以電腦斷層為基礎之電腦輔助定量法

A novel computed tomography-based computer-aided quantification method

施政廷(Cheng-Ting Shih)

指導教授 ： 莊克士

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2015年)

摘要

電腦斷層攝影（computed tomography, CT）能夠快速的提供高解析度的解剖斷層影像，現今在臨床上已成為一強而有力的診斷工具並廣泛的被使用已達成各種醫療目的，近年來台灣及美國的CT檢查數亦以逐年以超過10%的年增率快速成長。相對於其他放射線檢查，CT掃描將對病人帶來相對較高的輻射劑量，從輻射防護的觀點而言，醫療暴露有其正當性及必要性，但仍須遵循利大於弊的原則，然而，目前臨床上取得之CT影像主要用於視覺診斷，此外便無其他有效利用方式。除了視覺觀察外，透過以影像為基礎的定量分析求出之物理、生理組織參數亦可幫助臨床上的特定物理矯正及疾病診斷，然而，常見的單能CT掃描儀使用了連續能譜及積分式偵檢器，使得掃描物的組成、衰減資訊難以從投影資訊中求出，因此現今的影像定量方式主要透過使用各類等效材質達成，但在元素組成上，等效材質與實際人體組織仍有差異存在，使得計算獲得之組織參數多半為相對參考量。有鑑於此，本研究提出了一套以CT影像為基礎之電腦輔助定量（computer-aided quantification, CAQ）方法，以達成快速且準確的物理、生理組織參數定量，我們首先利用化學計量校準計算特定掃描儀之X光能譜特性參數，接著搭配適當的物理模型或轉換關係，將CT值轉換為具臨床價值的物理及生理組織參數。本研究分為兩個部分，在第一部分中，我們利用預先建立的轉換關係，將CT值轉換為臨床上物理矯正或劑量評估所需要的物理組織參數，包含物理密度、電子密度及有效原子序，並進一步利用物理模型計算質量衰減係數及質量能量吸收係數，在第二部分中，我們則利用提出的混合物模型將CT值轉換為骨骼生理參數。結果顯示，本方法能夠準確的將CT影像轉換為各種物理及骨骼組織參數分布圖，除此之外，亦可克服掃描儀間能譜特性差異的影響，以幫助掃描儀間影像的相互比較。我們認為此CAQ方法能夠廣泛的使用在臨床上，以準確將CT影像轉換為組織參數進而幫助達成各種臨床診療目的，並藉此有效提升病患在CT檢查當中能夠獲得的利益。

關鍵字

電腦斷層 ； 電腦輔助定量

並列摘要

Computed tomography (CT) can rapidly provide high resolution cross-section images. It has become one of the powerful tools in clinical and has been widely applied to achieve a variety of diagnostic and therapeutic purposes. In recent years, number of CT procedures increase year after year with an annual growth rate higher than 10% in Taiwan and in the U.S. Comparing to the other radiological examinations, CT scan delivers relatively high radiation dose to patients. From the view point of radiation protection, medical exposure is justified as long as it follows the ALARA principle. Currently, CT images are mainly used in visually diagnosis of various diseases but have no other way to effective utilize. In addition to the visual observation, tissue parameters, such as physical electron densities, effective atomic numbers and bone mineral densities, obtained from CT image-based quantification are also useful for several physical correction and diagnosis in clinical. However, common CT scanners employ polychromatic X-ray spectrum and cumulative detector, causes the composition and attenuation information of scanning objects are difficult to estimate from acquired projection data or reconstructed images. Therefore, present CT image-based quantification is mainly performed through various tissue equivalent materials (TEMs). Nevertheless, the differences between the elemental composition of tissue equivalent materials and actual human tissues results that the estimated parameters are a reference equivalent. In view of the above, a novel computer-aided quantification (CAQ) method was proposed to achieve fast and accurate tissue parameter quantification. In this method, a stoichiometric calibration was performed to acquire spectrum characteristic parameters (SCPs) that describe the energy spectrum of a specific CT scanner. The acquired SCPs were then used to convert CT number into clinically valuable physical and physiological tissue parameters (PTPs and PoTPs). This study was divided in to two parts. In the first part, the CT number was converted into the PTPs by using conversion relationships. In addition, these parameters were further used to calculate the mass attenuation coefficients (MACs) and mass energy transfer coefficients (MEACs) with physical models. In the second part, the CT numbers were converted into bone physiological parameters (BPPs) by using a novel mixture model. The results show that the proposed CAQ method can accurately convert the CT images into PTP and BPP maps. Moreover, the proposed method also reduce the influences of energy spectrum that is helpful in image exchanging and comparing between scanners. We conclude that the proposed CAQ method could be applied in the clinical to estimate several tissue parameters from CT image for various diagnostic and therapeutic purposes, whereby benefits for patients from CT examinations can be increased.

並列關鍵字

computed tomography ； computer-aided quantification

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