高解析度、對比增強的人腦磁化率權重造影

Abstract

磁化率權重造影是近期發展的T2*權重的梯度迴訊(gradient echo)序列，利用不同組織的磁化率差異會導致局部的非均勻性磁場。這項技術是起源於高解析度、血氧濃度依賴的靜脈造影(HRBV)，相較於傳統的造影技術而言，可以提升以往看不見的微小靜脈血管的可見度。因為越來越多的發展和應用並不侷限在靜脈血管造影方面，所以現在被重新賦予了名稱『磁化率權重造影』。 磁化率權重造影的主要機制是利用不同組織的磁化率差異作為對比特性，特別是去氧紅血球在大腦靜脈結構中的磁化率特性。雖然現今，磁化率權重造影已經內建於許多核磁共振機台的序列中，但我們仍然要去了解磁化率權重造影的重建方法和參數，唯有如此，我們才能再進一步地對磁化率權重造影最佳化。 演算法的處理步驟1)對原始相位影像的濾波過程，移除背景造成的低頻雜訊部分2)製作負的相位遮罩，對已濾波的相位影像去做負相位的萃取和正規化3)將負相位遮罩去乘上振幅影像，將相位影像對磁化率敏感度的優勢，映射至磁化率權重的影像中4)最小強度投影，讓磁化率權重能進一步地被突顯出來。 最後，本研究會將磁化率權重造影的技術嘗試在臨床的疾病。許多文獻也證明在磁化率權重造影中，出血性的組織部位將會比用傳統照影方式更容易被偵測到；這項特性促成了磁化率權重造影在醫學上的貢獻，因而能有效地評估病理的症狀，越來越多臨床疾病的技術應用也陸續地被發現。

Susceptibility-weighted imaging (SWI) is the recently T2*-weighted gradient echo sequence, taking advantage of magnetic susceptibilities of tissues because of local in-homogeneities in mainly magnetic field. It originates from “High –Resolution Blood oxygen level dependent Venography (HRBV)”, helping visualization of the venous structure in the brain. In this way, minute vessels can be seen by HRBV, not by conventional imaging technique. Because more and more applications are not restricted to venography, it is recalled “Susceptibility-weighted imaging” for its contrast mechanism. The essential theory of SWI is utilizing paramagnetic properties to enhance contrast and detection, especially deoxygenated hemoglobin in venous vasculature. Until now, SWI has become the scanner-derived image, but we should know post-processed reconstruction in order to improve SWI technique one day. The processing methods include filtering phase data, generating phase mask and multiplying magnitude image with phase mask. The study also promotes the clinical applications of SWI. Literatures prove that hemorrhagic lesions are more detectable and visible in the SWI than in traditional sequences. This can contribute SWI to evaluate pathology effectively and efficient.