Flat panel detector The DR detection system uses a flat panel detector as an image acquisition device, which has fast and convenient imaging and has higher quantum detection efficiency (DQE) than traditional film. The digital imaging of the flat panel detector has a wide winter range, low scattering loss, and fast and fast image acquisition.
Flat panel detectors can be divided into direct or indirect conversion according to energy conversion. The direct conversion type FPD uses a photoconductor material, which is converted into an electrical signal after X-ray exposure, stored through a thin film semiconductor array, and then subjected to A/D conversion to obtain a digital image. The indirect conversion type FPD uses a scintillation crystal. After X-ray exposure, it can be converted to visible light, then converted to an electrical signal using a photodiode array and converted to digital images line by line.
From the perspective of the flat panel detector itself, the performance of the vegetable is mainly evaluated from three aspects: spatial resolution, quantum detection efficiency and modulation transfer function.
(1) Spatial resolution. The image distinguishable minimum object diameter is determined by the spatial resolution. The period measurement method is measured by a large current and a lower voltage in the case of high contrast characteristics and low noise.
(2) Quantum detection efficiency. The quantum detection efficiency is calculated by the pixel size, which is also called the cutoff frequency, which represents the pixel center distance.
(3) Modulation transfer function. When the radiation enters the plate, it interacts with the plate and performs photoelectric conversion. At the time of conversion, the ray attenuation related information is modulated onto the analog voltage, and the image gradation value is formed by computer reading. In this process, the expression of the contrast change is performed by the MTF.
The MTF function is able to reflect the image detailing ability of the imaging system. The MTF can determine the loss of contrast when detecting objects through the detector, which is directly related to the physical structure of the detector. The MTF of the detector will also be greatly dissipated. For example, if the function has a spatial frequency distribution of 0 and a cutoff frequency, the MTF value of this image varies between 1 and 0. One of them indicates the effective conversion of contrast in the gray scale range, and 0 indicates the transition cutoff of this contrast, that is, this detail cannot be seen on the image. According to its definition, an imaging system with a spatial frequency of 0 has an MTF of 1, and if the MTF value decreases, it represents an increase in spatial frequency, which in turn determines the limit spatial frequency. MTF can map out the relationship between spatial resolution and contrast. The higher the MTF value, the more realistic the image information will be.
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