CT Phantom Research Articles

SU-E-T-507: Dosimetric Validation of An IntrabeamTM GATE Model, Based On Monte Carlo GEANT4 Toolkit, for IORT Applications

Purpose: Intra-operative Radiation Therapy (IORT) with Intrabeam™ is delivered using a miniaturized radiation device with applicators generating the prescribed dose directed to the tumor bed. However, the dose delivered to the patient is not well known. In this context, a Monte Carlo (MC) calculation can accurately assess the dose delivered to the patient by simulating the particle transport. The aim of this study is to create and validate an Intrabeam model on the GATE platform, a MC code based on Geant4. Methods: A model of the Intrabeam™ was created on GATE taking into account dimensions and materials of each element of the system as well as physical processes. This model was extended to include simulations based on patient CT images including the presence of applicators within the same simulation framework allowing their inclusion in the dose calculation. For breast cancer simple simulations in a water phantom were performed including the bare probe and the spherical applicators, using soft X-Ray PTW ionization chambers. For spinal metastases, the RANDO phantom was used to simulate a patient and realistic treatment conditions allowing dose delivery verifications using thermo-luminescent dosimeters (TLD). Finally a breast cancer patient CT acquisition, carried out during the operation with the applicator in place, was also used in the validation of the Intrabeam™ GATE model and associated MC based dosimetry using TLDs. Results: Depth dose curve and isotropy were compared showing a good agreement in water. For breast application, the comparison between TLD measurements and GATE simulations on patient and phantom CT images showed a good agreement with less than 1% of deviation (one billion simulated particles). Conclusion: The Monte Carlo GATE platform can simulate accurately relative dose distributions delivered by the Intrabeam system. The proposed model has been validated and will be used for personalized 3D dose prediction in IORT.

SU-D-105-01: Patient-Specific Quality Assurance for Monte Carlo-Calculated Lung SBRT On Cyberknife - Is It Necessary?

Purpose: In transitioning from ray-tracing (RT) to Monte Carlo (MC) dose computation algorithm for Cyberknife lung stereotactic body radiotherapy (SBRT), we realized large differences (10–20%) in PTV coverage to heterogeneous target regions which required dosimetric confirmation. Currently, our practice requires an independent second calculation for all plans. MuCheck software (Oncology Data Systems) is used in lieu of physical measurements for RT plans; however, there exists no commercial software for verifying MC plans. We determined all lung SBRT plans should utilize the MC algorithm and initially be confirmed by direct measurements. Methods: Lung treatment plans were first optimized with RT then recalculated using MC for final optimization and high-resolution dose computation. The MC plan was superimposed on a heterogeneous thorax phantom CT (Standard Imaging 91230) using the phantom overlay tool of the MultiPlan 3.5.2 software. Isodose distributions were manually shifted to place the ion chamber (0.053cc Exradin-A1SL) in a suitably homogeneous region within the CTV. The plan was then delivered to the thorax phantom with the ion chamber placed in the mediastinum insert location. Results: This methodology was used for 33 consecutive lung patients receiving Cyberknife SBRT with PTVs ranging from 10.7–185.9cm3. The mean deviation between measured and MC calculated doses was −2.31%±1.66%. The maximum deviation was −4.69%. Acceptable tolerance for patient-specific QA was considered ±5%. Conclusion: The MC algorithm provides improved accuracy over RT for heterogeneous dose calculation, confirmed by direct point measurements. Patient-specific QA using a heterogeneous lung phantom provided an acceptable anthropomorphic approximation of patient plans calculated with MC. Since the QA results establish satisfactory delivery of MC doses, patient-specific plans calculated on a homogeneous phantom comparing RT and MC algorithms may provide a suitable second check without direct measurement. Conversely, many users may continue direct measurement as a second check until commercial MC verification software becomes available.

SU‐E‐J‐95: Increasing Soft Tissue Contrast by Combining New CT Technologies and Imaging Dose for Radiation Treatment Planning and Delivery Guidance

Purpose: CT dose is normally minimized based on ALARA principle. For radiation therapy (RT), a slightly increased CT dose in exchange for improved soft‐tissue contrast may be justified. In this work, we investigate the impact of CT imaging dose and other acquisition and reconstruction parameters on CT quality for RT planning and delivery guidance. Methods: CT data of various phantoms were acquired using a CT‐on‐rails (Somaton Definition AS Open, Siemens) with a series of acquisition parameters, such as tube voltage and current, rotation time, pitch, imaging dose control methods, and reconstruction algorithms, and were analyzed based on contrast‐to‐noise ratio (CNR) and dose‐weighted CNR (CNRD) to identify image quality enhancement protocols (IQEP). The IQEPs were then used to acquire CT during daily IGRT for selected patients with head and neck, prostate, and pancreas cancers. The impact of the improved CT images on RT was assessed with (1) accuracy and reproducibility of auto‐segmentation using ABAS (Elekta) and (2) intra‐and inter‐variability of manual contouring of the relevant organs by 10 observers. Results: As expected, the CNR generally increases with mAs from phantom CT data, while CNRD experiences a slow increase with mAs or reaches a plateau and then drops above certain mAs depending phantom size and relative electron density or contrast level. There is complicated interplay between kVp and mAs, particularly with regards to perception. Visual inspection of the patient CTs obtained with IQEPs showed clearer organ boundaries as compared to those from the standard CT protocols. This improvement was supported by the reduced intra‐and inter‐variability of manual contouring. The CT doses with IQEPs were 2–3 times higher than those from the standard protocols. Conclusion: CT quality can be improved by optimizing parameters in CT protocols for modern scanners, leading to improved accuracy of organ delineation for RT planning and delivery.

Radiation dose reduction in medical x‐ray CT via Fourier‐based iterative reconstruction

A Fourier-based iterative reconstruction technique, termed Equally Sloped Tomography (EST), is developed in conjunction with advanced mathematical regularization to investigate radiation dose reduction in x-ray CT. The method is experimentally implemented on fan-beam CT and evaluated as a function of imaging dose on a series of image quality phantoms and anonymous pediatric patient data sets. Numerical simulation experiments are also performed to explore the extension of EST to helical cone-beam geometry. EST is a Fourier based iterative algorithm, which iterates back and forth between real and Fourier space utilizing the algebraically exact pseudopolar fast Fourier transform (PPFFT). In each iteration, physical constraints and mathematical regularization are applied in real space, while the measured data are enforced in Fourier space. The algorithm is automatically terminated when a proposed termination criterion is met. Experimentally, fan-beam projections were acquired by the Siemens z-flying focal spot technology, and subsequently interleaved and rebinned to a pseudopolar grid. Image quality phantoms were scanned at systematically varied mAs settings, reconstructed by EST and conventional reconstruction methods such as filtered back projection (FBP), and quantified using metrics including resolution, signal-to-noise ratios (SNRs), and contrast-to-noise ratios (CNRs). Pediatric data sets were reconstructed at their original acquisition settings and additionally simulated to lower dose settings for comparison and evaluation of the potential for radiation dose reduction. Numerical experiments were conducted to quantify EST and other iterative methods in terms of image quality and computation time. The extension of EST to helical cone-beam CT was implemented by using the advanced single-slice rebinning (ASSR) method. Based on the phantom and pediatric patient fan-beam CT data, it is demonstrated that EST reconstructions with the lowest scanner flux setting of 39 mAs produce comparable image quality, resolution, and contrast relative to FBP with the 140 mAs flux setting. Compared to the algebraic reconstruction technique and the expectation maximization statistical reconstruction algorithm, a significant reduction in computation time is achieved with EST. Finally, numerical experiments on helical cone-beam CT data suggest that the combination of EST and ASSR produces reconstructions with higher image quality and lower noise than the Feldkamp Davis and Kress (FDK) method and the conventional ASSR approach. A Fourier-based iterative method has been applied to the reconstruction of fan-bean CT data with reduced x-ray fluence. This method incorporates advantageous features in both real and Fourier space iterative schemes: using a fast and algebraically exact method to calculate forward projection, enforcing the measured data in Fourier space, and applying physical constraints and flexible regularization in real space. Our results suggest that EST can be utilized for radiation dose reduction in x-ray CT via the readily implementable technique of lowering mAs settings. Numerical experiments further indicate that EST requires less computation time than several other iterative algorithms and can, in principle, be extended to helical cone-beam geometry in combination with the ASSR method.

GATE Simulation of $^{12}{\rm C}$ Hadrontherapy Treatment Combined With a PET Imaging System for Dose Monitoring: A Feasibility Study

The GATE package is used to perform Monte Carlo hadrontherapy simulations of a cancer treatment combined with the complete description of an associated positron emission tomography (PET) imaging device for dose monitoring. This study aimed to demonstrate that the GATE platform has the capability to perform realistic simulations in the field of hadrontherapy, combining both dose and imaging system. We defined the simulation configuration as a carbon ion pencil beam scanning of a thorax CT phantom together with a complete PET imaging system. Two main positron emitters resulting from nuclear 12C reactions are considered in this study: 11C and 15O. We studied the produced data to analyse the interest in using a full PET system simulation instead of the usual Gaussian smoothing applied on the positron emitters map. We found differences in the distal position of the signal falloff of 20% between full PET system simulation and the Gaussian model. We also studied the influence of the 15O isotope in PET images and found the contribution to falloff of this isotope to be negligible (4%), which suggests the inclusion of 15O isotope in the simulation is not necessary. Finally, we analysed the impact of dose delivery on PET image quality and found a difference of 20% on the PET estimation falloff between doses of 10 Gy and 1 Gy. This study shows that GATE, implemented on a computing system with large number of CPUs (>;1000), has the potential to be used for quantitative evaluation of imaging protocols for radiation monitoring.

Effect of vertical positioning on organ dose, image noise and contrast in pediatric chest CT—phantom study

CT optimization has a special importance in children. Smaller body size accentuates the importance of patient positioning affecting both radiation dose and image quality. To determine the effect of vertical positioning on organ dose, image noise and contrast in pediatric chest CT examination. Chest scans of a pediatric 5-year anthropomorphic phantom were performed in different vertical positions (-6 cm to +5.4 cm) with a 64-slice CT scanner. Organ doses were measured with metal-oxide-semiconductor field-effect transistor (MOSFET) dosimeters. Image noise and contrast were determined from the CT number histograms corresponding to different tissues. Significant changes in organ doses resulting from vertical positioning were observed, especially in radiosensitive anterior organs. The breast dose increased up to 16% and the thyroid dose up to 24% in lower positions. The noise was increased up to 45% relative to the centre position in the highest and lowest vertical positions, with a particular increase observed on the anterior and posterior sides, respectively. Off-centering also affected measured image contrast. Vertical off-centering markedly affects organ doses and measured image-quality parameters in pediatric chest CT examination. Special attention should be given to correct patient centering when preparing patients for CT scans, especially when imaging children.

MCNPX simulation of proton dose distribution in homogeneous and CT phantoms

A dose simulation system was constructed based on the MCNPX Monte Carlo package to simulate proton dose distribution in homogeneous and CT phantoms. Conversion from Hounsfield unit of a patient CT image set to material information necessary for Monte Carlo simulation is based on Schneider's approach. In order to validate this simulation system, inter-comparison of depth dose distributions among those obtained from the MCNPX, GEANT4 and FLUKA codes for a 160MeV monoenergetic proton beam incident normally on the surface of a homogeneous water phantom was performed. For dose validation within the CT phantom, direct comparison with measurement is infeasible. Instead, this study took the approach to indirectly compare the 50% ranges (R50%) along the central axis by our system to the NIST CSDA ranges for beams with 160 and 115MeV energies. Comparison result within the homogeneous phantom shows good agreement. Differences of simulated R50% among the three codes are less than 1mm. For results within the CT phantom, the MCNPX simulated water equivalent Req,50% are compatible with the CSDA water equivalent ranges from the NIST database with differences of 0.7 and 4.1mm for 160 and 115MeV beams, respectively.

Dosimetric assessment of a novel metal artifact reduction method in CT images

The aim of this study was to assess the ability of metal artifact reduction (MAR) algorithm in restoring the CT image quality while correcting the tissue density information for the accurate estimation of the absorbed dose. A phantom filled with titanium (low‐Z metal) and Cerrobend (high‐Z metal) inserts was used for this purpose. The MAR algorithm was applied to phantom's CT dataset. Static intensity‐modulated radiation therapy (IMRT) plans, including five beam angles, were designed and optimized on the uncorrected images to deliver 10 Gy on the simulated target. Monte Carlo dose calculation was computed on uncorrected, corrected, and ground truth image datasets. It was firstly verified that MAR methodology was able to correct HU errors due to the metal presence. In the worst situation (high‐Z phantom), the image difference, uncorrected ground truth and corrected ground truth, went from −4.4±118.8 HU to 0.4±10.8 HU, respectively. Secondly, it was observed that the impact of dose errors estimation depends on the atomic number of the metal: low‐Z inserts do not produce significant dose inaccuracies, while high‐Z implants substantially influence the computation of the absorbed dose. In this latter case, dose errors in the PTV region were up to 23.56% (9.72% mean value) when comparing the uncorrected vs. the ground truth dataset. After MAR correction, errors dropped to 0.11% (0.10% mean value). In conclusion, it was assessed that the new MAR algorithm is able to restore image quality without distorting mass density information, thus producing a more accurate dose estimation.PACS numbers: 87.53.Bn, 87.55.K, 87.57.cp

Estimation of patient dose in 18 F-FDG and 18 F-FDOPA PET/CT examinations

To estimate specific organ and effective doses to patients resulting from the (18)F-FDG ((18)F-2-deoxy-D-glucose) and (18)F-FDOPA (6-fluoro-((18)F)-L-3, 4-dihydroxyphenylalanine) PET/CT examinations for whole body and brain. Three protocols for whole body and three for brain PET/CT were used. The CTDI values were measured using standard head and body CT phantoms and also computed using a software CT-Expo for dose evaluation from the CT component. OLINDA software based on MIRD method was used for estimating doses from the PET component of the PET/CT examination. The organ doses from (18)F-FDG and (18)F-FDOPA whole body and brain PET/CT studies were estimated. The total effective dose from a typical protocol of whole body PET/CT examination was 14.4 mSv for females and 11.8 mSv for male patients from (18)F-FDG, whereas it was 11 mSv for female and 9.1 mSv for male patients from (18)F-FDOPA. The total effective doses from a typical protocol for PET/CT studies of brain was 6.5 mSv for females and 5.1 mSv for males from (18)F-FDG whereas it was 3.7 mSv for females and 2.8 mSv for males from (18)F-FDOPA. The effective radiation doses from whole body PET/CT examination was approximately 4-8 times higher than the background radiation dose from both (18)F-FDG and (18)F-FDOPA scans, while it was 1-3 times the background radiation dose from PET/CT scans of brain.

Usefulness of the Virtual Monochromatic Image in Dual-Energy Spectral CT for Decreasing Renal Cyst Pseudoenhancement: A Phantom Study

The purpose of this study was to determine the effect that dual-energy spectral CT has on renal cyst pseudoenhancement with a renal cyst phantom. Two renal compartments within a CT phantom were filled with 40- and 240-HU solutions of diluted contrast material, mimicking the nonenhanced phase and nephrographic phase of MDCT, respectively. A saline-filled tapering cylindric cyst model was used, simulating cysts of varying diameters (range, 0.7-3.0 cm) suspended in the renal compartments. The phantom was scanned using dual-energy CT (DECT) (80 and 140 kVp) and conventional, single-energy CT (120 kVp) with the same 64-MDCT scanner. Attenuation increase was measured for a cyst model within 40- and 240-HU dilutions. We compared the attenuation increase obtained on virtual monochromatic images from DECT with conventional, single-energy MDCT images using a paired Student t test. Pseudoenhancement occurred in the conventional single-energy CT acquisitions, with water attenuation increase of 42.44 ± 4.03 HU measured at 120 kVp. In virtual monochromatic images of DECT, we observed less pseudoenhancement with water attenuation increase of 21.51 ± 6.18 HU at 70 keV. Both acquisitions yielded a p value less than 0.0001. We achieved less pseudoenhancement of renal cysts with virtual monochromatic images of DECT compared with conventional single-energy CT. This method may be useful when an accurate differentiation between small renal cysts and solid masses is critical.

A new evaluation method for image noise reduction and usefulness of the spatially adaptive wavelet thresholding method for CT images

We have proposed a direct evaluation method concerning preservation of noise-free components for image noise reduction. This evaluation method is to graphically estimate how well a noise-reduction method will preserve noise-free image components by using the normal probability plot of the image pixel value difference between an original image and its noise-reduced image; this difference is equivalent to the "method noise" which was defined by Buades etal. Further, by comparing the linearity of a normal probability plot for two different noise reduction methods, one can graphically assess which method will be more able to preserve the noise-free component than the other. As an illustrative example of this evaluation method, we have evaluated the effectiveness of the spatially-adaptive BayesShrink noise-reduced method devised by Chang etal., when applied to chest phantom CT images. The evaluation results of our proposed method were consistent with the visual impressions for the CT images processed in this study. The results of this study also indicate that the spatially-adaptive BayesShrink algorithm devised by Chang etal. will work well on the chest phantom CT images, although the assumption for this method is often violated in CT images, and the assumption postulated for the spatially-adaptive BayesShrink method is expected to have sufficient robustness for CT images.

Kilo-Voltage Imaging Doses to Organs-at-Risk in CBCT-guided Breast Radiation Therapy

Recently kilo-voltage cone beam computed tomography (kVCBCT) scans have been widely used in the external beam radiation therapy of breast cancer patients to improve daily setup accuracy so that high dose of tumoricidal radiation can be delivered accurately to the partial volume, as well as to the whole breast. Although early studies investigated the kV imaging doses from kVCBCT scans to critical structures in the pelvis and head regions, the knowledge of dose depositions to the various organs-at-risk (OARs) in thoracic kVCBCT scans of the patient chest is largely missing. Therefore, the goal of this study is to systematically investigate the kV imaging doses to the OARs during CBCT-guided radiation therapy of breast cancer patients. Twenty breast cancer patients in supine orientation were retrospectively assorted into two groups by the lesion site (left vs. right breast). Critical structures such as the heart, the lungs, the normal breast and the thyroid were delineated on the Eclipse treatment planning system based on the planning CT images, which together were subsequently converted into patient CT phantoms for Monte Carlo simulations with an in-house DICOM utility tool. A benchmarked EGS4 Monte Carlo code was used to calculate the 3D dose distributions in the patients scanned by the kVCBCT with a half-fan pelvis protocol pre-defined by the manufacturer. The correlation between the imaging doses received by the OARs and the patient size as well as the lesion site was studied. Furthermore, a similar study was carried out on an adult female anthropomorphic phantom as defined in ICRP report 110 with cortical bone and red bone marrow delineated in the ribs, scapulae, clavicles, thoracic and cervical spines. The mean doses deposited to the heart, the lungs, the normal breast and the thyroid were found to be 3.1, 3.2, 3.3, and 1.5 cGy per CBCT scan, respectively. There were no significant differences observed between the dose depositions to the left and the right lungs, due primarily to the 364° arc rotation defined for the half-fan pelvis scan. A strong correlation was observed between the patient size and the imaging doses, with lower organ doses in the patients of larger sizes. The surface dose for the skin of the breast could reach 7-8 cGy per scan due to proximity to the kV source. The ICRP phantom study indicated that the kVCBCT-contributed mean doses to the cortical bones and the red bone marrows in the thoracic region were 6.1 and 4.0 cGy, respectively for each scan. Compared to the negligible doses (< 0.2% of therapeutic dose except for ipsilateral lung) from the mega-voltage photon beams in breast radiation therapy, each kVCBCT scan contributed 4-10 times more doses to the various OARs on average. Caution should be exercised to avoid multiple kVCBCT acquisitions for each fraction in CBCT-guided breast radiation therapy.

An investigation into factors affecting electron density calibration for a megavoltage cone‐beam CT system

There is a growing interest in the use of megavoltage cone‐beam computed tomography (MV CBCT) data for radiotherapy treatment planning. To calculate accurate dose distributions, knowledge of the electron density (ED) of the tissues being irradiated is required. In the case of MV CBCT, it is necessary to determine a calibration‐relating CT number to ED, utilizing the photon beam produced for MV CBCT. A number of different parameters can affect this calibration. This study was undertaken on the Siemens MV CBCT system, MVision, to evaluate the effect of the following parameters on the reconstructed CT pixel value to ED calibration: the number of monitor units (MUs) used (5, 8, 15 and 60 MUs), the image reconstruction filter (head and neck, and pelvis), reconstruction matrix size (256 by 256 and 512 by 512), and the addition of extra solid water surrounding the ED phantom. A Gammex electron density CT phantom containing EDs from 0.292 to 1.707 was imaged under each of these conditions. The linear relationship between MV CBCT pixel value and ED was demonstrated for all MU settings and over the range of EDs. Changes in MU number did not dramatically alter the MV CBCT ED calibration. The use of different reconstruction filters was found to affect the MV CBCT ED calibration, as was the addition of solid water surrounding the phantom. Dose distributions from treatment plans calculated with simulated image data from a 15 MU head and neck reconstruction filter MV CBCT image and a MV CBCT ED calibration curve from the image data parameters and a 15 MU pelvis reconstruction filter showed small and clinically insignificant differences. Thus, the use of a single MV CBCT ED calibration curve is unlikely to result in any clinical differences. However, to ensure minimal uncertainties in dose reporting, MV CBCT ED calibration measurements could be carried out using parameter‐specific calibration measurements.PACS number: 87.59.bd

A comparison of the thresholding strategies of micro-CT for periodontal bone loss: a pilot study

Micro-CT provides three-dimensional details and has been widely used for biomedical assessments. This study aimed to determine the most appropriate threshold method for quantitatively assessing the dynamics of periodontal destruction. Inflammation was induced by submerging a silk ligature in the sulcus of the maxillary second molars of rats, and the animals were killed prior to ligature placement and after 7 and 21 days. The maxillae were examined for the bone resorptive activities by micro-CT, histology and tartrate-resistant acid phosphatase staining. The imaging threshold was determined by CT phantom, global and local algorithms. A bone fraction measurement from each threshold-determining technique was compared with histomorphometry. The reliability and reproducibility were examined by the intraclass correlation coefficient (ICC) and the coefficient of variation. Significant reduction of inflammatory infiltration (p < 0.01) and active osteoclastic resorption (p < 0.05) from Day 7 to Day 21 were noted. High inter- and intraexaminer agreement were demonstrated in both histomorphometric and micro-CT assessments (ICC > 0.98). The algorithm-based technique demonstrated stronger correlation to histomorphometry than phantom-based thresholds, and the highest agreement was presented by the local algorithm (ICC > 0.96). This, however, was considerably computationally expensive. The local threshold-determining algorithm is suggested for examining inflammation-induced bone loss. Further investigation will be aimed at enhancing computational efficiency.

Low tube voltage CT for improved detection of pancreatic cancer: detection threshold for small, simulated lesions

BackgroundPancreatic ductal adenocarcinoma is associated with dismal prognosis. The detection of small pancreatic tumors which are still resectable is still a challenging problem.The aim of this study was to investigate the effect of decreasing the tube voltage from 120 to 80 kV on the detection of pancreatic tumors.MethodsThree scanning protocols was used; one using the standard tube voltage (120 kV) and current (160 mA) and two using 80 kV but with different tube currents (500 and 675 mA) to achieve equivalent dose (15 mGy) and noise (15 HU) as that of the standard protocol.Tumors were simulated into collected CT phantom images. The attenuation in normal parenchyma at 120 kV was set at 130 HU, as measured previously in clinical examinations, and the tumor attenuation was assumed to differ 20 HU and was set at 110HU. By scanning and measuring of iodine solution with different concentrations the corresponding tumor and parenchyma attenuation at 80 kV was found to be 185 and 219 HU, respectively.To objectively evaluate the differences between the three protocols, a multi-reader multi-case receiver operating characteristic study was conducted, using three readers and 100 cases, each containing 0–3 lesions.ResultsThe highest reader averaged figure-of-merit (FOM) was achieved for 80 kV and 675 mA (FOM = 0,850), and the lowest for 120 kV (FOM = 0,709). There was a significant difference between the three protocols (p < 0,0001), when making an analysis of variance (ANOVA). Post-hoc analysis (students t-test) shows that there was a significant difference between 120 and 80 kV, but not between the two levels of tube currents at 80 kV.ConclusionWe conclude that when decreasing the tube voltage there is a significant improvement in tumor conspicuity.

Kidney Stone Volume Estimation from Computerized Tomography Images Using a Model Based Method of Correcting for the Point Spread Function

We propose a method to improve the accuracy of volume estimation of kidney stones from computerized tomography images. The proposed method consisted of 2 steps. A threshold equal to the average of the computerized tomography number of the object and the background was first applied to determine full width at half maximum volume. Correction factors were then applied, which were precalculated based on a model of a sphere and a 3-dimensional gaussian point spread function. The point spread function was measured in a computerized tomography scanner to represent the response of the scanner to a point-like object. Method accuracy was validated using 6 small cylindrical phantoms with 2 volumes of 21.87 and 99.9 mm(3), and 3 attenuations, respectively, and 76 kidney stones with a volume range of 6.3 to 317.4 mm(3). Volumes estimated by the proposed method were compared with full width at half maximum volumes. The proposed method was significantly more accurate than full width at half maximum volume (p <0.0001). The magnitude of improvement depended on stone volume with smaller stones benefiting more from the method. For kidney stones 10 to 20 mm(3) in volume the average improvement in accuracy was the greatest at 19.6%. The proposed method achieved significantly improved accuracy compared with threshold methods. This may lead to more accurate stone management.

4D CT image reconstruction with diffeomorphic motion model

Four-dimensional (4D) respiratory correlated computed tomography (RCCT) has been widely used for studying organ motion. Most current RCCT imaging algorithms use binning techniques that are susceptible to artifacts and challenge the quantitative analysis of organ motion. In this paper, we develop an algorithm for analyzing organ motion which uses the raw, time-stamped imaging data to reconstruct images while simultaneously estimating deformation in the subject's anatomy. This results in reduction of artifacts and facilitates a reduction in dose to the patient during scanning while providing equivalent or better image quality as compared to RCCT. The framework also incorporates fundamental physical properties of organ motion, such as the conservation of local tissue volume. We demonstrate that this approach is accurate and robust against noise and irregular breathing patterns. We present results for a simulated cone beam CT phantom, as well as a detailed real porcine liver phantom study demonstrating accuracy and robustness of the algorithm. An example of applying this algorithm to real patient image data is also presented.

SU‐C‐217BCD‐06: Analysis of Different CT Accreditation Programs

Purpose: This study will review two alternate pathways for CT accreditation ‐ the program of the American College of Radiology (ACR) and the program of Intersocietal Accreditation Commission (IAC). The study will compare and contrast the two programs. Methods: The requirements of the ACR‐CT accreditation and IAC ‐ICACTL programs are reviewed for both the clinical and technical aspect. Physics data collected on several multidetector CT scanners (MDCT), including 16, 64, 128, 320, and up to 750 detector arrays utilizing both the ACR CT phantom and the CATPHAN 600 CT Phantom (note, IAC does not require a specific phantom, thus this widely used commercial phantom) is used. Scanning protocols are based on the protocols as required by ACR or IAC. Data necessary to complete the data forms for alignment, CT# accuracy, slice width, low contrast resolution (uniformity and noise, distance accuracy and high contrast spatial resolution), obtained measurements were made as specified in the accrediting group's instructions using manual processes as well as automated analysis approaches. Results: The results show examples of significant differences in the required levels of physical performance of head and body scanning under the different programs, particularly in the high resolution modulation transfer function (MTF) scanning ability. Likewise, significant differences in clinical body scanning ability of approved devices are shown. Conclusions: Significant differences in the levels of CT performance required to pass the two different accreditation programs are shown. The data sets indicate that a single phantom requirement for an accreditation program is unnecessary because the portability of physics data should obviate the need to purchase duplicate phantoms for accreditations purposes, and allow for more ready comparison between accreditation programs, government and industry specifications. This paper may guide imaging professionals in decisions regarding accreditation pathways with CT.DJG serves as a consultant to The Phantom Labratory and Image OWL, Salem, NY

TU-E-217BCD-04: Spectral Breast CT: Effect of Adaptive Filtration on CT Numbers, CT Noise, and CNR.

Photon counting spectral breast CT is feasible in part due to using an adaptive filter. An adaptive filter provides flat x-ray intensity profile and constant x-ray energy spectrum across detector surface, decreases required detector count rate, and eliminates beam hardening artifacts. However, the altered x-ray exposure profiles at the breast and detector surface may influence the distribution of CT noise, CT numbers, and contrast to noise ratio (CNR) across the CT images. The purpose of this work was to investigate these effects. Images of a CT phantom with and without adaptive filter were simulated at 60kVp, 90kVp, and 120kVp tube voltages and 660 mR total skin exposure. The CT phantom with water content had 14cm diameter, contrast elements representing adipose tissue and 2.5mg/cc iodine contrast located at 1cm, 3.5cm, and 6cm from center of the phantom. The CT numbers, CT noise, and CNR were measured at multiple locations for several filter/exposure combinations: (1)without adaptive filter for 660mR skin exposure; (2)with adaptive filter for 660mR skin exposure along central axis (mean skin exposure across the breast was <660mR); and (3)with adaptive filter for scaled exposure (mean skin exposure was 660mR). Beam hardening (cupping) artifacts had 47HU magnitude without adaptive filter but were eliminated with adaptive filter. CNR of contrast elements was comparable for (1) and (2) over central parts but was higher by 20-30% for (1) near the edge of the phantom. CNR was higher by 20-30% in (3) as compared to (2) over central parts and comparable near the edges. The adaptive filter provided: uniform distribution of CT noise, CNR, and CT numbers across CT images; comparable or better CNR with no dose penalty to the breast; and eliminated beam hardening artifacts.

SU-E-T-530: Relative Electron Density Phantom Comparison.

Modern treatment planning systems require lookup tables to convert Hounsfield Units (HU) to relative electron density (RED) for use in heterogeneity corrections during dose calculations. The purpose of this work is to illustrate the impact of using different model CT phantoms to determine HU to RED curves for treatment planning. A GAMMEX model 467 tissue characterization phantom and a CATPHAN model 500 multipurpose CT phantom were imaged using CT scanners in four different cancer centers and the HU to RED curves derived from each phantom were imported into an Eclipse 8.10 treatment planning system. Dose calculation were performed on a heterogeneity phantom and then compared to measurements. A comparison of isodose and DVH were performed by calculating 3D and IMRT plans onto identical CT datasets with different HU to RED curves to determine the clinical significance. Analysis based on effective atomic number of the phantom inserts was also performed. The HU to RED curves from the GAMMEX and CATPHAN phantoms were found to be reasonably self-consistent across the different CT scanners. However, observable differences for higher density materials were observed between the two phantom models. The differences for the larger HU values can be attributed to the effective atomic number of the materials. In kV range of a CT scanner x-ray spectrum, photon interactions are partially due to the photoelectric effect which has a larger dependence on atomic number than Compton scatter which depends most directly on electron density. The HU to RED curve is more dependent on the phantom model than CT scanner. The HU to RED curve from the GAMMEX phantom produced better agreement between Eclipse AAA calculations and measured dose distributions on a heterogeneity phantom than that from the CATPHAN. However, DVH and isodose data on patient plans show small differences for common treatment sites.