使用顯影劑當屏蔽對影像品質的影響

在臨床上，頭部電腦斷層經常被用來追蹤病人頭部創傷後的恢復情形，或病人中風後的變化，檢查頻率很高，因此會造成病人接受到許多的輻射劑量，其中水晶體劑量也會過高，而水晶體劑量過高會造成水晶體病變，最常見的就是白內障，為了降低病人水晶體劑量，所以就使用平常沒使用完就要報廢的電腦斷層顯影劑Ultravist370放進裝有化妝棉的夾鏈袋均勻吸附當作輻射遮蔽物，利用水假體表面來模擬人類頭部，並將遮蔽物放置於水假體表面，為了解遮蔽物的厚度、遮蔽物與物體之間的間隔、不同管電流調控與在定位圖前後放置遮蔽物對影像的影響，所以利用11種掃描條件做掃描，將得到的影像分為前、中、後區域，分析影像的CT值、雜訊與影像的假影，觀察期前、中、後區域的影像與數值的變化並分析。發現單層遮蔽物前方的CT值比原始沒有遮蔽物的前方區域增加10.85HU(p<0.05)，而雙層遮蔽物的CT值更是增加了16.93HU(p<0.05)，而在遮蔽物與物體之間隔1.2cm比間隔0.6cm雜訊減少了0.19HU(p<0.05)，Auto mA 與ODM沒有遮蔽物時，前方區域的CT值無太大差異(p=0.378)但如果加上遮蔽物後，在掃定位圖前放置遮蔽物Auto mA 比ODM的 CT值增加了0.32HU(p<0.05)，而在掃定位圖後放置Auto mA 比ODM 的CT值減少1.09(p<0.05)。關於CTDIvolume劑量，掃定位圖前放置遮蔽物比掃定位圖後放置大約增加2%，而Auto mA 不管用哪一個技術操作，CTDIvolume劑量都比ODM高約5%。使用越厚的遮蔽物更可以使光子能量降低，在遮蔽物與物體之間增加間隔可以減少雜訊。在掃定位圖前放置遮蔽物，會使整體的劑量增加。

關鍵字

輻射劑量；水晶體；電腦斷層技術；輻射遮蔽物； CT值；雜訊

並列摘要

In a clinical scenario, head computed tomography is often used to follow-up for patients with head trauma or acute cerebral vascular accidents. Repeated exposures are essential to monitor the patient recovery; however, excess radiation exposure to the lens may lead to pathological changes and eventually cause cataracts. To decrease the patient's exposures to an excess dosage of radiation, we used contrast medium “Ultravist370” into cotton pad filled zipper bags to act as a shielding. Water phantom was used to mimic the head of patients, and we placed the shielding over the surface to stimulate and evaluate it protective functions.To fully understand the effect of radiation sheltering in regards to the thickness of the shielding, the distance of the shielding to the target, or the difference in tube current modulations. We established 11 different types of scanning protocols, separating into anterior/ middle/ posterior zones. We analyzed the CT number, the noise, and the artifacts in all three regions of the images. Our results indicated the CT number at the anterior portion of a single layer shielding increased by 10.85 HU (p < 0.05). For protocols using double layer shielding, the CT number was increased by 16.93 HU (p <0.05).The spacing between the shielding and the water phantom also demonstrated a decreased of 0.19 HU (p<0.05) when comparing 0.6 cm distance to 1.2 cm. Auto mA and ODM without shielding show no difference (p=0.378), but with shielding placed before to topography leads to increase in CT number of 0.32 HU (p<0.05). In addition, placement of Auto mA after topography demonstrated a decreased Auto mA CT number of 1.09 (p<0.05) when compared to ODM. In regards to radiation dosage (CTDIvolume), we found that placement of shielding before topography would increase about 2 %. Under Auto mA, regardless which types of technique, CTDIvolume dose is about 5% higher than ODM. In conclusion, utilizing thicker shielding can decrease photon energy expenditure; increased distance between the shielding and the target can decrease noise, and placement of shielding before topography leads to increased overall radiation dosage.