Metal Artifact Reduction Reconstruction Research Articles

Image quality evaluation of a new high-performance ring-gantry cone-beam computed tomography imager

Objective. Newer cone-beam computed tomography (CBCT) imaging systems offer reconstruction algorithms including metal artifact reduction (MAR) and extended field-of-view (eFoV) techniques to improve image quality. In this study a new CBCT imager, the new Varian HyperSight CBCT, is compared to fan-beam CT and two CBCT imagers installed in a ring-gantry and C-arm linear accelerator, respectively. Approach. The image quality was assessed for HyperSight CBCT which uses new hardware, including a large-size flat panel detector, and improved image reconstruction algorithms. The decrease of metal artifacts was quantified (structural similarity index measure (SSIM) and root-mean-squared error (RMSE)) when applying MAR reconstruction and iterative reconstruction for a dental and spine region using a head-and-neck phantom. The geometry and CT number accuracy of the eFoV reconstruction was evaluated outside the standard field-of-view (sFoV) on a large 3D-printed chest phantom. Phantom size dependency of CT numbers was evaluated on three cylindrical phantoms of increasing diameter. Signal-to-noise and contrast-to-noise were quantified on an abdominal phantom. Main results. In phantoms with streak artifacts, MAR showed comparable results for HyperSight CBCT and CT, with MAR increasing the SSIM (0.97–0.99) and decreasing the RMSE (62–55 HU) compared to iterative reconstruction without MAR. In addition, HyperSight CBCT showed better geometrical accuracy in the eFoV than CT (Jaccard Conformity Index increase of 0.02–0.03). However, the CT number accuracy outside the sFoV was lower than for CT. The maximum CT number variation between different phantom sizes was lower for the HyperSight CBCT imager (∼100 HU) compared to the two other CBCT imagers (∼200 HU), but not fully comparable to CT (∼50 HU). Significance. This study demonstrated the imaging performance of the new HyperSight CBCT imager and the potential of applying this CBCT system in more advanced scenarios by comparing the quality against fan-beam CT.

Evaluation of a Novel Metal Artifact Reduction Algorithm for Reconstruction of Cone Beam CT (CBCT) Images Acquired on a New Imaging System

The presence of metal implants, dental work or prosthetics can produce image artifacts and distort Hounsfield units (HU) in patient CBCT images. The purpose of this work was to characterize a novel metal artifact reduction (MAR) algorithm for reconstruction of CBCT images obtained by a new imaging system and compare improvements in image quality with a conventional iterative reconstruction algorithm. Preliminary analyses show that the novel reconstruction algorithm implemented on the new imaging system produces image quality and HU comparable to standard fan-beam CT MATERIALS/METHODS: A 30 cm (length) x 30 cm (width) x 11 cm (depth) solid water phantom was fitted with a central cavity measuring 5.5 cm x 5.5 cm x 0.5 cm. Metal inserts of copper, aluminum, stainless steel, and titanium (5.5 cm x 5.5 cm x 1 - 5 mm in thickness) and sample dental implant materials (Amalgam, Au, Co-Cr, and Ti 6Al-Yv) were sequentially used to fill the cavity. CBCT images were acquired and reconstructed using the novel MAR and conventional iterative algorithms both on the on the new system. Images were also taken with the cavity filled with solid water for comparison. Eight image quality metrics were measured in six volumes of interest (VOIs) surrounding the cavity and compared to baseline VOI metrics for solid water-only images: HU mean, signal-to-noise ratio, contrast-to-noise ratio, artifact index, standard deviation, range, skewness, and kurtosis. The mean and standard deviation of HU within a shell-structure surrounding the cavity were calculated to evaluate the dependence of these values on distance from the center of the cavity. The artifact volume of the phantom (excluding the cavity) was estimated by calculating the 5th and 95th percentiles of HU in the CBCT of the phantom with the solid water insert, then summing the volume of all voxels with HU values outside this range. The eight image quality metrics extracted from the chosen VOIs were closer to baseline when using the novel MAR compared to conventional iterative reconstruction. It was also determined that mean HU returned to expected solid water background at a shorter distance from metal sample in the novel MAR images, and the standard deviation remained lower for the novel MAR images at all distances from the insert. These experimental outcomes were obtained for all high-density material inserts except for aluminum. The artifact volume was decreased using the novel MAR algorithm for all metal inserts excluding aluminum (p = 0.00013), and all dental implants (p = 0.00028). The novel MAR reconstruction algorithm shows a much lower sensitivity to metal artifacts, and significant improvement in image quality for all metal samples except for aluminum. These results justify the use of the novel MAR reconstruction for patients with metal implants and highlight advantages of the new CBCT imaging system for more accurate imaging and targeted treatment.

Evaluation of radiation treatment planning by computed tomography metal artifact reduction algorithm

Introduction: The objective of this study was to evaluate the usefulness of Computed Tomography(CT) images acquired through repeated subtraction reconstruction algorithms to reduce metal artifacts in CT Treatment Planning System(TPS).Materials and methods: Origin images of Gammex phantom and Rando phantom and non orthopedic metal artifact reduction(O-MAR) images were obtained after high density implantation. O-MAR applied images were also obtained. For evaluation of images, regions of interest(ROI) were set at five tissue rods and three points directly affected by artifacts in Gammex phantom. CT number and noise were compared and analyzed. To investigate dose change according to CT number change of the surrounding artifact, a three virtual cylindrical target volume was formed on Rando phantom CT images. The average dose was then compared and analyzed.Results: CT number difference according to the application of O-MAR showed significant difference among lung and bone rod and 3 ROI directly affected. Noise difference according to O-MAR application was significantly different in rod except for bone rod. In the treatment plan using Rando phantom, non-O-MAR and O-MAR images showed -4.3 ~ 1.9% and -0.4 ~ 2.3% dose differences, respectively.Conclusion: Applying a metal artifact reduction reconstruction algorithm can reduce image distortion due to high density implantation, improve image quality, and correct CT number.

Benefits of iterative metal artifact reduction and dual-energy CT towards mitigating artifact in the setting of total shoulder prostheses.

To determine the utility of iterative metal artifact reduction and 130keV dual-energy virtual monoenergetic images to improve bone and soft tissue visualization in CT scans affected by metal artifacts. Thirteen females and 6 males with a history of total shoulder prosthesis who underwent dual-energy shoulder CT were included. Four sets of images were reconstructed for each patient: (1) original polychromatic kV images reconstructed with weighted filtered back projection; (2) polychromatic kV images with iterative metal artifact reduction; (3) 130keV dual-energy virtual monoenergetic; (4) combined iterative metal artifact reduction and 130keV dual-energy virtual monoenergetic. Three readers blindly reviewed all image sets and graded the extent of artifact and image quality. Mean artifact score and median overall image quality score were better in 130keV dual-energy virtual monoenergetic with iterative metal artifact reduction compared with those in original polychromatic kV images (3.02 vs 4.28, P < 0.001 and 3.00 vs 4.33, P < 0.001, respectively). The median difference in CT numbers between regions affected by artifacts and normal regions was lowest in 130keV dual-energy virtual monoenergetic with iterative metal artifact reduction compared with that in original polychromatic kV images (72.28 vs 252.08, P < 0.001 for bony regions and 15.09 vs 324.38, P < 0.001 for soft tissue). In patients with metal artifacts due to shoulder replacement surgery, the use of dual-energy monoenergetic images and iterative metal artifact reduction reconstruction significantly improves both subjective and objective indicators of image quality.

Unintended Consequences: Review of New Artifacts Introduced by Iterative Reconstruction CT Metal Artifact Reduction in Spine Imaging.

Metal hardware serves as a common artifact source in spine CT imaging in the form of beam-hardening, photon starvation, and streaking. Postprocessing metal artifact reduction techniques have been developed to decrease these artifacts, which has been proved to improve visualization of soft-tissue structures and increase diagnostic confidence. However, metal artifact reduction reconstruction introduces its own novel artifacts that can mimic pathology.

Metal About the Hip and Artifact Reduction Techniques: From Basic Concepts to Advanced Imaging.

Promising outcomes of hip replacement interventions in this era of aging populations have led to higher demands for hip arthroplasty procedures. These require effective methods and techniques for the detection of postoperative outcomes and complications. Based on the presence or absence of radiographic findings, magnetic resonance imaging (MRI) and computed tomography (CT) may be required to detect and further characterize different causes of failing implants. Yet metal-related artifacts degrade image quality and pose significant challenges for adequate image quality. To mitigate such artifacts in MRI, a set of techniques, collectively known as metal artifact reduction sequence (MARS) MRI, were developed that optimize the framework of the conventional pulse sequences and exploit novel multispectral and multispatial imaging methods such as Slice Encoding for Metal Artifact Correction (SEMAC) and Multi-Acquisition Variable-Resonance Image Combination (MAVRIC). Metal-induced artifacts on CT can be effectively reduced with virtual monochromatic reconstruction of dual-energy CT data sets, metal artifact reduction reconstruction algorithms, and postprocessing image visualization techniques.

Investigation of combined kV/MV CBCT imaging with a high-DQE MV detector.

Combined kV-MV cone-beam tomography (CBCT) imaging has been proposed for two potentially important image-guided radiotherapy applications: (a) scan time reduction (STR) and (b) metal artifact reduction (MAR). However, the feasibility of these techniques has been in question due to the low detective quantum efficiencies (DQEs) of commercially available electronic portal imagers (EPIDs). The goal of the work was to test whether a prototype high DQE MV detector can be used to generate acceptable quality pretreatment CBCT images at acceptable dose levels. 6MV and 100 kVp projection data were acquired on a Truebeam system (Varian, Palo Alto, CA). The MV data were acquired using a prototype EPID containing two scintillators (a) a standard copper-gadolinium oxysulfide (Cu-GOS) screen having a zero-frequency DQE (DQE(0)) value of 1.4%, and (b) a prototype-focused cadmium tungstate (CWO) pixelated "strip" with a DQE(0) = 22%. The kV data were acquired using the standard onboard imager (DQE(0) = 70%). The angular spacing of the MV projections was 0.81° and the source output was 0.03 MU/projection while the kV projections were acquired with an angular spacing of 0.4° at 0.3 mAs/projection. Image quality was evaluated using (a) an 18-cm diameter electron density phantom (CIRS, Norfolk, VA) with nine contrast inserts and (b) the resolution section of the 20-cm diameter Catphan phantom (The Phantom Laboratory, Greenwich, NY). For the MAR studies, two opposing CIRS phantom inserts were replaced by steel rods. The reconstruction methods were based on combining MV and kV data into one sinogram. The MAR reconstruction utilized mostly kV raw data with only those rays corrupted by metal requiring replacement with MV data (total absorbed dose = 0.7 cGy). For the STR study, projections from partially overlapping 105°kV and MV acquisitions were combined to create a complete dataset that could have been acquired in 18 sec (absorbed dose = 2.5 cGy). MV-only (4.3 cGy) and kV-only (0.3 cGy) images were also reconstructed. The average signal-to-noise ratio (SNR) of the inserts in the MV-only CWO and GOS CIRS phantom images were 0.62× and 0.12× the SNR of the inserts in kV-only image, respectively. The limiting spatial resolutions in the MV-only GOS, MV-only CWO, and kV-only Catphan images were 3, 6, and 8 lp/cm, respectively. In the combined kV/CWO STR reconstruction, all contrast inserts were visible while only two were detectable in the kV/Cu-GOS image due to high levels of noise (average SNRs of kV/CWO and kV/GOS inserts were 0.97× and 0.18× the SNR of the kV-only inserts, respectively). In the kV-MV MAR reconstructions, streaking artifacts were substantially reduced with all inserts becoming clearly visible in the kV/CWO image while only two were visible in the kV/Cu-GOS image (average SNRs of the kV/CWO and kV/Cu-GOS CIRS with metal inserts were 0.94× and 0.35× the SNRs of the kV-only CIRS without metal inserts). We have demonstrated that a high-DQE MV detector can be applied to generating high-quality combined kV-MV images for SRT and MAR. Clinically acceptable doses were utilized.

Dedicated CT and MRI Techniques for the Evaluation of the Postoperative Knee.

Advances in surgical techniques, orthopaedic implant design, and higher demands for improved functionality of the aging population have resulted in a high prevalence of patients with metallic implants about the knee. Total knee arthroplasty, knee-replacing tumor prostheses, and osteosynthesis implants create various imaging artifacts and pose special challenges for the imaging evaluation with computed tomography (CT) and magnetic resonance imaging (MRI). CT artifacts can be effectively mitigated with metal artifact reduction reconstruction algorithms, dual-energy data acquisition with virtual monoenergetic extrapolation, and three-dimensional postprocessing techniques, such as volume and cinematic rendering. Artifacts related to metal implants on MRI can be reduced via optimization of the scan parameters and using advanced techniques such as multi-acquisition variable-resonance image combination, and slice encoding for metal artifact correction.

Clinical evaluation of volume of interest imaging combined with metal artifact reduction reconstruction techniques in coiling and stent assisted coiling during neurointerventional procedures

PurposeThree-dimensional (3D) scans with flat detector angiographic systems are widely used for neurointerventions by providing detailed vascular information. However, its associated radiation dose and streak metal artifact generated by implanted...

SU‐F‐J‐74: High Z Geometric Integrity and Beam Hardening Artifact Assessment Using a Retrospective Metal Artifact Reduction (MAR) Reconstruction Algorithm

Purpose:To test the efficacy of a retrospective metal artifact reduction (MAR) reconstruction algorithm for a commercial computed tomography (CT) scanner for radiation therapy purposes.Methods:High Z geometric integrity and artifact reduction analysis was performed with three phantoms using General Electric's (GE) Discovery CT. The three phantoms included: a Computerized Imaging Reference Systems (CIRS) electron density phantom (Model 062) with a 6.5 mm diameter titanium rod insert, a custom spine phantom using Synthes Spine hardware submerged in water, and a dental phantom with various high Z fillings submerged in water. Each phantom was reconstructed using MAR and compared against the original scan. Furthermore, each scenario was tested using standard and extended Hounsfield Unit (HU) ranges. High Z geometric integrity was performed using the CIRS phantom, while the artifact reduction was performed using all three phantoms.Results:Geometric integrity of the 6.5 mm diameter rod was slightly overestimated for non‐MAR scans for both standard and extended HU. With MAR reconstruction, the rod was underestimated for both standard and extended HU. For artifact reduction, the mean and standard deviation was compared in a volume of interest (VOI) in the surrounding material (water and water equivalent material, ∼0HU). Overall, the mean value of the VOI was closer to 0 HU for the MAR reconstruction compared to the non‐MAR scan for most phantoms. Additionally, the standard deviations for all phantoms were greatly reduced using MAR reconstruction.Conclusion:GE's MAR reconstruction algorithm improves image quality with the presence of high Z material with minimal degradation of its geometric integrity. High Z delineation can be carried out with proper contouring techniques. The effects of beam hardening artifacts are greatly reduced with MAR reconstruction. Tissue corrections due to these artifacts can be eliminated for simple high Z geometries and greatly reduced for more complex geometries.

SU‐F‐J‐175: Evaluation of Metal Artifact Reduction Algorithms in Computed Tomography and Their Application to Radiation Therapy Treatment Planning

Purpose:High‐Z (metal) implants in CT scans cause significant streak‐like artifacts in the reconstructed dataset. This results in both inaccurate CT Hounsfield units for the tissue as well as obscuration of the target and organs at risk (OARs) for radiation therapy planning. Herein we analyze two metal artifact reduction algorithms: GE's Smart MAR and a Metal Deletion Technique (MDT) for geometric and Hounsfield Unit (HU) accuracy.Methods:A CT‐to‐electron density phantom, with multiple inserts of various densities and a custom Cerrobend insert (Zeff=76.8), is utilized in this continuing study. The phantom is scanned without metal (baseline) and again with the metal insert. Using one set of projection data, reconstructed CT volumes are created with filtered‐back‐projection (FBP) and the MAR and the MDT algorithms. Regions‐of‐Interest (ROIs) are evaluated for each insert for HU accuracy; the metal insert's Full‐Width‐Half‐Maximum (FWHM) is used to evaluate the geometric accuracy. Streak severity is quantified with an HU error metric over the phantom volume.Results:The original FBP reconstruction has a Root‐Mean‐Square‐Error (RMSE) of 57.55 HU (STD=29.19, range=−145.8 to +79.2) compared to baseline. The MAR reconstruction has a RMSE of 20.98 HU (STD=13.92, range=−18.3 to +61.7). The MDT reconstruction has a RMSE of 10.05 HU (STD=10.5, range=−14.8 to +18.6). FWHM for baseline=162.05; FBP=161.84 (−0.13%); MAR=162.36 (+0.19%); MDT=162.99 (+0.58%). Streak severity metric for FBP=19.73 (22.659% bad pixels); MAR=8.743 (9.538% bad); MDT=4.899 (5.303% bad).Conclusion:Image quality, in terms of HU accuracy, in the presence of high‐Z metal objects in CT scans is improved by metal artifact reduction reconstruction algorithms. The MDT algorithm had the highest HU value accuracy (RMSE=10.05 HU) and best streak severity metric, but scored the worst in terms of geometric accuracy. Qualitatively, the MAR and MDT algorithms increased detectability of inserts, although there is a loss of in‐plane resolution near the metallic insert.

Moving metal artifact reduction in cone-beam CT scans with implanted cylindrical gold markers.

Implanted gold markers for image-guided radiotherapy lead to streaking artifacts in cone-beam CT (CBCT) scans. Several methods for metal artifact reduction (MAR) have been published, but they all fail in scans with large motion. Here the authors propose and investigate a method for automatic moving metal artifact reduction (MMAR) in CBCT scans with cylindrical gold markers. The MMAR CBCT reconstruction method has six steps. (1) Automatic segmentation of the cylindrical markers in the CBCT projections. (2) Removal of each marker in the projections by replacing the pixels within a masked area with interpolated values. (3) Reconstruction of a marker-free CBCT volume from the manipulated CBCT projections. (4) Reconstruction of a standard CBCT volume with metal artifacts from the original CBCT projections. (5) Estimation of the three-dimensional (3D) trajectory during CBCT acquisition for each marker based on the segmentation in Step 1, and identification of the smallest ellipsoidal volume that encompasses 95% of the visited 3D positions. (6) Generation of the final MMAR CBCT reconstruction from the marker-free CBCT volume of Step 3 by replacing the voxels in the 95% ellipsoid with the corresponding voxels of the standard CBCT volume of Step 4. The MMAR reconstruction was performed retrospectively using a half-fan CBCT scan for 29 consecutive stereotactic body radiation therapy patients with 2-3 gold markers implanted in the liver. The metal artifacts of the MMAR reconstructions were scored and compared with a standard MAR reconstruction by counting the streaks and by calculating the standard deviation of the Hounsfield units in a region around each marker. The markers were found with the same autosegmentation settings in 27 CBCT scans, while two scans needed slightly changed settings to find all markers automatically in Step 1 of the MMAR method. MMAR resulted in 15 scans with no streaking artifacts, 11 scans with 1-4 streaks, and 3 scans with severe streaking artifacts. The corresponding numbers for MAR were 8 (no streaks), 1 (1-4 streaks), and 20 (severe streaking artifacts). The MMAR method was superior to MAR in scans with more than 8 mm 3D marker motion and comparable to MAR for scans with less than 8 mm motion. In addition, the MMAR method was tested on a 4D CBCT reconstruction for which it worked equally well as for the 3D case. The markers in the 4D case had very low motion blur. An automatic method for MMAR in CBCT scans was proposed and shown to effectively remove almost all streaking artifacts in a large set of clinical CBCT scans with implanted gold markers in the liver. Residual streaking artifacts observed in three CBCT scans may be removed with better marker segmentation.

Metal Artifact Reduction Software Used With Abdominopelvic Dual-Energy CT of Patients With Metal Hip Prostheses: Assessment of Image Quality and Clinical Feasibility

The objective of our study was to determine the feasibility of using Metal Artifact Reduction (MAR) software for abdominopelvic dual-energy CT in patients with metal hip prostheses. This retrospective study included 33 patients (male-female ratio, 19:14; mean age, 63.7 years) who received total hip replacements and 20 patients who did not have metal prostheses as the control group. All of the patients underwent dual-energy CT. The quality of the images reconstructed using the MAR algorithm and of those reconstructed using the standard reconstruction was evaluated in terms of the visibility of the bladder wall, pelvic sidewall, rectal shelf, and bone-prosthesis interface and the overall diagnostic image quality with a 4-point scale. The mean and SD attenuation values in Hounsfield units were measured in the bladder, pelvic sidewall, and rectal shelf. For validation of the MAR interpolation algorithm, pelvis phantoms with small bladder "lesions" and metal hip prostheses were made, and images of the phantoms both with and without MAR reconstruction were evaluated. Image quality was significantly better with MAR reconstruction than without at all sites except the rectal shelf, where the image quality either had not changed or had worsened after MAR reconstruction. The mean attenuation value was changed after MAR reconstruction to its original expected value at the pelvic sidewall (p < 0.001) and inside the bladder (p < 0.001). The SD attenuation value was significantly decreased after MAR reconstruction at the pelvic sidewall (p = 0.019) but did not show significant differences at the bladder (p = 0.173) or rectal shelf (p = 0.478). In the phantom study, all lesions obscured by metal artifacts on the standard reconstruction images were visualized after MAR reconstruction; however, new artifacts had developed in other parts of the MAR reconstruction images. The use of MAR software with dual-energy CT decreases metal artifacts and increases diagnostic confidence in the assessment of the pelvic cavity but also introduces new artifacts that can obscure pelvic structures.

SU-E-J-218: Evaluation of CT Images Created Using a New Metal Artifact Reduction Reconstruction Algorithm for Radiation Therapy Treatment Planning

Purpose: Metal in patients creates streak artifacts in CT images. When used for radiation treatment planning, these artifacts make it difficult to identify internal structures and affects radiation dose calculations, which depend on HU numbers for inhomogeneity correction. This work quantitatively evaluates a new metal artifact reduction (MAR) CT image reconstruction algorithm (GE Healthcare CT-0521-04.13-EN-US DOC1381483) when metal is present. Methods: A Gammex Model 467 Tissue Characterization phantom was used. CT images were taken of this phantom on a GE Optima580RT CT scanner with and without steel and titanium plugs using both the standard and MAR reconstruction algorithms. HU values were compared pixel by pixel to determine if the MAR algorithm altered the HUs of normal tissues when no metal is present, and to evaluate the effect of using the MAR algorithm when metal is present. Also, CT images of patients with internal metal objects using standard and MAR reconstruction algorithms were compared. Results: Comparing the standard and MAR reconstructed images of the phantom without metal, 95.0% of pixels were within ±35 HU and 98.0% of pixels were within ±85 HU. Also, the MAR reconstruction algorithm showed significant improvement in maintaining HUs of non-metallic regions in the images taken ofmore » the phantom with metal. HU Gamma analysis (2%, 2mm) of metal vs. non-metal phantom imaging using standard reconstruction resulted in an 84.8% pass rate compared to 96.6% for the MAR reconstructed images. CT images of patients with metal show significant artifact reduction when reconstructed with the MAR algorithm. Conclusion: CT imaging using the MAR reconstruction algorithm provides improved visualization of internal anatomy and more accurate HUs when metal is present compared to the standard reconstruction algorithm. MAR reconstructed CT images provide qualitative and quantitative improvements over current reconstruction algorithms, thus improving radiation treatment planning accuracy.« less

O-031 Metal artifact reduction for C-arm flat panel CT: preliminary results in neurointerventional applications

PurposeWith the advent of latest generation flat panel detectors, C-arm angiographic systems have the ability to provide high-resolution CT-like images of brain parenchyma, vasculature and deployed devices while the patient...

Metal artifact reduction image reconstruction algorithm for CT of implanted metal orthopedic devices: a work in progress

Despite recent advances in CT technology, metal orthopedic implants continue to cause significant artifacts on many CT exams, often obscuring diagnostic information. We performed this prospective study to evaluate the effectiveness of an experimental metal artifact reduction (MAR) image reconstruction program for CT. We examined image quality on CT exams performed in patients with hip arthroplasties as well as other types of implanted metal orthopedic devices. The exam raw data were reconstructed using two different methods, the standard filtered backprojection (FBP) program and the MAR program. Images were evaluated for quality of the metal-cement-bone interfaces, trabeculae < or = 1 cm from the metal, trabeculae 5 cm apart from the metal, streak artifact, and overall soft tissue detail. The Wilcoxon Rank Sum test was used to compare the image scores from the large and small prostheses. Interobserver agreement was calculated. When all patients were grouped together, the MAR images showed mild to moderate improvement over the FBP images. However, when the cases were divided by implant size, the MAR images consistently received higher image quality scores than the FBP images for large metal implants (total hip prostheses). For small metal implants (screws, plates, staples), conversely, the MAR images received lower image quality scores than the FBP images due to blurring artifact. The difference of image scores for the large and small implants was significant (p = 0.002). Interobserver agreement was found to be high for all measures of image quality (k > 0.9). The experimental MAR reconstruction algorithm significantly improved CT image quality for patients with large metal implants. However, the MAR algorithm introduced blurring artifact that reduced image quality with small metal implants.