If you don't remember your password, you can reset it by entering your email address and clicking the Reset Password button. You will then receive an email that contains a secure link for resetting your password
If the address matches a valid account an email will be sent to __email__ with instructions for resetting your password
Use of Slice Encoding for Metal Artifact Correction (SEMAC) sequencing improves diagnostic evaluation of graft integrity following anterior cruciate ligament reconstruction
Division of Orthopedic Surgery, University of Alberta, Edmonton, AB, CanadaGlen Sather Sports Medicine Clinic, University of Alberta, Edmonton, AB, Canada
To determine whether MRI with metal artifact reduction sequencing (MARS) is superior to conventional knee MRI in the evaluation of the injured anterior cruciate ligament graft where visualization on conventional MRI can be limited by metal artifact from fixation devices.
Methods
Eighteen patients who underwent conventional MRI sequence (PD) and two types of MARS MRI (WARP, SEMAC; Siemens) following secondary injury to their ACL reconstructed knee. Six raters with experience in knee MRI evaluation reviewed sagittal PD, WARP and SEMAC sequences, providing semi-quantitative grades for visualization and diagnostic confidence assessing the: ACL, PCL, menisci, tibial and femoral tunnel margins, and articular cartilage. Intraclass correlation coefficients for inter-rater reliability were evaluated. The 6-rater mean scores for visualization and diagnostic confidence derived from each sequence were compared using the Friedman test for multiple paired samples.
Results
No statistically significant difference in ACL visualization among the sequences was found (p = 0.193). Further subgroup analysis was performed in cases evaluated as “moderately blurry” or “indistinct ACL visualization” on PDFS (58% of cases). SEMAC significantly improved diagnostic confidence in ACL visualization (p=0.041) and ACL graft rupture (p=0.044) compared to PDFS.
There was no statistically significant difference in the interobserver reliability between sequences. The WARP sequence added 2.84 + 0.69 minutes while SEMAC added 2.95 + 0.40 minutes to the standard knee MRI scan time.
Conclusion
Use of the SEMAC metal reduction sequence significantly improved diagnostic accuracy and confidence in detection of ACL graft rupture in cases where the ACL was moderately blurry or indistinct on PD sequence. This sequence should be considered as an adjunct to conventional PDFS in cases where graft visualization is limited by metal artifact from fixation devices.
Use of Slice Encoding for Metal Artifact Correction (SEMAC) sequencing improves diagnostic evaluation of graft integrity following anterior cruciate ligament reconstruction.
•
Metal artifact reduction sequencing (MARS) aids in the diagnosis of graft rupture and evaluation of previous tunnel position providing important information for surgical planning of revision surgery.
•
A novel diagnostic algorithm is provided for the use of metal artifact reduction sequencing (MARS) as an adjunct to conventional MRI sequences in evaluating the integrity of the anterior cruciate ligament graft following re-injury in the acute injury phase or when clinical assessment is equivocal.
Introduction
The anterior cruciate ligament (ACL) is the most frequently reconstructed knee ligament, with 80% of knee ligament surgeries involving the ACL.[
] The rate of ACL reconstruction (ACLR) continues to increase. In the United States there was a 37% increase in surgeries from 1994 to 2006 representing an increase of 134,421 ACL reconstructions annually.[
Figure 1Comparison of ACL graft seen on PDFS (A), WARP (B), and SEMAC (C). The graft was reported as intact on all three sequences. The ACL tibial tunnel is best visualized with SEMAC. Other structures in the knee such as articular cartilage are better visualized with PDFS (A) and WARP (B) than with SEMAC* (C), which is a lower-resolution sequence.
*Note that the slight difference in slice between SEMAC compared to PDFS and WARP is due to the difference in slice thickness – SEMAC uses thicker 4mm slices versus 3mm slices for the other sequences.
Figure 2Comparison of medial meniscus seen on PDFS (A), WARP (B), and SEMAC (C). The tear in the posterior horn of the medial meniscus (arrow) is better visualized with PDFS (A) and WARP (B) than with SEMAC* (C). Diagnostic confidence of meniscal tears was not statistically different between the three sequences.
*Note that the slight difference in slice between SEMAC compared to PD and WARP is due to the difference in slice thickness – SEMAC uses thicker 4mm slices versus 3mm slices for the other sequences.
] In the native knee, MRI has a diagnostic accuracy of 83% to 95% sensitivity and 95% to 100% specificity for native ACL tears, and has a well-defined role in identifying meniscal tears, and cartilaginous injuries.[
] Many techniques for ACLR exist, all requiring some form of implant for graft fixation. Metallic implants such as interference screws, cortical buttons and staples are some of the most common implants used.[
] It can be difficult to ascertain the integrity of the reconstructed ACL graft vis-a-vis stress relaxation, complete ruptures, partial ruptures or fixation failure. Rates of ACL graft rupture are 3.9-11.1%, with an overall graft rupture rate of 7.9% at a minimum follow-up time of 10 years, as indicated in a systematic review by Magnussen et al.[
] Advanced imaging studies, particularly MRI, can assist in confirming graft rupture, identifying causes of failure, associated injuries and planning revision surgery. Complications following ACLR that can be seen at MRI include cyclops lesion (anterior arthrofibrosis), graft impingement, graft rupture, and tunnel widening.[
MRI assessment of ACL graft integrity following ACL reconstruction is challenging. Signal changes related to the ligamentization process of the ACL graft may be confused with partial or even complete graft rupture.[
] Magnetic susceptibility artifact can distort or obscure the graft and tunnels. This artifact is not only due to metal implants, but also to microscopic metallic debris remaining in the joint and tunnels following surgery.[
] This can lead to limited diagnostic value with MRI when evaluating graft integrity, which presents a diagnostic problem when trying to evaluate patients who have suffered another injury to their surgically reconstructed knee.[
] Imaging accuracy can be improved by noting secondary signs of ACL graft rupture such as anterior tibial translation, uncovered posterior horn of the lateral meniscus, PCL hyperbuckling, and an abnormal PCL line.[
] However, ideally the ACL graft would be clearly visualized using MRI despite metal artifacts.
Metal artifact reduction sequences (MARS) were originally developed to improve visualization of periprosthetic bone and soft-tissue structures in patients following joint arthroplasty.[
] MARS reduces the size and intensity of susceptibility artifacts from magnetic field distortion, promising to allow otherwise non-diagnostic studies due to metal artifact to be able to yield key diagnostic information.[
] Manufacturers have developed their own proprietary MAR sequences. For example, Siemens has developed WARP and SEMAC sequences; likewise, GE has developed VAT and MAVRIC with other manufacturers offering similar sequences. These generally use principles of MARS, including multidirectional view angle tilting to reduce in-plane distortion, and two-dimensional fast spin-echo sequence which is phase encoded in the third dimension, and reduces through-section distortion by using complex reconstruction algorithms.[
As the number of ACL injuries, reconstructions, and ruptures continues to increase, a validated method to reliably assess graft integrity becomes increasingly necessary.[
] The purpose of our study is to determine whether MRI with MARS is superior to conventional knee MRI sequences in the visualization and diagnostic accuracy for ACL graft rupture in knees that have previously undergone ACLR. We propose that the addition of metal reduction sequencing will reduce artifact distortion of the ACL graft, thus improving diagnostic accuracy of identifying graft rupture as well as tunnel visualization in the setting of ACLR.
Methods
Approval for this project from our institutional Research Ethics Board (Pro00086602).
Cohort
A computer search of all knee MRI examinations was conducted to identify patients who underwent scans with MARS following secondary injury to their ACL reconstructed knee. MARS were recommended to be added to these patients after senior author’s clinical examination, and the presence of metallic implants. Post ACLR patients who underwent MRI evaluation of their ACLR between November 2018 to April 2019 had additional MARS sequencing to conventional MRI sequences. We identified 18 patients, 19 knees (male, 6; female, 12; age, 33 + 11.9 years).
Imaging
All patients were examined with a 1.5T Siemens Avanto imaging system (Siemens AG, Munich). The MRI protocol included five routine triplanar spin-echo MR sequences plus two additional MAR sequences: WARP for view angle tilting, and slice encoding for metal artifact reduction (SEMAC). For this study, we compared 3 sequences: the routinely acquired sagittal proton density (PD) (repetition time TR/echo time TE=2350/38 ms; slice thickness ST 3 mm; field of view FOV=170x170 mm; matrix size 448x448; bandwidth BW = 140 Hz/voxel); PD with WARP artifact reduction (same quoted parameters as PD); and PD with SEMAC artifact reduction (TR/TE=3000/29, ST 4 mm, FOV = 170x170mm, matrix 256x256; BW=975). PD was chosen as the comparative sequence as it is the standard ACL MRI sequence provided at our institution. PD has been used as a sequence that visualizes the ACL well, and is widely used in the assessment of ACL tears.[
MRI assessment was completed by six raters (three orthopedic surgeons with fellowship training in sports medicine, two fellowship-trained musculoskeletal radiologists, and one medical student). All patient identifiers were removed from the scans, and raters were blinded to the patient’s diagnosis and clinical history. However, because the visual differences between the sequences are obvious, raters could not be blinded to the MRI sequence (PD, WARP, SEMAC). Scans from patients were analyzed to compare the efficacy between MRI sequences, and images were analyzed qualitatively and semi-quantitively using a Likert scale. The visualization of the ACL graft, PCL, medial and lateral menisci, tibial and femoral tunnel margins, and articular cartilage were rated on a scale of 1 to 5 (1 = cannot visualize, 2 = very blurry or indistinct, 3 = moderately blurry or indistinct, 4 = mildly blurry or indistinct, 5 = excellent visualization). Confidence in diagnosis of ACL rupture and meniscus tear were rated on a scale from -5 to 5 (-5 = definitely ruptured with high confidence, 0 = equivocal, 5 = definitely intact with high confidence). Cyclops lesions were also assessed on a similar scale from -5 to 5 (-5 = definitely present with high confidence, 0 = equivocal, 5 = definitely not present with high confidence). Lastly, assessment of X-rays was performed by each reader to determine ACL graft fixation type.
Statistical Analysis
Interobserver reliability was calculated with intraclass correlation coefficient (two-way random, single score) using SPSS. Using the mean scores of all 6 raters, differences in visualization and diagnostic confidence scores derived from PDFS, WARP, and SEMAC were evaluated using the non-parametric Friedman test for multiple repeated comparisons. Conover post-hoc analysis was performed to determine which of the sequences were significantly different from one another.
Results
Relative Visualization of Structures
The ACL graft was “mildly blurry or indistinct” with all sequences, with mean scores of 3.79/5, 3.96/5, and 3.97/5 for PD, WARP, and SEMAC respectively. The native PCL had “excellent visualization” with all sequences, with mean scores of 4.75/5, 4.81/5, and 4.63/5 for PD, WARP, and SEMAC respectively (see Table 1).
Table 1Patient demographics.
Gender
Male
6
Female
12
Age
15-24
5
25-34
5
35-44
4
45-54
4
Tibial Fixation
Metal suspensory (eg. Cortical buttons, staples, cross pins)
There was no statistically significant difference in visualization of the ACL graft between the 3 sequences (p=0.193) (Table 2). Further analysis was performed in 58% of cases (11/19 knees) where ACL graft visualization was rated “moderately blurry, indistinct or worse”. In these cases, a statistically significant increase in visualization with SEMAC was found compared to PDFS (p=0.041) (Table 2A). Native PCL visualization scores were best with PDFS and worst with SEMAC (p=0.006) (Table 2).
Table 2Comparison of the visualization scores of structures within the knee between PDFS, WARP, and SEMAC MRI using a semi-quantitative scale (1 = cannot visualize, 2 = very blurry or indistinct, 3 = moderately blurry or indistinct, 4 = mildly blurry or indistinct, 5 = excellent visualization) using 6-reader mean data.
Visualization of femoral and tibial articular cartilage was statistically significantly worse using the SEMAC and WARP sequences than PDFS (p<0.00001) (Table 2).
Bony tunnels
The femoral and tibial tunnels were better visualized from the WARP and SEMAC sequences than the PDFS sequence. This was statistically significant (p=0.001 femoral tunnel; p=0.002 tibial tunnel) (Table 2).
Menisci
Median visualization scores for the medial meniscus were best with PDFS and worst with SEMAC (p<0.00001). Lateral meniscus visualization was best with WARP and worst with SEMAC (p<0.00001) (Table 2).
Diagnostic Confidence
There were no statistically significant difference in diagnostic confidence for ACL rupture or cyclops lesion between the three sequences (Table 3). In the subset of cases with moderately blurry or indistinct ACL visualization score by at least one rater, diagnostic confidence scores for ACL graft rupture significantly improved with use of the SEMAC sequence compared to both WARP and PDFS (p=0.041) (Table 3A).
Table 3Comparison of diagnostic confidence scores between PDFS, WARP, and SEMAC MRI sequences using absolute value of 6-rater mean scores.
Diagnostic confidence scores were found to be statistically significantly lower for lateral meniscus tear using SEMAC than from PDFS or WARP (p=0.036). SEMAC and WARP sequences both produced poorer diagnostic confidence scores for medial meniscus tear than PDFS (p<0.0001) (Table 3).
Diagnostic Accuracy
There was no statistically significant difference in the interobserver reliability between each sequence. There was a trend of slightly higher interobserver reliability for all structures on PDFS sequences compared to WARP, which was also slightly higher than SEMAC, however these did not reach statistical significance. For PDFS, interobserver reliability was moderate for ACL graft (ICC 0.713) and medial meniscus (ICC 0.630), and poor for the lateral meniscus (ICC 0.439) (Table 4). Interobserver reliability for WARP was moderate for ACL graft(ICC 0.661) and medial meniscus (ICC 0.579), and poor for the lateral meniscus (ICC 0.392). Interobserver reliability for SEMAC was moderate for ACL (ICC 0.627) and medial meniscus (ICC 0.593), and poor for the lateral meniscus (ICC 0.374) (Table 4).
Table 4Interobserver correlation coefficient of diagnostic confidence of ACL graft rupture, and medial and lateral meniscus tear between PDFS, WARP, and SEMAC MRI.
The WARP sequence increased scan time by 2.84 + 0.69 minutes while SEMAC added 2.95 + 0.40 minutes to the standard knee MRI scan time at our institution.
Discussion
There is a paucity of literature regarding soft tissue knee imaging using metal artifact reduction sequencing (MARS) (see Fig. 3). This is the first study to evaluate the utility of MRI with MARS in the assessment of ACL grafts. The current literature demonstrates poor sensitivity (32-50%) and good specificity (90-100%) in evaluating for ACL graft rupture with MRI.[
] The primary outcome of this study was to identify whether graft integrity would be more confidently reported using MARS sequences (WARP or SEMAC) than conventional PDFS MRI. It is important to note that we only used these sequences on patients with clinically significant injury and in whom graft integrity was determined to be equivocal on clinical examination. We found that SEMAC significantly improved diagnostic confidence for ACL graft rupture versus conventional PDFS imaging, notably in cases where the ACL was felt to be moderately blurry or indistinct on the PDFS sequence. A secondary finding was that both WARP and SEMAC significantly improved visualisation of the femoral and tibial tunnels. This greatly helps in surgical planning and aids the clinician in deciding whether a multi-stage or single-stage revision procedure is needed. [
] The addition of a SEMAC sequence is superior to conventional MRI sequences in assessing the previous tunnels, and would be of benefit for pre-operative planning, potentially removing the need for some patients to undergo a CT scan to assess for tunnel widening and positioning. This is of significant benefit in maintaining the as low as reasonably achievable (ALARA) radiation exposure in young patients and adding no additional risk to patients.[
] Furthermore, WARP and SEMAC each added under 3 minutes of added scanning. Adding one of these sequences to selected post-ACLR knee MRI would generally have minimal or no effect on total scanner patient throughput, while providing valuable additional diagnostic information. However, there seems to be little diagnostic benefit from adding both WARP and SEMAC sequences to the same scan. Therefore, we propose the following clinical approach (Figure 4).
Figure 3Comparison of 3 consecutive slices on MRI of ACL graft seen on PDFS (A, B, C) and SEMAC (D, E, F). The ACL graft was reported as a complete rupture on PDFS, and reported to be intact on SEMAC. This highlights visually our quantitative finding that diagnostic confidence of ACL graft rupture was increased with metal suppression sequences.
*Note that the slight difference in slice between SEMAC compared to PDFS and WARP is due to the difference in slice thickness – SEMAC uses thicker 4mm slices versus 3mm slices for the other sequences.
Post-surgical MRI provides an assessment of graft integration, femoral and tibial tunnel positioning as well as other intraarticular injuries. The current literature supports our findings that metal suppression is beneficial in the assessment of patients who have undergone prior ACLR.[
] With the use of metal fixation devices, susceptibility artifacts make tunnel visualization as well as graft incorporation difficult especially in higher field strength such as 3.0 Tesla MR scanner.[
] Van Dyck et al. state that the presence of the metallic implant in the tibia usually necessitates the use of MARS, and that the reconstructed ACL is difficult to assess on MRI.[
] Others have suggested that metal implants only show metal artifacts and insufficient fat saturation right next to the implants, and that visualization of the ACL graft and the rest of the knee joint may be evaluated sufficiently.[
] We clearly demonstrate benefit in adding MARS to post ACLR MRI in the evaluation of ACL graft integrity.
McCauley et al. describe using MR arthrography (MRA) in the evaluation of ACL grafts. With MRA, there was a 100% sensitivity and 100% specificity in diagnosing graft tear and there was perfect agreement between MRA interpretations and surgical findings for the presence of ACL tear, however they did not compare their results to conventional MRI.[
] Vande Berg et al. describe using CT arthrography for assessing the postoperative knee and determined that there is improved visualization of the ACL graft as there are limited imaging artifacts related to the presence of microscopic metallic debris that may hinder MR imaging studies.[
] Despite the diagnostic advantages to CT or MR arthrography, these are costly and invasive procedures which can cause pain and also include a small risk of infection.[
] For these reasons, we believe it is impractical to image every patient with a secondary injury to their knee using arthrography.
The gold standard for evaluating graft rupture is through arthroscopy, which provides direct visualization of structures within the knee, especially in cases where there are equivocal findings on clinical exam and radiologic findings.[
] However, arthroscopy is an invasive procedure and has associated complications. In certain cases, arthroscopy is warranted in intractable cases with equivocal imaging. Horton et al. compared MRI to arthroscopy as the gold standard and found the diagnosis of full-thickness ACL graft tears to have poor sensitivity (50%), but excellent specificity (100%).[
] These studies support our notion that ACL graft integrity is not well visualized with routine knee MRI sequencing and that there is a clear role for MARS in these cases.
The MRI at our institution is a 1.5T coil and despite the smaller coil, we achieved good image quality. Artifacts from metal hardware are more extensive with higher magnetic field due to basic physics principles. The study by Jungmann et al. showed that a less powerful magnet actually gives excellent metal suppression and therefore improved image quality.[
] This demonstrates good generalizability for this technique including use in less developed countries or in countries with a publicly funded healthcare system, such as Canada. Whether these MARS MRI are also of benefit in 3T MRI is an area for further study.
Strengths and Limitations
To our knowledge, this is the first study evaluating the use of metal suppression MRI protocols specifically to assess ACL graft integrity. Our semiquantitative grading systems were straightforward to apply with high and consistent interobserver reliability for diagnosis of graft tears amongst 6 raters. We acknowledge that there is no validated rating scale in the assessment of the ACL graft in the post-injury setting. There is no validated rating scale for assessing ACL or ACL graft integrity on MRI and therefore we developed our scale to evaluate graft integrity and other intraarticular structures semi-quantitatively. Furthermore, all MRI scans were performed at a single institution using Siemens scanners, limiting our ability to draw conclusions about generalizability of results to other manufacturers, although this did allow for consistent image quality for evaluation. Lastly, we only assessed whether a graft was torn or not, without assessing completeness of the tear. Other studies that have assessed the utility of MRI post ACLR have commented on the ability to differentiate between partial and complete graft tears, which we did not do in this study. At our institution, partial graft tears would rarely proceed to revision surgery, therefore we felt that the key distinction relevant to clinical management was to diagnose a complete graft tear. In this study design, we wanted to see if there was any benefit to MARS and therefore decided against routine arthroscopy in all study patients due to the invasiveness and potential risks. Therefore, we were not able to verify the MRI findings with arthroscopy in all patients, however, all complete graft tears were verified at time of revision surgery. We also acknowledge the relatively small cohort size of 18 patients. Our sample size calculation determined a minimum sample of 10 scans to achieve statistical significance between the different sequences and we were able to study a larger sample size than what was determined in our sample size calculation.
We believe a prospective study evaluating graft integrity using MARS compared to arthroscopy may provide additional information on the true utility of these sequences in evaluating the injured post-ACL reconstructed knee. Future research with a larger patient cohort would allow a clearer distinction between the relative benefits of each MAR sequence, as well as possible stratification based on graft type and graft fixation.
Conclusion
We found that compared to conventional MRI imaging, sequences dedicated to metal artifact reduction significantly improved assessment of ACL graft integrity and associated bony tunnels which may provide clinicians important information for diagnosis and surgical planning with no risk of ionizing radiation to the patient and only minutes of increased scan time. When the key clinical question is ACL graft integrity, we recommend adding a SEMAC sequence to the routine knee MRI scan protocol.
Declaration of interests
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Brandon Zhao reports was provided by University of Alberta Division of Orthopaedic Surgery.
00472-8&id=gr1.jpg){kind=link}
00472-8&id=gr2.jpg){kind=link}
00472-8&id=gr3.jpg){kind=link}
00472-8&id=gr4.jpg){kind=link}