使用核磁共振逆影像於面回訊氫原子核磁共振頻譜影像重建

Abstract

核磁共振頻譜 (magnetic resonance spectroscopy, MRS) 能以非侵入式的方法去量測大腦代謝物分布，可用於臨床的預防及診斷。面回訊氫原子核磁共振頻譜影像 (proton echo planar spectroscopy imaging, PEPSI) 是一種快速的磁振頻譜成像技術，可大幅度地縮短二維磁振頻譜影像掃描時間至一分鐘左右，並能提升核磁共振頻譜於臨床的實用性。 核磁共振逆影像 (inverse imaging, InI) 是一利用高度平行化核磁共振射頻線圈同步收取核磁共振信號以達成含括全腦視區以及 100 毫秒的時間解析度的成像技術。其基本原理是省略了使用梯度線圈於空間編碼的步驟，藉由多組射頻線圈的空間敏感度分布得以從二維投影影像重建出三圍的空間訊息。 本篇論文結合了上述兩種技術，藉由使用多通道陣列線圈，希望得到高倍數加速的資料截取，用以縮短頻譜影像的成像時間。然而射頻線圈空間敏感度通常較平滑，若以最小範數估計解 (minimum-norm estimate) 重建的影像多半會模糊，因此還要加入許多數學上的限制。雖然無法得到預期高倍速加速的結果，但利用此方法仍能加速頻譜影像截取資料時間，並可得到可信的大腦代謝物資料以供臨床診斷之用。

Magnetic resonance spectroscopy (MRS) is a non-invasive technique that has been used to investigate the metabolic changes in living tissues. Fast magnetic resonance spectroscopy imaging (MRSI) using the proton-echo-planar-spectroscopy-imaging (PEPSI) technique can provide spatial distribution of metabolites in one single radio-frequency (RF) excitation. This method significantly reduces the scan time of 2-dimension MRSI down to 1 minute. Inverse imaging (InI) uses a highly parallel RF coil array to achieve 100-millisecond temporal resolution with the whole brain coverage. Combining PEPSI sequence with InI can further accelerate the MRSI data acquisition. InI reconstruction utilizes coil sensitivities to reconstruct the omitted partition/phase encoded data by solving an under-determined inverse problem. This study shows that the acceleration rate of the combined PEPSI and INI method is not as fast as expected. Specifically, using a 32-channel head coil array, we can achieve up to 6-fold acceleration in 2D PEPSI. Such scan time reduction can still increase the potential of applying MRSI to clinical applications.