Difference between amorphous silicon of planar detector receiver and crystalline silicon of detector receiver

The core part of DDR is flat panel detector. Generally speaking, it is a detector that directly converts X-ray energy into electrical signals and generates X-ray images by using semiconductor technology. Flat panel detector can replace various X-ray detectors, such as TV image enhancement system. Its most prominent feature is to output high-quality digital images. In the direct conversion type, its MTF characteristic is better than that of the image screen system, and its sensitivity is comparable to that of the TV enhancement system. The development and further improvement of flat panel detector will gradually replace the traditional X-ray detection equipment.

1. The types of flat panel detectors can be roughly divided into CCD type, amorphous silicon type and amorphous type. The main principle of CCD flat panel detector is that optical signals are received by CCD in the detector, read out and form digital images.

2. Amorphous silicon flat panel detector, the core of which is a matrix board composed of amorphous silicon and thin film transistors, and each cell of the matrix board contains a storage capacitor and an amorphous silicon field effect transistor. The whole digital matrix is packaged in a box like a "clip", which is mainly composed of scintillation layer, matrix board, glass substrate, readout circuit and so on. Its good density and spatial resolution represent the main development direction at present.

Cesium iodide (CsI) has high X-ray reception and visible light photon yield. Cesium is the best choice for X-ray receiver because of its high atomic number, so this metal is very suitable for inputting X-rays. It takes 20~25 electron volts to generate a photon. CsI doped with cesium excites light at 550nm, which is the peak of spectral sensitivity of amorphous silicon.

3. As a light guide material, there are two reasons: ① the high resolution characteristic of the photoresistor itself; (2) The thicker photoconductive absorption layer can obtain higher X-ray sensitivity. X-ray energy can be directly converted into electrical signals. Electron-hole pairs generated by X-ray irradiation on photoconductive layer are separated by electric field under 6KV bias voltage, and are collected by each pixel unit and converted into X-ray digital image data.

The matrix board includes a thin film transistor (TFT), an energy storage capacitor and a current collector, on which an amorphous silicon layer with a thickness of about 500 microns is deposited ... A plurality of pixels (139× 139μm) are arranged in a two-dimensional matrix, and the gate lines are arranged in rows. The size of TFT pixels directly determines the spatial resolution of the image. Each pixel has a charge receiving electrode, a signal storage capacitor and a signal transmitter, which are connected with the scanning circuit through a data network. Finally, the digital signal is read by the readout circuit and restored to an image.

Amorphous silicon is the most ideal material for X-ray receiver, because amorphous silicon is not damaged by radiation.

The above is the main structure of flat panel detector, and flat panel detector and X-ray tube constitute the main part of direct digital imaging. The other part is operation, quality control and post-processing. Most of the work is done by computers.

The real-time functions of the main processor include: offset correction and gain; Black level clamping; Stain interpolation; Frame accumulation and equalization. For image post-processing, auxiliary processor can be used to complete, including X-ray exposure control, image data windowing, scanning into normal mode, directory search, noise reduction and variable recursive filtering.

The best performance requires low noise, wide dynamic range and fast response.

DDR imaging system can be combined with the user's original X-ray machine to provide digital X-ray image data. Other auxiliary devices include: scanning controller, system controller, image monitor, etc. The scanning controller is mainly composed of a computer-controlled matrix scanning circuit, a recovery circuit and a scanning data conversion device. System scanner is a computer host system, including operating program, image processing program, image storage, printing network management and so on. The image monitor can display photographic images and provide photographic quality reference for staff.

The conversion from X-rays to electrical signals can be done directly and indirectly. In the direct conversion mode, a voltage is applied to the layer as a photoconductor, and the energy of X-rays is directly converted into an electrical signal. In the indirect conversion mode, X-rays are first converted into optical signals by flicker, and then the optical signals are converted into electrical signals by photodiodes.

The flat panel detector can be converted in the above two ways, and their main difference lies in their different manufacturing structures. The above two types of detectors have their own advantages and disadvantages. Due to the crystal structure of CsI scintillator layer, light scattering inevitably occurs while transmitting signals, and the absorption rate decreases, but it has little effect on the final image quality. Its high quantum detection efficiency (DQE) can obtain high-quality images with low dose exposure. Because of the high imaging speed, it can be used in the fields of perspective and time subtraction, which greatly increases the application scope of X-ray inspection. As a photoconductor, optical signals can be directly converted into electrical signals to avoid scattering. However, the X-ray absorption rate is low, and the image quality can not be well guaranteed at low dose. And the layer is sensitive to temperature, so the use conditions are limited.

The flat panel detectors of DR and DDR can be conveniently applied to conventional X-ray photography equipment and fixed on the filter of vertical buckywall stand or flat bed. The image can be displayed in a few seconds after exposure, and the cassette can be transported back and forth without virtual image; The system itself has a solid structure without any mechanical movement. At the same time of exposure, the photographic image can be displayed on the monitor of the operating room, and the staff can judge the projection quality, which shortens the examination time and avoids repeated examination of patients for technical reasons.