低特定吸收率與平行擷取之多頻激發寬頻磁振造影

Abstract

自從四十年前發明以來，快速磁振造影一直是個不間斷之研究方向。因其可縮短掃描時間，並在同樣時間內能夠服務更多的民眾，提供快速舒適的磁振造影檢查 。同時多截面造影是多種快速磁振造影技術中之一支備受矚目的研究方向；而其中的多頻激發寬頻磁振造影為我們實驗室團隊在台大所研發的新技術。本研究主要目的為整合平行磁振造影技術與多頻激發寬頻磁振造影，以更進一步縮短造影時間 ；並且發展射頻脈衝合成方法以降低特定吸收率。 為了克服多頻激發寬頻磁振造影中因相位偏移而產生的影像假影，我們應用 相位修正方法來修正信號相位不連續，以提升多頻激發寬頻磁振造影之影像品質；其次，我們將平行磁振造影技術結合多頻激發寬頻磁振造影，以進一步縮短造影時間，並提出降低其中之信號估計誤差的方法; 最後我們運用射頻脈衝設計技術，來降低同時多截面造影時之特定吸收率以及合成均勻的同時多截面激發輪廓。 相位修正方法大幅減少上述之影像假影，使加速後之多頻激發寬頻磁振造影的影像品質更接近於標準影像； 再者，我們將具相位修正之多頻激發寬頻磁振造影與平行磁振造影技術結合，將整體造影速度加速超過3倍； 至於射頻脈衝設計研究，在同時5切面仿體及3切面人腦造影，可將特定吸收率分別降低至32% 與 28%，且維持相同之影像品質。 總結而言，藉由導入相位修正、平行造影與低特定吸收率之射頻脈衝等方法，使得多頻激發寬頻磁振造影成為更佳的、更快速的以及更安全的磁振造影技術。本論文之結果與提出的方法對於多頻激發寬頻磁振造影為基礎，應用於同時多截面造影之研究與臨床應用將有所助益。

Since its invention four decades ago, faster magnetic resonance imaging (MRI) has always been a popular research topic. Fast MR imaging shortens the scan time, and provides swift as well as comfortable services for more subjects within the same period. Simultaneous multi-slice (SMS) MRI is a growing field of fast MRI methods; in which multiple-frequency excitation wideband magnetic resonance imaging (ME-WMRI) is a novel technique developed by our group in NTU. The main goal of this study is to integrate parallel imaging technique with ME-WMRI to achieve further scan time reduction while develop new pulse synthesis methods to minimize the specific absorption rate (SAR). To reduce the image artifacts resulting from phase offsets in ME-WMRI, we applied phase correction methods to fix the signal phase discontinuity and improve the image quality of ME-WMRI. We then added parallel MR imaging (pMRI) onto ME-WMRI to further reduce the MR scan time and proposed a reconstruction algorithm to minimize errors caused by the fitting problem. Finally, we adopted SLR algorithm for radiofrequency (RF) pulse synthesis to reduce the SAR and obtain the uniform excitation profile for SMS MRI. The phase correction method greatly reduces the image artifacts and causes the quality of accelerated ME-WMRI images to be closer to standard images. Furthermore, by integrating phase corrected ME-WMRI with pMRI, the total acceleration rate adds up to more than 3-fold. As for RF pulse study, the simultaneous 5-slice phantom imaging and 3-slice human brain imaging reduced the SAR values to 32% and 28% while providing similar image qualities. In summary, by introducing phase correction, pMRI reconstruction and low SAR RF pulse synthesis methods, ME-WMRI has been made better, faster, and safer. The results and proposed techniques would be helpful on the researches and clinical applications of ME-WMRI-based SMS MRI.