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Artifacts on multi-detector computed tomography (MDCT) Poster No.: C-2106 Congress: ECR 2012 Type: Educational Exhibit Authors: J. Yoon , A. Eftekhari , D. J. Hou , L. Louis , P. Scheffler , S. 1 2 2 1 2 2 1 2 Nicolaou ; Montreal, QC/CA, Vancouver, BC/CA Keywords: Artifacts, Technical aspects, Physics, CT-High Resolution, CT, Cardiac, Anatomy, Abdomen DOI: 10.1594/ecr2012/C-2106 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. www.myESR.org Page 1 of 19 Learning objectives • • Familiarize themselves with a variety artifacts commonly occuring with several multidtector computed tomography (MDCT) imaging protocols. Introduce some of the techniques employed by our centre to reduce these artifacts. Background The wide application of multi-detector computed tomography (MDCT) has led to substantial improvements in imaging. However, MDCT scanners are still subject to artifacts. Awareness of these artifacts is of crucial importance to radiologists in order to differentiate them from true pathologies and prevent errors in clinical decision-making. Artifact is a systematic discrepancy between CT numbers of reconstructed images and true attenuation coefficients. CT images are more prone to artifacts than conventional radiographs due to the process of reconstruction from millions of independent detector measurements. Any error in individual measurement can result in errors in reconstructed images. These errors can lead to decrease in overall quality of images and/or decrease the accuracy of a given scan. The artifacts can manifest as streaking, shading, rings, and distortion. In this exhibit, examples of commonly encountered artifacts such as motion artifacts, metallic artifacts, and beam-hardening artifacts will be examined. Imaging findings OR Procedure details Motion artifacts Patient's motion can cause misregistration and therefore shading or streak artifacts. This can be caused by lack of cooperation or by involuntary motions such as breathing, and cardiac motion (Fig 1 - 2). The motion artifacts can mimic true pathologies at times, making image interpretation. In Fig 3, motion artifact is creating illusion of aortic dissection. In another set of studies, motion artifacts is mimicking renal lasceration in a trauma patient (Fig 4). Page 2 of 19 Solution for motion artifacts Motion artifacts can be reduced by reducing patient movements or by shortening the scan time. Strategies to reduce motion can be categorized into operator-dependent and builtin feature of the scanner. The operator can achieve immobilization of the patient through positioning aids or sedation. Also, when scanning the chest, patient can hold the breath to reduce respiratory motion. The built-in manufacturer solutions include overscan and underscan mode, software correction, cardiac gating, and ultra high pitch protocols. 1) Overscan and underscan mode: In overscan mode, X-ray gantry rotates more than standard 360° to acquire additional projections. These additional images are averaged in order to reduce motion artifacts. Conversely, motion artifacts can be reduced by having partial scans by virtue of reduced scan time at the expense of resolution. 2) Software correction: Iterative Reconstruction in Image Space (IRIS) by Siemens is an image reconstruction algorithm that has the potential to reduce image noise and artifact whilst increasing the sharpness. (Fig 5, 6) 3) Cardiac gating: In order to image the rapidly moving heart, electrocardiogram-gated (ECG-gated) acquisition has been developed. The porjections are acquired only at a certain phase of cardiac cycle (e.g. diastole) capturing relatively constant image of the heart. 4) Ultra high pitch protocols: FLASH by Siemens is a method of increasing scanning speed by utilizing both X-ray tubes in Dual Source CT scan. It allows dramatic reduction in scanning time. Motion artifacts in non-sedated pediatric phantom is reduced by using FLASH (Fig 7, 8). In this abdominal CT scan, by using ultra high pitch protocol, motion artifact was significantly reduced (Fig 9, 10). Metallic artifacts Metallic material causes severe streak artifacts in multiple ways. The density of the metal is beyond the normal range of computer resulting in incomplete attenuation profile. Other artifacts such as beam-hardening, partial volume and aliasing artifacts contribute to the severity of degradation in image quality. In the following figures, dental amalgam is causing streak artifacts making assessment of the carotid artery more difficult (Fig 11) Page 3 of 19 Solution for metallic artifacts When feasible, all the metallic objects on the patient should be removed prior to the scan. In case of irremovable objects such as dental fillings, surgical clips, prosthesis or orthopedic appliances, the X-ray tube gantry angulations can be set in order to exclude these objects. Alternatively, increasing kVp will allow some penetration, and using thinner sections will reduce artifacts due to partial volume. As for software corrections, interpolation techniques to replace over-range values in attenuation profiles can be used. However, the use of this technique may be limited in metal artifact reduction because loss of detail around metal-tissue interface. Beam-hardening artifacts An X-ray beam is made of photons with a range of energy levels. As these photons go through a material, photons of relatively lower energy becomes preferentially attenuated resulting in "Beam Hardening". This phenomenon can cause two types of artifacts, streaking and cupping artifacts. Cupping artifacts manifest as apparent hypo-attenuation in the centre of reconstructed images. Because at the centre, there is more material to go through, there is a higher degree of beam hardening therefore decreased level of attenuation. Streaking artifacts occur between two dense objects in an image. They occur because when the X-ray goes through two dense objects, higher degree of beam hardening occurs compared to when the X-ray goes through one dense object depending on the tube position. In this intubated trauma patient, his vertebral arteries are obscured by beam-hardening artifact due to the endotracheal (ET) tube (Fig 12). Dense bony structures of the shoulders can also cause streak artifacts through beamhardening. In this patient with cervical spine fractures, beam-hardening artifacts from ET tube and the shoulder are obscuring the fracture (Fig 13) Solution for beam-hardening artifacts A flat piece of metal filter can be used to pre-harden the beam to filter out the lower energy component of the X-ray beam before passing through the patient. Page 4 of 19 CT scanner can be calibrated using phantoms to compensate the cupping artifact. Beam hardening correction software can be applied to bony regions. Avoid scanning bony regions by altering position or tilting gantry. Select appropriate scan field of view so that the scanner uses the correct calibration data and other measures to decrease the artifacts. Images for this section: Page 5 of 19 Fig. 1: Sagittal section of the chest CT showing motion artifact due to cardiac motion. Page 6 of 19 Page 7 of 19 Fig. 2: Coronal reformat of an abdominal CT scan in which the quality is significantly degraded by motion artifact. Fig. 3: Axial image of chest CT showing apparent intimal flap and false lumen in the aorta. This finding was a motion artifact due to aortic pulsation. Page 8 of 19 Page 9 of 19 Fig. 4: Coronal reformat of abdominal CT in a trauma patient. Motion artifact mimics renal lasceration (green arrow). Fig. 5: Sagittal reformat of a head CT. Streak artifacts above the level of the foramen magnum can be appreciated. Page 10 of 19 Fig. 6: Same image as Fig 5. reconstructed using IRIS. In comparison, there is lesser degree of streak artifacts, and noise levels. Page 11 of 19 Fig. 7: Pediatric phantom, simulating a non-sedated baby. Coronal reformat of standard spiral CT and Volume-rendered technique (VRT) showing poor image quality due to motion artifacts. Page 12 of 19 Fig. 8: Pediatric phantom, simulating a non-sedated baby as in Fig. 7. Coronal reformat of Flash spiral CT and Volume-rendered technique (VRT) showing significantly reduced level of motion artifacts due to rapid scanning. Page 13 of 19 Fig. 9: Coronal reformats of abdominal CT scan. The quality of the scan is reduced by motion artifacts. Page 14 of 19 Fig. 10: Abdominal CT scan obtained with a ultra high pitch protocol showing lack of motion artifacts in the same patient. Page 15 of 19 Fig. 11: CT angiogram of the carotid artery, sagittal reformat. Due to the streak artifacts from dental amalgam. Page 16 of 19 Fig. 12: Axial cut of CT angiogram. Beam-hardening artifacts from endotracheal tube is obscuring the left vertebral artery. Page 17 of 19 Fig. 13: Sagittal reformat of cervical spine CT showing vertebral fracture in C7 (green arrow) is obscured by beam-hardening artifacts from ET tube and the shoulders. Page 18 of 19 Conclusion Despite the continuing advances in CT imaging technology, MDCT remains susceptible to a variety of artifacts. A fundamental understanding of these artifacts is of critical importance to radiologists in order to ensure accurate diagnoses and prevent missing potentially critical injuries. Some of the measures to reduce artifacts include: 1) Optimizing arm positioning, preferably above head. 2) Minimizing metallic objects outside patient. 3) Ultra high pitch - reduce motion artifacts (bowel). 4) Software correction - post-processing reconstruction to reduce noise at constant dose. Personal Information References 1. Barrett, J.F. and N. Keat, Artifacts in CT: recognition and avoidance. Radiographics : a review publication of the Radiological Society of North America, Inc, 2004. 24(6): p. 1679-91. 2. Choi, S.I., et al., Recent developments in wide-detector cardiac computed tomography. The international journal of cardiovascular imaging, 2009. 25 Suppl 1: p. 23-9. 3. Fishman, E.K., K.M. Horton, and P.T. Johnson, Multidetector CT and threedimensional CT angiography for suspected vascular trauma of the extremities. Radiographics : a review publication of the Radiological Society of North America, Inc, 2008. 28(3): p. 653-65; discussion 665-6. 4. Kataoka, M.L., et al., A review of factors that affect artifact from metallic hardware on multi-row detector computed tomography. Current problems in diagnostic radiology, 2010. 39(4): p. 125-36. Page 19 of 19