bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron dose calculation using multiple‐scattering theory: A new theory of multiple scattering
Authors: Jette David
Published: March 2003

Starting from the Boltzmann–Fokker–Planck transport equation, we have developed a new theory of multiple scattering which incorporates the advances already made with our Gaussian multiple‐scattering theory for electron dose calculation. This incorporation has been accomplished in a natural way, by modifying the scattering power T and by adding a convolution term to the distribution‐function equation of the Gaussian theory. Our previous results concerning increasing the accuracy of the small‐angle approximation used and dealing with localized tissue inhomogeneities have thus been maintained, and we have arrived at a complete distribution function in both transverse spatial and angular variables. When integrated over the transverse angular variables, for a first‐order small‐angle approximation this distribution function for a pencil beam is essentially the same as the Molière multiple‐scattering distribution, which includes large‐angle single scattering. For a water phantom, we have used comparisons with EGS4 Monte Carlo calculations to demonstrate the greatly increased accuracy of our new multiple‐scattering theory over the Gaussian theory, which includes the usual Fermi–Eyges theory. We have also presented a fairly accurate Gaussian approximation to the pencil‐beam dose profiles given by our new theory, which can be used in order to maintain the mathematical simplicity of the predictions of the Fermi–Eyges theory.
A new radioimmunotherapy (RIT) treatment planning system employing a semi‐automated segmentation toolkit and fast Hartley transform (FHT) for 3D convolution
Authors: Erdi Alev Kutan
Published: March 2003

Efficient RIT depends on a priori knowledge of the radiation absorbed dose distribution. Straightforward application of published table of S values, which are for standard‐size homogeneous activity sources in a standard‐size body geometry, is not sufficient for accurate RIT dosimetry calculations. A new, fast, and functional treatment planning approach is developed that considers heterogeneous activity distributions. Calculation of the three‐dimensional absorbed dose distribution requires convolution of a cumulated activity distribution with a point‐source kernel; both represented by large matrices. To reduce the computation time required, a novel implementation of convolution using FHT is realized. Absorbed dose calculation time was reduced from 80 hours to 5 minutes. Volume measurement error has a direct impact on the absorbed dose calculation when mean activity information for an organ is used. Since identification of the tumor and other organs of interest is time‐consuming, a semi‐automated segmentation toolkit is developed. It is now possible to determine the volume of a structure in half the time with the same accuracy. The developed system was used to calculate and display as isodose contours the absorbed dose distribution in a patient's tumor and for a bone marrow sample. [For copies contact the author or University Microfilms, Inc., Ann Arbor, MI 48106 (Phone: 1‐800‐521‐0600).]
Mean energy, energy‐range relationships and depth‐scaling factors for clinical electron beams
Authors: Ding G. X. , Rogers D. W. O. , Mackie T. R.
Published: March 2003

Using Monte Carlo simulations we have studied the electron mean energy, Ēo, and the most probable energy, Eo,p, at the phantom surface and their relationships with half‐value depth, R50, and the practical range, Rp, for a variety of beams from five commercial medical accelerators with an energy range of 5–50 MeV. It is difficult to obtain a relation between R50 and Ēo for all electrons at the surface because the number of scattered lower‐energy electrons varies with the machine design. However, using only direct electrons to calculate Ēo, there is a relationship which is in close agreement with that calculated using monoenergetic beams by Rogers and Bielajew [Med. Phys. 13, 687–694 (1986)]. We show that the empirical formula describes accurately the relationship between Rp andEo,p for clinical beams of energies from 5 to 50 MeV with an accuracy of 3%. The electron mean energy, Ēd, is calculated as a function of depth in water as well as plastic phantoms and is compared both with the relation, Ēd=Ēo(1−d/Rp), employed in AAPM protocols and with values in the IAEA Code of Practice. The conventional relations generally overestimate Ēd over the entire therapeutic depth, e.g., the AAPM and IAEA overestimate Ēd at dmax by up to 20% for an 18 MeV beam from a Clinac 2100C. It is also found that at all depths mean energies are 1%–3% higher near the field edges than at the central axis. We calculated depth‐scaling factors for plastic phantoms by scaling the depth in plastics to the water‐equivalent depth where the mean energies are equal. The depth‐scaling factor is constant with depth in a given beam but there is a small variation (<1.5%) depending on the incident beam energies. Depth‐scaling factors as a function of R50 in plastic or water are presented for clear polystyrene, white polystyrene and PMMA phantom materials. The calculated depth‐scaling factor is found to be equal to . This is just the AAPM definition of effective density but there are up to 2% discrepancies between our calculated values and those recommended by the AAPM and the IAEA protocols. We find that the depth‐scaling factors obtained by using the ratio of continuous‐slowing‐down ranges are inaccurate and overestimate our calculated values by 1%–2% in all cases. We also find that for accurate work, it is incorrect to use a simple 1/r2 correction to convert from parallel beam depth‐dose curves to point source depth‐dose curves, especially for high‐energy beams.
Automatic correction of biplane projection imaging geometry
Authors: Close Robert , Morioka Craig , Whiting James S.
Published: March 2003

A novel method is presented for correcting errors in measurements of biplane projection imaging geometry without prior identification of corresponding points in the two images. For imaged objects that project onto both images, a constraint equation is obtained that relates weighted integrals along corresponding epipolar lines. The integrals are computed to first order in the angular beamwidth, which is assumed to be small. Starting from measured or estimated values, geometrical parameters are computed iteratively in order to maximize the correlation between epipolar line integrals in the two images. Improvement in the computation of corresponding epipolar lines is demonstrated on images of a wire phantom. The root mean square distance of the epipolar lines from the corresponding reference points is improved from 15 pixel widths to less than 4 pixel widths (1.3 mm). Convergence is demonstrated on phantom images for individual parameter variations up to 70% in relative magnification, a relative shift of the imaging planes by 50 pixels, or a relative rotation of at least 35° around either of two axes. Applicability to clinical images is demonstrated by using a biplane angiogram of a pig to align corresponding points determined from images of a Perspex cube acquired with the same geometry.
Flattening‐filter‐based empirical methods to parametrize the head scatter factor
Authors: Lam Kwok L. , Muthuswamy Moorthy S. , Ten Haken Randall K.
Published: March 2003

Parametrizing the collimator scatter factor, Sc (or head scatter factor), of a linear accelerator by the side of the equivalent square of the collimator setting at the isocenter does not accurately predict the change in Sc when the width and length of a rectangular field are exchanged. We have studied two methods based on measurements of square fields to predict Sc's of rectangular fields more accurately. The first method parametrizes Sc by the side of the equivalent square of the flattening filter region visible from the point of calculation. The Sc's of rectangular fields were predicted to an accuracy of 1% from measurements with square fields. The second method computes Sc of rectangular configurations by integrating radiation that can reach the point of calculation from a point source at the target and a second extended source at the flattening filter. The radial distribution of the extended source at the level of the flattening filter is computed from Sc of square fields measured at the isocenter. Effects of extended distance are modeled by separately performing inverse square law corrections for the two sources. This method also predicted the measured values to within 1% accuracy.
Lung compensation in total body irradiation: A radiographic method
Authors: Hussein Sherali , Kennelly Greg M.
Published: March 2003

Using megavoltage radiography and a composite chest phantom, exit dose measurements were carried out to establish an empirical relationship between the optical film density and the corresponding equivalent thickness of overlying phantom material. Results for Co‐60, 4‐MV, and 10‐MV photons show that the relationship depends on the sensitometric properties of the radiographic film and the photon beam quality. For an actual patient undergoing total body irradiation (TBI), a chest radiograph in treatment geometry provides the optical density information that is used to calculate the tissue deficit in the lung region. The compensators are made of lead whose thickness is chosen to replace the tissue deficit over the lung region. The validity of the method is established both by comparing its results to that obtained from multiple‐slice computed tomography (CT) data for 4‐ and 10‐MV photons, and by in‐phantom thermoluminescent dosimetry (TLD) for Co‐60, 4‐MV, and 10‐MV beams.
Evaluation of a high‐density scintillating glass for portal imaging
Authors: Bissonnette Jean‐Pierre , Munro P.
Published: March 2003

One of the main factors that limits the performance of T.V. camera‐based portal imaging systems is the poor light‐collection efficiency of the lens and T.V. camera. An x‐ray detector that produces more light per incident x ray would help overcome this limitation. We have been evaluating a high‐density (3.8 g/cm3), thick (12 mm) glass scintillator for its suitability as an x‐ray detector for T.V. camera‐based portal imaging systems. The light output and spatial resolution of the glass scintillator has been compared to that of a copper plate/phosphor screen detector using radiographic film and the T.V. camera of our portal imaging system. The film measurements show that the light output of the glass scintillator is 82% of that of the copper plate/phosphor screen, while the T.V. camera measurements show that this value is 48%. A theoretical model of light transport described in this paper suggests that this discrepancy is due to refraction at the glass–air interface. Our measurements of the modulation transfer function (MTF) show that the spatial resolution obtained with the glass scintillator is similar to that obtained with the copper plate phosphor screen. However, the spatial resolution obtained with the glass scintillator decreases as the angle of x‐ray incidence increases; this decrease, which is not observed for the copper plate/phosphor screen detector, is due to the large thickness of the glass scintillator. Due to the limited light output and the variable spatial resolution, the transparent glass scintillator, in its current form, is not suitable for portal imaging.
125I eye plaque dose distribution including penumbra characteristics
Authors: de la Zerda Alberto , Chiu‐Tsao Sou‐Tung , Lin Jun , Boulay Leslie L. , Kanna Ilham , Kim Jae Ho , Tsao Hung‐Sheng
Published: March 2003

The two main purposes of this work are (1) to determine the penumbra characteristics for 125I eye plaque and the relative influence of the plaque and eye–air interface on the dose distribution, and (2) to initiate development of a treatment planning algorithm for clinical dose calculations. Dose was measured in a newly designed solid water eye phantom for an 125I (6711) seed at the center of a 20 mm COMS eye plaque using thermoluminescent dosimeter (TLD) “cubes” and “minichips” inside and outside the eye, in the longitudinal and transverse central planes. TLD cubes were used in most locations, except for short distances from the seed and in the penumbra region. In the presence of both the plaque and the eye–air interface, the dose along the central axis was found to be reduced by 10% at 1 cm and up to 20% at 2.5 cm, relative to the bulk homogeneous phantom case. In addition, the overall dose reduction was greater for larger off‐axis coordinates at a given depth. The penumbra characteristics due to the lip collimation were quantified, particularly the dependence of penumbra center and width on depth. Only small differences were observed between the profiles in the transverse and longitudinal planes. In the bulk geometry (without the eye–air interface), the dose reduction due to the presence of the plaque alone was found to be 7% at a depth of 2.5 cm. The additional reduction of 13% observed, with the presence of eye–air interface (20% combined), can be attributed to the lack of backscattering from the air in front of the eye. The dose‐reduction effect due to the anterior air interface alone became unnoticeable at a depth of 1.1 cm (1.5 cm from the eye–air interface). An analytic fit to measured data was developed for clinical dose calculations for a centrally loaded seed. The central axis values of the dose rates multiplied by distance squared, Dr2, were fitted with a double exponential function of depth. The off‐axis profile of Dr2, at a given depth, was parametrized by a modified Fermi–Dirac function to model both the penumbra characteristics due the plaque lip collimation and the effect of oblique filtration by silastic.
Experimental determination of the dose kernel in high‐energy x‐ray beams
Authors: Ceberg Crister P. , Bjärngard Bengt E. , Zhu Timothy C.
Published: March 2003

A semiempirical method to characterize the pencil‐beam dose kernel is presented. Results from measurements are described by mathematical models of the applicable physical processes. The measurements were made with 6 and 25 MV x‐ray beams from a linear accelerator. Broad‐beam notations were used consistently, and the pencil‐beam quantities were obtained by differentiation. The results were compared to pencil‐beam kernels calculated by Monte Carlo techniques. The analysis of the measured data included a number of approximations. It was assumed that all the constituent pencil beams in the field are parallel, i.e., the divergence is ignored. Furthermore, the lateral variations of the incident photon fluence and the energy spectrum were disregarded. Monte Carlo calculations, on the other hand, are based on an average energy spectrum over the field, and are free from divergence and variations in the incident photon fluence. Measured and Monte Carlo calculated pencil beams nevertheless agreed well, and the approximations mentioned caused at maximum 2.7% discrepancies for the largest field size at 6 MV.
Calculation of portal dose using the convolution/superposition method
Authors: McNutt Todd R. , Mackie T. Rock , Reckwerdt Paul , Papanikolaou Nikos , Paliwal Bhudatt R.
Published: March 2003

The convolution/superposition method was used to predict the dose throughout an extended volume, which includes a phantom and a portal imaging device. From the calculated dose volume, the dose delivered in the portal image plane was extracted and compared to a portal dose image. This comparison aids in verifying the beam configuration or patient setup after delivery of the radiation. The phantoms used to test the accuracy of this method include a solid water cube, a Nuclear Associates CT phantom, and an Alderson Rando thorax phantom. The dose distribution in the image plane was measured with film and an electronic portal imaging device in each case. The calculated portal dose images were within 4% of the measured images for most voxels in the central portion of the field for all of the extended volumes. The convolution/superposition method also enables the determination of the scatter and primary dose contributions using the particular dose deposition kernels for each contribution. The ratio of primary dose to total dose was used to extract the primary dose from the detected portal image, which enhances the megavoltage portal images by removing scatter blurring. By also predicting the primary energy fluence, we can find the ratio of computed primary energy fluence to total dose. Multiplying this ratio by the measured dose image estimates the relative primary energy fluence at the portal imager. The image of primary energy fluence possesses higher contrast and may be used for further quantitative image processing and dose modeling.
Three‐dimensional reconstruction of vascular trees: Experimental evaluation
Authors: Henri C. J. , Peters T. M.
Published: March 2003

This paper is the second of two that together present a novel approach to the problem of reconstructing vascular trees from a small number of projections. Previously, we described the reconstruction algorithm and how it effectively circumvents the matching or “correspondence problem” found in most photogrammetric or computer‐vision‐based approaches. The algorithm is fully automatic and assumes that the imaging geometry is known, the vascular tree is a connected structure, and that its center‐lines have been identified in three or more images. It employs consistency and connectivity constraints and comprises three steps: The first generates a connected structure representing the multiplicity of solutions that are consistent with the first two views; the second assigns a measure of agreement to each branch in this structure based on one or more additional projections; and the third step employs this measure to distinguish between those branches comprising the vasculature and the accompanying artifacts. This paper addresses the issue of validation via simulations and experiments. In addition to a clinical case, we examine the performance of the algorithm when applied to simulated projections of two 3‐D vascular models, both representative of the complexity faced in coronary and cerebral angiography. The results in each instance are impressive and demonstrate that adequate reconstructions may be obtained with as few as three distinct views.
Estimation of fractal dimension in radiographs
Authors: Veenland J. F. , Grashuis J. L. , van der Meer F. , Beckers A. L. D. , Gelsema E. S.
Published: March 2003

In the last decade, the fractal dimension has become a popular parameter to characterize image textures. Also in radiographs, various procedures have been used to estimate the fractal dimension. However, certain characteristics of the radiographic process, e.g., noise and blurring, interfere with the straightforward application of these estimation methods. In this study, the influence of quantum noise and image blur on several estimation methods was quantified by simulating the effect of quantum noise and the effect of modulation transfer functions, corresponding with different screen–film combinations, on computer generated fractal images. The results are extrapolated to explain the effect of film‐grain noise on fractal dimension estimation. The effect of noise is that, irrespective of the noise source, the fractal dimension is overestimated, especially for lower fractal dimensions. On the other hand, blurring results in an underestimation of the dimensions. The effect of blurring is dependent on the estimation method used; the dimension estimates by the power spectrum method are lowered with a constant value, whereas the underestimation by the methods working in the spatial domain is dependent on the given dimension. The influence of the MTF and noise on fractal dimension estimation seriously limits the comparability of fractal dimensions estimated from radiographs which differ in noise content or MTF. Only when the power spectrum method is used, it is possible to correct for the influence of different MTFs of screen–film combinations. It is concluded that only when using the same object–focus distance, the same exposure conditions, the same digitizer at the same resolution, can fractal dimensions as estimated in radiographs be reliably compared.
Magnetic repulsion of linear accelerator contaminates
Authors: Butson Martin J. , Wong Tony P. Y. , Law Andy , Law Maria , Mathur Jagdish N. , Metcalfe Peter E.
Published: March 2003

Neodymium Iron Boron (NdFeB) rare earth permanent magnets have unique properties that enable them to fit easily onto the accessory mount of a clinical linear accelerator to partially sweep away electron contamination produced by the treatment head and block trays and thus increase skin sparing. Using such magnets the central axis entrance surface dose has been reduced by 11% for a 20×30 cm field size from 32% to 21% of maximum dose by the magnetic device. A reduction of 14% from 32% to 18% was seen for a 20×20 cm field size with a 6 mm perspex block tray positioned above the magnet. The magnetic device is light weight and thus clinically usable.
Evaluation of 14(R,S)‐[18F]fluoro‐6‐thia‐heptadecanoic acid (FTHA) as a positron emission tomography fatty acid tracer of beta‐oxidation in the heart
Authors: Pooley Robert A.
Published: March 2003

The radiolabeled long‐chain fatty acid analog, 14(R,S)‐[18F]fluoro‐6‐thia‐heptadecanoic acid (FTHA), was designed (DeGrado, 1991) to undergo metabolic trapping subsequent to its commitment to the beta‐oxidation (β‐ox) pathway. FTHA was evaluated under various conditions in two animal models for heart research: isolated perfused working rat heart and open‐chest extracorporeally perfused swine heart. FTHA was infused into working rat hearts. Linear radioactivity accumulation was measured by external coincidence detection; the slope was used to determine an estimate for the rate of β‐ox and was compared to the rate determined from the standard tracer, 3H‐palmitate (3H‐PA). Experiments were conducted under conditions of low and high workload, fed and fasted states, suppression of β‐ox, and various perfusate concentrations of palmitate and glucose. Swine LAD artery was cannulated and perfused from the femoral artery through a perfusion pump. FTHA and 3H‐PA were simultaneously infused during three 15 min segments of hyperemia, control, and β‐ox suppression. Blood samples were collected and assayed for 3H2O and 18F concentrations. Results indicate that FTHA uptake is specifically sensitive to β‐ox, sensitive to changes in energy demand, independent of flow, and can accurately determine in vivo rates of β‐ox in the normal clinical range of fatty acid concentrations. At concentrations higher than this range, FTHA showed a decreased sensitivity to β‐ox, indicating the required use of a correction factor in the clinic.
TLD measurements of invivo mammographic exposures and the calculated mean glandular dose across the United States
Authors: Gentry J. R. , DeWerd L. A.
Published: March 2003

An analysis is presented of the exposures received by TLDs placed on the breasts of 4400 women obtaining mammograms at 170 institutions across the United States. Mean glandular dose and exposure were examined as a function of compressed breast thickness. The exposure and mean glandular dose were found to increase linearly with breast thickness. The mean glandular dose typically delivered by the institutions was well below the limit of 3.0 mGy for the 4.5‐cm breast. However, some institutions tend to uniformly give higher doses. In such institutions approximately 25% of 4.5‐cm‐thick breasts received a mean glandular dose exceeding 3.0 mGy.
Image feature selection by a genetic algorithm: Application to classification of mass and normal breast tissue
Authors: Sahiner Berkman , Chan Heang‐Ping , Wei Datong , Petrick Nicholas , Helvie Mark A. , Adler Dorit D. , Goodsitt Mitchell M.
Published: March 2003

We investigated a new approach to feature selection, and demonstrated its application in the task of differentiating regions of interest (ROIs) on mammograms as either mass or normal tissue. The classifier included a genetic algorithm (GA) for image feature selection, and a linear discriminant classifier or a backpropagation neural network (BPN) for formulation of the classifier outputs. The GA‐based feature selection was guided by higher probabilities of survival for fitter combinations of features, where the fitness measure was the area Az under the receiver operating characteristic (ROC) curve. We studied the effect of different GA parameters on classification accuracy, and compared the results to those obtained with stepwise feature selection. The data set used in this study consisted of 168 ROIs containing biopsy‐proven masses and 504 ROIs containing normal tissue. From each ROI, a total of 587 features were extracted, of which 572 were texture features and 15 were morphological features. The GA was trained and tested with several different partitionings of the ROIs into training and testing sets. With the best combination of the GA parameters, the average test Az value using a linear discriminant classifier reached 0.90, as compared to 0.89 for stepwise feature selection. Test Az values with a BPN classifier and a more limited feature pool were 0.90 with GA‐based feature selection, and 0.89 for stepwise feature selection. The use of a GA in tailoring classifiers with specific design characteristics was also discussed. This study indicates that a GA can provide versatility in the design of linear or nonlinear classifiers without a trade‐off in the effectiveness of the selected features.
Sinusoidal modulation analysis for optical system MTF measurements
Authors: Boone John M. , Yu Tong , Seibert J. Anthony
Published: March 2003

The modulation transfer function (MTF) is a commonly used metric for defining the spatial resolution characteristics of imaging systems. While the MTF is defined in terms of how an imaging system demodulates the amplitude of a sinusoidal input, this approach has not been in general use to measure MTFs in the medical imaging community because producing sinusoidal x‐ray patterns is technically difficult. However, for optical systems such as charge coupled devices (CCD), which are rapidly becoming a part of many medical digital imaging systems, the direct measurement of modulation at discrete spatial frequencies using a sinusoidal test pattern is practical. A commercially available optical test pattern containing spatial frequencies ranging from 0.375 cycles/mm to 80 cycles/mm was used to determine the MTF of a CCD‐based optical system. These results were compared with the angulated slit method of Fujita [H. Fujita, D. Tsia, T. Itoh, K Doi, J. Morishita, K. Ueda, and A Ohtsuka, “A simple method for determining the modulation transfer function in digital radiography,” IEEE Trans. Medical Imaging 11, 34–39 (1992)]. The use of a semi‐automated profile iterated reconstruction technique (PIRT) is introduced, where the shift factor between successive pixel rows (due to angulation) is optimized iteratively by least‐squares error analysis rather than by hand measurement of the slit angle. PIRT was used to find the slit angle for the Fujita technique and to find the sine‐pattern angle for the sine‐pattern technique. Computer simulation of PIRT for the case of the slit image (a line spread function) demonstrated that it produced a more accurate angle determination than “hand” measurement, and there is a significant difference between the errors in the two techniques (Wilcoxon Signed Rank Test, p<0.001). The sine‐pattern method and the Fujita slit method produced comparable MTF curves for the CCD camera evaluated.
A multiple wavelength algorithm in color image analysis and its applications in stain decomposition in microscopy images
Authors: Zhou Ruixia , Hammond Elizabeth H. , Parker Dennis L.
Published: March 2003

Stains have been used in optical microscopy to visualize the distribution and intensity of substances to which they are attached. Quantitative measures of optical density in the microscopic images can in principle be used to determine the amount of the stain. When multiple dyes are used to simultaneously visualize several substances to which they are specifically attached, quantification of each stain cannot be made using any single wavelength because attenuation from the several stain components contributes to the total optical density. Although various dyes used as optical stains are perceived as specific colors, they, in fact, have complex attenuation spectra. In this paper, we present a technique for multiple wavelength image acquisition and spectral decomposition based upon the Lambert–Beer absorption law. This algorithm is implemented based on the different spectral properties of the various stain components. By using images captured at N wavelengths, N components with different colors can be separated. This algorithm is applied to microscopy images of doubly and triply labeled prostate tissue sections. Possible applications are discussed.
Super‐Monte Carlo: A 3‐D electron beam dose calculation algorithm
Authors: Keall Paul J. , Hoban Peter W.
Published: March 2003

An electron beam dose calculation algorithm has been developed which is based on a superposition of pregenerated Monte Carlo electron track kernels. Electrons are transported through media of varying density and atomic number using electron tracks produced in water. The perturbation of the electron fluence due to each material encountered by the electrons is explicitly accounted for by considering the effect of (i) varying stopping power, (ii) scattering power, and (iii) radiation yield. For each step of every electron track, these parameters affect the step length, the step direction, and the energy deposited in that step respectively. Dose distributions in both homogeneous water and nonwaterlike phantoms, and heterogeneous phantoms show consistent agreement with “standard” Monte Carlo results. For the same statistical uncertainty in broad beam geometries, this new calculation method uses a factor of 9 less computation time than a full Monte Carlo simulation.
Characteristics of a scintillator‐based daily quality assurance device for radiation oncology beams
Authors: Das Indra J. , Gazda Michael J. , Beddar A. S.
Published: March 2003

A plastic scintillator based device marketed as a “DCT 444” is investigated for daily quality assurance (QA) of radiation beams. Variations in DCT responses were studied for three identical devices with beam, energy, distance, dose, dose rate, and orientations. The effects of gantry, collimator, and patient support assembly (PSA) rotations were investigated at photon energies of 6, 10, and 18 MV and electron energies in the range of 5–20 MeV from three different linear accelerators. The short and long term linearity, reproducibility, and radiation damage were also investigated in kilovoltage and megavoltage beams. Results indicate that the DCT response is linear with dose and dose rate, but shows directional dependence for all beams. The response is dependent on its orientation and the angle of rotations of collimator, gantry, and PSA of the linear accelerator. When the collimator is rotated through 0–360°, the DCT response varied within ±4.5% and ±7.2% for the 6 MV and 18 MV beams, respectively. With PSA rotation, deviations up to ±11% were noted. The DCT response is symmetric across 0° and peaks at ±60° PSA angle for all accelerators and beam energies. The effect of backscattering medium on DCT response is noted for all beams but significant for kilovoltage beams. A deviation as high as 30% was observed when DCT was placed on a thick polystyrene phantom. The long term stability studied over 25 months showed varied signal losses possibly due to radiation damage. However, there is no noticeable signal loss over a short period of time (2–4 weeks). The lightweight DCT device is linear with dose and dose rate, stable, and easy to operate. It stores ten readings that can be downloaded to a personal computer. This study indicates that DCT‐444 is a suitable QA tool for central axis dose measurement. However, proper precaution is needed as this device exhibits directional and positional dependence. For a meaningful QA result of a radiation unit, the daily QA setup geometry should be identical to that used for calibration of the DCT device.
Alignment of multi‐segmented anatomical features from radiation therapy images by using least square fitting
Authors: Cai Jialing , Zhou Shao‐Qun , Hopkinson James , Saxena V. Amod , Chu James
Published: March 2003

A least square fitting algorithm for alignment of multi‐open‐curved segments and point pairs of anatomical features obtained from both portal images and simulation radiographs has been developed for patient position verification in radiation therapy. A coordinate system associated with each curved segment pair is constructed so that the discrepancies of each segment pair can be obtained easily. The algorithm allows users to select not only multiple curve pairs but also individual point pairs on the important anatomic landmarks. The misregistration is measured by the sum of the position deviation of individual point pairs and ordinate discrepancies of all segment pairs from their corresponding coordinate systems. A mathematical minimization method is applied to seek an optimal transformation for matching of all segment and/or point pairs involved. The reliability of the algorithm has been tested with both phantom and clinical images. The test results indicate that the typical errors are less than 1 mm and 1° in translation and rotation, respectively.
Measurements of magnetism‐related forces and torque moments affecting medical instruments, implants, and foreign objects during magnetic resonance imaging at all degrees of freedom
Authors: Planert J. , Modler H. , Vosshenrich R.
Published: March 2003

We designed a novel force/torque moment transducer with six degrees of freedom. This new sensor is used for measuring the forces and torque moments exerted by static magnetic resonance imaging (MRI) fields on various medical implants, instruments, and ballistic shell fragments. To determine the location of strongest effect the inhomogeneity of the magnetic field between the entrance of the MRI‐tube (portal), and the imaging section was measured. Force and torque moments of a sample of 45 objects were measured at two positions, at the portal, and in the center of the imaging section. We classified the objects regarding to the magnetism‐related forces measured in the homogeneous part of the MRI tube. About one‐third of the objects was non‐ or weakly magnetic, one‐third moderately, and the last third was strongly magnetic. Forces within the inhomogeneous area of the magnetic field at the entrance part of the tube were up to half a magnitude larger than forces inside the homogeneous imaging section. Here the greatest potential risks to patients might occur.
A comparison of mixed integer programming and fast simulated annealing for optimizing beam weights in radiation therapy
Authors: Langer M. , Morrill S. , Brown R. , Lee O. , Lane R.
Published: March 2003

Two competing methods for assigning intensities to radiation treatment beams were tested. One method was derived from mixed integer programming and the other was based on simulated annealing. The methods faced a common objective and identical constraints. The goal was to maximize the minimum tumor dose while keeping the dose in required fractions of normal organ volumes below a threshold for damage. The minimum tumor doses of the two methods were compared when all the dose‐volume constraints were satisfied. A mixed integer linear program gave a minimum tumor dose that was at least 1.8 Gy higher than that given by simulated annealing in 7 of 19 trials. The difference was ≥5.4 Gy in 4 of 19 trials. In no case was the mixed integer solution one fraction size (1.8 Gy) worse than that of simulated annealing. The better solution provided by the mixed integer program allows tumor dose to increase without violating the dose‐volume limits of normal tissues.
A generic genetic algorithm for generating beam weights
Authors: Langer Mark , Brown Richard , Morrill S. , Lane R. , Lee O.
Published: March 2003

A genetic algorithm for generating beam weights is described. The algorithm improves an objective measure of the dose distribution while respecting dose volume constraints placed on critical structures. The algorithm was used to select beam weights for treatment of abdominal tumors. Weights were selected for up to 36 beams. Dose volume limits were placed on normal organs and a dose inhomogeneity limit was placed on tumor. Volumes were represented as sets of several hundred discrete points. The algorithm searched for the beam weights that would make the minimum tumor dose as high as the constraints would allow. The results were checked using dose volume histograms with standard sized grids. Nineteen trials were created using six patient cases by changing the required field margin or allowed beam position in each case. The sampling of points was sufficiently dense to yield solutions that strictly satisfied the constraints when the prescribed dose was renormalized by a factor of less than 6%. The genetic algorithm supplied solutions in 49 min on average, and in a maximum time of 87 min. The randomized search does not guarantee optimality, but high tumor doses were obtained. An example is shown for which the solution of the genetic algorithm gave a minimum tumor dose 7 Gy higher than the solution given by a simulated annealing algorithm under the same set of constraints. The genetic algorithm can be generalized to admit nonlinear functions of the beam intensities in the objective or in the constraints. These can include tumor control and normal tissue complication probabilities. The genetic algorithm is an attractive procedure for assigning beam weights in multifield plans. It improves the dose distribution while respecting specified rules for tissue tolerance.
A quality control test for electronic portal imaging devices
Authors: Rajapakshe R. , Luchka K. , Shalev S.
Published: March 2003

A quality control (QC) test suitable for routine daily use has been developed for video based electronic portal imaging devices. It provides an objective and quantitative test for acceptable image quality on the basis of the high contrast spatial resolution and the contrast‐to‐noise ratio (CNR). The test uses a phantom consisting of five sets of high‐contrast rectangular bar patterns with spatial frequencies of 0.1, 0.2, 0.25, 0.4, and 0.75 lp/mm. Data obtained during a one month calibration period were used to determine a critical frequency fc for the relative square wave modulation transfer function and a critical contrast‐to‐noise ratio (CNRc). Subsequent measurements indicating significant deviations from these critical values result in warning messages to the operator indicating potential problems in system performance. Measurements over a period of two years show that the QC test provides a sensitive indication of imaging performance.
A restoration algorithm for P‐32 and Y‐90 bremsstrahlung emission nuclear imaging: A wavelet‐neural network approach
Authors: Qian Wei , Clarke Laurence P.
Published: March 2003

A novel wavelet‐based neural network (WNN) filter is proposed for image restoration as required for imaging of beta emitters by bremsstrahlung detection using a gamma camera. Quantitative imaging of beta emitters is important for the in vivo management of antibody therapy using either P‐32 or Y‐90. The theoretical basis for the general case for M‐channel multiresolution wavelet decomposition of the nuclear image into different subimages is developed with the objective of isolating the signal from noise. A modified Hopfield neural network (NN) architecture is then used for multichannel image restoration using the dominant signal subimages. The NN model avoids the common inverse problem associated with other image restoration filters such as the Wiener filter. The relative performance of the WNN for image restoration, for M=2 channel, is compared to a previously reported order statistic neural network hybrid (OSNNH) filter. Initially simulated degraded images of known structures with different noise levels are used. Quantitative metrics such as the normalized mean square error (NMSE) and signal‐to‐noise ratio (SNR) are used to compare filter performance. The WNN yields comparable results for image restoration with suggested slightly better performance for the images with higher noise levels as often encountered in bremsstrahlung detection. Attenuation measurements were performed using two radionuclides, 32P and90 Y as required for calibration of the gamma camera for quantitative measurements. Similar values for an effective attenuation coefficient were observed for the restored images using the OSNNH filters (32P: μ=0.122 cm−1, 90Y: μ=0.135 cm−1) and WNN (32P: μ=0.122 cm−1, 90Y: μ=0.135 cm−1) filters with slightly higher values obtained for the raw data (32P: μ=0.142 cm−1, 90Y: μ=0.142 cm−1) for a 3.5‐cm source size. The WNN, however, was computationally more efficient by a factor of 4 to 6 compared to the OSNNH filter. The filter architecture, in turn, is also optimum for parallel processing or VLSI implementation as required for planar and particularly for SPECT mode of detection.
Scatter reduction in mammography with air gap
Authors: Krol A. , Bassano D. A. , Chamberlain C. C. , Prasad S. C.
Published: March 2003

Scatter reduction by air gaps in mammography was investigated. We have experimentally demonstrated that, independently of the imaging geometry, scatter in air‐gap mammography can be well described by a virtual source of scatter (VSS) model. This model postulates that scatter radiation originates from a virtual point source of scatter placed on the central axis between the x‐ray source and the exit surface of a patient at distance δ and utilizes only two parameters: δ and (S/P)0. The (S/P)0 parameter represents scatter‐to‐primary ratio without an air gap and δ is the distance from the exit surface of a patient to the virtual source of scatter. We have experimentally determined the analytical form of the two independent parameters of the VSS model; δ exhibits a linear increase proportional to the radiation field size, does not depend on patient thickness, and is in the 10–30 cm range, while (S/P)0 increases with the field size as a power function and is in the 0.4–1.3 range. In the framework of the VSS model the selectivity, the contrast improvement factor, and the signal‐to‐noise improvement factor were employed to evaluate performance of air‐gap mammography systems. We have demonstrated that selectivity of an air gap rapidly deteriorates at some well‐defined critical value of scatter fraction that has profound consequences on air‐gap performance. Assuming fixed patient exposure, the results shows that, if a contrast limited detection system (such as film/screen mammography) is used, an air gap system can outperform a grid system only if a very large source‐to‐patient (SPD) distance is utilized, which might be possible with new laser‐based x‐ray sources. For the noise limited detection systems (such as digital mammography) even a small SPD (70 cm) and a small air‐gap (20 cm) system will outperform a grid system.
An automatic contour extraction algorithm for short‐axis cardiac magnetic resonance images
Authors: Zimmer Yair , Akselrod Solange
Published: March 2003

In this article an automatic process is presented for the extraction of the left ventricular contours. The only operator‐supplied information is the location of a single point within the left ventricle. The raw data for processing are short‐axis cardiac MR images, obtained using various MRI protocols. In this study we considered only protocols for which the blood appears dark. The proposed algorithm performs smoothing and thresholding of the original MR images, and uses the segmented images to form edge images. From those edge images the endocardial curve is extracted using a modified contour following approach. The algorithm can accommodate moderate noise and small gaps in the contour. A smooth curve that approximates the epicardium is then obtained using a method combining radial search and FFT based smoothing. The quality of the obtained contours was visually assessed by three observers. Their evaluation shows that the algorithm is successful when images of reasonable quality are processed. Further evaluation work is required in order to fully assess the performance and limitation of the suggested method.
Image compression in digital mammography: Effects on computerized detection of subtle microcalcifications
Authors: Chan Heang‐Ping , Lo Shih‐Chung B. , Niklason Loren T. , Ikeda Debra M. , Lam Kwok Leung
Published: March 2003

Our previous receiver operating characteristic (ROC) study indicated that the detection accuracy of microcalcifications by radiologists is significantly reduced if mammograms are digitized at 0.1 mm×0.1 mm. Our recent study also showed that detection accuracy by computer decreases as the pixel size increases from 0.035 mm×0.035 mm. It is evident that very large matrix sizes have to be used for digitizing mammograms in order to preserve the information in the image. Efficient compression techniques will be needed to facilitate communication and archiving of digital mammograms. In this study, we evaluated two compression techniques: full frame discrete cosine transform (DCT) with entropy coding and Laplacian pyramid hierarchical coding (LPHC). The dependence of their efficiency on the compression parameters was investigated. The techniques were compared in terms of the trade‐off between the bit rate and the detection accuracy of subtle microcalcifications by an automated detection algorithm. The mean‐square errors in the reconstructed images were determined and the visual quality of the error images was examined. It was found that with the LPHC method, the highest compression ratio achieved without a significant degradation in the detectability was 3.6:1. The full frame DCT method with entropy coding provided a higher compression efficiency of 9.6:1 at comparable detection accuracy. The mean‐square errors did not correlate with the detection accuracy of the microcalcifications. This study demonstrated the importance of determining the quality of the decompressed images by the specific requirements of the task for which the decompressed images are to be used. Further investigation is needed for selection of optimal compression technique for digital mammograms.