THE MOST PRACTICAL INTRAORAL DIGITAL X-RAY IMAGING

为了设计最实用的CCD（Charge-Coupled Device）口内数字X射线成像系统，首先对现有系统的暗电流特性、梯度和动态范围、粒度、管电压依赖性、管电流依赖性和对比度分辨率进行了评价。各系统在暗电流特性和动态范围上表现出明显的差异。为了消除暗电流的影响，灰度量化的比特数应大于12。动态范围为1：25-1：35。其次，利用CCD传感器性能评价试剂盒，研究了放大CCD传感器模拟输出信号的增益值与放大信号数字化后得到的动态范围之间的关系。动态范围可以改变到1：45，比现有系统的动态范围更宽，为了确定最小所需的动态范围和优化对X射线响应特性的梯度，分析了上颌磨牙区的X射线对比度，因为在口内摄影中，上颌磨牙区获得最大的X射线对比度。动态范围要求最大为1：12。因此，通过使动态范围变窄，可以获得比现有系统中的梯度更大的梯度，即更大的对比度分辨率。

Several investigations were carried out to design the most practical intraoral digital x-ray imaging system with CCD (Charge-Coupled Device) sensor under consideration of on the response properties to x-ray.Firstly, dark current property, gradient and dynamic range, granularity, tube voltage dependency, tube current dependency, and contrast resolution of commercially available systems were evaluated. Each system showed the difference especially in the dark current property and the dynamic range. The bit number for quantization of gray scale should be set at more than 12 to get rid of the effect of the dark current. The dynamic ranges were 1 : 25-1 : 35.Secondly, relations between the gain value to amplify the analog output signals from the CCD sensor and the dynamic range obtained with digitization of the amplified signals were investigated using the kit for evaluation of CCD sensor properties. The dynamic range could be changed to 1 : 45, which was wider than those of the existing systems.To decide the dynamic range required in minimum and optimize the gradient in response properties to x-ray, the x-ray contrast in the maxillary molar region was analyzed becouse the largest x-ray contrast in intraoral radiography is obtained in there. The dynamic range required was 1 : 12 at most. Therefore larger gradient, namely larger contrast resolution than those in the existing systems can be acquired by narrowing the dynamic range.