Abdominal CT with Single-Energy Metal Artifact Reduction (SEMAR): Initial Experiences

Transcription

1 Abdominal CT with Single-Energy Metal Artifact Reduction (SEMAR): Initial Experiences Poster No.: C-0674 Congress: ECR 2014 Type: Scientific Exhibit Authors: K. Sofue 1, T. Yoshikawa 1, N. Negi 1, Y. Ohno 1, N. Sugihara 2, T. Murakami 1, H. Koyama 1, M. Nishio 1, K. Sugimura 1 ; 1 Kobe/JP, 2 Ohtawara/JP Keywords: DOI: Foreign bodies, Artifacts, Surgery, Observer performance, Embolisation, Image manipulation / Reconstruction, CT, Vascular, Pancreas, Liver /ecr2014/C-0674 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 16

2 Aims and objectives INTRODUCTION Metal streak artifacts caused by metallic materials in the body introduced during various interventions degrade image quality and limit diagnostic values of abdominal CT. These artifacts occur because filtered back projection (FBP) assumes that data from each detector is equally accurate, although X-rays can be highly attenuated and have a greater error when it passes through metallic devices. There have been many researches for reducing metallic artifacts on CT images, however, none have achieved widespread clinical use because of their insufficient effects or new artifacts. Recently, new reconstruction techniques for CT images using iterative method are available and can be used for reducing metallic artifacts. However, there is no previous report on this issue in the abdomen. PURPOSE The purpose of this study was to assess single-energy metal artifact reduction (SEMAR) technique for abdominal CT in patients with metallic materials after interventions. Methods and materials Patients Nineteen patients (14 men and 5 women; mean age: 68.0 years old), who had histories of surgical or radiological interventions for hepatocellular carcinoma, and had metallic materials in the abdomen (clip, n=10, or vascular embolization coil, n=9), and underwent tri-phasic abdominal CT, were retrospectively analyzed. This retrospective study was approved by the local ethics committee and obtaining informed consent from participant was waivered. Imaging Techniques Dynamic CT was performed by using area-detector CT system (Aquilion One, Toshiba Medical System, Ohtawara, Japan) with conditions of a 0.35 second gantry rotation Page 2 of 16

3 speed, a tube voltage of 120kVp, collimations of x0.5mm. The tube current was set using automatic exposure control. The scanning protocol consisted of unenhanced and two enhanced scans with delay times of 40 and 70 seconds after the injection. Injection dose was 600 mg iodine per kg of BW and duration was fixed at 25 seconds. All three scans were extended from the diaphragm to the bifurcation of the aorta using wide volume scan (step-and-shoot scan) protocol with several rotations of "volume scan mode". Single-Energy Metal Artifact Reduction (SEMAR) (Fig. 1) 1. Segment the metal parts in the FBP original image. 2. Forward project the metal image to find the metal trace in the projection data. 3. Interpolate the metal trace in the projection data using the nearby measurements. 4. Reconstruct the interpolated projection data. 5. Segment the interpolation corrected image to further exclude residual metal artifacts. Forward project the image. Blend the original projection data with the forward projection of the segmented image on the metal trace. 6. Reconstruct the projection data. 7. Blend the reconstructed image with the metal image to obtain the final image. Image Reconstruction Source images with 0.5-mm thickness were reconstructed with and without SEMAR. Then, transverse and coronal MPR images with 5-mm thickness were reconstructed. Image Analysis The quantitative analysis was conducted by one observer on the images reconstructed with and without SEMAR. ROIs were placed in normal liver and pancreatic areas near the metallic materials (distance<2cm) on transverse and coronal MPR and source images. ROIs were also placed in artifact-free area in the liver. Mean and standard deviation (SD) of HU were measured on each dynamic phase in all subjects. Page 3 of 16

4 The ROIs were at least 3 cm2, while vessels were avoided as much as possible. The ROIs were placed in the same locations both on images with and without SEMAR, and the values were statistically compared between images with and without SEMAR. For the qualitative analysis, two abdominal radiologists independently evaluated metallic artifacts in liver and pancreatic areas near the metallic materials as well as visualizations of common and proper hepatic arteries and superior mesenteric and portal veins using 5-point scales below. 1: non-diagnostic, marked artifacts with impaired image quality 2: poor image quality, major artifacts with reduced diagnostic confidence 3: moderate image quality, moderate artifacts with limited diagnostic confidence 4: good image quality, minor artifacts with sufficient for diagnosis 5: excellent image quality, no artifacts Interobserver Agreements & Statistical analysis Interobserver agreements were analyzed by means of Kappa statistics. Then, consensus was established, and the scores were statistically compared between images with and without SEMAR. Image quality scores were compared between the two images using Wilcoxon singedrank test. For the comparison of continuous variables between the two images, paired-t test was used. P values of less than 0.05 were considered to indicate statistical significance. Images for this section: Page 4 of 16

5 Fig. 1 Page 5 of 16

6 Results All mean HUs and SDs with SEMAR were significantly lower than those without SEMAR in the areas near metallic materials with a few exceptions in the pancreas, although no significant difference of them were found in artifact-free area (figs. 2-5). Metallic artifacts on each phase in both organs and all vascular visualizations were significantly improved with exceptions in the arteries, when applied SEMAR (figs. 6-8). Representative cases are shown in figs 9 & 10. The # values for the two observers were ranged from 0.72 to 0.80, indicating that substantial to almost perfect agreements were obtained. Images for this section: Fig. 2 Page 6 of 16

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15 Conclusion DISCUSSION With SEMAR, HU values in the liver and pancreas measured near the metals were significantly lower than those without SEMAR. SEMAR did not affect HU values in the Artifact-free normal liver. In a majority of the qualitative parameters, image quality near the metals was significantly improved with SEMAR. In small number of quantitative and qualitative parameters, no significant deference was found, however, there were trends toward lower HU values and higher image qualities with SEMAR. These indicate that SEMAR can reduce metallic artifacts, correct HU values, and improve image quality without any new image degradation in abdominal CT. Limitations The numbers of patients involved were relatively small. We only evaluated limited abdominal organs and sites. Effect of differences in material or exposed radiation dose is unknown. Further studies with larger population are needed to confirm our results. CONCLUSION SEMAR is useful for reducing artifacts in patients with metallic materials after interventions. Personal information References Kataoka ML, et al. A review of factors that affect artifact from metallic hardware on multi-row detector computed tomography. Curr Probl Diagn Radiol 2010;39: Page 15 of 16

16 Morsbach F, et al. Metal artefact reduction from dental hardware in carotid CT angiography using iterative reconstructions. Eur Radiol 2013;23: Lell MM, et al. Frequency split metal artefact reduction in pelvic computed tomography. Eur Radiol 2013 ;23: Morsbach F, et al. Reduction of Metal Artifacts from Hip Prostheses on CT Images of the Pelvis: Value of Iterative Reconstructions. Radiology 2013;268: Meyer E, et al. Frequency split metal artifact reduction (FSMAR) in computed tomography. Med Phys 2012;39: Zhang Y, et al. A hybrid metal artifact reduction algorithm for x-ray CT. Med Phys 2013;40: Dong J, et al. Metal-induced streak artifact reduction using iterative reconstruction algorithms in x-ray computed tomography image of the dentoalveolar region. Oral Surg Oral Med Oral Pathol Oral Radiol 2013;115:e Boas FE, Fleischmann D. Evaluation of two iterative techniques for reducing metal artifacts in computed tomography. Radiology 2011;259: Bal M, Spies L. Metal artifact reduction in CT using tissue-class modeling and adaptive prefiltering. Med Phys.2006;33: Page 16 of 16