Rapid steady-state MR imaging and its clinical applications (2/2)

本研究展示穩定態自由旋進影像中水與 脂肪信號分離之可行性。其中心思想為，在採 用TE = TR/2 之穩定態自由旋進技術中，相位 角相對於射頻脈衝基本上只允許兩種角度的 可能：亦即00 與1800。利用此種特性，吾人得 以利用中心頻率偏移的細部調整，在相同的影 像條件之下達到水與脂肪同相（in-phase）或反 相（out-of-phase）的影像，因此，一般磁振造 影所使用的迪克遜法（Dixon method）便可以 應用於此種技術中，經由複數影像的相加與相 減達成水與脂肪信號的分離。在本計畫中，掃 瞄參數TR 與中心頻率偏移量經由理論計算加 以最佳化，同時並在3.0 Tesla 磁振造影系統上 取得八位志願受試者腹部之二維穩定態自由 旋進影像以驗證理論的推測。實驗結果證實本 方法幾可達成接近完全的脂肪信號抑制，並且 比商業化的技術具有更佳的軟組織對比。The feasibility of fat/water separation using the balanced steady-state free precession (SSFP) technique is demonstrated. The key idea is based on the observation that at the nominal values of TE = TR/2 in SSFP imaging, phase coherence could be achieved at essentially only two orientations, namely 00 and 1800 relative to the RF pulses in the rotating frame, under the assumption of TR << T2 and independent of the steady-state free precession angle. This property allows in-phase and out-of-phase SSFP images to be obtained by proper choices of the center frequency offset, hence permitting the Dixon subtraction method to be utilized for effective fat/water separation. The TR and frequency offset for optimal fat/water separation are derived from theories. Experimental results on healthy subjects using a 3.0 Tesla system show that nearly complete fat suppression can be accomplished