- 期刊
評估平面X光攝影在垂直與水平方向放大率之差異
Through-plane and In-plane Magnification Errors in Plain Radiography
摘要
目的：平面X光攝影(plain radiography)中有興趣的物體大小經常會受到攝影時，X光射源與影像接受器的距離(source to image receptor distance; SID)或被照體和影像接受器的距離(object to image receptor distance; OID)等不同所影響，而形成影像放大率(magnification)。本研究主要利用改變不同SID與OID等距離條件，來評估被照體在垂直與水平方向上，對於平面X 光攝影放大率的影響。方法：利用不同的SID與OID，對9個已知大小、不同位置的十元硬幣，分別進行攝影，比較SID與OID對於影像中間硬幣放大率的影響，也比較其與周圍八個硬幣與中心硬幣的放大率的差異。結果：結果發現，在OID為0的情況下，SID的降低(120至90公分)會使中央硬幣放大率的變化會由15%上升至18%(約增加3%)。另外一方面，當固定SID為100公分的情況下，我們發現增加OID(0至20公分)會使放大率由18%上升至49% (約增加31%)。另外、比較同水平方向硬幣的放大率變化，我們發現無論在任何的SID或OID的條件情況下，影像中9個硬幣的放大率差異變化都約在1~2%標準差以內。結論：OID的增加對於放大率的影響相較於SID的降低來的嚴重。代表當物體和影像接受器距離越大時，放大率造成的影響必須被考慮進去；然而對於相同平面、不同位置的放大率的變化，SID與OID的改變所造成的影響並不大，代表相同平面不同位置的放大率幾乎可忽略。
關鍵字
無資料並列摘要
PURPOSE: The magnification error in plain radiography is known to be dependent on distances between image receptor to x-ray source (SID) and that to imaging object (OID). These through-plane magnification errors have been reported at 15%~30%, but these errors within in-plane direction have not been assessed. The purpose of this study was to determine the range of magnification errors both on the through-plane and in-plane directions for digital plain radiography. METHODS: A routine X-ray machine (Hitachi, DHF-155HⅡ) with digital flat panel detector (CANON CXDI-401C) and known diameter (25mm) coins were used as experimental tools. The 9 coins were placed on the center and 4 borders and 4 corners of the imaging field to assess the through-plane and in-plane magnification errors. We performed 20 times radiographies by changing different SID (90,100,110 and 120cm) and OID (0, 5, 10, 15, 20cm) respectively. The diameter of each coin was measured for each image by using ImageJ (NIH, USA). The magnification error was calculated from the diameter of measured coin subtracted by 25mm, and then normalized by dividing 25mm. Standard deviation among each magnification error were also assessed to determine the magnification differences both at the through-plane and in-plane directions. RESULTS: Our results showed that the magnification errors are ranged from 15~18% at different SID (OID=0cm) and 18%~49% at different OID (SID=100cm). We noted that through-plane magnification difference is enlarged with changing the OID (SD: 9%~14%) than SID (1.7%~6.9%). In contrast, the difference within in-plane magnification errors is relative small (SD:1~2%). CONCLUSIONS: External landmark has been considered as a useful tool to assess the magnification errors for plain radiography. However, the positions of landmark where should be placed including through-plane and in-plane locations have not been recommended. In this study, we demonstrated that magnification errors for routine SID (90~120cm) is range from 15%~18%, and these errors will be increased if the OID is increased as well. In addition, we found that the magnification error at center is similar to that at borders or corners on in-plane directions. These results suggested the external landmark should be place at the same level as the lesion as possible but the location at in-plane direction is ignorable.