利用改良反向梯度磁場方法修正平面回訊影像的幾何扭曲：技術及其應用

Abstract

本論文展示一種平面回訊影像幾何扭曲的修正方法。這個方法是建基於反向梯度磁場方法，它是利用兩張影像在極性相反的相位編碼梯度磁場下，幾何扭曲在影像上的剛好呈現相反的表現，最後利用幾何扭曲在兩張影像上相反的特性所提供的資訊，來修正影像。但是，這個修正方法在修正一些低信號部份時，由於缺乏幾何扭曲的資訊，因此導致在這些低信號部份產生修正上的錯誤。在實行反向梯度磁場方法時，加入了位移表示圖像 (displacement map) 的概念去減低在低信號部份的修正錯誤。這個位移表示圖像的概念包括了除去位移表示圖像上錯誤的資訊、進行表面近似和濾波。在3T高磁場下實驗的結果顯示，相對於本來的反向梯度磁場方法，改良反向梯度磁場反法修正的影像展示出減低影像的幾何扭曲以及在低信號部份錯誤的修正。把改良反向梯度磁場法應用在螺旋槳式磁振平面回訊影像上，對每一個葉片資料作額外的修正，在高解析度擴散張量影像上有一定的優勢，可以提高影像的品質以及在臨床的應用。

A technique suitable for echo planar imaging (EPI) geometrical distortion correction is presented in this thesis. The method is based on reversed gradient method (RG) using phase-encoding gradient with reversed polarity that produces distortion in opposite direction, from which the information on the difference in spatial displacements is then used to correct for distortion. However, this method is problematic in regions with low signals because of a lack of information on distortions which leads an error correction in regions with low signals. The implementation of reversed gradient method induced a displacement map concept to reduce the error correction in regions with low signals. The displacement map concept consists of eliminating error displacement map, surface fitting and filtering. Experimental results on a 3.0 Tesla MR system showed that the modified reversed gradient method corrected images exhibit substantially reduced geometric distortions and correction error in regions with low signals compared with original reversed gradient method. Applied reversed gradient method in high-resolution DTI using PROPELLER EPI to extra correct each blade, shows benefits from the modified reversed gradient method. It can increase the image quality for routine diffusion tensor imaging applications in clinical practice.