最近的研究建議擴散加權梯度回波脈衝波序(diffusion-weighted gradient- echo) 可以去除來自血管內大部分的功能性磁振造影(fMRI)信號在場強1.5 特士拉，並且保留具有低顯著擴散因數(ADC)靠近組織的水自旋。另外，有些研究指出來自大血管外的信號只會貢獻給梯度回波功能性磁振造影技術，而不是自旋回波(spin-echo)功能性磁振造影，那是因為自旋回波功能性磁振造影的一百八十度射頻脈衝(RF)可以重新聚焦因為來自大血管的靜態磁場不均勻而導致的去相位效應。因為發生在接近神經元活化的微血管和小靜脈血流會比較早，為了增加時間的解析度和減少來自距離神經元活化遠的大血管內外信號，這個研究在場強1.5特士拉，只沿著切片選擇方向在自旋回波脈衝波序中實施兩極擴散梯度(bipolar diffusion gradient)，並給予短暫視覺刺激，然後在相同對雜比(CNR)上比較開始時間差異(onset time variance)在傳統梯度回波和非擴散加權自旋回波。結果顯示在擴散加權自旋回波信號偵測到的開始時間差異比梯度回波信號偵測的早，但是在擴散加權因數(b value)50秒每毫米平方和200秒每毫米平方並沒有差異。這結果顯示了擴散加權自旋回波信號被純化了，而且擴散加權因數50秒每毫米平方已經足以去除來自大血管的污染信號。除此之外，擴散加權因數50秒每毫米平方擴散加權自旋回波的開始時間領先梯度回波信號(2.23±0.11 毫秒vs. 2.92±0.08 毫秒, p<0.005)，而且擴散加權因數200秒每毫米平方擴散加權自旋回波的尖峰時間(time to peak)也比梯度回波信號早(4.62 ±0.07 毫秒vs. 4.97 ±0.19 毫秒, p<0.05)。同時，為了把擴散梯度的方向列入考量，這研究也同時在三個方向加入了雙極擴散梯度在自旋回波脈衝波序中，並且做了和前面相同的比較。結果表示在加權擴散自旋回波脈衝波序信號有比較早的開始時間、尖峰時間和開始時間差異，就如同單一方向的結果。這和我們所預期的是一致的，因為只有在單一像素中出現了強烈非均向的血流，梯度的方向才顯得重要。

Recent study had suggested diffusion-weighted (DW) gradient-echo (GE) pulse sequence null the majority of fMRI signal arise from intravascular(IV) at 1.5 T and retains water spins with lower ADC close to tissue water. Moreover, some groups indicate that extravascular (EV) component of large vessels contribute only to GE fMRI, not to spin echo (SE) fMRI, because the 180°RF pulse in SE fMRI can refocus the dephasing effect of the static field inhomogeneity around large vessels. Since blood flow occur earlier in capillaries and small venules near the activation site and gradually extend to larger draining vein, to improve temporal specify and reduce both the IV and EV signal contribution from large vessels distant to neuronal activity, this study apply bipolar diffusion gradients into SE pulse sequence only in slice selection direction at 1.5 T in response to brief visual stimulus and compare onset time variance at the same CNR to that detected in conventional GE and non-diffusion-weighted SE. The results show onset variance is smaller in DW SE than that of GE at the same CNR but no difference between DW SE b value of 50 s mm-2 and DW SE b value of 200 s mm-2. This illustrate DW SE signal is purified and b value 50 s mm-2 is sufficient to suppress contaminated signal from large vessels. Further, onset time of DW SE with b values of 200 s mm-2 precedes that of GE (2.23±0.11 vs. 2.92±0.08, p<0.005) and time to peak (TTP) of DW SE with b values of 200 s mm-2 is also earlier than that of GE (4.62 ±0.07 vs. 4.97 ±0.19, p<0.05). Meanwhile, to take the direction of applied diffusion gradients into consideration, this study also simultaneously incorporated 3 axes bipolar diffusion gradients into SE sequence and make the same comparison as previous did. The results show earlier onset time, time to peak, and smaller onset time variance in DW SE, as previous. This is consistent with what we expected because only in voxels with strong anisotropy of flow does the gradient direction become important.

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