在磁振造影中，某些特定的參數如反轉時間或是無線脈衝的頻率校準必須在進行臨床掃描前先確定。因此在一般臨床使用上，會有提供一組預掃描序列讓使用者由該影像群中選定最好的掃描參數。但是，這樣的預掃描序列會降低整體掃描流程的流暢性，這是因為使用者必須中斷掃描來手動的選擇最佳參數後才繼續接下來的掃描。在本論文中，我們提出了一套即時回授系統，該系統能夠自動且快速地最佳化所需要的參數。我們也將此系統應用在兩個不同的研究上：一個為使用z梯度修正感磁性假影的補償梯度之最佳化；另一個則為穩態自由旋進序列於心臟造影之無線脈衝頻率校準以修正黑帶假影。第一個應用中，最佳化的掃描序列和臨床使用於功能性磁振造影序列為相同之平面回訊造影序列。透過這套即時回授系統，影像會同步的傳送到電腦中進行z補償梯度最佳化的運算，並且造影儀會根據最新的z補償梯度來成像。在此研究中，我們能夠在20次的最佳化疊代內，同時決定了正的與負的z補償梯度，而往後要做的功能性磁振造影序列也會根據所覺得得這正負z補償梯度來交替成像。第二個應用，我們針對無線脈衝頻率校準提出一個新的預掃描脈衝序列，稱為頻帶轉移小角度平衡穩態自由旋進序列。使用這個新的預掃描脈衝序列可以透過電腦自動運算得到最佳的無線脈衝頻率，進而取代以往手動且主觀的選擇方式。之後，再經由這套即時回授系統，我們將頻帶頻率校準序列和常用的平衡穩態自由旋進動態心臟造影序列合併，並且能夠在一次閉氣的時間內完成。頻率校準一但完成，其最佳頻率值就會即時回授到造影儀，其後的動態心臟造影就會根據該頻率值進行掃描以得到無假影干擾之影像。根據統計的分析，經過頻率校準之影像的確有顯著的假影修正(P<.01)。結論上，我們提出了一套快速且完全自動化的即時回授系統，並且可根據情形調整預掃描序列與臨床使用之掃描序列，使其成為一套完整的預校準模組並解決需要參數最佳化之問題。

In MRI, some specific scan parameters such as inversion time or RF frequency offset have to be determined before applying clinical protocols. In general, the MR operators or doctors would utilize the developed scouting protocol and then select the proper imaging parameter according to the set of images manually. However, the additional scouting scan would diminish the fluency of clinical routine. In this thesis, we proposed a framework of real-time feedback system that could automatically and rapidly optimize the desired parameter. Two types of applications have been accomplished based on this real-time feedback system. The first application was to implement the optimization of compensative gradient moment for z-shim method in gradient-echo echo-planar-imaging (GE-EPI). In this type, the pre-scan and target sequences were identical. After about 20 iterations for evaluating optimal negative and positive z-shim gradient moments, the following z-shimmed GE-EPI for functional MRI (fMRI) study would contain images alternatively according to the inherited values. The second application is the calibration of RF frequency to suppress the dark banding artifact.in steady-state free precession (bSSFP) cardiac imaging at 3.0 T. The real-time feedback system combined two sequences: transition-band low flip-angle bSSFP (bSSSFP-L) for frequency calibration and conventional cine bSSFP. The optimal frequency obtained by real-time feedback calibration significantly reduced dark-band artifacts in cine bSSFP images (P < .01). The period of calibration and conventional protocol could be implemented within one breadth-hold. In conclusion, the proposed real-time feedback system is rapid and fully automatic and can thus serve as a pre-adjustment module in various MR researches which are in need of parameter calibration.

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