混合式骨減影頭部電腦斷層血管攝影及其臨床運用

Abstract

頭部電腦斷層血管攝影是臨床上常用來診斷許多血管異常及病變的重要工具，利用顯影劑的注射可以使得血管構造強烈顯影，進而評估其變化。骨頭在電腦斷層下亦為密度極高的構造，而頭部的血管會穿過骨頭而連接顱外與顱內血管，某些血管異常及病變恰巧發生在骨頭內的血管或是非常靠近骨頭的血管，這時使用傳統電腦斷層來評估血管將變得困難，影像的重組也會受到限制。 混合式骨減影頭部電腦斷層血管攝影是一種從電腦斷層影像中移除骨頭構造的方法，需要額外取得注射顯影劑前的影像。由於不同次掃描間通常會有些許位移，在骨減影之前須先進行位移校正，利用無顯影劑的影像取得骨頭的遮罩，在遮罩範圍內進行減影，並補上軟組織的密度值，即可得到一個消除骨頭後的電腦斷層影像。 利用混合式骨減影頭部電腦斷層血管攝影，原本會因為骨頭而影響診斷效果的疾病就有很高的機會可以在電腦斷層呈現。我們將之運用在兩種通常不會用電腦斷層來評估的腦血管疾病。其一是硬腦膜動靜脈瘻管，因為其發生位置緊鄰骨頭或在骨頭內，傳統電腦斷層不易偵測，但混合式骨減影頭部電腦斷層血管攝影卻得到很高的準確度，並且利於擬定血管內治療計畫。其二是頸動脈阻塞，阻塞段的遠端常位於顱底的骨頭內，傳統電腦斷層不易評估遠端位置，而利用混合式骨減影頭部電腦斷層血管攝影可以清楚的看到顱底段的阻塞位置，不同的阻塞位置，對於進行血管內重建的成功率及再阻塞率有顯著的差別。

Head Computed Tomography Angiography (CTA) is an important image modality for evaluation of various vascular diseases of head. With contrast injection, the vessels become high-density structures and can be evaluated. The bones are also high-density structures on CT. Many head vessels traverse the bones. Some vascular anomalies or disorders occur in or adjacent to the bones, and conventional CTA is difficult to evaluate these situations. Image reconstruction is also not easy if the key structures are obscured by the bones. Hybrid CTA is a technique for bone removal. An additional precontrast scan is needed. Motion correction between precontrast and postcontrast images is performed first. A bone mask is produced from precontrast images. Subtraction is performed inside the mask. A soft tissue value is added to the subtracted value for even background. A bone-eliminated CTA can be obtained. With hybrid CTA, vascular diseases that may be masked by bones can be better evaluated. Here, hybrid CTA was used for two diseases. The first one was dural arteriovenous fistula (AVF), which was adjacent to or in the bones. Conventional CTA had limited value in diagnosis of dural AVF, but hybrid CTA showed a good accuracy. The other was carotid chronic total occlusion, with its distal end occasionally in the skull base. Hybrid CTA could detect the distal end of occlusion, which was associated with successful rate in endovascular recanalization and re-occlusion rate after recanalization.