bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐C‐211‐02: Implementation of a Small Animal Phase Contrast Computed Tomography Imaging System: The MicroPCCT
Authors: Silvius A A , Price S , Izaguirre E W
Published: November 2011

Purpose: Implementation of a small animal phase contrast computed tomography scanner to obtain high resolution small animal anatomical images at dose levels that are acceptable for radiobiological preclinical experiments. Methods: The scanner is constructed using a micro focus x‐ray source, a rotating small animal bed, and high resolution amorphous silicon detector. The x‐ray source is a 50 W, 80kVp micro focus x‐ray source with a 10×10 um2 focal spot. The animal bed consists of a rotating base supporting a small animal holder and non‐rebreathing isoflurane anesthesia delivery system. The final component is a flat panel detector with 2048×1024 pixels with a pixel size of 48 um. The detector is mounted on two linear stages orthogonally positioned allowing for fine adjustment of detector positioning and to scan the detector on the image plane to acquire projection images with a matrix area of 2048×2048 pixels by adjoining complementary portions of a projection. The animal holder is located at 25 cm and the detector is at 70 cm from the micro focus source for a system with a magnification of 2.8. Results: MicroPCCT reconstructed tomographic data with a resolution of 50 μm is achievable using 512 projections. The images show the characteristic features of phase contrast images such as an increase in the contrast at bone/tissue and tissue interfaces when compared with standard microCT images. The images are reconstructed using an adaptation of the Feldkampˈs cone beam CT algorithm to enhance the boundaries definition of the phase contrast tomographic images. Conclusions: We evaluated the performance of a microPCCT system by imaging murine models for radiobiological experiments. The system demonstrates the feasibility of acquiring phase contrast images with bench top equipment and a maximum achievable resolution of 50 μm.
SU‐C‐211‐03: X‐Ray Imaging Properties of Two Highly Granular Spectroscopic Pixel Detectors Intended for Small Animal Imaging
Authors: Koenig T , Zuber M , Zwerger A , Fauler A , Nill S , Fiederle M , Oelfke U
Published: November 2011

Purpose: The purpose of this study was to determine the X‐ray imaging properties of two spectroscopic pixel detectors of the Medipix2 kind operated with 1 mm thick cadmium telluride (CdTe) sensors. With pixel pitches of 110 and 165 um and sizes of 2.8 × 2.8 cm̂2 and 4.2 × 2.8 cm̂2, these devices seem well suited for small animal imaging. Particular emphasis was placed on determining energy response functions (ERFs), modulation transfer functions (MTFs), and detective quantum efficiencies (DQEs). Methods: ERFs were measured for both pixel pitches using 60 keV monochromatic photons. The full widths at half maximum (FWHMs) of the corresponding photopeaks were calculated. The sensorsˈ presampling MTFs were determined using a tilted wedge technique for three different values of the detectorsˈ low energy threshold (10, 25 and 50 keV). Zero frequency DQEs were measured for photon energies of 60 and 122 keV by determining the average number of pixels responding to a single incident photon. Results: For the 110 and 165 um pixel pitches, FWHMs of 2.2 and 2.3 keV were found for a bias voltage of 500 V. However, the corresponding ERFs are heavily biased especially for the smaller pixel size due to charge sharing and characteristic X‐rays. The 5% levels of the MTF were measured to be 5.7, 7.2 and 9.0 / mm for the three energy thresholds chosen. The DQEs found were 0.77 (110 um, 60 keV), 0.84 (165 um, 60 keV), 0.37 (110 um, 122 keV) and 0.38 (165 um, 122 keV). Conclusions: The generation of characteristic X‐rays in the sensor by the incident beam leads to a distortion of the detectorˈs response to radiation, because a significant amount of these fluorescence photons are detected in neighbor pixels. Another consequence is that the spatial resolution can be effectively increased by discarding low energy events.
SU‐C‐211‐04: Micro‐CT Geometric Accuracy Phantom for Improved Fiducial Localization in Image‐Guided Needle Positioning Systems
Authors: Waring C , Bax JS , Samarabandu A , Holdsworth DW , Lacefield JC , Fenster A
Published: November 2011

Purpose: To quantify and correct the geometric accuracy of an x‐ray microCT scanner to improve fiducial localization in image‐guided needle positioning systems. Methods: A geometric accuracy phantom was constructed by creating a one‐inch cube consisting of a 4×4×4 3D matrix of calibrated ¼” diameter spherical beads alternating between two materials. The phantom was imaged at both the isocenter and at the edge of the field of view of a micro‐CT scanner. All bead positions were measured in sets of five images and averaged. An automated algorithm evaluated the geometric accuracy of the scanner by comparing the known position of the beads in the phantom to their positions in the images. The algorithm used a least‐squares solution to determine scaling factors in the horizontal, vertical and axial directions of the images to correct the geometric inaccuracies. The accuracy of localizing a fiducial marker mounted onto a needle positioning system was determined with and without using the calculated scaling factors. Results: Use of the scaling factors reduced the geometric error of the measured bead positions within the phantom by a factor of two at both the isocenter and the offset position. The mean bead position errors were reduced from 54 μm to 27 μm at the isocenter and from 68 μm to 26 μm at the offset position. Localization of the fiducial mounted onto the needle positioning system was also improved by applying the scaling factors. The mean error of the measured fiducial position was reduced from 108 μm to 38 μm. Conclusions: Application of a geometric accuracy phantom in micro‐CT image‐guided needle positioning systems can improve fiducial localization and should reduce the overall needle positioning error of these systems. Further study will be required to determine optimal use of the phantom and its utility in image‐guided interventions.
SU‐C‐211‐05: Improved Target Localization with Integrated X‐Ray/bioluminescent Tomography
Authors: Naser M , Pekar J , Wong J , Patterson M
Published: November 2011

Purpose: Bioluminescence tomography (BLT) is a promising approach to guide focal irradiation of pre‐palpable tumors in small animals, but is limited in resolution. In this study, we investigate the use of x‐ray and diffuse optical tomography tomography to provide spatial and optical information about the object to improve BLT reconstruction. Methods: In a 3‐step approach, x‐ray CBCT is used first to provide the anatomic spatial information about the object. Secondly, photons of different wavelengths are transmitted through the object for detection by a CCD camera. The optical properties of the anatomic regions are then found by solutions of the diffuse optical tomography problem. In the final step, the anatomic and optical “priors” are employed in BLT reconstruction, where optical filters are used again to extract the multi‐spectral information required to derive the position and power of the internal source. The approach was tested in an experimental setup where a 1.5 mm diameter (dia) optical fiber light source was inserted at 5 different lateral positions in a 12 mm dia optical heterogeneity within a 25 mm dia cylindrical tissue‐mimicking phantom. The reconstructed positions and powers were compared with the known inputs. Results: The average error in locating the source centroid was 0.13 mm, range 0.3 mm to 0 mm. The average error in source power estimations was 9%, range 14% to −1%. The results are an improvement over commercial systems that typically have an average localization uncertainty of 1 mm and error in power estimation of more than 20%. Conclusions: The spatial and spectral prior information provided by x‐ray CT and diffuse optical tomography improves the accuracy of BLT. Integrated x‐ray/BL tomographic capability on board new small animal radiation research systems can contribute significantly to accurate biological targeting in realistic animal models.

J Wong has a sponsored research agreement with Xstrahl Ltd on small animal radiation research.
SU‐C‐211‐06: High Resolution Pre‐Clinical PET System Based on Liquid Xenon Gamma Ray Detectors
Authors: Miceli A , Bryman D , Clements C , Glister J , Kurchaninov L , Retiere F , Sossi V , Zhu H
Published: November 2011

Purpose: The authors are developing a novel high‐resolution preclinical PET system based on twelve liquid xenon (LXe) detectors arranged in a ring geometry. Each detector consists of a LXe time projection chamber (TPC) viewed by two arrays of large area avalanche photodiodes (LAAPDs). The LXe detector that uses both ionization and scintillation signals measures the energy and coordinates of each photon. In this work, we evaluate the simulated imaging performance of the PET system according to a subset of NEMA‐NU4 measurements and we report on the measurements of the spatial resolution obtained with cosmic rays. Methods: The LXePET prototype was simulated with the Geant4 code. Positron range and photon non‐colinearity were integrated in the model. Instrumental responses were parametrized. Combined light‐charge energy was used to reject scatter events. A Compton reconstruction algorithm was employed to identify the first interaction point in multi‐site events. List‐mode data were reconstructed with the software for image reconstruction STIR. To evaluate sensitivity, scatter fraction, and spatial resolution of the LXePET system, a point source was simulated. To ascertain the image quality, a micro‐Derenzo phantom was used. To measure the detector spatial resolution, muon cosmic ray data were acquired with a LXePET sector.Results: The simulation studies indicated that the noise equivalent count rate peaks at 899 kcps at 212 MBq and 1312 kcps at 224 MBq for mouse and rat phantoms. The absolute sensitivity at the center of the field of view was 12.6% and the resolution in the trans‐axial plane was < 1 mm (FWHM). Experimental measurements with a LXePET sector show a spatial resolution better than 0.5 mm Conclusions: High sensitivity, spatial and energy resolutions, and image quality are achievable with PET scanners employing LXeTPC detectors. The use of new image reconstruction algorithms tailored for high‐resolution scanners could further improve image quality.
SU‐C‐211‐07: Quantitative Reconstruction of Multiplexed Multi‐Pinhole SPECT with Scatter and Attenuation Correction
Authors: Strydhorst J , Wells R Glenn
Published: November 2011

Purpose: Small animal microSPECT is an important pre‐clinical imaging modality. However, the quantitative accuracy is limited by photon attenuation in the subject and scatter in the subject and the collimator. In this study we investigate correcting for both scatter and attenuation in the reconstruction of a small animal sized phantom. Methods: A phantom consisting of a cold centre surrounded by a hot shell, surrounded by an outer layer of either water or air, was scanned in a multiplexing multi‐pinhole (MMP) SPECT scanner. Scan parameters were as follows: 9 pinholes/detector, 2.5 mm diameter; 48 projections; helical scan; 3 minutes per projection. Projection data was recorded for two energy windows, one centered at the photopeak (140 keV) and a second window below the photopeak centered at 110 keV. The phantom was also scanned with the built‐in CT scanner (45 keV, cone beam) and reconstructed using filtered backprojection. The SPECT data were reconstructed using an iterative ordered subsets expectation maximization (OSEM) algorithm with no correction, attenuation correction (AC) only, and both scatter (SC) and AC. Attenuation was estimated from the CT and incorporated into the system matrix as part of the reconstruction. Scatter was estimated using the dual energy window method and subtracted from the projections prior to reconstruction. Absolute quantification was derived from a scan of a point source with known activity. Results: With attenuation and scatter correction, the measured activity concentrations in the hot region of the phantom were within 12% of the true value with the external chamber of the phantom both full and empty. Scatter correction in addition to AC improves the accuracy over AC alone in the cold regions. Conclusions: Attenuation correction significantly reduces the subject‐size dependence of the quantitative accuracy in small‐animal MMP SPECT. Scatter correction may provide some additional benefit.

Heart and Stroke Foundation of Ontario (NA6374) National Science and Engineering Research Council of Canada (PGS D)
SU‐C‐211‐08: Deformable Registration and Analysis of Small Animal 18F‐FLT and 18F‐FDG PET/CT Images
Authors: Bass C , Axente M , He J , Gobalakrishnan S , Hirsch J , Hugo G , Zweit J , Pugachev A
Published: November 2011

Purpose: To utilize serial PET imaging it is important to minimize the effect of repositioning errors and anatomical changes. Compensating for these errors requires an objective and reliable method of deformable image registration. Here we report on the methodology used to deformably co‐register serial PET images using CT anatomy and compare intratumoral distributions of FDG and FLT as imaged in the same animal with PET/CT on two consecutive days. Methods: Nude mice bearing FaDu (human H&N) tumor xenografts were imaged with 18F‐FDG and 18F‐FLT on two consecutive days using a dedicated small animal PET/CT scanner (Siemens Inveon). Both data sets were reconstructed and loaded into Pinnacle 9.1. Despite careful repositioning of the animal using an animal‐specific pad with recorded landmarks, misalignment of FDG and FLT PET images hindered direct voxel‐by‐voxel analysis. To perform an objective co‐registration of the PET images, we relied on associated CT images. Animal bodies and tumors were contoured on both CT scans. A mesh was generated from the contours for visualization purposes. The CT images were deformably registered with a demons algorithm, and the resulting displacement vector field was applied to the FLT image, allowing for voxel‐by‐voxel analysis of co‐registered FLT and FDG PET images in Matlab. Results: Based off of visual inspection, the deformable image registration tools available in Pinnacle are adequate for co‐registration of the animal PET/CT images despite the deformations caused by repositioning. Voxel‐by‐voxel analysis of co‐registered FLT and FDG PET/CT images produced correlation coefficients ranging from .45 to .55, p<10–4. Conclusions: Using corresponding CT anatomy with the tools in Pinnacle 9.1 to generate deformation matrices is a viable approach to deformable PET image registration. The images produced facilitated a voxel‐by‐voxel comparison of FLT vs. FDG.
SU‐C‐211‐09: Translational Imaging Applications in Traumatic Brain Injury
Authors: Selwyn R , Tang H
Published: November 2011

Purpose: To develop a high‐resolution, multimodal and translatable imaging protocol to characterize experimental models of traumatic brain injury (TBI). Small animal imaging modalities include fluorodeoxyglucose (FDG)‐positron emission tomography (PET) with computed tomography (CT) and magnetic resonance imaging (MRI).Methods: Male Sprague‐Dawley rats were exposed to either lateral fluid percussion (LFP) or controlled cortical impact (CCI) TBI with mild or moderate severity. Baseline FDG‐PET and MRI scans were conducted prior to injury and at various times post‐TBI. FDG‐PET/CT images were acquired on a Siemens Inveon Multimodality scanner based on IV injection of 37 – 74 MBq (1 – 2 mCi) of FDG. Diffusion, perfusion, and spectroscopic images along with susceptibility and T1/T2‐weighted images were acquired on a Bruker Biospec 7T/20 cm magnet running Paravision 5.1. Results: For translatable FDG‐PET imaging, a tail vein injection of FDG with 45 minutes of anesthetized uptake and 30 minutes of scanning was optimal. With the brain centered in the field of view, uptakes (ipsilateral and contralateral) were normalized by cerebellar uptake since blood sampling was not conducted. The 2‐hour MRI protocol included fast spin echo (100 um), 3D multi‐echo gradient echo susceptibility (100 um), 4‐shot EPI‐based continuous arterial spin labeling (152 um), 8‐shot EPI‐based with three direction diffusion (100 um), and single voxel PRESS spectroscopy(3 mm) Conclusions: The impact of TBI on military service members is significant with more than 5,500 soldiers diagnosed with TBIs. Early diagnosis and development of an effective treatment for veterans with TBI is a priority for the military. A multimodal and multi‐parameter imaging protocol for experimental TBI that is both translatable and efficient (high throughput) is pivotal for studying neuroprotection, regeneration, and plasticity. An optimized small animal imaging protocol was developed to investigate longitudinal changes in glucose utilization (FDG‐PET/CT), cortical volume, diffuse axonal injury, perfusion, and metabolite concentration (MRI).

Funding provided by the Center for Neuroscience and Regenerative Medicine, Uniformed Services University
SU‐C‐301‐02: Enhanced Field Emission from Clustered TiO2 Nanotube Arrays for a High Flux X‐Ray Source
Authors: Alivov Y , Klopfer M , Molloi S
Published: November 2011

Purpose: Titanium dioxide (TiO2) nanotubes grown by electrochemical oxidation can be used as a cold cathode X‐ray source. They can potentially address some of the current limitations of carbon nanotubes. As an oxide, TiO2 is not affected by residual oxygen present in the tube, which otherwise can severely degrade the deviceˈs performance. Furthermore, TiO2 nanotubes have a good electrical contact with the conductive Ti sheet, which eliminates the heating effects. One important parameter for high performance X‐ray tubes is the electron field emission current. The current density is strongly dependent on the nanotubeˈs density due to electric field screening effects. The goal of this work was to develop a method for producing low density nanotube arrays and to investigate field emission properties. Methods: TiO2 nanotubes were grown via electrochemical oxidation of Ti sheets at 40 V using ethylene glycol, glycerol solvents and 2% NH4F acid. The field emission measurements were performed in a vacuum chamber with a base pressure of 6.6×10–5 Pa in a voltage range of 0–1000 V. A copper grid with 30 m diameter wire and 70% open area was used as an anode. Results: The samples grown in ethylene glycol + 2% NH4F revealed a clustered pattern with 1–2 μm spacing, unlike the regular pattern of dense, highly‐aligned tubular structure. Clustered nanotube samples had a significant improvement in field emission performance with reduced threshold voltage (>2 times), and increased field enhancement factor (>5 times) and current density (6 times), as compared to standard nanotube samples. This improvement is thought to be due to the reduction in electric field screening effects. Conclusions: Our results show the significant enhancement in field emission performance of clustered TiO2 nanotube arrays when compared to the standard highly‐aligned nanotubes. This implies possibility for further increase the X‐ray flux from the emission source.
SU‐C‐301‐04: Development of Iterative Image Reconstruction Algorithm for TBCT System
Authors: Kim J , Xu X , Gersten D , Zhang T
Published: November 2011

Purpose: Tetrahedron Beam Computed Tomography (TBCT) circumvents some problems of cone beam CT (CBCT) but still shares the same approximate reconstruction artifact with FDK algorithm. Recently, iterative image reconstruction has been widely employed in diagnostic CT scanners in order to suppress noise and reduce imaging dose. In this study we developed an iterative TBCT image reconstruction algorithm which is able to mitigate the cone beam reconstruction artifact as well as reduce image noise. Methods: Simultaneous algebraic reconstruction technique (SART) image reconstruction algorithms were developed based on our TBCT benchtop geometry. The algorithms were tested using both numerical and physical phantoms. The projection matrix was calculated using a distance driven method. Noise levels in reconstructed images were quantified. The approximate image reconstruction artifact at larger cone angle was evaluated. Results: With the same numerical and real scanned projection data, SART algorithm produces similar spatial resolution as the FDK algorithm. The cone artifact was significantly reduced as compared with FDK algorithm. The noise levels were reduced 16–20% by the iterative algorithm at different regions of the images. Conclusions: Iterative reconstruction algorithm further improves the image quality of the TBCT system. TBCT system with SART image reconstruction algorithm potentially may be used as a mobile CT scanner for diagnostics.

This work is partially supported by NIH grant 1R21CA130330‐01A1 and DOD Prostate Cancer Research Program W81XWH‐07‐0083.
SU‐C‐301‐05: Comparative Low‐Contrast Performance of Scan Equalization Digital Mammography (SEDM) v.s. Full‐Field Digital Mammography (FFDM): A Simulation Study with Micro‐Calcifications
Authors: Liu X , Lai C , Shaw C , Shen Y , Zhong Y , Yi Y , Ge S , You Z
Published: November 2011

Purpose: To investigate the effects of exposure equalization on the noise property and visibility of micro‐calcifications using simulated SEDM imaging experiments. Method: An anthropomorphic breast phantom was imaged at various exposure levels using a full‐field digital mammography system. A lead plate with two‐dimensional array of aperture holes was used to measure primary signals which were then subtracted from those obtained without the lead plate to separate scatter components from total image signals. Exposure equalization factors were determined from a standard FFDM image and these factors were then multiplied with images acquired at various exposure levels to simulate SEDM image. Two sets of image were acquired and subtracted from each other to estimate the noise properties for different imaging techniques. Images of simulated micro‐calcifications with different sizes were composed with primary signals and then added with scatter components to form FFDM and simulated SEDM images for visualization studies. Results: SEDM resulted in reduced noise level in dense area while increased noises level in less attenuating area (fatty area or at the board of the breast where the thicknesses are reduced) compared with the standard FFDM method. The visualization of simulated micro‐calcification is seen slightly improved in dense area of the breast phantom due to improved signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR). Conclusion: SEDM technique can improve image SNRs and CNRs in dense area of the breast, hence to improve the visibility of micro‐calcification in breast images.

This work was supported in part by grants CA104759, CA124585 and CA13852 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.
SU‐C‐301‐06: Assessment of Plaque Vulnerability Using Spectral CT and Two Contrast Materials
Authors: Baturin P , Alivov Y , Molloi S
Published: November 2011

Purpose: Spectral CT was used to investigate the feasibility of detection of plaque vulnerability using gold nanoparticles. Calcium concentration in a plaque has been proposed as a marker for plaque vulnerability to rupture. A new method for quantification of calcium concentration was introduced in presence of iodine and gold. Methods: Single‐slice spectral CT from the photon counting Cadmium‐Zinc‐Telluride detector was simulated for different mixture solutions of iodine, gold, and calcium. A 120 kVp spectrum was generated to produce photon fluxes sufficient for detecting contrast materials above the k‐edges of iodine (33.2 keV) and gold (80.7 keV). The concentrations of both iodine and gold were determined by material decomposition based on the maximum likelihood method. Furthermore, calibration curves of Hounsfield units with respect to the concentrations of different mixtures were used to eliminate signal from contrast material and different tissue from the mixtures while simultaneously assessing the calcium concentration. Results: The described method yielded reliable estimates of calcium mixed with water, blood, and muscle. The average relative errors were approximately 6%. Only in a few cases the uncertainty reached up to 15%, which was due to the 5–10% uncertainty in measurements of iodine and gold concentrations. Conclusions: The study showed that it is possible to quantify calcium concentration in the presence of iodine and gold. This demonstrated the potential feasibility of the method for assessing plaque vulnerability.
SU‐C‐220‐04: Electrostatic Focal Spot Correction for X‐Ray Tubes Operating in Strong Magnetic Fields
Authors: Lillaney P , Constantin D , Ganguly A , Fahrig R
Published: November 2011

Purpose: To evaluate the effectiveness of electrostatic mechanisms for controlling the focal spot position in X‐ray tubes operating in the fringe field of an MR bore, and to determine a cathode design to implement the correction mechanism while not compromising normal X‐ray tube operation.Methods: Using a combination of theoretical calculations and high voltage vacuum standoff constraints, a cathode design was derived that is practical for standard X‐ray tube geometry. The crucial part of the design consists of two voltage‐biased deflection electrodes placed adjacent to the cathode. Space charge beam simulations were performed for the design to determine current density changes and beam deflection in the presence of a magnetic field. Phase space information from the beam simulation was input into a Monte Carlo engine to determine the effect of the cathode design on the X‐ray photon energy spectrum.Results: For a 0.07T magnetic field, a 120 kV cathode‐anode potential, a 14.4 mm cathode‐anode separation distance, and a 35 kV electrode potential, the deflection of the X‐ray tube focal spot is within 2 mm of the original position with slight distortions to focal spot shape. However, the curvature of the electron trajectories is altered resulting in a significant velocity component tangential to the anode that is on average 10 times larger than in the control case. The electron velocity changes coupled with slightly lower current density on the anode reduces the total number of photons generated by 7.5% without significantly altering the energy spectrum of the X‐ray photons Conclusions: The beam simulations demonstrate that focal spot deflection can be controlled to within reasonable values. The generated spectrum is not significantly different from that of a standard X‐ray tube, with only a moderate decrease in overall photon fluence. Work is underway to evaluate the cathode design in an experimental setting.

Funding Sources: NIHR01 EB007626, Stanford Bio‐X Fellowship, and the Lucas Foundation
SU‐C‐220‐06: Comparison of the Absorption Coefficient and Diffraction Enhanced Imagingˈˈs Apparent Absorption Coefficient of Lung Tissue
Authors: Connor D , Cole E , Zhong Z , Dilmanian F , Wiebe S , Parham C , Pisano E
Published: November 2011

Purpose: Diffraction enhanced imaging (DEI) generates image contrast in a fundamentally different way than absorption x‐ray imaging—the refraction and extinction of x‐rays yield unique contrast. The purpose of this study was to measure the apparent absorption coefficient of the lung tissue in normal rats and to quantify the difference in contrast between DEIˈs peak image and radiography. Methods: Chest radiographs and DEI peak images (analyzer crystal aligned to the Bragg peak) were acquired of anesthetized and paralyzed Sprague Dawley rats (n=14) at Beamline X15A of the National Synchrotron Light Source at an x‐ray energy of 60 keV. The images were acquired in the anteroposterior (AP) direction. Intensity measurements were used to calculate an approximate apparent absorption coefficient of rat lung tissue. Measurements of the transmitted intensity of both the radiograph and DEI peak images were then used to calculate contrast within the lung. Results: The measured apparent absorption coefficient for the inflated lungs was 0.86 +/− 0.15 cm̂−1. To put this in context, the absorption coefficient of air, water and bone at 60 keV are 0.0002 cm̂−1, 0.21 cm̂−1 and 0.50 cm̂−1, respectively. Thus, the DEI contrast generated by the lung is about two times that generated by the same thickness of bone. DEI showed a significant gain in contrast over radiography (C_DEI/C_Rad) of 1.7 +/− 0.6. Conclusions: DEI generates 70% greater lung contrast than radiography at 60 keV. The large difference in the apparent absorption coefficient of DEI and the absorption coefficient of conventional radiography shows that DEI can be used to either enhance lung contrast for the same radiation dose, or could be used to substantially reduce the required radiation dose for a chest x‐ray while maintaining the same contrast‐to‐noise ratio.

Drs. Connor, Parham, Pisano and Zhong are equity holders in NextRay, Inc. Dr. Pisano is on the Board of Directors at NextRay, Inc. Drs. Connor, Parham and Zhong are consultants to NextRay, Inc.
SU‐D‐BRA‐01: The Impact of Continuous Variable Dose Rate (CVDR) on VMAT Plan Quality, Delivery Efficiency and Accuracy
Authors: Cao D , Shepard D
Published: November 2011

Purpose: Elekta VMAT utilizes a binned variable dose rate (BVDR) whereby the dose rate jumps between a series of discrete levels during the VMAT delivery. The use of discrete dose rate levels may limit the plan quality and delivery efficiency. A continuous variable dose rate (CVDR) delivery technique has now been developed with the goal of solving such limitations. This work studies the impact of CVDR on VMAT plan quality, delivery efficiency and accuracy. Methods: 10 cases covering a variety of treatment sites were selected for this study. Among them, three cases were selected for plan quality comparison. All VMAT plans were created using Pinnacle3 SmartArc® and were delivered on an Elekta Synergy linac equipped with an 80‐leaf conventional MLC. The delivery accuracy was checked using a 2D ion chamber array. Results: Similar dose distributions were observed in both the BVDR and CVDR plans. For the more complex H&N case, however, the CVDR plan had a slightly improved target dose coverage with an average V95 (target volume receives at least 95% prescribed dose) of 99.0% as compared with 98.4% for the BVDR plan. For all three cases, the average V95 for the BVDR and CVDR plans are 96.7% and 97.0%, respectively. The average delivery time per arc was 2.35 minutes for the BVDR plans. This value decreased 25% to 1.75 minutes for the CVDR plans. As to the plan delivery accuracy, the gamma analysis passing rate of the VMAT plan QA delivered using BVDR and CVDR remains essentially the same for each case. Conclusions: The availability of continuous variable dose rate represents a significant advancement in VMAT delivery on Elekta linacs providing a 25% reduction in treatment times while providing similar or slightly improved plan quality.

This work was sponsored in part through a grant from Elekta.
SU‐D‐BRA‐02: Penumbra Measurement for a Flattening‐Filter‐Free Artiste Linear Accelerator
Authors: Gayou O , Johnson M
Published: November 2011

Purpose: The primary benefit of removing the flattening filter of a linear accelerator is to increase the dose rate by a factor of 3 to 6. Additionally, change in scatter conditions in the head of the accelerator may also decrease the penumbra, which could lead to better target conformality. The purpose of this work is to evaluate this effect on a Siemens Artiste linear accelerator. Methods: Profiles of 10 ×10 cm2 flat and flattening‐filter‐free (FFF) fields were measured in water using a scanning diode with 0.3 mm steps. The penumbra created by both the 160‐leaf multileaf collimator (MLC) and the backup jaw (Y‐jaw) was measured. The profile of the FFF beam was renormalized so that for both flat and FFF beams, the penumbra was defined as the distance between the points with 20% and 80% of the central flat beam dose. The penumbra was measured every 5 cm along the major axes and around the central axis at four depths from 1.5 to 20 cm. Results: The average penumbra defined by the MLC and Y‐jaw was 4.9 mm and 5.9 mm, respectively for the flat beam, and 4.9 mm and 5.2 mm for the FFF beam. The flat beam penumbra increased with depth from 4.6 to 9.2 mm and 5.5 to 10.1 mm for the MLC and Y‐jaw respectively. The corresponding increase for the FFF beam penumbra was 4.6 to 9.2 mm and 4.6 to 8.8 mm. The average paired difference was <0.1 mm for the MLC and 0.8 mm for the Y‐jaw. Conclusion: The MLC penumbra was not affected by removing the flattening filter, while the decreased scattering in the FFF beam reduced the Y‐jaw penumbra by 0.8 mm. Removing the flattening filter does not clinically significantly improve the way the MLC can conform to the target.

This research was partially supported by Siemens Medical Solutions.
SU‐D‐BRA‐03: Gantry Angle Sensitivity for IMRT Plan Delivery
Authors: Atwal P , Cao F , Ansbacher W , Kohli K
Published: November 2011

Purpose: Intensity Modulated Radiation Therapy (IMRT) is currently the modality of choice for optimum dose conformability for head and neck (H&N) patients. We have quantified the sensitivity of H&N treatment delivery to errors in the gantry angle (GA) using EPIDose[1], an IMRT QA tool used in our centre. Methods: EPIDose was modified to read in the planned GA for each field, add an ‘error’ term, and continue with analysis. Systematic errors (shifting all fieldsˈ GA together by same value) and a subset of random errors (using different values for each field) up to 5 degrees were studied. Analysis consisted of recreating the dose based on the captured images (including the modified GAs), and creating Chi distributions (using 3% dose‐difference, 3mm distance‐to‐agreement parameters) between planned and reconstructed doses for two dose ranges: dose above 80% of prescription dose, for primary tumour volume, and the region with 40% to 80% of prescription dose, focusing on organ‐at‐risk. For the planned and reconstructed dose to be considered clinically equivalent at least 90% of the data‐points being compared in either region needed to have a Chi value between −1 and +1. Results: Based on 20 plans, the average systematic error that can be introduced into H&N IMRT plans without having the planned and reconstructed dose considered clinically different, via the metric discussed above, is 2.4±0.5 degrees. Random error affected 13 of these plans, with the average error being 4.4±0.8 degrees. Conclusion: During delivery of H&N treatments, discrepancies in the GA less than approximately 1 degree (3 standard deviations from average systematic error) are not flagged as clinically relevant by our QA procedure, suggesting no accuracy impact to IMRT plan delivery. Fortunately, this is greater than 0.5 degrees standard tolerance set by CAPCA (Canadian Association of Provincial Cancer Agencies).

Medical Physics 33 (2006) 3369‐3382 W.Ansbacher
SU‐D‐BRA‐04: Technical Evaluation of TomoTherapy Roll Correction
Authors: Laub S , Snyder M , Burmeister J
Published: November 2011

Purpose: Evaluate the effectiveness of the roll correction feature of the TomoTherapy Hi‐ART system. Methods: A roll‐sensitive plan was designed and delivered to a cylindrical solid water phantom to test the accuracy and utility of roll corrections. Cylindrical target structures containing coaxial inner avoidance structures were placed adjacent to the plane bisecting the phantom and 7cm laterally off central axis. The phantom was positioned at isocenter with the target‐plane parallel to the couch surface. Varying degrees of phantom roll were induced and dose to the targets and inner avoidance structures was measured using Kodak EDR2 films placed in the target‐plane. Normalized point doses were compared with baseline (no roll) data to determine the sensitivity of the test and the effectiveness of the roll correction feature. Gamma analysis between baseline, roll‐corrected and uncorrected films was performed using RIT software. Results: On average, significant deviations from baseline point dose measurements (±3 standard deviations) are observed beyond 0.4° of uncorrected phantom roll. Conversely, all point dose measurements with roll correction applied are indistinguishable from baseline (<3 standard deviations) up to our maximum tested roll of 5°. For measurements in the rolled phantom without roll correction, gamma pass rates drop below 90% starting at an average of 0.25°. Gamma pass rates for all measurements with roll correction applied were greater than 97%. Conclusion: Using our rigid phantom and roll‐sensitive test plan, rotational misalignment greater than 0.25° can be detected and results in observable differences between planned and delivered dose. Utilizing the roll correction feature accurately corrects for the effects of such misalignment. Further work is required to evaluate the efficacy of roll correction in clinical circumstances where deformation accompanies rotation and to assess the clinical impact of using rotational and translational corrections compared to translational corrections alone.
SU‐D‐BRA‐05: Evaluation of the Accuracy of 3DVH+Mapcheck2 Predictions of Dose to Virtual Ion Chamber and Film in Composite IMRT QA
Authors: Olch A
Published: November 2011

Purpose: Two common methods of performing patient‐specific IMRT QA are the composite dose (ion chamber and film) and beam‐by‐beam dose measurement (diode array). Neither method by itself gives a complete enough measure of the dose distribution. DVH+Mapcheck2 was evaluated for its ability to provide both types of doses with just the array measurement. Methods: The dosimetric results from 15 patient‐specific IMRT QA tests performed by composite irradiation of ion chamber and EDR film (in coronal plane) in phantom were compared to the predicted doses for the same locations based on Mapcheck 2 measured doses of each beam within each plan and interpretation by 3DVH software. The ratio of the measured to predicted ion chamber doses was calculated and the Gamma metric was used to compare the relationship between the film doses to the Eclipse or 3DVH predicted planar doses. Also, the 3D gamma value was calculated in the 3DVH software which compares the Eclipse to 3DVH predicted dose distributions. For the 2D and 3D Gamma metrics, 2% dose and 2mm DTA were used. In addition, a simple dose difference was performed using either a 2% or 3% dose difference tolerance. Results: The mean ratio(SD) of the chamber to 3DVH predicted dose was 1.013(.015) while the ratio of the measured to Eclipse calculated dose to the chamber was 1.003(.012). The mean percentage of pixels failing the 3D gamma=1 metric was 1.2%(1.4%) while the failure rate for the 2D Gamma metric (3DVH to Eclipse) was 1.1%(1%). The Gamma failure rate for the film vs. Eclipse was 1.6%(2%). Mean dose differences, highly sensitive measures of dosimetric difference when not paired with DTA, were not significantly different between 3DVH+Mapcheck2 or film vs. Eclipse. Conclusions: 3DVH+Mapcheck2 accurately predicts the doses one would have measured had one instead used an ion chamber and coronal film in plastic phantom.

Sun Nuclear has provided support for this project
SU‐D‐BRA‐06: In Vivo Quality Assurance of Breast Cancer IMRT Treatments Using TomoTherapyˈˈs Exit Detector Data
Authors: Rodriguez V , Zoberi I , Taylor M , Klein E , Richardson S , Pereira G , Rangaraj D , Goddu S
Published: November 2011

Purpose: The primary objective of this study was to explore the possibility of using the exit detector data (EDD) for in vivo quality assurance of our clinical breast cancer patient (BC) treatments. Methods: Eleven clinical BC patients were evaluated for dose delivery accuracy using TomoTherapyˈs Delivery‐Verification software on a GPU platform. Evaluations consisted of reconstructed delivery verification sinogram (DVS) and subsequent dose computation on MV‐CT datasets. The DVS reconstructions and dose computations were calculated utilizing MV‐CT scans covering the complete length of the treatment volume and the EDD files collected on the same day. Leaf‐opening‐time (LOT) differences were analyzed to investigate the frequency and magnitude of the sinogram differences between planned and reconstructed. Reconstructed dose distributions of fraction‐deliveries were scaled and convolved to give an approximation of the composite dose distributions. Composite dose distributions were compared against the original plans. Summed doses were based on rigid‐registrations of the fraction‐doses and do not account for soft‐tissue and dose deformations. Reconstructed DVHs are based solely on planning structures.Results: Characteristic average LOT differences were ∼5msec‐eq. It can be noted that large differences between individual fractions are small for composite dose distribution. For the reconstructed summed fractions, both minimal dose (D99%) and dose inhomogeneity (D5%–D95%) of the PTV were found to be within ±0.8Gy of the planned. Ipsi‐lateral lung volume receiving >20Gy and contra‐lateral lung volume receiving >5Gy were decreased by 0.5Gy%±0.4% and 3.2%Gy±1.9%, respectively. The heart mean dose and volume receiving >35Gy were decreased by 0.3Gy±0.4 and 0.3%Gy±0.3%, respectively. Conclusion: The results of study indicate that the EDDs are useful for DVS reconstruction and dose computation which can translate into being a great tool for in vivo quality assurance of BC treatments. Currently further studies are being investigated to account for the deformation of soft‐tissue and dose distributions.
SU‐D‐BRB‐01: On the Dependence of Scanned Proton Beam Dose Output Factors on the Field Size and the Accuracy of the Lateral Dose Profiles
Authors: Titt U , Mirkovic D , Anand A , Perles L , Sawakuchi G , Mohan R
Published: November 2011

Purpose: To evaluate the dependence of central axis doses and output factors of square fields formed by scanning proton beamlets on the lateral profiles of the beamlets. Method and Materials:Square field dose distributions in a water phantom, delivered by proton beamlets used for scanned beam proton therapy, were simulated with MCNPX. Normalized central axis doses as a function of field size and depths in the phantom, the output factors, were analyzed and compared to measured values. Lateral profiles of the simulated beamlets were adjusted to closer fit characteristics of measured beamlet profiles, and to improve the agreement of output factors for measured and simulated doses and output factors. Results: Our results revealed that small changes in the low dose parts of lateral profiles far from the central axis (i.e., the “halo”) of proton beamlets produced significant changes in the central axis doses of square fields. This indicates the importance of low‐dose halo around individual beamlets in the context of modeling beams for dose prediction purposes.Conclusion: Our findings suggest that the accuracy of dose prediction algorithms for scanned beam proton therapy is highly dependent on correct modeling of the halo doses far from the beam axis. Because contemporary dose prediction algorithms do not calculate doses with a large enough lateral extent for reasons of maintaining acceptable calculation times, results of this research may help develop novel and more accurate algorithms for dose computations for scanned charged particle beams.

This project was supported in part by P01CA021239 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.
SU‐D‐BRB‐05: A Framework for 4D and Adaptive Planning Workflow Incorporating Monte Carlo‐Based Optimization and Dose Calculation
Authors: Zhong H , Stanley N , Chetty I
Published: November 2011

Purpose: Development of tools for 4D and/or adaptive planning workflow in the clinical setting, incorporating advanced dose algorithms for inverse planning, has been evolving slowly. The purpose of this work was to develop a framework for 4D and/or adaptive treatment planning in the clinical setting, incorporating Monte Carlo‐based dose calculation for optimization and dose calculation. Methods: The framework incorporates several modules for 4D and/or adaptive planning, including, (a) planning and optimization on individual, respiratory‐correlated CT datasets using a Monte‐Carlo‐based dose engine (EGSnrc/BEAMnrc for the head model and modified DOSXYZnrc for the patient calculation); (b) deformable image registration based on a finite element method (FEM); (c) dose accumulation involving mapping of energy and mass to the reference phase; (d) plan evaluation. For MC‐based optimization, beamlet 3D doses are computed and the inverse problem is solved using quadratic objective functions. Beamlet intensities are optimized using a gradient‐based search method. The framework was applied to a simulated lung phantom using FEM to simulate lung deformation. Beamlet‐doses were substituted into the re‐optimization objective function, minimized on the initial CT dataset. With the resultant beam configuration, the dose was calculated on the deformed image, and accumulated/mapped to the reference image to compute the “warped” dose. The treatment plan was then re‐optimized. Results: The MC‐based optimization procedure was verified using square field depth doses and found to be within 2% of measurements. The optimization was also applied to a clinical lung patient plan and was found produced a significantly more homogeneous distribution than the conventional pencil‐beam algorithm. For the example case, results indicate that the 90% iso‐dose coverage to the PTV was 97%, 78% and 96% for the initially planned, delivered, and re‐optimized doses, respectively. Conclusion: Initial results suggest that the framework may help facilitate use of 4D and adaptive planning in the clinical setting.
SU‐D‐BRC‐02: Predicting Individual Lung Tumor Volume Change during Image‐ Guided Radiation Therapy for Adaptive Radiation Therapy
Authors: Tai A , Gore E , Li A X
Published: November 2011

Purpose: Changes in lung tumor volume can be observed from the daily images acquired during the image‐guided radiation therapy (IGRT). The purpose of this study is to predict patient‐specific lung tumor volume change in the early part of IGRT so that adaptive radiation therapy can be carried out in the late part of the treatment. Methods: The two‐component model, assuming that lung tumor cells comprise both radiosensitive and radioresistant components, was used. Three mechanisms are included in the model: (1) proliferation of radiosensitive cells, (2) transition of the radioresistant cells into radiosensitive cells, and (3) disintegration of radiation‐damaged cells. The model consists of 4 parameters: radiobiological parameter, alpha, potential doubling time for the radiosensitive cells, initial fraction of radiosensitive cells and the disintegration half‐life of radiation‐damaged cells. The clinical data of lung tumor volume change observed from the beginning up to the mid of the treatment course for a given patient were used to determine the model parameters. The model with the patient‐specific parameters was then used to predict tumor volume change for the rest of treatment. Results: The model predicts individual lung tumor volume change reasonably well. Significant variations of the model parameters were found between patients. For example, for the patient data studied, alpha varies between 0.02 to 0.10 Gy‐ 1, the doubling time changes from a few days to a few months, and the contents of radioresistant cell varies between 20% to 60%. The fact that tumor volume change and model parameters are patient specific indicates that the remaining optimal treatment should adapt to the patient morphological and radiobiological properties. Conclusions: The lung tumor volume change observed in the early part of IGRT can be used to predict individual tumor volume regression, allowing the adaptive radiotherapy to be carried out in the remaining of the treatment.