Index of content:
Volume 33, Issue 6, June 2006
- Imaging Moderated Poster Session: Exhibit Hall F
- Moderated Poster ‐ Area 3 (Imaging): Image Segmentation, Visualization, and Registration
TU‐EE‐A3‐01: 2D‐3D Registration of Portal Images with the Planning CT for Detection of Patient Positioning Errors33(2006); http://dx.doi.org/10.1118/1.2241599View Description Hide Description
Purpose: To compare the use of 2D‐3D automatic registration of portal images with the planning CT for detection of patient positioning errors and the use of 3D‐3D registration of MVCBCT with the planning CT.Method and Materials: Two prototype programs were used to carry out 2D‐3D and 3D‐3D image registrations. To assess the accuracy and robustness of these programs, 25 sets of 2D portal images, 25 sets of megavoltage conebeam CT (MVCBCT) images with known positioning shifts were acquired. A planning CT of the RANDO was also acquired. The known shifts between these image sets were ranged from −17mm to 4 mm, −20mm to 5mm and −12mm to 6mm, with uncertainty of 4.278mm, 5.359mm, 3.396mm along the latitude, longitude and vertical directions Results: The average differences between 2D‐3D method and the known shifts were −0.632±0.318 mm, −0.121±0.437 mm, −0.416±0.346 mm, compared to 3D‐3D method of 1.487±0.342 mm, −0.127±0.528 mm, 0.083±0.48 mm along the latitude, longitude and vertical directions. The average differences between 2D‐3D and 3D‐3D image registration methods were 0.86±0.286 mm, −1.39±0.347 mm, −0.33±0.303 mm Conclusion: Both 3D‐3D and 2D‐3D registration methods can detect positioning errors within 1 mm. For a rigid body, 2D‐3D method is sufficient. Conflict of Interest: This project is partly funded by SIEMENS.
33(2006); http://dx.doi.org/10.1118/1.2241600View Description Hide Description
Purpose: To implement and validate a 2D‐3D registration method for determining 3D patient position in external beam radiotherapy using orthogonal EPIDimages and megavoltage digitally reconstructedradiographs (MDRRs). To test the methods dependence on cost function, image pre‐processing and parameter space sample density, and determine the dependence of registered rotations on setup translations and vice versa. Method and Materials: Orthogonal EPIDimage of a humanoid phantom in different poses (3D rotations and translations) were acquired in anterior‐posterior and latero‐lateral view. The EPIDimages were registered with a data base of orthogonal MDRRs, calculated as projection images through the phantom's CT data set at rotation angles within ±5°. Registration results were compared for three different cost functions (least‐squares, cross‐correlation and mutual information), different image pre‐processing techniques (unsharp masking, histogram matching) and for isolated and combined rotations and translations. The influence of setup translations on registration results for rotations, and vice versa, was investigated and compared with a simple model. Results:Image pre‐processing improves registration precision by more than a factor 2. Three dimensional translations were registered with better than 0.5 mm (one standard deviation) when no rotations were present. Three‐dimensional rotations registered with a precision of better than 0.2° (1 SD) when no translations were present. Combined rotations and translations of up to 4° and 15 mm were registered with a precision of better than 0.4° and 0.7 mm respectively. Mutual information resulted in the most precise registration. Setup translations influence registered rotations, mostly following a simple theoretical model, but not vice versa. Conclusion: Precise registration requires image pre‐processing and benefits from interpolation of the parameter space. Influence of object translation on registration of out‐of‐plane rotations can be significant; these “pseudo rotations” can be corrected using the theoretical model when only one projection image is used for registration (e.g. fluoroscopy).
33(2006); http://dx.doi.org/10.1118/1.2241601View Description Hide Description
Purpose: Intraoperative quantitative C‐arm fluoroscopy guidance depends on discerning the relative pose of images (pose recovery). A possible method is to use radiographic fiducials visible in fluoro images [1,2]. We propose a robust and fast method for segmenting fiducials designed for brachytherapy applications. Methods and materials: The fiducial contains points, lines and ellipses made from BBs and wires. The algorithm integrates the a‐priori knowledge of fiducial's mechanical construction in a cleverly devised workflow. The BB segmentation is achieved using morphological top‐hat transform. This information serves as a heuristic input to line segmentation realized by a curve tracing algorithm which operates on edge image, followed by augmenting information from intensity image. Once the lines are segmented, this information feeds to the ellipse extraction step. For ellipse segmentation, intensity image is morphologically processed to eliminate background noise, followed by elimination of BB‐s and lines from the information obtained in prior steps. The resulting image consists of only ellipse segments. A fast variation of Hough transform is used to rectify the full ellipse from the segments. Results: The fiducial algorithm identified all the features (BBs, lines and ellipses) visible to human eye in all ten clinical images. Next the accuracy of fiducial segmentation was assessed numerically by feeding the results to the pose recovery algorithm of . The fiducial was moved on an accurate mechanical platform (as ground truth) while the C‐arm was stationary. We reconstructed the relative poses with an accuracy of 1.2 mm in translation and 0.3 degrees in rotational based on the segmented fiducials. Conclusions: The algorithm makes effective use of a‐priori knowledge and combines the techniques of morphological segmentation, curve tracing, and Hough transform, resulting in a novel curve segmentation strategy.
TU‐EE‐A3‐04: Massive Training Artificial Neural Network (MTANN) to Reduce False Positives Due to Rectal Tubes in Computer‐Aided Polyp Detection33(2006); http://dx.doi.org/10.1118/1.2241602View Description Hide Description
Purpose: One limitation of current computer‐aided detection (CAD) of polyps in CT colonography is a relatively large number of false positives. Rectal tubes are a common source of false positives and may distract the reader from less common polyps in the rectum. Our purpose was to develop a three‐dimensional massive‐training artificial neural network (3D MTANN) for reduction of false positives due to rectal tubes generated by a CAD scheme. Material and Methods: Our database consisted of CT colonography of 73 patients, scanned in both supine and prone positions. Fifteen patients had 28 polyps (15 polyps: 5–9 mm; 13 polyps: 10–25 mm). These cases were subjected to our previously reported CAD scheme that included shape‐based detection of polyps and reduction of false positives with a Bayesian neural network. With this scheme, 96.4% (27/28) by‐polyp sensitivity with 3.1 (224/73) false positives per patient was achieved. To eliminate false‐positive rectal tubes, we developed a 3D MTANN that was trained to enhance polyps and suppress rectal tubes. Results: In the output volumes of the trained 3D MTANN, various polyps were represented by distributions of bright voxels, whereas rectal tubes appeared as darker voxels. The 3D MTANN removed all 20 false‐positive rectal tubes produced by our original CAD scheme without removing any true positives. To evaluate the overall performance, we applied the 3D MTANN to the entire database containing 27 polyps (true positives) and 224 non‐polyps (false positives). The 3D MTANN eliminated 33% (73/224) of non‐polyps without removal of any true positives in an independent test. Conclusion: The 3D MTANN was able to improve the false‐positive rate of our original CAD scheme from 3.1 to 2.1 false positives per patient, while an original by‐polyp sensitivity of 96.4% was maintained. Conflict of Interest: HY, SGA: shareholders, R2 Technology, Inc.
33(2006); http://dx.doi.org/10.1118/1.2241603View Description Hide Description
Purpose: Over the last few years liquid crystal displays have replaced the traditional light box in medical imaging. When display systems are used for primary diagnosis they need to comply with the DICOM GSDF standard. Previous work already demonstrated that limited grayscale depth and viewing angle dependency of LCD results into lower calibration accuracy. As original contribution this paper investigates the impact of spatial noise and non‐uniformities on calibration accuracy. Additionally we will also quantify the relative importance of each of those three shortcomings of medicalLCDs.Method and Materials: To study the effect of non‐uniformity and spatial noise on DICOM GSDF compliance, we compare the observed transfer curve after calibration with the target DICOM GSDF curve and this for multiple locations across the display. The display used is a 5 Mega Pixel monochrome medicalLCD display. We also analyze the typical GSDF compliance metrics dL/L and JNDs/step and compare these plots for the other effects of viewing angle dependency and grayscale depth. Results: Current calibration practice is to characterize the native transfer curve of the display at only one position (often the centre). However there is significant variation in native transfer curve across the display surface and therefore calibration will only be accurate at the position where the characterization took place. On other display positions the average target luminance distortion compared to GSDF ranges from a few JNDs to over 20 JNDs. A technique of spatial noise‐reduction can solve this problem. We also observe that spatial‐noise is much more important than viewing angle and grayscale depth for typical usage. Conclusion: This paper has demonstrated that non‐uniformities and spatial noise can result into poor calibration accuracy. Also a possible solution to the problem has been described. Conflict of Interest: Research sponsored by Barco Medical Imaging Systems.
33(2006); http://dx.doi.org/10.1118/1.2241604View Description Hide Description
Purpose: To evaluate an innovative volumetric display for radiation treatment planning applications. Method and Materials: A volumetric, auto‐stereoscopic display (Perspecta Spatial 3D, Actuality‐Systems Inc., Bedford, MA) has been integrated with the Pinnacle3 TPS for treatment planning. The Perspecta 3D display renders a 25 cm diameter volume that is viewable from any side, floating within a translucent dome. In addition to display all the 3D data exported from Pinnacle, the system provides a 3D cursor and beam placement tools. A 125 point, 5 cm spaced grid centered at isocenter was created in Pinnacle and transferred to Perspecta. A Perspecta 3D ruler verified distances between any two points on the 3D grid. Ten teletherapy beams with various gantry/couch combinations were generated on Pinnacle and verified on Perspecta display. Doses at the same grid points were also compared. CTimages from a QUASAR phantom in 3 orientations were used on Perspecta to confirm beam field size, divergence, etc. Results: In general, the Perspecta system accurately depicted all 3D data exported from Pinnacle. When measured by the 3D ruler, distances between any two points using Perspecta agreed with Pinnacle within the measurement error (typically <0.5 mm). Beam angles were verified through Cartesian coordinate system measurements and also upon rotating the phantom. Field sizes,collimator angles, and beam divergence were similarly confirmed. Isodose surfaces and dose values chosen at arbitrary locations in Perspecta agreed with Pinnacle within ± 2% in an absolute sense, which was governed by human error in coinciding the points. Conclusions: These preliminary results indicate that the Perspecta device is capable of displaying consistent data from the Pinnacle radiotherapytreatment planning system, and may become a valuable tool for visualization and quantitative evaluations in radiation oncology. Conflict of Interest Statement: Actuality Systems Inc. provided the 3D display used in this study.
- Moderated Poster ‐ Area 3 (Imaging): Imaging for Therapy Guidance
33(2006); http://dx.doi.org/10.1118/1.2241650View Description Hide Description
Purpose: To study the imagingcharacteristics of thick, segmented, 2‐D CdWO4 crystal‐photodiode detectors as a function of crystal height, septa material and optical reflectivity, x‐ray beam spectrum and beam divergence using a two‐step Monte Carlo approach involving both x‐ray photon transport at megavoltage (MV) energies and the optical photon transport in scintillator and photodiodes.Method and Materials: We have studied the spatial frequency dependent detective quantum efficiency (DQE) of thick, segmented, 2‐D CdWO4 crystals in contact with silicon photodiode arrays. The energy deposited into the 3‐D voxels (1 × 1 × 1 mm3, septa thickness = 0.15 mm, fill factor = 72%) of the detector for each of the 6 and 3.5 MV x‐ray photons in a normally incident pencil beam was calculated using the DOSXYZnrc user code of the EGSnrc Monte Carol system. The isotropically emitted optical photons in each voxel were calculated using the average CdWO4 optical yield and transported to the photodiode array using DETECT2000 optical Monte Carlo code. A 10° beam divergence angle was also simulated. The detector DQE was calculated using the spatial distribution of optical photons.Results: The DQE increases with the crystal height only if the reflectivity of the septa material is high (0.975). For poor reflectivity (0.65 and 0.8), the increase in the DQE of the taller crystals to MV photons is seriously offset (from 42% to less than 20% for 3 cm tall crystals) by the decreased probability of detecting optical photons. Similarly, the increase in DQE due to the lower energy photons is obtained if the high reflectivity of septa material is maintained for the detector. Beam divergence in thick crystals also reduces the DQE. Conclusion: High reflectivity of the septa in thick, segmented scintillation detectors is very important to achieve high DQE.
33(2006); http://dx.doi.org/10.1118/1.2241651View Description Hide Description
Purpose: Functional and molecular (F&M) imaging onboard radiation therapy machines would allow for targeting of functional tumor volume, and avoiding of healthy tissue, by guiding and modifying radiation beams, in the treatment room, based on real‐time F&M information. This capability might be particularly critical when tumor is located within deformable internal organs. When deformable structures do not present adequate contrast in the CTimage, precise localization of tumor and healthy‐tissue can be difficult using onboard CT alone. Also, since some F&M properties can change on the time scale of an hour, onboard F&M imaging may be important for its temporal proximity to therapy. Given the benefits of onboard F&M imaging and the essentiality of onboard CT, the purpose of this study is to investigate the feasibility of using flat‐panel detectors (FPDs) to accomplish both CT and SPECT onboard therapy machines. Method and Materials: A 10 mCi point source of Tc99m was placed 100 cm from a bar phantom, with a FPD immediately behind the phantom. Data were acquired for 10 seconds. A signal‐to‐noise ratio (SNR) was computed with signal given by difference in mean activities in the exposed and bar‐covered regions and noise given by standard deviation of amplitudes in the 0.776‐mm‐wide exposed‐region pixels. Results: The bar phantom was clearly visualized. The measured SNR was 4. Conclusion: Additional experiments will be required to evaluate FPDSPECT when radiotracer is distributed over an extended source and a collimator is employed. Since FPDs have not been designed for SPECT, only limited SPECT performance can be expected. However, these initial characterizations of FPDSPECT may support FPD design modifications, such as thicker scintillators, that enable combined SPECT/CT imaging using FPDs mounted onto therapy machines.
TU‐FF‐A3‐03: Investigation of Dose Reduction Strategies for Image Guidance with KV‐CBCT in Radiation Therapy33(2006); http://dx.doi.org/10.1118/1.2241652View Description Hide Description
Purpose: To explore methods of minimizing the dose to eye (lens) and contra‐lateral breast during image guidance with kilo voltage cone beam CT (kVCBCT) in radiotherapy.Method and Materials: A set of high sensitivity MOSFET, with high bias, was utilized for dosemeasurements. The MOSFET calibration factors, in terms of cGy/mV, were determined by measuring the response at a depth of 2.0 cm in water against ion chamber. Dose to eye was measured using a head phantom for 360‐degree full rotation, 270‐degree and 195‐degree scans (half rotation plus fan angle). The eye dose was also measured for 195‐degree scan simulating x‐ray tube rotation anterior as well as posterior to the head. Dose to contra‐lateral breast was also evaluated with a Rando phantom. The dosemeasurements were performed for 120kV beams with mAs values of 0.5, 1, 2 and 3.2 per projection. The images obtained during these measurements were analyzed for image quality. Results: The dosemeasured on the surface of the eye was less by 50% for 270‐degree scan and by 75% for 195‐degree posterior scan compared to 360‐degree scan. The dose to contra‐lateral breast was less by 30% and 40% for 270 and 195‐degree scans. Excellent image quality was obtained with 0.5 mAs/ projection and 320 projections over a complete rotation scan, however, acceptable image quality also resulted with 195‐degree scan with 75% reduction in eye dose. Reducing the number of projections, over a given arc angle decreased the dose to critical organ, but resulted in artifacts. Conclusion: Reducing the scanning arc resulted in significant reduction in dose without much loss of image quality. A posterior scan reduced the dose to eye considerably without any significant change in the image quality. This work demonstrates that there are opportunities to minimize dose to critical organs without compromising the quality.
TU‐FF‐A3‐04: An In Vivo Comparative Study of the MV and KV Cone Beam Computed Tomography Image Quality of a Lung Patient33(2006); http://dx.doi.org/10.1118/1.2241653View Description Hide Description
Purpose: To compare image quality, reconstruction artifacts and tumorvisibility for kV and MV cone‐beam computed tomography(CBCT) scans reconstructed with the same algorithm. Method and Materials: A protocol lung‐cancer patient was set up in the identical treatment position for kV and MVCBCT using a Varian On‐Board ImagerCBCT and an inhouse MVCBCT imaging system. For both scans the gantry made a 1‐minute, 360° continuous rotation. For the MVCBCT, ∼460 projection images were acquired at 6MV for ∼13 MU; for kVCBCT ∼600 projections were acquired using 125 kVp, 80 mA and 25‐ms exposure time per projection, resulting in ∼2cGy at isocenter. Reconstruction was performed using the Feldkamp back projection algorithm. Both scans were registered to the treatment plan CT. The visibility of three selected regions (bronchus, vertebrae, heart) is compared using the corresponding signal‐to‐noise ratio (SNR). The contrast ratio (CR) and contrast‐to‐noise ratio (CNR) at the tumor are also compared for ease of tumor identification. Results: The SNR of bronchus, vertebrae and heart are 25, 34 and 33 respectively for MVCBCT while the corresponding values in kV scan are 17, 33 and 42. For tumor identifiability, CNR and CR are 11 and 2 respectively for MV scan, and 10 and 2 for kV scan. The CNR of the vertebrae in MV and kV cases are 2 and 6. Time to register the kV image is approximately 50% less than MV image. Similar breathing artifacts are present in both scans. Conclusions: Both kV and MV scans deliver usable images. The tumor can be discriminated from the lung background. Higher bone contrast in kV scan helps to reduce time required to register the scan with the planning CT.Conflict of Interest: Research sponsored by NCI Grant P01‐CA59017 and Varian Medical Systems; Research agreement with Varian Medical Systems.
33(2006); http://dx.doi.org/10.1118/1.2241654View Description Hide Description
Purpose: To quantify the dose calculation accuracy achievable with 3D anatomical images obtained by Megavoltage Cone‐Beam Computed Tomography (MVCBCT) for the head and neck region (H&N). Method and Materials: MVCBCT images of inserts of different density immersed in water were obtained. This allowed the tuning of the parameters used for the image reconstruction. A MVCBCT number versus material density curve was also extracted for dose calculation purposes. MVCBCT images of a Rando phantom head were then acquired on a linactreatment couch with two different gain imagecalibrations and in two different positions relative to the room isocenter. Voxel‐based and band‐pass filter cupping artifact reduction methods were applied on all MVCBCT images.Images of the same phantom were also obtained with a kVCT. All images were transferred to a treatment planning system and dose calculations performed with various beam configurations. The dose differences obtained with the kVCT images and the MVCBCT images were analyzed using a gamma index function. Results: At best, 96.1% and 98.8% of the dose points calculated with the MVCBCT images were within the dose calculated with the kVCT image by [2%, 2 mm] and [3%, 3 mm], respectively. The worst cases observed had fractions of 87.7% and 96.3% of the dose points that agreed within [2%, 2 mm] and [3%, 3 mm], respectively. The cupping artifact reduction methods tested did not significantly improve the dose calculation for most cases. Conclusion: With proper calibration, dose calculations with MVCBCT images in the H&N region are feasible with an accuracy of [3 %, 3 mm] or less. The cupping artifact for H&N imaging does not lead to important dose calculation errors. Dose calculation with patient MVCBCTs and treatment plans are ongoing. Conflict of Interest: Research sponsored by Siemens OCS.
33(2006); http://dx.doi.org/10.1118/1.2241655View Description Hide Description
Purpose:CTimage artefacts caused by the presence of metallic objects hinder organ delineation and preclude precise dose calculation. In contrast, the presence of high atomic number material has relatively little impact on the image quality of Megavoltage Cone‐beam CT (MVCBCT). The objective of this work was to determine if MVCBCT could be used for accurate dose calculation in the presence of metallic objects.
Material: Experiments were performed with a 16cm diameter cylindrical water‐equivalent phantom. MVCBCT and kVCT images of the phantom were acquired with and without a 1.5 cm diameter steel rod (3 mm wall thickness) sitting on the phantom. Using a treatment planning system, dose distributions were calculated for a 6 MV 10 cm square field AP beam for these configurations. Dose measurements were performed using an ion chamber placed at isocenter and mosfet detectors placed at 35 locations in the phantom. Measured and calculated dose values were compared. Results: Without metal, dose calculated in the phantom using either the kVCT or the MVCBCT image corrected for cupping artefact were all within 2% of each other and of the measured values, showing the ability to use the MVCBCT for dose calculation. In the presence of the metallic object, erroneous density values, around the rod, made dose calculation on the kVCT unreliable. MVCBCT provided correct density values (within 5%) and differences between measured and calculated dose values were on average 3.2% (SD 3.4%). The two largest differences (10.6% and 8.9%) were found in a high dose gradient region below the rod. Conclusion: MVCBCT can be used in a treatment planning system for dose calculation in the presence of metallic objects. Results for non‐cylindrical geometries will also be presented.
This research is supported by Siemens.
- Moderated Poster ‐ Area 4 (Imaging): Breast Imaging
SU‐DD‐A4‐01: Quantifying Skin Effects After Accelerated Partial Breast Irradiation Using Digital Infrared Imaging (DII): Preliminary Feasibility Data33(2006); http://dx.doi.org/10.1118/1.2240149View Description Hide Description
Purpose: Accelerated partial‐breast irradiation (APBI) is an emerging radiation technique that challenges standard whole breast irradiation. The larger fraction sizes used in these hypofractionated schedules may increase the risk of late normal tissueeffects. Identification of the causes of variability in radiation sensitivity and normal tissue reactions could have important implications for breast cancer therapy. For this, a quantitative method of estimating early and late skineffects is needed. DII is recording instantaneous skin temperatures that are directly correlated with the skinblood flow, a parameter known to be an indicator of skin reaction. Material and Methods: An infrared digital camera IRSnapShot® was used to image breast cancer patients treated with APBI to a total dose of 3850 cGy over 10 fractions. The plans consisted of multiple external non‐coplanar photons +/− electron beams. The patients were imaged in a controlled temperature room before and after each fraction. They were also be imaged at regular intervals during their follow‐up. Two sets of orthogonal DII images were taken. The images were then transferred into Matlab where a GUI is being developed for image registration, thresholding and data analysis. Results: Five patients were imaged as described, three treated with photons only, and two with a combination of photons and electrons. The increase in maximum skin temperature from the baseline (pre treatment) to treatment completion is on average 2 to 4 degrees and depends on the techniques used, higher for plans including electrons, as expected due to their way of depositing dose.Conclusions: DII generated skin temperature information is a promising quantitative tool to estimate early and late effects in irradiated breast cancer patients. Our goal is to generate an “Index of Radiosensitivity” based on the early pattern of change in skin temperature that will allow individualization of radiotherapeutic prescription.
33(2006); http://dx.doi.org/10.1118/1.2240150View Description Hide Description
Purpose: To design and develop a portable optical imager for early‐stage breast cancer diagnostics, providing great depth information, enhanced data acquisition rates, and minimal patient discomfort. Method and Materials: A unique measurement geometry of simultaneous multiple point source illumination was implemented in the design and development of the hand‐held based optical probe. Simultaneous multiple point detection was carried out using an intensified charge‐coupled camera (ICCD) that can be operated in the continuous wave and frequency domain measurement approaches. The hand‐held based imaging probe has been coupled to the ICCD detection system and the performance characteristics (in terms of measurement accuracy and precision) of the imager is characterized through initial phantom studies under homogeneous conditions. Results: Preliminary simulated studies using simultaneous multiple point illuminationmeasurement geometry over the universally used single point illumination geometry demonstrated an increase in the detected signal strength as well as total interrogated tissue volumes. An optimal number of source and detector fibers used to develop the probe head, minimized the dead volume and improved the data acquisition times. Conclusion: A novel fluorescence‐enhanced imagingsystem was developed using a hand‐held probe and an ICCD camera, enabling the flexible and rapid imaging of any given tissue volume. Further work involves phantom based experimental studies towards 3D optical imaging and tomographic analysis. The final goal is to translate the current laboratory‐based techniques into routine clinical use.
33(2006); http://dx.doi.org/10.1118/1.2240151View Description Hide Description
Purpose: Anti‐scatter grids have been commonly used to reduce the amount of scatter in mammography. However, using grids require increasing the radiation dose to the breast in order to have an acceptable exposure to the image receptor. We used Monte Carlo simulation to optimize liner grid design for mammographyimaging in a way to achieve best contrast improvement with lowest dose to the breast. Materials and Methods: We used Monte Carlo Simulation Code MCNP5 to determine the amount of Scatter to Primary Ratio (SPR) for different x‐ray tube peak voltage (kVp), breast thicknesses, and grid geometries. We used a Molybdenum target/Molybdenum filtered x‐ray spectra, materials and geometrical dimensions that closely mimic the clinical situation. We used a semicircular shaped breast phantom made of 50 % adipose and 50 % glandular tissue equivalent materials. The grid septa were made of lead and inter‐space was made of carbon fiber. Results: Our calculated SPR values agree within 5 % with previously published clinical data. We have obtained significant contrast improvement for low bucky factors. For a 5 cm thick breast equivalent phantom, we found an optimal septa height of 0.9mm, septa thickness of 12μm and an inter‐space thickness of 100μm gives an optimal combination of 0.2 SPR, a 2.43 bucky factor, and a 1.31 contrast improvement factor (8 % error). With this geometry, the maximum SPR was lowered from 0.58 without the grid to 0.2 with the grid. Conclusion: We have optimized the geometry of the linear grid and achieved very significant contrast improvement with low SPR while minimizing the bucky factor and hence the mean‐glandular dose to the breast.
33(2006); http://dx.doi.org/10.1118/1.2240152View Description Hide Description
Purpose: This study's aim was to develop an easily reproducible clinical protocol to predict the average glandular dose (AGD) delivered to patients during routine mammography screening. It incorporates an evaluation of patient specific features, including glandularity, to predict the clinically delivered dose for both the cranio‐caudal (CC) and the medio‐lateral oblique (MLO) views. Method and Materials: The development of a modified homogenous dosimetric breast tissue equivalent phantom series (BRTES‐MOD) based on anthropomorphic measurements of the screening mammography population is central in evaluating the patient's fibroglandular content. It has been constructed with reference to the breast tissue elemental composition tabulated in the International Commission on Radiation Units and Measurements ‐ Report 44, and simulates the compression and variable content of patient's tissuecharacteristics. This study calculates the average glandular dose using entrance skin exposure and dose conversion factors based on fibroglandular content, compressed breast thickness, volumetric and anatomical factors, mammographic unit parameters and modifiable parameters of the BRTES‐MOD phantom. Results: Dose conversion factors were successfully calculated from the patient's fibroglandular content, compressed thickness, unit parameters, and spectral half value layer. An anthropometric population study facilitated the derivation of clinically usable equations to determine patient whole breast area, estimate patient skin layer thickness, and assess optimal placement for the automatic exposure control ionization chamber location. Dose distributions for the study population are presented for both CC and MLO views and compare well with those derived from previous population studies. Conclusion: The designed protocol can be performed within the time of a typical mammography screening appointment, and allows the determination of patient‐specific average glandular dose. The BRTES‐MOD method also provides a quantitative measure of patient specific AGD for the multiple projections comprising screening mammography examinations.
SU‐DD‐A4‐05: Characterization of X‐Ray Scatter and Glandular Dose in Digital Tomosynthesis for Breast Imaging Using Monte Carlo Simulations33(2006); http://dx.doi.org/10.1118/1.2240153View Description Hide Description
Purpose: To study the characteristics of x‐ray scatter and glandular dose in digital tomosynthesis for breast imaging.Method and Materials:Monte Carlo simulations of x‐ray transport in breast tomosynthesis were performed using the Geant4 package [Agostinelli et al, Nucl Instrum Meth A 506: 250–303, 2003]. Scatter‐to‐primary ratio (SPR) maps, maximum SPR, scatter point spread functions (PSF) and glandular dose to the breast were computed at several projection angles while varying compressed breast size, thickness, glandularity and x‐ray spectrum. For validation, the SPR and scatter PSF for the planar mammography view (0 degrees) for various setups were compared with published values [Boone et al, Med Phys 27(10): 2408‐16, 2000 and 27(8): 1818–1831, 2000]. Results: SPR maps and PSF show variations with increasing projection angle, with apparent asymmetry appearing at projection angles beyond 10 degrees. When the projection angle is increased from 0 to 21 ‐degrees, while the breast thickness encountered by the central ray increases by 7.1%, the maximum SPR for a semi‐circular 10 cm radius breast increases by 10.1% and 18.8 % for breast thicknesses of 2 cm and 8 cm, respectively. Dose deposition shows a decrease, varying by 3.8–7.6% for the same thicknesses and projection angles. Conclusion: Since the use of an anti‐scatter grid is not easy to implement in tomosynthesisimaging, the development of software‐based post‐acquisition scatter reduction is important, which requires a good understanding of the scatter effects. This work characterizes the scatter signal present in tomosynthesisimages and shows that x‐ray scatter affects each projection angle differently and therefore each projection must be corrected separately, using appropriate prior knowledge. Decreased glandular dose with increasing projection angle must be taken into account when planning a tomosynthesis clinical protocol. Research supported in part by: NIH‐NIBIB Grant RO1‐ EB002123 and the Georgia Cancer Coalition.
33(2006); http://dx.doi.org/10.1118/1.2240154View Description Hide Description
Purpose: To evaluate the ability of a prototype breast CT scanner to detect micro‐calcifications, and to understand the influence that tube potential and radiation dose have on this. Method and Materials: Commercially available micro‐calcifications (μCa) of various sizes (200 to 425 μm) were embedded inside a 12.7 mm polyethylene tube filled with gelatin (to simulate glandular tissue). The gelatin tube was then placed inside a 14 cm diameter adipose equivalent cylindrical phantom and scanned using various tube potentials (60 to 100 kVp) and tube currents. CTimages were reconstructed with both Ramp and Shepp‐Logan filters, with a reconstructed voxel size of about 320×320×200 μm. The μCa were then evaluated quantitatively using signal‐to‐noise ratio (SNR) metric, and subjective appraisals were made as well. A dedicated breast CT visualization workstation was used for subjective evaluation. Results: Results for 250–280 mm μCa imaged at 80 kVp shown that the μCa are clearly visible when the rod is scanned by itself, but extremely difficult to locate when placed inside the 14 cm phantom. The visualization of the μCa improved overall for larger μCa, and overall visualization improves as the radiation levels are increased, as expected. Conclusion: These initial results suggested that the pixel size may not be a critical factor when determining the ability of the prototype system to visual micro‐calcifications, as the current objects scanned are only about 48% of the reconstructed voxel size. Maximum intensity projection (MIP) display for thick‐slice imaging was found to be most useful for subjective viewing of micro‐calcification clusters.
- Moderated Poster ‐ Area 4 (Imaging): Computed Tomography
33(2006); http://dx.doi.org/10.1118/1.2240233View Description Hide Description
Purpose: To investigate the effects of patient body size and lung size on the CT numbers of lung nodules measured with multi‐detector CTscanners and whether improved accuracy can be obtained with a dual‐energy technique. Method and Materials: Simulated lung nodules consisting of 9.5‐mm diameter spheres containing 50mg/cc and 100mg/cc CaCO3 in a water‐equivalent resin were scanned in two simulated thorax section phantoms with a GE VCT scanner. One phantom (A) represented the middle of the chest. It had large simulated lung regions and simulated ribs, heart and spine. The other (B) represented the upper chest. It had a much wider aspect ratio, smaller simulated lung regions, and simulated ribs, scapula, heart, and spine. Fat rings were added to the phantoms to simulate larger patients. Images were acquired on a GE VCT scanner with high‐resolution techniques (0.53:1 pitch, 0.625‐mm slice thickness and interval) at 80, 120 and 140kVp. Scans were repeated 3 times for reproducibility and analyzed using an automated technique. Results: Body size had a significant effect on the measured mean CT‐numbers of the nodules. For phantom‐A, adding fat rings decreased the overall average CT‐numbers of the 50mg/cc nodules at 120kVp by 15HU and those of the 100mg/cc nodules by 21HU. Corresponding reductions in phantom‐B were 9HU and 13HU. The dual‐energy approach (CT#80kVp‐CT#140kVp) reduces the variability, with a maximum difference of 4HU for all conditions. Lung size had a minimal effect with a maximum difference (nodule CT# phantom A ‐ nodule CT# phantom B) of 4.5 HU. Conclusion: Even with modern multi‐detector CTscanners, beam hardening and x‐ray scatter errors due to body size can result in underestimates of the true CT numbers of lung nodules. A dual‐energy approach compensates for these errors and should be considered especially if it can be implemented using a rapid kVp switching technique.
33(2006); http://dx.doi.org/10.1118/1.2240234View Description Hide Description
Purpose: Because diagnostic Computed Tomography(CT)imaging involves a tradeoff between image quality and radiation risk, there is great interest in determining the effects of stochastic noise on the utility of clinical tasks. The reconstruction processes used in CT result in noise properties that are non‐local and anisotropic in the image domain. A commonly used approximation for computing imagenoise from raw measurement data was empirically tested for validity. A noise variance mapping scheme was used to estimate stochastic noise in complex anatomical scenes and was compared to variance measurements of image simulations generated with controlled amounts of synthetic noise.Methods: The commonly assumed transformation between linear and log variance (σ2=1/Q) was tested for Poisson random numbers with means ranging from less than one to larger than 30. Noise variance maps were generated by filtered back projection using the square of the reconstruction kernel operating on sinogram variance estimates. A series of images was reconstructed by adding Poissonnoise to sinogram data, and the variance of regions of interest in the image sequence was calculated. Results: The approximation that log variance is proportional to the inverse number of quanta fails badly for N<10. Estimated variance maps were found to agree with empirical measurements of image variance. The noise variance in a CTimage is a slowly varying spatial function. Image simulations demonstrated that noise has a texture that is highly anisotropic and can mimic anatomic structures. Conclusions:CTnoise is a complex phenomenon. Variance maps are a useful tool for estimating noise in structured image regions where direct variance measurements fail. Fortunately most clinical scans operate at higher flux levels where the commonly used variance approximation is valid. Low‐dose protocols must be carefully evaluated to determine the effects of stochastic noise on diagnostic performance.