DOI: 10.1148/rg.273065087
Overcoming Artifacts from Metallic Orthopedic Implants at High-Field-Strength MR Imaging and Multi-detector CT1
Mi-Jung Lee, MD,
Sungjun Kim, MD,
Sung-Ah Lee, MD,
Ho-Taek Song, MD,
Yong-Min Huh, MD,
Dae-Hong Kim, PhD,
Seung Hwan Han, MD, and
Jin-Suck Suh, MD
1 From the Department of Diagnostic Radiology and Research Institute of Radiological Science (M.J.L., S.K., H.T.S., Y.M.H., J.S.S.) and Brain Korea 21 Project for Medical Science (J.S.S.), Yonsei University College of Medicine, 134 Sincheon-Dong Seodaemun-Gu, Seoul 120-752, Republic of Korea; Department of Diagnostic Radiology, Hanyang University College of Medicine, Kuri City, Kyunggi-do, Republic of Korea (S.K.); Department of Diagnostic Radiology, Seoul Medical Center, Seoul, Republic of Korea (S.A.L.); Korea Basic Science Institute, Daejeon, Republic of Korea (D.H.K.); and Department of Orthopedic Surgery, Ajou University College of Medicine, Suwon, Republic of Korea (S.H.H.). Presented as an education exhibit at the 2005 RSNA Annual Meeting. Received May 3, 2006; revision requested August 22 and received October 5; accepted November 13. All authors have no financial relationships to disclose.
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Figure 1a. Schematics show the mechanism that causes metal-related artifacts at MR imaging. (a) The spins in the main magnetic field predictably rotate at the same frequency. When the frequency encoding gradient is applied, the Larmor frequencies of the spins change along the gradient direction. The spins at locations of higher magnetic field strength (upper part of the triangle) rotate at higher frequencies. When a metallic object is placed in the region of interest (arrow inside the circle), the spins near the object rotate with higher frequency than those that are distant from it. (b) The region of interest near the metallic object is encoded as if it were at a higher gradient location (upper circle) than it actually is. The signal around the region of interest is summed at a higher gradient location (+) and subtracted at a lower one (–). The result is geometric distortion of the object.
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Figure 1b. Schematics show the mechanism that causes metal-related artifacts at MR imaging. (a) The spins in the main magnetic field predictably rotate at the same frequency. When the frequency encoding gradient is applied, the Larmor frequencies of the spins change along the gradient direction. The spins at locations of higher magnetic field strength (upper part of the triangle) rotate at higher frequencies. When a metallic object is placed in the region of interest (arrow inside the circle), the spins near the object rotate with higher frequency than those that are distant from it. (b) The region of interest near the metallic object is encoded as if it were at a higher gradient location (upper circle) than it actually is. The signal around the region of interest is summed at a higher gradient location (+) and subtracted at a lower one (–). The result is geometric distortion of the object.
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Figure 2a. Comparison of metal-related artifacts according to the physical composition and size of the metallic object and the MR pulse sequence used. (a) Schema shows the relative positions of a titanium-alloy screw (diameter, 4.5 mm) and two stainless steel screws (diameters, 3.5 and 4.5 mm) within the phantom. (b, c) Axial MR images of the phantom, obtained with a gradient-recalled echo (GRE) sequence (repetition time msec/echo time msec, 500/14; flip angle, 80°) (b) and a fast spin-echo (SE) sequence (500/14; echo train length, 16) (c), show that the titanium alloy screws produced smaller artifacts than did the stainless steel screws, regardless of the sequence used. The artifacts are smaller on c than on b. The arrow in b shows the direction of the frequency encoding gradient.
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Figure 2b. Comparison of metal-related artifacts according to the physical composition and size of the metallic object and the MR pulse sequence used. (a) Schema shows the relative positions of a titanium-alloy screw (diameter, 4.5 mm) and two stainless steel screws (diameters, 3.5 and 4.5 mm) within the phantom. (b, c) Axial MR images of the phantom, obtained with a gradient-recalled echo (GRE) sequence (repetition time msec/echo time msec, 500/14; flip angle, 80°) (b) and a fast spin-echo (SE) sequence (500/14; echo train length, 16) (c), show that the titanium alloy screws produced smaller artifacts than did the stainless steel screws, regardless of the sequence used. The artifacts are smaller on c than on b. The arrow in b shows the direction of the frequency encoding gradient.
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Figure 2c. Comparison of metal-related artifacts according to the physical composition and size of the metallic object and the MR pulse sequence used. (a) Schema shows the relative positions of a titanium-alloy screw (diameter, 4.5 mm) and two stainless steel screws (diameters, 3.5 and 4.5 mm) within the phantom. (b, c) Axial MR images of the phantom, obtained with a gradient-recalled echo (GRE) sequence (repetition time msec/echo time msec, 500/14; flip angle, 80°) (b) and a fast spin-echo (SE) sequence (500/14; echo train length, 16) (c), show that the titanium alloy screws produced smaller artifacts than did the stainless steel screws, regardless of the sequence used. The artifacts are smaller on c than on b. The arrow in b shows the direction of the frequency encoding gradient.
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Figure 3a. Effect of hardware orientation on metal-related artifacts at fast SE MR imaging. (a) Schematics demonstrate the orientation of a screw in the vertical position (0°) and at successive angles of 30°, 60°, and 90°. (b) Coronal T1-weighted fast SE images (500/17; echo train length, three), obtained in the phantom with sequential changes in the angular orientation of the screw (as specified in a), show increases in artifact size that are proportional to the increase in the angle between the long axis of the screw and the direction of the main magnetic field.
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Figure 3b. Effect of hardware orientation on metal-related artifacts at fast SE MR imaging. (a) Schematics demonstrate the orientation of a screw in the vertical position (0°) and at successive angles of 30°, 60°, and 90°. (b) Coronal T1-weighted fast SE images (500/17; echo train length, three), obtained in the phantom with sequential changes in the angular orientation of the screw (as specified in a), show increases in artifact size that are proportional to the increase in the angle between the long axis of the screw and the direction of the main magnetic field.
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Figure 4a. Effect of fat signal suppression method on metal-related artifacts at MR imaging. A comparison of images obtained with a frequency-selective fat saturation sequence (3000/62) (a) and with a STIR sequence (repetition time msec/echo time msec/inversion time msec, 3000/33/210) (b) shows a smaller artifact on the STIR image.
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Figure 4b. Effect of fat signal suppression method on metal-related artifacts at MR imaging. A comparison of images obtained with a frequency-selective fat saturation sequence (3000/62) (a) and with a STIR sequence (repetition time msec/echo time msec/inversion time msec, 3000/33/210) (b) shows a smaller artifact on the STIR image.
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Figure 5a. Effect of magnetic field strength on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained at 1.5 T (a) and at 3.0 T (b) shows a smaller artifact on a, the image obtained with lower magnetic field strength.
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Figure 5b. Effect of magnetic field strength on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained at 1.5 T (a) and at 3.0 T (b) shows a smaller artifact on a, the image obtained with lower magnetic field strength.
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Figure 6a. Effect of field of view on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained on a 3.0-T system with a field of view of 14 cm (a) and 24 cm (b) shows a smaller artifact on a, the image obtained with a smaller field of view.
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Figure 6b. Effect of field of view on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained on a 3.0-T system with a field of view of 14 cm (a) and 24 cm (b) shows a smaller artifact on a, the image obtained with a smaller field of view.
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Figure 7a. Effect of image matrix on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained with matrix dimensions of 128 x 126 (a), 512 x 508 (b), and 1024 x 1016 (c) shows a smaller artifact on b and c, the images obtained with a high-resolution matrix.
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Figure 7b. Effect of image matrix on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained with matrix dimensions of 128 x 126 (a), 512 x 508 (b), and 1024 x 1016 (c) shows a smaller artifact on b and c, the images obtained with a high-resolution matrix.
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Figure 7c. Effect of image matrix on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained with matrix dimensions of 128 x 126 (a), 512 x 508 (b), and 1024 x 1016 (c) shows a smaller artifact on b and c, the images obtained with a high-resolution matrix.
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Figure 8a. Effect of section thickness on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained with section thicknesses of 1 mm (a), 4 mm (b), and 7 mm (c) shows reduced artifacts on a and b (a shows the least artifact), the images obtained with thinner sections.
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Figure 8b. Effect of section thickness on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained with section thicknesses of 1 mm (a), 4 mm (b), and 7 mm (c) shows reduced artifacts on a and b (a shows the least artifact), the images obtained with thinner sections.
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Figure 8c. Effect of section thickness on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90; echo train length, 10) obtained with section thicknesses of 1 mm (a), 4 mm (b), and 7 mm (c) shows reduced artifacts on a and b (a shows the least artifact), the images obtained with thinner sections.
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Figure 9a. Effect of echo train length on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90) obtained with echo train lengths of four (a), 10 (b), and 20 (c) shows reduced artifacts on b and c, the images obtained with increased echo train length.
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Figure 9b. Effect of echo train length on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90) obtained with echo train lengths of four (a), 10 (b), and 20 (c) shows reduced artifacts on b and c, the images obtained with increased echo train length.
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Figure 9c. Effect of echo train length on metal-related artifacts. A comparison of T2-weighted fast SE images (3600/90) obtained with echo train lengths of four (a), 10 (b), and 20 (c) shows reduced artifacts on b and c, the images obtained with increased echo train length.
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Figure 10a. Effect of metal composition on metal-related artifacts at multidetector CT. (a) Scout view of an ex vivo model of a pig femur with a 4.5-mm titanium alloy screw (arrow) and a 4.5-mm stainless steel screw (arrowhead). (b) CT image obtained through the short axis of the screws, with 140 kVp, 120 mAs, 0.6-mm collimation, and 1.0-mm reconstructed section thickness, shows more severe streak artifacts from the stainless steel screw than from the titanium alloy screw.
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Figure 10b. Effect of metal composition on metal-related artifacts at multidetector CT. (a) Scout view of an ex vivo model of a pig femur with a 4.5-mm titanium alloy screw (arrow) and a 4.5-mm stainless steel screw (arrowhead). (b) CT image obtained through the short axis of the screws, with 140 kVp, 120 mAs, 0.6-mm collimation, and 1.0-mm reconstructed section thickness, shows more severe streak artifacts from the stainless steel screw than from the titanium alloy screw.
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Figure 11a. CT images obtained through the short axis of a stainless steel screw in an agarose gel phantom. Image obtained with optimal parameters (140 kVp, 300 mAs, 0.6-mm collimation, 3.0-mm reconstructed section thickness, a low kernel value [B30f], and the extended CT scale) (a) shows a less pronounced artifact than that in b, which was obtained with less advantageous parameters (80 kVp, 80 mAs, 1.2-mm collimation, 1.5-mm acquired section thickness, 1.5-mm reconstructed section thickness, and a high kernel value [B70f], without the extended CT scale).
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Figure 11b. CT images obtained through the short axis of a stainless steel screw in an agarose gel phantom. Image obtained with optimal parameters (140 kVp, 300 mAs, 0.6-mm collimation, 3.0-mm reconstructed section thickness, a low kernel value [B30f], and the extended CT scale) (a) shows a less pronounced artifact than that in b, which was obtained with less advantageous parameters (80 kVp, 80 mAs, 1.2-mm collimation, 1.5-mm acquired section thickness, 1.5-mm reconstructed section thickness, and a high kernel value [B70f], without the extended CT scale).
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Figure 12a. Effect of peak voltage on metal-related artifacts. A comparison of CT images obtained through the short axis of the screws in the pig femur model, with 140 kVp (a) and 80 kVp (b) and with other parameters the same (120 mAs, 0.6-mm collimation, and 1.0-mm reconstructed section thickness), shows a less obtrusive artifact on a, the image obtained with a higher peak voltage.
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Figure 12b. Effect of peak voltage on metal-related artifacts. A comparison of CT images obtained through the short axis of the screws in the pig femur model, with 140 kVp (a) and 80 kVp (b) and with other parameters the same (120 mAs, 0.6-mm collimation, and 1.0-mm reconstructed section thickness), shows a less obtrusive artifact on a, the image obtained with a higher peak voltage.
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Figure 13a. Effect of tube charge on metal-related artifacts. CT images obtained through the short axis of the screws in the pig femur model, with tube charge of 80 mAs (a) and 120 mAs (b) and with other parameters the same (140 kVp, 0.6-mm collimation, and 1.0-mm reconstructed section thickness), show similar streak artifacts.
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Figure 13b. Effect of tube charge on metal-related artifacts. CT images obtained through the short axis of the screws in the pig femur model, with tube charge of 80 mAs (a) and 120 mAs (b) and with other parameters the same (140 kVp, 0.6-mm collimation, and 1.0-mm reconstructed section thickness), show similar streak artifacts.
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Figure 14a. Effect of collimation on metal-related artifacts. A comparison of CT images obtained through the short axis of the screws in the pig femur model, with collimation of 0.6 mm (a) and 1.2 mm (b) and with other parameters the same (140 kVp, 120 mAs, and 1.5-mm reconstructed section thickness), shows a less obtrusive artifact on a, the image obtained with narrow collimation.
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Figure 14b. Effect of collimation on metal-related artifacts. A comparison of CT images obtained through the short axis of the screws in the pig femur model, with collimation of 0.6 mm (a) and 1.2 mm (b) and with other parameters the same (140 kVp, 120 mAs, and 1.5-mm reconstructed section thickness), shows a less obtrusive artifact on a, the image obtained with narrow collimation.
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Figure 15a. Effect of reconstruction algorithm (kernel) on metal-related artifacts. A comparison of CT images of the short axis of the screws in the pig femur model, obtained with a standard reconstruction algorithm (B30f, medium smooth kernel) (a) and a higher kernel value (B70f, very sharp kernel) (b) and with other parameters the same (140 kVp, 120 mAs, 0.6-mm collimation, and 1.0-mm reconstructed section thickness), shows less severe artifacts but also a loss of sharpness in a.
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Figure 15b. Effect of reconstruction algorithm (kernel) on metal-related artifacts. A comparison of CT images of the short axis of the screws in the pig femur model, obtained with a standard reconstruction algorithm (B30f, medium smooth kernel) (a) and a higher kernel value (B70f, very sharp kernel) (b) and with other parameters the same (140 kVp, 120 mAs, 0.6-mm collimation, and 1.0-mm reconstructed section thickness), shows less severe artifacts but also a loss of sharpness in a.
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Copyright © 2007 by the Radiological Society of North America.
