bluuebunny/crossref_metadata_2025 huggingface dataset:
Post-Combustion Evolution of Soot Properties in an Aircraft Engine
Authors: Dakhel Pierre M. , Lukachko Stephen P. , Waitz Ian A. , Miake-Lye Richard C. , Brown Robert C.
Published: November 2008

Recent measurements have suggested that soot properties can evolve downstream of the combustor, changing the characteristics of aviation particulate matter (PM) emissions and possibly altering the subsequent atmospheric impacts. This paper addresses the potential for the post-combustion thermodynamic environment to influence aircraft non-volatile PM emissions. Microphysical processes and interactions with gas phase species have been modeled for temperatures and pressures representative of in-service engines. Time-scale arguments are used to evaluate the relative contributions that various phenomena may make to the evolution of soot, including coagulation growth, ion-soot attachment, and vapor condensation. Then a higher-fidelity microphysics kinetic is employed to estimate the extent to which soot properties evolve as a result of these processes. Results suggest that limited opportunities exist for the modification of the size distribution of the soot, its charge distribution, or its volatile content, leading to the conclusion that the characteristics of the turbine and nozzle of an aircraft engine have little or no influence on aircraft non-volatile emissions. Combustor processing determines the properties of soot particulate matter emissions from aircraft engines, setting the stage for interactions with gaseous emissions and development as cloud condensation nuclei in the exhaust plume.
Development of Hybrid Power Systems Based on Direct Fuel Cell/Turbine Cycle
Authors: Ghezel-Ayagh Hossein , Sanderson Robert , Walzak Jim
Published: November 2008

FuelCell Energy Inc. (FCE) is developing ultra high efficiency Direct FuelCell/Turbine® (DFC/T®) hybrid power plants. Present activities are focused both on the demonstration of the DFC/T concept in small packaged hybrid power generation units for distributed generation, and the design of multi-megawatt (Multi-MW) hybrid systems for the wholesale electric power market. The development of Multi-MW DFC/T systems has been focused on the on the design of power plants with efficiencies approaching 75% (LHV of natural gas). The design efforts included thermodynamic cycle analysis of key gas turbine parameters such as compression ratio. The power plant designs were studied for near-term deployment utilizing the existing commercially available gas turbines and long-term deployment requiring advanced gas turbine technologies. A new fuel cell cluster concept was developed for mechanical design of Multi-MW systems. The concept utilizes the existing one-MW fuel cell modules as the building block for the Multi-MW hybrid systems.
The Design and Integration of the Rolls-Royce Fuel Cell Systems 1MW SOFC
Authors: Agnew Gerry , Bozzolo Michele , Moritz Robert R. , Berenyi Steve
Published: November 2008

Rolls-Royce Fuel Cell Systems (RRFCS) is developing a 1MW fuel cell hybrid system package, supported by a prototype demonstration of a 250kW generator module in 2006. The ongoing design process has been focused from the early stages towards simplicity as a key to achieve the demanding cost targets for an effective market entry. This paper describes how the specifically designed components are being integrated as a system. A description of the progress against the demonstration plan is provided. The baseline plant is expected to provide excellent part-load performance in an extensive range of operating conditions, while a modular architecture guarantees availability and reliability. Finally, opportunities for further increases in efficiency and power density are discussed, as the technology evolves towards a next generation product.
Equivalent Initial Flaw Size Distribution Determination for Gas Turbine Blades Based Upon Inspection Data
Authors: Johnson W. S. , Wilson A. C. , Highsmith S.
Published: November 2008

A fracture mechanics-based methodology is presented for determining equivalent initial flaw size (EIFS) distributions from field inspection data accounting for different service histories of fracture critical components. The EIFS distribution based on inspection data from a subset of the fleet allows for an assessment of initial material/manufacturing quality and enables a probabilistic fracture mechanics-based fatigue life prediction for the fleet as a whole. The methodology is demonstrated on an industrial gas turbine blade experiencing in-service cracking.
A Study of On-Line and Off-Line Turbine Washing to Optimize the Operation of a Gas Turbine
Authors: Boyce Meherwan P. , Gonzalez Francisco
Published: November 2008

This paper highlights the procedure followed in order to establish an effective on-line and off line water wash program on a fleet of 36 small industrial turbines. To determine the efficacy of water washing a program of tests under controlled conditions was organized. With proper condition monitoring techniques, a set of tests were developed in order to identify the proper water wash frequency and the dissolving agent used to water wash. The goal of the water wash program is to maximize turbine power, and efficiency; while minimizing maintenance labor, and material. The Gas Turbine Compressor Isentropic Efficiency, the overall heat rate, and the overall thermal efficiency were used to compare the tests and evaluate the performance of different water wash frequencies and solvents. 8760 points defined each test as the data was taken over a one year time period, at a one hour interval.
Upgraded M501G Operating Experience
Authors: Arimura H. , Iwasaki Y. , Fukuizumi Y. , Shiozaki S. , Kallianpur V.
Published: November 2008

In 2003 the M501G at Mitsubishi’s in-house combined cycle power plant facility located at Takasago (T-Point) was upgraded. The upgrade was accomplished by replacing some of the existing hardware in the M501G gas turbine, for further improvements in output and heat rate. The verification testing at this power plant has been continuing with MHI’s latest upgraded combustor technology, that has successfully demonstrated NOx levels at 15ppm and 9ppm or lower emission levels in Mitsubishi’s G and F gas turbines, respectively. The upgraded M501G has been officially designated as the M501G1 gas turbine. This paper describes the upgraded hardware and the operating experience at the T-Point power plant.
Smart Combustors: Just Around the Corner
Authors: Zinn Ben T.
Published: November 2008

This paper reviews the state of the art of active control systems (ACS) for gas turbine combustors. Specifically, it discusses the manner in which ACS can improve the performance of combustors, the architecture of such ACS, and the designs and promising performance of ACS that have been developed to control combustion instabilities, lean blowout and pattern factor. The paper closes with a discussion of research needs, with emphasis on the integration of utilized engine ACS, health monitoring and prognostication systems into a single control system that could survive in the harsh combustor environment.
Design and Development of a Low-Cost, High Temperature Silicon Carbide Micro-Channel Recuperator
Authors: Wilson Merrill A. , Recknagle Kurt , Brooks Kriston
Published: November 2008

Typically, ceramic micro-channel devices are used for high temperature heat exchangers, catalytic reactors, electronics cooling, and processing of corrosive streams where the thermomechanical benefits of ceramic materials are desired. These benefits include: high temperature mechanical and corrosion properties and tailorable material properties such as thermal expansion, electrical conductivity and thermal conductivity. In addition, by utilizing Laminated Object Manufacturing (LOM) methods, inexpensive ceramic materials can be layered, featured and laminated in the green state and co-sintered to form monolithic structures amenable to mass production. In cooperation with the DOE and Pacific Northwest National Labs, silicon carbide (SiC) based micro-channel recuperator concepts are being developed and tested. The performance benefits of a high temperature, micro-channel heat exchanger are realized from the improved thermal efficiency of the high temperature cycles and the improved effectiveness of micro-channels for heat transfer. In designing these structures, the heat and mass transfer within the micro-channels are being analyzed with heat transfer models, computational fluid dynamics models and validated with experimental results. As an example, a typical micro-turbine cycle was modified and modeled to incorporate this ceramic recuperator and it was found that the overall thermal efficiency of the micro-turbine could be improved from about 27% to over 40%. Process improvements require technical advantages and cost advantages. These LOM methodologies have been based on well-proven industry standard processes where labor, throughput and capital estimates have been tested. Following these cost models and validation at the prototype scale, cost estimates were obtained. For the micro-turbine example, cost estimates indicate that the high-temperature SiC recuperator would cost about $200 per kWe. The development of these heat exchangers is multi-faceted and this paper focuses on the design optimization of a layered micro-channel heat exchanger, its performance testing, and fabrication development through LOM methodologies.
General properties of different models used to predict normal tissue complications due to radiation
Authors: Kuperman V. Y.
Published: November 2008

In the current study the author analyzes general properties of three different models used to predict normal tissue complications due to radiation: (1) Surviving fraction of normal cells in the framework of the linear quadratic (LQ) equation for cell kill, (2) the Lyman–Kutcher–Burman (LKB) model for normal tissue complication probability (NTCP), and (3) generalized equivalent uniform dose (gEUD). For all considered cases the author assumes fixed average dose to an organ of interest. The author's goal is to establish whether maximizing dose uniformity in the irradiated normal tissues is radiobiologically beneficial. Assuming that NTCP increases with increasing overall cell kill, it is shown that NTCP in the LQ model is maximized for uniform dose. Conversely, NTCP in the LKB and gEUD models is always smaller for a uniform dose to a normal organ than that for a spatially varying dose if parameter in these models is small (i.e., ). The derived conflicting properties of the considered models indicate the need for more studies before these models can be utilized clinically for plan evaluation and/or optimization of dose distributions. It is suggested that partial‐volume irradiation can be used to establish the validity of the considered models.
Investigation of physical image characteristics and phenomenon of edge enhancement by phase contrast using equipment typical for mammography
Authors: Yamazaki Asumi , Ichikawa Katsuhiro , Kodera Yoshie
Published: November 2008

A technique called phase contrast mammography (PCM) has only recently been applied in clinical examination. In this application, PCM images are acquired at a magnification using an x‐ray tube for clinical use, and then reduced to the real size of the object by image processing. The images showed enhanced object edges; reportedly, this enhancement occurred because of the refraction of x rays through a cylindrical object. The authors measured the physical image characteristics of PCM to compare the image characteristics of PCM with those of conventional mammography. More specifically, they measured the object‐edge‐response characteristics and the noise characteristics in the spatial frequency domain. The results revealed that the edge‐response characteristics of PCM outperformed those of conventional mammography. In addition, the characteristics changed with the object‐placement conditions and the object shapes. The noise characteristics of PCM were better than those of conventional mammography. Subsequently, to verify why object edges were enhanced in PCM images, the authors simulated image profiles that would be obtained if the x rays were refracted and totally reflected by using not only a cylindrical substance but also a planar substance as the object. So, they confirmed that the object edges in PCM images were enhanced because x rays were refracted irrespective of the object shapes. Further, they found that the edge enhancements depended on the object shapes and positions. It was also proposed that the larger magnification than 1.75 in the commercialized system might be more suitable for PCM. Finally, the authors investigated phase‐contrast effects to breast tissues by the simulation and demonstrated that PCM would be helpful in the diagnoses of mammography.
An empirical method for lag correction in cone‐beam CT
Authors: Mail N. , Moseley D. J. , Siewerdsen J. H. , Jaffray D. A.
Published: November 2008

Image lag degrades image quality in cone‐beam CT (CBCT), resulting in contrast reduction, lack of CT number accuracy and uniformity, and skin‐line artifacts. The magnitude of such degradation and the extent to which imaging performance can be improved by means of a lag correction method were investigated. Measurements were performed using a radiotherapy CBCT guidance system (Elekta Synergy XVI, Elekta Oncology Systems, Atlanta, GA), for which the imaging system is based upon a RID1640‐AL1 flat‐panel imager (Perkin Elmer, Wiesbaden, Germany). Image lag and its relationship to various parameters including signal magnitude, photon energy, and frame number were investigated, and an empirical lag correction method was developed to manage lag artifacts. The correction method was simply the subtraction from the current frame by previous frames weighted by the temporal response function. The CatPhan 500 phantom (The Phantom Laboratory, Salem, NY) within an irregularly shaped body annulus was used to demonstrate the magnitude of artifacts with and without lag correction. CBCT images after correction demonstrated improvement in skin‐line reconstruction, CT number accuracy, image uniformity, and contrast‐to‐noise ratio. Lag artifacts can be reduced by means of algorithmic correction of the projection images. Lag correction is most important for all shapes of objects having contrast inserts. For circular/cylindrical objects, lag correction does not improve the skin‐line artifact but can improve low contrast visibility adjacent to high contrast objects.
Variable beam dose rate and DMLC IMRT to moving body anatomy
Authors: Papież Lech , Abolfath Ramin M.
Published: November 2008

Derivation of formulas relating leaf speeds and beam dose rates for delivering planned intensity profiles to static and moving targets in dynamic multileaf collimator (DMLC) intensity modulated radiation therapy (IMRT) is presented. The analysis of equations determining algorithms for DMLC IMRT delivery under a variable beam dose rate reveals a multitude of possible delivery strategies for a given intensity map and for any given target motion patterns. From among all equivalent delivery strategies for DMLC IMRT treatments specific subclasses of strategies can be selected to provide deliveries that are particularly suitable for clinical applications providing existing delivery devices are used. Special attention is devoted to the subclass of beam dose rate variable DMLC delivery strategies to moving body anatomy that generalize existing techniques of such deliveries in Varian DMLC irradiation methodology to static body anatomy. Few examples of deliveries from this subclass of DMLC IMRT irradiations are investigated to illustrate the principle and show practical benefits of proposed techniques.
The Jacobian as a measure of planar dose congruence
Authors: Paniak L. D. , Charland P. M.
Published: November 2008

We propose a new starting point for comparing planar dose distributions using a Jacobian‐based measure. The measure is normalization independent, i.e., is sensitive to dose patterns while not being biased by global dose scaling. The measure is free of tunable parameters and is bounded between zero and one. We also propose an integrated version of the measure, normalized over the integrated area: the Jacobian index. We demonstrate analytically and verified numerically that the Jacobian index tends to a unique value of when comparing unrelated dose distributions. We present a preliminary demonstration of the sensitivity and general characteristics of this measure. Examples of intensity modulated photon dose distributions from a linac as predicted with the convolution‐superposition algorithm for various scenarios of irradiation mismatch and a film measured distribution are compared.
Spectrally resolved bioluminescence tomography using the reciprocity approach
Authors: Dehghani Hamid , Davis Scott C. , Pogue Brian W.
Published: November 2008

Spectrally resolved bioluminescence optical tomography is an approach to recover images of, for example, Luciferase activity within a volume using multiwavelength emission data from internal bioluminescence sources. The underlying problem of uniqueness associated with nonspectrally resolved intensity‐based bioluminescence tomography is demonstrated and it is shown that using a non‐negative constraint inverse algorithm, an accurate solution for the source distribution can be calculated from the measured data. Reconstructed images of bioluminescence are presented using both simulated complex and heterogeneous small animal models as well as real multiwavelength data from a tissue‐simulating phantom. The location of the internal bioluminescence source using experimental data is obtained with accuracy and it is shown that small ( diameter) sources of up to deep, within a complex mouse model, can be resolved accurately using a single view data collection strategy. Finally, using the reciprocity approach for image reconstruction, a dramatic improvement in computational time is shown without loss to image accuracy with both experimental and simulated data, potentially reducing computing time from .
Equivalent phantom sizes and shapes for brachytherapy dosimetric studies of and
Authors: Granero Domingo , Perez‐Calatayud Jose , Pujades‐Claumarchirant MCarmen , Ballester Facundo , Melhus Christopher S. , Rivard Mark J.
Published: November 2008

The impact of phantom size and shape in brachytherapy dosimetry was assessed using Monte Carlo methods in liquid water for and point sources. This is needed since differences in published dosimetry data, both measurements and simulations, employ a variety of phantom sizes and shapes which can cause dose differences exceeding 30% near the phantom periphery. Spheres of radius, , were examined to determine the equivalent spherical phantom size to a variety of cylinder and cube sizes, and , respectively. These sizes ranged from . The equivalent for a given size cylinder or cube was determined using a figure of merit (FOM) function to minimize differences between radial dose functions, . Using the FOM approach, a linear fit was obtained for the equivalent for a given size cylinder or cube. The equivalent phantom for a cylinder, of diameter and length , is a sphere of in radius and the equivalent phantom for a cube of on each side is a sphere of 17.5 in radius. When normalizing all results to for comparisons of phantom shape, the absolute dose rates were equivalent within 0.1% for for both and . Correlation factors to permit comparisons of unbounded data for in 20 published datasets resulted in agreement generally within 2%. Residual differences with four datasets were attributed to methodological uncertainties in the published references.
Evaluation of a new IR‐guided system for mechanical QA of linear acceleratorsa)
Authors: Lyatskaya Yulia , Kadam Dnyanesh , Levitsky Gennady , Hacker Fred , Chin Lee
Published: November 2008

The authors report the development of a new procedure for mechanical quality assurance of linear accelerators using an infrared‐guided system. The system consists of an infrared (IR) camera and an IR‐reflective marker that can be attached to a gantry, a collimator, or a treatment table. The trace of this marker can be obtained in three dimensions (3D) for a full or partial rotation of the mechanical devices. The software is written to localize rotational axes of the gantry, collimator, and the treatment table based on the marker traces. The separation of these axes characterizes the size of the sphere defining the mechanical isocenter. Additional information on anomalies in gantry movement such as degree of gantry sag and hysteresis can also be obtained. An intrinsic uncertainty of the system to localize rotational axis is or less. Tests on a linear accelerator demonstrated the ability of this system to detect the separation between rotational axes of less than and to confirm orthogonality of the planes of gantry, collimator, and table rotation.
Dual energy exposure control (DEEC) for computed tomography: Algorithm and simulation study
Authors: Stenner Philip , Kachelrieß Marc
Published: November 2008

DECT means acquiring the same object at two different energies, respectively two different tube voltages and . The raw data and undergo a decomposition process of type . The raw data are reconstructed to obtain monochromatic images of the attenuation , of the object density , or of a specific material distribution. Recent advances in DECT focus on noise reduction techniques [S. Richard and J. H. Siewerdsen, Med. Phys. (2), 586–600 (2008)] and enable high performance DECT such as lung nodule detection [Shkumat et al., Med. Phys. (2), 629–632 (2008)]. Given and a raw data‐based projection‐wise patient dose estimation the authors determine the optimal tube current curves and , with being the view angle, which minimizes image noise for a given patient dose level. DEEC can perform online; and can be determined during the scan. Simulation studies using semianthropomorphic phantom data were carried out. In particular, functions that generate ‐images and density images were evaluated. Image quality was compared to standard scans at (clinical CT) and (micro‐CT) that were taken at the same dose level and identical spatial resolution. Appropriate choice of allows to obtain ‐images that show fewer artifacts and yield image noise levels comparable to the noise of the standard scan. The authors compared the standard scan to ‐images at , which is the effective energy used in clinical CT, and found optimal results with ‐images at for micro‐CT. Nonoptimal choice of the decomposition function will, however, significantly increase image noise. In particular ‐images at , as needed for PET/CT attenuation correction, exhibit more than twice as much image noise as the standard scan. With DEEC, which guarantees best dose usage possible, monochromatic images are generated with only slightly increased noise levels at the same dose compared to a standard scan. The benefit of significantly decreased artifacts appears to allow using DEEC‐generated monochromatic images in daily routine. Furthermore, DEEC is not restricted to DECT and the inherent tube current modulation algorithm may also be applied to single energy CT.
Dosimetric uncertainty in prostate cancer proton radiotherapy
Authors: Lin Liyong , Vargas Carlos , Hsi Wen , Indelicato Daniel , Slopsema Roelf , Li Zuofeng , Yeung Daniel , Horne Dave , Palta Jatinder
Published: November 2008

Purpose: The authors we evaluate the uncertainty in proton therapy dose distribution for prostate cancer due to organ displacement, varying penumbra width of proton beams, and the amount of rectal gas inside the rectum. Methods and Materials: Proton beam treatment plans were generated for ten prostate patients with a minimum dose of 74.1 cobalt gray equivalent (CGE) to the planning target volume (PTV) while 95% of the PTV received 78 CGE. Two lateral or lateral oblique proton beams were used for each plan. The authors we investigated the uncertainty in dose to the rectal wall (RW) and the bladder wall (BW) due to organ displacement by comparing the dose‐volume histograms (DVH) calculated with the original or shifted contours. The variation between DVHs was also evaluated for patients with and without rectal gas in the rectum for five patients who had of visible rectal gas in their planning computed tomography (CT) imaging set. The uncertainty due to the varying penumbra width of the delivered protons for different beam setting options on the proton delivery system was also evaluated. Results: For a anterior shift, the relative change in the RW volume receiving 70 CGE dose was 37.9% (5.0% absolute change in 13.2% of a mean ). The relative change in the BW volume receiving 70 CGE dose was 20.9% (4.3% absolute change in 20.6% of a mean ) with a inferior shift. A penumbra difference in beam setting options on the proton delivery system resulted in the relative variations of 6.1% (0.8% absolute change) and 4.4% (0.9% absolute change) in of RW and BW, respectively. The data show that the organ displacements produce absolute DVH changes that generally shift the entire isodose line while maintaining the same shape. The overall shape of the DVH curve for each organ is determined by the penumbra and the distance of the target in beam's eye view (BEV) from the block edge. The beam setting option producing a sharper penumbra at the isocenter can reduce the magnitude of maximal doses to the RW by 2% compared to the alternate option utilizing the same block margin of . The dose to of the femoral head on the distal side of the lateral‐posterior oblique beam is increased by 25 CGE for a patient with of rectal gas. Conclusion: Variation in the rectal and bladder wall DVHs due to uncertainty in the position of the organs relative to the location of sharp dose falloff gradients should be accounted for when evaluating treatment plans. The proton beam delivery option producing a sharper penumbra reduces maximal doses to the rectal wall. Lateral‐posterior oblique beams should be avoided in patients prone to develop a large amount of rectal gas.
The effect of respiratory motion variability and tumor size on the accuracy of average intensity projection from four‐dimensional computed tomography: An investigation based on dynamic MRI
Authors: Cai Jing , Read Paul W. , Sheng Ke
Published: November 2008

Composite images such as average intensity projection (AIP) and maximum intensity projection (MIP) derived from four‐dimensional computed tomography (4D‐CT) images are commonly used in radiation therapy for treating lung and abdominal tumors. It has been reported that the quality of 4D‐CT images is influenced by the patient respiratory variability, which can be assessed by the standard deviation of the peak and valley of the respiratory trajectory. Subsequently, the resultant MIP underestimates the actual tumor motion extent. As a more general application, AIP comprises not only the tumor motion extent but also the probability that the tumor is present. AIP generated from 4D‐CT can also be affected by the respiratory variability. To quantitate the accuracy of AIP and develop clinically relevant parameters for determining suitability of the 4D‐CT study for AIP‐based treatment planning, real time sagittal dynamic magnetic resonance imaging (dMRI) was used as the basis for generating simulated 4D‐CT. Five‐minute MRI scans were performed on seven healthy volunteers and eight lung tumor patients. In addition, images of circular phantoms with diameter 1, 3, or were generated by software to simulate lung tumors. Motion patterns determined by dMRI images were reproduced by the software generated phantoms. Resorted dMRI using a 4D‐CT acquisition method (RedCAM) based on phantom or patient images was reconstructed by simulating the imaging rebinning processes. AIP images and the corresponding color intensity projection (CIP) images were reconstructed from RedCAM and the full set of dMRI for comparison. AIP similarity indicated by the Dice index between RedCAM and dMRI was calculated and correlated with respiratory variability and tumor size . The similarity of percentile intrafractional motion target area (IMTA), defined by the area that the tumor presented for a given percentage of time, and MIP‐to‐percentile IMTA similarity as a function of percentile were also determined. As a result, AIP similarity depends on both respiratory variability and tumor sizes. The AIP similarity correlated linearly with the respiratory variability normalized by tumor sizes ( equal to 0.82 and 0.91 for the phantom study and the patient study, respectively). For both studies, MIP derived from RedCAM was close to the area that the tumor presented 90% or more of the time and missed the region where the tumor appeared less than 10% of the time. In conclusion, the accuracy of composite images such as AIP and MIP derived from 4D‐CT to define the tumor motion and position is affected by patient‐specific respiratory variability and tumor sizes. Based on our study, normalized respiratory variability appears to be a pertinent parameter to assess the suitability of a 4D‐CT image set for AIP‐based treatment planning.
Breast cancer imaging: A perspective for the next decade
Authors: Karellas Andrew , Vedantham Srinivasan
Published: November 2008

Breast imaging is largely indicated for detection, diagnosis, and clinical management of breast cancer and for evaluation of the integrity of breast implants. In this work, a prospective view of techniques for breast cancer detection and diagnosis is provided based on an assessment of current trends. The potential role of emerging techniques that are under various stages of research and development is also addressed. It appears that the primary imaging tool for breast cancer screening in the next decade will be high‐resolution, high‐contrast, anatomical x‐ray imaging with or without depth information. MRI and ultrasonography will have an increasingly important adjunctive role for imaging high‐risk patients and women with dense breasts. Pilot studies with dedicated breast CT have demonstrated high‐resolution three‐dimensional imaging capabilities, but several technological barriers must be overcome before clinical adoption. Radionuclide based imaging techniques and x‐ray imaging with intravenously injected contrast offer substantial potential as a diagnostic tools and for evaluation of suspicious lesions. Developing optical and electromagnetic imaging techniques hold significant potential for physiologic information and they are likely to be of most value when integrated with or adjunctively used with techniques that provide anatomic information. Experimental studies with breast specimens suggest that phase‐sensitive x‐ray imaging techniques can provide edge enhancement and contrast improvement but more research is needed to evaluate their potential role in clinical breast imaging. From the technological perspective, in addition to improvements within each modality, there is likely to be a trend towards multi‐modality systems that combine anatomic with physiologic information. We are also likely to transition from a standardized screening, where all women undergo the same imaging exam (mammography), to selection of a screening modality or modalities based an individual‐risk or other classification.
PET/CT in radiation oncology
Authors: Pan Tinsu , Mawlawi Osama
Published: November 2008

PET/CT is an effective tool for the diagnosis, staging and restaging of cancer patients. It combines the complementary information of functional PET images and anatomical CT images in one imaging session. Conventional stand‐alone PET has been replaced by PET/CT for improved patient comfort, patient throughput, and most importantly the proven clinical outcome of PET/CT over that of PET and that of separate PET and CT. There are over two thousand PET/CT scanners installed worldwide since 2001. Oncology is the main application for PET/CT. Fluorine‐18 deoxyglucose is the choice of radiopharmaceutical in PET for imaging the glucose uptake in tissues, correlated with an increased rate of glycolysis in many tumor cells. New molecular targeted agents are being developed to improve the accuracy of targeting different disease states and assessing therapeutic response. Over 50% of cancer patients receive radiation therapy (RT) in the course of their disease treatment. Clinical data have demonstrated that the information provided by PET/CT often changes patient management of the patient and/or modifies the RT plan from conventional CT simulation. The application of PET/CT in RT is growing and will become increasingly important. Continuing improvement of PET/CT instrumentation will also make it easier for radiation oncologists to integrate PET/CT in RT. The purpose of this article is to provide a review of the current PET/CT technology, to project the future development of PET and CT for PET/CT, and to discuss some issues in adopting PET/CT in RT and potential improvements in PET/CT simulation of the thorax in radiation therapy.
Fourier rebinning algorithm for inverse geometry CT
Authors: Mazin Samuel R. , Pelc Norbert J.
Published: November 2008

Inverse geometry computed tomography (IGCT) is a new type of volumetric CT geometry that employs a large array of x‐ray sources opposite a smaller detector array. Volumetric coverage and high isotropic resolution produce very large data sets and therefore require a computationally efficient three‐dimensional reconstruction algorithm. The purpose of this work was to adapt and evaluate a fast algorithm based on Defrise's Fourier rebinning (FORE), originally developed for positron emission tomography. The results were compared with the average of FDK reconstructions from each source row. The FORE algorithm is an order of magnitude faster than the FDK‐type method for the case of 11 source rows. In the center of the field‐of‐view both algorithms exhibited the same resolution and noise performance. FORE exhibited some resolution loss (and less noise) in the periphery of the field‐of‐view. FORE appears to be a fast and reasonably accurate reconstruction method for IGCT.
Whole‐procedure clinical accuracy of Gamma Knife treatments of large lesionsa)
Authors: Ma Lijun , Chuang Cynthia , Descovich Martina , Petti Paula , Smith Vernon , Verhey Lynn
Published: November 2008

The mechanical accuracy of Gamma Knife radiosurgery based on single‐isocenter measurement has been established to within . However, the full delivery accuracy for Gamma Knife treatments of large lesions has only been estimated via the quadrature‐sum analysis. In this study, the authors directly measured the whole‐procedure accuracy for Gamma Knife treatments of large lesions to examine the validity of such estimation. The measurements were conducted on a head‐phantom simulating the whole treatment procedure that included frame placement, computed tomography imaging, treatment planning, and treatment delivery. The results of the measurements were compared with the dose calculations from the treatment planning system. Average agreements of for the isodose lines ranging from 25% to 90% of the maximum dose were found despite potentially large contributing uncertainties such as imaging resolution, dose grid size, frame registration, multi‐isocenter deliveries, etc. The results of our measurements were found to be significantly smaller than the calculated value based on the quadrature‐sum analysis. In conclusion, Gamma Knife treatments of large lesions can be delivered much more accurately than predicted from the quadrature‐sum analysis of major sources of uncertainties from each step of the delivery chain.
EGSnrc Monte Carlo calculated dosimetry parameters for and brachytherapy sources
Authors: Taylor R. E. P. , Rogers D. W. O.
Published: November 2008

This study presents the results of EGSnrc Monte Carlo calculations of the dose distribution surrounding a high dose rate brachytherapy source and 14 high dose rate and pulsed dose rate brachytherapy sources. Energy‐weighted spectra of emitted photons, a full set of TG‐43 dosimetry parameters, along‐away dose tables, and a description of the materials and geometry used for each source are provided. In addition to this, separate tallies are made of the dose contributed from primary, single‐scattered, and multiply‐scattered photons. Separation of dose in this manner allows one to use convolution/superposition methods to calculate the dose surrounding a brachytherapy source accounting for a non‐homogeneous medium. The effect of phantom size on TG‐43 dosimetry parameters and scattered dose is also investigated for the microSelectron v2 HDR source. This paper describes the calculation methods and presents the dose rate constants calculated for each source with the full set of dosimetry data being available online at the Carleton Laboratory for Radiotherapy Physics brachytherapy database (http://www.physics.carleton.ca/clrp/seeḏdatabase/).
Comparison of model and human observer performance for detection and discrimination tasks using dual‐energy x‐ray images
Authors: Richard Samuel , Siewerdsen Jeffrey H.
Published: November 2008

Model observer performance, computed theoretically using cascaded systems analysis (CSA), was compared to the performance of human observers in detection and discrimination tasks. Dual‐energy (DE) imaging provided a wide range of acquisition and decomposition parameters for which observer performance could be predicted and measured. This work combined previously derived observer models (e.g., Fisher‐Hotelling and non‐prewhitening) with CSA modeling of the DE image noise‐equivalent quanta (NEQ) and imaging task (e.g., sphere detection, shape discrimination, and texture discrimination) to yield theoretical predictions of detectability index and area under the receiver operating characteristic . Theoretical predictions were compared to human observer performance assessed using 9‐alternative forced‐choice tests to yield measurement of as a function of DE image acquisition parameters (viz., allocation of dose between the low‐ and high‐energy images) and decomposition technique [viz., three DE image decomposition algorithms: standard log subtraction (SLS), simple‐smoothing of the high‐energy image (SSH), and anti‐correlated noise reduction (ACNR)]. Results showed good agreement between theory and measurements over a broad range of imaging conditions. The incorporation of an eye filter and internal noise in the observer models demonstrated improved correspondence with human observer performance. Optimal acquisition and decomposition parameters were shown to depend on the imaging task; for example, ACNR and SSH yielded the greatest performance in the detection of soft‐tissue and bony lesions, respectively. This study provides encouraging evidence that Fourier‐based modeling of NEQ computed via CSA and imaging task provides a good approximation to human observer performance for simple imaging tasks, helping to bridge the gap between Fourier metrics of detector performance (e.g., NEQ) and human observer performance.
An evaluation of iterative reconstruction strategies based on mediastinal lesion detection using hybrid Ga‐67 SPECT images
Authors: Pereira Nicholas F. , Gifford Howard C. , Pretorius P. Hendrik , Smyczynski Mark , Licho Robert , Schneider Peter , Farncombe Troy , King Michael A.
Published: November 2008

Using psychophysical studies, the authors have evaluated the effectiveness of various strategies for compensating for physical degradations in SPECT imaging. The particular application was Ga‐67‐citrate imaging of mediastinal tumors, which was chosen because Ga‐67 is a particularly challenging radionuclide for imaging. The test strategies included compensations for nonuniform attenuation, distance‐dependent spatial resolution, and scatter applied in various combinations as part of iterative reconstructions with the rescaled block iterative–expectation maximization (RBI‐EM) algorithm. The authors also evaluated filtered backprojection reconstructions. Strategies were compared on the basis of human‐observer studies of lesion localization and detection accuracy using the localization receiver operating characteristics (LROC) paradigm. These studies involved hybrid images which were obtained by adding the projections of Monte Carlo‐simulated lesions to disease‐free clinical projection data. The background variability in these images can provide a more realistic assessment of the relative utility of reconstruction strategies than images from anthropomorphic digital phantoms. The clinical datasets were obtained using a GE‐VG dual‐detector SPECT system with CT‐estimated attenuation maps. After determining a target lesion contrast, they conducted pilot LROC studies to obtain a near‐optimal set of reconstruction parameters for each strategy, and then conducted the strategy comparison study. The results indicate improved detection accuracy with RBI‐EM as more compensations are applied within the reconstruction. The relative rankings of the test strategies agreed in most cases with those of previous studies that employed simulated projections of digital anthropomorphic phantoms, thus confirming the findings of those studies.
Measurement of neutron ambient dose equivalent in passive carbon‐ion and proton radiotherapies
Authors: Yonai Shunsuke , Matsufuji Naruhiro , Kanai Tatsuaki , Matsui Yuki , Matsushita Kaoru , Yamashita Haruo , Numano Masumi , Sakae Takeji , Terunuma Toshiyuki , Nishio Teiji , Kohno Ryosuke , Akagi Takashi
Published: November 2008

Secondary neutron ambient dose equivalents per the treatment absorbed dose in passive carbon‐ion and proton radiotherapies were measured using a rem meter, WENDI‐II at two carbon‐ion radiotherapy facilities and four proton radiotherapy facilities in Japan. Our measured results showed that (1) neutron ambient dose equivalent in carbon‐ion radiotherapy is lower than that in proton radiotherapy, and (2) the difference to the measured neutron ambient dose equivalents among the facilities is within a factor of 3 depending on the operational beam setting used at the facility and the arrangement of the beam line, regardless of the method for making a laterally uniform irradiation field: the double scattering method or the single‐ring wobbling method. The reoptimization of the beam line in passive particle radiotherapy is an effective way to reduce the risk of secondary cancer because installing an adjustable precollimator and designing the beam line devices with consideration of their material, thickness and location, etc., can significantly reduce the neutron exposure. It was also found that the neutron ambient dose equivalent in passive particle radiotherapy is equal to or less than that in the photon radiotherapy. This result means that not only scanning particle radiotherapy but also passive particle radiotherapy can provide reduced exposure to normal tissues around the target volume without an accompanied increase in total body dose.
A convenient alignment approach for x‐ray imaging experiments based on laser positioning devices
Authors: Zhang Da , Donovan Molly , Wu Xizeng , Liu Hong
Published: November 2008

This study presents a two‐laser alignment approach for facilitating the precise alignment of various imaging and measuring components with respect to the x‐ray beam. The first laser constantly pointed to the output window of the source, in a direction parallel to the path along which the components are placed. The second laser beam, originating from the opposite direction, was calibrated to coincide with the first laser beam. Thus, a visible indicator of the direction of the incident x‐ray beam was established, and the various components could then be aligned conveniently and accurately with its help.
Quantification of radiotracer uptake with a dedicated breast PET imaging system
Authors: Raylman Raymond R. , Smith Mark F. , Kinahan Paul E. , Majewski Stan
Published: November 2008

Tomographic breast imaging techniques can be used to quantify radiotracer uptake in breast and tumor tissue. However, physical processes common to PET imaging can confound accurate quantification. In this investigation, we assessed the effects of these phenomena and tested correction schemes for our new positron emission mammography–tomography system (PEM–PET). The PEM–PET scanner utilizes two sets of rotating planar detector heads. Each unit consists of a array of Hamamatsu H8500 flat panel position sensitive photomultipliers coupled to a array of LYSO detector elements . Image reconstruction is performed with a 3D‐OSEM algorithm parallelized to run on a multiprocessor computer system. The reconstructed field‐of‐view is . Much of the testing procedures were based on NEMA‐NU2/2001 protocols. Count rate losses due to pulse pile‐up, image contamination due to acceptance of random coincidences and Compton scatter, and image artifacts produced by photon attenuation were measured. It was found that the system was susceptible to count rate losses when moderate levels of radiation were present in the scanner due to the current design of the event trigger electronics. Application of corrections for Compton scattering, photon attenuation and dead time resulted in improved estimations of concentration in simplified phantom studies. Results from these preliminary studies indicate that the PEM–PET scanner will be useful for the quantification of radiotracer uptake in breast tumors, possibly facilitating early assessment of cancer treatments.
The use of EPID‐measured leaf sequence files for IMRT dose reconstruction in adaptive radiation therapy
Authors: Lee Louis , Mao Weihua , Xing Lei
Published: November 2008

For intensity modulated radiation treatment (IMRT) dose reconstruction, multileaf collimator (MLC) log files have been shown applicable for deriving delivered fluence maps. However, MLC log files are dependent on the accuracy of leaf calibration and only available from one linear accelerator manufacturer. This paper presents a proof of feasibility and principles in (1) using an amorphous silicon electronic portal imaging device (aSi‐EPID) to capture the MLC segments during an IMRT delivery and (2) reconstituting a leaf sequence (LS) file based on the leaf end positions calculated from the MLC segments and their associated fractional monitor units. These EPID‐measured LS files are then used to derive delivered fluence maps for dose reconstruction. The developed approach was tested on a pelvic phantom treated with a typical prostate IMRT plan. The delivered fluence maps, which were derived from the EPID‐measured LS files, showed slight differences in the intensity levels compared with the corresponding planned ones. The dose distribution calculated with the delivered fluence maps showed a discernible difference in the high dose region when compared to that calculated with the planned fluence maps. The maximum dose in the former distribution was also 2.5% less than that in the latter one. The EPID‐measured LS file can serve the same purpose as a MLC log files does for the derivation of the delivered fluence map and yet is independent of the leaf calibration. The approach also allows users who do not have access to MLC log files to probe the actual IMRT delivery and translate the information gained for dose reconstruction in adaptive radiation therapy.