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1 Changes in CT number of high atomic number materials with field of view when using an extended CT number to electron density curve and a metal artifact reduction reconstruction algorithm Poster No.: R-0094 Congress: 2014 CSM Type: Scientific Exhibit Authors: V. Nelson, A. Gray; CAMPBELLTOWN/AU Keywords: Radiation physics, CT, Radiation therapy / Oncology, Radiotherapy techniques DOI: /ranzcr2014/R-0094 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 RANZCR/AIR/ACPSEM's endorsement, sponsorship or recommendation of the third party, information, product or service. RANZCR/AIR/ ACPSEM 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 RANZCR/AIR/ACPSEM 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,.doc documents and any other multimedia files are not available in the pdf version of presentations. Page 1 of 13

2 Aim In external beam radiation therapy treatment planning CT numbers are used to perform tissue heterogeneity corrections. The presence of metallic hip prostheses in patients with pelvic malignancies produces artifacts which alters the CT numbers. Philips Healthcare provides an image reconstruction algorithm, O-MAR, which reduces artifacts caused by metal objects in CT images (Fig.1). In addition to this a 16-bit reconstruction is also provided which extends the range of CT numbers. The purpose of this study was to investigate the impact of bit depth reconstruction and the OMAR correction on the CT number of high atomic number (Z) materials. Images for this section: Page 2 of 13

3 Fig. 1 Page 3 of 13

4 Methods and materials CT scans were performed with a Philips Brilliance 16 slice CT scanner on a tissue characterisation phantom (CT number to electron density (ED) phantom) (Fig.2). The center of the phantom was aligned with the center of the bore. All scans used the same scan range to ensure that the scans and their reconstructions would geometrically coincide in order to facilitate data analysis. Three sets of scans were made using our clinical scanning parameters (120kVp, 250 mas/slice, 2 mm slice width, standard filter 'C' for filtered back projection). In the first scan, the phantom was scanned in its homogenous configuration and the images were reconstructed without O-MAR (12-bit depth) to obtain the metal-free baseline CT number values. The second set of scans was performed with 3 a titanium (#=4.59 g/cm ) rod inserted in the phantom and images were reconstructed without and with O-MAR (Figs. 3(a) and 3(b)) using 12 and 16-bit depths. The third set of scans was performed with a steel insert (Figs. 3(c) and 3(d)) and the image reconstructions were performed as for the second scan. The scans were completed using two clinical CT scanning protocols, brain and pelvis. CT number to ED curves derived from this data were compared for a change in the CT number due to the O-MAR reconstruction and the 16-bit depth reconstruction. A planning study was also performed on a commercial radiotherapy treatment planning system using a water equivalent slab 2 phantom at 100 cm SSD with a 10x10 cm 6MV photon beam for a fixed MU delivery. A 1 cm diameter cylindrical region of interest (ROI) was created in the phantom at a depth of 5 cm. Doses were computed at five points beyond the ROI (Fig 4). In the first plan the ROI was assigned a density of 4.59, with a CT to ED file generated from 12-bit images with a titanium insert. For the planning system used in the study, 4.59 was the maximum density that could be assigned when the CT-ED curve based on 12-bit images was used. This plan was then recalculated using a CT to ED file generated from 16-bit images with titanium and steel inserts. In the second plan the ROI was assigned a density of 8.1 to represent steel and an extended CT to ED file, generated from 16-bit images with titanium and steel inserts, was used. The plan with the maximum allowable density of 4.59 and the 12-bit image CT-ED curve was used for comparison with this plan. Images for this section: Page 4 of 13

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8 Results The mean CT numbers from the 16-bit with and without O-MAR reconstructions were in good agreement with the baseline values from the 12-bit without O-MAR scans, with tissue equivalent materials being within 20 for most cases (Tables 1 and 2). For 12-bit images, the CT numbers for titanium and stainless steel saturated at 3052 Hounsfield units (HU) for O-MAR uncorrected images and 3049 for O-MAR corrected images. For 16bit depth images, the mean CT numbers of titanium and steel were higher than for the 12bit images, but the difference between the values for the with and without O-MAR scans were within one standard deviation of each other. The CT to ED curves produced from the 12-bit and 16-bit reconstruction were similar below electron densities of approximately 1.7, but differed significantly beyond an electron density of 2.0 (Figure 5). In the planning study it was found that the doses at all the measurement points beyond the high Z object location were equivalent when the ROI density was assigned to 4.59 for the 12 and 16bit CT-ED curves. However, when the ROI density was assigned to 8.1 the doses were lower when the extended CT to ED file was used when compared with doses calculated using the CT to ED file generated with 12-bit image reconstruction data (Table 3). This difference was more than 7% at 2 and 5 cm beyond the calculation point. However, there was no significant difference in doses at surface point and at 1 cm in front of ROI. Images for this section: Page 8 of 13

9 Table 1 Page 9 of 13

10 Table 2 Page 10 of 13

11 Fig. 5 Table 3 Page 11 of 13

12 Conclusion 16-bit O-MAR-corrected images were found to result in an increased range of maximum CT number allowable and as a result may provide a more accurate estimation of doses in tissues surrounding high Z materials. Provided the planning system is able to accept this image type, 16-bit images are recommended. Applying the O-MAR correction did not have a large impact on the CT numbers so the CT to ED curve for the 16 bit images can be applied for images with or without the correction applied. Personal information Vinod Nelson Medical Physicist MacArthur Cancer Therapy Centre Campbelltown NSW 2560 Alison Gray Medical Physics Specialist Liverpool & MacArthur Cancer Therapy Centre Campbelltown NSW 2560 References Glide-Hurst C, Chen D., Zhong H and Chetty I J. Changes realized from extended bit-depth and metal artifact reduction in CT. Med. Phys Volume 40 (No 6): p Spadea M F, Verburg J, Baroni G and Seco J. Dosimetric assessment of a novel metal artefact reduction method in CT images. J/ Appl/ Clin/ Med. Phys Volume 14 (No 1): p 299 Philips Health Care Metal Artifact Reduction for Orthopedic Implants (O-MAR). USA. Philips CT Clinical Science. Available from: clinical.netforum.healthcare.philips.com/us_en/explore/white-papers/ct/ Metal-Artifact-Reduction-for-Orthopedic-Implants-%28O-MAR%29. Page 12 of 13

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