Magic angle artifact in MRI of the patellar ligament: preliminary comparison between conventional and weightbearing

Transcription

1 Magic angle artifact in MRI of the patellar ligament: preliminary comparison between conventional and weightbearing MRI Poster No.: C-1017 Congress: ECR 2012 Type: Scientific Exhibit Authors: R. Piccazzo 1, M. Revelli 1, E. Aleo 2, C. Puppo 2, A. Del Prato 1, V. Keywords: DOI: Prono 2, A. Muda 1, G. Garlaschi 1 ; 1 Genova/IT, 2 Genoa/IT Musculoskeletal system, Anatomy, MR physics, MR, Diagnostic procedure, Artifacts /ecr2012/C-1017 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 7

2 Purpose The "Magic Angle" is an artifact in Magnetic Resonance seen as increased signal, in Short Echo Time sequences (as SE T1-weighted or Proton Density-weighted) [1] and with a high flip angle [2], in tissues with well-ordered collagen fibers in one direction. This artifact occurs when the angle formed by such fibers with the magnetic field is equal to 55 [3,4,5,6]. This interaction results in an increased signal (that is not to be confused with a pathological signal alteration [7]) in anatomical structures, such as tendons, which usually show no MRI-signal in physiological conditions. The effects of the "Magic Angle" can be reduced by using sequences with higher echo time or by changing the orientation of the examined structure [8]. Our study focused on the evaluation of the MRI signal in the patellar ligament (especially in the proximal insertion), both in healthy patiens and in ones with tendinous pathology [9], performing the examination in supine and weight-bearing position. In the latter, leg muscles shrink and change the orientation of patellar ligament, which assumes a more parallel position to the magnetic field, reducing the appearance of the artifact. Methods and Materials Twenty-eight patients were evaluated (16 males, 12 females; mean age 36.32±9.05 standard deviation): 14 were asymptomatc, while 14 were referring for patellar ligament pain or had a previous diagnosis of a tendinous pathology. They were evaluated performing two SE T1-weighted sequences (TR=1890 ms, TE=24 ms; slice thickness=4.5 mm) on sagittal plane. Scans were performed both in standard supine position and in weight-bearing position (Fig. 1 and Fig. 2), using an open rotating MRI system (0,25 T,G-Scan, Esaote Biomedica, Genova, Italy). This machine has coils and sequences specific for musculoskeletal imaging and has a rotating mechanism from 0 to 90 which allows performing the examination in orthostatic position. We preferred starting our examinations with orthostatic scans to avoid the effects of vagal hypertone when the patients assumed the upright position after they had remained in supine position for a few minutes, as already demonstrated by a study performed with the same MRI machine [10]. To improve the patient comfort and further avoid this possibility, orthostatic rotation was initially carried out up to an angle of 44 and afterwards, only after been sure of the absence of symptoms like nausea, dizziness or syncope, up to 82, an angle at which the gravitational load on lower limbs is the same as physiological. We preferred not using the 90 rotation to avoid that patients, being at a height greater than the real one, felt unbalanced and unable holding a static position. All examinations were evaluated by a radiologist with 15 years of experience in musculoskeletal imaging, who assigned to each enthesis a value among 0 (negative Page 2 of 7

3 tendinous insertion), 1 (low grade signal alteration) and 2 (high grade signal alteration). The same scale was used both for supine and weight-bearing examinations. Cohen's Kappa and McNemar's test were used for statistic analysis. Images for this section: Fig. 1: Asymptomatic 28 years old patient. T1- weighted sequences on sagittal plane. (A) The image was obtained during the examination in supine position and shows a low grade signal alteration at the proximal insertion of patellar ligament. During the examination in weight-bearing position (B), the tendon loses its signal intensity, which was caused by the "Magic Angle" artifact. Page 3 of 7

4 Fig. 2: 36 years old patient with cronic tendinous pathology. T1- weighted sequences on sagittal plane. Patellar ligament has a high grade signal alteration, both in supine (A) and weight-bearing (B) examination. Signal intensity does not change with orientation of the examined structure because it was caused by the the cronic inflammatory pathology. Page 4 of 7

5 Results Our data (Tab. 1) show that most of the patients had a remarkable signal alteration at the proximal insertion of the patellar ligament in the supine position scans (asymptomatic patients median value=1; 25th-75th percentile=0-1; symptomatic patients median value=1; 25th-75th percentile=0-2, p=n.s.) (Fig.1 and Fig.2). In weight-bearing scans, enthesis signal intensity reduced or disappeared in all asymptomatic patients (median value=0, 25th-75th percentile=0-1, p<.001) (Fig.1), while it remained the same in the group of symptomatic patients (median value=1; 25th-75th percentile=0-2, p=n.s. compared to baseline, p<.001 compared to asymptomatic subjects) (Fig. 2). Images for this section: Table 1 Page 5 of 7

6 Conclusion All the data we got from our study suggest that weight-bearing MRI allow to have a more reliable detection of patellar ligament enthesis, compared to conventional MRI, thanks to its ability to reduce or erase the effects of the "Magic Angle" artifact by changing the orientation of the tendon when it is in physiological weight-bearing situation. Further investigations on a larger sample are needed to confirm our preliminary data. References 1 Peh WC, Chan JH. The magic angle phenomenon in tendons: effect of varying the MR echo time. Br J Radiol Jan;71(841): Zurlo JV, Blacksin MF, Karimi S. The influence of flip angle on the magic angle effect. Skeletal Radiol Oct;29(10): Li T, Mirowitz SA. Manifestation of magic angle phenomenon: comparative study on effects of varying echo time and tendon orientation among various MR sequences. Magn Reson Imaging Sep;21(7): Erickson SJ, Cox IH, Hyde JS, Carrera GF, Strandt JA, Estkowski LD. Effect of tendon orientation on MR imaging signal intensity: a manifestation of the "magic angle" phenomenon. Radiology Nov;181(2): Bydder M, Rahal A, Fullerton GD, Bydder GM. The magic angle effect: a source of artifact, determinant of image contrast, and technique for imaging. J Magn Reson Imaging Feb;25(2): Hayes CW, Parellada JA. The magic angle effect in musculoskeletal MR imaging. Top Magn Reson Imaging Feb;8(1): Khanna S, Crues JV 3rd. Complexities of MRI and false positive findings. Ann N Y Acad Sci Feb;1154: Peh WC, Chan JH. Artifacts in musculoskeletal magnetic resonance imaging: identification and correction. Skeletal Radiol Apr;30(4): Van der Worp H, van Ark M, Roerink S, Pepping GJ, van den Akker-Scheek I, Zwerver J. Risk factors for patellar tendinopathy: a systematic review of the literature. Br J Sports Med Apr;45(5): Page 6 of 7

7 10 Mauch F, Jung C, Huth J, Bauer G. Changes in the Lumbar Spine of Athletes From Supine to the True-Standing Position in Magnetic Resonance Imaging. Spine 2010;35: Personal Information Page 7 of 7