How does distortion correction correlate with anisotropic indices? A diffusion tensor imaging study

한글 요약

확산텐서영상(DTI)에서 영상 왜곡을 보정하는 방식이 임상 평가에 쓰이는 이방성 지표에 어떤 영향을 미치는지를 살핀 연구입니다. 연구팀은 건강한 성인의 DTI 자료에 보정을 하지 않는 경우와 SPM 및 AIR 소프트웨어 기반의 선형·비선형 정합을 각각 적용한 뒤, 텐서 적합 오차와 관심 영역별 지표, 구상 다발과 뇌량의 신경섬유 추적 결과로 성능을 비교했습니다. AIR 기반 비선형 정합이 왜곡을 가장 잘 줄였으며, 왜곡을 보정하면 분할 비등방도(FA)가 낮아지고 비선형 보정 후 가장 낮아진 반면 평균 확산도에는 차이가 없었습니다. 이는 왜곡을 보정하지 않거나 부적절하게 보정하면 FA가 실제보다 높게 추정될 수 있음을 뜻하며, 정확한 확산 지표를 얻으려면 비선형 왜곡 보정이 필요하다는 점을 보여 줍니다.

PURPOSE: The purpose of this study was to determine a suitable registration algorithm for diffusion tensor imaging (DTI) using conventional preprocessing tools [statistical parametric mapping (SPM) and automated image registration (AIR)] and to investigate how anisotropic indices for clinical assessments are affected by these distortion corrections.

MATERIALS AND METHODS: Brain DTI data from 15 normal healthy volunteers were used to evaluate four spatial registration schemes within subjects to correct image distortions: noncorrection, SPM-based affine registration, AIR-based affine registration and AIR-based nonlinear polynomial warping. The performance of each distortion correction was assessed using: (a) quantitative parameters: tensor-fitting error (Ef), mean dispersion index (MDI), mean fractional anisotropy (MFA) and mean variance (MV) within 11 regions of interest (ROI) defined from homogeneous fiber bundles; and (b) fiber tractography through the uncinate fasciculus and the corpus callosum. Fractional anisotropy (FA) and mean diffusivity (MD) were calculated to demonstrate the effects of distortion correction. Repeated-measures analysis of variance was used to investigate differences among the four registration paradigms.

RESULTS: AIR-based nonlinear registration showed the best performance for reducing image distortions with respect to smaller Ef (P<.02), MDI (P<.01) and MV (P<.01) with larger MFA (P<.01). FA was decreased to correct distortions (P<.0001) whether the applied registration was linear or nonlinear and was lowest after nonlinear correction (P<.001). No significant differences were found in MD.

CONCLUSION: In conventional DTI processing, anisotropic indices of FA can be misestimated by noncorrection or inappropriate distortion correction, which leads to an erroneous increase in FA. AIR-based nonlinear distortion correction would be required for a more accurate measurement of this diffusion parameter.