https://scholars.lib.ntu.edu.tw/handle/123456789/83321
標題: | 超高速磁振造影技術之研究:渦電流自動補償系統之設計與應用 | 作者: | 陳志宏 | 關鍵字: | 核磁共振影像;渦型電流;功能性磁振造影;擴散權重影像;MRI;eddy current;pre-emphasis system;fMRI;EPI;genome study | 公開日期: | 31-七月-2004 | 出版社: | 臺北市:國立臺灣大學醫學工程學研究所 | 摘要: | 功能性磁振造影目前使用的脈衝序列平面迴訊成像序列(Echo Planar Imaging; EPI),其 梯度磁場的切換速度需在1kHz 以上,這樣的要求遠高於一般脈衝序列,所以其渦型電流也 比一般的脈衝序列更強、影響更嚴重。擴散權重迴訊平面成像 (Diffusion Weighted Echo-Planar Imaging;DW-EPI),因為需要強度非常大的擴散梯度,所以受到渦型電流的影 響程度也是非常嚴重,影像會因此產生1/2 N 假影、變形及位移。利用preemphasis 系統補 償渦型電流可以有效解決渦型電流的影響,但若以手動調整則需要技術經驗而且耗時。為 了改善上述手動耗時麻煩的缺點,我們發展一套新技術:當核磁共振儀擷取影像的同時, 即時的量測實際梯度磁場大小與K 空間軌跡,以了解渦型電流所出現的時間與強度。配合 這些資訊,設計出一套自動調整preemphasis 參數系統之方法,取得最佳之preemphasis 系 統參數,此方法的優點是不需要設計渦型電流模型來調整系統參數,避免為了建立模型時, 需採用近似與假設而造成之誤差以及條件限制。 本研究已建立一套可自動校正Preemphasis 參數之系統, 並已確認可應用於EPI 脈衝序列 之上。本研究可自動執行MRI 感應電流的補償校正,改進主磁場中梯度波形,取代傳統手 動調節方式。將這項技術套用於超快速磁振造影之脈衝序列,即EPI 與DW-EPI 之上,預 期將會對功能性磁振造影的研究與臨床診斷有所助益。 Echo planar imaging (EPI) is a unique imaging method that can collect an MR image from a single free induction decay signal (FID). This rapid imaging technique is especially useful in the applications, which is performed MRI scanning repeatedly, such as function MRI (fMRI), diffusion weight MRI (DWI). During EPI acquisition, large field gradients have to be generated and switched quickly at the rate of about 1 kHz. In this situation, eddy currents are induced on the innermost conducting layer of the magnet significantly. It makes imaging geometric distortions that will consist of N/2 ghost, shearing, or shift. In order to compensate this eddy currents effects, the gradient waveform needs a preemphasis treatment. The preemphasis adjustment requires manual operation, needs considerable skill, and could be difficult to achieve. In contrast, automated preemphasis adjustments can be faster and more reliable, and can be applied without operator intervention. We present a method for optimizing gradient preemphasis, suitable for fast fMRI sequences on a 3T scanner, by fully automated numerical control instead of manual adjustments. Downhill simplex method and k-space trajectory measurements are combined into an automated procedure of preemphasis adjustment that is simple to implement, and is found to be reliable and easy to use. The computations require no specific models relating actual gradient waveforms to preemphasis parameters. Expectably, it is very useful to MRI study and clinic diagnosis that applies this technique to fast MR imaging, such as EPI and DW-EPI. |
URI: | http://ntur.lib.ntu.edu.tw//handle/246246/22388 | 其他識別: | 922218E002018 | Rights: | 國立臺灣大學醫學工程學研究所 |
顯示於: | 醫學工程學研究所 |
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