bluuebunny/crossref_metadata_2025 huggingface dataset:
Decaying source method for scintillation camera resolving times
Authors: Woldeselassie Tilahun
Published: July 2002

An earlier paper by the author showed that scintillation camera systems can be described effectively using the resolving times and of the dominant nonparalyzable and paralyzable components, that is, the detector system and the computer interface, respectively. When used with a full spectrum window, the camera has a lower nonparalyzable and an upper paralyzable operating range with normalized threshold input rate where Correct determination of and requires that both and in the normalized two‐source equations and come from the nonparalyzable and paralyzable ranges, respectively. A serious constraint of the two‐source method, therefore, is that the large ratio can lead to an input rate range which includes the threshold point and in which neither nor can be measured correctly. The decaying source method constitutes a refinement of the two‐source method, which enables smaller ratios to be used, and also includes the two‐source method as a special case This new method requires just two consecutive readings on a single decaying source, as opposed to three measurements on two sources of activity, for each determination of or thus also minimizing staff exposure. The fact that only count rates and time intervals need now be recorded greatly simplifies computerization of the data acquisition and analysis activities, and the potential for real‐time applications is obvious. The method enables and to be measured accurately and with sufficient resolution to reveal possible variations with input rate. Long measurement times using a decaying source can be avoided, if required, by using a set of decaying sources simultaneously to cover different portions of the count rate range. The application of the measurement procedure in real‐time and the use of the resolving times in the accurate correction of deadtime losses, also in real‐time, are treated separately in another paper.
Human subjects exposed to a specific pulsed extremely low frequency magnetic field: Effects on standing balance
Authors: Thomas Alex W.
Published: July 2002

Extremely low frequency pulsed magnetic fields (±200 μT, ∼400 mT/s, 0–1 kHz) may produce electric currents similar to action potentials, which in turn would be expected to produce detectable physiological or behavioral effects in human subjects. This thesis reports on the effects of pulsed extremely low frequency magnetic fields (ELFMFs) on human standing balance by measurement of postural sway on a force platform utilizing a double blind protocol with four randomly assigned 2 min exposure conditions (eyes open/closed, sham/ELFMF) within each subject's session. Historically, and in current research by others, normal postural sway has been well characterized. Standing balance in humans is a largely automatic behavior and with “eyes open” is a robust behavior not easily disrupted by small perturbations. However, with “eyes closed” standing balance becomes very sensitive to perturbation by a wide variety of stimuli. Since we have shown that ELFMF perturbs human postural sway, measurements of standing balance may be an ideal method of investigating bioelectromagnetic coupling mechanisms in humans (i.e., peripheral versus central nervous system mechanisms). Ideal because, being under automatic control, standing balance is an objective measure not subject to placebo effects. The coupling mechanisms can be studied by examining the effects related to different anatomical location of ELFMF exposure, or studying patients with different pathology. This thesis has established that normal standing balance is perturbed by weak ELFMF for both eyes open/eyes closed conditions. Also, it has been established that this human assay may indeed be appropriate to study coupling mechanisms, since the ability of ELFMF to affect sway varies between patients with diseases affecting different anatomical and physiological centers.
A monitor unit “odometer” for measuring linac workload
Authors: Evans M. D. C. , Larkin J. J. , Léger P. , Podgorsak E. B.
Published: July 2002

The annual linac workload is often required by regulatory agencies to assess compliance with license conditions. Summation of the monitor units produced by the machine is generally used for this purpose. Various methods of estimating this value have inherent inaccuracies. We have built an integrating Monitor Unit “odometer” that is able to automatically accumulate all MUs delivered by the linac and segregate the total by mode (photon or electron) and energy. The device has been used to record clinical linac MU workloads for 10 months, and was installed in a new dual‐energy linac during the acceptance and commissioning process.
Comparison of light and x‐ray sensitometric responses of double‐emulsion films for different processing conditions
Authors: Blendl Christian , Buhr Egbert
Published: July 2002

The effects of different film processing conditions on light and x‐ray sensitometric responses were compared for a variety of double‐emulsion x‐ray films. The processing conditions were altered by changes of the developer temperature. Three different exposure variants were applied: x‐ray sensitometry using two stepped neutral density attenuators between film and screens, simultaneous double‐sided light sensitometry, and single‐sided light sensitometry. 13 different types of double‐emulsion x‐ray films were investigated, among them three asymmetric films. In the special case of exposing the asymmetric films with the single‐sided light sensitometer, a method was investigated where each side of the film is exposed at different locations and the sum effect is analyzed. From each sensitometric curve shape two parameters, the logarithmic speed and the average gradient (G), were evaluated. The results of this study can be summarized as follows: (1) Single‐sided and double‐sided light sensitometers revealed almost equal changes of when the processing conditions are altered. Thus, single‐sided light sensitometers can serve as a substitute for double‐sided light sensitometers provided that suited exposure methods are used and appropriate sensitometric parameters are evaluated. (2) Light sensitometry quantitatively indicated changes of the film processing that affect the x‐ray speed. Hence, light sensitometry is a useful method to monitor changes in film processing.
Measurement of absorbed dose with a bone‐equivalent extrapolation chamber
Authors: DeBlois François , Abdel‐Rahman Wamied , Seuntjens Jan P. , Podgorsak Ervin B.
Published: July 2002

A hybrid phantom‐embedded extrapolation chamber (PEEC) made of Solid Water™ and bone‐equivalent material was used for determining absorbed dose in a bone‐equivalent phantom irradiated with clinical radiation beams (cobalt‐60 gamma rays; 6 and 18 MV x rays; and 9 and 15 MeV electrons). The dose was determined with the Spencer–Attix cavity theory, using ionization gradient measurements and an indirect determination of the chamber air‐mass through measurements of chamber capacitance. The collected charge was corrected for ionic recombination and diffusion in the chamber air volume following the standard two‐voltage technique. Due to the hybrid chamber design, correction factors accounting for scatter deficit and electrode composition were determined and applied in the dose equation to obtain absorbed dose in bone for the equivalent homogeneous bone phantom. Correction factors for graphite electrodes were calculated with Monte Carlo techniques and the calculated results were verified through relative air cavity dose measurements for three different polarizing electrode materials: graphite, steel, and brass in conjunction with a graphite collecting electrode. Scatter deficit, due mainly to loss of lateral scatter in the hybrid chamber, reduces the dose to the air cavity in the hybrid PEEC in comparison with full bone PEEC by 0.7% to ∼2% depending on beam quality and energy. In megavoltage photon and electron beams, graphite electrodes do not affect the dose measurement in the Solid Water™ PEEC but decrease the cavity dose by up to 5% in the bone‐equivalent PEEC even for very thin graphite electrodes (<0.0025 cm). In conjunction with appropriate correction factors determined with Monte Carlo techniques, the uncalibrated hybrid PEEC can be used for measuring absorbed dose in bone material to within 2% for high‐energy photon and electron beams.
Theoretical analysis and experimental evaluation of a CsI(Tl) based electronic portal imaging system
Authors: Sawant Amit , Zeman Herbert , Samant Sanjiv , Lovhoiden Gunnar , Weinberg Brent , DiBianca Frank
Published: July 2002

This article discusses the design and analysis of a portal imaging system based on a thick transparent scintillator. A theoretical analysis using Monte Carlo simulation was performed to calculate the x‐ray quantum detection efficiency (QDE), signal to noise ratio (SNR) and the zero frequency detective quantum efficiency [DQE(0)] of the system. A prototype electronic portal imaging device (EPID) was built, using a 12.7 mm thick, 20.32 cm diameter, CsI(Tl) scintillator, coupled to a liquid nitrogen cooled CCD TV camera. The system geometry of the prototype EPID was optimized to achieve high spatial resolution. The experimental evaluation of the prototype EPID involved the determination of contrast resolution, depth of focus, light scatter and mirror glare. Images of humanoid and contrast detail phantoms were acquired using the prototype EPID and were compared with those obtained using conventional and high contrast portal film and a commercial EPID. A theoretical analysis was also carried out for a proposed full field of view system using a large area, thinned CCD camera and a 12.7 mm thick CsI(Tl) crystal. Results indicate that this proposed design could achieve DQE(0) levels up to 11%, due to its order of magnitude higher QDE compared to phosphor screen–metal plate based EPID designs, as well as significantly higher light collection compared to conventional TV camera based systems.
Tomographic imaging of the angular‐dependent coherent‐scatter cross section
Authors: Westmore M. S. , Fenster A. , Cunningham I. A.
Published: July 2002

A new special‐purpose computed tomographic (CT) imaging system is described which produces images based on measurements of the low‐angle x‐ray diffraction properties of an object. Low‐angle scatter in the diagnostic x‐ray energy range is dominated by coherent scatter, and the system uses first‐generation CT geometry to acquire a diffraction pattern for each pencil beam. The patterns are used to reconstruct a series of images which represent the coherent‐scatter intensity at a series of scatter angles. To demonstrate the potential of coherent‐scatter CT (CSCT), the scanner has been built and used to image a phantom consisting of a water‐filled Lucite cylinder containing rods of polyethylene, Lucite, polycarbonate, and nylon. In this paper, the system is described and a sequence of CSCT images of this phantom is shown. Coherent‐scatter cross sections of these materials are generated for each pixel from this sequence of images and compared with cross sections measured separately. The resulting excellent agreement shows that the angular‐dependent coherent‐scatter cross section can be accurately imaged in a tomographic slice through an object. These cross sections give material‐specific information about the object. The long‐term goal of this research is to make measurements of bone‐mineral content for every pixel in a tomographic slice
Comparison of dose response of radiochromic film measured with He‐Ne laser, broadband, and filtered light densitometers
Authors: Reinstein L. E. , Gluckman G. R.
Published: July 2002

Dose response curves for GAFChromic MD‐55‐2 film were measured using three different densitometer systems: a He‐Ne laser densitometer, a broadband (white light) densitometer, and a filtered red light densitometer. These were found to differ significantly; the dose needed to achieve a net optical density of 1 (DNOD1) was greater than 100 Gy for the white light densitometer, 56 Gy for the He‐Ne densitometer, and only 14.8 Gy for the filtered red light densitometer. This represents approximately a fourfold increase in response for the filtered red light versus the He‐Ne laser densitometer, which is a significant improvement. For some patient prescriptions this enables us to achieve an accuracy and precision sufficient to verify daily dose to within 5%.
Electronic portal imaging for dosimetric purposes
Authors: Hansen Vibeke N.
Published: July 2002

Dosimetric aspects of Electronic Portal Imaging Devices (EPIDs) have been explored. Three models to extract the primary signal from EPIDs have been studied. The first simple model removes the scatter component using a method based on the scatter‐to‐primary ratio (SPR). This model has several limitations. Two more complex models based on convolution of Pencil Beam Spread Kernels (PBSK) have been studied, one analytical approximation and one iterative solution. Both models based on PBSK were tested on a large set of Monte Carlo generated EPID signals. The primary signal was extracted to within 2% of the true primary signal, using either model, even when SPR was 0.3 in the original image. On the basis of the primary signal of the EPID, generated from any of the above models, two new applications were developed and studied: Transit dosimetry,1 this is verification of the dose delivered using an EPID; and the design of intensity modulated beams (IMB) for tangential breast fields. The IMBs were implemented using either a physical compensator or a set of fields shaped with a multileaf collimator. Dose measurements in an anatomical phantom showed that both implementations gave better dose uniformity than the standard treatment. 1V. N. Hansen, P. M. Evans, and W. Swindell, Med. Phys. 23, 713–721 (1996).
Oncologic image compression using both wavelet and masking techniques
Authors: Yin Fang‐Fang , Gao Qinghuai
Published: July 2002

A new algorithm has been developed to compress oncologic images using both wavelet transform and field masking methods. A compactly supported wavelet transform is used to decompose the original image into high‐ and low‐frequency subband images. The region‐of‐interest (ROI) inside an image, such as an irradiated field in an electronic portal image, is identified using an image segmentation technique and is then used to generate a mask. The wavelet transform coefficients outside the mask region are then ignored so that these coefficients can be efficiently coded to minimize the image redundancy. In this study, an adaptive uniform scalar quantization method and Huffman coding with a fixed code book are employed in subsequent compression procedures. Three types of typical oncologic images are tested for compression using this new algorithm: CT, MRI, and electronic portal images with matrix size and 8‐bit gray levels. Peak signal‐to‐noise ratio (PSNR) is used to evaluate the quality of reconstructed image. Effects of masking and image quality on compression ratio are illustrated. Compression ratios obtained using wavelet transform with and without masking for the same PSNR are compared for all types of images. The addition of masking shows an increase of compression ratio by a factor of greater than 1.5. The effect of masking on the compression ratio depends on image type and anatomical site. A compression ratio of greater than 5 can be achieved for a lossless compression of various oncologic images with respect to the region inside the mask. Examples of reconstructed images with compression ratio greater than 50 are shown.
Lateral electron transport in FFT photon dose calculations
Authors: Wong Eugene , Van Dyk Jake , Zhu Yunping
Published: July 2002

Previously published FFT dose calculations and their verifications have been performed for field sizes greater than with photon energies up to 18 MV [Zhu and Boyer, Phys. Med. Biol. 35, 351–368 (1990)]. With a smaller field size at megavoltage energies, lateral electronic disequilibrium exists in low density inhomogeneities which is not handled properly by the FFT calculations. In this paper, we extend FFT dose calculations to address this problem without significantly increasing the computational time. We employ lateral scaling of the field size at each depth along with an inverse scaling of the resultant dose distribution. The concept of effective density is introduced for the lateral and inverse scaling. We demonstrate that electron transport can be adequately captured in the presence of inhomogeneities, thus improving 3D FFT dose calculations in inhomogeneous media.
A three‐dimensional volumetric test object for geometry evaluation in magnetic resonance imaging
Authors: McRobbie Donald W.
Published: July 2002

The design of a test object for the volumetric evaluation of geometric image quality parameters in magnetic resonance imaging (MRI) is presented. Two‐dimensional parameters: linearity, distortion, slice position, slice width, slice oblique angle, spatial resolution, line spread function (LSF) are measurable in all slice planes within a volume corresponding to a typical head examination. Additionally, parameters particular to three‐dimensional Fourier transform (3‐DFT) imaging are measurable: 3D ghosting and 3D LSF. Parameter measurements are correctable with regard to test object positioning through the determination of the six degrees of freedom of the test object within the scanner's field of view.
Biophysical studies of pulsed photothermal radiometry in tissues and tissuelike media
Authors: Vitkin I. Alex
Published: July 2002

Pulsed photothermal radiometry (PPTR) is a remote sensing technique which monitors the temperature transient at the surface of a sample following the absorption of a brief optical pulse. In this work, it was used to investigate optical and thermal processes occurring in laser‐irradiated, turbid, inhomogeneous media such as tissue. An optical diffusion approximation and heat conduction equation were used to derive the turbid PPTR model, for both homogeneous and layered semi‐infinite media. These models were tested experimentally in a series of aqueous gel phantoms of known optical and thermal properties, and were found to be in good agreement. A study of discrete optical absorbers in a transparent medium necessitated the inclusion of the thermal radiative effects as an additional mode of heat transfer, and a corresponding PPTR model was derived and tested experimentally. Studies of intact biomolecules and of in vivo tissue have also been performed to test and verify the derived formulations, and to determine the practical utility of PPTR for specific biomedical applications. The theoretical and experimental results presented in the thesis lay the groundwork for further inclusion of PPTR into the diagnostic arsenal of laser medicine.
Validation of a new virtual wedge model
Authors: Desobry G. E. , Waldron T. J. , Das I. J.
Published: July 2002

Results of a validation study of a commercial virtual wedge device recently installed at our institution are presented. The wedge simulation produces an energy fluence from the treatment head that is equivalent to the primary energy fluence attenuated through a wedge‐shaped slab of water with the central axis fluence set to unity. A simple exponential formula used to compute off‐axis wedge factors is compared to beam profiles measured in a water phantom. A fast Fourier transform (FFT) convolution dose calculation is compared to measured dose profiles. Measured and calculated central axis wedged/open field ratios as a function of depth are also compared.
A quality assurance test tool for high dose‐rate remote afterloading brachytherapy units
Authors: Li Zuofeng , Mitchell Thomas P. , Palta Jatinder R. , Liu Chihray
Published: July 2002

A QA test tool is designed to quantitatively measure the HDR source positioning error, and to facilitate quick and dependable HDR timer linearity test and daily output constancy check. The test tool consists of two concentric disks. The lower disk has a cutout for inserting an HDR catheter, and the upper disk accepts a diode or miniature ionization chamber and can rotate relative to the lower disk. Ionization readings from the source (transferred to the center of the disks) are obtained at two rotational positions of the upper disk which houses the detector. The ratio of the readings is used to determine the source‐positioning error of the HDR unit relative to the nominal source position by a simple triangulation principle. Experimental measurements confirm that the QA test tool is sensitive to approximately 0.2 mm variance in source positioning errors. In addition, the QA test tool is suitable for other common HDR QA tests such as the source travel step size test, the daily HDR unit output constancy check, and the timer linearity test. Its simple and robust design permits routine clinical use and provides a high confidence level in the accurate operation of HDR units.
Automated seed localization from CT datasets of the prostate
Authors: Brinkmann Debra H. , Kline Robert W.
Published: July 2002

With the increasing utilization of permanent brachytherapy implants for treating carcinoma of the prostate, the importance of accurate post‐treatment dose calculation also increases for assessing patient outcome and planning future treatments. An automatic method for seed localization of permanent brachytherapy implants, using CT datasets of the prostate, has been developed and tested on a phantom using an actual patient planned seed distribution. This method was also compared to results with the three‐film technique for three patient datasets. The automatic method is as accurate or more accurate than the three film technique for 1 mm, 3 mm, and 5 mm contiguous CT slices, and eliminates the inter‐ and intra‐observer variability of the manual methods. The automated method improves the localization of brachytherapy seeds while reducing the time required for the user to input information, and is demonstrated to be less operator dependent, less time consuming, and potentially more accurate than the three‐film technique.
Optimum tube voltage for chest radiographs obtained by psychophysical analysis
Authors: Asai Yoshiyuki , Tanabe Yumi , Ozaki Yoshiaki , Kubota Hideaki , Matsumoto Masao , Kanamori Hitoshi
Published: July 2002

Many kinds of x‐ray films having various characteristic curves have been developed for chest radiographs. In general, a phototiming device for determination of a mAs value which gives a proper exposure has been used for a chest radiography. For each film, however, the x‐ray tube voltage has been determined by the subjective evaluation of radiologists or radiological technologists. In this paper, we propose a new method for determining the optimum tube voltage for chest radiographs using psychophysical analysis. The optimum density and the optimum density range of a screen/film system are obtained from the gradient curve of film and the minimum perceptible contrast [Acta Radiol. Diagnos. 4, 463–476 (1966)]. The optimum tube voltage, by which the lowest density of a mediastinum and the highest density of a lung field just cover the optimum density range, is obtained using the x‐ray photon spectrum and sensitivity spectrum of the screen. This objective method does not depend on personal subjective evaluation, therefore it is available for the determination of optimum tube voltage for chest radiographs to be observed by many doctors of various departments.
Radiographic characterization and energy spectrum of the IoGold source Model 3631‐A
Authors: Wierzbicki Jacek G. , Waid David S. , Rivard Mark J. , Arterberry V. Elayne
Published: July 2002

Despite the growing popularity of permanent implants using encapsulated seeds, only two types were commercially available, Models 6702 and 6711. Recently, the Food and Drug Administration (FDA) approved a new type of seed, Model 3631‐A, called IoGold (apparent activity 0.28–0.63 mCi). Due to the large demand for seeds for prostate implants, IoGold seeds will be available in January 1998. In this paper, the basic properties of the IoGold seeds have been evaluated in comparison to the established Models 6702 and 6711.
Monte Carlo calculations to characterize the source for neutron therapy facilities
Authors: Bohm Tim D. , Deluca Paul M. , Cox Lawrence J. , Maughan Richard L. , Jones D. T. L. , Lennox Arlene
Published: July 2002

Modern radiation treatment planning for photons includes full 3D modeling of the adsorbed dose distribution, accurate inclusion of the patient anatomy, and consideration of significant changes in material density and composition. Such efforts are founded in an accurate description of the radiation source and the beam delivery system. Modern fast neutron therapy facilities employ highly penetrating beams and isocentric beam delivery. Treatment planning is largely based on analytic models adapted from photon codes and interaction cross sections normalized to macroscopic attenuation. However, the recent PEREGRINE initiative at Lawrence Livermore Laboratory offers the possibility of fully stochastic modeling if the neutron source can be adequately described. In this article we report neutron source modeling of three high energy facilities. Neutron production is based on the intra‐nuclear cascade model of the LAHET code while neutron transport through the beam delivery system is managed by MCNP using cross section libraries extended to 100 MeV neutron energy. PEREGRINE is then used to transport the neutron beam through typical phantoms. The resulting neutron sources are in excellent agreement with the limited experimental information and the measured phantom data are well described by the PEREGRINE transport using the LAHET/MCNP determined neutron sources.
Distance–dose curve for a miniature x‐ray tube for stereotactic radiosurgery using an optimized aperture with a parallel‐plate ionization chamber
Authors: Biggs P. J. , Beatty J. , Yasuda T.
Published: July 2002

A recently developed miniature x‐ray tube operating at 40 kV has been used in a randomized trial for the treatment of small intracranial lesions. The diameter of these lesions ranges from 10 to 30 mm. A thin window parallel‐plate ionization chamber was used to calibrate the output of the x‐ray tube, modified by the addition of a thin platinum aperture to reduce the charge collecting area of the chamber. The effect of such an aperture on the measurement of dose versus distance from the x‐ray tube in a phantom has been examined as a function of aperture diameter. Aperture diameters were varied between 0 and 5 mm and dose measurements were made for distances between the x‐ray source and the front surface of the chamber of 5–30 mm in water. The ratio of doses measured with and without an aperture, when normalized to unity at a distance of 10 mm, differs significantly from unity, for distances between 7.5 and 15 mm, for aperture diameters <1.5 mm and differs from unity, but less significantly, for apertures ⩾3 mm. For intermediate diameters, however, this dose dependence is minimized, indicating an aperture diameter that provides a similar distance–dose curve as the measurement taken without an aperture over this range of distances. This diameter was found to be between 2 and 2.5 mm with a dose variation of less than ±1%. For distances <7.5 mm, measurements made with a 1.5‐mm‐diam aperture agree better with those taken with a 1.7‐mm‐diam chamber compared with a 5.2‐mm‐diam chamber.
Computer‐aided diagnosis for detection of changes in sequential chest radiographs (in Japanese)
Authors: Sanada Shigeru
Published: July 2002

The subtraction image of digital chest radiographs taken at different times improved the diagnostic accuracy of radiologists viewing screening chest radiographs. The accurate registration of two chest images using “lung markings” instead of “rib edges” enables us to detect subtle changes in the “difference image.” The “lung markings” are the shadows of blood vessels traversing lung substance. Most of the morbid changes grow in lung substance not in the rib cage. Twenty‐nine pairs of posteroanterior chest radiographs with and without temporal morbid changes were selected from approximately 200 sequential cases. The images were 1760×1760 matrix and 1024 gray scale with a computed radiography system (Fuji Medical System, Tokyo). Image registration employed the matching of “lung markings” in the two radiographs. Observer performance tests with and without the use of the “difference image” were carried out by eight radiologists. Six of the eight observers diagnosed with higher sensitivity. The mean for eight observers increased from 43.9% to 55.3% with a statistical significance of The mean area under the AFROC (Alternative Free‐Response Receiver Operating Characteristics) curve improved from 0.596 to 0.647 with a statistical significance of The false positive response was comparable with and without the use of the subtraction image. We conclude that the use of the subtraction image with the new registration technique improves the diagnostic accuracy for detection of subtle changes between radiographs, such as nodule, infiltrate, and cardiomegaly of radiographic findings.
Quantitative planar imaging method for measurement of renal activity by using a conjugate‐emission image and transmission data
Authors: Kojima Akihiro , Ohyama Yoichi , Tomiguchi Seiji , Kira Mitsuko , Matsumoto Masanori , Takahashi Mutsumasa , Motomura Nobutoku , Ichihara Takashi
Published: July 2002

We are proposing a method to accurately measure renal activity in renography using Tc‐99m labeled tracers. This method uses a conjugate‐view image and transmission data for attenuation correction, the triple energy window (TEW) method for scatter correction, and background correction techniques that consider the source volume for accurate background activity correction. To examine this method in planar imaging, we performed two renal phantom studies with various uniform background activity concentrations. One study used two ideal box‐shaped kidney phantoms with a thickness of 2 or 4 cm in a water tank and the other study employed two real kidney‐shaped phantoms in a fillable abdominal cavity. For these studies the kidney phantom‐to‐background activity concentration ratio (S) was changed from 5 to ∞. The transmission data were obtained with an external Tc‐99m line array source. The anterior‐ and posterior‐view emission images were acquired with a dual‐headed gamma camera simultaneously and the TEW method was used to correct scatter for the emission and transmission images. The results showed that this method with both the accurate background correction and scatter correction could give depth‐independent count rates and could estimate the true count rate with errors of less than 5% for all S values. However, if either accurate background correction or scatter correction was performed alone, the absolute error increased to about 50% for the smaller S values. Our proposed method allows one to accurately and simply measure the renal radioactivity by planar imaging using the conjugate‐emission image and transmission data.
On scatter in megavoltage x‐ray transmission imaging (in English)
Authors: Spies Lothar
Published: July 2002

This thesis considers the role of scatter in electronic portal imaging devices (EPIDs). The thesis involved a direct measurement of scatter in an EPID. An already established Monte Carlo code, developed at the Royal Marsden Hospital, Sutton/UK, was extended and agreed with the experimental data for phantom‐to‐detector distances larger than 10 cm. An analytical scatter model was developed on the basis of the Monte Carlo analysis. In this model scatter was estimated by convolving the input beam profile with an analytical scatter kernel. The model was tested over a wide range of clinical parameters. For a 6 MV x‐ray beam impinging upon a water‐equivalent phantom, the absolute error in the scatter‐to‐primary ratio was better than 1% for a 100 field. The accuracy dropped to 4% for a 400 field. The model was implemented for a camera‐based EPID with a converter comprising a 1‐mm‐thick copper plate bonded to a 134 phosphor foil. Algorithms were developed for correcting for scatter contributions in megavoltage CT (MVCT) and for reconstruction of input beam profiles. For the latter an overall accuracy of better than 4% was obtained for fields of 100 in size. In MVCT the yield in contrast resolution after a scatter correction was small. However, a scatter correction reduced the discrepancy of 30% between true and reconstructed electron densities to 10%. In both applications the limit in accuracy is a direct consequence of the performance of the converter, which strongly overestimates multiply scattered photons of low energy. Within the framework of a convolution model multiply scattered radiation exiting an inhomogeneous phantom cannot be calculated with an accuracy exceeding the limits given.
Technical note: Improved analytical fit to the TG‐43 radial dose function,
Authors: Moss Dale C.
Published: July 2002

An improved analytical fit for the TG‐43 radial dose function, is presented using a modified sigmoid function. Fit coefficients and graphs are provided for comparison with the polynomial fits given in TG‐43.
Treatment planning algorithm corrections accounting for random setup uncertainties in fractionated stereotactic radiotherapy
Authors: Zavgorodni S. F.
Published: July 2002

A number of relocatable head fixation systems have become commercially available or developed in‐house to perform fractionated stereotactic radiotherapy (SRT) treatment. The uncertainty usually quoted for the target repositioning in SRT is over 2 mm, more than twice that of stereotactic radiosurgery (SRS) systems. This setup uncertainty is usually accounted for at treatment planning by outlining extra target margins to form the planning target volume (PTV). It was, however, shown by Lo et al. [Int. J. Radiat. Oncol., Biol., Phys. 34, 1113–1119 (1996)] that these extra margins partly offset the radiobiological advantages of SRT. The present paper considers dose calculations in SRT and shows that the dose predictions could be made at least as accurate as in SRS with no extra margins required. It is shown that the dose distribution from SRT can be calculated using the same algorithms as in SRS, with the measured off‐axis ratios (OARs) replaced by “effective” OARs. These are obtained by convolving the probability density distribution of the isocenter positions (assumed to be normal) and the original OARs. An additional output correction factor has also been introduced accounting for the isocenter dose reduction (2.4% for a 7 mm collimator) due to the OARs “blurring.” Another correction factor accommodates for the reduced (by 1% for 6 MV beam) dose rate at the isocenter due to x‐ray absorption in the relocatable mask. Mean dose profiles and the standard deviations of the dose (STD) were obtained through simulating SRT treatment by a combination of normally distributed isocenters. These dose distributions were compared with those calculated using the convolution approach. Agreement of the dose distributions was within 1%. Since standard deviation reduces with the number of fractions, N, as the planning predictions in fractionated stereotactic radiotherapy can be made more accurate than in SRS by increasing N and using “effective” OARs along with corrected dose output.
Clinical reference dosimetry: Comparison between AAPM TG‐21 and TG‐51 protocols
Authors: Ding G. X. , Cygler J. E. , Kwok C. B.
Published: July 2002

We compare the results of absorbed dose determined at reference conditions according to the AAPM TG‐21 dose calibration protocol and the new AAPM TG‐51 protocol. The AAPM TG‐21 protocol for absorbed dose calibration is based on ionization chambers having exposure calibration factors for gamma rays, The new AAPM TG‐51 dosimetry protocol for absorbed dose calibration is based on ionization chambers having absorbed dose‐to‐water calibration factor, This study shows that the dose changes are within 1% for a cobalt beam, 0.5% for photon energies of 6 and 18 MV, and 2%–3% for electron beams with energies of 6 to 20 MeV. The chamber primary calibration factors, and are traceable to the Canadian primary standards laboratory (NRCC). We also present estimated dose changes between the two protocols when calibration factors are traceable to NIST in the United States.
Development of a time‐resolved optical tomography system for neonatal brain imaging
Authors: Schmidt Florian E. W.
Published: July 2002

This Ph.D. project involved the development and evaluation of a prototype medical optical tomography system. The time‐resolved 32‐channel instrument is presented, and various instrumental aspects and performance issues are discussed. It has been designed primarily as a continuous bedside monitor for obtaining functional images of premature infants' brains that are at an increased risk of injury due to dysfunction in cerebral oxygenation or haemodynamics. The fully automated device employs 32 source fibers that sequentially deliver near infrared pulsed laser radiation of picosecond duration to the tissue. Transit time measurements of very high temporal resolution (∼100 ps FWHM) and stability (∼5 ps/h drift, and jitter of similar magnitude) are made between these sources and 32 detector optodes also located on the skin surface. Photons transmitted diffusely through the tissue and collected by the optodes are transmitted to four ultra‐fast eight‐anode Microchannel Plate Photomultiplier Tube detectors. Thirty‐two fully simplex time‐correlated single photon counting channels simultaneously record histograms of the photon flight times at rates of up to about 300,000 counts per second per channel. These so‐called temporal point spread functions represent the raw data for the image reconstruction. Separate maps of the internal absorption and scattering properties can be reconstructed from purely temporal data without recourse to reference or baseline measurements. The effectiveness of this instrument has been demonstrated by successfully imaging various tissue‐equivalent phantoms and an adult forearm. A preliminary post‐mortem study suggested that a sufficient number of photons can be transmitted across the head of the neonate at safe exposure levels and durations to obtain an adequate signal.
Dynamic and omni wedge implementation on an Elekta SL linac
Authors: Phillips Mark H. , Parsaei Homayoon , Cho Paul S.
Published: July 2002

The concommitant use of a multileaf collimator (MLC) and a wedge can result in conflicts in the optimal collimator angle if both MLC and wedge are fixed relative to one another. This is particularly true of linacs in which a single wedge orientation is provided. In this paper, a solution is provided that makes use of two orthogonal universal wedges (omni wedge). Although this technique can be applied regardless of the means by which the wedged fields are implemented, the measurements reported in this paper were performed using a fixed, internal mechanical wedge coupled with a dynamic wedge, formed by the motion of one of the backup jaws. An implementation of a dynamic wedge for the Elekta SL series of linear accelerators is presented. Results of measurements of the dosimetric characteristics of both the particular implementation of the dynamic wedge and of the omni field are presented. For the dynamic wedge, measurements were made of the wedge factor and dose profile as a function of field size and depth. In addition, the effects of variables, such as dynamic delivery technique and direction of diaphragm motion, on the dynamic wedge profiles were studied and discussed. For the omni wedge, measurements were made of the degree to which the mathematical formalism for describing an omni wedge matches the measured isodose distributions. Comparisons between mechanical wedge dose distributions and the omni wedge were also made.
Relativistic precession of quantum elliptical states in the Coulomb potential
Authors: Crawford Michael G. A.
Published: July 2002

A special relativistic perturbation to nonrelativistic quantum mechanics is shown to lead to the classical special relativistic prediction for the rate of precession for quantum states in the Coulomb potential. This behavior is illustrated using SO(4) coherent states as examples. These states are localized on Kepler ellipses and precess in the presence of a relativistic perturbation.
Measurement of a third-order elastic constant in silicon by a comparison method
Authors: Duquesne J.-Y.
Published: July 2002

The third-order elastic constant C111 of silicon is measured with a comparison method, based on the interaction between longitudinal acoustic waves. The value is in good agreement with published values and demonstrates the validity of the method.
Image shift caused by strong lateral reflections, and its relation to inter-aural cross correlation
Authors: Okano Toshiyuki
Published: July 2002

Conditions of sound fields were studied to avoid the occurrence of image shift, an acoustical phenomenon caused by extremely strong lateral reflections. Three types of image shift are defined as variations of the normal condition, in which there is only one auditory image perceived in the source direction. From a series of listening tests in electrically reproduced sound fields, each of the three types of image shift was found to have its own specific frequency range in which the image shift occurred. They were also found to relate to each of three measures which are all derived from the inter-aural cross correlation function (IACF) of binaural impulse responses of a sound field. The results of measurement made in three existing concert halls suggested acceptable ranges for the measures, in which the image in the source direction is not lost in the 500 Hz and higher frequency bands. However, the 250 Hz and lower frequency bands were found not so important in evaluating sound fields of concert halls in relation to the image shift. Finally, 0.85 is found to be the upper limit for the preferable range of [1−IACCE3].
Novel graph model and analysis method for piezoelectric thickness-drive transducers
Authors: Hsueh Wen-Jeng
Published: July 2002

This study develops a new graph model and analysis scheme for representing and analyzing a piezoelectric thickness-drive transducer. The proposed TWSF graph model, expressed via electrical and acoustical variables instead of electromechanical analogy, simplifies understanding of the physical mechanism of the transducer. The primary advantage of the graph model analogy is that performance characteristics, such as the electrical and mechanical impedance, and the force and velocity responses of the transducer system can be calculated directly using the topology scheme based on the graph model. The dynamic equations do not need to be formulated according to the analogy model for design and analysis. The proposed method also does not require computation schemes to solve the equations for obtaining the characteristics and the responses. Compared to the classical method, the present graph and analysis scheme offers a more efficient technology to obtain precise results with less analysis. Moreover, the derived results are expressed as both analytical and closed-form formulas, making them convenient to apply in analysis and design.
An illustration of analytical/numerical matching with finite-element analysis for structural vibration problems
Authors: Franzoni Linda P. , Park Christopher D.
Published: July 2002

Analytical/numerical matching (ANM) is an accurate and efficient method for solving many types of problems with discontinuities. The method separates local and global effects, and solves separate subproblems using high resolution around the discontinuity and low resolution away from the discontinuity. The work presented in this manuscript demonstrates a methodology for applying ANM to a dynamic structure using finite-element analysis (FEA) for the solution of the high-resolution (local) and the low-resolution (global) subproblems. The ANM method is illustrated on a thick, two-dimensional beam having several displacement constraints attached to its lower surface. Ordinarily (and here, for verification purposes) this problem would be solved using two-dimensional plane elements due to the local discontinuities around the constraints and the thickness of the beam. Using ANM, these discontinuities and through-thickness effects are modeled in the geometrically compact local problem using a high-resolution mesh of two-dimensional eight-node plane elements. The much larger global problem contains no discontinuities and is reduced to the solution of a low-resolution finite-element mesh of two-node Bernoulli–Euler beam elements. A third subproblem (matching) is solved analytically (no computational overhead). The agreement between the ANM solution and the purely FEA solution is excellent, and the computational savings are significant.
Modeling vibrational energy transmission at bolted junctions between a plate and a stiffening rib
Authors: Bosmans Ivan , Nightingale Trevor R. T.
Published: July 2002

An analytical model is presented for structure-borne sound transmission at a bolted junction in a rib-stiffened plate structure. The model is based on the wave approach for junctions of semi-infinite plates and calculates coupling loss factors required by statistical energy analysis. The stiffening rib is modeled as a plate strip and the junction is represented by an elastic interlayer with a spatially dependent stiffness. Experimental verification is carried out on a series of Plexiglas plate structures with varying rib depth and bolt spacing. A well-defined connection length at the junction was created by inserting thin spacers between the plate and the rib at each bolt. Comparison between numerical and experimental data for this case showed good agreement. Measured results for the bolted junction without spacers suggested that structure-borne sound transmission could be modeled as a series of connections characterized by a finite connection length. This concept is explored further by determining an equivalent connection length which gives the best agreement between numerical and experimental data.
Vortex sound generation due to a flow impedance discontinuity on a flat surface
Authors: Tang S. K. , Li K. M.
Published: July 2002

The sound generated by the unsteady motion of a vortex filament moving over a flat boundary with a sharp flow impedance discontinuity is studied theoretically. Theoretical results show that the vortex filament undergoes significant accelerating or decelerating motions and radiates sound at the instant when it moves across the plane of impedance discontinuity. The accelerations and decelerations of the vortex filament are shown to be the major mechanisms of sound generation. The sound so produced has a large low-frequency content such that the change in the flow impedance affects only the sound generation process but not the subsequent sound propagation to the far field.
Finite-element modeling of lead magnesium niobate electrostrictive materials: Dynamic analysis
Authors: Coutte Jocelyne , Debus Jean-Claude , Dubus Bertrand , Bossut Régis
Published: July 2002

A finite-element model is proposed for the time-domain analysis of electrostrictive materials. Hom’s material model, developed for lead magnesium niobate (PMN) ceramics, is used. It includes the quadratic dependence of strain with polarization, the saturation of polarization, assumes constant temperature, and excludes hysteresis. The theoretical formulation is justified by the principle of virtual works. The numerical model is obtained after discretization in space and time. The validation is performed by comparing numerical results with semianalytical results for an electrostrictive spherical shell subjected to a step in voltage or in charge. From these results, a method to compute the coupling coefficient of electrostrictive materials, based on Ikeda’s definition, is proposed and applied to a bar with parallel electric field.