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Fig. 2. Sagittal localizer image of ACR phantom. Table 1. ACR Pulse Sequence Acquisition Parameters Image TR(ms) TE(ms) Matrix FOV(cm) Slice Thickness(mm) Gap(mm) NEX Sagittal Localizer 0200 20 256256 25 20 1 T1 Spin Echo 0500 20 256256 25 05 5 1 T2 Spin Echo 2000 20/80 256256 25 05 5 1 A Fig. 1. Oblique (A) and lateral (B) views of ACR phantom. B 48
A B Fig. 4. A. Slice 1 with resolution insert and hole array pairs indicated. B. Magnification view of high contrast resolution insert from slice 1 shows three sets of two arrays of holes. A B C Fig. 3. A. Positioning of length measurement on sagittal localizer (arrow). B. Slice 1 with diameter measurements illustrated (arrows). C. Position for diameter measurements on slice 5 (arrows). 49
Fig. 7. Image of slice 7 showing size and placement of the large ROI. Fig. 5. Magnified portion of slice 1 showing slice thickness signal ramps. The length measurements for the ramps are shown on the image (arrows). Fig. 6. Magnified portion of slice 1 illustrating slice position error. The arrows indicate the bar length difference measurement. Fig. 8. Image of slice 7 illustrating four small ROIs placement. 50
Fig. 9. Image of slice 11 showing the circles of low contrast objects. Table 2. MRI Instruments Symbol Site* Year MR Magnetic field Manufactured Manufacturer strength(tesla) A 3 2004 Ailab 0.3 B 3 1998 Toshiba 1.5 C 3 2000 Medison 1.0 D 2 1996 Fornor 0.35 E 2 1995 Picker 1.0 F 2 1997 Toshiba 1.5 G 1 1997 Picker 1.5 H 1 1994 Siemens 1.0 I 4 2003 GE 1.5 J 1 2002 GE 3.0 K 1 2003 GE 1.5 L 1 1997 Siemens 1.5 M 2 2001 Hitachi 0.3 N 3 2002 Isol 0.3 O 1 2000 GE 1.5 P 1 1995 GE 1.5 Q 1 1995 GE 1.5 R 4 1997 Medinus 1.5 S 4 1997 Elscint 0.5 T 2 2003 Hitachi 0.3 * 1: University hospitals, 2: General hospitals, 3: Hospitals, 4: Local clinics GE: General electric 51
Table 3. Results of MRI Phantom Test Test Slice Item Normal Range Mean Test Instruments Failed Instruments Failure Rate (No.) (No.) (%) Geometric Localizer End-to-End 145150 mm 146.95 19 2 10.5 accuracy ACR T1 #1 Top-to-Bottom 187193 mm 190.07 19 3 15.8 Left-to-Right 189.79 19 3 15.8 ACR T1 #5 Top-to-Bottom 189.70 19 4 21.1 Left-to-Right 189.68 19 2 10.5 High contrast ACR T1 #1 Left-to-Right 1.0 0.99 20 2 10.0 spatial resolution Top-to-Bottom 1.00 20 2 10.0 ACR T2 #1 Left-to-Right 1.01 20 3 15.0 Top-to-Bottom 1.01 20 3 15.0 Slice thickness ACR T1 #1 Slice thickness 4.35.7 mm 5.24 18 4 22.2 accuracy ACR T2 #1 Slice thickness 4.94 18 1 5.6 Slice position ACR T1 #1 Length difference 5 mm 2.99 19 2 10.5 accuracy ACR T1 #11 Length difference 2.17 19 1 5.3 ACR T2 #1 Length difference 2.80 18 3 16.7 ACR T2 #11 Length difference 2.09 18 3 16.7 Image intensity ACR T1 #7 PIU 87.5 94.74 17 1 5.9 uniformity ACR T2 #7 PIU 93.43 16 1 6.3 Percent signal ACR T1 #7 Ghosting ratio 0.025 0.016 19 3 15.8 ghosting Low contrast ACR T1 #8#11 9 27.03 12 1 8.3 object ACR T2 #8#11 25.53 12 1 8.3 detectability Site T1 #8#11 29.07 10 1 10.0 Site T2 #8#11 24.62 10 1 10.0 ACR: American College of Radiology PIU: Percent integral uniformity 52
Table 4. Inter-Observer Variation Test Slice Item Measurements Variation (p Value) Failure Variation (p Value) Geometric accuracy Localizer End-to-End 0.521 1.000 ACR T1 #1 Top-to-Bottom 0.999 0.844 Left-to-Right 0.850 1.000 ACR T1 #5 Top-to-Bottom 0.983 0.858 Left-to-Right 0.520 0.800 High contrast spatial resolution ACR T1 #1 Left-to-Right 0.908 1.000 Top-to-Bottom 1.000 0.409 ACR T2 #1 Left-to-Right 0.678 0.844 Top-to-Bottom 0.678 0.844 Slice thickness accuracy ACR T1 #1 Slice thickness 0.928 0.502 ACR T2 #1 Slice thickness 0.788 1.000 Slice position accuracy ACR T1 #1 Length difference 0.851 0.844 ACR T1 #11 Length difference 0.971 0.762 ACR T2 #1 Length difference 0.973 0.858 ACR T2 #11 Length difference 0.971 1.000 Image intensity uniformity ACR T1 #7 PIU 0.833 0.997 ACR T2 #7 PIU 0.762 0.580 Percent signal ghosting ACR T1 #7 Ghosting ratio 0.675 0.656 Low contrast object detectability ACR T1 #8-#11 0.913 1.000 ACR T2 #8-#11 0.767 1.000 53
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Usefulness of ACR MRI Phantom for Quality Assurance of MRI Instruments 1 Jung Whee Lee, M.D., Kook Jin Ahn, M.D., Seung Koo Lee, M.D. 2, Dong Gyu Na, M.D. 3, Chang Hyun Oh, Ph.D. 4, Yong Min Chang, Ph.D. 5, Tae Hwan Lim, M.D. 6 1 Department of Radiology, College of Medicine, The Catholic University of Korea 2 Department of Radiology, Yonsei University, College of Medicine 3 Department of Radiology, Seoul National University 4 Department of Electronics and Information Engineering, Korea University 5 Department of Diagnostic Radiology, College of Medicine, Kyungpook National University 6 Department of Diagnostic Radiology, Asan Medical Center, University of Ulsan Purpose: To examine whether the ACR phantom could be used in quality standards for magnetic resonance imaging (MRI) instruments in Korea. Materials and Methods: We conducted the phantom test using the ACR MRI phantom in 20 MRI instruments currently used in Korea. According to ACR criteria, we acquired the phantom images which were then assessed by the following seven tests: geometric accuracy, high spatial resolution, slice thickness accuracy, slice position accuracy, image intensity uniformity, percent signal ghosting, and low contrast object detectability. The phantom images were interpreted by three experienced radiologists according to ACR criteria. Then, we examined the failure rate of each test and evaluated the inter-observer variation in the measurements and test failure. Results: The failure rate of each test could be broken into the following components: geometric accuracy (11 21%), high contrast spatial resolution (1015%), slice thickness accuracy (622%), slice position accuracy (5 17%), image intensity uniformity (6%), percent signal ghosting (16%), and low contrast object detectability (8 10%). In this series, all the failure rates were less than 30%. In addition, no inter-observer variation was seen in the measurements and test failure. Conclusion: ACR MRI phantom promises to be established as the standard phantom for MRI instruments in Korea because of its objectivity in assessing the phantom images. Index words : Magnetic resonance (MR), quality assurance Phantoms Address reprint requests to : Kook Jin Ahn, M.D., Department of Radiology, College of Medicine, The Catholic University of Korea, St. Mary s Hospital, 62 Youido-dong, Youngdeungpo-gu, Seoul 150-713, Korea. Tel. 82-2-3779-2017 Fax. 82-2-3779-5288 E-mail: ahn-kj@catholic.ac.kr 55