膝关节摄影中照射野大小对DDR系统影像质量的影响

目的评价在膝关节摄影中，照射野的选择对直接数字化X线摄影（DDR）系统影像质量的影响。方法随机抽取50例膝关节患者，在摄影条件相同的情况下，对左右侧膝关节用不同的照射野进行摄影，在工作站分别对膝关节影像进行灰度值测量，测量点为胫骨内外髁连线，获得连线的灰度值曲线。小照射野组使用DDR系统进行膝关节正位摄影，焦一片距100cm，摄影条件为55kV、25mAs，照射野包括软组织边缘1～2cm。大照射野组是对小照射野组相同病例的对侧膝关节使用相同的摄影条件，照射野与探测器板的大小一致。结果小照射野组的软组织灰度值曲线呈逐渐变化，显示层次增加；大照射野组的软组织灰度值曲线较平缓，接近骨骼组织时变化陡峭，显示高反差。小照射野组的骨骼组织灰度值曲线形态与大照射野组基本一致，但灰度值的大小整体增大，骨骼组织与软组织的灰度值差异减少。结论DDR系统对面积较小的部位进行摄影时，应缩小照射野，可以获得较理想的影像质量。

Effect of exposure field size on the imaging quality during direct digital radiography of the knee joints

Objective To evaluate the effect of exposure field size on the imaging quality during direct digital radiography(DDR)of the knee joints.Methods Fifty patients with knee joint disease,who were randomly selected,underwent DDR of bilateral knee joints with different exposure field size under same radiographic condition.The gray scalae value of all digital images was measured along a connecting line from tibial internal condyle to external condyle on workstation so as to obtain the curve of gray scalae.According to exposure field size,the patients were divided into two grous.In small exposure field group,bilateral knee joints were examined by 55 kV,25 mAs and 10 cm of focus-film distance,simultaneously,the exposure field included 1-2 cm of soft tissue border.In large exposure field group,contralateral knee joints were examined by the radiographic condition similar to small exposure field group but their exposure fields were coincident with detecting plate size.Results gray scalae curve of soft tissue in small exposure field group was gradually changed and appeared as increasing layers,while that in large exposure field group was slowly changed but nearby bone it was rapidly changed with high reflectance.Gray scalae curve shape of bone in small exposure field group was basically similar to that in large exposure field group but its value was larger than latter.The difference of gray scalae values between bone and soft tissue was redued due to its whole enlargement. Conclusion DDR of small locations should select small exposure field,which can obtain better imaging quality.