement, provided that the points are distributed over about 15% of the image area.
Superheated drop detector for determination of neutron dose equivalent to patients undergoing high‐energy x‐ray and electron radiotherapy
Authors: Nath Ravinder , Meigooni Ali S. , King Christopher R. , Smolen Stuart , d'Errico Francesco
Published: March 2003

The superheated drop detector (SDD) consists of thousands of superheated drops dispersed in a small vial of gel, which vaporize upon exposure to high LET radiation, thereby providing a directly observable indication of neutron dose. This detector possesses high sensitivity to neutrons and insensitivity to high‐energy photons and electrons, making it suitable for the determination of neutron dose equivalent rates around high‐energy photon and electron radiotherapy beams. In the present work, the SDD was used to measure the neutron dose equivalent in and around the radiotherapy beams produced by a 32‐MeV linear accelerator. For both x‐ray and electron beams, the neutron dose profiles were observed to follow the photon/electron radiotherapy beam profiles. For 25‐MV x rays, the neutron dose equivalent per photon dose on the central axis increased by a factor of about 3 as field size increased from 5×5 to 30×30 cm. However, the neutron dose equivalent rate at 50 cm off‐axis in the patient plane was essentially independent of field size. The neutron dose equivalent per electron dose was essentially zero for electron beams with energies below 15 MeV, but increased rapidly above 15 MeV. For 25‐MeV electrons, neutron dose equivalent on the central axis was about that for 25‐MV x rays. Analogous to the data for 25‐MV x rays, the neutron dose equivalent rate on the central axis of a 25‐MeV electron beam exhibited a similar field size dependence and outside the beam it was essentially independent of field size. Compared with P2O5 phosphorous activation detector data [Med. Phys. 7, 545–548 (1980)], the SDD neutron dose equivalent rate reading is about 20% higher on the central axis of a 25‐MV x‐ray beam. At points off‐axis in the patient plane, agreement between SDDs and phosphorous activation detectors is excellent. The SDD provides an easy‐to‐use and accurate method for the determination of neutron dose in the presence of high‐energy photons and electrons generated by medical accelerators.
The need for investigating the optical transfer functions in several field orientations for nonisotropic radiographic systems
Authors: Schiabel Homero , Frére Annie France , Azevedo Marques Paulo M.
Published: March 2003

In this paper, it is demonstrated that obtaining OTF's for only two directions, parallel and perpendicular to the x‐ray tube axis, is insufficient to completely describe the performance of nonisotropic radiographic systems. By performing experiments with three radiological systems in which slit images were obtained for ten different directions, we confirmed that OTF's vary in a nonlinear fashion for directions that are intermediate to the parallel and perpendicular ones. Moreover, we have also identified for each system a range of field orientations—referred to as “Optimum Region”—where sharper images can be obtained. These experimental results can be accounted for by the transfer functions theory.
A vectorized Monte Carlo code for modeling photon transport in SPECT
Authors: Smith Mark F. , Floyd Carey E. , Jaszczak Ronald J.
Published: March 2003

A vectorized Monte Carlo computer code has been developed for modeling photon transport in single photon emission computed tomography (SPECT). The code models photon transport in a uniform attenuating region and photon detection by a gamma camera. It is adapted from a history‐based Monte Carlo code in which photon history data are stored in scalar variables and photon histories are computed sequentially. The vectorized code is written in FORTRAN77 and uses an event‐based algorithm in which photon history data are stored in arrays and photon history computations are performed within DO loops. The indices of the DO loops range over the number of photon histories, and these loops may take advantage of the vector processing unit of our Stellar GS1000 computer for pipelined computations. Without the use of the vector processor the event‐based code is faster than the history‐based code because of numerical optimization performed during conversion to the event‐based algorithm. When only the detection of unscattered photons is modeled, the event‐based code executes 5.1 times faster with the use of the vector processor than without; when the detection of scattered and unscattered photons is modeled the speed increase is a factor of 2.9. Vectorization is a valuable way to increase the performance of Monte Carlo code for modeling photon transport in SPECT.
Beam quality specification for photon beam dosimetry
Authors: Kosunen A. , Rogers D. W. O.
Published: March 2003

It is argued that %dd(10), the percentage depth dose at 10 cm in a 10×10 cm2 photon beam at a SSD of 100 cm, is a better beam quality specifier for radiotherapy beams than the commonly used values of or nominal accelerating potential (NAP). For radiation dosimetry purposes, is not an ideal beam quality specifier because (i) stopping‐power ratios for the same value of can vary by up to 0.7% for thick‐target bremsstrahlung beams; (ii) the value of becomes insensitive to beam quality changes for high‐energy beams; and (iii) it has little intuitive meaning. In contrast, %dd(10) in a pure photon beam specifies stopping‐power ratios within 0.2% for all thick‐target bremsstrahlung beams, maintains its sensitivity for high‐energy beams, and has a simple physical and clinical meaning. It is shown that for all thick‐target bremsstrahlung beams the spr (water/air)=1.2676−0.002 224[%dd(10)] with a rms deviation of 0.1%. The effects of electron contamination in typical high‐energy clinical beams can be corrected for using previously published experimental results or by reducing electron contamination using lead scattering foils.
Use of an octree‐like geometry for 3‐D dose calculations
Authors: McShan D. L. , Fraass B. A.
Published: March 2003

A representation used for 3‐D graphical objects, the “octree,” has been applied to the geometrical calculations needed to perform photon beam dose calculations for radiotherapy treatment planning. This representation allows the algorithm to attempt to minimize the number of distinct geometrical calculations that are needed to perform dose calculations to a particular resolution. In this way, the calculational time can be minimized, since the geometrical part of the dose calculations is often the most time‐intensive part of the calculation process. The octree‐like system used here has sped up the photon dose calculations described here by up to a factor of 10.
History, current status, and trends of radiation protection standards
Authors: Hendee William R.
Published: March 2003

Quantitative standards for protection against exposure to ionizing radiation were first formulated in the 1930s. Since that time, standards have been restated periodically in different radiation units and conceptual frameworks that reflect improved understanding of the biological effects of radiation interactions and their consequences for human health. In the 1970s the expression of protection standards shifted from a dose‐ to a risk‐based approach, with dose limits established to yield risks to radiation workers comparable with those for workers in other “safe” industries. Over the years, radiation protection standards have exhibited a downward trend to more rigorous limits that require increased commitments of personnel and resources for their enforcement. There are several reasons for this trend, including increased recognition of the long‐term health effects of radiation, improved protective measures that permit radiation use at lower levels of exposure, growing numbers of persons exposed occupationally to radiation, and probably a greater intolerance to involuntary risks in society, with radiation targeted as a highly visible source of involuntary risks in the form of nuclear power plants and radioactive waste sites. In the past few years, reports of the Radiation Effects Research Foundation, United Nations Scientific Committee on the Effects of Atomic Radiation, and the National Research Council of the U.S. National Academy of Sciences have presented increased risk estimates for radiation exposure as a consequence of ongoing epidemiological analyses of human populations exposed to ionizing radiation. These risk estimates have enhanced public concern about radiation exposure and set the stage for discussions about the desirability of further reductions in exposure standards for radiation workers and members of the public. Such reductions would directly affect the professional activities, educational responsibilities, and administrative burdens of most medical and health physicists. These persons should understand the process of deriving risk estimates from epidemiological data, the factors that influence the risk estimates, how risk estimates are integrated into radiation protection standards, and the possible impact of more rigorous standards on the use of radiation in medicine.
“Zero‐field” dose data for 60Co and other high‐energy photon beams in water
Authors: Nizin Paul , Qian Guo‐Xin , Rashid Harunor
Published: March 2003

A procedure of separating the primary‐ and scatter‐dose components in therapeutic photon beams is examined. It is based on the observation that the scatter‐dose component is proportional to the variable z=rd/(r+d), where r and d are beam radius and depth in phantom, respectively. It is, therefore, possible to express an absorbed dose in the form of a linear equation D(r)=P+Nz, where at a fixed depth d, both primary dose P and coefficient N are constant. A method of linear extrapolation of an absorbed dose D(r) to “zero‐field” size, i.e., r=0, is utilized. Since Monte Carlo technique is capable of scoring separately the primary‐ and scatter‐dose components, it is used to evaluate the accuracy of the linear extrapolation method within the range of 60Co ‐ 15‐MV nominal photon energies. The results demonstrate that this method is sufficiently accurate to obtain the primary dose component in photon beams. For 60Co γ radiation in water, tabulated sets of measured depth‐dose data are analyzed by the linear extrapolation method to review “zero‐field” dose values [percentage depth dose (PDD) and tissue–air ratio (TAR) tables of the British Journal of Radiology, Suppl. 17]. The “zero‐field” PDD data are found to be accurate within limits of experimental uncertainties. Inconsistencies in the TAR table are illustrated and discussed.60Co tables of relative doses, D(r,d)/P(dmax), including “zero‐field” values for both fixed SSD and isocentric geometries, are generated. Dose calculation in irregular fields is considered. The linear extrapolation method is recommended as a standard procedure for separating primary dose from depth‐dose data in high‐energy photon beams.
Evaluation of fixed‐ versus variable‐modulation treatment modes for charged‐particle irradiation of the gastrointestinal tract
Authors: Daftari Inder , Petti Paula L. , Collier John M. , Castro Joseph R. , Pitluck Samuel
Published: March 2003

The clinical usefulness of variable‐modulation dose delivery of neon ion and proton beams over fixed‐modulation beams is evaluated for several patients with tumors in the gastrointestinal tract by comparing dose distributions, dose volume histograms, and predictions of normal tissue complication probabilities calculated with the two methods. Both techniques provide excellent coverage of the target volume with neon ion and proton beams. The advantage of variable modulation is that less dose is delivered proximal to the target volume. For tumors in the gastrointestinal tract, this implies that less dose is given to the liver, gut, kidneys, and lungs. For the ten patients considered in this study, variable‐modulation reduced the total integral dose by an average of 17% for neon ion beams and by 18% for protons as compared to fixed‐modulation. If the tumor volume is excluded, the reduction in the integral dose to normal tissues ranged from 15% to 32% for neon ions and from 18% to 34% for proton beams. These gains are larger than those anticipated on the basis of an analytic study by Goitein and Chen [Med. Phys. 10, 831–840 (1983)], which predicted integral dose reductions of the order of 10% for protons and 14% for neon ions. They are also larger than those reported in a similar study by Urie and Goitein [Med. Phys. 16, 593–601 (1989)] for proton irradiation of skull‐base tumors. This is probably because the tumors in the GI tract considered in this study were more irregularly shaped than Goitein and Chen's analytic model assumes. The results of this study also suggest that due to increased sparing of normal tissues, the number of different portal directions required to achieve a satisfactory treatment plan will be reduced for variable‐modulation beam delivery systems. This implies that variable‐modulation treatment plans will be easier to execute than current fixed‐modulation plans.
Uniform oscillating gradient produced by spherical birdcage resonator
Authors: Harpen Michael D.
Published: March 2003

The theory and operation of a resonator which produces a uniform oscillating magnetic field gradient is described. When the resonator is operated at audio frequency, it will produce the oscillating magnetic field gradient required for use in spatial encoding in rapid acquisition magnetic resonance imaging.
Dosimetry for 125I seed (model 6711) in eye plaques
Authors: Chiu‐Tsao Sou‐Tung , Anderson Lowell L. , O'Brien Keran , Stabile Leonard , Liu John C.
Published: March 2003

The effect of eye plaque materials (gold backing and silastic seed‐carrier insert) on the dose distribution around a single 125I seed has been measured, using cubic lithium fluoride thermoluminescent dosimeters (TLDs) 1 mm on an edge, in a solid water eye phantom embedded in a solid water head phantom. With an 125I seed (model 6711) positioned in the center slot of the silastic insert for a 20‐mm plaque of the design used in the collaborative ocular melanoma study (COMS), dose was measured at 2‐mm intervals along the plaque central axis (the seed's transverse axis) and at various off‐axis points, both with and without the COMS gold backing placed over the insert. Monte Carlo calculations (MORSE code) were performed, as well, for these configurations and closely the same geometry but assuming a large natural water phantom. Additional Monte Carlo calculations treated the case, both for 20‐ and 12‐mm gold plaques, where the silastic insert is replaced by natural water. Relative to previous measurements taken in homogeneous medium of the same material (without the eye plaque), the dose reduction found by both Monte Carlo and TLD methods was greater at points farther from the seed along the central axis and, for a given central‐axis depth, at larger off‐axis distances. Removal of the gold backing from the plaque did not make measurable difference in the dose reduction results (10% at 1 cm).
Method of simulated screen sensitometry for asymmetric, low crossover medical x‐ray films
Authors: Dickerson Robert E. , Haus Arthur G. , Baker Charles W.
Published: March 2003

Recognition of the importance of performing simulated screen‐light sensitometry of medical x‐ray films for the purpose of processor quality control has increased over the past several years. As a result there is a greater need to provide new techniques for performing simulated screen‐light sensitometry. Medical films with reduced intensifying screen‐light crossover intended to achieve reduced blur and higher spatial resolution pose particular problems in doing simulated screen‐light sensitometry if care is not taken to choose a proper simulated light sensitometer with the capability of simultaneous double‐sided exposures. Misleading and incorrect sensitometric data can be obtained for film contrast evaluation if a single side exposure is used. Asymmetric, near‐zero crossover films pose even greater problems as proper orientation of the film and proper degree of light output asymmetry need be achieved in order to obtain correct sensitometry. The films used in this study were three double emulsion films varying in crossover from 3% to 24%. Of the two very‐low‐crossover films, one had symmetric emulsion layers while the second featured emulsion layers which were asymmetric in terms of contrast and speed. Sensitometric data show several curve shapes with significant distortions, depending on orientation, for the asymmetric, low‐crossover film when exposed using a single‐sided exposure. Only by using a double‐sided exposure and an appropriate neutral density filter to simulate the degree of screen‐light asymmetry in this system could one achieve a characteristic curve comparable to that achieved by inverse square sensitometry.
A miniature cesium iodide‐photodiode detector for ambulatory monitoring of left ventricular function
Authors: Millaire A. , Hossein‐Foucher C. , Rousseau J. , Bedoui H. , Ducloux G. , Marchandise X.
Published: March 2003

The physical characteristics of a portable nonimaging scintillation probe system for continuous ambulatory monitoring of the left ventricular function are described. The detector of the equilibrium radionuclide labeled blood pool is a single cesium iodide (CsI) crystal coupled to a silicium photodiode and interfaced to a microcomputer. The spatial properties of this small CsI crystal (1×1×1 cm3) were evaluated with various single‐hole collimators. Linearity was studied in nonattenuating medium. Saturation began at 3000 cps, count loss was 10% at 4000 cps, maximal count rate was 24 000 cps. In attenuating medium, isocount curve of 5% of the maximal count rate was 100 mm deep and 160 mm wide. The most appropriate tested lead collimator to record the global ejection fraction of the left ventricle was a disc‐shaped (thickness 5 mm, diameter 41 mm) single‐hole (proximal aperture 8 mm, distal aperture 18 mm) collimator. Sensitivity was similar to the sensitivity of a sodium iodide nuclear probe. The detection performance appeared comparable to other available detector systems. Our results indicate that such a CsI‐photodiode probe is a promising candidate for left ventricular function monitoring. The application to an ambulatory multicrystal detector system is presented and discussed.
Computerized detection of clustered microcalcifications in digital mammograms using a shift‐invariant artificial neural network
Authors: Zhang Wei , Doi Kunio , Giger Maryellen L. , Wu Yuzheng , Nishikawa Robert M. , Schmidt Robert A.
Published: March 2003

A computer‐aided diagnosis (CAD) scheme has been developed in our laboratory for the detection of clustered microcalcifications in digital mammograms. In this study, we apply a shift‐invariant neural network to eliminate false‐positive detections reported by the CAD scheme. The shift‐invariant neural network is a multilayer back‐propagation neural network with local, shift‐invariant interconnections. The advantage of the shift‐invariant neural network is that the result of the network is not dependent on the locations of the clustered microcalcifications in the input layer. The neural network is trained to detect each individual microcalcification in a given region of interest (ROI) reported by the CAD scheme. A ROI is classified as a positive ROI if the total number of microcalcifications detected in the ROI is greater than a certain number. The performance of the shift‐invariant neural network was evaluated by means of a jackknife (or holdout) method and ROC analysis using a database of 168 ROIs, as reported by the CAD scheme when applied to 34 mammograms. The analysis yielded an average area under the ROC curve (Az) of 0.91. Approximately 55% of false‐positive ROIs were eliminated without any loss of the true‐positive ROIs. The result is considerably better than that obtained in our previous study using a conventional three‐layer, feed‐forward neural network. The effect of the network structure on the performance of the shift‐invariant neural network is also studied.
Unfolding patient motion with biplane radiographs
Authors: Li Shidong , Pelizzari Charles A. , Chen George T. Y.
Published: March 2003

A method of unfolding patient movement between biplane radiographs is presented. This method requires biplane projections of two points with a known separation to determine a translation motion or biplane projections of two or three point pairs to determine a general patient motion. Such a point pair can be provided by two ends of a line source or two radiopaque marks, with a fixed distance between them. The point pairs can be taped on skin, implanted in the target volume, or embedded in the bony structures. After obtaining the point locations on each film, the three‐dimensional coordinates of points of interest at the instant of each view can be reconstructed, with an error of less than 0.5 mm. The motion parameters, characterized by a pseudotranslation vector and three axial rotational angles for the region of interest relative to the machine coordinate system, are also provided. In comparing with some other reconstruction algorithms, the point pair method can significantly improve source localization or point reconstruction in the presence of patient motion. Computer simulations and phantom experiments demonstrates the feasibility and accuracy of the method.
Optimal radiographic magnification for portal imaging
Authors: Bissonnette Jean‐Pierre , Jaffray David A. , Fenster Aaron , Munro Peter
Published: March 2003

Two approaches to estimate the optimal radiographic magnification for a TV camera‐based portal imaging system and portal films have been used. The first approach optimizes signal transfer while the second optimizes signal‐to‐noise ratio (SNR) transfer. In order to perform these optimization calculations, the physical characteristics of the imaging system (modulation transfer function and noise power spectrum) as well as the sizes of the radiation sources of our medical linear accelerators have been measured. Using these data, the optimal magnification considering signal transfer alone (Msignal) has been calculated to range between 2.0 and 2.3 for the TV camera‐based imaging system and is about 1.0 for portal films. Conversely, the optimal magnification considering SNR transfer (MSNR) has been calculated to range between 1.5 and 1.7 for the TV camera‐based imaging system and is about 1.0 for portal films. The results suggest that most portal imaging systems are operated close to their optimal radiographic magnification.
The spectral dependence of electron central‐axis depth‐dose curves
Authors: Deasy J. O. , Almond P. R. , McEllistrem M. T.
Published: March 2003

Electron linac fields are usually characterized by the central‐axis practical range in water, Rp, and the depth of half maximum dose, R50, for dosimetry, quality assurance, and treatment planning. The quantitative relations between the range parameters and the intrinsic linac beam's energy structure are critically reviewed. The spectral quantity 〈E0〉* is introduced which is defined as the mean energy of the incident spectral peak, termed the “peak mean energy.” An analytical model is constructed to demonstrate the predicted relation between polyenergetic spectral shapes and the resulting depth‐dose curves. The model shows that, in the absence of electrons at the patient plane with energies outside about 〈E0〉*±0.1〈E0〉*, Rp and R50 are both determined by 〈E0〉*. This analytical approximation is confirmed by a Monte Carlo calculation comparing two different idealized incident spectra. The effect of contaminant lower energy or wide‐angle scattered electrons is also discussed. The effect of the width of the intrinsic energy spread on the shape of the depth‐dose curve is investigated using Monte Carlo depth‐dose simulations based on measured linac energy spectra having energy spreads (full width at half maximum) as large as 20%. These simulations show that the energy spread has only a small effect on the shape of the central‐axis depth‐dose curve.
A solution to the Yang equation with electron energy loss following Harder's formula
Authors: Tulovsky V. , Sandison G. A. , Papiez L. S.
Published: March 2003

The Yang diffusion transport equation for charged particles was modified to allow the linear angular scattering power to vary with penetration depth in the scattering medium. Assuming charged particle energy loss to be a linear function of depth, conditional solutions to this transport equation have been found for the two cases of interest specified by Yang. The normalized excess path length distributions predicted for a 10‐MeV electron beam show a shift toward larger excess path lengths compared to Yang's solutions.
Convolution calculations of dose in the buildup regions for high energy photon beams obliquely incident
Authors: Dupont S. , Rosenwald J. C. , Beauvais H.
Published: March 2003

The central dose received from high energy photon beams that are obliquely incident on an absorber is markedly different from that absorbed when such beams are normally incident at the surface. In this work, we calculate the dose differences using a convolution of energy deposition kernels in a phantom presenting an oblique entrance surface. The dose distributions were calculated with oblique incident 1.25, 3, and 6 MeV monoenergetic and polyenergetic photon beams angled from 0° to 80°. In order to study the dose variations with surface obliquity, we introduced an obliquity factor. For each energy, the obliquity factor was calculated as a function of depth and field size. We found that, ignoring the electron contamination from the air and from the treatment machine head, the influence of obliquity can be described in terms of upstream and downstream contribution of the electrons set in motion by the primary photons. In actual beams, especially for large field sizes, the electron contamination becomes significant and tends to reduce the influence of surface obliquity. Results indicate that the obliquity factor is highly dependent on the beam energy and depth, and are in good agreement with our experimental results measured for 10 and 25 MV x‐ray beams. In this paper a theoretical explanation of these dose variations due to oblique incidence of the beam is presented.
An improved bolus configuration for commercial multielement ultrasound and microwave hyperthermia systems
Authors: Diederich C. J. , Stauffer P. R. , Bozzo D.
Published: March 2003

A simple modification is presented for two commercially available hyperthermia applicators which dramatically improves the regulation and dynamic control of the temperature at the bolus/tissue interface. This alteration requires the addition of a variable speed pump, bubble trap, simple heat exchanger, and a few minor changes to the existing system. With this modified design, the water within the bolus is directly circulated and temperature controlled. The convective nature of the circulating system ensures uniform temperature throughout the extended bolus and increases the thermal energy transfer at the bolus/tissue interface. This modification also provides significantly improved flexibility in controlling the treatment temperature distributions since the bolus/tissue interface temperature can now be dynamically varied during a treatment, in addition to adjusting the applicator power output and frequency.
Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator
Authors: Niroomand‐Rad Azam , Rodgers James E. , Taylor Paula , Hames Brian
Published: March 2003

A set of standard wedge filters has been modified for use with half‐collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half‐collimated field coverage (up to 20×30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off‐axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.
The measurement of radiation dose profiles for electron‐beam computed tomography using film dosimetry
Authors: Zink Frank E. , McCollough Cynthia H.
Published: March 2003

The unique geometry of electron‐beam CT (EBCT) scanners produces radiation dose profiles with widths which can be considerably different from the corresponding nominal scan width. Additionally, EBCT scanners produce both complex (multiple‐slice) and narrow (3 mm) radiation profiles. This work describes the measurement of the axial dose distribution from EBCT within a scattering phantom using film dosimetry methods, which offer increased convenience and spatial resolution compared to thermoluminescent dosimetry (TLD) techniques. Therapy localization film was cut into 8×220 mm strips and placed within specially constructed light‐tight holders for placement within the cavities of a CT Dose Index (CTDI) phantom. The film was calibrated using a conventional overhead x‐ray tube with spectral characteristics matched to the EBCT scanner (130 kVp, 10 mm Al HVL). The films were digitized at five samples per mm and calibrated dose profiles plotted as a function of z‐axis position. Errors due to angle‐of‐incidence and beam hardening were estimated to be less than 5% and 10%, respectively. The integral exposure under film dose profiles agreed with ion‐chamber measurements to within 15%. Exposures measured along the radiation profile differed from TLD measurements by an average of 5%. The film technique provided acceptable accuracy and convenience in comparison to conventional TLD methods, and allowed high spatial‐resolution measurement of EBCT radiation dose profiles.
Photon propagation and detection in single‐photon emission computed tomography—An analytical approach
Authors: Riauka Terence A. , Gortel Zbigniew W.
Published: March 2003

An analytical theory of photon propagation and detection in single‐photon emission computed tomography (SPECT) for collimated detectors is developed from first principles. The total photon detection kernel is expressed as a sum of terms due to the primary and the Compton scattered photons. The primary as well as contributions due to every order of Compton scattering are calculated separately. The model accounts for the three‐dimensional depth dependence of the collimator holes as well as for nonhomogeneous attenuation. No specific assumptions about the boundary or the homogeneity of the attenuating medium are made. The energy response of the detector is also modeled by the theory. Analytical expressions are obtained for various contributions to the photon detection kernel, and the multidimensional integrals involved are calculated using standard numerical integration methods. Theoretically calculated projections and scatter fractions for the primary and the first through second scattering orders are compared with our own experimental results for a small cylindrical primary radiation source immersed at various positions in a uniform cylindrical phantom. Also, theoretically calculated scatter fractions for a small spherical (pointlike) source in a uniform elliptic phantom are compared with experimental and Monte Carlo simulation results taken from the recent literature. The results from the analytical method are essentially exact and are free from the inaccuracies inherent in the numerical simulation methods used to deal with the photon propagation and detection problem in SPECT so far. The method developed here is unique in the sense that it provides accurate theoretical predictions of results averaged over an infinite number of simulations or experiments. We believe that our theory enhances an intuitive understanding of the complex image formation process in SPECT and is an important step toward solving the inverse problem, that of reconstructing the primary radiation source distribution from the measured gamma camera projections.
Analytic modeling of photon scatter from flattening filters in photon therapy beams
Authors: Ahnesjö Anders
Published: March 2003

Analytic models for calculation of scatter distributions from flattening filters in therapy photon beams are presented. It is shown that the amount of scatter with high atomic number filters can vary from 2% in 4‐MV beams to 10% for 24‐MV beams. The use of low atomic number filters can increase the amount of scatter by a factor of 2. The dependence on the opening angle of the primary collimator is quite large since a larger opening angle requires a thicker filter, which increases the scattered fraction of the filtered beam. The scatter makes the filter act as an extended source of extra‐focal radiation. The source distribution for monomedia filters is shown to be almost triangular. Integration in the calculation‐point's eye view over the visible part of the filter scatter source yields the scatter fraction of the total energy fluence incident upon the patient. The experimentally well‐known “tilt” of dose profiles for asymmetrical fields is explained by the model. For complete modeling of head scatter distributions in treatment planning, the model presented must be combined with models also describing the scatter from the collimators, auxiliary modulators such as wedges and compensating filters, and collimator backscatter to the beam monitor.
Dose calculation for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations
Authors: Chui Chen‐Shou , LoSasso Thomas , Spirou Spiridon
Published: March 2003

A dose calculation algorithm has been developed for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations. First, an in‐air fluence distribution is constructed based on the dynamic motion of the jaws or leaves, taking into account the variation of output with field size defined by the jaws. The fluence distribution is then convolved with the appropriate pencil beam kernel to give correction factors which are used to calculate the dose distribution for an intensity‐modulated photon field. The proposed algorithm is strictly valid in homogeneous media only, patient heterogeneity correction is accounted for in an approximate manner. Dose distributions at several depths and for several field sizes were calculated for 6‐ and 15‐MV x‐ray beams for a set of standard wedges produced by dynamic jaws. Measurements were made with film and an ion chamber. Comparisons between calculated and measured data show good agreement (within 2%) for both dose profiles and wedge factors. Similar calculations and measurements were also made for a 25‐MV intensity‐modulated photon field produced by dynamic motion of a multileaf collimator. Agreement between calculations and measurements is also good (within 3%). The “tongue‐and‐groove” effect associated with a multileaf collimator design is also examined using a ring‐shaped field produced by matching two component fields. The computation time for a dynamic‐collimated field is the same as that for an irregular field shaped by conventional blocks. The algorithm is applicable to any pattern of jaw or multileaf motions. The strengths and remaining problems of the algorithm are discussed.
Dosimetry of centrally shielded electron beams
Authors: Gosselin Micheline , Podgorsak Ervin B. , Evans Michael D. C.
Published: March 2003

In electron beams, the dose in phantom under the central shielding depends on electron‐beam energy, depth in phantom, and shield area and thickness. In our experiments, all shield thicknesses were larger than the range of electrons in the shield material. At a given depth, the dose under the shield never exceeds the open field value; however, it can attain quite a large proportion of the open field value despite a shield thickness which exceeds the range of electrons in the shield material. The effects of shield area on the dose under the central shields were studied in detail and dose distributions are given as a function of shield lateral dimensions and electron‐beam energy. It is shown that in clinical use of central shielding, the best approach to dose estimation under the shield is direct measurement in phantom under conditions of the actual clinical setup.
Improving the convergence of iterative filtered backprojection algorithms
Authors: Lalush David S. , Tsui Benjamin M. W.
Published: March 2003

Several authors have proposed variations of the iterative filtered backprojection (IFBP) reconstruction algorithms claiming fast initial convergence rates. We have found that these algorithms are trying to minimize an unusual squared‐error criterion in a suboptimal way. As a result, existing IFBP algorithms are inefficient in the minimization of the criterion, and may become unstable at higher iteration numbers. We show that existing IFBP algorithms can be modified to use the steepest descent technique by simply optimizing the step size at each iteration. Further gains in convergence rates can be achieved with conjugate gradient IFBP algorithms derived from the same criterion. The steepest descent and conjugate gradient IFBP algorithms are guaranteed to converge, unlike some IFBP algorithms, and will do so in fewer iterations than existing IFBP algorithms.
Theoretical limits of SAR distributions of a four‐element square array of dipole‐type antennas
Authors: Fan C. J. , Leybovich L. B. , Devanna W. G. , Kurup R. G.
Published: March 2003

Four‐element dipole microwave antenna arrays with square insertion patterns are commonly used clinically for interstitial hyperthermia. One major disadvantage with this type of antenna array is the presence of a large dead length at the tips because the current gradually decreases from maximum at the junctions to zero at the tips. This dead length is usually 1.5–2 cm along the central axis of a 2×2 cm array of regular dipole antennas. Many attempts to improve the performance of dipole antenna arrays have been made by designing antennas with increased current at the tips. While some dipole antennas of new design show negligible dead lengths at close proximity in the single‐antenna configuration, phantom experiments have demonstrated that these antennas exhibit at least 1.1 cm dead space along the central axis of the four‐antenna array. Therefore, there seems to be a limit to which the array dead length can be reduced by the improvements in the dipole‐type antenna design. The goal of this work is to find the theoretical minimum of the array dead length. This was done by assuming a uniform current distribution along the entire antenna. The specific absorption rate (SAR) patterns were calculated for an array with an insertion depth of 7 cm (resonant length for 915 MHz) and a variable spacing between antennas (1–3 cm). It was found that there is a dead length of 6 mm along the central axis of the 2×2‐cm array with the uniform current distribution, which can be considered as the theoretical limit of the dead length for this array. It was also found that the SAR patterns in the midplane of the array are nearly independent of the separation between antennas and the lateral extensions of the SAR patterns are independent of the current distributions.
Use of capillary optics as a beam intensifier for a Compton x‐ray source
Authors: Tompkins Perry A. , Abreu Carmen C. , Carroll Frank E. , Xiao Qi‐Fan , MacDonald C. A.
Published: March 2003

The use of Kumakhov capillary optics will significantly enhance the performance of near‐monochromatic, Compton backscattered x‐ray programs. The Vanderbilt University Medical Free‐Electron Laser Center is developing the capability to create these tunable x rays for medical imaging. The present transport has only reflection optics, and the beam is quite large in diameter at the laboratory. Low loss collimation of this beam would allow higher x‐ray intensities after transport. This article describes experimental and computer simulation results which predict the expected performance for a multifiber Kumakhov collimator for use in the x‐ray beam transport. Estimates from our research are that a multifiber optic formed of individual polycapillary fibers could be used to capture the full 7 mrad of the Vanderbilt x‐ray beam and collimate it to a 1–2 mrad divergence with approximately 40%–50% transmission efficiency. This optic should increase the x‐ray intensity at the laboratory level by a factor of ≥5 by decreasing the beam divergence and subsequent spot size. Additionally, analysis of monolithic optics of fused multicapillary fibers predicts an increase in the intensity of the x rays at the laboratory by a factor of 55. These optics can have tapered channels that greatly decrease their exit divergence. This will greatly enhance the capabilities of this unique x‐ray source. This article reports the initial results from a collaboration between Vanderbilt, The Center for X‐Ray Optics at University at Albany, SUNY, and X‐Ray Optical Systems in Albany, NY.
Performance of glass fiber antiscatter devices at mammographic energies
Authors: Fahrig Rebecca , Mainprize James G. , Robert Normand , Rogers Arthur , Yaffe Martin J.
Published: March 2003

Using fiber optic manufacturing techniques, it is possible to produce a radiographic grid that discriminates against scattered radiation in two dimensions. Such grids consist of septa composed of glass with a high lead content; the interspace material is air, so that approximately 80% of the grid area is open. In this way, effective high ratio grids can be produced with relatively low Bucky factors. The performance of samples of such grid material is characterized in terms of both scatter rejection and dose efficiency for application in digital mammography in both slot–beam and area–beam geometry. For area beams, five‐ to tenfold improved scatter rejection relative to conventional grids was observed. In slot configurations, such grids could provide improved SNR/dose performance and more effective utilization of the heat loading capability of the x‐ray source.
Theoretical considerations on the validity of the Stewart–Hamilton principle in measuring cycle‐averaged flows via histogram of indicator in the pulsating compartment
Authors: Eterović Davor , Dujić Željko
Published: March 2003

It has been heuristically shown that the Stewart–Hamilton principle, adapted to external counting observables of system indicator histogram, A(t), its cycle‐averaged equilibrium count rate, Aequ, and indicator volume of distribution in the body, Vbody, is F/Vbody=Aequ/(t)dt, where F is the cycle‐averaged cardiac output. Since the method lacks the theoretical plausibility, it remained unclear whether it is an approximation and what conditions warrant its usability. This paper presents an exact derivation of the above equation. To fulfill it the generalizations of the stationary theory of indicator kinetics were set up that allowed for the conditions of pulsatile flows and volumes and the dependence of the distribution of transit times of indicator on the phase of the cardiac cycle. The assumptions utilized were that the tracer enters the compartment well mixed and convectively carried by the blood in concentrations that do not vary in the single cycle to a material extent. The method yields the cardiac output, even when the flow to a compartment is only a part of it, provided that the fraction of indicator that traversed the system equals the fraction of cardiac output that perfuses the compartment. It was shown that, when applied to a regurgitant ventricle, the method obtains the forward flow and that separate application of the method to each of the ventricles provides the theoretical basis for evaluation of the central‐circulatory shunts.