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Usefulness of slice encoding for metal artifact correction (SEMAC) technique for reducing metal artifacts after total knee arthroplasty

  • Ahmed Jawhar
  • Miriam Reichert
  • Michael Kostrzewa
  • Mathias Nittka
  • Ulrike Attenberger
  • Henning Roehl
  • Frederic Bludau
Original Article • KNEE - ARTHROPLASTY
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Abstract

Purpose

To evaluate the usefulness of a novel MRI sequence strategy in the assessment of the periprosthetic anatomical structures after primary total knee arthroplasty.

Methods

Two MR sequences were retrospectively compared for the imaging of 15 patients with implanted cruciate-retaining/fixed-bearing TKAs (DePuy, PFC Sigma): a slice encoding sequence for metal artifact correction (SEMAC) and a standard sequence. Images were acquired on a 1.5-T system. The degree of artifact reduction was assessed using several qualitative (Likert-type scale) (artifact size, distorsion, blur, image quality, periprosthetic bone, posterior cruciate ligament, lateral collateral ligament, medial collateral ligament, patella tendon, popliteal vessels) and quantitative (artifact volume, Insall–Salvati index, length of patella/tendon, prosthesis dimensions) parameters by blinded reads performed by four investigators. The SEMAC sequences were statistically compared with the standard sequence using Wilcoxon test. Additionally, the intraclass correlation coefficient (ICC) for interobserver agreement was calculated.

Results

Higher levels of blurring were found with SEMAC compared to standard sequences (p < 0.001). All other qualitative parameters improved significantly with the application of SEMAC. In comparison with conventional sequences, the artifact volume was reduced by 59% utilizing SEMAC. Thus, the artifact reduction improved the precision of measurements such as Insall–Salvati index and length of patella/tendon (p < 0.001). The dimension of the tibial component (Ti alloy/polyethylene) revealed accurate values with both MRI sequences. A sufficient interobserver agreement among all readers was found with SEMAC, qualitatively ICC 0.9 (range 0.8–1) as well as quantitatively ICC 0.95 (range 0.92–0.98).

Conclusions

SEMAC effectively reduces artifacts caused by metallic implants after total knee arthroplasty relative to standard imaging. This allows for an improved assessment of periprosthetic anatomical structures. This might enable an improved detectability of postoperative complications in the future.

Level of evidence

Diagnostic Study Level III.

Keywords

Total knee arthroplasty Ligaments Periprosthetic bone SEMAC Artifact reduction 

Introduction

The long-term success after primary total knee arthroplasty (TKA) depends on patient selection and surgical technique ensuring an accurate implant positioning with balanced soft tissue [17]. Despite efforts to improve surgical techniques, 20% of patients worldwide remain dissatisfied with their results after primary TKA due to substantial pain [16]. Thirty-five percentage of early TKA revisions are performed due to unbalanced or insufficient periprosthetic soft tissue [5]. The unbalanced soft tissue in turn could clinically explain anterior knee pain, stiffness/instability [20], recurrent effusion [15] and early component loosing [3]. The number of TKA revisions is estimated to grow significantly until 2030 [7]. The diagnosis leading to revision surgery is usually based on native radiographs/CT scans/bone scintigraphy and the subjective clinical orthopedic examination [12]. To clarify the etiology of the symptoms after primary TKA more accurately, MR imaging might be useful, especially in cases with synovitis, periprosthetic osteolysis, fractures, tendon/muscle injuries, instability, arthrofibrosis and mechanical component damages [18].

However, the efficiency of standard MR imaging for the evaluation of the periprosthetic tissues is limited due to susceptibility artifacts of the implant components, which consecutively contributes to poor-quality images. Therefore, MR imaging has not been routinely used in the presence of knee endoprosthesis [10].

Recently, numerous MR imaging strategies were introduced and evolved to reduce metal artifacts. The SEMAC sequence, which has been previously described in detail [9, 14], corrects both in-plane and through-plane metal-implant-induced artifacts [9]. The SEMAC technique effectively reduced artifacts generated by different implants compared to conventional imaging at 1.5 T [14].

However, the limited data [1, 2, 19] did not provide a solid base for clinical decision making: whether the SEMAC technique is useful for human imaging of periprosthetic tissue in the presence of highly paramagnetic TKAs.

To our knowledge, the present study is the first one assessing the usefulness of the SEMAC technique in primary, cruciate-retaining, fixed-bearing TKA (DePuy, PFC Sigma).

The aim of the present study was to compare SEMAC with standard sequences concerning the qualitative and quantitative assessments of the periprosthetic tissue after primary TKA.

Patients and methods

The present retrospective study was carried out in accordance with the Declaration of Helsinki. Retrospective MR data analysis was approved by our Institutional Ethics Committee (file reference 2008-338N-MA).

Patients suffering from unspecific postoperative knee symptoms after TKA were scheduled to undergo MRI with SEMAC and standard sequences at the end stage of their routine clinical workup.

Fifteen patients (11 females and 4 males) with a mean age of 76 years (range 52–89 years) with complete data sets were included in the retrospective study. They underwent TKA from 12 to 24 months prior to MR imaging.

Patients had received a cemented (SmartSet Bone cement, DePuy/Synthes, Warsaw, IN, USA) fixed-bearing, cruciate-retaining TKA (DePuy, PFC Sigma). The femoral component consists of CoCrMo alloy, whereas the tibial plateau consists of Ti alloy and a polyethylene insert.

MR imaging technique

Patients were placed supine on the MRI table. A 1.5-T MRI scanner (MAGNETOM Avanto, Siemens Healthcare, Erlangen, Germany) was used for all cases. The MR imaging protocol applied T1 coronar, T2 transversal and PD sagittal sequences. SEMAC imaging was performed with a prototype software package (Siemens Healthcare, Erlangen, Germany), providing a 2D TSE sequence with View Angle Tilting (VAT), Slice Encoding for Metal Artifact correction (SEMAC), and optimized for high-bandwidth imaging parameters. A standard 2D TSE sequence served as a reference. The details of the image parameters are described in Table 1.
Table 1

Image parameter at 1.5 T

 

T1

T2

PD

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

TR (ms)

512

650

5310

3300

3000

4570

TE (ms)

11

6

79

74

14

36

Readout BW (kHz/pixel)

150

651

189

651

127

651

Matrix

320 × 300

320 × 256

320 × 256

320 × 256

256 × 256

320 × 256

Slice thickness (mm)

3

2

3

4

3

2

Acquisition time (min:s)

04:09

11:22

04:32

11:31

2:28

3:17

TR repetition time, TE echo time, BW readout bandwidth, PD proton density

Qualitative analysis of the MRI

The MR images from the knee joints were retrospectively assessed independently by a total of four readers. Two experienced orthopedic surgeons and two experienced diagnostic radiologists evaluated the images. The readers were not aware of the sequence type at the time of evaluation/measurement.

Images with SEMAC and standard sequences were compared qualitatively side by side and with the same contrast according to defined qualitative parameters: artifact size; image distorsion; image blur; visualization of the bone marrow/bone cortex/soft tissue; posterior cruciate ligament, lateral collateral ligament, medial collateral ligament, patella tendon, popliteal vessels, periprosthetic bone (Figs. 1, 2, 3, 4). The images were graded as described (Tables 2, 3).
Fig. 1

Measurement of periprosthetic artifact volume after a standard and b SEMAC sequence MRI

Fig. 2

Measurement of the patella height with a standard and b SEMAC sequence using the formula: Insall–Salvati index = length of patella tendon/diagonal length of patella. The identification of several periprosthetic anatomical landmarks such as quadriceps tendon, patella, patella tendon, tibial tuberosity, posterior cruciate ligament is markedly improved with SEMAC

Fig. 3

Measurement of mediolateral tibia plateau and tibia stem width with sagittal a standard and b SEMAC sequences

Fig. 4

Measurement of mediolateral tibia plateau and tibia stem width with transversal a standard and b SEMAC sequences

Table 2

Overall qualitative evaluation between MR imaging modalities by four readers for the metal artifact reduction

a

Surgeon 1

Surgeon 2

Radiologist 1

Radiologist 2

Reliability (ICC)

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

Artifact size

Mean

2.0

1.0

2.0

1.0

2.0

1.0

2.0

1.0

1

1

SD

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

Imaging distortion

Mean

2.0

1.0

2.0

1.0

2.0

1.0

2.0

1.0

1

1

SD

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

Imaging blur

Mean

1.1

1.9

1.1

1.9

1.0

2.0

1.0

2.0

0.80

0.80

SD

0.3

0.3

0.3

0.3

0.0

0.0

0.0

0.0

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

Visualization of bone marrow/bone cortex and soft tissue

Mean

1.9

1.1

2.0

1.0

2.0

1.0

2.0

1.0

0.95

0.95

SD

0.3

0.3

0.0

0.0

0.0

0.0

0.0

0.0

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

Overall image quality

Mean

2.0

1.0

2.0

1.0

2.0

1.0

2.0

1.0

1

1

SD

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

a1 = best; 2 = worst

Table 3

Qualitative evaluation between MR imaging modalities by four readers with regard to identification of clinically relevant structures

a

Surgeon 1

Surgeon 2

Radiologist 1

Radiologist 2

Reliability (ICC)

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

PCL

Mean

3.0

1.2

3.0

1.0

2.9

1.0

2.9

1.0

0.90

0.90

SD

0.0

0.4

0.0

0.0

0.3

0.0

0.3

0.0

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

MCL

Mean

2.7

1.3

2.1

1.1

2.0

1.3

2.1

1.2

0.34

0.80

SD

0.5

0.6

0.3

0.3

0.0

0.4

0.3

0.4

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

0.6

< 0.001

LCL

Mean

2.2

1.4

2.1

1.3

2.0

1.3

2.1

1.3

0.37

0.87

SD

0.4

0.5

0.3

0.4

0.0

0.4

0.3

0.4

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

0.16

< 0.001

Patella tendon

Mean

2.1

1.1

1.9

1.0

1.9

1.0

1.9

1.0

0.68

0.99

SD

0.5

0.3

0.3

0.0

0.3

0.0

0.3

0.0

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

Popliteal vessels

Mean

2.2

1.1

1.5

1.1

1.4

1.2

1.5

1.2

0.83

0.87

SD

0.6

0.3

0.5

0.3

0.5

0.4

0.6

0.4

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

Periprosthetic bone

Mean

2.0

1.0

2.0

1.0

2.0

1.2

2.0

1.1

1.0

0.80

SD

0.0

0.0

0.0

0.0

0.0

0.4

0.0

0.3

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

a1 = completely visible; 2 = partially visible; 3 = not visible

Quantitative analysis of the MRI

The artifact volume generated by the implant components was measured by an experienced radiologist (Fig. 1). The artifact volume was determined offline using a dedicated workstation (GE Healthcare, Advantage Workstation AW4.2, Erlangen, Germany). The reliable method of artifact volume quantification first described by Lee et al. [8] was utilized for the analysis of each sequence.

Four readers independently compared SEMAC and standard sequence images side by side and with the same contrast according to defined quantitative parameters: Insall–Salvati index [11], length of patella, length of patella tendon, diameter of tibial stem, medial–lateral width of the tibia plateau (Figs. 2, 3, 4; Table 4).
Table 4

Quantitative comparison between MR imaging modalities by four readers with regard to measurement of clinically relevant parameter

 

Surgeon 1

Surgeon 2

Radiologist 1

Radiologist 2

Reliability (ICC)

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

Standard

SEMAC

Insall–Salvati index a

Mean

0.79

1.01

0.80

1.05

0.97

1.13

0.87

1.11

0.72

0.93

SD

0.1

0.2

0.1

0.1

0.1

0.2

0.1

0.2

  

p value

 

< 0.001

 

< 0.001

 

< 0.001

 

< 0.001

< 0.001

< 0.001

Length of patella (mm)

Mean

42.0

40.2

44.1

39.1

41.3

36.9

42.0

37.7

0.92

0.92

SD

13.8

3.5

8.9

2.9

9.0

3.5

6.9

3.8

  

p value

 

> 0.05

 

< 0.05

 

< 0.05

 

< 0.05

< 0.001

< 0.001

Length of patella tendon (mm)

Mean

31.8

40.0

32.5

40.2

37.9

40.7

36.5

41.1

0.79

0.98

SD

10.0

6.0

3.7

5.7

6.2

6.3

7.0

6.5

  

p value

 

< 0.001

 

< 0.001

 

< 0.05

 

< 0.001

< 0.001

< 0.001

Tibial stem diameter b (mm)

Mean

15,8

14.8

15.8

16.4

16.5

16.9

16.7

16.4

0.92

0.94

SD

1,7

4.4

2.1

1.4

1.6

2.0

1.5

1.6

  

p value

 

> 0.05

 

> 0.05

 

> 0.05

 

> 0.05

< 0.001

< 0.001

Width of tibia plateau c (mm)

Mean

71.2

73.0

71.0

71.5

72.2

71.5

72.5

72.1

0.92

0.97

SD

4.9

3.4

3.2

3.2

5.1

3.2

3.1

3.6

  

p value

 

> 0.05

 

> 0.05

 

> 0.05

 

> 0.05

< 0.001

< 0.001

aInsall–Salvati index as determined on standard radiographs averaged 1.02 ± 0.2

bTibial stem diameter measured intraoperatively averaged 15.3 ± 0.7 mm

cWidth of tibia plateau measured intraoperatively averaged 70.5 ± 3 mm

Statistical analysis

Statistical analysis was performed using commercial statistical software package (SPSS for Windows, version 23). The results are presented with mean ± standard deviation (SD). Nonparametric Wilcoxon test was used for the statistical comparison among different imaging protocols. As evaluated by four readers, intraclass correlation coefficient (ICC) was used to calculate the interobserver agreement for the SEMAC and standard sequences separately. A p < 0.05 was set to be statistically significant.

Results

Qualitative analysis of the MRI

Increased blurring was noted with SEMAC compared to standard sequences. A statistically significant superiority of SEMAC was found for all other qualitative parameters (artifact size, distortion, image quality, periprosthetic bone, posterior cruciate ligament, lateral collateral ligament, medial collateral ligament, patella tendon, popliteal vessels (all p < 0.001). Mean ICC was 0.9 (range 0.8–1) for the SEMAC sequences and 0.8 (range 0.34–1) for the standard sequences (Tables 2, 3).

Quantitative analysis of the MRI

In comparison with the standard method, the artifact volume was reduced significantly by 59% with SEMAC sequences (p < 0.001) (Fig. 5). With the reduction in the artifact volumes, the precision of the measurements such as Insall–Salvati index and length of patella/tendon improved. The mediolateral width of the tibial component, which consists of a Ti alloy/polyethylene insert, revealed accurate measurement values with both methods as compared to real dimensions provided by the manufacturer. The mean ICC of the SEMAC sequence was 0.95 (range 0.92–0.98). The ICC of the standard sequence averaged 0.85 (range 0.72–0.92; Table 4).
Fig. 5

Comparison of the artifact volume in each case between a standard and b SEMAC sequences

Discussion

The principal finding of the present study was that SEMAC sequence MRI successfully reduced the artifact volume and thus improved the quantitative/qualitative evaluation of periprosthetic tissue after primary TKA.

Sutter et al. [19] implanted a different prosthesis design/manufacturer and described a significant reduction in artifact size with SEMAC. Similar to previously published studies [1, 14], we found increased blurring with SEMAC compared to standard sequences.

The results of the present study suggest that MR imaging with SEMAC allows for adequate identification and evaluation of the collateral ligaments as well as PCL. Failing to obtain balanced periprosthetic ligaments or to retain the PCL in corresponding prosthesis designs results in early instability, anterior knee pain and revision surgery [20]. Also posttraumatic/degenerative tear of MCL, LCL, PCL or patella tendon could occur after primary TKA [13]. In cases of instability or insufficiency of the extensor mechanism, the visualization of the periprosthetic soft tissue complimentary to the subjective clinical findings might help the orthopedic surgeon to establish the therapeutic management more precisely.

Similar to postoperative radiographic measurements, Insall–Salvati index [11], length of patella and length of patella tendon were detected and measured reliably with SEMAC sequences. The etiology behind patellar height changes with reduced range of movement after TKA remained unknown. Several prominent explanation models exist, e.g., scar tissue/new bone formation, intratendinous fibrosis, tendon adhesion to infrapatellar bursa or proximal part of tibia, trauma and ischemia [6, 21]. Metal-artifact-reduced MR imaging could be useful to increase understanding about the etiology of patella tendon shrinking/elongation and to accurately quantify the changes of patellar height.

The detection and quantification of periprosthetic osteolysis around TKA with native radiographs were reported to be inferior due to the projection of the implant components on the periprosthetic bone [22].

SEMAC sequences have been proposed to be included in the diagnostic algorithm of patients with a painful TKA, since the detection of clinically relevant periprosthetic osteolysis significantly improved [19].

In summary, MR imaging with SEMAC sequence provided promising detailed diagnostic information about all periprosthetic tissues. The improved characterization and depiction of the periprosthetic anatomical structures due to reduced artifact might contribute to a more reliable and conclusive diagnosis prior to revision surgeries.

The most important limitations of SEMAC sequences are the increased acquisition time (delta 15 min) with consecutively possible patient discomfort and motion artifacts. However, recent data suggest the possibility to reduce the acquisition time by using compressed sensing SEMAC [4]. Residual artifacts sometimes limit a detailed evaluation of bone–cement and cement–metal interfaces.

Future studies with higher sample sizes are required to elucidate the diagnostic value of SEMAC sequences to detect further clinically relevant parameters such as implant position, polyethylene wear, osteolysis, fracture, infection and mechanical complications.

Conclusion

SEMAC effectively reduces artifacts caused by metallic implants after total knee arthroplasty relative to standard imaging. This might allow for an improved detectability of complications after TKA.

Notes

Compliance with ethical standards

Conflict of interest

Mathias Nittka is an employee of Siemens Healthcare GmbH. The remaining authors declare that there were no conflicts of interest.

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Copyright information

© Springer-Verlag France SAS, part of Springer Nature 2018

Authors and Affiliations

  • Ahmed Jawhar
    • 1
  • Miriam Reichert
    • 2
  • Michael Kostrzewa
    • 4
  • Mathias Nittka
    • 3
  • Ulrike Attenberger
    • 2
  • Henning Roehl
    • 1
  • Frederic Bludau
    • 1
  1. 1.Department of Orthopaedics and Trauma Surgery, University Medical Center Mannheim, Medical Faculty MannheimHeidelberg UniversityMannheimGermany
  2. 2.Department of Clinical Radiology and Nuclear Medicine, Institute of Clinical Radiology and Nuclear Medicine, University Medical Center Mannheim, Medical Faculty MannheimHeidelberg UniversityMannheimGermany
  3. 3.Siemens Healthcare GmbHErlangenGermany
  4. 4.Division of Vascular and Interventional Radiology, Department of Medical Imaging, Toronto General Hospital and Mount Sinai HospitalUniversity of TorontoTorontoCanada

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