bluuebunny/crossref_metadata_2025 huggingface dataset:
Accurate reflectometry for extreme-ultraviolet lithography at the National Institute of Standards and Technology
Authors: Grantham S. , Tarrio C. , Lucatorto T. B.
Published: December 2002

The most demanding application of extreme ultraviolet multilayer optics is in lithography. The optics in current alpha-class tools are large, and the multilayer coatings must have both optimized reflectance and extremely high uniformity. At the National Institute of Standards and Technology we have recently commissioned a very large sample chamber at the Synchrotron Ultraviolet Radiation Facility storage ring. This dedicated reflectometer is capable of carrying out complete surface maps of alpha- and beta-class tools for the extreme-ultraviolet lithography community. We have conducted a rigorous characterization of the system by varying the output spectrum of our storage ring in conjunction with multiple filter and detector combinations. This exercise has resulted in an accurate model of the effects of out-of-band radiation and scattering on the measured reflectivity. The model can be directly applied to raw data measured under any typical operating conditions to reduce the uncertainty of our measurements to 0.3%. We will report the findings of the characterization along with the resulting model and sample measurements, which show that the requisite accuracy and precision in reflectivity required for beta-class tools has been achieved. We will also present measurements of large-sized curved multilayer samples. Finally, we will report on an upgrade to the beamline’s monochromator, which is currently in progress. This will improve the stability that will yield the wavelength precision needed for all planned extreme-ultraviolet lithography tools.
Parallel maskless optical lithography for prototyping, low-volume production, and research
Authors: Gil Dario , Menon Rajesh , Tang Xudong , Smith Henry I. , Carter D. J. D.
Published: December 2002

Earlier we reported on a proof-of-concept maskless-lithography system that used an array of Fresnel zone plates to focus multiple beams of 442 nm light onto a substrate, and micromechanics for multiplexing light to the several zone plates, enabling patterns of arbitrary geometry, at 350 nm linewidth, to be written. We referred to the technique as zone-plate-array lithography (ZPAL). We also demonstrated zone-plate-array microscopy. Here, we report on a “preprototype” ZPAL system operating at an exposure wavelength of 400 nm, capable of quick-turn-around, maskless lithography. We describe the lithography results with this system as well the development of high-speed data delivery systems, high-numerical-aperture zone plates (up to 0.95), and a multiplexing scheme that will enable us to move to a “full-prototype” system capable of 210 nm feature sizes at a moderate but useful throughput of ∼0.25 cm2 in 20 min.
Lithographic aerial-image contrast measurement in the extreme ultraviolet engineering test stand
Authors: Lee Sang H. , Tichenor Daniel A. , Naulleau Patrick
Published: December 2002

An important metric in determining both system and optic performance is aerial-image contrast. Here, we describe aerial-image contrast characterization for the extreme ultraviolet (EUV) Engineering Test Stand prototype EUV stepper. The system has been characterized operating with a developmental projection optic (the set-1 optic). Characterization was performed using the resist-clearing (Kirk) method and the results were compared to simulations based on the known system parameters. In this method, the image contrast is determined from the dose required to clear the bright features and the dose required to clear the dark features in a pattern of dense lines and spaces. Comparisons to simulations have shown discrepancies that increase as the contrast dose is reduced. Although various potential sources exist for these discrepancies (as described herein), analysis points to finite resist resolution as being the primary cause. An empirical model that accounts for finite resist resolution in aerial-image contrast measurements is presented.
Nanoimprinting over topography and multilayer three-dimensional printing
Authors: Bao L.-R. , Cheng X. , Huang X. D. , Guo L. J. , Pang S. W. , Yee A. F.
Published: December 2002

We have developed a simple imprinting technique that allows patterning over a nonflat substrate without the need for planarization. In this process, a polymer film is spin coated onto the mold and then transferred to a patterned substrate by imprinting. By selecting polymers with different mechanical properties, either suspended structures over wide gaps or supported patterns on raised features of the substrate can be obtained with high uniformity. It is found that imprinting at a temperature well above the glass transition temperature (Tg) of the polymer causes the thin residue film between features to dewet from the mold, which can greatly simplify the subsequent pattern transfer process. Multilayer three-dimensional polymer structures have also been successfully fabricated using this new imprinting method. The yield and dimensional stability in the multilayer structure can both be improved when polymers with progressively lower Tg are used for different layers. Compared to existing techniques for patterning on nonflat substrates, the current method has a number of advantages, including simplicity, versatility, high resolution, and low pattern distortion.
Near-surface electronic defects and morphology of CuIn1−xGaxSe2
Authors: Strzhemechny Y. M. , Smith P. E. , Bradley S. T. , Liao D. X. , Rockett A. A. , Ramanathan K. , Brillson L. J.
Published: December 2002

CuIn 1−x Ga x Se 2 (CIGS) is a leading candidate for high-efficiency solar cells, yet the defects and electronic structure that define performance are relatively unexplored. We employed low-energy depth-resolved cathodoluminescence to measure the local band and defect properties of epitaxial CIGS films having (002), (220)/(204), or (112) orientations on GaAs substrates. Along with a near-band-edge emission, room-temperature luminescence spectra for all epitaxial samples reveal a well-defined deep-level transition confined within ∼100 nm of the surface. The intensity of this defect-related feature reaches a maximum at the surface relative to the near-band-edge peak. Polycrystalline CdS/CIGS/Mo/glass and CIGS/Mo/glass solar-cell layers were also studied. These exhibit a weaker deep-level emission modulated by CIGS-thickness-related optical cavity effects. The integrated intensity of this emission relative to the near-band-edge peak intensity is also a maximum at the surface and decreases into the sample. A strong deep-level emission due to CdS was also observed decaying with depth relative to the near-band-edge emission of the CIGS. The CdS emission was the only significant change in the polycrystalline emission spectrum with and without the CdS layer. The observed behavior is consistent with the existence of a nanoscale near-surface defective region whose properties can impact charge generation and recombination in solar energy-generating structures.
Evaluation of siloxane and polyhedral silsesquioxane copolymers for 157 nm lithography
Authors: Bellas V. , Tegou E. , Raptis I. , Gogolides E. , Argitis P. , Iatrou H. , Hadjichristidis N. , Sarantopoulou E. , Cefalas A. C.
Published: December 2002

Siloxane and silsesquioxane copolymers have been synthesized and first evaluated as potential components of 157 nm resist materials. In block copolymers of dimethylsiloxane and tert-butyl methacrylate negative imaging chemistry dominates at 157 nm, due to the presence of the siloxane component, although positive imaging in aqueous base developers via chemical amplification was obtained at longer wavelengths (248 nm). The same behavior is observed in graft copolymers of dimethylsiloxane and tert-butyl methacrylate. On the other hand, random copolymers of polymerizable polyhedral oligomeric silsesquioxane monomers with various acrylates, including partially fluorinated, can be used as components of resist formulations that provide positive imaging at 157 nm, aqueous base development, and physicochemical properties which resemble those of conventional poly(meth)acrylates. Pattern transfer properties depend on the selection of the silsesquioxane component. Polymers containing 30% or higher w/w ethyl-substituted silsesquioxane cages provide the necessary etch resistance as well as low surface roughness to oxygen plasma at 100 nm film thickness, allowing bilayer 157 nm lithography, even without further absorbance optimization.
Photoinduced organic nanowires from self-assembled monolayers
Authors: Qiao Ying-Hong , Zeng Qing-Dao , Tan Zhong-Yin , Xu Shan-Dong , Wang Dong , Wang Chen , Wan Li-Jun , Bai Chun-Li
Published: December 2002

The self-assembled diacetylene molecular monolayer was prepared on highly oriented pyrolitic graphite and observed by scanning tunneling microscopy. Upon applying ultraviolet irradiation, the self-assembled conjugated polydiacetylene nanowires resulting from photoinduced topochemical reaction of diacetylene molecules were obtained. Control of the spacings between nanowires was achieved by a simple coadsorption method. The result presented here may supply a general method to obtain spacing-controllable organic nanowires.
Operational model for pattern and probe based aberration monitors
Authors: Robins Garth , Neureuther Andrew
Published: December 2002

A one-dimensional treatment of pattern and probe based aberration monitors is discussed, yielding significant insight into the characteristics of optimum targets for measuring Zernike aberrations. One key new result is that each target possesses a “double-width” thick ring, resulting from the stationary phase condition between the aberration and the wave front incident upon the pupil. The characteristic radius of the ring is given by R≈(0.186×n+y0)λ/NA. The use of simple on/off mask openings to synthesize targets proves to be beneficial, increasing sensitivity by a factor of 4/π, but it is also shown to increase cross contamination between aberrations on the order of 10%. A full algebraic formulation of the principle of operation including amplitude and phase effects is presented, demonstrating that effects of target phase variation outweigh the effect of amplitude variation by a factor of 2π/λ.
Method for manufacturing nanoscale structures in transition metal layers
Authors: van Delft Falco C. M. J. M. , Ketelaars W. S. M. M. , Kroon M. , Lambregts J.
Published: December 2002

In most transition metals, steeply profiled nanoscale structures can not easily be obtained by dry etching, due to the redeposition of the etch products. An alternative method is presented here, which comprises the deposition of a plating base, electron-beam writing of nanoscale structures in negative tone hydrogen silsesquioxane resist, and electroplating of the desired metal layer between the resist lines. This method is used for making transmittive and reflective masks, alignment marks, and other optical gratings, consisting of features down to 40 nm wide that are embedded in transition metal layers. These gratings can, e.g., be applied in extreme-UV lithographic sensors. More generally, other devices, e.g., magnetic recording heads, can also be manufactured using this technology.
Improvement in brightness of multicusp-plasma ion source
Authors: Ji Q. , Jiang X. , King T.-J. , Leung K.-N. , Standiford K. , Wilde S. B.
Published: December 2002

The beam brightness of a multicusp-plasma ion source has been substantially improved by optimizing the source configuration and extractor geometry. The current density of a 2 keV He+ beam extracted from a 7.5-cm-diameter source operating at 2.5 kW rf power is ∼100 mA/cm2, which is ∼10 times larger than that of a beam extracted from a 5-cm-diameter source operating at 1 kW rf power. A smaller focused beam spot size is achieved with a counter-bored extractor instead of a conventional (“through-hole”) extractor, resulting another order of magnitude improvement in beam current density. Measured beam brightness can be as high as 440 A/cm2Sr, which represents a 30 times improvement over prior work.
Nanostructure fabrication by direct electron-beam writing of nanoparticles
Authors: Griffith Saul , Mondol Mark , Kong David S. , Jacobson Joseph M.
Published: December 2002

Direct additive-layer fabrication of nanostructures is a widely sought goal, which is not possible using traditional layered resist optical and electron-beam lithographic techniques. However, recently, it has been shown that certain metallic and semiconducting nanoparticles capped with protective organic groups are promising “inklike” resist materials for patterning a variety of electronic and mechanical structures [C. A. Bulthaup et al., Appl. Phys. Lett. 79, 1525 (2001)]. Several groups have successfully patterned single-layer gold nanoparticle films by means of direct electron-beam writing [X. M. Lin, R. Parthasarathy, and H. M. Jaeger, Appl. Phys. Lett. 78, 1915 (2001); T. R. Bedson, R. E. Palmer, T. E. Jenkins, D. J. Hayton, and J. P. Wilcoxon, Appl. Phys. Lett. 78, 1921 (2001); L. Clarke et al., Appl. Phys. Lett. 71, 617 (1997)]. In this work, we apply these materials in a new lithographic mode, using an electron beam to cause direct sintering of these 2–10 nm nanoparticles, building structures of multiple layers and multiple materials with linewidth resolutions of 80–100 nm.
Density estimation for amorphous carbon nanopillars grown by focused ion beam assisted chemical vapor deposition
Authors: Ishida Masahiko , Fujita Jun-ichi , Ochiai Yukinori
Published: December 2002

The mechanical resonant frequency and spring constant of amorphous carbon nanopillars grown by focused ion beam assisted chemical vapor deposition (FIB CVD) were measured to evaluate the pillar density. The measured density ranged from 2.5 to 4.2×3 kg/m3, depending on the growth conditions. Annealing the pillars at 600 °C completely removed the Ga incorporated in the pillar during FIB CVD. The pillar density after annealing decreased to about 1.9×103 kg/m3, which indicated the density excess was mainly due to the implantation of Ga. At lower growth rates, a sharp decrease in the Ga content suggested higher sputtering rate of Ga through ion beam irradiation.
Characterization of the Surface of Ba(NO3) 2 Powder by XPS
Authors: Schmitz Peter J.
Published: December 2002

This work reports survey and core level x-ray photoemission spectra obtained from the surface of a barium nitrate powder. The high purity (99.999%) barium nitrate specimen was commercially obtained from Alfa/AESARS. The XPS spectra were measured with a Kratos Axis 165 instrument using monochromatic Al Kα radiation. The survey spectrum was measured with an 80 eV pass energy and the core level spectra (Ba 3d, N 1s, O 1s, and C 1s) were acquired using a 20 eV pass energy corresponding to analyzer resolutions of 1.12 and 0.28 eV, respectively. The data are consistent with the surface being composed of primarily barium nitrate with a small contribution from barium nitrite. Carbon was the only contaminant species observed.
Characterization of the Surface of BaSO4 Powder by XPS
Authors: Schmitz Peter J.
Published: December 2002

This work reports survey and core level x-ray photoemission spectra obtained from the surface of a barium sulfate powder. The high purity (99.998%) barium sulfate specimen was commercially obtained from Alfa/AESARS. The XPS spectra were acquired with a Kratos Axis 165 instrument using monochromatic Al Kα radiation. The survey spectrum was measured with an 80 eV pass energy and the core level spectra (Ba 3d, S 2p, O 1s, and C 1s) were acquired using a 20 eV pass energy corresponding to analyzer resolutions of 1.12 and 0.28 eV, respectively. The data are consistent with the surface being predominately barium sulfate. Adventitious carbon and a small nitrogen component were the only contaminants observed.
Analysis of Poly(amino acids) by Static Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)
Authors: Samuel Newton T. , Wagner M. S. , Dornfeld K. D. , Castner David G.
Published: December 2002

This study presents the static time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra of 15 poly(amino acids), solvent or spin cast onto either mica or silicon substrates. These poly(amino acid) spectra are useful for interpreting the complex static TOF-SIMS spectra obtained from adsorbed protein films and peptide-functionalized surfaces. Previous studies have reported poly(amino acid) spectra acquired with a quadrupole SIMS instrument. The spectra obtained with a TOF-SIMS instrument in this study have significantly higher sensitivity and mass resolution, which are essential for producing good, high-quality reference spectra.
Chemical Vapor Deposited Fe2O3 Thin Films Analyzed by XPS
Authors: Barreca Davide , Battiston Giovanni A. , Berto Davide , Gerbasi Rosalba , Tondello Eugenio
Published: December 2002

In the present study, x-ray photoelectron spectroscopy (XPS) was employed to investigate the principal core levels of a Fe2O3 thin film. The sample was deposited on glass in a N2+O2 atmosphere by chemical vapor deposition (CVD), using Fe(acac)3 (Hacac = 2,4-pentanedione) as the source compound. XPS surface data for the Fe 2p3/2, O 1s, and C 1s photoelectrons, obtained by an Al Kα monochromatized excitation source, are reported. Deconvolution of the O 1s signal revealed the presence of –OH groups along with the major component ascribed to lattice oxygen. The relatively high carbon content in the film indicated an incomplete decomposition of the used precursor.
γ-Al2O3 by Au 4f7/2-referenced XPS and XAES
Authors: Lippitz A. , Boese O. , Kemnitz E. , Unger W. E. S.
Published: December 2002

Powder x-ray diffraction (XRD) defined γ-Al2O3 has been investigated by x-ray photoelectron spectroscopy (XPS) and x-ray excited Auger electron spectroscopy (XAES). The Au 4f7/2 orbital is used for static charge referencing. This feature originates from well-defined and chemically inert 20 nm highly dispersed gold particles deposited on the sample surface. The particles, with a narrow particle size distribution, were obtained from a highly purified colloidal solution (sodium concentration below 1 mg/l). This procedure allows a determination of gold-referenced XPS and XAES data sets.
CrO2 by XPS: Comparison of CrO2 Powder to CrO2 Films on TiO2(110) Single Crystal Surfaces
Authors: Bullen Heather A. , Garrett Simon J.
Published: December 2002

Chromium dioxide is a ferromagnetic metallic oxide utilized in data storage applications. Here, the XP spectrum of CrO2 bulk powder is compared to a CrO2(110) film grown on a TiO2(110) single crystal surface by chemical vapor deposition. The spectra compare the Cr 2p and O 1s regions for the two materials. In both materials, the Cr 2p region (2p3/2 = 576.4 ± 0.1 eV and Cr 2p1/2 = 578.6 ± 0.1 eV) is asymmetric and broadened to the high binding energy side, consistent with metallic material. The O 1s envelope for both materials consists of two components. The lower BE component, at 529.2 ± 0.1 eV, has been assigned to the oxide and the higher binding energy component, at ∼531.5 eV, is believed to be associated with an adsorbed water or hydroxide layer.
Manganese Dioxide (MnO2) by XPS
Authors: Militello Maria C. , Gaarenstroom Stephen W.
Published: December 2002

XPS spectra of manganese dioxide (MnO2) powder are presented. The spectra were collected using monochromated Al Kα x-rays. Spectra include survey scan, the Mn 2p, 3p, and 3s regions, the O 1s region, and the Mn LMM Auger transitions.
Lithium Manganese Oxide (LiMn2O4) by XPS
Authors: Militello Maria C. , Gaarenstroom Stephen W.
Published: December 2002

XPS spectra of lithium manganese oxide (LiMn2O4) are presented. The spectra were collected using monochomated Al Kα x-rays. Spectra include survey scan, the Li 1s region, the Mn 2p, 3p, and 3s regions, the O 1s region, and the Mn LMM Auger transitions.
Y
2
O
3
Thin Films Characterized by XPS
Authors: Barreca Davide , Battiston Giovanni A. , Berto Davide , Gerbasi Rosalba , Tondello Eugenio
Published: December 2002

Y2O3 thin films were synthesized by chemical vapor deposition (CVD) using Y(acac)3.xH2O (Hacac= 2,4-pentanedione) as precursor. This work is focused on the x-ray photoelectron spectroscopy (XPS) characterization of an Y2O3 thin film deposited on glass substrate at 450 °C in N2+O2 atmosphere and subsequently annealed in air at 1000 °C. Besides the wide scan spectrum, charge corrected binding energies for the Y 3d5/2, O 1s, and C 1s surface photoelectron signals are reported.
Development of a three‐dimensional radiation dosimetry system
Authors: Bero Mamdouh A.
Published: December 2002

A radiochromic gel dosemeter in which ionizing radiation causes a color change in the visible wavelength region was produced. The roles of the dosemeter's active components were quantified with special consideration given to their effect on the system sensitivity and stability. Optimal composition was found to be 0.5 mM ferrous sulfate, 0.1 mM xylenol orange, and 25 mM sulfuric acid. The dose response is linear in the range 0.1–30 Gy and the dosemeter sensitivity was where A is the optical absorbance measured at a wavelength of 585 nm. An optical tomography scanner with parallel light beam and charged coupled device detector was designed, built, and used to interrogate cylindrical samples of the dosemeter. The current version of the scanner can cope with samples up to 10 cm in diameter limited by the width of the light beam. A maximum of 512×512 pixels per slice images are obtainable with this scanner. Slice thickness as small as 0.14 mm was achieved that is much narrower than the value of about 6 mm achieved when using the MRI method of three‐dimensional (3D) readout. The time to acquire a stack of 100 cross‐sectional images with a resolution of 128×128 pixels per slice is about 20 min. The 3D imaging performance was tested with collimated x‐ray beams, and the scanner high‐resolution presentation demonstrated.
Verification of multileaf collimator leaf positions using an electronic portal imaging device
Authors: Samant Sanjiv S. , Zheng Wei , Parra Nestor Andres , Chandler Jason , Gopal Arun , Wu Jian , Jain Jinesh , Zhu Yunping , Sontag Marc
Published: December 2002

An automated method is presented for determining individual leaf positions of the Siemens dual focus multileaf collimator (MLC) using the Siemens BEAMVIEW(PLUS) electronic portal imaging device (EPID). Leaf positions are computed with an error of 0.6 mm at one standard deviation (σ) using separate computations of pixel dimensions, image distortion, and radiation center. The pixel dimensions are calculated by superimposing the film image of a graticule with the corresponding EPID image. A spatial correction is used to compensate for the optical distortions of the EPID, reducing the mean distortion from 3.5 pixels (uncorrected) per localized x‐ray marker to 2 pixels (1 mm) for a rigid rotation and 1 pixel for a third degree polynomial warp. A correction for a nonuniform dosimetric response across the field of view of the EPID images is not necessary due to the sharp intensity gradients across leaf edges. The radiation center, calculated from the average of the geometric centers of a square field at 0° and 180° collimator angles, is independent of graticule placement error. Its measured location on the EPID image was stable to within 1 pixel based on 3 weeks of repeated extensions/retractions of the EPID. The MLC leaf positions determined from the EPID images agreed to within a pixel of the corresponding values measured using film and ionization chamber. Several edge detection algorithms were tested: contour, Sobel, Roberts, Prewitt, Laplace, morphological, and Canny. These agreed with each other to within ⩽1.2 pixels for the in‐air EPID images. Using a test pattern, individual MLC leaves were found to be typically within 1 mm of the corresponding record‐and‐verify values, with a maximum difference of 1.8 mm, and standard deviations of <0.3 mm in the daily reproducibility. This method presents a fast, automatic, and accurate alternative to using film or a light field for the verification and calibration of the MLC.
History by history statistical estimators in the BEAM code system
Authors: Walters B. R. B. , Kawrakow I. , Rogers D. W. O.
Published: December 2002

A history by history method for estimating uncertainties has been implemented in the BEAMnrc and DOSXYZnrc codes replacing the method of statistical batches. This method groups scored quantities (e.g., dose) by primary history. When phase‐space sources are used, this method groups incident particles according to the primary histories that generated them. This necessitated adding markers (negative energy) to phase‐space files to indicate the first particle generated by a new primary history. The new method greatly reduces the uncertainty in the uncertainty estimate. The new method eliminates one dimension (which kept the results for each batch) from all scoring arrays, resulting in memory requirement being decreased by a factor of 2. Correlations between particles in phase‐space sources are taken into account. The only correlations with any significant impact on uncertainty are those introduced by particle recycling. Failure to account for these correlations can result in a significant underestimate of the uncertainty. The previous method of accounting for correlations due to recycling by placing all recycled particles in the same batch did work. Neither the new method nor the batch method take into account correlations between incident particles when a phase‐space source is restarted so one must avoid restarts.
Cone‐beam CT with megavoltage beams and an amorphous silicon electronic portal imaging device: Potential for verification of radiotherapy of lung cancer
Authors: Ford E. C. , Chang J. , Mueller K. , Sidhu K. , Todor D. , Mageras G. , Yorke E. , Ling C. C. , Amols H.
Published: December 2002

We investigate the potential of megavoltage (MV) cone‐beam CT with an amorphous silicon electronic portal imaging device (EPID) as a tool for patient position verification and tumor/organ motion studies in radiation treatment of lung tumors. We acquire 25 to 200 projection images using a EPID. The acquisition is automatic and requires 7 minutes for 100 projections; it can be synchronized with respiratory gating. From these images, volumetric reconstruction is accomplished with a filtered backprojection in the cone‐beam geometry. Several important pre‐reconstruction image corrections, such as detector sag, must be applied. Tests with a contrast phantom indicate that differences in electron density of 2% can be detected with 100 projections, 200 cGy total dose. The contrast‐to‐noise ratio improves as the number of projections is increased. With 50 projections (100 cGy), high contrast objects are visible, and as few as 25 projections yield images with discernible features. We identify a technique of acquiring projection images with conformal beam apertures, shaped by a multileaf collimator, to reduce the dose to surrounding normal tissue. Tests of this technique on an anthropomorphic phantom demonstrate that a gross tumor volume in the lung can be accurately localized in three dimensions with scans using 88 monitor units. As such, conformal megavoltage cone‐beam CT can provide three‐dimensional imaging of lung tumors and may be used, for example, in verifying respiratory gated treatments.
Contrast enhancement in dense breast images using the modulation transfer function
Authors: Nunes Fátima L. S. , Schiabel Homero , Benatti Rodrigo H.
Published: December 2002

This work proposes a method aimed at enhancing the contrast in dense breast images in mammography. It includes a new preprocessing technique, which uses information on the modulation transfer function (MTF) of the mammographic system in the whole radiation field. The method is applied to improve the efficiency of a computer‐aided diagnosis (CAD) scheme. Seventy‐five regions of interest (ROIs) from dense mammograms were acquired in two pieces of equipment (a CGR Senographe 500t and a Philips Mammodiagnost) and were digitized in a Lumiscan 50 laser scanner. A computational procedure determines the effective focal spot size in each region of interest from the measured focal spot in the center for a given mammographic equipment. Using computational simulation the MTF is then calculated for each field region. A procedure that enlarges the high‐frequency portion of this function is applied and a convolution between the resulting new function and the original image is performed. Both original and enhanced images were submitted to a processing procedure for detecting clustered microcalcifications in order to compare the performance for dense breast images. ROIs were divided into four groups, two for each piece of equipment—one with clustered microcalcifications and another without microcalcifications. Our results show that in about 10% of the enhanced images more signals were detected when compared to the results for the original dense breast images. This is important because the usual processing techniques used in CAD schemes present poor results when applied to dense breast images. Since the MTF method is a well‐recognized tool in the evaluation of radiographic systems, this new technique could be used to associate quality assurance procedures with the processing schemes employed in CAD for mammography.
Smoothing intensity‐modulated treatment delivery under hardware constraints
Authors: Ma Lijun
Published: December 2002

A method is developed to smooth intensity‐modulated beam delivery while considering hardware constraints. The method uses matrix algebra to model MLC leaf motion and hardware constraints such as interleaf digitization, synchronization requirements, and intersegment field abutment to deliver intensity modulated beam sequences. It was implemented and demonstrated for simulated and clinical intensity modulated cases. The simulated cases included intensity distributions from 3×3 to 30×30 in dimension with 1000 intensity levels. The clinical examples included four treatments sites: prostate, head and neck, lung, and esophagus cancers. In particular, we studied the relationships between the intensity levels, segment numbers and their influence on the dose‐volume histograms of the inversely optimized IMRT treatment plans. It was found that a significant reduction in the leaf segment numbers was achieved by applying the smoothing approach. The effects were found to be more significant for the unconstrained delivery than for the constrained delivery. The effects were also found to be more sensitive for the complex intensity distributions such as the head‐and‐neck cases than for the less complex ones such as the prostate cases. These results demonstrated the feasibility of maintaining inversely optimized treatment plans under acceptable tolerance levels while smoothing IMRT treatments with hardware constraints.
Particle in cell simulation of laser‐accelerated proton beams for radiation therapy
Authors: Fourkal E. , Shahine B. , Ding M. , Li J. S. , Tajima T. , Ma C.‐M.
Published: December 2002

In this article we present the results of particle in cell (PIC) simulations of laser plasma interaction for proton acceleration for radiation therapy treatments. We show that under optimal interaction conditions protons can be accelerated up to relativistic energies of 300 MeV by a petawatt laser field. The proton acceleration is due to the dragging Coulomb force arising from charge separation induced by the ponderomotive pressure (light pressure) of high‐intensity laser. The proton energy and phase space distribution functions obtained from the PIC simulations are used in the calculations of dose distributions using the GEANT Monte Carlo simulation code. Because of the broad energy and angular spectra of the protons, a compact particle selection and beam collimation system will be needed to generate small beams of polyenergetic protons for intensity modulated proton therapy.
Using multiple‐electrode impedance measurements to monitor cryosurgery
Authors: Hartov Alex , LePivert Patrick , Soni Nirmal , Paulsen Keith
Published: December 2002

We outfitted cryoprobes with electrodes and used them in conjunction with a multiple channel electrical impedance tomography (EIT) system to record data during freezing experiments in a shallow saline tank. We made measurements using electrodes mounted on the probes and the tank's periphery. Reconstructed images based on both sets of electrodes indicate a significant improvement in the appearance of the ice ball over using tank electrodes alone. The size of the ice balls was varied by deliberately altering the cooling rate. We found a positive correlation between the measured size of the ice ball and the sizes of isocontour lines in the reconstructed impedance maps. Similarly, the shape of the ice balls was altered by circulating the saline about the probe. Two‐dimensional reconstructed impedance contours indicated a deformation in agreement with the shape of the ice ball during the experiments. These findings suggest that using multielectrode impedance sensing may constitute a means for monitoring cryosurgery.
Treatment planning for the layer‐stacking irradiation system for three‐dimensional conformal heavy‐ion radiotherapy
Authors: Kanematsu Nobuyuki , Endo Masahiro , Futami Yasuyuki , Kanai Tatsuaki , Asakura Hiroshi , Oka Hiroyoshi , Yusa Ken
Published: December 2002

We have upgraded a heavy‐ion radiotherapy treatment‐planning system to adapt for the layer‐stacking irradiation method, which is to conform a variable spread‐out Bragg peak to a target volume by means of dynamic control of the conventional beam‐modifying devices. The biophysical model, the beam‐setup logic, and the dose‐calculation algorithm implemented for the layer‐stacking method are described and the expected clinical usability is discussed. The layer‐stacking method was integrated in perfect accordance with the ongoing conventional treatments so that the established protocols, which are the clinically optimized dose fractionation schemes, will still be valid. On the other hand, a simulation study indicated a substantial improvement of dose distribution with the layer‐stacking method though the significance may depend on the size, shape, and location of the tumor. The completed treatment system will provide an option for improved conformal radiotherapy without interfering with the conventional method and we expect a gradual expansion of the clinical cases applicable to the layer‐stacking method.
Performance evaluation of a pinhole SPECT system for myocardial perfusion imaging of mice
Authors: Wu Max C. , Hasegawa Bruce H. , Dae Michael W.
Published: December 2002

The increasing use of transgenic mice as models of human physiology and disease has motivated the development of dedicated in vivoimaging systems for anatomic and functional characterization of mice as an adjunct to or a replacement for established ex vivotechniques. We have developed a pinhole single photon emission computed tomography (SPECT) system for high resolution imaging of mice with cardiovascular imaging as the primary application. In this work, we characterize the system performance through phantom studies. The spatial resolution and sensitivity were measured from images of a line source and point source, respectively, and were reported for a range of object‐ to‐pinhole distances and pinhole diameters. Tomographic images of a uniform cylindrical phantom, Defrise phantom, and grid phantom were used to characterize the image uniformity and spatial linearity. The uniform phantom image did not contain any ring or reconstruction artifacts, but blurring in the axial direction was evident in the Defrise phantom images. The grid phantom images demonstrated excellent spatial linearity. A novel phantom modeling perfusion of the left ventricle of a mouse was designed and built with perfusion defects of varying sizes to evaluate the system performance for myocardial perfusion imaging of mice. The defect volumes were measured from the pinhole SPECT images and correlated to the actual defect volumes calculated according to geometric formulas. Linear regression analysis produced a correlation coefficient of demonstrating the feasibility for measurement of perfusion defect size in mice using pinhole SPECT. We have performed phantom studies to characterize the spatial resolution, sensitivity, image uniformity, and spatial linearity of the pinhole SPECT system. Measurement of the perfusion defect size is a valuable phenotypic assessment and will be useful for hypothesis testing in murine models of cardiovascular disease.
Artifacts associated with implementation of the Grangeat formula
Authors: Lee Seung Wook , Cho Gyuseong , Wang Ge
Published: December 2002

To compensate for image artifacts introduced in approximate cone‐beam reconstruction, exact cone‐beam reconstruction algorithms are being developed for medical x‐ray CT. Although the exact cone‐beam approach is theoretically error‐free, it is subject to image artifacts due to the discrete nature of numerical implementation. We report a study on image artifacts associated with the Grangeat algorithm as applied to a circular scanning locus. Three types of artifacts are found, which are thorn, wrinkle, and V‐shaped artifacts. The thorn pattern is created by inappropriate extrapolation into the shadow zone in the radon domain. If the shadow zone is filled in with continuous data, the thorn artifacts along the boundary of the shadow zone can be removed. The wrinkle appearance arises if interpolated first derivatives of the radon data are not smooth between adjacent detector planes. In particular, the nearest‐neighbor interpolation method should not be used. If the number of projections is not small, the bilinear interpolation method is effective to suppress the wrinkle artifacts. The V‐shaped artifacts on the meridian plane come from the line integrations through the transition zones where derivative data change abruptly. Two remedies are to increase the sampling rate and suppress data noise.
Optimum momentum transfer arguments for x‐ray forward scatter imaging
Authors: Leclair Robert J. , Johns Paul C.
Published: December 2002

In our research program we have shown through modeling, related numerical calculations, and experimental measurements that there exists a potential use of scattered radiation for medical x‐ray imaging. Each incident photon of wavelength λ which scatters at a small angle θ with respect to its initial direction of travel has a change in momentum characterized by the photon momentum transfer argument In this work, we show that in order to maximize the signal‐to‐noise ratio (SNR) obtained with scattered x rays, one must detect photons with specific values. Using a photon counting detector to distinguish 2‐cm‐thick polymethyl methacrylate and nylon targets situated within a 15‐cm‐diam spherical water phantom with an 80 kV beam yields experimentally when using the photons between and Here is the air collision kerma and the average momentum transfer argument, is calculated by weighting by the incident photon fluence distribution. The model predicts a value of If we choose to form the signal with the range in extended to be from 0.5 to then, despite the detection of more scattered photons, experimentally decreases by 38% to The model predicts a value of Results for energy integrating detectors are in general similar to those for photon counters, but there exist cases where a significant decrease in SNR can occur. For example, for measurements in air with the two plastics at the SNR for an energy integrator was found to be 52% that of a photon counter. Numerical calculations predict that the effects of spectral blur can be significant when a narrow angular range is used for detection. Preliminary numerical predictions for breast tissues suggest a potential use of x‐ray scatter in the field of mammography.
Energy response simulation of 4π gamma ionization chambers using Monte Carlo technique
Authors: Aleissa Khalid A.
Published: December 2002

The energy response of 4π gamma re‐entrant well‐pressurized ionization chambers was calculated using the MCNP4B Monte Carlo computer code. Two different chambers were simulated—one filled with argon and the other with nitrogen gases at 2 MPa pressure. The MCNP4B results were in good agreement with the experimental measurements and the EGS4 Monte Carlo theoretical results. Energy response formulas for both chambers were obtained with respect to the 1.173 MeV gamma energy line ofDetailed energy response in the low energy region was facilitated by proper curve fitting of the simulation results. The MCNP4B code was also used to examine the effect of the axial displacement of the standard gamma source in the chamber well. The highest chamber response occurs when the source is located at the center position of the well and it decreases with increasing distance from the center. The effect of the source axial position displacement on the chamber energy response is noticeable at medium and high‐energy photon energies.
The water equivalence of solid materials used for dosimetry with small proton beams
Authors: Schneider Uwe , Pemler Peter , Besserer Jürgen , Dellert Matthias , Moosburger Martin , de Boer Jorrit , Pedroni Eros , Boehringer Terence
Published: December 2002

Various solid materials are used instead of water for absolute dosimetry with small proton beams. This may result in a dose measurement different to that in water, even when the range of protons in the phantom material is considered correctly. This dose difference is caused by the diverse cross sections for inelastic nuclear scattering in water and in the phantom materials respectively. To estimate the magnitude of this effect, flux and dose measurements with a 177 MeV proton pencil beam having a width of 0.6 cm (FWHM) were performed. The proton flux and the deposited dose in the beam path were determined behind water, lucite, polyethylene, teflon, and aluminum of diverse thicknesses. The number of out‐scattered protons due to inelastic nuclear scattering was determined for water and the different materials. The ratios of the number of scattered protons in the materials relative to that in water were found to be 1.20 for lucite, 1.16 for polyethylene, 1.22 for teflon, and 1.03 for aluminum. The difference between the deposited dose in water and in the phantom materials taken in the center of the proton pencil beam, was estimated from the flux measurements, always taking the different ranges of protons in the materials into account. The estimated dose difference relative to water in 15 cm water equivalent thickness was −2.3% for lucite, −1.7% for polyethylene, −2.5% for teflon, and −0.4% for aluminum. The dose deviation was verified by a measurement using an ionization chamber. It should be noted that the dose error is larger when the effective point of measurement in the material is deeper or when the energy is higher.
Predicting film dose to aid in cassette placement for radiation therapy portal verification film images
Authors: Keys Richard A. , Marks James E. , Haus Arthur G.
Published: December 2002

EC film has improved portal localization images with better contrast and improved distinction of bony structures and air‐tissue interfaces. A cassette with slower speed screens was used with EC film to image the treatment portal during the entire course of treatment (verification) instead of taking separate films after treatment. Measurements of film density vs source to film distance (SFD) were made using 15 and 25 cm thick water phantoms with both 6 and 18 MV photons from 1 to 40 cm past the phantom. A characteristic (H & D) curve was measured in air to compare dose to film density. Results show the reduction in radiation between patient and cassette more closely follows an “inverse cube law” rather than an inverse square law. Formulas to calculate radiation exposure to the film, and the desired SFD were based on patient tumor dose, calculation of the exit dose, and the inverse cube relationship. A table of exposure techniques based on the SFD for a given tumor dose was evaluated and compared to conventional techniques. Although the film has a high contrast, there is enough latitude that excellent films can be achieved using a fixed SFD based simply on the tumor dose and beam energy. Patient diameter has a smaller effect. The benefits of imaging portal films during the entire treatment are more reliability in the accuracy of the portal image, ability to detect patient motion, and reduction in the time it takes to take portal images.
A quality management program in intravascular brachytherapy
Authors: Chakri Abderrahim , Thomadsen Bruce
Published: December 2002

While simple, intravascular brachytherapy (IVB) presents a considerable potential for harm to the patient. The medical physicist maintains the responsibility to minimize the likelihood of operational problems or dosimetric errors. The principals for safe operation remain the same as with any radiotherapy treatment: to deliver the correct dose, to the correct location, safety. To develop an effective and comprehensive quality management (QM) program for IVB, a physicist should utilize proven risk assessment techniques rather than simply thinking of things to check, and follow guidances such as ISO9001:2000. The proposed QM program includes the following: Procedures designed to assure the safety of the patient: Identification of the patient; tests of the integrity and patency for the delivery catheter, operation of the source train, and patency of the catheter in the treatment position; a check for recovery preparations; and verification of source recovery. Procedures to assure positional accuracy of the treatment: Verification of the positioning the catheter in the artery and of the sources in the catheter. Procedures to assure dosimetry accuracy: Acceptance testing of the device, including verification of the source strength and uniformity, and of the treatment duration tables; verification of the treatment prescription and duration for each patient; and control measures that minimize the likelihood of errors removing the source at the correct time.
Maximum likelihood fitting of FROC curves under an initial‐detection‐and‐candidate‐analysis model
Authors: Edwards Darrin C. , Kupinski Matthew A. , Metz Charles E. , Nishikawa Robert M.
Published: December 2002

We have developed a model for FROC curve fitting that relates the observer's FROC performance not to the ROC performance that would be obtained if the observer's responses were scored on a per image basis, but rather to a hypothesized ROC performance that the observer would obtain in the task of classifying a set of “candidate detections” as positive or negative. We adopt the assumptions of the Bunch FROC model, namely that the observer's detections are all mutually independent, as well as assumptions qualitatively similar to, but different in nature from, those made by Chakraborty in his AFROC scoring methodology. Under the assumptions of our model, we show that the observer's FROC performance is a linearly scaled version of the candidate analysis ROC curve, where the scaling factors are just given by the FROC operating point coordinates for detecting initial candidates. Further, we show that the likelihood function of the model parameters given observational data takes on a simple form, and we develop a maximum likelihood method for fitting a FROC curve to this data. FROC and AFROC curves are produced for computer vision observer datasets and compared with the results of the AFROC scoring method. Although developed primarily with computer vision schemes in mind, we hope that the methodology presented here will prove worthy of further study in other applications as well.