臺灣博碩士論文加值系統

電腦斷層掃描(CT)在臨床上已是十分重要且運用廣泛的醫療診斷儀器，由CT延伸出來的錐狀射束電腦斷層掃描(CBCT)也廣泛普及的運用於顱面、四肢及牙科。降低CBCT輻射劑量仍是個重要的研究主題，許多研究致力於降低CBCT的輻射劑量，文獻中提到利用有限角度重建方法，可直接減少照射物所接收到的輻射劑量，而疊代重建演算法在有限角度方法中仍具有高品質影像的效果。目前雖然已有許多疊代的重建演算法，但未有研究客觀且完整地比較這些疊代演算法，本研究將比較在有限角度下，現在臨床上使用的非疊代及五種常見的疊代演算法，評估重建之影像品質。方法：我們使用X光機、偵檢器、馬達和工作站建立原型的CBCT系統，整合及控制三個硬體的控制軟體，並利用開放軟體基於GPU加速斷層掃描重建涵式庫 (TIGRE)中的非疊代演算法及疊代演算法，將收到的投影資料重建出來，使用CT仿體和沙氏變色蜥樣本這兩種拍攝物體，之後利用對比訊號比(CNR)、訊號雜訊比(SNR)、通用性影像品質指標(UQI)、均方根誤差(RMSE)及半高全寬(FWHM)做為影像品質評估的標準，比較及評估各演算法的結果。結果：以冠狀及水平切面來評估重建出的結果，不管是以肉眼判斷或是以上述的參數去評估，MLEM、OS-SART和OSC-TV這三種在有限角度的狀況下還是有保持住一定的影像品質，相較FDK有明顯的改進，在仿體結果中，以CNR及SNR的平均值為例，由較好到較差的排序為OS-SART、OSC-TV、MLEM；在沙氏變色蜥樣本的結果中，CNR跟SNR的平均值由較好到較差的依序是OSC-TV、MLEM和OS-SART，結論：在本研究所評估的疊代演算法中，MLEM、OS-SART和OSC-TV這三種演算法表現較好，由於他們的性能會因拍攝物的不同造成差異，在重建演算法的選擇上會因拍攝物的不同而改變且須進一步研究，本研究可提供未來研究上，測試與評估疊代重建架構於CBCT系統之效果。

Computed tomography (CT) is already an indispensable tool and widely applied in clinical diagnosis, so is cone beam computed tomography (CBCT) in maxillofacial, limb and dental applications. Radiation dose reduction of CBCT remains to be an active topic of research. In the literature, image reconstruction with limited angles can directly reduce the radiation dose, and the iterative reconstruction algorithms have superior performance on limited angles reconstruction. Although there are many kinds of iterative reconstruction algorithms, there is no systematic and objective comparison of these iterative reconstruction algorithms. In this work, we compared five common iterative reconstruction algorithms to the non-iterative algorithm, the L. A. Feldkamp, L. C. Davis, and J. W. Kress (FDK) algorithm, which is common in clinical use, to evaluate which iterative reconstruction method is preferred in limited angle reconstruction. Methods: To set up a prototype CBCT system, we integrated three hardware components: X-ray tube, flat-panel detector and a motor with in-house controlling software. Acquired projection data were used to reconstruct image volumes with tomographic iterative GPU-based reconstruction toolbox (TIGRE), an open source toolbox of various image reconstruction methods. A micro-CT bar pattern phantom and anolis sagrei cadaver were used as irradiated objects. We analyzed the reconstructed images with image quality criteria of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), universal quality Index (UQI ), root mean of the squared error (RMSE) and full width at half maximum (FWHM). Results: Inspecting the coronal and sagittal planes of the reconstructed results, MLEM, OS-SART and OSC-TV were able to maintain a good image quality with limited angle reconstruction. In the micro-CT bar pattern phantom results, the top three best-performing algorithms are OS-SART, OSC-TV, and MLEM. In the anolis sagrei cadaver results, the best algorithms are OSC-TV, MLEM, and OS-SART. Conclusion: In the iterative reconstruction algorithms tested, MLEM, OS-SART and OSC-TV have superior performance than other algorithms. Since their performances in image quality vary between imaged objects, the choice of reconstruction algorithm may be task-dependent and requires further investigation. This study provides future studies with a framework of testing and evaluation for determining the ideal iterative reconstruction method in the CBCT system development.

Chapter 1 Overview 1
Chapter 2 Literature Survey 3
2.1 Background of CBCT 3
2.2 Radiation dose 6
2.3 Image reconstruction in CBCT 7
2.4 Limited angle reconstruction under limited angle reconstruction 11
2.5 Evaluation of image quality 14
2.6 Specific aims 16
Chapter 3 Materials and Methods 19
3.1 Prototype CBCT system 19
3.1.1 Hardware 20
3.1.2 Controlling Software 24
3.2 Reconstruction algorithms 26
3.2.1 FDK 28
3.2.2 Gradient descend-based 29
3.2.3 Total variation regularization 32
3.2.4 Maximum likelihood expectation–maximization algorithm 35
3.3 limited angle reconstruction 36
3.4 Evaluation of image quality 37
Chapter 4 Results 41
4.1 CBCT system 41
4.1.2 Hardware Setting 41
4.1.2 Controlling Software 42
4.2 Experiment 45
4.2.1 micro-CT bar pattern phantom results 47
4.2.2 Anolis sagrei cadaver results 56
Chapter 5 Discussion 66
Chapter 6 Conclusion 67
References 69

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