bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐DD‐A2‐03: Assessment of Four‐Dimensional CT Image Acquisition Quality
Authors: Starkschall G , Desai N , Balter P , Prado K , Luo D , Cody D , Pan T
Published: July 2006

The purpose of the present work is to develop and validate a series of tests to assess the quality of four‐dimensional (4D) computed tomography (CT) imaging as applied to radiation treatment planning. Using a commercial motion phantom and two programmable moving platforms with a CT phantom, we acquired 4D CT datasets on two commercial multislice helical CT scanners using different approaches to 4D CT image reconstruction. 4D CT image data sets were obtained as the platform moves in different patterns designed to simulate various breathing patterns. Known inserts were contoured and statistics were generated to evaluate properties important to radiation therapy, namely phase‐binning accuracy, geometric accuracy, volume accuracy, and CT number accuracy. Phase‐binning accuracy varied by as much as 5% for a 4D procedure in which images were reconstructed, then binned, but exhibited no variation for a 4D procedure in which projections were binned prior to reconstruction. Geometric distortion was found to be small as was volume error. Partial volume effects in the direction perpendicular to the axial planes of reconstruction affected volume accuracy, however. CT numbers were reproduced accurately, but 4D images exhibited significantly more noise than static CT images. Characterization of such properties can be used to better understand and optimize the various parameters that affect 4D CT image acquisition.
SU‐DD‐A2‐04: A Simple Method to Reconstruct a Representative Mid‐Ventilation CT Scan From 4D Respiration Correlated CT Scans for Radiotherapy Treatment Planning of Lung Cancer Patients
Authors: Wolthaus J , Schneider C , Sonke J‐J , van Herk M , Belderbos J , Rossi M , Lebesque J , Damen E
Published: July 2006

Purpose: Four‐dimensional (4D) imaging techniques can be used to obtain (respiration) artifact‐free CT images of the thorax. However, its use in radiotherapy is limited since clinical treatment planning systems are currently not able to use the full 4D data. The purpose of this study was to reconstruct a representative single 3D CT scan from the 4D data set (with tumor closest to the mean position) for use in radiotherapy planning of lung tumors to enable reduction of treatment error margins. Method and Materials: After acquisition of the 4D CT scan (10 frames), the tumor is manually segmented (roughly) in the first frame and automatically (gray‐value) registered to the tumor in the subsequent frames. This gives the motion of the tumor during the respiratory cycle in 3D. Subsequently, from the cranio‐caudal (CC) tumor motion curve, the mean tumor position and its corresponding mid‐ventilation (MV) time‐percentage are calculated. The CT scan for planning is reconstructed at this time‐percentage. As indication of the merit of this concept, its effect on margins from CTV to PTV and on the PTV volume was calculated covering respiratory motion, respiratory baseline variation and setup errors (systematic and random). Results: Based on 13 patients, the worst tumor position accuracy (with respect to the mean tumor position) in the mid‐ventilation CT scan occurred in the anterior‐posterior direction: −0.7±0.8 mm (due to hysteresis). For these patients, the errors in conventional free‐breathing CT were estimated to be 0±3.4 mm (CC) and 0±1.4 mm (AP). The mid‐ventilation concept resulted in margin reduction up to 45% and a PTV volume reduction up to 35%. Conclusion: The mid‐ventilation concept, based on tumor motion, is a simple method to obtain an artifact‐free CT scan with smaller systematic errors compared to conventional CT scans. Significant reduction of the PTV volume can be achieved.
SU‐DD‐A2‐05: Impact of Fiducial Marker Placement for the Purpose of Phase Definition of the Respiratory Cycle for 4D‐CT Image Reconstruction
Authors: Killoran J , Allen A , Berbeco R , Lyatskaya Y
Published: July 2006

Purpose: Most 4D‐CT acquisition methods rely on an externally measurable quantity proportional to the breathing cycle (e.g. chest wall excursion), for 4D‐CT image reconstruction. Typically, the position of a single reflective marker placed on the patient's chest is monitored. The marker location is often chosen primarily to maximize the measurable motion irrespective of proximity to tumor location. We examine the behavior of motion of multiple markers, at different locations, placed on the patient surface during 4D‐CT acquisition and evaluate the impact of marker location on respiratory cycle phase definition for 4D‐CT reconstruction and its subsequent application to radiotherapy planning. Method and Materials: An infrared guided positioning system (iGPS), capable of tracking multiple reflective fiducials in 3 dimensions, has been adapted to provide respiratory phase information for 4D‐CT reconstruction. Data for 3‐5 marker positions, placed at different locations on the patients chest, from 10 patients receiving 4D‐CT was examined. Results: For most patients (9/10) motion of 3–5 markers is reasonably well synchronized suggesting no significant effect of the fiducial location. For one patient, we observed a marker on the abdomen switch from being completely in‐phase to being completely out of phase relative to a marker on the center of the chest. This dramatically illustrates that the phase of a specific external marker may not correspond to the motion, external or internal, near the volume of interest. Conclusions: The position of a fiducial marker may affect not only the amplitude of motion but also the observed phase for some patients. The importance of this phase shift depends on how the resulting 4D‐CT is ultimately applied to radiotherapy planning. In particular, if specific phases (e.g. extremes) are selected for radiotherapy target definition, special attention should be paid to the location of the fiducial marker and its role in image reconstruction.
SU‐DD‐A2‐06: The Effect of Respiratory Rate and Radiation Timing On Dose Coverage in Dynamic Breast IMRT
Authors: Ding C , Li X , Huq M , Saw C , Heron D , Yue N
Published: July 2006

Purpose: IMRT has been shown to be capable of delivering plans with desirable homogeneous dose distribution for breast cancer treatment. However, the dose distribution may be influenced by interplay between dynamic MLC and respiratory motion. The purpose of this study was to investigate the impact of respiratory rate and radiation timing on the dose distribution of breast dynamic IMRT. Method and Materials: Using similar setup configuration, a helical CT and 4DCT image sets for six breast cancer patients were collected and contoured. Dynamic IMRT plans were designed using the helical CT images. The planned MLC sequence was segmented according to the respiratory phases with a series of respiratory rates (7.5–30/min) and radiation timing (evenly distributed in respiratory cycles). The segmented dynamic MLC sequences were applied to the radiation fields on the corresponding 4DCT phases. A program was developed to calculate the cumulative dose distribution from all the phases. Results: For normal breathing rates (15–20/min), the dose coverage didn't change significantly regardless of radiation starting time. The change of target V90 was less than 2%. However, for extremely slow respiratory rates (7.5–10/min), the dose distribution and V90 changed significantly depending on the radiation timing. The change of target V90 was more than 10%. There was no significant dose coverage change for the underlying heart regardless respiratory rate or radiation timing. Conclusions: For breast patients treated with dynamic IMRT, if the respiratory rate of the patient is within the “normal” range then the impact of such respiration on dose coverage of the target was found to be statistically insignificant. However, the dose distribution may change significantly when patient has a slow breathing rate. Respiratory gating may be required to obtain satisfactory dose coverage for such cases. There was no significant dose distribution change for heart regardless respiratory rate or radiation timing.
SU‐DD‐A3‐01: Kernel Classification for Assessing Inter‐Fraction Motion in IGRT
Authors: Robison B , Ramsey C , Seibert R , Garvey D
Published: July 2006

Purpose: To develop a method that identifies an IGRT imaging session as either normal or problematic based solely on the amount of right‐left, anterior‐posterior, and superior‐inferior repositioning of the patient over the treatment session. Methods and Materials: A retrospective data set containing over 1100 anterior‐posterior, right‐left lateral, and superior‐inferior patient shift values for 29 prostate patients was examined using a non‐parametric kernel regression classification method to determine if a patient was “normal” or “problematic.” The treatment sessions were grouped as either being “normal”, or affected because they were “overweight”, or had “rectal filling”, or were both “overweight and had rectal filling”. In kernel regression, constants are fitted using a locally weighted criterion. The basis of kernel regression is to estimate a response using a weighted average of points, in a training set, which are local to the query point. A bandwidth is used to determine the definition of local. Leave one out cross validation (LOOCV) was used to select the optimal bandwidth and also evaluate the technique's performance. Results: The method correctly classified 24 of the 29 patients using their respective shift data sets, with four of the misclassifications occurring when the technique correctly identified non‐normal datasets, but assigned them to the wrong problem group. Only one patient was classified as normal incorrectly. Conclusion: Using readily accessible shift data, the kernel regression classification method was able to correctly identify the cause behind IGRT positioning problems for individual prostate patients. This technique is fully automated and can be implemented on a treatment planning computer to determine the reason a patient is having positioning errors early during treatment.
SU‐DD‐A3‐02: Evaluation of Helical Tomotherapy Megavoltage CT System for Daily Automatic Patient Setup Correction and Manual Prostate Gland Motion Correction
Authors: Han C , Schultheiss T , Liu A , Wong J
Published: July 2006

Purpose: To evaluate the efficacy of helical tomotherapy megavoltage CT system (MVCT), and to study patient setup uncertainty and inter‐fractional internal organ motion for prostate cancer patients during the course of external beam treatment. Method and Materials: 34 prostate cancer patients that received definitive helical tomotherapy treatments were included in this study. MVCT images were registered with planning CT images using automatic bone registration followed by manual registration based on soft tissue match. Patient setup corrections and internal organ motion corrections in the medial‐lateral (ML), superior‐inferior (SI), anterior‐posterior (AP) directions, and rotations around the longitudinal axis were obtained from 1345 daily MVCT image registrations. Results: The mean and standard deviation of patient setup corrections were 3.1±7.3 mm in the ML direction, −0.8±4.9 in the SI direction, −0.2±6.4 in the AP direction, and 0.8±1.3° for rotations around the longitudinal axis. The mean and standard deviation of internal organ motion corrections were −0.1±0.8 mm in the ML direction, −0.1±0.7 mm in the SI direction, and 0.0±1.9 mm in the AP direction. The fraction of manual registrations that did not have adjustment in the ML, SI, or AP direction was 84%, 95%, and 71%, respectively. The prostate motion variability did not change during the course of treatment. Conclusion: Patient setup uncertainty dominated target position uncertainty. Helical tomotherapy MVCT system was effective in correcting patient setup errors and internal organ motions in the ML and AP directions, but provided limited soft tissue resolution in the SI direction.
SU‐DD‐A3‐03: A Dose‐Guided Adaptive Therapy Process for Treatment Evaluation and Correction
Authors: Ruchala K , Olivera G , Lu W , Haimerl J , Schnarr E , Chen Q , Meeks S , Langen K , Kupelian P
Published: July 2006

Purpose: To develop an adaptive therapy process for off‐line radiotherapy evaluation and modification. Method and Materials: An adaptive therapy process was developed to analyze and adjust a patient treatment. This included:

Daily Online Processes

• Daily CT imaging

• Patient repositioning

Weekly Offline Processes

• Automatic dose recalculation on each daily image

• Automatic deformable registration of each image with the planning image

• Automatic deformation‐based recontouring of each image

• Automatic deformation‐based dose accumulation

• Cumulative plan evaluation

• Replanning, as needed

The on‐line imaging and repositioning was performed with the TomoTherapy MVCT system and integrated registration software. The off‐line processes were performed on a standalone workstation.

Head and neck cases were studied with this process. Cumulated doses were typically analyzed at the end of each week, and modifications were performed mid‐course. Remaining treatments were then performed with the adapted plan. Results: It was found that CT‐guided soft‐tissue positioning alone did not protect against dosimetric changes due to patient weight loss. Without plan adaptation, the right parotid gland would have received a dose of 10 Gy above the plan, due to its medial motion towards the target region. However, since mid‐course adaptive replanning was used, the dose was only 2.5 Gy above the original plan. The use of an additional plan adaptation could have further reduced this discrepancy. Conclusion: An adaptive therapy process was developed for off‐line contouring, dose recalculation, dose accumulation, and replanning. This process was applied to clinical head and neck patients to evaluate on‐going treatments, adjust plans, and retrospectively assess the results. This process reduced unexpected parotid dose for these patients. This process also indicated that ability to recalculate to accumulate doses on daily CT images is important for addressing systematic errors, such as anatomical changes, that may arise during a treatment.

Supported by TomoTherapy, Inc.
SU‐DD‐A3‐05: Evaluation of Respiration‐Correlated Digital Tomosynthesis for Soft Tissue Visualization
Authors: Kriminski S , Lovelock D , Mageras G , Yorke E , Rosenzweig K , Mostafavi H , Ling C , Amols H
Published: July 2006

Purpose: To find optimal parameters for digital tomosynthesis (DTS) image acquisition, assess DTS imaging for soft tissue visualization and patient positioning, and determine if DTS can be acquired fast enough to avoid blur caused by the respiratory motion Methods and Materials: We have used Varian gantry‐mounted kV on‐board imaging system to acquire DTS images as well as reference cone‐beam CT (CBCT) scans. An external respiratory monitor system recorded patient respiration together with the x‐ray on/off signal during imaging for retrospective sorting of projections based on respiration phase. DTS reconstruction used backprojection followed by a deblur. For a lung tumor subject to the respiratory motion we also reconstructed DTS images during a short time interval (∼ 1 s = 6° arc at 1 rpm) around the end‐exhalation.

Results: Phantom studies indicate that image quality increases with DTS arc length; however, longer arc lengths cause image blur and degradation. Optimal DTS arc length is 10–20.° Patient studies also indicate that at approximately 15° arc length image quality, as judged visually, is the best. For longer arcs image blur increases, while for shorter arcs out of plane objects become more pronounced. For all arc lengths tumor visualization was possible. Both manual and automatic 2D registrations of DTS and CBCT were possible in most cases. For short (6°) or long (30°) arc lengths manual registration became more challenging and automatic registration less precise, but still possible. Registration of a respiratory correlated DTS image over a 6,° non‐optimal arc, was possible. Conclusions: DTS is capable of soft tissue and bone visualization and can be an efficient imaging modality for image‐guided radiotherapy. DTS can be acquired, with some tradeoff in image quality, during a ∼1s time interval, allowing reduction of respiratory motion artefacts. Conflict of interest: Research sponsored by NCI Grant P01‐CA59017 and Varian Medical Systems.
SU‐DD‐A3‐06: Assessment of Lung Tumor and Diaphragm Motion Using Cone‐Beam CT and 4DCT
Authors: Purdie T , Bissonnette J , Moseley D , Franks K , Bezjak A , Sharpe M , Jaffray D
Published: July 2006

Purpose: To determine the variability in diaphragm and tumor motion assessed at planning and for each treatment fraction for patients with inoperable early stage non‐small cell lung cancer treated using stereotactic body radiation therapy (SBRT). Method and Materials: Six patients with upper lobe lung tumors were treated using SBRT. For each patient, tumor motion was assessed initially using fluoroscopy on a conventional simulator. The subsequent scanning session involved a helical scan and a four‐dimensional CT (4DCT) scan to assess tumor motion at the inspiration and expiration phases of breathing. Each of the three treatments was performed under on‐line cone‐beam computed tomography (CBCT) image guidance. Similarly to 4DCT, the projections for the CBCT scans were sorted based on an internal surrogate of the breathing cycle to provide volumetric datasets at the inspiration and expiration phases of breathing for each treatment fraction. Results: For five of the six patients treated, lung tumor and diaphragm motion measured on the treatment unit using CBCT was consistent with the tumor motion measured by 4DCT at planning. However, for the sixth patient, diaphragm motion, and to a lesser extent tumor motion, was markedly different between planning and the three treatment fractions. For this patient, the average tumor motion difference observed between 4DCT and projection‐sorted CBCT scans was 4.5 mm (A/P) and the average diaphragm motion difference was 12.0 mm (A/P) and 9.5 mm (S/I), respectively. Conclusion: Our study has shown that the relative motion and position of the tumor at the time of treatment may not match that of the planning scan. Application of breathing motion data acquired at scanning time to modulate or gate radiation therapy may not be suitable for all patients. Conflict of Interest: Princess Margaret Hospital is part of the Elekta Synergy Consortium. The research is supported, in part, by Elekta Inc.
SU‐DD‐A4‐03: Optimization of Mammography Linear Grid Geometry
Authors: Al Kafi A , Maalej N , Naqvi A
Published: July 2006

Purpose: Anti‐scatter grids have been commonly used to reduce the amount of scatter in mammography. However, using grids require increasing the radiation dose to the breast in order to have an acceptable exposure to the image receptor. We used Monte Carlo simulation to optimize liner grid design for mammography imaging in a way to achieve best contrast improvement with lowest dose to the breast. Materials and Methods: We used Monte Carlo Simulation Code MCNP5 to determine the amount of Scatter to Primary Ratio (SPR) for different x‐ray tube peak voltage (kVp), breast thicknesses, and grid geometries. We used a Molybdenum target/Molybdenum filtered x‐ray spectra, materials and geometrical dimensions that closely mimic the clinical situation. We used a semicircular shaped breast phantom made of 50 % adipose and 50 % glandular tissue equivalent materials. The grid septa were made of lead and inter‐space was made of carbon fiber. Results: Our calculated SPR values agree within 5 % with previously published clinical data. We have obtained significant contrast improvement for low bucky factors. For a 5 cm thick breast equivalent phantom, we found an optimal septa height of 0.9mm, septa thickness of 12μm and an inter‐space thickness of 100μm gives an optimal combination of 0.2 SPR, a 2.43 bucky factor, and a 1.31 contrast improvement factor (8 % error). With this geometry, the maximum SPR was lowered from 0.58 without the grid to 0.2 with the grid. Conclusion: We have optimized the geometry of the linear grid and achieved very significant contrast improvement with low SPR while minimizing the bucky factor and hence the mean‐glandular dose to the breast.
SU‐DD‐A4‐04: Measurement of Patient Specific Doses in Mammography
Authors: Moloney W , Benevides L , Hintenlang D
Published: July 2006

Purpose: This study's aim was to develop an easily reproducible clinical protocol to predict the average glandular dose (AGD) delivered to patients during routine mammography screening. It incorporates an evaluation of patient specific features, including glandularity, to predict the clinically delivered dose for both the cranio‐caudal (CC) and the medio‐lateral oblique (MLO) views. Method and Materials: The development of a modified homogenous dosimetric breast tissue equivalent phantom series (BRTES‐MOD) based on anthropomorphic measurements of the screening mammography population is central in evaluating the patient's fibroglandular content. It has been constructed with reference to the breast tissue elemental composition tabulated in the International Commission on Radiation Units and Measurements ‐ Report 44, and simulates the compression and variable content of patient's tissue characteristics. This study calculates the average glandular dose using entrance skin exposure and dose conversion factors based on fibroglandular content, compressed breast thickness, volumetric and anatomical factors, mammographic unit parameters and modifiable parameters of the BRTES‐MOD phantom. Results: Dose conversion factors were successfully calculated from the patient's fibroglandular content, compressed thickness, unit parameters, and spectral half value layer. An anthropometric population study facilitated the derivation of clinically usable equations to determine patient whole breast area, estimate patient skin layer thickness, and assess optimal placement for the automatic exposure control ionization chamber location. Dose distributions for the study population are presented for both CC and MLO views and compare well with those derived from previous population studies. Conclusion: The designed protocol can be performed within the time of a typical mammography screening appointment, and allows the determination of patient‐specific average glandular dose. The BRTES‐MOD method also provides a quantitative measure of patient specific AGD for the multiple projections comprising screening mammography examinations.
SU‐DD‐A4‐06: Visualization of Micro‐Calcifications in a Prototype Breast CT Scanner
Authors: Kwan A , Yang K , Lindfors K , Nelson T , Boone J
Published: July 2006

Purpose: To evaluate the ability of a prototype breast CT scanner to detect micro‐calcifications, and to understand the influence that tube potential and radiation dose have on this. Method and Materials: Commercially available micro‐calcifications (μCa) of various sizes (200 to 425 μm) were embedded inside a 12.7 mm polyethylene tube filled with gelatin (to simulate glandular tissue). The gelatin tube was then placed inside a 14 cm diameter adipose equivalent cylindrical phantom and scanned using various tube potentials (60 to 100 kVp) and tube currents. CT images were reconstructed with both Ramp and Shepp‐Logan filters, with a reconstructed voxel size of about 320×320×200 μm. The μCa were then evaluated quantitatively using signal‐to‐noise ratio (SNR) metric, and subjective appraisals were made as well. A dedicated breast CT visualization workstation was used for subjective evaluation. Results: Results for 250–280 mm μCa imaged at 80 kVp shown that the μCa are clearly visible when the rod is scanned by itself, but extremely difficult to locate when placed inside the 14 cm phantom. The visualization of the μCa improved overall for larger μCa, and overall visualization improves as the radiation levels are increased, as expected. Conclusion: These initial results suggested that the pixel size may not be a critical factor when determining the ability of the prototype system to visual micro‐calcifications, as the current objects scanned are only about 48% of the reconstructed voxel size. Maximum intensity projection (MIP) display for thick‐slice imaging was found to be most useful for subjective viewing of micro‐calcification clusters.
SU‐EE‐A1‐01: Calibration of a Cobalt‐60 Irradiator for Stereotactic Radiosurgery Following the AAPM TG51 Protocol
Authors: Drzymala R , Wood R
Published: July 2006

Purpose: Compare calibration of the Leksell Gamma Knife according to the American Association of Physicists in Medicine Task Groups 21 and 51 protocols. Materials and Methods: The Gamma Knife calibration phantom (The Phantom Laboratory, Inc., Salem, NY) is designed to fill with water and support an Exradin (Standard Imaging, Inc., Middleton, WI), model A‐16, ionization chamber positioned at its center. The phantom and chamber assembly was mounted in a Leksell stereotactic ring. The location of chamber's sensitive volume was determined using computed tomography and the Leksell fiducial frame. The chamber‐phantom assembly was attached to the 18 mm helmet in the Gamma Knife by the stereotactic ring. The phantom's geometry allowed radiation beams from each of the 201 Gamma Knife cobalt‐60 sources to converge along an 8 cm path to the ionization chamber's sensitive volume. This is equivalent to the arrangement by which one calibrates the Gamma Knife using the manufacturer‐supplied polystyrene phantom. Results: The phantom could be attached to the Gamma Knife so that the ionization chamber was reproducibly positioned at the convergence of the beams. Because of the phantom's design, either trunnions or automatic patient positioning system could attach the phantom. Comparisons using different phantoms and protocols resulted in the following calibration ratios for TG‐21 in the polystyrene sphere phantom, TG‐21 in the water phantom and TG‐51 in the water phantom, respectively: 1.00, 1.010, 0.996. Transmission measurements using a block of identical material indicate that the phantom's 2mm plastic shell would result in an error of approximately 0.6% if ignored. Conclusions: Calibration of the Gamma Knife can be performed in liquid water using the AAPM TG‐51 protocol, thereby eliminating any uncertainties with respect to the composition of the polystyrene from. Calibration values for the Gamma Knife that were obtained using the three methods for our phantoms agree to within 1.4%.
SU‐EE‐A1‐06: IMRT Dose Gradients For Extracranial Stereotactic Radiosurgery
Authors: Robison B , Ramsey C , Seibert R
Published: July 2006

Purpose: To compare IMRT treatment techniques for a simulated para‐spinal mass located in the thoracic spine of an anthropomorphic phantom and to measure the accuracy of Megavoltage‐CT (MVCT) images for localizing spinal anatomy in the T‐Spine region. Methods and Materials: Treatment planning CT images were acquired on a kilovoltage CT simulator of a whole body anthropomorphic RANDO phantom and used to create a planning target volume (PTV) covering the T7 to T9 vertebral bodies. The fixed gantry IMRT cases were planned using the Pinnacle treatment planning system and delivered on a Varian 21EX with a 120‐leaf multileaf collimator. Inverse treatment plans were created with 9 and 12 equally spaced fixed fields starting at 0‐degrees (IEC Scale). Inverse planning was performed using Direct Machine Parameter Optimization (DMPO), and gradient decent optimization with sliding window leaf sequencing. Helical tomotherapy cases were planned using the TomoTherapy HI‐ART treatment planning system. Relative dose measurements were made using calibrated film placed in the RANDO phantom. MVCT images of the RANDO phantom were acquired with a tomotherapy system and fused with the treatment planning CT images. The phantom was then correctly positioned, and the fusion error was measured by imaging the T7 and T9 phantom vertebrae. A principal component analysis was used to determine the largest factors in image registration. Results: The 9‐Field DMPO and helical tomotherapy cases had PTV uniformities of 10% and maintained a large dose gradient. Conclusions: Helical tomotherapy and 9‐Field DMPO treatments yielded similar dose gradients (10%/mm) and PTV dose uniformity indices (10%). The sliding window treatment deliveries were consistently worse in cord sparing and dose uniformity. Anthropomorphic phantom studies indicated that megavoltage CT images were capable of imaging the spine for placement at isocenter within 1‐mm of the desired position.
SU‐EE‐A2‐01: Minimizing Residual Target Motion for Imaging Guided IMRT Delivery
Authors: Chuang C , Xia P , Verhey L , Ma L
Published: July 2006

Purpose: IMRT delivery could be affected by residual target motion, the intrafractional target uncertainties resulting from motion that follows image‐guided procedures such as dual x‐ray image acquisitions with patient positional corrections. This study investigates the 6D spatial characteristics of residual target motion and a method for minimizing potential IMRT delivery errors. Method and Materials: A recursive dose painting algorithm was developed to reorder MLC segments, so that most segments were delivered repetitively at a fraction of its prescribed MU. The rationale is that there could be residual target motion that would cause significant delivery errors, so to “paint” the target voxels recursively in small dose segments instead of painting the whole area once with a larger dose segment, could produce a more accurate delivery. The residual target motions used were determined from implanted spine fiducials detected using the dual x‐ray images taken every 10–20 seconds during treatment (Cyberknife, Accuray). Six‐degree of freedom spatial characteristics of the residual target uncertainties were extracted. These residual motions were then fed into the recursive dose painting method for dose evaluation. Results: The residual target motion study showed that although the average was less than 3 mm during the whole delivery, there existed large irregularities in distributions in both rotational and translational directions. For most cases, there were also systematic motion errors of 0.5–1 mm and large spurious motions, sometimes of 5 mm or more. When using the actual error‐time curves, the recursive delivery significantly reduced dose variations caused by such errors (by as much as 27%). For the majority of cases studied, a cycle of three to six repetitive dose paintings was found sufficient to achieve such improvements. Conclusion: Residual target motion is irregularly distributed as treatment time progresses. Recursive dose painting provides a solution to decrease the dose errors caused by such uncertainties.
SU‐EE‐A2‐02: Feasibility of Tracking Head Position Under An Obscuring Immobilization Mask Using a Bite Block and a 3‐D Surface Imaging System
Authors: Drzymala R , Wood R
Published: July 2006

Purpose: To assess the accuracy and feasibility of measuring head motion under a thermoplastic immobilization mask when using a 3‐D surface imaging system. Materials and Methods: Small dome‐shaped objects were arranged asymmetrically on a styrofoam platform that was attached to a bite block system. The assembly was mounted on the couch of a linear accelerator to a micromanipulator with slow motion controls. This arrangement is operationally similar to a bite block affixed to the maxilla of a intracranial radiation therapy patient. The micromanipulator allowed for motions with six degrees of freedom. We were able to achieve sub‐millimeter translational adjustments of the bite block assembly and angulation. The platform and objects were “tracked” with the AlignRT® 3‐D surface imaging system (VisionRT, London, UK) in order to compare mechanical translational and rotational movements of known magnitude with the changes reported by the AlignRT system. While translational motions are only reported with millimeter resolution on the computer control screen, we obtained the system records sub‐millimeter from stored data file records. Results: Translational agreement between the micromanipulator and the AlignRT system was 0.1±0.1 mm in all three axes. Rotational agreement was within 0.5 degrees for pitch and roll. Agreement for yaw was not determined, however the display for couch rotation is 0.1 degree and has a stability of ±0.1 degrees. Conclusion: The AlignRT surface imaging system has superior accuracy that is sufficient for stereotactic radiosurgery guidance using a bite block as we have designed the experiment. Combining a bite block similar to what we designed with the AlignRT 3‐D system shows promise in monitoring head position under an occluding immobilization mask.
SU‐EE‐A2‐05: Direct Aperture Optimization for On‐Line Adaptive Radiation Therapy
Authors: Mestrovic A , Milette M , Nichol A , Otto K
Published: July 2006

Purpose: To investigate a novel technique for on‐line adaptive radiation therapy (ART) that uses direct aperture optimization (DAO) Methods and Materials: A model simulating the geometry of a prostate case was created. The prostate, rectum and bladder are represented by an ellipsoid, cylinder and sphere, respectively (the dimensions and positions of these structures are based on patient image data). This configuration represented the “original geometry” and was used to create the original IMRT treatment plan. The plan was created using an in‐house DAO system with seven beams (40, 80, 110, 250, 280, 310, 355 gantry angles) and six apertures per beam. Four different “deformed geometries” were created by systematically deforming the original geometry to various degrees (0.25, 0.50, 0.75 and 1.00 cm maximum deformations of rectum and prostate). For each deformed geometry, a new treatment plan was created by modifying (adapting) the original treatment plan using DAO. The quality of the resulting plans, together with the optimization time efficiency of the plan adaptation, was used to assess the suitability of DAO for on‐line ART. The effects of altering different DAO parameters were investigated by varying the maximum leaf step size, maximum aperture weight change and optimization cooling schedule. Results: The plans created by adapting the original treatment plan met the imposed dose constraints for all four deformed geometries. Adapting the original treatment plan was much faster than performing a completely new re‐optimization. Furthermore, by appropriately limiting selective DAO parameters the convergence to an acceptable plan was significantly accelerated. The optimal choice of DAO parameter limits was correlated to the degree of geometry deformation. Conclusion: This study demonstrated that DAO is highly suitable for online ART. The treatment plan adaptation was efficient and the resulting plans met the imposed dose constraints.
SU‐EE‐A4‐02: Stochastic Noise in CT Images
Authors: Whiting B , Massoumzadeh P , Snyder D , Williamson J
Published: July 2006

Purpose: Because diagnostic Computed Tomography (CT) imaging involves a tradeoff between image quality and radiation risk, there is great interest in determining the effects of stochastic noise on the utility of clinical tasks. The reconstruction processes used in CT result in noise properties that are non‐local and anisotropic in the image domain. A commonly used approximation for computing image noise from raw measurement data was empirically tested for validity. A noise variance mapping scheme was used to estimate stochastic noise in complex anatomical scenes and was compared to variance measurements of image simulations generated with controlled amounts of synthetic noise. Methods: The commonly assumed transformation between linear and log variance (σ2=1/Q) was tested for Poisson random numbers with means ranging from less than one to larger than 30. Noise variance maps were generated by filtered back projection using the square of the reconstruction kernel operating on sinogram variance estimates. A series of images was reconstructed by adding Poisson noise to sinogram data, and the variance of regions of interest in the image sequence was calculated. Results: The approximation that log variance is proportional to the inverse number of quanta fails badly for N<10. Estimated variance maps were found to agree with empirical measurements of image variance. The noise variance in a CT image is a slowly varying spatial function. Image simulations demonstrated that noise has a texture that is highly anisotropic and can mimic anatomic structures. Conclusions: CT noise is a complex phenomenon. Variance maps are a useful tool for estimating noise in structured image regions where direct variance measurements fail. Fortunately most clinical scans operate at higher flux levels where the commonly used variance approximation is valid. Low‐dose protocols must be carefully evaluated to determine the effects of stochastic noise on diagnostic performance.
SU‐EE‐A4‐06: A Novel Approach for Metal Artifacts Reduction Due to Tooth Filling
Authors: Yazdi M , Beaulieu L
Published: July 2006

Introduction: The aim of this study is to present a conceptually new method for metal artifact reduction (MAR), especially for patients who have multiple metal objects with small sizes. Metallic implants such as dental fillings cause serious artifacts in reconstructed CT images. Although the previous methods based on conventional projection‐interpolation successfully reduced artifacts in the case of large metal objects such as hip prostheses, their performance appears to depend highly on the complexity of the structures examined and they are very sensitive in correctly detection of missing projections resulting still many artifacts in the final reconstruction for the case of multiple‐near metal objects. Methods and Materials: The proposed method is based on modifying the raw CT data acquired during patient's examination. First, the projection data affected by metal objects (missing projections) are detected in sinogram using a simple thresholding algorithm. Then, the missing projections are replaced by corresponding 180 degrees projections, which are not affected by metal objects. The idea beyond the replacing scheme is due to the fact that the two projections along the same path but in the opposite sides would be the same in the absence of table motion. So, in the presence of table motion, like an helical CT exam, the opposite side projections still constitute very good approximations for the corresponding missing projections. In order to make the replacing scheme more reliable, we start the process simultaneously from each side of missing projections area. Finally, the modified sinogram is transferred back to the CT scanner device where CT slices are regenerated using the built‐in reconstruction operator.

Experimental Results: The resulting tomography by the proposed approach show significant improvements in image quality, especially for regions near the metallic implants, compared to those by interpolation‐based approaches.
SU‐FF‐I‐03: Computation‐Efficient Cone Beam Image Reconstruction for Image‐ Guided Radiation Therapy Applications Using 3D Weighted Filtered Backprojection (CB‐FBP) Algorithm
Authors: Tang X , Hsieh J , Dong F
Published: July 2006

Purpose: To extend the 3D weighted cone beam filtered backprojection (CB‐FBP) algorithm for diagnostic CT imaging to image‐guided radiation therapy (IGRT) applications. Method and Materials: 3D isotropic spatial resolution is one of the most attracting features of state‐of‐the‐art volumetric CT for diagnostics imaging. However, in IGRT treatment planning, the CT image slice thickness is usually larger than what is determined by detector row width (namely thin image). A straightforward way to generate thicker image is the combination of weighted adjacent thin images in image domain, which is computationally expensive because each thin image has to go through a computation expensive 3D backprojection. Another way is to carry out cross‐row filtering in projection domain, which may cause shading/glaring artifacts and uneven slice thickness as isotropic 3D geometry is distorted. To optimize both image quality (IQ) and computational efficiency, a virtual reconstruction plane (RP) based algorithm is proposed and implemented. By using the 3D weighted CB‐FBP algorithm, a thick image is still a weighted combination of adjacent thin images, but the combination is implemented in projection domain using virtual RPs. To maintain the IQ of thick image, the 3D weighted CB‐FBP algorithm is applied by tracking re‐sampled projection data. The tracking process is to improve computational efficiency further by making use of the projection data corresponding to involved virtual RPs only, while the re‐sampling process is to improve IQ by increasing the in‐plane sampling‐rate in virtual RPs. Results: By using a helical body phantom, spatial resolution phantom and 20 cm water phantom, the performance of the proposed algorithm, such as suppression of artifacts, uniformity of slice thickness and noise characteristics, are experimentally evaluated and verified. Conclusion: The experimental evaluation shows the proposed algorithm is indeed an optimized image reconstruction solution for IGRT applications in terms of both image quality and computational efficiency.
SU‐FF‐I‐04: A Fast Variable‐Intensity Ring Suppression Algorithm
Authors: Star‐Lack J , Starman J , Munro P , Jeung A , Richters J , Mostafavi H , Pavkovich J
Published: July 2006

Purpose: Gain drifts and nonlinearities in amorphous silicon flat‐panel x‐ray detectors can produce ring artifacts in reconstructed cone‐beam computed tomography (CBCT) images. We have found that the magnitude of these artifacts can exceed 50 HU in clinical situations, and that the intensity of a given ring may not be uniform throughout an image. In some cases (e.g. half‐fan pelvic scans), discrete arcs may be produced. The goal of this study was to develop a post‐processing algorithm to efficiently suppress such variable‐intensity rings in axial slices. Method and Materials: Our approach builds upon the work of Sijbers and Postnov who showed that constant‐intensity rings can be estimated via radial median filtering of the input image after its transformation to polar coordinates. To characterize variable‐intensity rings and arcs, we developed a 2‐D estimation technique that uses a combination of row‐based (radial) and column‐based (angular) filters operating in the polar domain. The 2‐D estimates were transformed back to Cartesian space for subtraction from the original image. The new algorithm was implemented in C++ and tested on clinical and phantom CBCT images acquired using a Varian 4030CB detector. Results: Correction times (3.2GHz Intel Pentium4 processor), including coordinate transformations, averaged 55 msec/slice for 512×512 matrix sizes. Rings and arcs were reduced in intensity by more than an order of magnitude to levels well below the background noise intensity. By subtracting ring estimates in Cartesian space, the polar matrix size could be reduced without sacrificing spatial resolution in the final image. This permitted for a 4× reduction in execution time compared to the original Sijbers‐Postnov approach where subtraction occurs in polar space. Conclusion: The Sijbers‐Postnov algorithm ring suppression algorithm was modified to provide improved image quality and fast execution times suitable for clinical implementation. Conflict of Interest: Funding provided by Varian Medical Systems.
SU‐FF‐I‐06: A Portable Test Platform for Image Acquisition and Calibration for Cone Beam Computed Tomography (CBCT) and Region of Interest CBCT (ROI‐CBCT) On a Commercial X‐Ray C‐Arm System
Authors: Rangwala H , Chityala R , Rudin S , Hoffmann K
Published: July 2006

Purpose: We have developed a unique portable test platform (PTP) which enables CBCT for specimens and phantoms on standard commercial clinical x‐ray systems. This PTP can be used to acquire ROI‐CBCT projection images, where a lower resolution, lower dose image peripheral to a high resolution ROI is acquired. This is achieved either by acquiring an image using an Image Intensifier (II) with an ROI filter in the x‐ray beam or by combining images acquired separately with low and high resolution x‐ray detectors. Method and Materials: The CBCT images are acquired as the object rotates on the computer‐controlled rotary table of the PTP. For ROI‐CBCT, a micro‐angiography (MA) detector or an ROI filter is mounted on the PTP. The PTP also provides for relative X, Y, Z adjustments. After coarse alignment adjustments of the PTP, fine translational and angular adjustments are made based on fluoroscopic imaging of a cylindrical calibration phantom. Results: The PTP allows quick assembly of the parts required for CBCT or ROI‐CBCT reconstruction, reduces initial setup time to < 45 min, and provides for setup reproducibility. The system can be aligned to within one pixel (43 micron for the MA detector), with angular alignments of pitch and roll of the object better than 0.7° and 0.1° respectively. Conclusion:, The PTP allows fast and reliable set‐up and alignment of CBCT specimens, for standard and for ROI‐CBCT applications. The PTP may enable wider use of CBCT and ROI‐CBCT for specimens and phantoms without a costly dedicated system.

(Partial support from NIH Grants R01‐NS43924, R01‐EB02873, R01‐HL52567, R01‐EB02916, and Toshiba Medical Systems Corporation).
SU‐FF‐I‐10: Compensators for Management of Dose and Scatter in Cone‐Beam CT
Authors: Graham S , Moseley D , Siewerdsen J , Jaffray D
Published: July 2006

Purpose: Compensating filters have been considered for use in flat‐panel based cone‐beam computed tomography (CBCT) for the primary purpose of reducing the range of exposure reaching the detector. The use of compensators can also have other benefits, including reducing the magnitude of x‐ray scatter reaching the detector, as well as reducing the x‐ray scatter induced dose within the patient. The magnitude of these effects is characterized for a set of compensators constructed for clinical cone‐beam CT imaging geometries. Method and Materials: A set of copper compensators has been constructed with a range of modulation (1:1, 4:1, 8:1) for compensation of a cylindrical phantom (16 cm diameter). Investigations were performed on a bench‐top flat‐panel CBCT system that matches the imaging geometry (source‐to‐axis distance of 100 cm, magnification of 1.4 to 1.85) of clinical cone‐beam CT systems used in radiation therapy. The influence of the compensators on scatter‐to‐primary ratio (SPR) at the detector (Paxscan 4030A, Varian Medical Systems) and dose within the phantom (NE 2571 0.6cc ion chamber) was measured. The influence of the compensators on reconstructed CBCT image quality (uniformity, accuracy) was assessed for cylindrical water baths. Results: Depending upon phantom size (16 or 32 cm) and imaging geometry compensation reduced x‐ray scatter at the detector by up to a factor of two. Dose at the center of phantom was reduced by 25 to 35% for the same primary fluence along the beam midline. The reduction of scatter was correlated with a reduction in cupping artifacts, however, spectral hardening by the compensator introduced other non‐uniformities in the water bath reconstructions. Conclusion: The implementation of simple compensators in CBCT has great potential for reducing x‐ray scatter at the detector and patient dose. Further investigations will focus on determining the optimal compensation scheme for patient and task specific CBCT imaging.