雙光子斷層掃描系統應用於前哨淋巴結位置的偵測 : 蒙地卡羅模擬研究

Application of A Dual Photons Emission Computed Tomography (DuPECT) System in the Sentinel Lymph Node Location Detection : A Monte Carlo Simulation Study

陸冠百(Kuan-Pai Lu)

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/碩士(2012年)

https://doi.org/10.6843/NTHU.2012.00420

摘要

臨床上都是利用99mTc核醫藥物聚集在前哨淋巴結的特性來偵測乳癌細胞的轉移，一般都是醫生於手術中，手持加馬探頭(Gamma probe)偵測核種聚集區域，即前哨淋巴結位置，並據以切除。但此技術易受雜訊影響、且無提供即時影像與深度資訊，因此近年來許多團隊致力於開發一套適用於手術中的影像探頭，希望能克服加馬探頭的缺點。然而傳統SPECT影像使用單光子射源如99mTc，需要旋轉偵檢器180度，才能重建三維影像，不可能在短時間內完成，提供即時影像，因此我們實驗室提出DuPECT方法，使用能同時發出兩個光子的雙光子射源，利用成對的準直儀限制光子路徑，與同符線路判斷收到的兩顆光子為同一衰變時發出，再由兩光子飛行軌跡的交點進行影像重建。因此DuPECT系統不需全角度的投影資訊可即時成像，並提供三維空間資訊。
然而DuPECT系統最大限制即靈敏度不高，因此我們嘗試以聚焦式準直儀的扇形準直儀(Fan-beam collimator)與錐狀射束準直儀(Cone-beam collimator)增加靈敏度。為了探討系統可行性，我們以蒙地卡羅軟體-GATE進行驗證，卻發現模擬時間耗時，且無法模擬焦距太小的幾何，因此建立一套更有效率、兼具彈性的幾何設計系統有其必要性。而GATE模擬時間中，光子與物理模組作用占了很大的比例，因此我們自行設計一套物理模組並結合能快速模擬光子在像素化假體遷移(transport)與作用的SimSET組成一套系統稱之 Virtual collimator System。
為了驗證Virtual Collimator System的準確度，我們與GATE比較，分別使用99mTc與75Se模擬隙縫式準直儀(Slat collimator)與扇形準直儀。確定Virtual Collimator System可行性後，我們即以此系統模擬DuPECT系統，先決定理想的聚焦式準直儀焦距，再分別測試使用扇形與錐狀射束準直儀搭配隙縫式準直儀時的系統靈敏度與解析度。
實驗結果顯示Virtual Collimator System與GATE在模擬隙縫式準直儀與扇形準直儀上，系統解析度與系統靈敏度有很高的一致性。而在模擬隙縫式準直儀時，可以增高2~3倍的模擬效率; 模擬扇形準直儀時，更可以增高20~60倍的模擬效率，因此Virtual Collimator System對於模擬複雜幾何構造時，能大大增進模擬效率，並提供準確結果。
而在DuPECT系統探討中，發現錐狀射束準直儀有較扇形準直儀更佳的系統解析度與系統靈敏度，使用錐狀射束準直儀時，當射源在焦點，靈敏度為5.42 cps/kBq，解析度在xyz方向分別為2.12×2.12×4.38毫米。研究證實，DuPECT系統應用於手術中前哨淋巴結偵測是可行的，儘管系統最大限制為系統表現深受物體所在位置影響，但相信未來非常有機會進入臨床測試階段。

關鍵字

雙光子斷層掃描系統 ； 蒙地卡羅 ； 前哨淋巴結 ； 扇形準直儀 ； 錐狀射束準直儀

並列摘要

In clinic, the detection of breast metastasis utilizes the aggregating effect of 99mTc labeled pharmaceuticals in sentinel lymph node (SLN). The doctor hold the gamma counting probe to locate the most radioactive (“hottest”) node (i.e. SLN) during operation and perform the resection. But the gamma counting probe technique is susceptible to noise and lack of depth information. In recent years, many groups devoted to develop an imaging probe suitable for the use in the operating room, hoping to overcome the drawbacks of the gamma counting probe.
However, traditional single photon emission computed tomography (SPECT) needs to scan 180 degrees to enable the reconstruction of the three dimensional image. As a result, it is impossible to perform real-time imaging for operation purpose. Our laboratory proposes the DuPECT (dual photons emission computed tomography) system employing the radioisotopes that emit two photons simultaneously. The DuPECT system uses the collimator pairs to restrict the direction of incoming photons and the coincidence circuit to determine whether the detected two photons are emitted in the same decay. The source position is simply the intersecting of the trajectory of the detected photons. The DuPECT system provides three-dimensional spatial information without requiring the rotation of the system.
Since the biggest limitation of the DuPECT is its low sensitivity, we try to take advantage of the converging collimators (including the fan-beam collimator and cone-beam collimator) to improve the sensitivity. We utilized the GATE Monte-Carlo simulation (MCS) to validate the feasibility of the DuPECT. However, GATE is time-consuming and can’t simulate the geometry with small focal length. So it is essential to develop a more efficient and flexible geometry-based simulation system. In GATE, the interaction between the photons and collimator takes a large portion of the simulation time. Therefore in this study, we design a new Virtual collimator combined with the SimSET to speed up the simulation.
The projection data of the Virtual collimator are compared with GATE to validate the accuracy of the proposed system. Both 99mTc and 75Se are used in the slat collimator and the fan-beam collimator simulation. After demonstrating the feasibility of the Virtual collimator, it was used to construct the DuPECT system. In the study, the optimal focal length of the converging collimator was determined first, and then testing the sensitivity and resolution of fan-beam collimator and cone-beam collimator separately to pair with the slat collimator.
According to both slat and fan-beam collimator experimental results, we demonstrate that Virtual collimators are in good agreement with GATE simulation in terms of resolution and sensitivity. In Comparison to GATE, the Virtual collimator improve 2~3 and 20~60 times simulating efficiency for slat collimator and fan-beam collimator, respectively. We conclude that the Virtual collimator boost simulating efficiency and produce the accurate results even on complex geometry.
The preliminary results suggest the use of Cone-beam collimator for the DuPECT system as it show better resolution and sensitivity than fan-beam collimator. The system sensitivity is 5.42 cps/kBq with resolution of 2.12*2.12*4.38 mm at x, y, and z direction when the source is placed at the focal point. The results demonstrate that the DuPECT system is feasible for intraoperative detection of the SLN. Although the system is strongly position dependent, we believe that the DuPECT-SLN Probe System will come into clinical trial stage in the future.

並列關鍵字

DuPECT ； Monte Carlo ； Sentinel Lymph Node ； Fan-beam collimator ； Cone-beam collimator

參考文獻

Aarsvold, J. N. and Alazraki, N. P. (2005). "Update on detection of sentinel lymph nodes in patients with breast cancer." Semin Nucl Med 35: 116-128.

Albertini, J., Lyman, G., Cox, C., et al. (1996). " Lymphatic mapping and sentinel node biopsy in the patient with breast cancer." JAMA 276: 1818-1822.

Atkins, H., Hayward, J., Klugman, D., et al. (1972). "Treatment of early breast cancer: a report after ten years of a clinical trial." British medical journal 2: 423-432.

Beekman, F. and van der Have, F. (2007). "The pinhole: gateway to ultra-high-resolution three-dimensional radionuclide imaging." Eur J Nucl Med Mol Imaging 34: 151-161.

Boisson, F., Bekaert, V., El Bitar, Z., et al. (2011). "Characterization of a rotating slat collimator system dedicated to small animal imaging." Phys Med Biol 56: 1471-1485.