多投影核磁共振逆影像

利用高度平行化核磁共振射頻線圈，核磁共振逆影像(magnetic resonance inverse imaging, InI) 可以達到全腦造影視野與100毫秒的時間解析度。此一快速動態影像技術是基於省略了使用梯度線圈於空間編碼的步驟，¬而改藉由射頻線圈的空間敏感度分布，從二維投影影像重建出三維的空間訊息。然而射頻線圈的空間敏感度分布因過於平滑, 所以不足以完全確切地解出三維影像，因此通常還要依靠數學上的限制以求出唯一解，若以要求解出之三維影像要有最小範數 (minimum l^2-norm)，則影像多會模糊。本論文提出結合多個投影影像，在維持100毫秒的時間解析度下，減少核磁共振逆影像空間解析度損失的方法。我們利用32通道頭部相列線圈於不同回合的功能性磁振造影中，分別擷取冠狀 (coronal)、矢狀 (sagittal)、與橫切面 (transverse) 投影影像, 再加以組合出三維影像。模擬的結果顯示，與核磁共振逆影像使用最小範數的影像重建相比較，多投影核磁共振逆影像(multi-projection InI, mInI) 可以顯著的增進影像空間解析度。當合併三組投影影像時，由點擴散函數的半高全寬值來量化的空間解析度，可由一組投影影像時的2.6像素提升至1.4像素 (每像素為4微米解析度)。若我們進一步考量點擴散函數的形狀，等效空間解析度將由16.9像素提升至4.7像素。本論文所發展之多投影核磁共振逆影像也應用於人腦功能性核磁共振影像實驗上，當受試者執行二選項反應時間 (two-choice reaction time) 工作時，多投影核磁共振逆影像顯示出與平面迴訊影像 (echo-planar imaging, EPI) 一致的視覺與感覺-運動(somatosensory) 腦區活化空間分佈。然而多投影核磁共振逆影像提供了100毫秒的空間解析度與全腦造影視野。使用三個投影的投影核磁共振逆影像資料揭露了側膝狀核 (lateral geniculate nucleus, LGN) 與視覺區之血液動態變化 (hemodynamic response) 分別比感覺運動區的血液動態變化早了1300毫秒與700毫秒。我們預期多投影核磁共振逆影像將可應用於血氧濃度變化對比之功能性核磁共振影像研究上以提供高時間與空間解析度的動態資訊幫助瞭解人腦功能。

關鍵字

核磁共振；核磁共振影像；功能性核磁共振影像；相列線圈；射頻線圈；逆影像；視覺；運動；逆算問題；投影

並列摘要

Using highly parallel radiofrequency (RF) detection, magnetic resonance inverse imaging (InI) can achieve 100-millisecond temporal resolution with the whole brain coverage. This is achieved by trading off partition encoding steps and thus the spatial resolution for a higher acquisition rate. The reduced spatial information is typically estimated by solving under-determined inverse problems using RF coil sensitivity information. The reconstructed InI images under the minimum l-2-norm constraint typically demonstrate a lower spatial resolution. Here we propose the multi-projection inverse imaging (mInI) method to combine different projection images to reduce the loss of spatial resolution of InI. Specifically, coronal, sagittal, and transverse projection images are acquired from different runs of the functional MRI (fMRI) acquisitions using a 32-channel head coil array. Simulations show that, compared to the InI reconstruction using the minimum l-2-norm, mInI improves the spatial resolution of the reconstructed image significantly. Going from one projection to three projections, the spatial resolution quantified by the full-width–half-maximum of the point-spread function (PSF) is improved from 2.6 pixels to 1.4 pixels (4 millimeter per pixel nominal resolution). Considering the shape of the PSF, the effective spatial resolution improves from 16.9 pixels to 4.7 pixels. In vivo fMRI experiments using a two-choice reaction time task shows visual and sensorimotor cortical activity spatially consistent with typical EPI data, yet mInI offers the 100 millisecond temporal. The mInI data with three projections reveal that the hemodynamic response at the lateral geniculate nuclei (LGN) and at the visual cortex precedes that at the sensorimotor cortex by 1300 ms and 700 ms respectively. mInI can be applied to BOLD-contrast fMRI experiments to characterize the dynamics of the activated brain areas with a high spatiotemporal resolution.

並列關鍵字

event-related ； fMRI ； InI ； visual ； motor ； MRI ； inverse problem ； projection

參考文獻

Belliveau, J.W., Kennedy, D.N., Jr., McKinstry, R.C., Buchbinder, B.R., Weisskoff, R.M., Cohen, M.S., Vevea, J.M., Brady, T.J., Rosen, B.R., 1991. Functional mapping of the human visual cortex by magnetic resonance imaging. Science 254, 716-719.

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Buxton, R.B., 2002. Introduction to functional magnetic resonance imaging : principles and techniques. Cambridge University Press, Cambridge, UK ; New York.

Chou, C.H., Kuo, T.F., Lin, C.C., Tsai, J.C., Lin, F.H., 2008. Glycosaminoglycan Synthesis of Chondrocytes in Fibrin Glue with Ghc6s Particles. Biomedical Engineering-Applications Basis Communications 20, 329-335.

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多投影核磁共振逆影像

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