Metal Artifact Reduction by Dual Energy CT

Transcription

1 Metal Artifact Reduction by Dual Energy CT Poster No.: C-0108 Congress: ECR 2011 Type: Authors: Keywords: DOI: Scientific Paper T. Johnson, F. Bamberg, A. Dierks, H.-C. Becker, M. F. Reiser; Munich/DE Physics in radiology, Musculoskeletal system, CT, Technical aspects /ecr2011/C-0108 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 8

2 Purpose Metal artifacts pose a significant clinical problem in CT. In the presence of metallic joint prostheses or osteosynthetic material, it would be desirable to evaluate the metal implant itself, the interface between implant and bone and the surrounding tissue. This is mostly impossible due to metal artefacts appearing as distorted contours of the implant and hypodense streaks in the surrondung tissue [1,2]. The extent of these artefacts depens mainly on the thickness and alloy of the metal [3]. High tube voltages [4] and imagebased postprocessing approaches [5,6] can help to reduce metal artefacts to some degree. With Dual Source CT, examinations can be performed in Dual Energy technique, allowing postprocessing for various purposes [7]. Physically, metal artefacts comprise two components, photon starvation and beam hardening. While the former represent a lack of data and are difficult to correct, the latter differ in Dual Energy acquisitions and may therefore be compensated by extrapolating the beam hardening properties to high photon energies [8]. The aim of this study was to assess the performance and diagnostic value of such a dual energy CT approach to reduce metal artifacts. Methods and Materials 31 patients were examined using a dual energy CT protocol with a filtered 140 kvp and a 100 kvp spectrum on a Dual Source CT (Somatom Definition Flash, Siemens). Tube current relation was 3:1 in favor of the high energy spectrum. 100 kvp at 90 mas eff /rot and Sn140kVp at 270 mas eff /rot were applied at 32 x 0.6 mm collimation in the trunk of the body, while 60 and 180 mas eff /rot, respectively, with 40 x 0.6 mm collimation were used in the extremities. Specific post-processing was applied to generate images simulating the energies of standard 120 and 140 kvp spectra as well as a filtered 140 kvp spectrum with mean photon energies of 64, 69 and 88 kev, respectively, and an optimized hard spectrum of kev. Image quality and diagnostic value were assessed on a five point rating scale and the density of the artifacts was measured in the different reconstructions. Results All CT examinations were acquired without any complications and images were postprocessed successfully. There were 22 implants in the trunk of the body (6 spine, 12 hip, 4 femur) and 9 in the extremities (2 humerus, 2 radius / ulna, 5 ankle). Image quality was rated superior to the standard image in 29/31 high energy reconstructions; the diagnostic value was rated superior in 27 patients. Image quality and diagnostic value scores improved from 3.5 to 2.1 and from 3.6 to 1.9, respectively (Fig. 1). In several Page 2 of 8

3 exams decisive diagnostic features such as loosening of implants or fractures were only discernible in the high energy reconstructions (Figs. 2-4). The density of the artifacts decreased from -882 to -341 HU. Images for this section: Fig. 1: Reconstructions of spine and tibia at 64, 69, 88, 105 kev and an optimal energy setting. Note that the thin layer of bone covering the left screw are only discernible in the two reconstructions at the highest energy. Similarly, the screw in the tibia is optimally depicted in the high energy image. Page 3 of 8

4 Fig. 2: Gamma nail in the left femur reconstructed at 109 kev. There are only minimal artefacts allowing to sufficiently delineate the fracture in the femoral neck. Page 4 of 8

5 Fig. 3: Multiplanar reformat at 115 kev in the plane of pedicle screws after dorsal spondylodesis. Note the loosening of both screws. Page 5 of 8

6 Fig. 4: Locked intramedullary nail in the left femur reconstructed at 105 kev. There are minimal residual artefacts, but the pseudarthrosis is clearly discernible. Page 6 of 8

7 Conclusion High energy reconstructions of Dual Energy CT datasets can significantly reduce metal artefacts and improve image quality and diagnostic value. The evaluation of metallic implants and adjacent bone or tissue is considerably enhanced. References 1 White LM, Buckwalter KA (2002) Technical considerations: CT and MR imaging in the postoperative orthopedic patient. Semin Musculoskelet Radiol, 6(1): Lee MJ, Kim S, Lee SA, et al. (2007) Overcoming artifacts from metallic orthopedic implants at high-field-strength MR imaging and multi-detector CT. Radiographics, 27(3): Haramati N, Staron RB, Mazel-Sperling K, et al. (1994) CT scans through metal scanning technique versus hardware composition. Comput Med Imaging Graph, 18(6): Barrett JF, Keat N (2004) Artifacts in CT: recognition and avoidance. Radiographics, 24(6): Yu L, Li H, Mueller J, et al. (2009) Metal artifact reduction from reformatted projections for hip prostheses in multislice helical computed tomography: techniques and initial clinical results. Invest Radiol, 44(11): Watzke O, Kalender WA (2004) A pragmatic approach to metal artifact reduction in CT: merging of metal artifact reduced images. Eur Radiol, 14(5): Johnson TR, Krauss B, Sedlmair M, et al. (2007) Material differentiation by dual energy CT: initial experience. Eur Radiol, 17(6): Krauss B, Schmidt B, Sedlmair M, Flohr T (2010) Evaluation of an image-based method to calculate monoenergetic images from dual energy images. Abstract Book, European Congress of Radiology European Society of Radiology, Vienna, pp S201 Personal Information Address correspondence to: Page 7 of 8

8 Thorsten R.C. Johnson, MD Associate Professor of Radiology University of Munich - Grosshadern Campus Marchioninistr. 15, Munich, Germany thorsten.johnson@med.uni-muenchen.de Page 8 of 8