Harada T, Nishikawa K, Shibuya H, Hayakawa Y, Kuroyanagi K
Department of Oral and Maxillofacial Radiology, Tokyo Dental College, Chiba, Japan.
Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 1995 Jul;80(1):120-3. doi: 10.1016/s1079-2104(95)80028-x.
This study examined the relationship between x-ray beam quality and image characteristics with the use of the Sens-A-Ray (Regam Medical Systems, Sundsvall, Sweden). To evaluate the x-ray response, properties of the image sensor, dark current and statistical noise, sensitivity, and contrast resolution were measured. Tube voltage, tube current, and exposure time were varied. Exposures were determined with the use of an ionization chamber. Pixel values beneath a lead strip were used to calculate dark current effects. Contrast was measured with pixel values from the images of a step-wedge. Dark current noise increased in proportion to exposure time. Although the sensitivity at each tube voltage was linear and proportional to the exposure, the gradient was inversely proportional to increased tube voltage. After subtraction of the dark current and statistical noise, the gradient of the sensitivity at low tube voltage remained steeper than at high tube voltages. Hence, sensitivity decreased with increased tube voltage. Sensitivity gradients at each different tube current were similar. Irrespective of exposure, contrast resolution between adjoining step-wedge steps > or = 9 mm were too small to distinguish at 50 kVp. The gray levels for each step below 12 mm in thickness were distinguishable with 90 kVp. Increased dark current noise consequent to long exposure times narrowed the available gray scale range. Lower tube voltage resulted in higher sensitivity with larger gradients than higher tube voltages.
本研究使用Sens - A - Ray（瑞典松兹瓦尔Regam医疗系统公司）来检测X射线束质量与图像特征之间的关系。为评估X射线响应，对图像传感器的特性、暗电流和统计噪声、灵敏度以及对比分辨率进行了测量。管电压、管电流和曝光时间均有所变化。曝光量通过电离室来确定。利用铅条下方的像素值来计算暗电流效应。对比度通过阶梯楔形块图像的像素值来测量。暗电流噪声与曝光时间成正比增加。尽管在每个管电压下灵敏度呈线性且与曝光量成正比，但梯度与管电压升高成反比。减去暗电流和统计噪声后，低管电压下灵敏度的梯度仍比高管电压下更陡。因此，灵敏度随管电压升高而降低。每个不同管电流下的灵敏度梯度相似。无论曝光情况如何，在50 kVp时，相邻阶梯楔形块步长≥9 mm之间的对比分辨率过小，无法区分。厚度低于12 mm的每个阶梯的灰度级在90 kVp时可区分。长时间曝光导致的暗电流噪声增加使可用灰度范围变窄。较低的管电压比较高的管电压具有更高的灵敏度和更大的梯度。
