臺灣博碩士論文加值系統

由於其優越之信噪比特性，平衡穩定態磁振造影序列逐漸成為一種廣為使用之掃描序列。非平衡穩定態造影序列亦逐漸受到重視，尤其在於此種掃描序列在臨床上的應用。舉例而言，雙重迴訊穩定態造影序列能同時接收到兩種截然不同特性之信號，並可藉由分析這兩種不同來源之信號而擷取出組織特性。

此份論文旨在利用雙重迴訊穩定態造影序列，藉以研究、分析其所收集之信號特性。我們提出了一種雙重迴訊穩定態之變體造影序列，並應用於腦部磁振造影。利用我們的方法，可以從該序列所收取到之兩種特性迥異的信號中，同時解析出兩種橫向磁緩速率以及組織內部磁場擾動。而解析出之組織特性在腦部鐵質含量定量上有相當大的助益。

此外，為了能獲得最佳的掃瞄參數，我們運用了數值分析模擬了解該此兩種信號的演進，以及掃描序列之特性。在此篇論文的最末一章，我們針對可能影響組織參數特性解析之因素加以討論；同時，我們也提出了兩種可能適用此改良序列的應用。我們期待此篇論文能貢獻于非平衡穩定態掃描序列的發展；同時，也希望我們的改良序列亦能在其他應用上發揮貢獻。

As balanced steady-state sequence becomes a popular MR imaging modality for its terrific efficiency in terms of SNR per TR, unbalanced steady-state sequences draw more and more attention for their clinical applicability. In particular, dual-echo in the steady-state (DESS) sequence acquires diverse signals of two different signal pathways, so that tissue properties may be revealed.

In this thesis, we aim to exploit the sampling ability of unbalanced steady-state sequence to investigate signal evolutions of different pathways. A variant of DESS sequence is designed and applied to brain imaging. To be specific, transverse relaxation rates and the internal field perturbation are simultaneously extracted from signals acquired by our proposed compact method, and the extracted information could be valuable in estimating brain iron concentration.

To obtain a proper set of scanning parameters, numerical simulations are employed to evaluate sequence performance. In addition, potential image artifacts are discussed, and two possible applications are proposed in a later chapter in this thesis. We expect this work may advance the current development of unbalanced steady-state sequences, and the proposed sequence would be a handy imaging sequence for other applications as well.

Table of Contents
中文摘要 ...................................................................................................................................... I
Abstract ........................................................................................................................................ II
Table of contents ...................................................................................................................... III
Table of figures and table ..................................................................................................... V

Chapter 1 Introduction ............................................................................................................ 1

Chapter 2 Unspoiled steady-state sequences ............................................................. 3
2.1 Extended phase graph, signal pathways, and echo formation ..................... 4
2.2 Balanced steady-state free-precession (bSSFP) ................................................. 11
2.3 Break the balance: unbalanced steady-state free-precession (ubSSFP).. 16

Chapter 3. Simultaneous relaxometry and internal field perturbation in the brain
3.1 Introduction: brain iron accumulation and parametric mapping in the brain ...................................................................................................................................... 25
3.2 Theory .................................................................................................................................. 27
3.3 Sequence optimization .................................................................................................. 36
3.4 Experimental design and data processing ............................................................ 43
3.5 Results .................................................................................................................................. 53
3.6 Discussions and conclusions ...................................................................................... 57

Chapter 4. Discussions and conclusions ........................................................................ 61
4.1 Potential problems .......................................................................................................... 62
4.2 Possible applications ...................................................................................................... 73
4.3 Conclusions ........................................................................................................................ 78

References ....................................................................................................................................... 79

Table of figures and table
Figure 2.1 Extended phase diagram ............................................................................. 2-5
Figure 2.2 Four different gradient configurations for signal acquisition .... 2-8
Figure 2.3 Sequence diagram of standard bSSFP sequence .............................. 2-11
Figure 2.4 Signal variations of bSSFP with different flip angles ...................... 2-13
Figure 2.5 Simplified sequence sketches of FISP and PSIF sequences .......... 2-18
Figure 2.6 Sequence diagram of DESS sequence .................................................... 2-21

Figure 3.1 Dual pathway steady-state sequence with multiple acquisition and echo formations ..................................................................................... 3-5
Figure 3.2 Comparison of two spectral distributions of isocromats .............. 3-11
Figure 3.3 Simulations of FISP and PSIF signals, and the SNR performance in the fitted frequency ...................................................... 3-16
Figure 3.4 Simulated SNR in fitted R2 and R2′ .......................................................... 3-18
Figure 3.5 Coil combination workflow ........................................................................ 3-22
Figure 3.6 Comparisons of two different approaches to estimate relaxation rates ............................................................................................... 3-26
Figure 3.7 Workflow of both phase and magnitude processing ...................... 3-28
Figure 3.8 Comparisons of the internal field perturbations with two different TRs ..................................................................................................... 3-30
Figure 3.9 Comparisons of R2 and R2* mappings with two different TRs ... 3-31
Table 3.1 Results of R2 and R2* values in brain regions ................................... 3-32

Figure 4.1 Simulations of b-factor using designed gradient scheme ............. 4-5
Figure 4.2 Signal void artifact in images acquired with different sequences .......................................................................................................... 4-8
Figure 4.3 Illustration of flow displacement artifact ............................................ 4-9
Figure 4.4 Flow-related displacement artifact in images ................................... 4-10

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