利用低解析度映象和空間頻率能量頻譜修正梯度平面回訊影像扭曲

Abstract

本文提出一個修正平面回訊影像磁場不均勻幾何扭曲的修正方法。這個方法分析頻率空間的能量頻譜計算能量在頻率空間的位移，得到位移表示圖像，並由能量位移和磁場梯度的相關性求得磁場梯度分布圖，可以由磁場梯度分布圖算出磁場分布圖。利用磁場分布圖所提供的磁場資訊來修正影像。但是這個方法在計算磁場分布圖的時候沒有辦法取得磁場的偏移量，並且在積分近似的過程中，會有誤差累積的產生。 在取樣的過程中，利用重複取樣中間低頻部分頻率空間的方法，得到兩張低頻的影像，在兩次取樣中影像相位的改變可計算出低解析度的磁場分布圖。以此低解析度的磁場分布圖為參考值，來修正磁場梯度map的積分近似，這個方法可以計算出磁場的偏移量和修正誤差的累積，得到較為準確的磁場分布圖。實驗結果顯示，平面回訊影像的幾何扭曲的修正結果良好。

The geometric distortion in echo-planar imaging (EPI) and its correction are systematically investigated in this thesis. EPI is one of the fastest MRI acquisition pulse sequence and is widely used for dynamic studies. However, in the presence of the field inhomogeneities, the EPI k-space energy peaks are displaced, which result in geometric distortions in the reconstructed images. Here we use the k-space energy spectrum analysis to calculate the k-space energy displacement in EPI. The calculated k-space energy displacement map can be converted to the field inhomogeneity map, which can then be applied to correct the EPI distortions using a phase modulation post-processing procedure. The EPI data corrected with the previously developed k-space energy spectrum based method, however, may have residual distortions due to an unknown B0 reference and the accumulated errors during the integration procedure. To further improve the accuracy and reliability of EPI distortion correction, we propose to combine the k-space energy spectrum analysis and an altered EPI acquisition strategy, in which the central part of the k-space is double sampled. In this approach, the B0 reference can be obtained from the embedded low-resolution double-TE data. In addition, the error accumulation in the integration procedure can be minimized. Experimental results show that the new method can effectively remove EPI geometric distortions.