bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐C‐211‐05: A New Solid State X‐Ray Image Intensifier (SSXII) with a 1×2 Modular Array and an Acquisition, Correction, and Display System
Authors: Huang Y , Qu B , Panse A , Wang W , Bednarek D , Titus A , Cartwright A , Rudin S
Published: November 2011

Purpose: To provide an extended field‐of‐view (FOV) with higher resolution images, a new SSXII imager with a two‐detector modular array was built. An acquisition, correction, and display system tiles two image modules in real‐time with geometric and brightness matching at the boundary. Methods: Two fiber‐optic tapers (FOTs) were fitted together with minimum dead‐space and embedded in liquid Sylgard 184 silicone elastomer which is allowed to cure and solidify; this 1×2 FOT array was fitted into an optical “head‐box.” The EMCCD sensors were fixed to the small‐ends of the two FOTs while a single CsI x‐ray phosphor plate was fastened onto the large‐ends. The optical head‐box was then combined with custom‐built electronics boards that control the driving clocks and sample the output. The digitized frames were transmitted to a PC through a CameraLink port accommodating a rate of 30 fps for both modules simultaneously. Customized software enabled acquisition, correction (dealing with the rotational and translational misalignments between the sensors), and display of the x‐ray images in real‐time. Other functions including acquisition control, save options, temporal filters, flat‐field correction, and mode selection for fluoroscopy, roadmapping, digital angiography (DA) and digital subtraction angiography (DSA), were also implemented. Results: The processed high‐resolution image was aligned properly along the boundary. The match up error was less than 1 pixel although there was a coupling loss of just under 7% due to chamfers at the edges of the FOTs; remachining the FOTs in future versions will reduce this loss. The two aligned images from each module exhibited balanced brightness between the sensors and flat‐field correction to eliminate fixed patterns introduced by the FOTs. Conclusions: The custom‐built SSXII detector and its acquisition, correction, and display system provides higher spatial resolution and nearly seamless images for the array in real‐time. Larger arrays are planned for future SSXII implementations.

Support: NIH Grants R01‐EB008425, R01‐EB002873
TU‐C‐211‐06: Practical Operational Considerations for Clinical Usage of the Micro‐ Angio Fluoroscope (MAF) in Neuro‐Vascular Interventions
Authors: Ionita C , Wang W , Jain A , Siddiqui A , Levy E , Hopkins N , Bednarek D , Rudin S
Published: November 2011

Purpose: To demonstrate a C‐arm mounted, high‐resolution, microangiographic‐fluoroscope (MAF) during neuro‐vascular clinical interventions (NVCI) with increased x‐ray tube output while still using the small focal spot. Method and Materials: The MAF consisting of a 300 μm CsI input phosphor coupled to a micro‐channel‐plate light image‐ intensifier, in turn coupled to a CCD sensor through a minifying fiber‐optic taper was mounted on a clinical C‐arm. To take advantage of the MAFˈs high resolution, the small focal spot must be used. In general NVCIs require x‐ray tubes to use near maximum available mAs during fluoroscopy and angiography even with a larger focal spot. To tackle the problem we implemented a variable temporal filter (TF) and modified imaging protocols to enable use of the small focal spot. For DSA we increased the frame rate to 15 fps with the maximum mAs available and implemented a 3‐frame TF. For short but critical fluoroscopy durations where high resolution was essential for guiding a NVCI such as stent deployment, we used the small focal spot with a low exposure angiography protocol at (2– 4)X fluoroscopy settings which we refer to as High Definition (HD) mode. Results: The variable temporal filtering was used successfully during fluoroscopy to compensate for mAs limitations. Depending on patient motion we used TF weights between 3 and 7. Temporally‐filtered, high‐speed angiograms allowed visualization of fine vasculature details better than the standard imager. HD mode was used during stent deployments allowing visualization of detailed stent structure which normally is not visible using standard x‐ray imagers. Conclusions: X‐ray output limitations during NVCI were overcome by a combination TF and x‐ray protocol modification to meet the MAF high‐resolution requirements. The imaging improvements were significant with patients benefiting from more accurate treatments or in one case the elimination of an additional costly procedure.

NIH Grants R01‐EB008425, R01‐EB002873.
TU‐C‐211‐10: Optimized Knot Placement for B‐Splines in Deformable Image Registration
Authors: Jacobson T , Murphy M
Published: November 2011

Purpose: The current trend of incorporating multi‐modality and multi‐ instance imaging in treatment planning for external beam radiotherapy has highlighted the need for fast and accurate deformable image registration techniques. One current implementation of non‐rigid registration uses uniform B‐splines to parametrically represent the displacement vector field (DVF). The piecewise B‐spline segments are joined at “knots” that define the local region of support and influence for each segment. This approach has limited flexibility and can require a fairly large number of control points to describe local complexity in the DVF. By preferentially placing knots in regions of high complexity, we can more efficiently utilize the control points. The authors present the implementation of an automatic knot optimization scheme. Methods: We developed a two‐step approach to fit a known one‐dimensional DVF. An initial fit was made with uniform knot spacing. The error generated by this fit was then assigned as an attractive force pulling on the knots, acting against a resistive spring force in an iterative equilibration scheme. To demonstrate the accuracy gain of knot optimization over uniform knot placement, we compared the sum of the squared errors and the frequency of large errors. Results: Fits were made to artificial surface DVFs as well as real‐world datasets. For each dataset, the optimized B‐spline fit reduced both the sum of the squared error and the frequency of errors larger than one standard deviation of the mean. Conclusion: Non‐uniform B‐splines offer an attractive alternative to uniform B‐splines in modeling the DVF. They carry forward the mathematical compactness of B‐splines while simultaneously introducing new degrees of freedom. The increased adaptability of knot placement gained from the generalization to NURBS offers increased local control as well as the ability to explicitly represent topological discontinuities.
TU‐C‐220‐04: Photoacoustic Scanning Tomography (PHAST) with Coded Optical Excitation: Theory and Experiment
Authors: Telenkov S , Mandelis A
Published: November 2011

Photoacoustic (PA) imaging of biological tissues is emerging as a novel diagnostic modality that relies on noninvasive detection of tissue optical contrast, which may be related to specific diseases. Generation of acoustic waves in response to intensity‐modulated laser irradiation of targeted tissues constitutes the basic principle of photoacoustic imaging. A number of clinically important applications are concerned with the maximum imaging depth and image contrast that can be achieved using the PA method. In the present talk, various modes of laser‐induced acoustic wave generation are reviewed with particular attention given to coded optical excitation and frequency‐domain signal processing methods. Advantages of specific forms of optical modulation with emphasis on the signal‐to‐noise ratio, imaging contrast, and maximum imaging depth will be analyzed theoretically and compared with experimental data. The dual‐mode (time and frequency domain) photoacoustic scanner utilizing a multi‐element transducer array will be presented with results obtained using tissue‐simulating phantoms and an ex‐vivo animal model.

Learning objectives:

1. Understand the principles of photoacoustic imaging with coded optical excitation of tissue chromophores.

2. Learn about instrumentation, signal processing and image formation using the PA response to custom optical modulation waveforms.

3. Understand issues related to PA system design and potential clinical applications.
TU‐E‐BRB‐05: Streamlining Monte Carlo Dose Calculations for Routing Clinical Use in Proton Therapy
Authors: Schümann J , Perl J , Shin J , Faddegon B , Paganetti H
Published: November 2011

Purpose: To design and validate a tool for routine verifications of clinical treatment plans using Monte Carlo (MC) simulations.Methods: We focus on the clinical implementation for patient dose calculations using TOPAS (TOol for PArticle Simulation), a user‐friendly interface to the Geant4 MC toolkit. Gantry and patient positioning follow guidelines of the IEC and phase‐space output is compliant with the IAEA format. Different parameterization schemes for voxel navigation have been tested to improve computing speed. Patient geometry and alignment are controlled by text files. CT slice input is currently supported for DICOM and XiO data files. Delivered doses, particle fluxes, energy deposits and many other properties can be scored. This new tool is now available for routine proton treatment planning at MGH. In many instances MC studies are preferred over pencil‐beam algorithms. Results: We present results from a cohort of MGH patients. A reduction in CPU time of up to 30% in patient dose calculations has been achieved with a specialized voxel navigation for CT volumes with 30 million voxels. Clinically required accuracy has been reached and verified in a water phantom with uncertainties of +1/−1.5mm in range, 3mm in modulation width and 2% for the flatness of spread‐out Bragg‐peaks with a variance comparable to measurements. For patient dose calculations, up to 8% of the voxels in patients were found to have larger than 2% and 2mm difference when comparing MC and a pencil‐beam algorithm. Conclusions: Monte Carlo simulations are still an exception in treatment planning, requiring detailed work by medical physicists who are also MC software experts. TOPAS supports an easy‐to‐use interface to set up nozzle and patient simulations without programming knowledge. This study demonstrates routine patient simulations with the MC method and supports the value and feasibility of using MC simulation as a standard in proton treatment planning.

The project described was supported by Award Number R01CA140735 (“PBeam: A Fast and Easy to Use Monte Carlo System for Proton Therapy”) from the National Cancer Institute.
TU‐E‐BRB‐11: Using CAD Description for Accurate Modelling in Radiation Therapy Advanced Dose Calculation
Authors: DˈAmours M , Verhaegen F , Beaulieu L
Published: November 2011

Purpose: Advanced model‐based dose calculation algorithms such as Monte Carlo and others rely heavily on the accurate representation of the simulation geometry and the associated material composition. The lower the photon energy, the more critical is this step. Nowadays, manufacturers all use Computer Aided Design (CAD) in their fabrication process. In this work, a method is presented that allows arbitrary hardware to be incorporated in a Monte Carlo simulation code from its CAD descriptions. Methods: Brachytherapy source and applicator parts were exported from CAD software to the STEP format. FASTRAD (TRAD, Toulouse, France) is used to import the CAD and set parameters related to the simulation. This information is then exported in a GEANT4 compatible format through the GDML Module. This process takes only a few minutes. Simulations were conducted to compare the CAD description of the Nucletron V2 HDR source to a simpler model, obtained after labour‐intensive trials and error modelling, previously validated by our group against TG43. Results: Using this process, all geometries were successfully imported in GEANT4. All properties were kept accurate and consistent through the process. Most solids were defined as polygon meshes which enable precise reproduction of the most complex brachytherapy applicator parts. We were also able to reproduce the TG43 parameters using the microSelectronV2 (Nucletron, Veenendaal, the Netherlands) STEP file. Conclusions: This study demonstrates the use of a complete CAD description in a GEANT4 simulation for medical application. This allows to take explicitly into account the intricate designs of certain objects such as applicators and sources for which the accurate representation will have a clinical impact on the final dose distribution, in particular at low photon energy. Given the standard nature of the STEP file format, it could be hoped that such import feature be extended to other dosimetry tools.
TU‐E‐BRC‐02: A Novel Framework of Treating Moving Tumors Using Min‐IP Based PTV and Variable Respiratory‐Synchronized Dose Rates
Authors: Sheng K , Yang W , Read P , Larner J
Published: November 2011

Purpose: To improve the robustness and duty cycle of treating a moving tumor, the minimum intensity projection (min‐IP) is used as the basis of planning target volume (PTV). Variable dose rates synchronized to the respiratory signal are adapted to overcome the dose heterogeneity resulted from the non‐uniform probability density function (PDF). Methods: 4DCT of a lung cancer patient with gross tumor volume of 43 cm3 is used to compute both maximum intensity projection (MIP) and min‐IP. Two RapidArc plans were generated based on MIP and min‐IP based PTVs. The tumor dose is the prescription dose in the case of MIP plan. For the min‐IP based plan, 3 PDFs, including a linear, a sinusoidal and an actual patient PDF were included in computing the tumor dose. In order to achieve prescription, which is 60 Gy to 95% of the tumor, the dose of the min‐IP plan is first renormalized. In the second method, variable dose rate synchronized to the respiratory signal is assumed. The dose rate is inversely proportional to the dwell time or PDF.Results: Compared with the plan based on MIP, significantly less lung volume is treated by the primary beam with the min‐IP plan, resulting in 62% lower mean lung dose without renormalization. However, with uniform renormalization, the change in mean lung dose is −33%, −8% and 3% respectively for the 3 different PDFs. More significant gains at 28.3% and 25.2% for the sinusoidal and actual patient PDFs in the mean lung dose were observed when variable dose rates were used. Conclusions: Instead of chasing the tumor using a moving aperture, we tested a novel approach to instead let the tumor pass through a smaller static aperture. Variable dose rates synchronized to the respiratory signal substantially improved lung sparing without compromising tumor dose.

NIH 1R21CA144063
TU‐E‐BRC‐05: “Step and Shoot MRI:” a Simple Acquisition Method to Reduce Gradient Nonlinearity‐Induced Geometric Distortions for Radiation Treatment Planning
Authors: Paulson ES , Prost RW , Li XA
Published: November 2011

Purpose: Magnetic Resonance Imaging (MRI) offers great promise in improving target delineation during radiation treatment planning. However, it is well known that MR images may be hampered by geometric distortion. One potentially large source of geometric distortion arises from gradient nonlinearities, which can result in anatomical compression along the superior‐inferior direction, anatomical dilation in the anterior‐posterior and right‐left directions, and aliasing (arising from spins at different spatial locations being mapped to the same frequency). The purpose of this study was to determine the effectiveness of moving table acquisitions in diminishing gradient nonlinearity‐induced geometric distortion. Methods: A custom 20×30×50 cm grid phantom was imaged on a Siemens 3T Verio scanner. Conventional 3D FLASH images were collected in the sagittal plane and reconstructed without and with vendor‐provided 2D and 3D distortion correction. The prescribed volume was then broken into four segments and, immediately prior to acquisition of each segment, the scanner table was moved such that the center of each segment aligned with the scanner isocenter. Following acquisition, images from each segment were combined into a single series. Results: Gradient nonlinearities produced geometric distortion in all three dimensions. The magnitude of the distortion exceeded 2 cm at the periphery of the field of view in the uncorrected images. Although the vendor‐provided 3D correction algorithm was effective at reducing distortion, residual distortion at the periphery of the field of view remained. The images acquired using the proposed step and shoot approach exhibited minimal distortion along the entire length of the field of view. Conclusions: The proposed step and shoot MRI approach is a simple and effective method to diminish gradient nonlinearity‐induced geometric distortion. By breaking up the imaging volume into smaller segments, images in each segment can be acquired in the region of highest gradient linearity.

Research supported by Siemens Healthcare and the MCW Cancer Center Fotsch Foundation.
TU‐E‐BRC‐07: Accurate Tumor Motion Delineation with 4D Cone Beam CT Using Prior Image Constrained Compressed Sensing (PICCS‐4DCBCT)
Authors: Chen G , Qi Z
Published: November 2011

Purpose: To validate that 4D cone beam CT using the Prior Image Constrained Compressed Sensing (PICCS) method enables accurate extraction of tumor motion profiles using a 1‐minute data acquisition scheme. Methods: Two methods with known ground truth were used in this validation study. The first method is numerical simulations based on a 4DCT dataset of a human subject, and the second one is a physical motion phantom. In both cases, 600 projections were either simulated from human subject 4DCT data or experimentally acquired using a on‐board CBCT system (Trilogy, Varian Medical Systems, Palo Alto, California) over a 1‐minute gantry rotation through 360°. Images were reconstructed using the PICCS algorithm for all respiratory phases. The motion profile of a selected ROI was extracted using a free‐form deformable registration method. The extracted motion trajectories were compared with the ground truth (the first method uses the motion trajectory from 4DCT images, and the second uses the programmed motion profile) for both methods to evaluate their accuracy. The root mean square error (RMSE) was used to quantify the error of motion trajectory over all phase bins. Results: Results from both methods show excellent agreement between the extracted motion profiles from PICCS‐4DCBCT images and the ground truth. The RMSEs for the first method are 0.68 mm, 0.23 mm and 0.24 mm in x, y and z directions, respectively. The RMSEs for the second method are 0.58 mm and 0.47 mm in AP and SI directions, respectively. The accuracy of motion delineation is at sub‐pixel level. Conclusions: PICCS‐4DCBCT using a standard 1 minute acquisition protocol has been validated to provide tumor motion delineation with high accuracy.
TU‐E‐BRC‐09: An Automated Scoring Method for Motion Artifacts in Cine Four‐Dimensional Computed Tomography Images
Authors: Cui G , Jew B , Hong J , Loo B , Maxim P
Published: November 2011

Purpose: The aim of this work is to develop an automated method to objectively score the severity of image artifacts in four‐dimensional (4D) computed tomography (CT). Methods: We developed a metric based on the difference of the two‐dimensional normalized correlation coefficients (NCC) between the edge slices at couch transitions and the NCC between edge slices and their neighbors within their respective couch positions. We applied our method to the 4D‐CT images of ten patients that were initially retrospectively sorted using the breathing trace with miscalculated phase values, and then re‐sorted using the same breathing trace with recalculated phase values after manually correcting the peaks of inhalation. The metric was calculated for both sets of 4D‐CT images and a score was determined to assess the severity of the image artifacts in both sets. The accuracy of our method to identify the 4D‐CT image set with fewer or lesser artifacts was validated by two human observers who evaluated three coronal and two sagittal cross‐sections from the same locations in both image sets. The two sets were displayed side by side and each observer independently scrolled through ten respiratory phases of both sets simultaneously (blinded to the selection method of each set), and marked the image set that appeared to have fewer or lesser artifacts. Results: Our method determined that the 4D‐CT sets using the breathing trace with recalculated phase values after manually correcting the peaks of inhalation to result in images with fewer or lesser artifacts. This is in good agreement with the findings of both observers. The observers agreed forty times and disagreed one time among total fifty selections. Conclusions: In this study we have demonstrated proof of principle that our method is able to replicate the findings of two human observers in determining 4D‐CT images with fewer or lesser artifacts.

The authors greatly acknowledge the support from Mr. and Mrs. T. Archer Morgan Fellowship in Radiation Oncology at Stanford University.
TU‐G‐BRB‐01: Investigation of Dose Perturbations and Radiographic Visibility of Potential Fiducials for Proton Radiation Therapy of the Prostate
Authors: Huang J , Newhauser W , Zhu X , Lee A , Kudchadker R
Published: November 2011

Purpose: To identify fiducial markers that are good candidates for use in proton therapy of the prostate by evaluating each marker based on three criteria: radiographic visibility on kV portal images, minimal perturbation of therapeutic dose in a clinical proton beam (<=10% near the middle of the SOBP and <=15% near the end of range), and no streak artifact production in CT treatment planning images. Methods: Twelve total fiducials, one that is routinely used at our institution and eleven prototype, non‐commercially available markers, were evaluated. The radiographic visibility of each marker was verified by acquiring kV plane radiographic images of the fiducials in a pelvic RANDO® phantom using an on‐board imager (OBI) kV imaging system. CT images of the pelvic phantom containing the markers were also acquired and analyzed in order to quantify the CT streak artifacts caused by each marker. To measure the dose perturbations caused by each fiducial, the fiducials were loaded into a PMMA phantom along with radiochromic film and irradiated using a clinical proton beam (225 MeV, 10 cm SOBP). Measurements were taken with the fiducials located at the middle of the SOBP as well as near the end of range. Results: Five prototype fiducials along with the clinical fiducial, met our criteria for clinically acceptable fiducials markers; all of the passing markers were composed of carbon‐coated ZrO2. Conclusions: This work indicates that carbon‐coated ZrO2 is a judicious choice for fiducial markers in proton therapy because this material represents a good compromise between two conflicting requirements, radiographic visibility and minimal dose perturbations. Furthermore, several general recommendations can be extracted from this study (mid‐Z material, minimize mass thickness while maintaining good radiographic visibility) that can be applied to the design of more exotic fiducials as well as novel applications of fiducial markers in radiation therapy.
TU‐G‐214‐03: Functional Imaging for Radiotherapy Guidance and Treatment Monitoring
Authors: Das S
Published: November 2011

Functional imaging in radiotherapy has great promise to guide radiation delivery as well as to monitor response to treatment. Normal tissues as well as tumor tissues may be imaged with functional imaging. This talk will discuss positron emission tomography (PET), single photon emission tomography (SPECT) and Magnetic Resonance Imaging (MRI) in the context of radiation therapy for thoracic and head‐and‐neck cancers. PET for hypoxia imaging presents an especially appealing way to guide dose painting, wherein doses to hypoxic regions may be increased with the aim of achieving local control. This talk will describe PET hypoxia imaging with F18‐FMISO and, importantly, a novel approach that uses kinetic analysis with serial Cu62‐ATSM/PTSM imaging to map the distribution of oxygen availability. Radiotherapy planning incorporating PET hypoxia imaging for tumor local control will be discussed, as well as normal tissue dose avoidance using SPECT perfusion imaging to reduce dose to functionally active healthy tissue. Other than radiotherapy guidance, functional imaging may also be used to monitor response to chemoradiotherapy. Functional images taken during the initial treatment period, if predictive of overall treatment response, allows modification of the treatment plan to improve efficacy. Specifically, this talk will discuss F18‐FDG PET, dynamic contrast enhanced (DCE) MRI and diffusion MRI to monitor treatment related changes. The intrinsic temporal variability of tumor glucose metabolism versus treatment‐induced changes from chemoradiation will be demonstrated. For reliable utilization of FDG PET imaging in early treatment response assessment, intrinsic variability must be less than treatment‐induced changes. The leakiness of both tumor and normal tissue blood vessels, as measured with the transfer coefficient obtained from DCE‐ MRI, may also be used as a measure of early treatment response. Decreased transfer coefficient values in the tumor may be positively interpreted as corresponding to normalization of vasculature. Conversely, increasing transfer coefficient values in normal tissues likely indicate undesirable damage to vasculature. Increase in the tumor apparent diffusion coefficient (diffusion MRI) signals an expansion in the extracellular space, possibly identifying responders. In summary, this talk will discuss functional imaging in the context of guiding radiotherapy planning as well as treatment monitoring.

Learning objectives:

1. Familiarization with the utilization of selected PET, SPECT and MRI techniques in radiotherapy.

2. Functional imaging to guide radiotherapy planning.

3. Treatment response monitoring with functional imaging.
TU‐G‐110‐03: Optimal Techniques for Dual Energy and Spectral Breast Computed Tomography with an Iodinated Contrast Agent
Authors: Ducote J L , Behroozi F , Molloi S
Published: November 2011

Purpose: To investigate the optimal techniques for dual energy and spectral breast computed tomography with an iodinated contrast agent.Methods: Simulations were performed for three CT systems. The first system was based on a CsI flat panel detector and used a dual kVp technique. The second and third systems were based on a CZT photon counting detector with energy resolution and single kVp technique. The first CZT system used a single threshold to separate photons from one acquisition into either low or high energy images. The second CZT system used multiple thresholds to separate photons from one acquisition into one of five energy images. Iodine images for the dual kVp system and the two energy CZT system were generated under the assumption that a linear three material decomposition contains glandular tissue, adipose tissue and iodine. For the second CZT system, iodine images were generated using an iterative likelihood maximization algorithm. Beam energies ranging from 40 to 140 kVp were simulated for all systems. A figure‐of‐merit (FOM), defined as the dual energy signal‐to‐noise ratio of contrast in the iodine image divided by the square root of mean glandular dose, was used to quantify imaging performance. Results: The highest FOM, normalized to 1.0, was seen with the two energy CZT system at a beam energy of 40 kVp. Performance from the five energy CZT system was highest at 46 kVp with an FOM value of 0.75. The FOM of the dual kVp was maximized at beam energies of 48 kVp and 140 kVp with a value of 0.28. Conclusion: The results of this study indicate that a CZT based CT system of either configuration could potentially produce images of significantly higher quality than that of a comparable flat panel based dual energy CT system.
TU‐G‐110‐04: Material Decomposition in Cardiovascular CT Imaging with Photon Counting Detector
Authors: Baturin P , Alivov Y , Molloi S
Published: November 2011

Purpose: The feasibility of material decomposition in x‐ray spectral computed tomography for detection of vulnerable plaque was investigated.Methods: Simulations were performed for imaging of iodine, gold, calcium, and lipid with a photon counting Cadmium‐Zinc‐Telluride detector. Iodine is used to image the arterial tree, while the gold nanoparticles are used to detect plaque vulnerability to rupture. The concentrations of calcium and lipid in a plaque have been identified to be indicators of plaqueˈs vulnerability to rupture. The four‐material decomposition was performed with the least square minimization technique, including prior calibration for each material. The measured concentrations using decomposition were compared to the known concentrations. Results: The results showed reliable identification of the materials of interest and good agreement in assessment of their concentrations. Measured lipid concentrations (Lm) were related to known lipid concentrations (Lk) as Lm=1.0319Lk−0.0031%. Conclusions: Spectral CT in conjuction with the least square minimization technique can potentially be used for detection of vulnerable plaque.
TU‐G‐220‐01: Quantitative Elasticity Imaging with Acoustic Radiation Force Induced Shear Waves
Authors: Nightingale K , Palmeri M , Rouze N , Rosenzweig S , Wang M , Trahey G
Published: November 2011

Purpose: Shear waves can be generated in soft tissues using impulsive acoustic radiation force (ARFI), and the resulting shear wave speeds (SWS) can be related to the shear moduli (G) by SWS = sqrt(G/rho). Time‐of‐flight (TOF) algorithms can be used to track shear wave positions through time to estimate SWS. The spatial resolution of these methods is inherently limited by the size of the regression kernel, and, improved spatial resolution comes at the cost of increased variance in the SWS estimate. Here existing methods are reviewed, and a novel SWS reconstruction method that employs a spatial homogeneity constraint (SHC method) to obtain improved spatial resolution in ARFI‐based SWS images is presented. Methods: The SHC SWS reconstruction method performs an iterative maximum a posteriori estimation that weights each TOF SWS estimate in the image based upon its similarity to surrounding pixels. Next, ARFI images of relative tissue stiffness are used to identify homogeneous regions for which TOF based SWS estimates are expected to be accurate and precise in matched datasets. These regions are then used to derive a nonlinear mapping between the quantitative SWS estimates, and the the higher spatial resolution ARFI image. Both FEM simulations and experiments were performed to compare the SHC and TOF methods. Results: The spatial resolution of SWS estimates was directly related to the reconstruction kernel size in TOF methods, and the precision increased with increasing kernel size. The spatial resolution of the combined SHC images was comparable to that in the ARFI images, and was significantly better than that in the TOF SWS images. Conclusions: ARF‐induced TOF based SWS estimation methods provide quantitative stiffness information with spatial resolution that is dictated by the ARF shear wavelength, and the reconstruction kernel size. Improved accuracy and precision is obtained with larger reconstruction kernels in homogeneous media. The SHC reconstruction method provides improved contrast over qualitative ARFI images, and improved spatial resolution over TOF based quantitative SWS images.

Learning objectives:

1. Define acoustic radiation force phenomenon

2. Describe its use for elasticity imaging

3. Explain tissue response

4. Describe wave arrival time and shear wave speed estimation methods

5. Provide examples of clinical use
WE‐A‐BRB‐01: Accurate Dosimetric Characterization of a Fully Loaded 20 Mm COMS I‐125 Eye Plaque Using Specially Designed GAFCHROMICTM Film
Authors: Trichter S , Chiu‐Tsao S , Zaider M , Sabbas A , Kulidzhanov F , Chang J , Cohen G , Nori D , Chao K
Published: November 2011

Purpose: The majority of eye plaques used in the U.S. are of Collaborative Ocular Melanoma Study (COMS) design with I‐125 brachytherapy seeds. Treatment planning is usually done using the TG‐43 formalism, not accounting for the gold alloy backing and Silastic insert, which significantly reduce the dose, as calculated by BrachyDose Monte Carlo (MC) (www.physics.carleton.ca/clrp/eye_plaque/). Dosimetric characterization of a fully loaded 20 mm COMS plaque using radiochromic film is reported. Methods: A 20 mm COMS eye plaque was uniformly loaded with 24 6711 I‐125 seeds at 6.75 U/seed. Measurements were performed using specially designed single layer GAFCHROMIC EBT1 film sandwiched perpendicular to the plaqueˈs central axis at depths 0 – 19 mm between inserts of a Solid Water “eye” phantom located at the center of a 30×30×30 cm3 full scatter Solid Water phantom. The films were calibrated using a 6711 I‐125 seed in Solid Water, applying the TG‐43 formalism. Results: Dose distributions in planes perpendicular to the central axis of the plaque are radially isotropic and uniform. Measured depth dose on central axis as well as off‐axis profiles, agree well with the results of MC. At 3 mm from the inner sclera, the measured dose rate is 12.5% lower than TG‐43 dose rate in homogenous water, similar to 11.9% for MC. The thin flexible film enabled measurements of the dose at the surface of the plaque. Results on duplicate films agree within 5.5%. Conclusions: GAFCHROMIC film in a Solid Water phantom is a convenient, accurate, and reproducible dosimeter for I‐125 eye plaque dosimetry. Dose measurements on the inner surface of the plaque enable precise assessment of the scleral dose and its homogeneity, and of the active area of the plaque for coverage determination. This work validates the MC simulations about the dose reduction effect by the plaqueˈs backing and insert.

In‐Kind support of materials provided by Oncura, Inc., Arlington Heights, IL and by International Specialty Products, Inc., Wayne, NJ
WE‐A‐BRB‐05: AAPM Guidelines for Image‐Guided Robotic Brachytherapy: Progress Report from Task Group 192
Authors: Podder T , Beaulieu L , Caldwell B , Cormack R , Crass J , Dicker A , Fenster A , Fichtinger G , Meltsner M , Moerland M , Nath R , Rivard M , Salcudean T , Song D , Thomadsen B , Yu Y
Published: November 2011

Purpose: To report progress made by AAPM Task Group 192 which was charged to review the state‐of‐the‐art systems for robotic interstitial brachytherapy and to recommend commissioning and quality assurance procedures for the safe and consistent clinical use of these systems. Methods: In the last decade, there have been significant developments in medical robots and automation tools, which have been integrated into brachytherapy systems. These developments have led to higher precision and reproducibility in source placement, optimization of source locations, improvement in consistency, elimination of clinicianˈs fatigue as well as further reduction of radiation exposure to medical staff. Most of the applications of these technologies have been in the implantation of seeds in patients with early stage prostate cancer. Nevertheless, the techniques apply to any clinical sites where interstitial brachytherapy is appropriate. The TG‐192 has reviewed all the available pertinent robotic systems, and is testing a procedure for commissioning and quality assurance for the safe and reliable clinical use of these systems. The committee addressed both the characteristics of robotic behaviors, and the interactions between the robots and the clinicians in an operational radiotherapy environment. Results: Existing robotic brachytherapy systems are capable of achieving a spatial accuracy of about 1 mm for source placement in a phantom. Considering that manual source placement with a rigid template has an estimated accuracy of 2–3 mm and source placement may vary from ideal within a patient due to multiple factors such as tissue deformation, source displacement, and edema. This task group recommends that robotic systems should have a spatial accuracy of source placement in phantom of <1 mm. Conclusions: Preliminary recommendations are that during clinical commissioning, specified tests should be conducted to ensure that this level of accuracy is maintained. The recommended tests mimic the real operating procedure as closely as possible.
WE‐A‐BRC‐01: Improving Patient Safety in Radiation Therapy
Authors: Ford E , Fraass B , Mutic S , Dunscombe P , Sutlief S , Holmberg O
Published: November 2011

The need to improve the safety and quality of radiotherapy treatments is broadly recognized. Significant efforts are underway by a variety of groups to provide guidelines, tools and techniques that can be employed in the clinic to improve safety and quality. This symposium will review recent work in this area including: 1) The use of incident reporting systems in radiation therapy, 2) Database elements and design underlying such systems, 3) Education and training of medical physicists in error management. The initiatives of AAPM, ASTRO and other affiliate groups will be described. Learning objectives of this symposium are to understand current developments in patient safety improvement and to gain familiarity with safety improvement tools available to the clinical medical physicist.
WE‐A‐301‐03: Enhancement of Four‐Dimensional Cone‐Beam Computed Tomography by Compressed Sensing with Bregman Iteration
Authors: Choi K , Fahimian B , Li T , Suh T , Xing L
Published: November 2011

Purpose: In four‐dimensional (4D) cone‐beam computed tomography (CBCT), there is a spatio‐temporal tradeoff. The aim of this study is to develop a Bregman iteration based formalism for high quality 4D CBCT image reconstruction from low‐dose projections. Methods: The 4D CBCT problem is first divided into multiple 3D CBCT subproblems by grouping the projection images corresponding to the phases. To maximally utilize the information from the under‐sampled projection data, a compressed sensing method is employed for solving each subproblem. We formulate an unconstrained lasso (least absolute shrinkage and selection operator) problem based on least‐square criterion regularized by total‐variation. The least‐square criterion reflects the inconsistency between the measured and the estimated line integrals. Furthermore, the unconstrained lasso problem is updated and solved repeatedly by Bregman iterations. Results: The performance of the proposed algorithm is demonstrated through a series of phantom experiments, and the results are compared to those of conventional filtered back‐projection (FBP). The simulation studies have shown that artifact suppressed images can be obtained with as small as 41 projections per phase, which is adequate for clinical 4D CBCT reconstruction without slowing down the gantry rotation. With such small number of projections, the conventional FBP failed to yield meaningful 4D CBCT images. Conclusions: The proposed method significantly reduces the radiation dose and scanning time to achieve the high quality images compared to the conventional 4D CBCT imaging based on FBP technique.
WE‐B‐214‐01: CT Dose Reduction Strategies
Authors: Mahesh M
Published: November 2011

The number of CT examinations performed in the United States has been growing at an annual rate of approximately 10% leading to more than 70 million procedures in 2008. The apparent increase in the number of CT procedures,new protocols and the associated radiation dose and risks has drawn considerable attention. It appears that the so‐called ‘slice wars’ with regard to the number of slices provided per CT gantry rotation may be reaching a plateau and increasing concerns about radiation dose due to CT examinations are fueling the efforts to reduce radiation dose and leading to the beginning of the so‐called ‘dose wars’.A number of radiation dose reduction strategies are been considered and many new are been developed. At the same time, efforts to optimize CT protocols, dose modulation techniques and further improvement in image quality and image quantificationsare gaining ground.

There is an increasing demand for the medical physicists to assist clinicians in developing, reviewing and recommending changes to the clinical protocols. Therefore, it is critical for the medical physicists to become familiar with the various dose reduction strategies that are currently available and thenew technologies that are been developed. In this lecture, variousCT dose reduction strategies will be discussed.

Educational Objectives:

1. To become familiar with radiation dose reduction strategies in CT.

2. To understand various image quality improvements in CT.
WE‐C(SAM)‐211‐01: Physics of MR Safety
Authors: Stafford R
Published: November 2011

Magnetic resonance imaging (MRI) is a non‐invasive and non‐ionizing imaging modality that has an excellent track record for safety in its capacity as a diagnostic imaging device for over 20 years. MRI devices utilize a strong static magnetic field to polarize a sample, radiofrequency pulsed magnetic fields to excite the sample, and rapidly switched magnetic field gradients to spatially and temporally modulate the magnetization in the sample for image encoding. Because of their potential interaction with biological tissue as well as with medical implants or ancillary objects in the room, each of these fields presents a unique potential risk to the patient, staff or family members in the MRI environment that must be mitigated via education and constant vigilance.

Additionally, innovations in MRI design and use can significantly impact safety considerations in and around the suite. MRI is no longer exclusively confined to a diagnostic imaging environment as its use as a treatment planning, interventional or intra‐operative imaging device expands. In addition to the safety challenges associated with performing procedures in these unique environments, technological advancements also constantly engage a re‐evaluation of MRI safety principles as they apply to monitoring of patients and objects in the MR suite. The use of higher field strengths as well as innovative bore designs which impact the spatial magnetic field gradients, such as wide or short bore magnets, may interact more strongly with implants or objects previously regarded as safe for use in the MRI environment. More powerful gradient systems invite much more rapid or higher resolution imaging acquisitions, which can increase the energy imparted to the patient or implant as well as push the limits of acoustic noise and peripheral nerve stimulation.

The purpose of this continuing education review is to provide an overview for the Medical Physicist of the underlying physics which motivate many of the safety concerns associated with operating in the MRI environment. The primary intent is to provide additional perspective to, and promote a better understanding of, the current safety guidelines and recommendations as well as aid in addressing safety questions and concerns related to developing, operating and working in an MRI suite.

Learning Objectives:

1. Understand the major safety risks associated with MRI from a physics perspective

2. Understand the potential impact MRI technology may have on safety considerations

3. Review current FDA MR safety guidelines
WE‐C‐BRB‐01: Does IMRT Treatment Plan Complexity or Mismatched Dosimetry Data Contribute to Dose Delivery Errors Detected Using an IMRT H&N Quality Assurance Phantom?
Authors: Tonigan J , Kry S , Dong L , Purdie T , White R , Ibbott G , Followill D
Published: November 2011

Purpose: Investigate whether increased intensity modulated radiation therapy (IMRT) treatment plan complexity or mismatched accelerator dosimetry data are responsible for or contribute to failures with the Radiological Physics Centerˈs IMRT H&N QA phantom. Methods: Eight H&N IMRT plans with a range of total MU (1460–3466), number of segments (54–225), and modulation complexity scores (MCS) (0.181–0.609) were created in Pinnacle v.8 and delivered to the RPCˈs H&N phantom on a single Varian Clinac. One of the IMRT plans (1851 MU, 88 segments, and MCS=0.469) was equivalent to the median H&N plan from ∼1000 RPC H&N phantom irradiations. This average IMRT plan was also delivered on four matched Varian Clinac machines and the dose distribution calculated using a different 6MV beam model. Radiochromic film and TLD within the phantom were used to analyze the dose profiles and absolute doses. Results: Increasing the treatment plan complexity by varying the MU, number of segments, or the MCS resulted in no clear trend toward an increase in dosimetric error using either ±7%/4mm or ±5%/3mm gamma index criteria. Varying the delivery machines as well as the beam model (use of a Clinac 6EX 6MV beam model vs. Clinac 21EX 6MV model), also did not show any clear trend towards an increased dosimetric error using the criteria indicated above. Conclusions: Accurate IMRT dose delivery requires a complicated chain of events involving numerous steps, each of which might contribute to dose delivery errors. IMRT modulation complexity and mismatched dosimetry data have been postulated as contributing to IMRT dose delivery errors but our results indicate otherwise. Other components of the IMRT delivery chain such as MLC performance and modeling uncertainty in setup, machine mechanical integrity, etc. should be investigated to determine whether they contribute significantly to IMRT dose delivery errors.

Work supported by PHS CA010953 and CA081647, awarded by NCI, DHHS.
WE‐C‐BRB‐04: Dosimetric Effect of Rotaional Positioning Errors on IMRT Treatment of Endometrial Cancer
Authors: Yu N , Xia P
Published: November 2011

Purpose:Clinical target volumes (CTV) for patients with endometrial cancer treated with external beam radiotherapy are large and irregularly shaped. The frequent obesity endemic to this patient population makes precise treatment positioning often difficult to achieve. Without a six‐degree treatment couch, rotational errors cannot be directly corrected by patient shifts. This study investigates the dosimetric effects of these residual, uncorrected rotational errors. Methods: Fifteen patients, who underwent IMRT with daily kilo‐voltage cone beam CT (CBCT) image guidance, were included in this IRB approved study. A total of 374 daily CBCT were co‐registered with the corresponding planning CT and translational and rotational shifts were recorded. Clinically, rotational errors > 50 prompted patient repositioning, while those < 50 were ignored. For each fraction, the planning dose matrix was rotated to evaluate dosimetric effects of the ignored rotation. Subsequently, all rotated dose matrices were combined to obtain a cumulative dose distribution. Studentˈs t‐test was used to estimate the 95% confidence interval of the percentage change of the dosimetric endpoints D99 and D95 of the CTV. Results:For each fraction, the D99 and D95 of the CTV varied from −8.7% to 3.3%, and −5.5% to 1.3%, respectively compared to the corresponding planning values. For the entire treatment course, the cumulative D99 and D95 of the CTV varied between −2.2% − 0.4% (95% CI‐2% − 1.6%), and −1.3% − 1.0% (95% CI ‐1% − 0.9%), respectively. Conclusions: Despite large target volumes and sharp dose gradients inherent in IMRT plans, the dosimetric effect of less than 50 of uncorrected rotational error is not clinically significant for the entire treatment course, despite variations in daily fractional doses of more than 5%. Our IGRT protocol and the planning margin for this group of patients are adequate.