CT in Patients With External Fixation for Complex Lower Extremity Fractures: Impact of Iterative Metal Artifact Reduction Techniques on Metal Artifact Burden and Subjective Quality

BACKGROUND. Lower extremity external fixators have complex geometries that induce pronounced metal artifact on CT. Iterative metal artifact reduction (iMAR) algorithms help reduce such artifact, although no dedicated iMAR preset exists for external fixators. OBJECTIVE. The purpose of our study was to compare iMAR presets for CT examinations in terms of quantitative metal artifact burden and subjective image quality in patients with external fixators for complex lower extremity fractures. METHODS. This retrospective study included 72 CT examinations in 56 patients (20 women, 36 men; mean age, 56 +/- 18 [SD] years) with lower extremity external fixators (regular, hybrid, or monotube). Examinations were reconstructed without iMAR (hereafter referred to as "noMAR") and with three iMAR presets (iMAR(spine), iMAR(hip), iMAR(extremity)). A radiology resident quantified metal artifact burden using software. Two radiology residents independently assessed overall image quality and diagnostic confidence using 4-point scales (4 = excellent [highest quality or highest confidence]). Techniques were compared using Bonferroni-corrected post hoc tests. Interreader agreement was assessed by intraclass correlation coefficients (ICCs). A post hoc multinomial regression model was used for predicting overall image quality. RESULTS. Mean quantitative metal artifact burden was 100,816 +/- 45,558 for noMAR, 88,889 +/- 44,028 for iMAR(spine), 82,295 +/- 41,983 for iMAR(hip), and 81,956 +/- 41,890 for iMAR(extremity). Overall image quality yielded an ICC of 0.94 or greater. Using pooled reader data, median overall image quality score for the regular fixator was 2 (noMAR), 3 (iMAR(spine) and iMAR(hip)), and 4 (iMAR(extremity)); for the hybrid fixator, 1 (noMAR), 2 (iMAR(s)(pine)), and 3 (iMAR(hip) and iMAR(extremity)); and for the monotube fixator, 2 (noMAR), 3 (iMAR(spine) and iMAR(h)(ip)), and 4 (iMAR(extremity)). Metal artifact burden was lower and overall image quality was higher (p < .05) for iMAR(hip) and iMAR(extremity) than noMAR and iMAR(spine) for all fixators (aside from image quality of iMAR(hip) and iMAR(extremity) vs iMAR(spine) for regular fixators) but were not different (all, p > .05) between iMAR(hip) and iMAR(extremity). Median diagnostic confidence was 4 for all fixators and reconstructions. Independent predictors of overall quality relative to noMAR were iMAR(s)(pine) (odds ratio [OR] = 1.92-5.51), iMAR(hip) (OR = 5.56-31.10), and iMAR(extremity) (OR = 7.07-38.21). All iMAR presets introduced new reconstruction artifacts for all examinations for both readers. CONCLUSION. For the three fixator types, iMAR(hip) and iMAR(extremity) achieved greatest metal artifact burden reduction and highest subjective image quality, although both introduced new reconstruction artifacts. CLINICAL IMPACT. CT using the two identified iMAR presets may facilitate perioperative management of external fixators.