bluuebunny/crossref_metadata_2025 huggingface dataset:
TH‐C‐AUD‐09: A Proposal for a Novel Compact Intensity Modulated Proton Therapy System Using a Dielectric Wall Accelerator
Authors: Mackie T , Caporaso G , Sampayan S , Chen Y , Blackfield D , Harris J , Hawkins S , Holmes C , Nelson S , Paul A , Poole B , Rhodes M , Sanders D , Sullivan J , Wang L , Watson J , Reckwerdt P , Schmidt R , Pearson D , Flynn R , Matthews D , Purdy J
Published: July 2007

Purpose: A novel compact CT‐guided intensity modulated proton radiotherapy (IMPT) system is introduced. The system is being designed to deliver motion‐managed IMPT to large target volumes. The system will be ideal for large and complex target volumes in young patients. Method and Materials: The basis of the design is the dielectric wall accelerator (DWA) system being developed at Lawrence Livermore National Laboratory (LLNL). The DWA will use fast switched high voltage transmission lines to generate pulsed electric fields on the inside of a high gradient insulating (HGI) acceleration tube. High electric field gradients are achieved alternating insulators and conductors and short pulse times. The system will produce individual pulses that can be varied in intensity, energy and spot width, all of which will be optimized in the IMPT planning system. It is anticipated that no magnets will be required and the neutron contamination will be very low. The system will be capable of being sited in a conventional linac vault. Results: The design specifications have been met in some component tests. Gradients of 100 MV/m have been achieved in small HGI samples. Optical switches based on fast laser switched SiC has been achieved. Feasibility tests of an optimization system for selecting the position, energy, intensity and spot size for a collection of spots comprising the treatment are underway. A prototype is being designed and concept designs of the envelope and environmental needs of the unit has commenced. Conclusion: The DWA accelerator represents breakthrough technology for intensity modulated proton therapy. The system is being designed from the ground up to be capable of CT‐guided intensity modulated proton therapy and to be housed in a conventional linac vault. Conflict of Interest:Some of the authors have financial interest in TomoTherapy Inc., which has licensed the DWA technology from LLNL.
TH‐C‐M100E‐04: Cerenkov Light From Phantom Cassettes in Absolute Dose Measurements Using Radiographic Film
Authors: Perrin D , Hogstrom K , Cheek D
Published: July 2007

Purpose: To determine the impact of Cerenkov light on radiographic film response and to recommend a methodology for correcting absolute dose measurements when using bare film, but calibrated using prepackaged film. Method and Materials: The Gammex RMI Film Dosimetry Cassette Model 436‐AST (Solid Water) and two in‐house cassettes (white opaque, high‐impact polystyrene) were studied using Kodak XV or EDR2 film. Films were exposed perpendicular to 16‐MeV electron (15×15 cm2) or 6 MV x‐ray (20×20 cm2) beams. Films were oriented such that quadrant ♯1 had bare film; quadrant ♯2 had film covered by the prepackaged white paper; quadrant ♯3 had film in its prepackaged container; and quadrant ♯4 had film covered by the prepackaged carbon jacket. To account for beam asymmetry, dose response for each quadrant was normalized to that in the corresponding quadrant of a film irradiated in the carbon jacket, which blocked phantom‐produced Cerenkov light. A prepackaged film, irradiated using a multi‐exposure technique, provided the dose‐response calibration. Results: The “carbon jacket only” dose values averaged 96.1% of the “prepackaged” dose values, indicating that the prepackaged white paper produced Cerenkov light that increased film response by 4.0%. No significant difference due to radiation modality or film type was evident. The “white paper only” dose values ranged from 103.6–107.5% of the “prepackaged” dose values, indicating that Cerenkov light from the phantom material contributed to an increased film response. For white opaque, high‐impact polystyrene the “bare film” dose values ranged from 102.2–109.6% of the “prepackaged” dose values, depending on phantom and modality. For Solid Water the “bare film” dose value was 117.3% of the “prepackaged” dose values. Conclusion: When making absolute dose measurements using bare film and calibrating using prepackaged film, a correction for excess film response arising from Cerenkov light is required, and the reported quadrant method is recommended.
TH‐C‐M100E‐06: Determining Optimal Respiratory Gating Parameters for Passively Scattered Synchrotron Based Proton Irradiation
Authors: Tsunashima Y , Vedam S , Dong L , Umezawa M , Balter P , Mohan R
Published: July 2007

Purpose: Respiratory gated irradiation offers potential for margin reduction and dose escalation for treating moving tumors in the thorax or abdomen. Unfortunately, for synchrotron‐based proton irradiation, it may not be efficient. We have determined the optimal respiratory gating parameters for passively scattered proton irradiation on a synchrotron through a simulation study. Method and Materials: An in‐house software program was developed to investigate the interaction of the respiratory gating intervals with different synchrotron magnet excitation cycle patterns. Test data was obtained by using the recorded respiratory trace of 94 patients who underwent 4DCT. A typical magnet excitation cycle, Tcyc consists of proton acceleration, flat top and deceleration periods. Proton beam delivery occurs only during the flat top portion of each such excitation cycle. Respiratory gating was simulated at expiration for a 30% duty cycle around peak exhalation. The time required to deliver 100 MUs was estimated for the following scenarios: (a) Ungated irradiation with Tcyc set to the minimum value (2.7sec) and (b) Gated irradiation with Tcyc set to (i) the minimum value, (ii) approximately equal each patient's average respiratory cycle, and (iii) a variable value according to each individual respiratory cycle. Overall treatment time and efficiency of treatment delivery were studied in each case. Results: Average times required to deliver 100 MUs were 1.1 minutes for ungated irradiation; and 3.7 (1.7 – 6.0), 3.2 (1.6 – 7.1), 2.3 (1.4 – 3.1) minutes respectively for gated irradiations at various scenarios mentioned above. For gated irradiation, variable Tcyc mode of operation yielded least overall treatment time and greatest efficiency of proton beam delivery. Conclusion: Respiratory gated passively scattered proton delivery using a synchrotron‐based system is feasible without significantly increasing treatment time. Based on above results, variable Tcyc mode of operation offered least overall treatment time and greatest efficiency for respiratory gated irradiation.
TH‐C‐M100E‐07: A CT Based Total Body Irradiation Technique Using Intensity Modulated Beams
Authors: Heaton R , van Prooijen M , Cheung F , Islam M , La Macchia N , Amin M , Tsang R , Millar B , Hodgson D
Published: July 2007

Purpose: To report experience with a novel total body irradiation (TBI) technique. 3D planning techniques are used to deliver a uniform dose to a patient using a conventional linear accelerator in a standard bunker. Manually segmented intensity modulated fields are employed to provide dose compensation for contour variation, tissue heterogeneity, inverse square law effects and junction dose stability. Methods and Materials: The technique uses a conventional Elekta Synergy linear accelerator together with a custom designed floor couch. The couch, positioned 102.5 cm below the machine isocentre, provides treatment distances near 180 cm SSD. The couch is oriented in the gantry rotation plane, with couch motion along the cranial‐caudal axis enabling a match of beam divergence through patient translations and gantry rotations. Treatment is delivered by a set of 2 to 3 divergence matched abutting fields, with field modulation feathering junctions through 4 cm on the patient. Treatment plans are created using conventional beam models in Pinnacle 7.6C and whole body CT scan data. Independent plans for supine and prone orientations are constructed to deliver a uniform dose at mid‐separation throughout the patient and create a composite uniform dose. Segmentation is used to adjust the dose at mid‐plane, correcting for effects of patient thickness, inverse square law, and lung density. Results: A total of 11 patients have been treated with this new technique. Dosimetry measurements in phantom at extended distance and in‐vivo measurements have demonstrated an accurate dose delivery. Composite AP‐PA dose assessments based on contributions to uniquely identified anatomical points have shown that a dose within 10% of the prescribed dose is achieved throughout the treatment volume. Conclusions: A new TBI technique has been implemented which employs modern imaging and delivery methods to achieve a uniform patient dose. The technique utilizes standard equipment, and does not require specialized bunker design.
TH‐C‐M100E‐10: A Correction Method for the MOSFET Energy Dependence Response to Therapeutic Proton Beams
Authors: Hsi Wen C , Hallil Abdelbasset , Cai L , Wang L
Published: July 2007

The energy‐dependence response of the metal oxide semiconductor field‐effect transistor (MOSFET) dosimeter has been investigated with regard to therapeutic proton beams. The MOSFET configurations used include the commercial standard MOSFET (TN‐502RD), the microMOSFET (TN‐502RDM) dosimeters, and a prototype MOSFET with no encapsulation. Proton beams of non‐modulated pristine and modulated Spread‐Out Bragg Peak (SOBP) of 5 cm and 10 cm widths were used with beam ranges of 8.9cm, 15.9cm and 25.9cm in water. Each MOSFET dosimeter was calibrated at the center of the modulated 10 cm SOBP proton beam with conditions of 1 cGy per MU. The MOSFET energy‐dependence response was quantitatively evaluated by the ratio of measured doses between the MOSFET and an ionization chamber in a same condition. The three dosimeters showed a similar response for the pristine proton beams at various beam ranges. This indicates that the variation in dosimeter response is dominated by the change of the linear energy transfer (LET) for the used proton beam and not by the MOSFET encapsulation thickness. The observed MOSFET trends for various pristine proton beams have been modeled by an analytical function , where Rres is the residue range (distance to the distal 90% level dose), and A, B, C are constants. A similar modeling has been performed for modulated SOBP proton beams at different widths and for various beam ranges. The k (Rres) function has been used as a correction factor for patient dose measured by MOSFETs for corresponding residue ranges; this resulted in dose measurements with an uncertainty of less than 3.0% for various proton beams.
TH‐C‐M100F‐03: One Vision of the Next Generation of Helical Tomotherapy
Authors: Kissick M , Flynn R , Westerly D , Jeraj R , Mackie T
Published: July 2007

Purpose: The current Hi‐Art™ helical TomoTherapy system is dynamic in the following sense: the gantry and couch move at constant speed during treatment. However, the dose calculations approximate this motion with 51 discrete angles projected with constant pitch. The couch speed and jaws remain fixed during treatment, but if their dynamics could be made to vary within a fraction, then real time organ motion corrections would be possible. We estimate the benefits that could be obtained with these enhanced dynamics. Method and Materials: Three key studies manifest the road to an enhanced TomoTherapy system. The studies are based on computer simulations. The first study explores longitudinal dose delivery resolution with the TomoTherapy system on a lung tumor with large avoidance structure adjacent superior and inferior. In the second study, an MTF‐like or Delivery Transfer Function (DTF) approach is used to determine the constant gantry source motion blurring on the off‐axis delivery — this effect has yet to be included in the treatment planning. Finally, the effects of longitudinal intra‐fractional organ motion are explored with respect to typical delivery patterns. Results: Equal importance specified adjacent to a tumor longitudinally can lead to few percent Gibbs‐like inhomogeneities. Ideally, we should use multiple jaw settings at edges. The DTF estimate of the gantry motion dose blurring is that the 6.25mm MLC projection at isocenter can double for full intensity of that leaf 15cm off axis. For typical treatment parameters, intra‐fraction organ motion leads to blurring of (motion amplitude)+(jaw width). If the couch speed and jaws could be variable within a fraction, then this blurring could be greatly reduced. Conclusion: If the couch and jaws are made dynamic, and motion is incorporated into the treatment planning, then very effective control of intra‐fraction organ motion and longitudinal resolution control would be achieved.
TH‐C‐M100F‐05: APT4D, An Integrated Treatment Planning Platform for 4D Radiotherapy
Authors: Lin L , Shi C , Xu X , Swanson G , Papanikolaou N
Published: July 2007

Purpose: To develop and implement an integrated treatment planning platform for 4D radiotherapy Method and Materials: 4D CT images of a thoracic dynamic phantom were acquired and resorted into 10 respiratory phases. A software platform called Automatic Post‐processing Tool for 4D treatment planning (APT4D) was developed using Matlab 6.5. Demons‐based deformable registration was performed to map the end‐exhale fixed phase (50%) to moving phases such as the end‐inhale phase (0%). The quality of image registration was evaluated by calculating the correlation coefficient slice by slice. Contours were automatically propagated into other phases using the deformation field computed from the image registration. Two indices (the volume intersection and the volume difference) from the manual contour were used to assess the quality of the contour propagation. The dose distribution of the moving phase was transformed to the common fixed phase and averaged with equal weighting factor. The 4D DMLC pattern was written by APT4D ready for the 4D delivery according to the 3D DMLC pattern for each respiratory phase. Results: The correlation coefficient ranged from 0.992 to 0.999 for the re‐sampled deformed moving phase image against the fixed phase image. Volume intersections between manual and automatic contouring were about 97% and 96%, while volume differences were around 2.5% and 3% for GTV and PTV respectively. Compared with individual phase dose distribution, dosimetric reductions for the spinal cord and total lungs were found for the 4D dose distribution. Conclusions: An integrated 4D treatment planning platform, APT4D, was developed and Preliminary test involving a thoracic dynamic phantom demonstrated that APT4D integrates the whole process of 4D treatment planning including deformable image registration, contour propagation, dose map transformation and MLC combination. The automatic functionality and feasibility can facilitate an increased workload for 4D treatment planning.
TH‐C‐M100F‐07: The Feasibility of a Hybrid Active Breath‐Hold/Gating Technique for Lung Radiotherapy
Authors: Doemer A , Hugo G
Published: July 2007

Purpose: Deep inspiration active breath‐hold enables a reproducible tumor position but a long breath‐hold is not practical for many patients with lung disease. Respiratory gating generally requires a surrogate (tracking blocks, strain gauges, etc.) that may not always correspond with actual tumor motion. In this study, we evaluated the feasibility of a hybrid system that used a short shallow inspiration active breath‐hold to gate a linear accelerator. Methods and Materials: Breath‐hold volume deviation and tolerance for the hybrid breath‐hold/gating technique were evaluated in four subjects. The subjects underwent breathing exams on an active breath‐hold device, breathing at normal and fast inhalation flow rates and two different breath‐hold volumes (0.6 L and 1.5 L). Latency in the active breath hold system was also measured with a simulated flow signal and a pressure switch. Gated beam stability (Elekta Synergy) was evaluated with an ion chamber and commercial diode arrays. Results: The short breath‐hold paradigm was tolerated well by all subjects. Active breath hold latency was measured to be 157.0 ± 7.8 milliseconds. The deviation between measured and intended breath‐hold volume was dependent on the inhalation flow rate of the patient (r2 = 0.9363). The percentage dose difference between nongated and gated delivery was less than 0.5% for breath‐holds longer then 3 seconds. Flatness and symmetry measured on SunNuclear's Profiler was relatively unaffected by gated delivery. Only 4% and 2% of diodes failed strict evaluation parameters in Mapcheck for 6 MV and 18 MV IMRT fields respectively. Conclusions: The results of this study show that using a short active breath‐hold for gating a linear accelerator is feasible. Acknowledgement: Supported in part by NIH R01 CA116249.
TH‐C‐M100F‐08: HDR Brachytherapy and Online Image‐Guided Adaptive IMRT for Dose Escalation in Prostate Cancer: Comparison of Brachytherapy and IMRT Boosts
Authors: Fatyga M , Williamson J , Dogan N , Todor D , Siebers J , George R , Hagan M , Barani I
Published: July 2007

Purpose: A course of one to three large fractions of HDR interstitial brachytherapy (HIB) is an attractive alternative to intensity modulated radiation therapy (IMRT) for delivering boost dose to the prostate in combination with additional external beam irradiation for intermediate risk disease. The purpose of this work is to quantitatively compare single‐fraction HIB boost to biologically equivalent fractionated IMRT, assuming idealized image‐guided delivery (igIMRT) and conventional delivery (cIMRT). Materials & Methods: For 9 prostate patients, both 7‐field IMRT and HIB boosts were planned. The Linear‐Quadratic model (alpha/beta = 3Gy) was used to compute biologically equivalent dose (BED) prescriptions: (a) HIB boost delivered 9Gy in a single fraction, (b) igIMRT boost delivered 20.25Gy to the CTV (prostate gland) in 9 fractions, (c) cIMRT boost delivered 20.25Gy in 9 fractions to the PTV (10mm expansion, 6mm posteriorily). The cIMRT plan was evaluated as a static plan, and with a simulated random and setup errors. The plan evaluation endpoints were: (a) for Tumor Control, the CTV Equivalent Surviving Fraction (ESD index) and (b) for Bladder and Rectum toxicity the Generalized Equivalent Uniform Dose (gEUD) and BED VH parameters. Results: HIB delivery produces outcomes comparable to or better than the idealized igIMRT delivery. On average, the ESD is 16% higher in the HIB delivery than it is in the IMRT delivery. For the HIB, the bladder/rectal gEUD values are strongly influenced by high dose DVH tails. A saturation BED, beyond which no further injury can occur, must be assumed. The gEUD values in HIB delivery are comparable, or better, than those for the IMRT delivery. Modeling of organ motion uncertainties yields mean outcomes similar to static plan outcomes. Conclusion: HIB offers therapeutic gains which exceed even the most optimum igIMRT.
TH‐C‐M100F‐09: Design and Dosimetry Validation for Micro‐Irradiator Using Monte Carlo Simulation
Authors: Rodriguez M , Jeraj R
Published: July 2007

Purpose: New tools are needed to improve the effectiveness of small specimen irradiation performed in radiobiology research. This work is focused to support the design and dosimetry of a new micro‐irradiator, capable to deliver fields as small as 1 mm, using Monte Carlo simulation. Method and Materials: A miniature x‐ray source was introduced into a brass collimator with 3.0 cm in diameter and 3.0 cm in length. A pinhole of 1.0 mm in diameter in the central axis of the collimator allowed a pencil photon beam. MCNP5 was used to characterize this collimated beam. Pulse height, flux and point detector were the tallies used to find dose rate at the surface of water, energy spectrum, PDD curves, penumbra, source position effect and radiation levels at 50 cm from the source at determined collimator‐surface distance (CSD). Relative importance in cell sub‐division, population control and angular distribution biasing were used to reduce computation time and keep the statistical fluctuation within 2%. Results: The designed micro‐irradiator has the following characteristics: the photon energy fluence was proportional to the source area projected in the pinhole and the average energy was 26.6 keV. At CSD=6.0 mm, the dose rate may go up to 1 Gy/min and it decayed about 10% per millimeter depth in water. The penumbra in the beam increases about 40 μm per millimeter away from the collimator. A source displacement of 0.5 mm respect to the pinhole axis resulted in a dose rate reduction of 20%. Radiation level at 50 cm was 10−7 Sv/min. Conclusion: The characteristics of the photon beam produced by the miniature x‐ray source and the pinhole collimator system are adequate to irradiate total and partial body of zebrafish embryos, cell cultures and other small specimens used in radiobiology research.
TH‐C‐L100E‐01: PET CT Workshop — Accreditation and Quality Assurance
Authors: Kemp B , Mawlawi O , Esser P , Anderson J
Published: July 2007

The objectives of this workshop are to provide an overview of the different commercially available PET/CT scanners and to gain the necessary knowledge of PET/CT QC/QA procedures as well as the criteria needed for PET scanner accreditation and acceptance testing. The first objective will be met during the first day whereby three presentations will be provided covering the PET/CT scanner offerings from GE Health Care, Siemens Medical Solutions, and Philips Medical Systems respectively. The presentations will focus on PET scanner design, performance, and capabilities and will be given by respective users of the equipment rather than representatives of the manufacturers. The second objective will be met during the second day of the workshop, whereby four speakers will discuss topics on PET/CT QC/QA procedures, PET NEMA acceptance testing, PET ACR accreditation, and PET site shielding calculations and requirements.
TH‐D‐L100J‐04: Dose and Image Quality of Flat‐Panel Detector Volume Computed Tomography for Sinus Imaging
Authors: Alspaugh J , Christodoulou E , Goodsitt M , Stayman J
Published: July 2007

Purpose: CT of the sinuses is commonly used for diagnosis and assessment of chronic sinusitis and preoperative planning for otolaryngological and dental procedures. Sinus imaging is a high‐contrast procedure requiring high spatial resolution. The purpose of this study is to compare the dose and image quality for typical sinus CT clinical protocols using conventional 16‐ and 64‐slice CT scanners with a flat‐panel volume CT scanner. Method and Materials: Three systems were chosen: a MiniCAT (Xoran Technologies, Inc.) flat‐panel detector unit, a Lightspeed 16 Pro, and a Lightspeed VCT (GE Medical Systems). A typical sinus technique of 120 Kvp, 150 mAs, and 2.5 mm slice thickness was selected for the GE scanners. The MiniCAT used a preset sinus technique of 120 kVp, 45 mAs, and 0.4 mm voxel size. The doses from 20 TLDs (Landauer, Inc.) inserted in a Rando head phantom were used to estimate the effective dose for each technique. The spatial resolution and slice sensitivity profiles (SSP) were measured for these same techniques. Results: The flat‐panel detector volume CT scanner gave an effective dose of 0.17 mSv while the 16‐ and 64‐slice scanners gave 0.96 and 0.87 mSv, respectively. The Lightspeed scanners had a trans‐axial spatial resolution of 7 lp/mm (standard filter) and 11 lp/cm (bone filter). The flat‐panel CT scanner had isotropic spatial resolution of 12 lp/cm. The FWHM of the SSP along the z‐axis was 0.49 mm for the MiniCAT, and 2.32 and 2.34 mm for the 16‐ and 64‐slice scanners, respectively. Conclusion: Flat‐panel volume CT with submillimeter isotropic spatial resolution is a low dose alternative for high‐contrast sinus imaging. While the conventional CT scanners have advantages for low‐contrast resolution, this can be sacrificed for a five‐fold dose reduction and increased spatial resolution in a volume image. Conflict of Interest: Research sponsored by Xoran Technologies, Inc.
TH‐D‐L100J‐07: Practical Implications of Cone‐Beam Artifact On Edge Resolution
Authors: Bartolac S , Jaffray D
Published: July 2007

Purpose: Cone‐beam CT utilizing circular trajectories fails to collect sufficient information to satisfy the inverse Radon problem. The lack of information has been characterized as a shift‐variant cone of missing frequency that scales with distance from the source trajectory plane. This cone has been previously demonstrated in localized, simple geometrical objects (spheres and disks). The aim of this study was to verify the cone in larger, clinically relevant objects and examine the relative impact of the resulting artifact on edge resolution. Methods: An excised rabbit spine with ribs and soft tissue was imaged at varying distances with respect to the source plane. Reconstructions were made using a modified Feldkamp filtered backprojection algorithm and were registered and compared for consistency. Inconsistent regions were examined with respect to surface orientation and planar extent. In addition, a section of human vertebra was similarly imaged. 3D Fourier Transforms of the vertebra were examined and regions of lacking frequency information were compared with theory. Results: In both specimens, large planar surfaces oriented such that their dominant frequencies lied within the expected null cone degraded most notably with increased distance from the source plane. In particular, upper and lower surface resolution of the rabbit vertebrae were most affected and associated with increased streaks. In contrast, cellular regions of trabecular bone, as well as bony and soft tissue with surfaces of varying orientation showed largely consistent reconstruction regardless of vertical placement. Finally, the conical region of missing frequency power predicted by theory was clearly identified in the Fourier Transforms of the human vertebra. Conclusions: Results support that cone‐beam artifact associated with circular trajectories preferentially affects a small well‐defined subset of surface orientations. In practice, cone‐beam CT using circular trajectories is expected to be largely edge preserving, since surfaces that belong to this subset are limited in extent.
TH‐D‐L100J‐08: Imaging Performance of a Mobile Cone‐Beam CT C‐Arm for Image‐Guided Interventions
Authors: Daly M , Siewerdsen J , Moseley D , Cho Y , Ansell S , Wilson G , Jaffray D
Published: July 2007

Purpose: To characterize the imaging performance of a mobile cone‐beam CT (CBCT) C‐arm for image‐guided interventions. This work reports on 3D image quality of a flat‐panel detector with multiple gain modes (Varian PaxScan 4030CB), radiation dose, and robust methods for geometric calibration and artifact management. Method and Materials: A prototype imaging system based on a mobile C‐arm (Siemens PowerMobil) has been developed to provide flat‐panel CBCT. Three readout modes (fixed‐, dual‐, and dynamic‐gain) were evaluated in CBCT phantom images across a range of dose (0.6–18.8 mGy). An analytic (non‐iterative) geometric calibration method capable of determining all nine degrees of freedom in source‐detector geometry was implemented. Image artifacts associated with x‐ray scatter and lateral truncation were characterized, and methods for artifact management (scatter estimation and projection extrapolation, respectively) were evaluated. Results: CBCT images exhibit soft‐tissue visibility (∼20 HU) and high spatial resolution (∼1 mm) at dose (∼10 mGy) sufficiently low as to permit repeat intraoperative imaging. Dynamic‐gain readout demonstrated the highest level of soft‐tissue and bony‐detail visibility across all doses, whereas fixed‐gain was degraded at high dose due to pixel saturation, and dual‐gain was degraded due to image noise. The C‐arm exhibits large geometric non‐idealities (>15 mm departure from semicircular orbit) due to mechanical flex; however, the geometric calibration restored image quality (e.g., 0.77 mm FWHM) and was reproducible to sub‐pixel precision. Lateral truncation artifacts were effectively minimized via mixed linear‐exponential extrapolation of projections at the detector edges, and x‐ray scatter was managed to a large extent by subtraction of 2D scatter fluence estimates based on the measured detector signal (patient thickness). Conclusion: The prototype C‐arm demonstrates sufficient image quality for guidance at doses low enough for repeat intraoperative imaging. The C‐arm is currently being deployed in patient protocols ranging from brachytheraphy to chest, breast, spine and head‐and‐neck surgery.
TH‐D‐L100J‐09: CT Truncation Artifact Removal Using Water‐Equivalent Thicknesses Derived From Truncated Projection Data
Authors: Maltz J , Bose S , Shukla H , Bani‐Hashemi A
Published: July 2007

Purpose: Large patient anatomies and limited imaging field‐of‐view (FOV) lead to truncation of CT projections. Truncation introduces serious artifacts into reconstructed images, including central cupping and bright external rings. FOV may be increased using laterally offset detectors, but this requires advanced imaging hardware and full angular scanning. When linacs equipped with mini‐MLCs are used for megavoltage cone beam CT imaging, truncation is inevitable. We propose a novel method to complete truncated projections based on the observation that the thickness of the patient at all angles may be estimated along the projection rays by calculating water‐equivalent thicknesses (WET). These values are not at all affected by truncation and constitute valuable auxiliary information. Methods: We parameterize a pair of points along each ray that intersects the unknown object boundary. These points are separated by the measured WET value (obtained from projections that have been corrected for scatter and beam‐hardening). We assume, for all large body parts, that the patient outline may be roughly approximated as an ellipse. We use a deterministic optimization algorithm to simultaneously estimate the point positions and ellipse parameters by minimizing the distance between point sets and the ellipse boundary. The ellipse obtained as solution is used to complete the truncated projections before reconstruction. We apply the algorithm to a severely truncated CT dataset of a typical abdomen. Results: The RMS error between non‐truncated and truncated reconstructions is 176%. The correction algorithm reduces this error to 1.0%. Conclusion: Even thought the algorithm assumes an elliptical patient cross‐section, truly impressive increases in quantitative image quality are observed. The presence of pelvic bone in the image does not appreciably bias the ellipse position even though it produces incorrect thickness estimates for some rays. The algorithm incurs low computational burden and is suitable for on‐line clinical work flows. Supported by Siemens.
TH‐D‐L100F‐01: Who's Minding the Dose?
Authors: Frush D , Boone J , McNitt‐Gray M , Cody D
Published: July 2007

This three‐part symposium will provide an update on the important topic of CT radiation dose, which continues to receive increased scrutiny from a growing proportion of the medical field.

The first talk is entitled “Radiation dose considerations in pediatric CT imaging,” and will be delivered by Dr. Donald Frush, an eminent expert in pediatric radiology. There are unique considerations when performing CT in infants and children. These considerations include the sensitivity of children to radiation, radiation dose exposure and dose assessment for this technology. The special technical considerations make pediatric CT in many ways more demanding than in adults. Accurate dose assessment is important for determining potential risks essential for the risk‐benefit ratio in this younger population.

The second talk is entitled “CT measurements: The good, the bad, and the ugly,” and will be delivered by Dr. John Boone, an esteemed medical physicist. The theory behind the current method for assessing radiation dose in CT (the CT dose index) will be reviewed. This method has many positive features (“the good”), and a growing number of limitations (“the bad”). Several groups have been formed to develop a different approach to assessing CT radiation dose, and many issues must be carefully considered (“the ugly”. Dr. Boone will provide his perspective on these and other interesting developments in the area of CT dose assessment.

The third talk is entitled “Custom patient‐based CT radiation dose estimations,” and will be delivered by Dr. Michael McNitt‐Gray, also an esteemed medical physicist. An approach that appears to be gaining in popularity for CT patient dose estimation is the use of computer models and scan simulations. This type of methodology requires a substantial investment in program development, but can result in a very flexible approach for exploring alternative technique design strategies. The use of computer modeling may find a more widespread role in CT dose assessment in the future, so it is important to understand its limitations in addition to its advantages.
TH‐D‐M100F‐01: An Evaluation of Noise in Radiotracer Emission Imaging Using Flat‐Panel Detectors
Authors: Bowsher J , Yin F , Chawla A , Greer K , Song H , Samei E , Willett C
Published: July 2007

Purpose: Single Photon Emission Computed Tomography (SPECT) onboard radiation therapy machines could enable the use of functional and molecular imaging in guiding the therapy beam and in monitoring disease. Costs and spatial constraints of onboard SPECT might be minimized if SPECT could be accomplished using the same flat‐panel detectors (FPDs) employed for onboard transmission imaging. This may require some re‐engineering of FPDs, and it is therefore important to understand the emission imaging performance of present‐day FPDs. Here we evaluate the electronic and offset‐and‐gain (OG) noise of current FPDs relative to the quantum noise characteristic of SPECT imaging. Method and Materials: A standard SPECT collimator was placed over a FPD. A flat circular radiotracer phantom was constructed with an outer region of 18 μCi/ml and a smaller inner region of 53 μCi/ml. (Typical human dose is 1 μCi/ml.) The phantom was placed on the collimator/FPD. Large low‐frequency variations in FPD response were removed by fitting each region (no activity, low activity, high activity) of the emission image with low‐order polynomials. Noise (standard deviation) was computed within each region as a function of number of acquisition frames, enabling separate estimates of electronic, OG, and quantum noise. Results: Electronic and quantum noise were comparable at 12 times the radiotracer doses typical of human SPECT imaging. For the particular OG correction considered, OG noise exceeded quantum noise after about 8 seconds (4 frames) of acquisition. Conclusion: For the FPD considered, electronic noise is about an order of magnitude above the quantum noise that would arise from radiotracer concentrations typical of human imaging. OG noise is also significant. These conclusions should be considered in the context of other aspects of FPD design. For example, thicker scintillators could boost emission‐imaging signal, thereby alleviating requirements on noise reduction.
TH‐D‐M100F‐03: A PET Head and Neck Tumor Delineation Approach Based On Adaptive Region‐Growing and Dual‐Front Active Contours
Authors: Li H , Thorstad W , Biehl K , Laforest R , Donnelly E , Low D , Lu W
Published: July 2007

Purpose: To present a novel, semi‐automatic hybrid method for PET tumor delineation utilizing adaptive region‐growing and dual‐front active contours. Method and Materials: The process begins when an experienced radiation oncologist manually draws a rough region of interest (ROI) that encloses a tumor. The image voxel having the highest intensity is chosen as a seed point. A region growing algorithm successively appends to the seed point all neighboring voxels whose intensities >= T% of the mean of the current region, which is updated after each voxel is added. When T varies from 100 to 0, the resulting volumes increase from a small region to the entire rough ROI. With this criterion, a sharp volume increase has been observed at certain T values. The process uses the sharp increases as landmarks that are used to define the image intensity level that is used to define the tumor boundary.

However, the tumor volume is found to be slightly larger than the anatomically defined volume possibly due to the partial volume effect. Therefore, we use a dual‐front active contour model to refine it automatically. We assign all points on the initial tumor boundary one label, and the seed point another label. Then we propagate the labeled boundaries towards each other. The evolution stops at meeting points that form the final tumor boundary. Results: A cylindrical phantom with seven hollow spheres of varying size (8–32mm) was constructed and scanned using a Siemens PET scanner under 12 various conditions. The method was tested on these 84 images with errors of volume overlap metric of 57.4% to 99.0% between the detected volumes and the actual volumes. Conclusion: This hybrid method guarantees a reproducible result for a specific image. Experimental results demonstrate the robustness, accuracy, reproducibility, and its potential usefulness in clinical radiation therapy planning.
TH‐D‐M100F‐04: A Biomechanical Phantom for Validation of Deformable Multimodality Image Algorithms
Authors: Yang D , Hubenschmidt J , Goddu S , Parikh P , Deasy J , Low D , El Naqa I
Published: July 2007

Purpose: Quantitative validation of multimodality deformable registration and segmentation algorithms is a challenging problem. To address this issue, we built a deformable multimodal imaging phantom system based on a preserved swine lung and a computer controlled airflow system. Comparing to most other digital and physical phantoms, this phantom is truly deformable, MRI compatible, biologically similar to human tissues, and more representative of clinical conditions. The phantom was imaged with CT and MRI protocols and yielded high resolution benchmark images. Our goal is to utilize these benchmark images to provide realistic and robust validation of medical image processing algorithms in radiotherapy applications, including multimodal deformable image registration, segmentation, tissue, as well as 4D‐CT imaging and reconstruction methods. Method and Materials: After the preserved swine lung leakage problems were addressed, the lung was inflated using a computer controlled airflow system and then imaged. 3D‐CT and MRI scans were taken at certain tidal volumes. For 4D‐CT imaging, the lung was scanned according to 4DCT protocol (Low, Med Phys v30, p1254–63, 2003) while the airflow cycle in the lung was computer‐controlled. For demonstration, we performed deformable CT‐CT registration and CT‐MR manual fusion on the collected images. 3D‐CT images were acquired at different spatial resolutions, up to image dimension of 1024×1024×530 and voxel volume of 0.244×0.244×0.67 mm. Results: 3D‐CT images allow us to clearly identify up to the 6th airway bifurcation. MR images were also of adequate quality. Conclusion: We have built the deformable phantom system and obtained images of high resolution with 3D‐CT, 4D‐CT and MRI. The images are expected to be very useful for validation of different medical image processing algorithms. We conclude that this system is promising tool for investigating and validating deformable image algorithms for radiotherapy.
TH‐D‐M100F‐05: Online Region‐Of‐Interest Delineation of Daily Head and Neck Images
Authors: Zhang T , Chi Y , Drake D , Yan D , Meldolesi E
Published: July 2007

Purpose: Online imaging modalities, such as cone beam computed tomography (CBCT) or CT on‐rail provide online volumetric images. A fast, automatic and robust region‐of‐interest (ROI) delineation method is highly desired in image guided radiation therapy (IGRT). We have developed such a method and tested it via segmentation of head and neck (HN) fan beam CT and CBCT images. Material and Methods: ROIs on planning CT images were manually delineated using commercial treatment planning system. A variational‐based deformable image registration algorithm was implemented to register planning CT images to daily CT images. ROIs on planning CT images were automatically mapped to daily images using voxel matching information between planning and daily image datasets. The results were quantitatively and qualitatively validated by comparing to manual delineation. In order to accelerate computing speed, we paralleled the algorithm using message passing interface (MPI) on a Beowulf cluster with 16 processing elements (PE). Speed improvement was benchmarked. Results: The discrepancies between automatically and manually delineated ROIs on fan beam images were mostly within 2mm. Automatic segmentation of CBCT images was acceptable by visual inspection. Benchmark results showed that paralleling efficiencies were above 95% and speedup factors were approximately equal to the number of PE used. With 16 PEs online delineation of HN images took about 1 minute. Conclusion: The online ROI delineation method we have developed is robust, fast and is suitable for HN online adaptive radiation treatments.

This research was partially supported by the Department of Defense Prostate Cancer Research Program under award number W81XWH‐07‐0083.
TH‐D‐M100J‐01: Deformable Registration of KV/MV Projection Images for Quantifying Patient Setup Offsets and Anatomical Deformations in Head and Neck IMRT
Authors: Xiang H , Cormack R , Tishler R , Maxion T , Makrigiorgos M , Chin L , Court L
Published: July 2007

Purpose: To develop an intensity‐based deformable image registration solution for detecting and quantifying both rigid and non‐rigid variations typically seen in head‐neck IMRT treatment, including patient position offsets, involuntary organ movements and anatomical deformations. Methods & Materials: A two‐step deformable image registration solution was developed by implementing a BSpline‐based non‐rigid representation of head‐neck anatomy and similarity metrics of mean square differences and normalized mutual information. kV and MV X‐ray projection images were registered in two successive steps. First, rigid‐body registrations were performed to determine patient position offsets in terms of translations and in‐plane rotations. The outputs were then used to initiate deformable registrations from which non‐rigid local displacements between reference and target images were extracted in the form of deformation vector fields. Validation studies were performed for patient CT simulation data, phantom images, and setup images of 12 head‐neck IMRT patients. The accuracy of the registrations were examined by comparing registration results with known variations in simulation, phantom images, a set of pre‐shift/post‐shift confirmation images, and with a feature‐based registration by subtracting coordinates of well‐identified anatomical points in patient setup images of different fractions. Results: For all three data sets with known changes, the mean (SD) error in rigid‐body registration was 0.3 mm (0.3 mm) for translations and 0.1° (0.1°) for in‐plane rotations. The error in deformable registration of image pairs with known changes was 0.5 mm (0.9 mm). For patient images with unknown non‐rigid local displacements, the agreements between deformable registration and the feature‐based registration were within 2.0 mm (96.5% of registered points). Conclusion: A 2‐step deformable image registration solution was developed and validated for registering kV/MV X‐ray projection images. The accuracy of the registration is adequate for detecting and quantifying both rigid and nonrigid variations typically seen during patient setup and target anatomy localization for head‐neck IMRT delivery.
TH‐D‐M100J‐05: A Phantom and Software Analysis Tool for Quality Assurance (QA) of LINAC with Onboard KV X‐Ray Imaging Device
Authors: Mao W , Xing L
Published: July 2007

Purpose: LINAC with integrated kV imaging device is emerging as an important clinical tool for radiation therapy. A novel strategy is developed to examine the system geometric and mechanical accuracies to ensure the adequate use of this new technology. Method and Materials: Three major issues that are crucial to the performance of the LINAC with onboard imager are in mind when designing the phantom system: (1) the coincidence of kV and MV beam isocenters; (2) the positional accuracy of kV and MV X‐ray sources; and (3) the positional and directional accuracy of the kV and MV imagers. After a computer simulation to maximize the detection sensitivity of these three sets of parameters, a QA phantom was built by placing 13 ball bearings (BBs) on the surface of the rectangular phantom (18×18×20 cm). A Varian Trilogy was used to validate the QA system design. The kV and MV projection data were collected with gantry rotating over 360°. The software analysis tool then computes those three sets of parameters by comparing the measured and predicted BB locations on each projection. Results: The performance of the QA package was assessed by intentionally introducing a number of errors. As a result, our QA system showed great sensitivity and accuracy in error detection. Specifically, it was found that the accuracy of the system in detecting angular uncertainties of the kV/MV sources was better than 0.2°. For spatial uncertainties, such as the kV/MV iso‐center mis‐alignment, the source position, or the detector position, the demonstrated accuracy was better than 1 mm. Furthermore, the system was able to reveal any combinational error of the angular and spatial variables at any gantry angle. Conclusions: Iso‐centers, source positions, and imager positions and directions for kV and MV can be examined accurately with this QA system.
TH‐D‐M100J‐06: Evaluation of Breast Density Using Sound Speed Measurements
Authors: Glide C , Duric N , Littrup P
Published: July 2007

Purpose: Women with high mammographic breast density are at a 4‐ to 6‐fold increased risk of developing breast cancer compared to women with fatty breasts. The purpose of this investigation was to confirm preliminary acoustic velocity measurements for breast density assessment, and to evaluate them with the current standard of care. Method and Materials: A sample of ∼100 patients was imaged with our computed ultrasound tomography clinical prototype with each dataset comprised of 45–75 tomograms ranging from near the chest wall through the nipple region. Whole breast acoustic velocity was determined by creating image stacks and evaluating the sound speed frequency distribution. The acoustic measures of breast density were evaluated by comparing these results to two mammographic density measures: (1) qualitative, as determined by a certified radiologist using the BI‐RADS Categorical Assessment based on a 1 (fatty) to 4 (dense) scale, and (2) quantitative, via digitization and computerized analysis of archival mammograms. The former involved a radiologist's visual assessment of each mammogram, while the latter required digitizing cranio‐caudal (CC) and mediolateral oblique (MLO) mammograms and implementing semi‐automatic segmentation routines. Results: Approximately 140 m/s difference in acoustic velocity was found between the fatty and dense BI‐RADS Categories, with statistically significant differences in mean sound speed demonstrated between each category (ANOVA, α= 0.05). In addition, an increased sound speed was exhibited with increased BI‐RADS Category. Furthermore, moderately strong positive associations were demonstrated between mean acoustic velocity and quantitative measures of mammographic percent breast density for both MLO and CC views (r2 = 0.65–0.76), respectively. Conclusion: These results confirm the hypothesis that utilizing acoustical velocity can be used as an indicator of breast tissue density. Having an operator independent and non‐ionizing means of whole‐breast density evaluation has important clinical implications which include tracking temporal changes, identifying high‐risk populations, and evaluating treatment response.