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Flat Panel Detectors for X-Ray Imaging

5 January, 2012 | Digital Radiography, imaging equipment, X-ray

For far too long, the effective design and manufacture of a reliable and affordable digital X-ray radiographic imaging system has been in process, but not completely achieved. Finally, at the end of the last century, commercially available flat panel digital X-ray detectors using amorphous silicon found their permanent place in the X-ray imaging world.
In the current digital era, we are collecting, storing, analyzing and using more and more information at a faster and faster pace. X-ray imaging is no exception. The forces behind the digital X-ray revolution are much the same as those powering home and office technologies. Digital devices are smaller and more robust and once an image is digital, it becomes portable. The x-ray image can easily be made available in multiple locations at the same time, as it can be transmitted over long distances in real-time. Digital images make it possible to have computer-assisted diagnoses. Digital images are far simpler to archive and much less costly than their analog counterpart, film. Digital images, video sequences and even volumetric data sets are easily linked to a patient’s electronic record. Just as digital technologies have dramatically improved home audio and video fidelity, digital X-ray technology offers significant improvement in image quality and dose utilization.

Medical modalities, such as CT, PET, SPECT, MRI and ultrasound are naturally digital. However, standard X-ray radiography and fluoroscopy are still mainly based on analog technologies; specifically, screen/film and the image intensifier. Flat panel detectors (FPDs) have emerged as the next generation digital X-ray technology. Flat panel X-ray imagers are based on solid-state integrated circuit (IC) technology, similar in many ways to the imaging chips used in visible wavelength digital photography and video.

A number of detector technologies have been developed based on amorphous silicon TFT (Thin Film Transistor) arrays, but the most successful and widely used detectors are called “indirect” detectors. These detectors are based on amorphous-silicon TFT/photodiode arrays coupled with X-ray scintillators.

With indirect digital X-ray imaging, an X-ray tube sends a beam of X-ray photons through a target. X-ray photons not absorbed by the target strike, a layer of scintillating material that converts them into visible light photons. These photons then strike an array of photodiodes which converts them into electrons that can activate the pixels in a layer of amorphous silicon. The activated pixels generate electronic data that a computer can convert into a high-quality image of the target, which is then displayed on a computer monitor.

The most common scintillators are the same ones used in the conventional screens in radiography and fluoroscopy. The success of this indirect X-ray photons conversion to electric signals stems from the fact that both scintillator and amorphous silicon technologies were previously developed for other applications.

The less -sed screens are based on the “indirect” approach. With the indirect approach, the flat panel detector consists of a sheet of glass with a thin layer of silicon that is in an amorphous, or disordered state. On a microscopic scale, the silicon has been imprinted with millions of transistors arranged in a highly ordered array.
Each of these TFTs is attached to a photon-absorbing diode, making up an individual pixel. Photons striking the diode are converted into two carriers of electrical charge, called electron-hole pairs. An electron-hole pair consists of a negatively charged electron and a positively charged hole (a vacant energy space that acts as if it were a positively charged electron).

Since the number of charge carriers produced will vary with the intensity of incoming light photons, an electrical pattern is created that can be swiftly read and interpreted by a computer to produce a digital image. Since the sensitivity of such an array is not good enough, a scintillator is added in front of the diode (which are “photodiodes”) to enhance the electric signal output.
The performance of the imagers is rated as the value of DQE. DQE is defined as the “Detective Quantum Efficiency“. Quantum efficiency (QE) of a detector is intended as the average fraction of the input quanta which is used in the formation of the output signal, no matter if the single input quantum generates a distinct output signal or not.
The DQE is generally defined by the ratio of the squared output signal-to-noise ratio of the imaging detector.