Reduction of metal artifacts in MR imaging of patients with total hip arthroplasty

Abstract: Aims of the studies: To apply and compare MR imaging metal artifact reducing sequences (MARS) including subtraction imaging after contrast application in patients with metal-on-metal (MoM) hip prostheses, investigate the prevalence and characteristics of periprosthetic abnormalities, as well as their relation with pain and risk factors. [Paper I]. To evaluate the distortion and artifact area of metal in MR images and to compare artifact reduction using different MARS in patients with MoM and polyethylene-on-metal (PoM) total hip prostheses (THA). [Paper II]. To develop methods for qualitative and quantitative evaluation of MRI artifacts near metallic prostheses, and to compare the efficiency of different artifact suppression techniques with different types of hip prostheses. [Paper III]. To investigate prevalence and characteristics of periprosthetic changes in PoM total hip arthroplasty patients using MR metal artifact reducing sequences (MARS). [Paper IV] Results: TSE VAT detected the highest number of osteolyses. Soft tissue mass (STM), effusion, and capsular thickening were common, whereas osteolyses in acetabulum and femur were less frequent. Contrast enhancement occurred in bone, synovia, joint capsule, and the periphery of STM. There was no significant relation between MRI findings and pain or risk factors. [Paper I]. The T1 VAT+SEMAC sequence showed the least distortion compared to T1 VAT and T1-hiBW (150°, 127° and 102°, p<0.001, in MoM; 152°, 143° and 128°, p≤0.014, in PoM). The artifact area was smaller in MoM prostheses using the T1 VAT sequence compared to T1 hiBW and T1 VAT+SEMAC (2506 mm compared to T1-hiBW (4296 mm STIR-hiBW (MoM 4559 mm by up to 50% compared to TSE, but did not reduce through-plane artifacts. SEMAC reduced through-plane artifacts by 60-80% compared to TSE and VAT. SEMAC in-plane artifacts were from 20 % higher (6 encoding steps) to 50% lower (16 steps) than VAT. Total artifacts were reduced by 60-80% in the best sequence (SEMAC, 16 steps) compared to the worst (TSE). The titanium prosthesis produced 3-4 times lower artifact scores than the other prostheses. [Paper III]. Prevalence of STM was numerically higher in the symptomatic than in the asymptomatic group (64 % versus 36 %), as was osteolysis in acetabular (73% vs. 36%) and femoral bones (55% vs. 36%), although these differences were not statistically significant. Acetabular osteolysis was significantly larger in the symptomatic group (p=0.04), whereas the size and capsular thickness of the STM, and the osteolysis of femur were not significantly different. [Paper IV] Conclusions: MARS and gadolinium subtraction imaging are useful for evaluation of complications to MoM prosthesis. TSE VAT had the highest sensitivity for osteolysis. Contrast enhancement might indicate activation of aseptic lymphocyte dominated vasculitis associated lesion (ALVAL). Pain, small head, or steep prosthesis inclination angle are not useful predictors of periprosthetic abnormalities, and wide indications for MR follow-up are warranted. [Paper I]. Metal artifacts in MR imaging examinations of hip prostheses can be evaluated for distortion using a distortion angle (ARA) and the degree of signal artifact as determined by measuring the largest cross-sectional artifact area. T1 VAT+SEMAC showed the least distortion; T1 VAT and STIR-mRFp were most efficient for reduction of the artifact area. [Paper II]. A rectilinear grid phantom is useful for qualitative and quantitative evaluation of artifacts provoked by different MR imaging protocols and prostheses models. VAT and SEMAC were superior to TSE with high bandwidth. A proper number of z-encoding steps in SEMAC was critical. The titanium prosthesis caused least artifacts. [Paper III]. MARS detected osteolyses in acetabular and femoral bone preoperatively with good precision. Acetabular osteolysis was significantly larger in the symptomatic group. There were numerically more periprosthetic changes in the symptomatic compared to the asymptomatic control group, but this latter difference was not statistically significant. Finally, nearly exclusively the fluid content of the soft tissue masses was homogeneous. [Paper IV]