正子/電腦斷層造影劑量與衰減之研究

Dose and Attenuation Studies in PET/CT Imaging

吳東信(Tung-Hsin Wu)

指導教授 ： 朱鐵吉

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2006年)

https://doi.org/10.6843/NTHU.2006.00662

摘要

近年來，正子/電腦斷層攝影系統(PET-CT dual-modality)已在國內各大醫學中心蔚為風潮，將PET與CT結合在單一掃描系統中，不但可以同時獲取功能性資訊(functional information)與解剖影像(anatomic image)，且能在一次的掃描中將此二種影像融合呈現(image fusion)。此外，基於CT的X光球管具備高光子通量率(high photon flux)的特性(約為傳統鍺-68穿透式掃描的104倍)，利用CT影像來作PET 射出式掃描影像(emission scan)的衰減校正(attenuation correction)會比使用傳統穿透式掃描鍺-68(germanium-68)有更短的掃描時間及提供更佳的影像解析(spatial resolution)，如此不但可以提昇診斷價值(diagnostic value)，更可增加就診人數(patient throughput)，就現行臨床的PET/CT診斷流程而言，CT穿透掃描(CT transmission scan)幾乎已全面取代傳統鍺-68衰減校正的功能。然而，高品質的CT影像雖能提供較佳的影像品質與診斷資訊(diagnostic information)，但相對帶給病人的卻是較高的輻射劑量，尤其是對大範圍的全身性掃描來說，CT的掃描範圍涵蓋顱頂至骨盆腔，這樣大範圍的全身性掃描模式是更是傳統CT診斷中罕見的。完整的劑量評估對接受正子造影之病患是有其必要性的，在不影響臨床造影程序與保有原有掃描速度的前題下，如何得到最多的診斷資訊並又能大幅降低病患輻射劑量是本研究最重要的課題，其研究結果將為此目標奠定重要基礎，以減少一般民眾對正子造影檢查高風險、高輻射劑量的過度疑慮，預期將能為推廣正子造影檢查帶來很大的助益。
在此研究中我們將探討幾個重要的議題：(1)比較CT和傳統鍺-68作衰減校正掃描時病人在不同掃描模式下，如腦部、心臟及全身性掃描所接受的輻射劑量，(2)在不影響現行的臨床造影的原則下，如何在影像品質與輻射劑量間取得一平衡點，以求得最佳化的造影準則(optimizing protocols)，都將在本研究中探討，(3)如何有效的降低輻射劑量，並提昇現行PET/CT系統的診斷優勢？以及預期能夠降低多少輻射劑量？將是本研究另一重要的課題。透過本研究的進行，除了可建立一套應用在正子造影上輻射的劑量測定術(radiation dosimetry)與風險評估(radiation risk)的參考標準外，更可以幫助擴展正子掃描的應用層面，以減少因劑量約束所帶來的應用限制。

關鍵字

正子/電腦斷層造影 ； 衰減校正 ； 診斷資訊 ； 輻射劑量 ； 熱發光計量劑

並列摘要

In the current PET/CT in-line system, high quality CT images not only provide better diagnostic values for anatomic delineation but also offer a shorter scanning time for the transmission scan. However, under standard operating conditions, the photon flux from a CT scan is greater than that from a typical PET transmission measurement obtained with germanium-68 rod sources by a factor of at least 104. This approach would potentially introduce more radiation burden owing to the higher radiation exposure. This situation is particularly severe for a whole-body protocol in which CT scans are obtained from the head to the pelvic region, though this is a relatively rare practice in typical diagnostic CT procedures. Ideal optimal operation protocols for routine PET/CT imaging must take diagnostic values, image quality, radiation doses, scanning time and practicability into consideration, and the question of how to reconcile all these factors is at present a hotly debated topic in nuclear medicine. By studying the potential effects on the corrected emission images obtained using various CT-based attenuation correction parameter settings, we can get more understanding of the relationship between image quality and radiation dose for different PET/CT diagnostic imaging purposes.
The objectives of this study were: (1) to measure the radiation doses by the same technique during germanium-based and CT-based transmission scanning and to compare the doses received by brain, cardiac and whole-body scans, (2) to propose the imaging parameters that could best reduce these radiation doses to levels that permit scanning with all the advantages of current PET/CT imaging and yet without significantly degrading the accuracy of the diagnostic information that such scans provide, and (3) to estimate the expected decrease in dose. In order to improve the current clinical protocols, a new operational protocol is suggested based on the results of our present study. It is our belief that the revised PET/CT imaging protocol is able to maximize diagnostic information and minimize radiation risks.

並列關鍵字

PET/CT ； attenuation correction ； diagnostic information ； radiation dosimetry ； thermoluminescent dosimeters

參考文獻

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