bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐I‐11: Site‐Specific Image‐Gain Calibration for MV CBCT
Authors: Maltz J , Gangadharan B , Hristov D , Bani‐Hashemi A
Published: July 2006

Purpose: MV CBCT is an image guidance modality which yields a 3D dataset representative of the patient anatomy in treatment position. During the acquisition, a series of low‐dose projection images are generated by exposing a high detection efficiency flat panel to short bursts of the linear accelerator beam. In order to avoid image artifacts, the projection images need to be “gain” corrected for variations in pixel intensity unrelated to traversed patient anatomy. These variations arise from differences in individual pixel sensitivities, as well as from the spectral and the intensity non‐uniformity of the incident beam. In this work, we propose and validate two treatment site‐specific gain correction (GC) strategies. Method and Materials: The first GC approach employs an open‐field in‐air image of the beam. Each subsequently acquired image is then divided by this GC image (GCI) to remove the effects of differing pixel gains as well as of incident beam non‐uniformity. The second approach acquires a GCI using a flattened beam. A tray with a 1/2 inch uniform lead plate is inserted in the accessory tray holder prior the gain image acquisition. The presence of the lead plate in the beam path results in spectral and spatial homogeneity of the beam incident on the flat panel imager. Results: The clinical scope of the two correction approaches was investigated by clinical MV CBCT acquisitions. In‐air gain calibration is well suited for head and neck imaging since the underlying spatial and spectral properties of the beam remain unaltered. However, for prostate cases the second calibration approach significantly reduces image artifacts related to the coupled phenomena of beam hardening and profile flattening. Conclusion: Site‐specific acquisition protocols that employ GCIs generated under different conditions result in substantial MV CBCT image quality improvement. Conflict of Interest: Supported by Siemens.
SU‐FF‐I‐13: Dose Delivered to Patients for Megavoltage Cone‐Beam CT Imaging
Authors: Morin O , Bellerose J , Descovich M , Aubin M , Chen J , Aubry J , Chen H , Pouliot J
Published: July 2006

Purpose: Megavoltage Cone‐Beam CT (MVCBCT) has recently been introduced in the clinic to improve patient alignment prior to dose delivery. The objective of this research was to evaluate the dose delivered to patients for MVCBCT acquisition. We also studied the possibility of making simple plan modifications to compensate for the dose delivered by daily MVCBCT imaging. Method and Materials: Because MVCBCT uses the treatment beam, conventional CT scans (pelvis and head and neck patients) were imported in a treatment planning system (Phillips, Pinnacle) to simulate an MVCBCT acquisition. To validate the dose obtained from Pinnacle, a simple water‐equivalent cylindrical phantom with spaces for MOSFETs and an ion chamber was used to measure the actual dose delivered during MVCBCT. Results: The MVCBCT dose delivered to the phantom, calculated from Pinnacle, was within 3% to all the MOSFET measurement points. The difference between Pinnacle and the ion chamber was 0.2%. For a typical MVCBCT (arc: 270° to 110°) the delivered dose forms an anterior‐posterior gradient. Head and neck patients receive dose ranging from 0.7 to 1.2 cGy per MVCBCT monitor unit (MU). The range is 0.6 to 1.2 cGy per MVCBCT MU for pelvis patients. The total dose for daily positioning using MVCBCT can be reduced and made uniform by alternating between two opposed imaging arcs. Dose‐volume histograms of a compensated plan for a pelvis patient imaged with 10 MU MVCBCTs for 40 fractions show no additional dose to the target and small increases at low doses. Conclusion: Given that clinical MVCBCTs are currently performed at doses ranging from 2–15 MU, simple plan modifications, such as reducing the total number of MU, can be used to nearly eliminate the dose used for daily positioning. Results for other body sites will also be presented. Conflict of Interest: Research sponsored by Siemens OCS.
SU‐FF‐I‐14: Computed Tomography with Separate Primary and Scattered Radiation
Authors: Ma F
Published: July 2006

Purpose: Separating primary and scattered radiation in cone‐beam Computed Tomography to improve image quality. Method and Materials: Earlier generations of CT uses narrow beams or fan beams of x‐rays, which suffer little from scattered x‐ray photons. Newer generations of CT systems, however, use cone‐beamed x‐ray beams are associated with large amount of scatter radiations that tend to blur the reconstructed images based on the radiation absorbed by the detectors. In the low‐energy range (∼100 kV) of x‐ray photons involved in diagnostic imaging, the scatter‐to‐primary (SPR) is on the order of 1, as compared to megavoltage x‐rays, where typical SPR is on the order of 0.1. Swindell and Evans (Med. Phys., vol. 23, p. 63, 1996) had computed that the central axis SPR is almost linear with beam area, and is also almost linear with depth in water for a 6MV beam. Three methods are proposed to separate primary and scattered radiation in cone‐beam CT based on the ideas in the field of radiation therapy: (1) using a relationship between SPR and distance to the radiation source, in conjunction with a two‐layer detector array; (2) using a pencil beam to sample primary dose; and (3) using Monte Carlo simulations to reconstruct primary and scattered radiation. Results: Simulations suggest that separate primary image and scattered images can be obtained, with the primary image at an improved image quality as compared with the image from the total radiation. Conclusion: Separate primary and scattered images can be obtained through mathematical means and a simple upgrade of detector array to achieve better images and more information.
SU‐FF‐I‐16: Volume‐Of‐Interest (VOI) Cone Beam CTwith Dual Resolution Image Acquisition
Authors: Chen L , Shaw C , Meng Y , Altunbas M , Wang T , Tu S , Liu X , Lai C
Published: July 2006

In this study, we investigate the feasibility of using VOI projection data acquired at high resolution in conjunction with full width projection data acquired at low resolution to reconstruct cone beam CT images for the VOI. To simulate cone beam CT with dual resolution image acquisition, flat panel images of a mastectomy specimen, acquired in the non‐binning mode, were converted into low resolution full width projection data. High resolution VOI projection data were directly extracted from the original data. To prepare for reconstruction, the low resolution projection data were first interpolated, re‐sampled to fill in the truncated space outside the VOI. The dual resolution full width projection data, consisting of true high resolution data in the VOI and interpolated data outside the VOI, were then used to reconstruct the 3‐D image for the VOI. Reconstructed images obtained with dual resolution projection data were compared with those obtained with low resolution data and those obtained with high resolution data for the visibility of small calcifications. We have successfully demonstrated the use of dual resolution projection data for VOI cone beam CT imaging. While the low resolution full width projection data did not allow smaller calcifications to be seen in the reconstructed images, addition of high resolution projection data for the VOI only could make them visible. The use of interpolated low resolution projection data to pad the truncated space outside the VOI did not affect the spatial resolution of reconstructed images inside the VOI. With the dual resolution technique, it would be possible to selectively image a VOI at very high resolution without requiring excessively long acquisition and reconstruction or unnecessarily overexposing the patient outside the VOI.

(This work was supported in part by a research grant CA104759 from the NCI and a research grant EB‐00117 from the NIBIB).
SU‐FF‐I‐17: Reduction of Set‐Up Error Using Cone Beam‐CT in Patients Undergoing Partial Breast Irradiation
Authors: Fatunase T , Wang Z , Yoo S , Yin F , Marks L
Published: July 2006

Purpose: Partial breast irradiation with multiple external beams requires accurate alignment of the target volume and treatment isocenter. On‐board plannar kV/MV imaging can verify set‐up based on boney landmarks. On‐board cone beam CT (CBCT) may provide additional soft‐tissue‐based information to further improve set‐up accuracy. In this study, we assess the utility of CBCT in patients receiving partial breast radiation therapy, who have been aligned with on‐board plannar kV/MV imaging. Method and Materials: Patients undergoing partial breast irradiation were imaged using an on‐board‐imager attached to the gantry of a Varian 21EX machine. Each patient was aligned to skin marks in the treatment position. Orthogonal kV images were acquired and registered to digitally reconstructed radiographs from the planning CT. Subsequently, a CBCT image data set was acquired and compared to the planning CT using both boney anatomy and soft tissue information which yielded estimations of residual setup error. Results: 10 patients were studied under an IRB protocol. Each patient had an average acquisition of 9 pre‐RT CBCT images. After 2D kV image registration based on boney anatomy, the average soft tissue residual error, based on the CBCT vs. planning CT, was 4, 2, and 3 mm in the Ant/Post, Sup/Inf, and Rt/Lt directions respectively. The ranges were 0–15, 0–9, and 0–7 mm, respectively. In one case, 30% of the planning tumor volume (PTV) would have been outside of the therapeutic isodose volume based on the pre‐RT CBCT images. Conclusion: CBCT is a practical tool for image registration while visualizing the soft tissue of the tumor bed. It provides additional anatomic information and soft‐tissue detail beyond that provided by planar radiographs. Such increased accuracy can significantly improve dosimetric coverage of the planning target volume. Conflict of Interest: Partially supported by a Varian research grant.
SU‐FF‐I‐19: A Study of Megavoltage Beam Tomosynthesis
Authors: Zhang J , Godfrey D , Yin F
Published: July 2006

Purpose: To study the impact of projection numbers on the quality of tomosynthesis images using megavoltage beams. Method and Materials: It has been shown that isocentric kilovoltage tomosynthesis images can be generated with no more than 50 degree scan angle. With only one seventh of scan time and dose of CBCT, this new technology can generate on‐board images with comparable quality as CBCT to guide patient's positioning. In this study, we used megavoltage (MV) beam projections to reconstruct tomosynthesis images. The MV projection images were acquired by using a MV portal imager mounted on a Varian‐21EX LINAC. The high‐resolution mode was used for acquisition and it required less than 1 cGy dose for each projection. The raw projections data needed background and floor image corrections and was then taken to generate tomosynthesis images by using the FDK cone‐beam reconstruction algorithm. Results: Compared with kV tomosynthesis images, MV tomosynthesis images also give good contrast for bones. However, for soft tissue, their contrast is relatively lower. We calculate normalized mutual information (NMI) between MV and kV tomosynthesis images and studied its dependence on scan angle and angular interval. The experiment shows that with the angular interval fixed, NMI increases as the scan angle grows and with the scan angle fixed, NMI decreases as the angular interval increases. For our study on a spine phantom, 50‐degree scan angle and 1‐degree angular interval gives good trade‐off between images quality and projection numbers. Conclusion: MV beam can be used to generate good quality tomosynthesis images.
SU‐FF‐I‐20: Digital Tomosynthesis Mammography (DTM) : Dependence of Reconstruction Image Quality On Number and Angular Range of Projection Views
Authors: Chan H‐P , Zhang Y , Roubidoux MA , Helvie MA , Goodsitt MM , Carson P , Sahiner B , Hadjiiski L , Wei J
Published: July 2006

Purpose: We have previously found in a phantom study that a simultaneous algebraic reconstruction technique (SART) could achieve similar image quality to maximum likelihood reconstruction, but with fewer iterations. In this study, we applied SART to patient and breast phantom images and evaluated the reconstructed image quality when the number and the angular range of the projection views (PVs) were varied. Method and Materials: A second generation GE prototype DTM system was used for image acquisition. PVs are acquired from 21 angles in 3° increments over a ±30° range in less than 8‐sec. The DTM system uses an Rh/Rh x‐ray source for all exposures. The total dose of the 21 PVs is set to be about 1.5X of that of a single‐view mammogram at the corresponding breast thickness. The phantom is composed of four 1‐cm‐thick breast‐shaped slabs of heterogeneous or homogeneous mixtures of fibroglandular‐and‐fatty‐tissue‐mimicking material with embedded masses. A 5×6 array of contrast‐detail disk‐shaped holes were drilled on one of the homogeneous slabs. DTMs were reconstructed with all 21 PVs, or subsets of the PVs to simulate different angular ranges and increments. The use of subsets of PVs resulted in a reduction in the total dose of the reconstructed DTM in this study. The image quality of the reconstructed DTMs under different conditions was compared. Results: For both the phantom and patient DTMs, image sharpness and contrast decreased with decreasing numbers of PVs used in the reconstruction. The interplane artifacts increased with decreasing angular range of the PVs. Conclusion: The image quality of DTMs depends on the number of PVs and the angular range used in image acquisition. Further investigation is needed to evaluate trade‐offs between the angular increment and angular range, as well as the effects of reconstruction parameters on image quality when the number of PVs varies.
SU‐FF‐I‐21: Two‐Dimensional Shift‐And‐Add (SAA) Algorithm for Digital Breast Tomosynthesis Reconstruction
Authors: Chen Y , Lo J , Baker J , Dobbins J
Published: July 2006

Purpose: To investigate a two‐dimensional Shift‐And‐Add algorithm for three‐dimensional digital breast tomosynthsis reconstruction to correct for defects existing in the traditional Shift‐And‐Add algorithm that calculates only one‐dimensional shift amount along the axis of x‐ray tube's motion. Method and Materials: With the traditional Shift‐And‐Add (SAA) algorithm for breast tomosynthesis reconstruction, shift amounts for each projection plane are calculated only along the axis of x‐ray tube's movement. As a result, small objects such as microcalcifications appear slightly blurred in the direction perpendicular to the direction of tube motion. In this project, a two‐dimensional SAA method was developed to correct for this phenomenon. Shift amounts for every pixel location on each reconstruction plane were computed, taking into account the 2D arc projection location of reconstructed objects in each plane. Bilinear interpolation was used for partial pixel locations. Impulses at different 3‐D locations were simulated and a few human subject tomosynthesis sequences were acquired for investigation. Results: Two‐dimensional SAA demonstrated the improvement in the direction that is perpendicular to the tube motion direction. For human subjects, the appearance of calcifications from 2D SAA was sharper than traditional SAA at the direction orthogonal to the tube motion direction. The out‐of‐plane artifacts of calcifications changed from curved to be straight. Conclusion: Two‐dimensional SAA is an effective method to reconstruct 3D tomosynthesis images of the breast. Compared with the traditional SAA, the new method corrects for 2D shift amounts coming from the isocentric tube motion. This provides more accurate and reliable results compared with other SAA algorithms. Conflict of Interest: Research sponsored in part by a research grant from Siemens Medical Solutions.
SU‐FF‐I‐22: Analysis of Residual Geometric Artifacts From 4DCT
Authors: Wolfgang J , Mui K , Chu A , Sharp G , Chen G
Published: July 2006

Purpose: 4DCT has been shown to provide improved imaging of the thoracic and abdominal region, reducing the temporal artifacts observed from traditional “free‐breathing” helical CT methods. However, the reconstruction accuracy of a 4DCT exam is dependent on the reproducibility of a patient's respiratory cycle (amplitude, period). Variation of this respiratory can introduce residual geometric uncertainty in the resulting 4DCT data. This work examines the geometric uncertainties introduced in phase resorted 4DCT imaging arising from the variability of patient respiration, comparing computational simulation with phantom measurements. Method and Materials: Examples of residual 4DCT artifacts were obtained by scanning a moving phantom capable of reproducing patient respiratory motion along the patient superior‐inferior and lateral axes. Motion of the phantom stage was driven by RPM signals recorded from actual patient 4DCT scans. Geometric dimensions of the target volumes scanned on the moving phantom were compared to phase reconstructed 4DCT target images. A new computational tool was developed to examine the continuous variation of patient respiration upon cine CT image reconstruction. This tool reproduces basic 4DCT acquisition, allowing variation of patient and scan parameters such as scan start time relative to the RPM signal, multi‐slice CT dimensions, amplitude of patient respiration and target volume dimensions. Results: Variation of 4DCT target volume has been observed to be as great as 13% from measured values. Spherical phantoms have shown as much as 17% deviation from the known value when compared to 4DCT reconstructed images. Conclusion: While 4DCT provides superior reconstruction of respiratory motion, it is not completely free from artifacts. A complete understanding of residual motion artifacts from 4DCT imaging is necessary before incorporating this data into patient treatment planning, especially with respect to techniques involving mid‐phase (between exhale and inhale) images where the motion artifacts are most significant.
SU‐FF‐I‐24: Evaluation of Time Sampling and Its Interpolation Effect Based On Deconvolution Technique for Quantification of Cerebral Perfusion by Dynamic Contrast Enhanced CT
Authors: Lee S , Kim J , Kim K , Park S , Choi B
Published: July 2006

Purpose: To investigate time sampling and its interpolation effect on cerebral perfusion measurements for proving the possibility of low‐radiation‐dose CT examination than usual. Method and Materials: Immediately after non‐enhanced transverse scanning of a patient's brain, contrast material‐enhanced scanning was performed every 1 second for 40 seconds at a single‐slice level. Uniform time sampling was executed from 1/2 to 1/7 rate of total time series, respectively. Thirty tissue concentration time‐course data were collected, and arterial input curve data were fitted by gamma‐variate function. The sinc function was introduced for interpolation. Deconvolution analysis based on singular value decomposition was performed for quantification of CBF. The lowest singular value corresponding to the minimum difference between residue and its exponential curve‐fitted function was considered as the optimal threshold value. The CBF values were calculated from the maximum of the scaled residue function. The perfusion values through time sampling and interpolation were compared with the original perfusion values by independent samples t‐test. Results: The CBF values without interpolation were underestimated with a decrease of sampling rate, and with interpolation had a tendency fluctuated around the original CBF values. The CBV values through time sampling were not statistically different from the original CBV values regardless of the existence of interpolation. The MTT values without interpolation were overestimated with a decrease of sampling rate, and with interpolation had a tendency fluctuated inversely to CBF change around the original MTT. Time sampling without interpolation was statistically possible up to 1/2 sampling rate, and with interpolation up to 1/5 sampling rate. Conclusion: The perfusion values through time sampling with interpolation are acceptable up to some less sampling rate, and more accurate than without interpolation. This study will help in selecting reasonable image acquisition time interval for low‐radiation‐dose CT examination. Conflict of Interest: Young Investigator Competition.
SU‐FF‐I‐25: Optimizing Dose and Image Quality in Pediatric Computed Tomography
Authors: Jones A , Huston A , Falkenstien P , Williams J , Staton R , Hintenlang D , Arreola M
Published: July 2006

Purpose: The goal of this work was to accurately quantify the doses delivered to pediatric patients during computed tomography (CT) exams while simultaneously evaluating the image quality of images obtained with the same protocols, and thus identify potential dose‐saving protocols that maintain adequate image quality. Method and Materials: A tomographic newborn physical phantom was constructed from tissue‐equivalent materials for use in evaluating the doses delivered to pediatric patients as a result of diagnostic imaging. Fiber optic‐coupled (FOC) dosimeters were used along with the physical phantom to measure average organ doses during CT exams across a wide range of protocol parameters (80–120 kV, 50–150 mAs, 12 mm and 24 mm collimated beam widths, pitches of 0.75, 1.0, and 1.25, for both head and body protocols). Then, images of the Catphan CTP515 low contrast module were acquired using the same protocols, and scored automatically with a custom‐written scoring routine with threshold contrast‐to‐noise ratios (CNR) based on radiologists' scoring of similar phantom images. Results: Measured effective dose values for head exams ranged from 0.33 mSv to 4.3 mSv, depending on protocol selection. These effective dose values are driven by doses to the bone marrow, bone surface, brain, and thyroid gland. Measured effective doses for CAP exams ranged from 1 mSv to 14.3 mSv, depending on protocol selection. A collimated beam width of 24 mm (16 × 1.5) was determined to be the optimal setting for both head and CAP imaging in terms of both image quality and dose. Conclusions: Tube potential and tube current‐time product are the two major contributors to both dose and image quality. However, the use of pitch values less than 1.0 does involve a substantial dose penalty (approximately 30 percent greater dose) without providing any significant gains in image quality for general imaging tasks.
SU‐FF‐I‐26: Tissue Equivalent Phantoms for the Evaluation of Tube Current Modulated CT Dose and Image Quality
Authors: Fisher R , Hintenlang D
Published: July 2006

Purpose: To develop and test new, flexible, tissue‐equivalent phantoms for the evaluation of tube current modulation dose reduction and image quality in CT. The developed phantom material also has applications for mammographic and 4‐D time varying phantoms. Method and Materials: A compressible, flexible, urethane‐based tissue equivalent phantom material was developed and utilized in the production of ellipsoid shaped phantoms for CT imaging. Multiple phantoms were created, each with different major (26–40 cm) and minor (18–28 cm) axes, in order to model patients of varying dimensions and thicknesses. All phantoms were made to be integrated with a Catphan CT image quality phantom as well as CTDI dose assessment phantoms. Image evaluation software was utilized in order to evaluate several image quality parameters in CT images taken using the phantoms in order to quantify the effects of tube current modulation in CT scanners for varying techniques. Ion chamber and gated fiber optically coupled dosimeters were also used with the phantoms in order to evaluate the effects of tube current modulation on dose to patients of varying dimensions. Results: It was found that the use of ellipsoid shaped phantoms allowed for more accurate observation of dose and image quality effects of tube current modulation in CT scanners over traditional circular acrylic phantoms. Patient doses were found the be less for studies in which tube current modulation was in use as compared to standard CT techniques, however the degree of dose reduction was found to be largely influenced by the major and minor axes of the ellipse. Conclusion: This work shows potential of ellipsoid phantoms to aid in dose reduction in CT scanning through tube current modulation by allowing more accurate modeling of actual patient dimensions.
SU‐FF‐I‐28: A Critical Look at the Numerical Coefficients in CTDIVOL
Authors: Bakalyar DM
Published: July 2006

Purpose: To critically examine the assumptions leading to the formula for CTDIVOL, which uses measured CTDI values at the center and periphery to estimate the overall average dose to a homogeneous cylindrical phantom. Method and Materials: CTDIVOL is a widely used figure of merit that estimates the average dose delivered by a CT scan to standard homogeneous cylindrical cylinders and offers a means of comparing doses delivered by different machines, or by the same machine using different scanning parameters. The calculation of CTDIVOL requires the measurement of two CTDI values: CTDIE measured at the edge of the cylinder and CTDIC measured at the center. . The following three assumptions are made in order to determine a and b: 1): The phantom is a homogeneous cylinder positioned coaxially with the gantry. 2) The cylinder is exposed to a constant beam of radiation during the rotation of the gantry. 3) At any one instant during the scanner's rotation, the radiation density within the cylinder varies smoothly with position. Following a 360° rotation, assumptions 1 – 3 imply that the dose distribution is also cylindrically symmetric, smoothly varying, and at an extremum at the center. Results: The simplest result following from assumptions 1 – 3 is that a = b = 1/2. These do not agree with the values of a and b (2/3 and 1/3, respectively) that are used conventionally; these conventional values have the unphysical implication that the dose gradient at the center is discontinuous. Conclusion: CTDIVOL, as it is currently calculated, underestimates the relative contribution made by the central CTDI value to the average dose, leading to a significant systematic error in a number used by regulatory and accreditation agencies. Equal weighting provides a more accurate value and should be employed.
SU‐FF‐I‐30: Dose Indices of Radiation to Skin in Invasive Cardiology Procedures
Authors: Chu R , Schechter E , Chu N
Published: July 2006

Purpose: Tissue injury depends on the extent as well as the intensity of the assault. It would be helpful to develop skin dose indices that are more descriptive of the skin area receiving radiation above a threshold value of potential injury. Method and Materials: For quality control, a GafChromic XR Type R film was placed close to the skin of 36 patients undergoing cardiac catheterization procedures. A hand‐held densitometer was used to measure the peak radiation dose. With the approval of the Institutional Review Board, these films were scanned. Contours were drawn at the increment of 100 cGy. Using each contour value as a threshold, the area exceeding this threshold, and the average dose within this area were computed. Results: For 3 patients who had skin dose exceeding the 200 cGy threshold, but less than 300 cGy, the total area of the skin with this amount of irradiation ranged from 1 cm2 to 27 cm2. The averages doses within these areas were not greater than 221 cGy. One patient had incurred a peak entrance dose of 409 cGy. For this patient, the area exceeding 300 cGy was 82 cm2 and the area exceeding 400 cGy was 4 cm2. The limited skin areas associated with the high skin entrance radiation exposures might explain the lack of significant skin injury in this sample of patients. Conclusion: If we define “skin dose loadthreshold” to be the area exceeding a chosen threshold value, the skin dose load200 would be the area exceeding 200 cGy and having the potential of skin injury. Summary statistics describing the dose distribution within this area might also be helpful. An example parameter would be the average dose within the area described by the “skin dose loadthreshold”. This investigation has demonstrated the technical feasibility of providing such dose indices.
SU‐FF‐I‐31: Diagnostic Utility Evaluation of a ASi:H/CsI(Tl) Flat‐Panel Based Digital Radiography System
Authors: Kim S , Lee H , Kim D , Yoon J , Suh T , Choe B
Published: July 2006

Rationale and objectives: The aim of this work is to quantitatively evaluate the diagnostic utility of our digital radiography system based on a aSi:H/CsI(Tl) flat‐panel. Materials and Methods: Two commercially available amorphous silicon flat‐panel imaging systems are tested together using a chest phantom. This phantom contains the low contrast objects in the lung, heart and subdiaphragm regions, and the spatial resolution pattern in the lung region. Digital chest phantom images are acquired under the same environment, e.g., exposure condition, a focal spot, and a source‐to‐image receptor distance and a stationary antiscatter grid. All images are post‐processed by each image processing unit. Results: The signal‐to‐noise ratio(SNR) of the nine low contrast objects in three chest regions and the spatial resolution are analyzed. No significant difference is showed in three different image processing units based on the same detector. Conclusions: The resulting SNRs are in the range of Rose Model for human observers to detect a contrast object signal. Finally, the simple procedure employed in this work eliminates observer subjectivity and can be applied for the detectability measurement of low contrast objects in other digital diagnostic modalities.
SU‐FF‐I‐32: Performance and Stability of the Varian Cone‐Beam CT System
Authors: Kim G , Pawlicki T , Luxton G
Published: July 2006

Purpose: A cone‐beam CT (CBCT) system was implemented on a Varian Trilogy linac in our department in May 2005. Weekly CBCT quality assurance has been performed. The purpose of this work is to present clinical CBCT performance and determine parameters that are predictive of reduced performance. Method and Materials: From July 2005 to February 2006, a quality assurance program was conducted on a weekly basis to include both image quality and geometrical characteristics of the system. Cone‐beam CT software version 1.2 was used during this study. Both large scan volume ‘half‐fan’ and small scan volume ‘full‐fan’ CBCT scans were included for analysis. Parameters checked as part of our quality assurance procedures were: constancy of Hounsfield units (HU) for different materials, image uniformity, high contrast spatial resolution, low contrast resolution, and spatial linearity. Data was analyzed globally using the descriptive statistics of mean and range to give an overall statement of CBCT performance and was analyzed in a time‐ordered fashion using process behavior charts to identify predictive parameters. Results: Geometrical accuracy was within 1 mm for both scan types and was reproducible throughout the study period. The average and (minimum, maximum) data for image uniformity, average HU difference for 7 materials, and high contrast spatial resolution were as follows: large scan volume; 73.7 HU (−21.2, 103.8), 0 HU (−32, 39), 0.55 mm (0.45, 0.63) and for small scan volume; 92.1 HU (49.7, 116.1), −1 HU (−32, 26), 0.48 mm (0.45, 0.56). For image uniformity, action levels by time‐ordered process behavior analysis were identified. Conclusion: Apart from a single data point, the Varian CBCT system was stable over the study period. Image uniformity was predictive for a mis‐performing CBCT system.
SU‐FF‐I‐34: How Well Do Ctdi Data Obtained in a Body Phantom Predict Patient and Embryo Doses in Abdominal Ct?
Authors: Huda W , Ogden K , Boone J , Nickoloff E
Published: July 2006

Purpose: In this work, we compare central dose measurements in anthropomorphic phantoms with corresponding Computed Tomography Dose Index (CTDI) values obtained at the center of the body CTDI phantom. Method and Materials: An adult Rando phantom was used to determine patient/fetal dose during an abdominal/pelvic CT examination acquired with clinically relevant techniques. TLD's were placed in multiple locations in the phantom to measure the dose in the abdomen/pelvis region. The corresponding CTDIFDA and CTDI100 values were determined using an ionization chamber placed at the center of a 32 cm diameter acrylic dosimetry phantom. Results: A pregnant patient, whose size is comparable to the Rando phantom, undergoing a CT examination on a commercial scanner receives an embryo dose of 16 mGy/100mAs, whereas published CTDI values for this scanner are lower by factor of 2.9 to 4.0. This large discrepancy can be accounted for by the following three factors: (a) specification of a tissue dose, as opposed to an air or acrylic dose; (b) use of a realistic phantom size; (c) inclusion of the total x‐ray scatter in the tails of CT dose profiles. Conclusion: To obtain accurate body patient doses from any specified body CTDI data, it is essential that soft tissue doses be obtained rather than air/acrylic doses, and with appropriate correction factors that account for the scan length and for the size of the patient.
SU‐FF‐I‐35: Do Lead Aprons Reduce Patient CT Doses?
Authors: Groat G , Huda W , Lavallee R , Ogden K
Published: July 2006

Purpose: Hitherto, the Radiology community has assumed that scatter radiation in CT originates within the patient, and that any use of lead aprons would have no measurable effect on patient (scatter) doses. In this study, we investigated the validity of this assumption, and quantified the potential for reducing patient (scatter) doses in CT when lead aprons are placed adjacent to the directly irradiated region. Method: Thermoluminscent dosimeters (TLDs) were used to measure dose distributions in three anthropomorphic phantoms (newborn, 10‐year old, and adult male). Phantoms were scanned on a 4‐slice GE Lightspeed CT scanner using 140 kV, 900 mAs, and contiguous axial scanning. We performed a chest CT scan of each phantom with approximately 45 TLDs distributed through the directly irradiated chest region, and distal regions (head and abdomen). A second CT was also performed with the head and abdomen regions wrapped with a 0.5 mm equivalent lead apron. We estimated the effect of the lead apron on regional doses, as well as the change in total energy absorbed. Results: As expected, the addition of a lead apron had no effect on doses in the directly irradiated chest region. The addition of a lead apron reduced head doses by about 40%, and abdominal doses by about 25%. However, the scatter radiation in the “head” accounts for ∼3% of the total energy imparted to the patient, and the corresponding value for the abdomen is ∼7%. As a result, the reduction in total energy imparted to the patient from the introduction of lead aprons was only ∼5%. Conclusions: Our results show that lead aprons reduce patient doses from “external scatter” by about 5%. The origin of “external scatter” in CT merits additional study.
SU‐FF‐I‐36: Use of Tissue Air Ratios to Obtain Organ Doses in CT
Authors: Huda W , Ogden K
Published: July 2006

Purpose. To develop a practical method for obtaining organ doses in CT. Method. We measured tissue air ratios (TAR) as the ratio of the air kerma at a specified location in an anthropomorphic phantom to the corresponding iso‐center free‐in‐air kerma. Measurements were obtained using a pitch ratio of 1, identical radiographic techniques (kV/mAs), and a scan length to ensure the full contribution from scattered radiation. TAR values were measured at selected head and body locations in a Rando phantom on a GE LightSpeed CT scanner operated at 80 and 120 kV. Results. The average iso‐center free‐in‐air kerma for modern CT scanners operating at 120 kV is 0.25 ± 0.05 mGy/mAs for head imaging, and 0.23 ± 0.08 for body imaging. At 120 kV, the average TAR value in the head of a Rando phantom was ∼0.70 and the average body TAR ratios was ∼0.64. Head TAR values showed only minor spatial variations, whereas body TAR ratios were highest on the anterior surface and lowest at the lateral chest surface and the abdomen center. The average ratio of TAR values at 120 kV to those at 80 kV was 1.28. Conclusions. TAR offer a direct method for obtaining organ doses in CT, and can be readily measured for any CT scanner or patient size. Modern CT scanners operated at 120 kV have result in tissue doses in the directly irradiated region of ∼0.19 mGy/mAs for head examinations and ∼0.16 mGy/mAs for body examinations.
SU‐FF‐I‐38: Calibration of An ASi Electronic Portal Imaging Device for Dosimetric Evaluation of Intensity Modulated Radiation Therapy
Authors: Peace T , Godson H , Ravindran P
Published: July 2006

Purpose: To calibrate an amorphous Silicon (aSi) Electronic Portal Imaging Device (EPID) for dosimetric evaluation of Intensity Modulated Radiation Therapy (IMRT). Methods and Materials: This study was performed using a 6 MV photon beam of a Primus linear accelerator with an aSi EPID. A water‐filled penta step‐phantom of perspex was fabricated for calibrating the EPID. Absolute dosimetry was performed using a calibrated 0.14 cc ion chamber. A polystyrene slab with 21 bores (each separated by 1 cm) to accommodate the ion chamber was used for measuring dose below each step of the phantom. The EPID was calibrated at a source to detector distance of 150 cm. The calibration procedure involved: i) Acquisition of Electronic Portal Images (EPIs) with the centre of the step‐phantom positioned at isocentre for 1, 2 and 3 monitoring unit settings. ii) Measurement of the dose profile below the step‐phantom with the ion chamber placed at pre‐determined positions on the polystyrene slab iii) Acquisition of an EPI with the centre of the step‐phantom at isocentre and the polystyrene slab fixed onto the surface of the flat panel detector. This was used to locate the positions of the ion chamber with respect to the step‐phantom. iv) Plotting the calibration graph for pixel values (averaged over ROI of 10 × 12 pixels) and dose. EPIs of IMRT segments were acquired with the EPID in the ‘port‐during’ mode for each field. All the individually acquired segments were added using codes developed in MATLAB (version 6.5) to get the fluence map. Results: The pixel values of the EPIs were found to increase linearly with dose. The fluence maps acquired using the calibrated EPID were verified with the planned fluence maps. Conclusion: Therefore it is concluded that EPID could be used as a dosimetric verification tool for IMRT.
SU‐FF‐I‐39: Estimating Conversion Coefficient of KERMA Free in Air to Glandular Dose in Mammography: A Comparison Between BR12 Model and a Realistic Voxel Model
Authors: Hoff G , de Almeida C , Drexler G
Published: July 2006

Purpose: To compare conversion coefficient of KERMA free in air to glandular dose, in mammography, simulated to BR12 model and a realistic breast voxel model. Method and Materials: We simulate the glandular dose (Dg) and KERMA free in air (Kai), using the Monte Carlo program MCNP (version 4B) to estimate the conversion coefficient (cg) of KERMA free in air at entrance skin in glandular dose. The computational universe generated to simulate a mammographic procedure mimics LORAD III mammographic equipment. The focal spot of molybdenum irradiates photons isotropically in a solid angle of 16.8°. The bucky is 0.6190 m far from de focal spot. Above the model there is a PMMA compress paddle 0.002 m thicker. The add filtration (30 μm Mo thicker and 25 μm Rh thicker) was located at 0.050 m far from the focal spot. Tow spectra were used in voxel model simulations: 28 kVp with Mo add filtration and 30 kVp with Rh add filtration. Results: The cg presented on Mo/Rh simulation was 1.5 times larger than the presented on Mo/Mo simulation. Comparing the voxel model to the BR12 model we have actually a super estimation on both simulated cg values: 3.4 times considering the simulation with Mo/Mo target/filter combination, and 2.4 times considering the simulation with Mo/Rh target/filter combination. Conclusion: The cg values show a decrease of 58.7% considering the Mo/Rh target/filter combination and a decrease of 70.2% considering the Mo/Mo target/filter combination, to the realistic breast model as comparative pattern. These variation on cg are probably caused by the definition of a non‐anthropomorphic model composed by an homogeneous distribution of tissues as pattern, that makes unviable the observation of the absorbed energy by each tissue; and because this model do not consider the position of glandular tissue in the real breast geometry.
SU‐FF‐I‐41: A Series of 4D Pediatric Hybrid Phantoms Developed From the UF Series B Tomographic Phantoms
Authors: Lee C , Lee C , Lodwick D , Bolch W
Published: July 2006

Purpose: To develop a series of the pediatric hybrid computational phantoms based on the non‐uniform rational B‐spline (NURBS) technology by converting the existing series of UF pediatric tomographic phantoms. Method and Material: The series of UF tomographic phantoms, newborn female, 9‐month male, 4‐year female, 8‐year female, 11‐year male, 14‐year male, which were developed by the researchers at University of Florida, were employed for this study. The tomographic phantoms were imported to the 3D‐DOCTOR (Able Software Corp., Lexington, MA) segmentation and 3D rendering software, and polygon mesh models representing internal organs and body contour were generated. The polygon mesh models were imported to the Rhinoceros software (McNeel, Seattle, WA) based on NURBS‐technology, and Smooth NURBS surfaces were developed for organ and tissue contours, and the NURBS‐based organ models were generated organ‐by‐organ. The NURBS organ models were integrated into hybrid human phantoms by the Rhinoceros software. Results: A total of 6 hybrid human phantoms were developed from the existing 6 UF tomographic phantoms. The cube‐shaped organ contours in the tomographic phantoms were innovatively smoothed in the resulting hybrid phantoms based on NURBS surfaces. The organ volumes calculated from tomographic and hybrid phantoms were in agreement within 5%. Conclusion: The resulting phantoms are deformable and can thus be used to represent 25th or 75th percentile subjects through the adjustment of control points surrounding each organ and body contour. The NURBS‐based pediatric phantoms developed in this study can be imported into Monte Carlo calculation code, and broadly utilized for dosimetry calculation. The techniques developed in this study will be also applied to the development of NURBS‐based 3D phantoms representing 50th and other percentile adult male and female subjects for use in radiation protection applications, as well as occupational or medical exam dose reconstruction.
SU‐FF‐I‐43: Air‐Kerma Length Calibration of CT Ionization Chambers
Authors: Jensen A , DeWerd L , Micka J , Davis S , Culberson W
Published: July 2006

Purpose: To develop a slit‐based calibration method, for the quantity air‐kerma length, for CT ionization chambers which combines the radiation response and response length properties of the chamber. Method and Materials: CT ionization chamber response profiles are measured for 3 models, and the response length is calculated. Air‐kerma length calibration coefficients require a correction for radiation that is scattered from and transmitted through the aperture. The Monte Carlo transport code MCNP5 is used to generate this correction factor, Cst. A variable‐width aperture is used to test a previously published, linear‐regression based correction factor, Creg. Results: The response lengths of several CT ionization chambers are calculated based on the chamber's response profile. The response lengths vary between 9.9cm and 12.1cm, which results in errors of −20% to +1%. The Cst correction factor yields an air‐kerma length calibration coefficient that varies by 0.1% for aperture widths ranging from 5mm to 90mm. Creg yields air‐kerma length calibration coefficients that vary by 1% over the same range of slit width. Conclusion: The proper way to calibrate a CT ionization chamber is via air‐kerma length calibration, which combines the radiation response and response length in the calibration coefficient. Currently CT ionization chambers are calibrated with a large x‐ray field that exposes the entire collecting volume, which only characterizes the radiation response of the chamber. The response length is then assumed to be 10cm, which is not always true and can cause errors of up to 20% compared to using an air‐kerma length calibration coefficient.
SU‐FF‐I‐46: Acceptance Testing and Verification of Automatic Brightness Control Logic of A Fluoroscopic System
Authors: Lin P
Published: July 2006

Purpose: To verify the functionality and operation of fluoroscopic automatic brightness control employed in a modern cardiovascular angiography system, equipped with flat panel detector, as part of an acceptance testing of a new installation, and to obtain the fluoroscopic “patient” entrance exposure (air kerma) for reference information. Method and Materials: A Siemens AXIOM Artis angiography system was acceptance tested under a fixed geometric arrangement. The PMMA phantom was employed to simulate the patient thickness varying from 0.635 cm ( nominal) to 35.56 cm (14″ nominal) in increments of 0.635 cm. The fluoroscopic imaging parameters; “kVp”, “mA”, “pulse width”, “spectral shaping filter; copper filters”, “ half value layer; HVL”, “patient air kerma; PAK”, and “flat panel input air kerma; FPIAK” were recorded as the phantom thickness was increased. Upon completion of the data acquisition, the imaging parameters were plotted against the phantom thickness. The graphs were then analyzed, and compared with a typical preprogrammed fluoroscopy operation curve supplied by the manufacturer. Results: The primary variable of this fluoroscopy system is the tube potential in conjunction with the spectral shaping filter selection. Depending on the Flat Panel Detector Signal (FPDS) level, the tube current and the pulse width are varied until the predefined FPDS level is achieved. The graphs show the operation logic indeed followed the preprogrammed logic and the PAK increased exponentially, ranging from 0.2 mGy/min to 100 mGy/min, while the FPIAK was maintained at a constant level of 0.7 to 0.8 μGy/sec. Conclusion: Due to the extensive use of spectral shaping filters and the sophisticated fluoroscopic operation logic design, the fluoroscopic tube potential is kept at optimum levels to provide good image quality by ensure high image contrast and proper penetration for a wide range of patient (PMMA) thickness. The graphs confirmed and verified that the system “behaved” as designed.