Improving Spatial Accuracy of MRI in the Presence of Metal-induced Geometric Distortions.

Objectives: To identify sequences and protocols for minimal metal-induced geometric distortion (MD) for improved spatial accuracy in MRI.

Materials and methods: A 3D lattice phantom containing a stainless-steel bracket or a crown-supported titanium implant was scanned using 6 MRI sequences (TSE, SEMAC, CS-SEMAC, SPACE, VIBE, and research sequence MSVAT-SPACE) in a 3T system. MD was assessed at 9360 crossing points as Euclidean distance using a standardized algorithm. MD was analyzed by total MD, affected volume (AV) at various thresholds, and directional dependency. Statistical analysis was performed by one-way ANOVA.

Results: For the stainless-steel bracket, TSE showed the highest total MD among all sequences (2187±297 mm, P<0.01) and maximum displacement (>4 mm), with 467 mL AV at MD>0.5 mm. CS-SEMAC and SEMAC yielded the lowest MD among all sequences (469±75 mm and 502±154 mm, P<0.01) and the smallest AV (55 mL and 45 mL) at MD>0.5 mm. 3D sequences exhibited intermediate performance with no significant difference (MSVAT-SPACE/VIBE/SPACE: 1569±204 mm/1137±71 mm/1513±143 mm; P>0.08). For the crown-supported titanium implant, all sequences showed reduced MD (<440 mm) and AV (<71 mL at MD>0.5 mm), while VIBE yielded a comparable AV (65 mL) but the highest MD (615 mm). MD was direction-dependent, particularly for the stainless-steel bracket, being highest along frequency-encoding direction (P<0.002); TSE also showed significantly higher MD in slice direction (P=0.0071), while distortions in phase direction were consistently lower.

Conclusions: Susceptibility artifact reduction sequences, particularly SEMAC and CS-SEMAC, effectively reduce total-MD by 79% and AV by 90%. Distortion varies by encoding direction and is most severe along the frequency-encoding axis, highlighting the importance of sequence and parameter selection for accurate MRI near metal implants.