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首頁> 外文學(xué)位 >Circular reconstruction and scatter correction in X-ray cone-beam CT.
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Circular reconstruction and scatter correction in X-ray cone-beam CT.

機(jī)譯：X射線錐形束CT中的圓形重建和散射校正。

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With the development of advanced computing and flat panel detector technology, volumetric X-ray computed tomography (CT) systems using large-area detectors, such as cone-beam CT (CBCT), are becoming more popular. This dissertation describes work aimed at reducing three types of image artifacts in CBCT.; Artifacts appear in the reconstruction when insufficient projection data are measured or the data redundancy is not properly handled. In practical CBCT systems, the circular trajectory is commonly used due to the ease of implementation on existing hardware. However, due to Tuy's data sufficiency condition, an exact reconstruction is possible only in the plane of the source trajectory. In a circular full scan, the standard FDK algorithm has been shown to be close to the optimal if unmeasured data are assumed to be zero. This algorithm usually results in cone-beam (CB) artifacts in the reconstruction, such as the intensity drop along the axial direction. An algorithm is proposed for estimating the unmeasured data. The algorithm is derived from the Radon transform and Grangeat's formula. In a circular short scan (pi plus fan angle), the modified FDK algorithm using Parker's weighting is commonly used. This algorithm handles data redundancy using approximation. Besides the axial intensity drop, the algorithm results in another type of CB artifact, mainly streaks around dense objects. An improved algorithm is derived using the central slice theorem and geometry transformation. Both the proposed full-scan and short-scan algorithms are verified using computer simulations and physical experiments.; Even if sufficient data are acquired and exact reconstruction is used, the projection data are corrupted by detected scatter, and cupping and shading artifacts appear in the reconstruction. Scatter is a large problem in CBCT systems since the geometry with a large-area detector has high scatter-to-primary ratios (SPR's). A scatter correction algorithm is introduced that provides effective scatter correction but does not require additional patient exposure. The algorithm uses a hardware-based modulation technique to separate scatter signal from the primary signal. The performance of this method is evaluated using computer simulations and preliminary physical experiments. With no loss of resolution, substantial reduction of scatter artifacts is shown using this approach.
機(jī)譯：隨著先進(jìn)的計(jì)算技術(shù)和平板檢測(cè)器技術(shù)的發(fā)展，使用大面積檢測(cè)器的體積X射線計(jì)算機(jī)斷層掃描（CT）系統(tǒng)（例如錐束CT（CBCT））變得越來越流行。本文介紹了旨在減少CBCT中三種圖像偽影的工作。當(dāng)測(cè)量的投影數(shù)據(jù)不足或未正確處理數(shù)據(jù)冗余時(shí)，將在偽像中出現(xiàn)偽影。在實(shí)際的CBCT系統(tǒng)中，由于易于在現(xiàn)有硬件上實(shí)現(xiàn)，因此通常使用圓形軌跡。但是，由于Tuy的數(shù)據(jù)充分條件，僅在源軌跡的平面上才可能進(jìn)行精確的重構(gòu)。在循環(huán)全掃描中，如果未測(cè)數(shù)據(jù)假定為零，則標(biāo)準(zhǔn)FDK算法已顯示接近最佳值。該算法通常會(huì)在重建過程中導(dǎo)致錐束（CB）偽影，例如沿軸向方向的強(qiáng)度下降。提出了一種估計(jì)未測(cè)數(shù)據(jù)的算法。該算法源自Radon變換和Grangeat公式。在圓形短掃描中（pi加扇形角），通常使用使用Parker加權(quán)的改良FDK算法。該算法使用近似值處理數(shù)據(jù)冗余。除了軸向強(qiáng)度下降，該算法還導(dǎo)致另一種CB偽像，主要是圍繞密集對(duì)象的條紋。使用中心切片定理和幾何變換導(dǎo)出了一種改進(jìn)的算法。所提出的全掃描和短掃描算法均通過計(jì)算機(jī)仿真和物理實(shí)驗(yàn)進(jìn)行了驗(yàn)證。即使獲取了足夠的數(shù)據(jù)并使用了精確的重建，投影數(shù)據(jù)也會(huì)由于檢測(cè)到的散射而損壞，并且在重建中會(huì)出現(xiàn)杯形和陰影偽影。散射是CBCT系統(tǒng)中的一個(gè)大問題，因?yàn)榫哂写竺娣e檢測(cè)器的幾何形狀具有很高的散射與原始比率（SPR）。引入了散射校正算法，該算法可提供有效的散射校正，但不需要額外的患者照射。該算法使用基于硬件的調(diào)制技術(shù)將散射信號(hào)與主要信號(hào)分離。使用計(jì)算機(jī)仿真和初步的物理實(shí)驗(yàn)來評(píng)估此方法的性能。在不降低分辨率的情況下，使用這種方法顯示了散射偽影的顯著減少。

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