Musculoskeletal Imaging Original Research

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1 Musculoskeletal Imaging Original Research MRI of Total Knee rthrolasty Musculoskeletal Imaging Original Research Reto Sutter 1,2 Roman Hodek 1,2 Sandro F. Fucentese 2,3 Mathias Nittka 4 Christian W.. Pfirrmann 1,2 Sutter R, Hodek R, Fucentese SF, Nittka M, Pfirrmann CW Keywords: metal artifacts, MRI, osteolysis, slice-encoding for metal artifact correction, total knee arthrolasty DOI: /JR Received January 2, 2013; acceted after revision March 11, M. Nittka is emloyed by Siemens Healthcare and was involved in the develoment of the slice-encoding for metal artifact correction sequence. He had no control of the study data. 1 Deartment of Radiology, Orthoedic University Hosital algrist, Forchstrasse 340, Zurich 8008, Switzerland. ddress corresondence to R. Sutter (reto.sutter@balgrist.ch). 2 Faculty of Medicine, University of Zurich, Zurich, Switzerland. 3 Deartment of Orthoedic Surgery, Orthoedic University Hosital algrist, Zurich, Switzerland. 4 Healthcare Sector, Imaging & Theray Division, Magnetic Resonance, Siemens Healthcare, Erlangen, Germany. JR 2013; 201: X/13/ merican Roentgen Ray Society Total Knee rthrolasty MRI Featuring Metal rtifact : Reduction of rtifacts for STIR and Proton Density Weighted Sequences OJECTIVE. The urose of this article is to comare slice-encoding for metal artifact correction (SEMC) sequences versus otimized standard MRI sequences in atients with total knee arthrolasty (TK). SUJECTS ND METHODS. Forty-two atients with TK underwent 1.5-T MRI. Sequences otimized for metal imlant imaging (SEMC) were comared with standard sequences otimized with high bandwidth for STIR and roton density (PD) weighted images. In 29 atients, CT was available as reference standard. Signal void and insufficient fat saturation were quantified. Qualitative criteria (anatomy, distortion, blurring, and noise) were assessed on a 5-oint scale (1, no artifacts; 5, severe artifacts) by two readers. bnormal imaging findings were noted. Student t test and a Wilcoxon signed rank test was used for statistics. RESULTS. Signal void areas and insufficient fat saturation were smaller for the SEMC sequences than for the otimized standard sequences ( for all comarisons). Deiction of anatomic structures was better on STIR with SEMC versus standard sequences otimized with high bandwidth (score range, vs ) and on PD-weighted imaging with SEMC versus standard sequences otimized with high bandwidth (score range, vs ), which was statistically significant ( < to = for different structures). Distortion and noise were lower for SEMC than for the standard sequences ( 0.001), whereas no technique had a clear advantage for blurring. Detection of abnormal imaging findings was markedly increased for the SEMC technique ( < 0.001) and was most ronounced for STIR images (98 and 74 findings for STIR with SEMC for readers 1 and 2, resectively, vs 37 and 37 findings for readers 1 and 2, resectively, for STIR with standard sequences otimized with high bandwidth). Sensitivity for detection of erirosthetic osteolysis was imroved for STIR with SEMC (100% and 86% for readers 1 and 2, resectively) comared with STIR with standard sequences otimized with high bandwidth (14% and 29% for readers 1 and 2, resectively). CONCLUSION. SEMC sequences showed a statistically significant artifact reduction. The detection of clinically relevant findings such as erirosthetic osteolysis was markedly imroved. U to 28% of atients undergoing total knee arthrolasty (TK) have substantial residual ain 1 year after TK, and the reason for this ain often is unclear [1, 2]. In the ast decade, MRI has occasionally been used to evaluate the bone and eriarticular soft tissues in atients with TK, but the large metal hardware induced artifacts are often detrimental to image quality and diagnostic information [3, 4]. Moreover, the use of fluid-sensitive sequences for the evaluation of TK is uncommon because sectral fat saturation relies on a very homogeneous field, which cannot be maintained in the vicinity of metal imlants [4]. lso, the more robust STIR sequence may fail in regions where the metal-induced slice rofile distortion leads to an incomlete inversion [5]. On the other hand, fluid-sensitive sequences might be esecially helful to diagnose erirosthetic osteolysis or bone marrow edema, if reliable fat saturation could be achieved in atients with TK [6, 7]. Radiograhs are the standard of care when evaluating atients with a ainful TK or atients with susected TK loosening, and some institutions also erform CT [8, 9]. lthough some techniques for reducing metal artifacts at MRI examinations of atients JR:201, December

2 with metal hardware have been in clinical use for several years, such as a high-readout bandwidth or view-angle tilting [5, 10], MRI of joint relacements has seen a considerable imrovement with new techniques that not only reduce artifacts in the image lane but also reduce distortion of the image lane in the third dimension (so-called through-lane artifact reduction), such as slice-encoding for metal artifact correction (SEMC) or multile-acquisition with variable resonance image combination [11 16]. ecause SEMC includes view-angle tilting, it can rovide a correction of bulk distortion in all dimensions [11]. SEMC has been imlemented for imaging of the knee in a study that featured 14 volunteers with TK and 11 atients with a variety of metal hardware [17] and in another study where different metal hardware in knee hantoms and two volunteers was assessed [18]. To our knowledge, there is no study in the literature that investigates the SEMC technique in a substantial number of atients with TK or with a CT reference standard. The objective of our study was to comare the SEMC sequences versus otimized standard MRI sequences in atients with TK. Subjects and Methods Patients Our study was submitted to the regional institutional review board, and a waiver was issued. s art of their hosital admission, all atients signed informed consent. We evaluated 48 consecutive atients with TK for inclusion in our study. ll atients were referred for an MRI examination of the knee at our institution during a eriod of 8 months as art of their worku for a ainful TK. One atient had a history of trauma 15 months before the MRI examination, and in two atients a TK infection was clinically susected. The following exclusion criteria were used: inability to use a dedicated knee coil for the MRI examination because of obesity of the atient, because the sequences used in this study would have to be altered substantially for a multiurose coil (six atients were excluded for this reason); regnancy (no atients excluded); and lack of informed consent (no atients excluded). Our study oulation consisted of 42 individuals (mean age, 66.1 years; range, years), including 23 women (mean age, 67.2 years; range, years) and 19 men (mean age, 64.7 years; range, years). Of the 42 atients included in our study, 29 (69%) also underwent CT for clinical indications (evaluation of bone structure or rotational ositioning of the rosthesis comonents) within a eriod of 3 months from the MRI examination. In this grou of atients, CT data were retrosectively used as a reference standard for the assessment of abnormal imaging findings. MRI 1.5-T MRI scanner (Magnetom vanto, Siemens Healthcare) was used for all examinations. The atients were ositioned suine on the examination table, and a dedicated knee coil (eight receiver channels) was used for image acquisition. Imaging was erformed with a SEMC sequence (WRP- TSE, Siemens Healthcare) that includes SEMC and view-angle tilting in combination with highbandwidth arameters both for signal readout and radiofrequency excitation. To rovide a robust STIR contrast in the resence of magnetic field distortions near the imlant, the SEMC sequence also includes an increased bandwidth of the inversion ulse that matches the bandwidth of the excitation ulse. Four different sequences were acquired. First was a coronal STIR 2D sequence with the SEMC technique, with TR/TE of 6000/35, section thickness of 4 mm, refocusing fli angle of 150, FOV of 20 cm, matrix of , one signal acquired, excitation bandwidth of 1.8 khz, readout bandwidth of 620 Hz/ixel, echo sacing of 6.98 ms, echo-train length of 21, inversion time of 145 ms, 23 slices, hase-encoding direction of right to left, arallel imaging using generalized autocalibrating artial arallel acquisition (GRPP) factor of 2, eight slice-encoding stes, and acquisition time of 5 minutes 44 seconds. Second was a standard coronal STIR 2D sequence otimized with high bandwidth with TR/TE of 6000/35, section thickness of 4 mm, refocusing fli angle of 150, FOV of 20 cm, matrix of , one signal acquired, excitation bandwidth of 1.3 khz, readout bandwidth of 620 Hz/ ixel, echo sacing of 8.66 ms, echo-train length of 8, inversion time of 145 ms, 23 slices, hase-encoding direction of right to left, no arallel imaging, no slice-encoding stes, and acquisition time of 3 minutes 32 seconds. The third sequence was a sagittal roton density (PD) weighted 2D sequence with the SEMC technique with TR/TE of 5010/15, section thickness of 4 mm, refocusing fli angle of 135, FOV of 18 cm, matrix of , one signal acquired, excitation bandwidth of 1.8 khz, readout bandwidth of 531 Hz/ixel, echo sacing of 7.56 ms, echo-train length of 25, 23 slices, hase-encoding direction of anterior to osterior, arallel imaging using GRPP factor of 3, 12 slice-encoding stes, and acquisition time of 5 minutes 7 seconds. The fourth sequence was a standard sagittal PDweighted 2D sequence otimized with high bandwidth with TR/TE of 5010/14, section thickness of 4 mm, refocusing fli angle of 135, FOV of 18 cm, matrix of , one signal acquired, excitation bandwidth of 2.0 khz, readout bandwidth of 485 Hz/ixel, echo sacing of 7.18 ms, echo-train length of 7, 23 slices, hase-encoding direction of anterior to osterior, no arallel imaging, no sliceencoding stes, and acquisition time of 3 minutes 42 seconds. For the SEMC sequences, arallel imaging was alied along the conventional hase-encoding direction only, not along the additional slice-encoding dimension. In our routine rotocol, we also acquire a transverse STIR sequence with an otimized inversion ulse and a coronal T1-weighted sequence otimized with high bandwidth for artifact reduction; these sequences were not used in our study. CT In the grou of atients who underwent CT, a 40-MDCT scanner was used (rilliance, Philis Healthcare). The atients were laced suine on the examination table. The following imaging arameters were used: tube voltage, 120 kv; tube current time roduct, 250 ms; collimation, ; itch, 0.924; matrix, ; thickness, mm; and reconstruction slice thickness, 2.0 mm. Transversal, coronal, and sagittal reconstructions of the CT data were erformed. Quantitative nalysis radiologist quantified the area of signal void on the image. Signal void was defined as the area that contained no anatomic information because of the low- and high-signal artifacts that are resent because of the metallic hardware. oth on coronal STIR images and sagittal PD-weighted images, the first area of signal void consisted of the femoral comonent, and the second area consisted of the tibial comonent. For all measurements, the image at the level of the center of the tibial eg was chosen. In addition, on both STIR sequences, the area of insufficient fat saturation was measured at the level of the greatest extent of the artifact. For this measurement, all areas with slightly or markedly increased signal intensity and blurred edges outside of the signal void area were included into one cumulative measurement [19]. Qualitative nalysis Two radiologists indeendently assessed all SEMC sequences and the corresonding conventional MRI sequences otimized with high bandwidth. oth readers were blinded to the clinical diagnosis of the atients. ga of 2 months was observed between the qualitative and the quantitative reading session to reduce recollection bias. Deiction of anatomic structures On the coronal STIR images, the medial collateral ligament and es anserine were assessed as anatomic structures adjacent to the medial art of the TK, and 1316 JR:201, December 2013

3 MRI of Total Knee rthrolasty the lateral collateral ligament and bices femoris tendon were assessed on the lateral side of the TK. On the sagittal PD-weighted images, the atella and Hoffa fat ad were assessed directly anterior of the TK, and the medial and lateral origins of the gastrocnemius muscle were assessed directly osterior of the TK. The anatomic structures were rated on a 5-oint scale: 1, good delineation of anatomic structure; 2, anatomic structure fully visible, with slight blurring of borders; 3, anatomic structure fully visible, with substantial blurring of borders; 4, anatomic structure only artially visible; and 5, anatomic structure not visible. Image quality 5-oint scale was used to rate geometric image distortion, blurring, and noise at MRI: 1, no artifacts; 2, hardly visible artifacts; 3, clearly visible artifacts, but without imairment of diagnostic quality; 4, substantial artifacts with moderate imairment of diagnostic quality; 5, severe artifacts and nondiagnostic-quality image. bnormal imaging findings bnormal findings, such as knee joint effusion, oliteal cysts, or erirosthetic osteolysis, were recorded. The number of abnormal imaging findings er sequence was recorded. nalysis of erirosthetic osteolysis In a searate assessment of CT examinations that took lace 2 months after the MRI reading session, the resence or absence of a erirosthetic osteolysis Fig. 1 Differences in metal artifact size on STIR and roton density (PD) weighted images., STIR imaging. rea of signal void for femoral and tibial comonent and area of insufficient fat saturation was significantly smaller for STIR with slice-encoding for metal artifact correction (SEMC) images than for STIR images with standard high-bandwidth sequences ( < for all three comarisons). Whiskers denote SD., PD-weighted imaging. rea of signal void for femoral and tibial comonent was significantly smaller for PD-weighted images with SEMC than for PD-weighted images with standard high-bandwidth sequences (femur, < 0.001; tibia, = 0.005). Whiskers denote SD. Fig year-old woman with total knee arthrolasty (TK) of left knee. and, MRI artifacts induced by rosthesis (arrowheads, and ) are larger on standard coronal STIR sequence with high bandwidth () than on coronal STIR image with slice-encoding for metal artifact correction (SEMC) (). Medial collateral ligament (long arrows, ), lateral collateral ligament (short arrows, ), and small joint effusion (asterisk, ) are visible on STIR with SEMC () but are not discernible on standard STIR sequence () because of larger artifact size. was recorded in all atients where a CT examination of the TK of the same knee was available (29 atients). This assessment was done in consensus, and the findings detected at CT were subsequently used as the reference standard for the assessment of erirosthetic osteolysis. Statistics statistical software ackage (SPSS for Windows, release 17.0, SPSS) was used for all analyses. Signal void size and areas with insufficient fat saturation were comared with descritive statistics, and mean values and SDs were calculated. The differences in quantitative measurements were assessed by the aired Student t test, with a value below 0.05 indicating significance. The differences in qualitative data were assessed with a Wilcoxon signed rank test. The number of discordant cases of abnormal imaging findings between the two MRI techniques was analyzed with a McNemar test. Results Quantitative nalysis STIR images Signal void for the femoral comonent was significantly smaller for STIR images with SEMC versus standard images (18.9 ± 4.1 cm 2 vs 25.3 ± 5.7 cm 2 ; < 0.001). lso for the tibial comonent, signal void was significantly smaller for STIR rea of rtifact (cm 2 ) SEMC High andwidth Signal Void: Femur Signal Void: Tibia Insufficient Fat Saturation images with SEMC versus standard images otimized with high-bandwidth (21.3 ± 3.5 cm 2 vs 28.7 ± 6.1 cm 2 ; < 0.001). The area of insufficient fat saturation was almost eight times smaller for STIR images with SEMC versus standard images otimized with highbandwidth (7.4 ± 4.9 cm 2 vs 56.5 ± cm 2 ; < 0.001) (Fig. 1). PD-weighted images Signal void for the femoral comonent was significantly smaller for PD-weighted images with SEMC versus standard images (15.5 ± 7.4 cm 2 vs 19.3 ± 8.5 cm 2 ; < 0.001). lso for the tibial comonent, the area of insufficient fat saturation was significantly smaller for PD-weighted images with SEMC versus standard images (12.9 ± 3.3 cm 2 vs 13.7 ± 3.2 cm 2 ; = 0.005) (Fig. 1). Qualitative nalysis Deiction of anatomic structures The SEMC sequences were substantially better in deicting the anatomic structures when comared with the standard sequences (Fig. 2), both for the STIR sequence ( < for all comarisons) and slightly less ronounced but still statistically significant for the PDweighted sequence ( < to = for different comarisons). The deiction of anatomic structures is summarized in Table 1. rea of rtifact (cm 2 ) Signal Void: Femur SEMC High andwidth Signal Void: Tibia JR:201, December

4 TLE 1: Deiction of natomic Structures, by Reader, Tye of Imaging, and Sequence Reader 1 Reader 2 Tye of Imaging, natomic Structure Metal rtifact Qualitative assessment of metal artifacts For most criteria, image quality was less affected by the metal hardware using the SEMC sequences comared with the standard sequences, as shown in Table 2. This effect was more ronounced for the STIR technique (Fig. 3). The PD-weighted sequences had significantly less distortion and noise on the SEMC images versus the standard images ( < to = for different comarisons) (Table 2). However, there was more blurring for the SEMC technique versus the standard technique for the PD-weighted sequences ( < for reader 1 and = for reader 2). bnormal imaging findings Significantly more abnormal imaging findings were noted on STIR images with SEMC than on standard STIR images with high-bandwidth both for reader 1 (98 findings vs 37 findings; < 0.001) andwidth and for reader 2 (74 findings vs 37 findings; < 0.001) (Table 3). ll abnormal findings noted on the standard sequences were also recognized on STIR sequences with SEMC. In the whole study oulation, erirosthetic osteolysis was more often detected on STIR with SEMC versus standard high-bandwidth sequences; however, this was only statistically significant for reader 1 ( = 0.002) and not for reader 2 ( = 0.06) (Fig. 4). lso for the PD-weighted images, significantly more abnormal imaging findings were noted for the SEMC technique than for the conventional technique, both for reader 1 (88 findings vs 55 findings; < 0.001) and for reader 2 (65 findings vs 35 findings; < 0.001) (Table 3). ll abnormal findings noted on the standard sequence were also recognized with the SEMC technique (Fig. 5). Metal rtifact andwidth STIR imaging Medial collateral ligament 2.8 ± ± 0.5 < ± ± 0.4 < Pes anserine 2.8 ± ± 0.8 < ± ± 0.7 < Lateral collateral ligament 3.2 ± ± 0.6 < ± ± 0.3 < ices femoris tendon 2.3 ± ± 0.7 < ± ± 0.6 < Proton density weighted imaging Hoffa fat ad 2.7 ± ± 0.8 < ± ± 0.8 < Patella 3.3 ± ± ± ± Medial gastrocnemius head 3.2 ± ± 0.6 < ± ± Lateral gastrocnemius head 3.3 ± ± 0.6 < ± ± Note natomic structures were assessed on a 5-oint scale from 1 (good) to 5 (not visible). Data are mean ± SD score. TLE 2: Qualitative ssessment of Metal rtifacts, by Reader, Tye of Imaging, and Sequence Tye of Imaging, Parameter Metal rtifact Reader 1 Reader 2 andwidth Metal rtifact andwidth STIR imaging Distortion 2.9 ± ± 0.5 < ± ± 0.4 < lurring 3.0 ± ± 0.7 < ± ± 0.7 < Noise 2.9 ± ± ± ± 0.7 < Proton density weighted imaging Distortion 2.8 ± ± 0.6 < ± ± 0.6 < lurring 2.8 ± ± 0.5 < ± ± Noise 2.2 ± ± 0.5 < ± ± Note Distortion, blurring, and noise were assessed on a 5-oint scale from 1 (no artifacts) to 5 (severe artifacts). Data are mean ± SD score. Perirosthetic osteolysis In the grou of atients where a CT scan of the knee was available, seven of 29 atients (24%) had a erirosthetic osteolysis found on CT. Sensitivity for the detection of osteolysis was markedly higher for the STIR images with SEMC (100% and 85.7% for readers 1 and 2, resectively), comared with the standard STIR images (14.3% and 28.6% for readers 1 and 2, resectively) (Table 4). ccuracy was also higher for the STIR images with SEMC (100% and 96.6% for readers 1 and 2, resectively), comared with the STIR images with high-bandwidth (79.3% and 82.8% for readers 1 and 2, resectively) (Fig. 4). On STIR images with SEMC, reader 2 missed one case with osteolysis, whereas reader 1 correctly detected all cases with osteolysis that were seen on CT. On STIR images with high JR:201, December 2013

5 MRI of Total Knee rthrolasty bandwidth, a substantial number of cases seen on CT were missed on MRI (six and five falsenegative cases for readers 1 and 2, resectively) (Table 4). No false-ositive cases were resent for both MRI techniques. Sensitivity for the detection of erirosthetic osteolysis was higher for the PDweighted images with SEMC (57.1% for both readers), comared with the PD-weighted images with high-bandwidth (42.9% and 28.6% for readers 1 and 2, resectively), but C the advantage for the PD-weighted images with SEMC was less ronounced than for the STIR images with SEMC when comared with the standard images (Table 4). ccuracy for the detection of osteolysis was similar for both techniques, with a slight advantage for PD-weighted images with SEMC (89.7% for both readers), comared with PD-weighted images with highbandwidth (86.2% and 82.8% for readers 1 and 2, resectively) (Figs. 5 and 6). The Fig year-old man with total knee arthrolasty (TK) of left knee at MRI. and, rtifacts are substantially larger on coronal STIR image with standard high-bandwidth sequence () than on coronal STIR image with slice-encoding for metal artifact correction (SEMC) (). Pes anserine bursitis (solid arrowheads, ) and normal roximal tibiofibular joint (oen arrowhead, ) are deicted on STIR image with SEMC () but are obscured by artifacts on STIR standard high-bandwidth sequence (). C and D, Prosthesis comonents (asterisks, C and D) are distorted on sagittal roton density (PD) weighted image with standard high-bandwidth sequence (C) but are well demarcated on sagittal PD-weighted SEMC image (D). Fibrosis of Hoffa fat ad (arrow, C and D) is barely visible on PD-weighted image with standard highbandwidth sequence (C) but is clearly delineated on PD-weighted SEMC image (D). D number of cases with a missed osteolysis at MRI comared with CT was lower for PDweighted images with SEMC (three cases for each reader) than for PD-weighted images with high-bandwidth (four and five cases for readers 1 and 2, resectively) (Table 4). No false-ositive cases were resent for both MRI techniques. Discussion The long-term outcome of TK is good for most atients [1]. However, residual ain is common in many atients after uncomlicated TK [1]. etween 7% and 28% of atients have substantial ain at rest or during walking 1 year after TK surgery [2, 20, 21]. Imaging in atients with a ainful TK is tyically erformed with standard radiograhs to assess the osition and integrity of the rosthesis comonents and to detect signs of wear or erirosthetic fracture [1]. Some institutions erform additional weightbearing radiograhs of the whole leg to measure the mechanical axis, and, if TK infection is susected, an asiration will be erformed to obtain a cell count and samles for microbiological assessment [1, 22, 23]. For the next ste of the diagnostic algorithm, some authors favor CT and others refer bone scintigrahy to search for signs of loosening or infection [23 26]. MRI has not been widely used in the last decade to assess atients with TK because of the large metal artifacts induced by the orthoedic hardware [3, 27]. However, several recent studies have shown a ossible role for MRI in evaluating atients with TK with otimized standard sequences [28, 29]. In addition, advanced MRI of the knee with through-lane distortion correction has been used in a study with the multile-acquisition with variable resonance image combination technique in 21 atients with knee arthrolasty [15] and in two studies using the SEMC technique in 14 volunteers with TK [17] as well as in knee hantoms and one volunteer with TK [18]. In the resent study, we imlemented the SEMC technique in a clinical setting: Comared with standard MRI sequences otimized with high bandwidth, the SEMC sequence resulted in a statistically significant imrovement for the deiction of anatomic structures and a statistically significant reduction of distortion and image noise. It should be noted that residual artifacts are resent with SEMC, in articular in the close vicinity of the imlant, and therefore the bone-metal interface is not comletely JR:201, December

6 TLE 3: Comarison of bnormal Imaging Findings, by Reader, Tye of Imaging, and Sequence Reader 1 Reader 2 Tye of Imaging, Finding Metal rtifact artifact free. These residual artifacts may be due to the limited number of slice-encoding stes that were chosen to achieve a scan duration suitable for clinical use and also due to the limited ability of SEMC to correct for very strong local field variations [30]. The acquisition scheme of SEMC greatly increases the scan time. This was counteracted by using arallel imaging and a larger echotrain length. s a trade-off, this may lead to a decreased signal-to-noise ratio and increased blurring of short T2 tissue, even though we did not observe a major imact on the images. nother trade-off was the use of only eight and 12 slice-encoding stes (for STIR and PD-weighted images, resectively) when using SEMC. Even though a larger number of andwidth stes would cover a larger range of off-resonant signal, the increased scan time would not be feasible for clinical use. Our results suggest that, although there are residual artifacts with these scan arameters, the bulk of distortions are corrected and this may be an accetable comromise for clinical use. The detection of abnormal imaging findings was markedly increased for the SEMC sequence; this effect was most ronounced for STIR images, with an imroved detection of findings such as soft-tissue edema or bone marrow edema, which is esecially helful in evaluating atients with collateral ligament injury or stress reactions of the bone [7, 31]. The assessment of erirosthetic osteolysis due to article disease is an essential roblem Metal rtifact andwidth STIR imaging Joint effusion < Politeal cyst one infarction Perirosthetic osteolysis Ganglion cyst Synovitis N Soft-tissue edema one marrow edema Pes anserine bursitis Tendon abnormalities 2 0 N 3 0 N Other ll abnormal imaging findings < a < a Proton density weighted imaging Joint effusion Politeal cyst Fibrosis of Hoffa fat ad 21 8 < < one infarction N Perirosthetic osteolysis Patellar cartilage defect 1 0 N 1 0 N Osseous defect Ganglion cyst Synovitis 4 0 N 1 0 N Tendon abnormalities 0 0 N Other ll abnormal imaging findings < a < a Note Data are absolute numbers of abnormal imaging findings in all 42 atients. N = not alicable. < 0.05 denotes statistical significance. a Overall significance. in clinical routine, because osteolysis may lead to remature loosening of a TK [23]. Radiograhs are routinely used for follow-u of atients with TK, but they have been shown to underestimate the extent of erirosthetic osteolysis because the rosthesis comonents may obscure osteolytic zones, esecially if the radiograhs are not acquired recisely arallel to the rosthesis comonents [32 34]. Several modalities can be used for the imaging worku in atients with TK with a susected osteolysis: study with 31 atients after TK reorted that CT was suerior to radiograhs for detecting osteolysis, with only eight lesions detected on the radiograhs versus 48 lesions detected on CT [32]. Zotti et al. [33] examined simulated erirosthetic oste JR:201, December 2013

7 MRI of Total Knee rthrolasty olysis in three cadavers with TK with radiograhs and CT and found in a receiver oerating curve analysis that CT erformed better than radiograhs at detecting osteolysis (area under the curve, 0.82) comared with radiograhs (area under the curve, for different radiograhs). Fig year-old man with total knee arthrolasty (TK) of left knee. and, Metal artifacts at MRI are markedly larger on coronal STIR image with standard high-bandwidth sequence () than on coronal STIR image with slice-encoding for metal artifact correction (SEMC) (). Small erirosthetic osteolysis (arrowhead, ) is deicted in medial tibial lateau, and large osteolysis is deicted around tibial eg (large arrow, ) and adjacent to osteotomy screws (small arrows, ) on STIR SEMC image (), but osteolytic zones are obscured by large metal artifacts on STIR image with standard high-bandwidth sequences (). Small joint effusion (asterisk, and ) is seen both on STIR standard high-bandwidth sequence () and STIR with SEMC (). C, Coronal CT reconstruction of left knee confirms erirosthetic osteolysis in medial tibial lateau (arrowhead), around tibial eg (large arrow), and adjacent to osteotomy screws (small arrows). Fig year-old woman with total knee arthrolasty (TK) of left knee at MRI. and, Prosthesis comonents (asterisk, and ) are distorted on sagittal roton density (PD) weighted standard high-bandwidth sequence image () but are sharly delineated on sagittal PD-weighted image with slice-encoding for metal artifact correction (SEMC) (). Fibrosis of Hoffa fat ad (thin arrow, and ) is visible but distorted on standard PD-weighted image with high-bandwidth (), whereas it is better visible on PD-weighted image with SEMC (). Perirosthetic osteolysis around tibial eg (arrowheads, ) and joint effusion (large arrow, ) are visible on PD-weighted image with SEMC (), but not on standard PD-weighted image with high-bandwidth (). In a study ublished in 2006, Vessely et al. [4] retrosectively reviewed radiograhs and MRI examinations of 11 atients after TK and reorted that, although erirosthetic osteolysis was susected on radiograhs in all 11 atients, MRI revealed additional osteolytic zones in five atients. In a recent study by Hayter et al. [15] that assessed MRI after arthrolasty, erirosthetic osteolysis was detected in five of 21 atients with TK on multile-acquisition with variable resonance image combination images, whereas no osteolysis was detected on conventional fast sin-echo images in these atients; however, because no reference C JR:201, December

8 standard was available, sensitivity and secificity were not determined in that study. Solomon et al. [29] reorted in a study of six knee cadavers that overall sensitivity for the detection of simulated erirosthetic osteolysis was higher for MRI (89%) than for CT (83%) and radiograhs (66%), whereas secificity was higher for CT (98%) than for MRI (90%) and radiograhs (51%). In our study, CT was used as the reference standard. The STIR images with the SEMC sequence showed a markedly imroved sensitivity for the detection of erirosthetic osteolysis (86 100%) when comared with STIR TLE 4: Detection of Perirosthetic Osteolysis, by Reader, Tye of Imaging, and Sequence Tye of Imaging, Finding Metal rtifact images with high-bandwidth (14 29%). lso PD-weighted imaging with SEMC had an imroved sensitivity for the detection of erirosthetic osteolysis (57%) when comared with PD-weighted imaging with high-bandwidth (29 43%), but the effect was less ronounced than for the STIR images. andwidth Reader 1 Reader 2 Reader 1 Reader 2 STIR imaging True-ositive (no. of atients) True-negative (no. of atients) False-ositive (no. of atients) False-negative (no. of atients) Sensitivity, % (95% CI) 100 ( ) 85.7 (73 99) 14.3 (2 27) 28.6 (12 45) Secificity, % (95% CI) 100 ( ) 100 ( ) 100 ( ) 100 ( ) Positive redictive value, % (95% CI) 100 ( ) 100 ( ) 100 ( ) 100 ( ) Negative redictive value, % (95% CI) 100 ( ) 95.7 (88 100) 78.6 (64 94) 81.5 (67 96) ccuracy, % (95% CI) 100 ( ) 96.6 (90 100) 79.3 (65 94) 82.8 (69 97) Proton density weighted imaging True-ositive (no. of atients) True-negative (no. of atients) False-ositive (no. of atients) False-negative (no. of atients) Sensitivity, % (95% CI) 57.1 (39 75) 57.1 (39 75) 42.9 (25 61) 28.6 (12 45) Secificity, % (95% CI) 100 ( ) 100 ( ) 100 ( ) 100 ( ) Positive redictive value, % (95% CI) 100 ( ) 100 ( ) 100 ( ) 100 ( ) Negative redictive value, % (95% CI) 88.0 (76 100) 88.0 (76 100) 84.6 (72 98) 81.5 (67 96) ccuracy, % (95% CI) 89.7 (79 100) 89.7 (79 100) 86.2 (74 99) 82.8 (69 97) Note The analysis was erformed in the grou of 29 atients where CT was available as a reference standard. Fig year-old man with total knee arthrolasty (TK) of right knee at MRI. and, Prosthesis comonents (asterisk, and ) are distorted on sagittal roton density (PD) weighted image with high-bandwidth () but are well demarcated on sagittal PD-weighted image with slice-encoding for metal artifact correction (SEMC) (). Perirosthetic osteolysis around tibial eg (arrowheads, and ) and lobulated ganglion cyst (small arrows, and ) are visible both on PDweighted image with high-bandwidth () and on PD-weighted image with SEMC (). Joint effusion and synovitis (large arrow, and ) are visible on both images, but synovitis is better discernible on PD-weighted image with SEMC () than on PDweighted image with high-bandwidth () JR:201, December 2013

9 MRI of Total Knee rthrolasty Our results show that the increased image quality and diagnostic erformance of the STIR sequence is a substantial advantage of the SEMC technique. To date, several grous have focused on non-fat-suressed sequences when evaluating atients with TK [15, 28], because these sequences are less suscetible to the field inhomogeneities induced by the metal than are fat-suressed or STIR images [4, 5, 35]. Using the SEMC technique, we have shown that it is ossible to obtain STIR images in atients with TK to assess the bone-rosthesis interface, as well as the eriarticular ligaments and soft tissues. n alternative otion for diagnosing rosthesis loosening is bone scintigrahy, but data from a study of 80 atients showed a sensitivity of 92%, secificity of 76%, and a diagnostic accuracy of 81% in cases of an unequivocally normal or abnormal bone scintigram [26]. The STIR with SEMC sequence erforms favorably with an accuracy of % in our study. SPECT/CT may have some clinical value in the evaluation of atients with a ainful TK [36], but an analysis of 77 atients who underwent TK or unicomartmental knee rosthesis roduced only moderate results for SPECT/CT, with a sensitivity of 81% and a secificity of 75% [37]. Furthermore, nuclear medicine scans are hamered by a markedly increased hysiologic utake in asymtomatic atients for the first 2 years after TK [38]. moderately increased utake in the medial tibial comartment may ersist for u to 4 years [39]. MRI has further advantages comared with CT or nuclear medicine scans for evaluating the TK, such as the deiction of bone marrow edema and the assessment of soft tissues, including synovial abnormalities [40], and MRI might be esecially helful in evaluating atients in the first 2 years after TK, when nuclear medicine scans are often nondiagnostic [26, 38]. With the advances in image quality and the imroved deiction of abnormal imaging findings, including erirosthetic osteolysis, we roose to include MRI in the diagnostic algorithm for the evaluation of atients with a ainful TK. t our institution, we have included the coronal STIR sequence with SEMC in the standard rotocol for evaluating atients with a ainful TK, combined with a transverse STIR sequence with an otimized inversion ulse and a coronal T1-weighted and sagittal PDweighted sequence with high bandwidth. sagittal PD-weighted sequence with SEMC is also acquired in selected cases. The following limitations are resent in our study. First, although we were able to use CT as a reference standard for the detection of erirosthetic osteolysis in 29 atients, there was no reference standard for the remaining 13 atients included in the study. s mentioned already, CT is widely used for detecting erirosthetic osteolysis, but it has limited accuracy. Surgery would be a more solid reference standard to assess erirosthetic osteolysis. Finally, in some cases, through-lane distortion might influence signal void area measurements, because signal intensity or anatomic structures from an adjacent slice may be visible on the image where the measurements are erformed. ecause more distortion is resent with standard sequences otimized with high bandwidth than with SEMC, this could result in a relative underestimation of the signal void area on the standard images using high-bandwidth, comared with the SEMC images. Our data show a statistically significant reduction of signal void for the SEMC images versus the standard images with high-bandwidth, and with the effect described already, the reduction of signal void for SEMC might even be larger. In conclusion, the SEMC sequences showed a statistically significant artifact reduction comared with standard MRI sequences otimized with high bandwidth. 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