bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of thermal noise on measured bond lengths in force measurements using dynamic atomic force microscopy
Authors: Hoffmann Peter M.
Published: April 2010

The use of dynamic methods in atomic force microscopy (AFM) has lead to spectacular advances in force measurements and imaging. There has been a gradual shift to stiffer cantilevers and smaller amplitudes, resulting in higher resolution imaging and three-dimensional force mapping. However, when amplitudes become much smaller than 1 Å, they can approach the same order as the thermal noise of the cantilever. In this article, we explore the effect of thermal noise on force measurements using small-amplitude, dynamic AFM. He restricts himself to off-resonance, amplitude-modulation AFM, as this is easiest to model and analyze. He finds that position and force noise increase roughly with the square root of temperature, as expected from the equipartition theorem; however, a closer look reveals a more complicated behavior due to nonlinearities in the system and the competition of position and force noise in these systems.
Donor-acceptor-pair photoluminescence in Ga-doped ZnO thin films grown by plasma-assisted molecular beam epitaxy
Authors: Yang Z. , Liu J. L.
Published: April 2010

Three dominant emission lines: neutral GaZn donor bound exciton I8 (3.359 eV); ionized GaZn donor bound exciton I1 (3.368–3.371 eV), and donor-acceptor-pair (DAP) IDA (3.313–3.321 eV) were observed in the 9 K photoluminescence (PL) spectra from a series of Ga-doped ZnO thin films with electron carrier concentration (n) ranging from 3.6×1018 to 3.5×1019 cm−3. As n increases, the dominant PL line changes from I1 to IDA, and finally to I8. Characteristic blueshifts of IDA PL lines were observed with increasing n, with increasing excitation power in power-dependent PL spectra, and with increasing temperature in temperature-dependent PL spectra. The experimental results of IDA lines in Ga-doped ZnO are generalized to a proposed model to explain the possibilities of the widely observed 3.30–3.32 eV PL lines in ZnO as DAP transitions, which are associated with a shallow donor (e.g., Ga, Al, In, H, etc) with an ionization energy of ∼44–65 meV and a deep acceptor VZn with an ionization energy of ∼180 meV.
SU‐DD‐A1‐01: Improving Delivery Accuracy of Volumetric Modulated Arc Therapy (VMAT)
Authors: Ye J , Rao M , Chen F , Cao D
Published: April 2010

Purpose: To evaluate the effect of VMAT sequencer, MLC leaf motion range and MU difference for adjacent control points in VMAT plan, and gantry interval used for dose calculation on the treatment delivery accuracy. Material and Methods: VMAT plans with leaves moving across the field in alternate open‐close pattern for every 10 degrees of rotation were created on Pinnacle SmartArc 8.1v and Ergo++ V1.7.1, where VMAT DICOM RT plans were generated with different sequencers. Various MLC motion range and MU with different ratios were assigned to the adjacent control points. The plans were delivered on Elekta Synergy. Measurements with Matrixx ion‐chamber array were analyzed. Gamma passing rate were evaluated using dose calculated by interpreting the MU and MLC shapes between control points with different gantry intervals. Results: DICOM plans transferred from SmartArc had higher gamma passing rate than those from Ergo++. Passing rate decreased as the MU ratio increased for Ergo++ because its VMAT sequencer averaged the MU from two adjacent control points. Less than 2% difference was observed between dose calculated with 1 and 2 degree gantry interval. Compared to plans with 2.5cm MLC motion, passing rate for 5.0cm plans decreased by 5–15%. Verification results degraded as MU ratio increased from 5:5 to 1:9 if dose was calculated using the gantry interval of 2 degrees or larger. Gamma pass rate decreased as the calculation interval increased from 2 to 10 degrees for VMAT plans were calculated assuming MLC leaves were stationary between control points. Conclusion: Smaller MLC leaf motion range can improve VMAT delivery accuracy. A sequencer capable of keeping original MU for each control point should be used to generate VMAT DICOM RT plan. Fine gantry interval should be used for dose calculation to show the VMAT plan that reflects the delivered dose accurately.

Research supported by Elekta.
SU‐DD‐A2‐03: Anisotropic Edema Modelling for Permanent Prostate Implants
Authors: Sloboda R , Clements C
Published: April 2010

Purpose: To develop an edema model for application in permanent prostate implant dosimetry that reflects edema spatial anisotropy and time‐resolution behaviors observed in clinical MRI data. Method and Materials: The basic manifestation of edema was represented as a trio of variations in Cartesian components of the distance, ri(t); i=1:3, from a radioactive seed to a dose calculation point; specifically . Here (r0)i are distance components in the absence of edema, fi(t) edema time‐resolution functions, and αi quantifiers of the directional contributions to edema volume subject to the constraint . Serial MRI data from our institution for n=40 prostate implant patients is well characterized by the parameters , and the function ; t<T; i=1:3. Hence these parameters and time‐resolution function were incorporated in the model. Next, the cumulative dose from a seed to a calculation point was expressed according to the TG‐43 formalism (using the anisotropy constant) as , where λ is the radionuclide decay constant and the other variables are defined as per TG‐43. The integrand was then expanded in even powers of |r⃗(t)|, and the resulting integral in each term of the expansion evaluated to yield either a closed‐form analytic function or a convergent series. Numerical evaluation of constituent terms in the dose expansion was done in MatLab to assess functional behavior. Results: The number of terms in the expansion of D(r⃗) are few, and all are well‐behaved numerically. For those terms involving convergent series, convergence is rapid and so only a small number of terms need to be evaluated. Based on these favorable properties, MatLab‐based dose calculation software is currently being developed to investigate applications to clinical dosimetry. Conclusion: A new edema model for permanent prostate implant dosimetry incorporating spatial anisotropy has been formulated. Clinical application is pending.
SU‐DD‐A2‐06: Reliability Growth of a Fully Automated Robotic IGBT System
Authors: Podder T , Buzurovic I , Huang K , Dicker A , Yu Y
Published: April 2010

Purpose: To evaluate the reliability growth in seed delivery by TJU's EUCLIDIAN, a fully automated robotic system, developed for image‐guided brachytherapy (IGBT). Method and Materials: Important steps in reliability growth analysis are: identification and isolation of failures, classification of failures, and trend analysis. For any one‐of‐a‐kind product, like EUCLIDIAN, reliability enhancement is accomplished through test‐fix‐test cycles of the product. Failure Mode, Effect and Criticality Analysis (FMECA) were used for collection and analysis of reliability data by identifying and categorizing the failure modes. Failures were classified according to severity. Failures that occurred in EUCLIDIAN operations were considered as Non‐Homogenous Poisson Process (NHPP), and the trend was analyzed using Laplace test. For analyzing and predicting reliability growth, commonly used and widely accepted models, Duane's model and Crow's model, were applied. Mean Time Before Failure (MTBF) was used as an important measure for assessing reliability. Results: During pre‐clinical testing, 360 seeds (in 10 cases) were deposited automatically and 6 critical failures were encountered. The majority (5 failures) of which occurred during the first three cases. The Laplace test index was −1.134 (<0), indicating significant trend in failure data, and the failure interval values were gradually becoming larger. Since the failure occurrence interval was increasing, the system reliability exhibited an increasing trend. System's failures distribution followed both the Duane's and Crow's postulations of reliability growth. MTBF was 129.4 seeds, which meant that a full brachytherapy case could be performed without any critical failure. The shape parameter for MTBF was 0.4638 (<1), suggesting positive reliability growth of the EUCLIDIAN. At 0.95 confidence, the lower and upper bounds of the MTBF were 42.8 and 612.9, respectively. Conclusion: Analyses of failure mode strongly indicated a gradual improvement in EUCLIDIAN's reliability. For better consistency in MTBF and reliability prediction, more data are being collected.

Acknowledgement: supported by NCI‐R01‐CA091763.
SU‐DD‐A3‐05: Radiosensitization of Endothelial Cells Model Using Gold Nanoparticles for Microbeam Radiotherapy
Authors: Rahman W , Ackerly T , Bishara N , Jackson P , Wong C , Davidson R , Dep P , Yagi N , Geso M
Published: April 2010

Purpose: Microbeam radiotherapy (MRT) is a technique that use array parallel thin (<100μm) slices of synchrotron generated x‐ray beam. In this study, we investigated the radio‐sensitizing effects of gold nanoparticles (AuNps) on endothelial cells culture model in combination with irradiation of MRT. Method and Materials: Bovine aortic endothelial cells (BAECs) were cultured as a confluent monolayer on a 2 well chamber slides with 0mM, 0.5 mM and 1.0mM of AuNps. The cells were irradiated with 10 Gy of synchrotron generated x‐ray beam of median energy 150 kVp. Each microbeam is approximately 30 microns wide with a spacing of 200 microns between adjacent microbeams. Gafchromic films were attached to the cells culture slide to verify the dose received by the cells. The experiments were performed on the BL28B2 beamline at the SPring8 Synchrotron Japan. The cells were then fixed at 6, 12 and 24 hours after irradiation and were imaged using phase contrast microscope. Cells viability assays using tryphan blue exclusion method were performed after 24 hours of irradiation. Results: The observations under phase contrast microscopy show all the cells were dead at the area of irradiation. Samples with AuNps clearly showed the path of microbeam which is visible as a straight line compared to the samples without AuNps. There are some neighbouring cells start to migrate to the irradiated area, filling the gap for both samples. The cells viability results showed the dose enhancement effects where only 52% cells survived for 0.5 mMol AuNps and only 18% survive for 1mMol AuNps when irradiated with microbeam. These results were expressed as percentage relative to the control samples. Conclusion: The results demonstrate that the AuNps are effective radiosensitizer that will increase the therapeutic efficacy of MRT.
SU‐DD‐A4‐01: The Estimation of the Uncertainty of Activity Concentrations in PET Images
Authors: Schmidtlein C , Bailey D , Akhurst T , Wang W , Beattie B , Gonen M , Kirov A , Humm J
Published: April 2010

Purpose: A methodology is proposed to estimate the standard deviation (STD) within lesion voxels for clinical PET images. Method and Materials: Three series of list‐mode dynamic PET images (8×3‐minute scans) of a seven chamber phantom (seven one‐liter bottles arranged hexagonally, 1120 ml) were acquired in 2D and 3D to estimate the STD of the images voxels. Filtered‐Back‐Projection (FBP) reconstruction with random, scatter, and attenuation correction is used instead of iterative reconstruction due to its improved signal‐to‐noise ratio (SNR) in hot lesions and the associated low count convergence issues of iterative reconstruction. In addition, the noise in FBP within each image is spatially invariant, which is not true for iterative reconstruction. The phantom fill ratios were 2×1:1, 2×2:1, 4:1, 8:1, and 16:1 with an initial activity concentration of 5.96 kBq/ml (1.26 mCi). The standard deviations are estimated by comparing the 8 3‐minute acquisitions to one another and by examining an ROI drawn within each image. The 3‐minute FBP images are compared to the 24‐minute image to estimate the standard deviation of the 24‐minute frame. Results: The following were verified: With FBP the STD of the voxels within a slice is spatially invariant, and FBP has superior SNR to iterative reconstruction for high contrast lesions. In addition, post reconstruction summed FBP images are statistically similar to sinogram summed images with maximum errors of the mean and STD of 0.2%, and 2.1%, in 2D and 2.1%, and 6.1% in 3D respectively. Finally, the STD for the summed image is inversely proportional to the square root of the number of frames. Conclusion: In phantom studies a single STD for an entire slice is shown to be representative of the STD of the individual voxels within that slice. Furthermore, in FBP the STD scales with . This methodology should extend well to patient studies.
SU‐DD‐A4‐05: Effects of Pixel Size and OSEM Iteration Parameters On Tc‐99m SPECT Resolution
Authors: Kappadath S , Erwin W
Published: April 2010

Purpose: To evaluate the effect of pixel size and OSEM iterative reconstruction parameters on radial (RR) and tangential (TR) Tc‐99m SPECT resolution versus distance from isocenter. Method and Materials: Ten high‐concentration Tc‐99m point sources of size <2mm3 were positioned coplanar 0–9 cm from isocenter in a cylindrical phantom with low‐concentration background. Emission scans were acquired on a SPECT/CT system (Symbia T6, Siemens Medical Solutions) with LEHR collimation in continuous (C) and step‐and‐shoot (SS) modes for 360 views over 360° at 0.9, 1.8 and 3.6 mm/pixel. Data were iteratively reconstructed with 3D‐OSEM incorporating resolution, CT‐based attenuation, and scatter modeling, for different combinations of iterations and subsets (IT_SUB): 1_18, 10_18, 20_18, 30_18, 30_36, 30_60, 30_90. SPECT resolution was estimated using a Gaussian fit of the radial and tangential profiles through each point source. Results: TR was consistently better than RR. Anisotropy was independent of pixel size and scan mode but decreased with IT times SUB (e.g., TR/RR=0.78 and 0.62 for 1_18 and 30_90 with 0.9 mm/pixel in SS). Both TR and RR improved linearly with distance away from isocenter. The center‐to‐periphery resolution differences decreased with IT times SUB (e.g., slopes of resolution versus radius were −0.74 and −0.45 for 20_18 and 30_36 with 0.9 mm/pixel in SS) and with smaller pixel sizes (e.g., slopes of resolution versus radius were −0.89, −0.82 and −0.74 for 3.6, 1.8 and 0.9 mm/pixel for 20_18 in SS). TR and RR improved as a power‐law with IT times SUB. The rate of improvement showed no obvious dependence on pixel size. TR and RR were similar between SS and C. Conclusion: Spatial resolution of SPECT images reconstructed iteratively exhibited power‐law dependence on IT times SUB, linear dependence on radial position, and exhibited TR/RR anisotropy — modeling of which are important for accurate SPECT quantification.
SU‐EE‐A1‐01: Development of a System to Measure Neutron Microdosimetry Spectra in a Mixed Proton‐Neutron Field
Authors: Diffenderfer E , Maughan R , Egan A , McDonough J
Published: April 2010

Purpose: To develop a miniature tissue‐equivalent proportional counter (TEPC), timing‐coincidence veto, and pulse‐height analysis system for the purpose of measuring neutron Microdosimetry spectra in a proton beam. Method and Materials: TEPC's were constructed with active volume geometry consisting of a 2.5mm right circular cylinder and employing a 10μm diameter stainless steel wire as the detector anode centered on the axis of an A150 tissue‐equivalent plastic cylinder with 2mm thick wall. Stainless steel field tubes define the active volume which is filled with propane based tissue‐equivalent gas to 168torr simulating a tissue density volume with 2μm diameter. Two fully depleted transmission‐type silicon detectors with diameter of 31.6mm will be operated in timing coincidence with the proportional counter as a charged particle veto system. Detector pulses are digitized in a 60MS/s sampling ADC and the data acquisition software is written in the LabVIEW graphical programming language which acquires and writes pulse waveforms to disk where pulse heights and timing information are extracted for further analysis. Results: A TEPC was tested in a mixed field produced in a proton treatment room with the proton beam incident on a closed tungsten MLC. The timing and energy resolutions for the TEPCs can be estimated from alpha spectra taken with a FWT LET‐1/2 detector attached to the data acquisition system. Timing and lineal energy resolution for the TEPCs are estimated to be 200ns and >15keV/μm, respectively. Conclusion: Preliminary testing shows that the TEPC and silicon detector timing‐coincidence veto system is a viable method for extracting neutron Microdosimetry spectra from the mixed fields present in a proton therapy treatment room. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17‐W81XWH‐07‐2‐0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.
SU‐EE‐A1‐03: The Neutron Dose Evaluation and Shielding at the Maze Entrance of the Varian Clinac 2300EX
Authors: Wang X , Esquivel C , Shi C , Nes E , Papanikolaou N , Charlton M
Published: April 2010

Purpose: After the installation of a Varian Clinac 2300EX accelerator, neutron and photon doses at the outer maze entrance were measured and compared with several empirical calculations. The measurements were taken before and after borated polyethylene (BPE) boards were installed on the maze wall as neutron absorption material. Method and Materials: The accelerator delivered an 18 MV photon beam at 600 cGy/min dose rate. An NRC NP‐2 type neutron REM‐meter and a Ludlum 14C survey meter with a 44‐6 type of probe were used to measure the neutron dose and photon dose, respectively. The measurements were taken at the center axis of the maze, 0.8 m above the floor, 0.3 m away from the maze door. Results: With the gantry head tilted close to the inner maze entrance with closed jaws, both neutron dose and photon dose reach their maximum. The measurement data were compared with empirical calculation results obtained by Kersey's method, modified Kersey's method and a newly proposed method by R.C. Falcão el al. The estimation from Kersey's method is about 2 – 4 times of the measurement (Ratio ≈ 2.4–3.8). Falcão's method has estimation about 22 times of the measurement (Ratio ≈ 21.9). The modified Kersey's method has the best prediction of the dose (Ratio ≈ 1.0). The McGinley's photon dose equation gives estimation about 80% of the measurement. After applying borated polyethylene boards as lining on maze wall, the neutron dose and the photon dose at maze entrance were decreased by 41% and 58%, respectively. Conclusion: This work indicates that Kersey's method overestimates the neutron dose by about 2 to 4 times for this study. Falcão's method largely overestimates the neutron dose. The modified Kersey's method is recommended to be used in neutron shielding calculation for the 2300EX accelerator.
SU‐EE‐A2‐01: Spatial Dose‐Volume Histogram (sDVH) — Incorporating Spatial Dose Information Back Into the DVH
Authors: Zhao B , Joiner MC , Liu Q , Burmeister J
Published: April 2010

Purpose: To incorporate spatial information into dose‐volume histograms (DVH). Methods and materials: For clinical prostate and head/neck cases, initial plans were created using dose‐volume constraints for planning target volume (PTV) and organs at risk in the Eclipse treatment planning system. Dicom RT files were exported to in‐house Matlab‐based software which categorizes target dose voxels by their location within the PTV. For this study, we arbitrarily chose three regions: center, middle, and periphery. Spatial dose‐volume histograms (sDVH) were created by color‐coding the three regions within the differential DVH. Spatially optimized plans were then created using the sDVH concept to preferentially push cold spots from the target center to the periphery. Results: For both prostate and head/neck cases, the DVH and dose distributions are different for the two plans. Using DVH only, the initial plan appears more homogeneous than the spatially optimized plan, however, inspection of the dose distribution reveals that the spatially optimized plan may be clinically favorable since there is less underdosing in the center of the target. The more homogeneous initial plan has a sharper peak in the sDVH. However, it also leaves a significantly larger fraction of the center region underdosed in comparison to the spatially optimized plan (5% vs 0.5% for prostate; 11% vs 3% for head/neck). Conclusion: Treatment plan evaluation based on DVH analysis alone excludes spatial information about the dose distribution. Plans with superior DVHs might be clinically inferior once spatial dose information is considered. The sDVH is a very simple and intuitive way to incorporate spatial dose information back into the DVH format. The sDVH provides more information than DVH alone and may be more consistent with dose distribution analysis for plan ranking purposes, thus making it a useful tool for plan quality evaluation and optimization.

Conflict of interest: Supported by Varian Medical Systems.
SU‐FF‐I‐07: The Effects of Temporal Sub‐Sampling On Estimates of CT Perfusion Parameters in Abdominal Tissues
Authors: Chandler A , Wei W , Herron D , Anderson E , Yao J , Ng C
Published: April 2010

Purpose: To assess the effects of temporal sampling interval (SI) on estimates of blood flow (BF) and blood volume (BV) parameters derived from CT perfusion (CTP) in abdominal tissues. Method and Materials: CTP data was acquired in ten patients with liver tumors on a 64 row CT (8×5mm thickness, 30s end‐inspiration breathold cine acquisition, 50cc intravenous contrast). For each dataset, the original series (0.5s SI) was reformatted to SIs of 1.0, 2.0 and 3.0s. BF and BV parametric maps for the original and three reformatted series' were computed using commercially available CT perfusion software (CT Perfusion 4, General Electric Healthcare). For each dataset, tumor, normal liver, spleen and kidney regions of interest (ROI's) were defined on a slice‐by‐slice basis on the original 0.5s SI series. For each ROI and each reformatted series, BF and BV values were recorded and plotted against 0.5s SI values for that ROI. Linear regression slope coefficients and R2 values were was used to compute the correlation and fit. Results: In total, 228 ROI were analyzed (108 tumor; 46 normal liver; 53 spleen; 21 kidney). BF showed relatively stable behavior as SI was increased from 0.5s to 3s, with slope coefficients close to unity, in the range 0.974 to 1.022, and with R2 values in the range 0.982 to 0.997. BV showed similar behavior with slope coefficients in the range 0.973 to 1.013, and R2 values in the range 0.972 to 0.976. Conclusion: SI as large as 3s permits relatively accurate estimates of BV and BF in abdominal tissue compared to those obtained with 0.5s intervals. By increasing the SI in such studies, the dose delivered in a liver CTP protocol can be reduced. Conflict of Interest: Research sponsored by GE Healthcare.
SU‐FF‐I‐10: Evaluation of a Generalized Contrast Enhancement Protocol for 4D‐CT Imaging of Liver Tumors
Authors: Najmaei S , Poenisch F , Beddar S , Briere TM
Published: April 2010

Purpose: Intravenous contrast is needed to enhance tumor visibility in the liver. Unfortunately, conventional diagnostic CT protocols cannot be used due to the extended acquisition time required. We present results of a generalized contrast enhancement protocol for 4D‐CT imaging of liver tumors. Method and Materials: 92 patients (metastases: 39, cholangiocarcinomas: 44 hepatocellular carcinomas: 9) were enrolled in the study. All 4D‐CT images were acquired on a GE Discovery ST PET/CT scanner in cine mode. 150 mL of intravenous contrast was delivered at a rate 5 mL/s. Liver metastases and cholangiocarcinomas were imaged in the portal venous phase with a delay of 45 seconds while hepatocellular carcinomas were imaged in the delayed phase at 270 seconds. A region of interest inside and outside of the tumor was created, and the average CT numbers were analyzed. Results: The protocol produced enhancement of greater than 35 CT numbers in 72% of patients with liver metastases and cholangiocarcinomas and 44% in patients with hepatocellular carcinomas. The image quality of the metastases and cholangiocarcinomas patients was improved considerably (vessels and the tumor borders could be more easily distinguished) but modest improvement was detected in hepatocellular carcinomas. Tumor contrast enhancement was not directly correlated with body surface area, but rather with the inherent contrast of the tumor. Average absolute delay time of tumor imaging was 70 seconds for cholangiocarcinomas (σ=15 s) and 66 seconds for metastases (σ=13 s). In this range the degree of enhancement in metastases was more sensitive to the absolute delay time compared to the cholangiocarcinomas and for longer delays less contrast enhancement was observed. Conclusion: The protocol works well for the majority of patients with cholangiocarcinomas and liver metastases. For tumors located well below the dome of the liver, a reduction in delay time may enhance tumor visibility.
SU‐FF‐I‐12: A Total Variation Based Iterative Reconstruction Algorithm for Cone Beam Breast CT with Under‐Sampled Projection Views
Authors: Zhong Y , Lai C , Han T , Chen L , Shen Y , Liu X , Wang T , Ge S , Yi Y , You Z , Shaw C
Published: April 2010

Purpose: To implement and investigate a total variation based reconstruction algorithm for cone beam breast CT. The algorithm is focused on reconstruction using under‐sampled projection views. Method and Materials: The implementation of the algorithm includes two major steps. In the reconstruction step, an algebraic reconstruction method is adopted. In the next step, the total variation is calculated and minimized. The initial image is updated and the iteration repeats. A segmented breast model was used to test the algorithm in this study. The cone beam CT images of a mastectomy breast specimen were corrected, filtered and segmented to form a breast model which consists of a 3D map of dense tissue structures. Re‐projection of the model was conducted with 9 degree angular increment to generate 40 projection images over 360°. The total variation based algorithm was then used to reconstruct the breast model with the under‐sampled projection views. The reconstructed images were compared with the original breast model and the percentage deviation was computed for the evaluation. Results: Iterative reconstruction with total variation minimization was performed on under‐ sampled re‐projections of the breast model. The structures of the breast were satisfactorily reconstructed, compared to the breast model. The percentage deviation was found to be 7.0%. Further improvement and optimization of the algorithm are under progress. Conclusion: The preliminary reconstruction result of a breast model demonstrates that under‐sampled breast CT projections can be reconstructed using the total variation iterative method, which provides novel opportunity for breast CT study.
SU‐FF‐I‐17: Methodology for Image Quality Characterization of Cone‐Beam CT Systems Using a Novel Adaptation of Fundamental Imaging Metrics
Authors: Gopal A , Samant S
Published: April 2010

Purpose: A methodology to characterize reconstructed image quality in cone‐beam computed tomography (CBCT) is presented in which, imaging performance is characterized using fundamental imaging metrics: modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Traditionally, imaging MTF, NPS and DQE have only been used to describe spatial resolution, noise and contrast in 2D imaging. In a novel adaptation to 3D imaging, the MTF and NPS are measured using a conventional CatPhan phantom. An effective DQE parameter is then evaluated by normalization to incident beam flunece characteristics using a dose index. Method and Materials: The modulations from CatPhan line‐pairs are used to calculate the MTF using a technique that we recently validated for megavoltage imaging. Blank/uniform slices are used to calculate the NPS, while the DQE is obtained from MTF and NPS measurements normalized by a quantum fluence term. While the fluence normalization is relatively straightforward with unscattered beams in 2D imaging, its evaluation in CBCT imaging is complicated by the effects of collimators, compensators, and scatter. In our calculations, we obtain an effective incident fluence term for each beam combination based on its weighted CT dose index (CTDIw) and by using Monte Carlo simulations to generate a transmitted fluence to CTDI dose ratio. The methodology was implemented for clinical CBCT systems to observe the influence of image acquisition parameters. Results: CatPhan based MTF, NPS and DQE measurements of clinical CBCT systems were found to be useful in evaluating the effects of various image acquisition and reconstruction parameters. The technique was extended to near automated metric analysis with minimal hindrances from scatter and associated CBCT artifacts. Conclusions: The use of fundamental imaging metrics to characterize CBCT image quality can be effective in optimizing image acquisition parameters and ensure optimal imaging performance at the lowest possible dose.
SU‐FF‐I‐18: Organ Doses and Projection Angles in Cone Beam CT
Authors: He W , Tavrides E , Huda W , Yao H
Published: April 2010

Purpose: To calculate how organ doses varies with projection angle for two types of cone beam CT geometry. Method: We used the PCXMC software package to compute organ doses to patients. We modeled a typical x‐ray spectrum using 120 kV x‐ray tube voltage and 6 mm Al filtration. Two cone beam irradiation geometries were modeled: (a) a standard CT gantry; and (b) a C‐arm. Both systems made use of a detector with dimensions of 30 cm × 40 cm. For the CT gantry system, the source to isocenter distance was 57 cm, and the SID was 93 cm. For the C‐arm system, the source to isocenter distance was 78.5 cm, and the SID was 120 cm. Organ dose to the five most radiosensitive organs (breast, colon, lung, red bone marrow, and stomach) were obtained for each cone beam arrangement at 15 degree intervals as the x‐ray tube rotates through 360 degrees around the patient. Results: On average, patient doses using the C‐arm were ∼60% of those obtained using the CT gantry. Ratios of maximum to minimum doses as a function of x‐ray tube angle ranged from ∼2.3:1 for the lung to ∼31:1 for the stomach. For the breast, colon, and stomach the highest doses were obtained for AP projections, whereas for the lung and red bone marrow, PA projections resulted in the highest organ doses. In all cases, dose minima were obtained when the x‐ray beam entered the patient in a lateral orientation. Conclusions: The high maximum to minimum dose ratios for radiosensitive organs such as the stomach and breast indicate that modulation of the x‐ray tube current with projection angle could result in substantial dose reductions in cone beam CT.
SU‐FF‐I‐34: Effect of Motion On High Contrast Vessel‐Like Objects for Volumetric DCE‐CT
Authors: Owrangi A , Keller H , Coolens C
Published: April 2010

Purpose: This study aims to assess the impact of motion on high‐contrast vessel‐like objects in volumetric CT scans as a step towards volumetric DCE‐CT on a 320‐slice CT scanner. Material and Methods: An acrylic cylindrical phantom was constructed to investigate influence of motion on contrast‐enhanced cylindrical structures (capsules) mimicking contrast‐bearing blood vessels. The phantom consists of 12 Teflon capsules of varying diameters (1, 2, 5 and 10 mm) embedded at predefined positions and in different orientations (longitudinal, diagonal and axial). A motor‐driven platform provided uniform phantom motion speeds of 0, 0.5, 1.0 and 2.0 cm/s along the longitudinal axis of the scanner. Gantry rotation speeds TG was varied between 0.35 and 3 s. All scans were acquired on a Toshiba Aquilion ONE CT scanner with a field‐of‐view of 16 cm in one rotation. Results: For a given phantom motion speed, HU decreased with increasing TG. Increasing the phantom motion speed from 0 to 2.0 cm/s reduced the HU by 9% for TG = 0.35 s and by 21% for TG = 3 s. Measured HU values also decreased with increasing TG for the other capsule orientations, however, it was least pronounced for the longitudinal orientation. The discrepancy between longitudinal and axial orientation is 2% for TG = 0.35 s and 9% for TG = 3 s. These differences are more pronounced for smaller capsules. Conclusions: A systematic study was performed to quantify the impact of motion on dynamic contrast‐enhanced CT measurements. It was found that contrast in vessel‐like objects is affected by orientation and motion. However, for fast gantry rotations (< 0.5 s) motion affects the contrast measurement by generally less than 10% for different phantom speeds and less than 2% for different vessel orientations. With this phantom, optimization of different scan parameters is possible.
SU‐FF‐I‐38: Automated Segmentation and Interpolation in Sinograms for Metal Artifact Suppression in CT
Authors: Veldkamp W , Joemai R , Geleijns J
Published: April 2010

Purpose: Metal implants cause artifacts in CT images. Purpose was to design a fast artifact suppression scheme directly applicable to original sinograms, to improve CT image quality for patients with metal hip or shoulder implants. Method and Materials: Segmentation of implants is performed in original sinograms. This contributes to a fast algorithm and enables artifact correction even caused by metal implants outside the field of view. The artifact suppression technique incorporates metal implant segmentation and replacement of the segmented region by interpolated values. Metal implant segmentation is achieved by a Markov Random Field Model. To preserve the structure of adjacent projections in the interpolation, corresponding pixel sites are determined at the different sides of the implant based on characteristic features. After interpolation, the processed sinograms are backprojected to obtain reconstructed images. Artifact reduction was assessed using CT scans of five patients with metal implants and CT scans of a PMMA phantom with teflon, PVC and titanium inserts. Qualitative improvement was assessed by comparing clinical images with artifact suppressed images. Quantitative accuracy was determined by comparing mean CT values and standard deviation (SD) values as a measure of distortion in phantom images with titanium (original and suppressed) and without titanium insert. Results: Artifacts in CT data of five patients were automatically suppressed and subjective rating showed improved image quality. The general visibility of structures improved, quantitative analyses showed that mean+/−SD values for teflon (with titanium insert) were 885+/−91 versus 895+/−49 with suppression (without titanium insert: 900+/−47). Values for PVC inserts (with titanium insert) were −60+/−75 versus −49+/−33 with suppression (without titanium insert: −62+/−35). Conclusion: The artifact suppression design is fast; it reduces artifacts in clinical and phantom images. In phantom images, the technique showed reduced SD values close to the SD values without titanium insert and associated improved image quality.
SU‐FF‐I‐41: Accuracy and Computing Time of a Ray‐Driven Projector/back‐Projector for Simulation and Reconstruction in Tomosynthesis and Cone Beam CT Imaging
Authors: Han T , Zhong Y , Chen L , Lai C , Liu X , Shen Y , Ge S , Yi Y , Shaw C
Published: April 2010

Accurate and fast projector/back‐projector (PBP) is the key to successful simulation and iterative reconstruction of tomosynthesis and cone beam CT. In this study, the accuracy and computation speed of a ray‐driven PBP operator were investigated.

To simulate x‐ray tomography imaging, a ray‐driven projector was developed and implemented on a 64 computing node PC Cluster. Each x‐ray path is represented by sampling points in the object. Their μ values are summed up to compute the integral attenuation along the path. To evaluate the accuracy of the reprojection algorithm as the function of sampling ratio (Sampling length / voxel length), reprojections obtained with a significantly smaller sampling ratio were used as reference. To minimize the additional loss of accuracy from pixelization, the pixel size of the projection images was selected to be one third of the voxel size projected back to the image plane. Error images were formed by subtracting the re‐projection from the reference re‐projection and used to compute the percentage errors.

SART were implemented for image reconstruction in tomosynthesis and CBCT imaging. To evaluate its accuracy, CBCT images reconstructed with 300 projection views over 360 degree were compared to the original CBCT images. Errors were computed from the subtraction images and plotted together the computing time as the function of the sampling ratio.

With reduced sampling ratio, the accuracies of both the re‐projection and SART algorithms were improved at the expense of longer computing time. The improvement was accelerated with smaller sampling ratios. Aliasing artifacts were visible when the sampling ratios were greater than 0.5.

We have demonstrated that the accuracy of the re‐projection and SART reconstructions improved with reduced the sampling ratio at the expense of longer computing time for a ray‐driven PBP. The tradeoff between accuracy and computing time should be determined by the imaging requirement.
SU‐FF‐I‐49: X‐Ray Scatter Reduction in Tomosynthesis Imaging of the Breast
Authors: Sechopoulos I , Feng S
Published: April 2010

Purpose: To develop and test a novel x‐ray scatter reduction algorithm for breast tomosynthesis. Method and Materials: The x‐ray scatter signal normalized by the primary signal in breast tomosynthesis projections has been shown to be a weak function of breast glandularity, size and x‐ray energy. Based on this, we propose a software‐based, post‐acquisition method that uses pre‐computed Monte Carlo estimates of the scatter field in projections of homogeneous, standard‐shaped breasts and an automated image registration algorithm (based on thin plate splines interpolation) to estimate the scatter field in clinically acquired tomosynthesis projections. The estimated scatter field can then be subtracted from the acquired projections and the results reconstructed using any tomosynthesis reconstruction algorithm. Tests of the registration algorithm, of the similarity of the estimated field with the true scatter field, and of the applicability of the scatter field from homogeneous breast volumes to projections of breasts with realistic texture were performed. The impact of the scatter correction method on the visibility of lesions in simulated tomosynthesis images was determined. Results: The registration algorithm was found to be successful at registering the “pre‐estimated” images and the “acquired” images. The estimated scatter fields were found to be good approximations of the true scatter field (normalized mean absolute error = 9.8%). Objective metrics on the visibility of the simulated lesions after scatter correction were found to mostly increase considerably, although some reduction in signal difference‐to‐noise ratio (SDNR) was found (mean signal difference increase = 54%, mean contrast increase = 34%, mean SDNR decrease = 9%). Conclusion: These results point to the feasibility of a scatter reduction method for breast tomosynthesis, which does not involve any increase in number of projections, dose and acquisition and reconstruction time. Further testing with clinical images is being performed.
SU‐FF‐I‐54: Development and Validation of An Algorithm for Facilitating Observer Studies On the Effect of Radiation Exposure On Diagnostic Accuracy in CT Acquisitions
Authors: Joemai R , Geleijns J , Veldkamp W
Published: April 2010

Purpose: In computed tomography (CT), there is increasing concern about detrimental effects of radiation and thus a special need for optimization of image quality and radiation dose. Purpose of this study was to develop and validate software for facilitating observer studies on the effect of radiation exposure on diagnostic accuracy in CT acquisitions taking into account the requested diagnosis. Method and Materials: A low dose CT simulator was developed which adds Gaussian noise to the raw data of the CT scanner. For validation two phantoms were used; a cylindrical water phantom with five different inserts and an anthropomorphic phantom. Scans of both phantoms were performed at different dose levels by changing only the tube current of the acquisition protocol (500‐300‐150‐80‐40‐20 mA). Low dose CT simulations were performed for 500 mA downwards to 20mA in above mentioned steps. Hounsfield Units (HU) and noise (standard deviation of HUs) were measured within six regions of interest (ROIs) in the cylindrical phantom, corresponding to the five different inserts and water; and additionally noise power spectra (NPS) were measured in water. Noise was measured in soft‐tissue equivalent material of the anthropomorphic phantom. Measurements in actually scanned and simulated CT images were compared. Results: The low dose CT simulator yielded similar image quality compared to actual low dose CT acquisitions. Mean differences over all comparisons in noise between actual scanned and simulated images was 5.0±4.2% for the cylindrical phantom and 3.3±2.6% for the anthropomorphic phantom. Evaluation of the NPS showed that spectra are similar with regard to general shape and intensity for the scanned and simulated images. Conclusion: The low dose CT simulator creates images that accurately represent image quality of CT acquisitions at lower dose levels and is suitable to be applied in clinical observer studies.
SU‐FF‐I‐60: Reduction in Breast Dose in Routine Chest CT Through the Use of Bismuth Breast Shields As Measured by DOT Dosimeters
Authors: Liang Y , Emerson S
Published: April 2010

Purpose: The goal of our research is to determine the effectiveness of commercially available Bismuth (Bi) breast shields for reduction of breast dose in routine chest computed tomography (CT) exams without having a negative impact on image quality, through the use of DOT dosimeters. Methods & Materials: We used a Siemens 16‐slice CT scanner, an acrylic chest phantom with cork lungs and one acrylic breast (Phase I), a RANDO phantom (Phase II), Bi breast shields from AttenuRad (0.06 mmPb equivalent) with the two included ¼ inch thick foam pads, and aluminum oxide DOT dosimeters. These DOT dosimeters are optically stimulated luminescent (OSL) dosimeters read by the Landauer InLight® microStar system. Each exposure was made with our routine chest protocol using Caredose4D. Five dosimeters were placed on the breast, one each at 12, 3, 6, and 9 o'clock positions (12 being superior to 9) and one central to the others. Doses were read both with and without the Bi shield to calculate the dose reduction with three time reading repetition. Results: Our initial study with a Radcal ionization chamber showed an exposure to the center of the breast of about 1.92 R unshielded and 1.52 R shielded (21% reduction). Results of Phase I found an average dose of 1.58 rad unshielded and 1.14 rad shielded (28% reduction). Use of foam pads between the shield and the phantom surface reduced streaking artifacts that occurred where the shield previously made direct contact. The expected increase in CT number of underlying tissues was not seen. Conclusion: Our results from Phase I showed a significant reduction of the breast skin entrance dose with the use of Bi breast shields. We will repeat the tests with the RANDO phantom, and investigate whether the thicker foam pads are able to further decrease image artifacts.
SU‐FF‐I‐63: Can Patient Shielding Reduce Fetal Dose During Thoracic CT Scans?
Authors: Donnelly K Greene , Ogden K , Roskopf M
Published: April 2010

Purpose: To determine the potential for reducing fetal dose in early pregnancy during thoracic computed tomography by shielding the patient's abdomen and pelvis. Method and Materials: An anthropomorphic phantom (Rando) representing a medium sized adult was used to measure relative tissue doses in the abdomen/pelvis during thoracic CT scanning. TLD's were used to measure the dose along the central axis of the phantom from the level of the adrenals to the location of the uterus. The phantom was scanned on a GE Lightspeed VCT 64 slice scanner using 120 kVp, 750 mAs, and pitch of 0.984 to increase the TLD signal. Scans were performed with no shielding, with shielding (lead apron) on the anterior aspect of the phantom over the abdomen and pelvis, and with shielding on both the anterior and posterior aspects of the phantom. Tissue doses were normalized to the in‐scan value measured at the level of the adrenals. Results: Tissue doses decreased exponentially with increasing distance from the bottom of the scanned anatomy. The rates of decay were −0.18 ± 0.010 cm−1, −0.21 ± 0.0024 cm−1, and −0.23 ± 0.0018 cm−1 for the no shielding, half shielding, and full shielding cases, respectively. The dose values at the level of the uterus were 0.99%, 0.65%, and 0.51% of the dose at the level of the adrenals, for the no shielding, half shielding, and full shielding cases, respectively. Conclusion: These data show that the total absolute dose received by a fetus early in the pregnancy may be reduce by approximately 1/2 during thoracic CT by use of shielding on the abdomen/pelvis. For a clinical technique of 120 kVp, 150 mAs, and pitch of 1.375, this would reduce the fetal dose from an estimated 0.1 mGy to 0.05 mGy.
SU‐FF‐I‐64: Shielding Practice for Volume CT Scanner
Authors: Hodefi D , Yang Z
Published: April 2010

Purpose: To report our experience concerning in‐room scatter and shielding for a wide coverage multi‐slice scanner. Our aim is to inform the medical physics community that special attention must be paid to the protocols used when performing scatter measurements and to the assumptions made when preparing shielding plans. Methods and Materials: A Victoreen 450B‐RYR ionization chamber and a RadCal 180 cc scatter probe were used to perform a shielding survey. Parameters of 140 kVp and 400 mAs were set. The largest collimation available, 80 mm coverage, was selected. A helical scan was prescribed. In‐room scatter measurements were performed with the Victoreen 450B‐RYR ionization chamber, RadCal probe, and an RTI solid‐state R100 dose probe. Both helical and axial modes of acquisition were tested for collimations of 80 mm and 40 mm coverage. The vendor's phantom, the FDA CTDI body phantom and a thorax phantom were used. Results: The shielding survey revealed that two walls, from below 6 feet to the ceiling line, failed the goal for an uncontrolled area (100 mrem/yr). Readings indicated that the dose was 2∼10 times the limit. The ceiling readings, 2∼15 times the limit, also failed. In‐room scatter measurements revealed that scatter to multiple points are more than doubled when a helical scan is prescribed as compared to an axial scan. Furthermore, the thorax phantom generated 1.4 times more scatter than the FDA CTDI body phantom. Conclusions: Radiation protection for volume scanners requires special attention. It is vital to consider differences in the scatter generated by axial and helical scanning modes. Scatter measured during helical acquisition is more clinically relevant. Choosing a phantom that best mimics the clinical scenario is important. Shielding planners for this type of unit should carefully consider ceiling shielding and wall shielding that extends beyond 7 ft.