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Flat Panel Detector Working Principle

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Do you often find it difficult to comprehend the working principles of flat panel detectors?

Flat panel detectors have become the modern favorite when it comes to X-ray imaging detecting systems. The reason can be attributed to the numerous benefits it offers over other image detecting systems including image intensifiers and X-ray film plates. Nonetheless, how the flat panel detector works seems a bit difficult to understand to many individuals and details like its functional units and detecting units are not so clear to other people. 

This guide explains the working principles of the flat panel detector and gives you an understanding of its functions and its detecting units. This knowledge will give you more confidence when next you are using the flat panel detector or when purchasing one. 

You want to know more? Then let’s dive in. 


I. What is the Functional Unit of the Flat Panel Detector?

For starters, a flat-panel detector is a modular composition of individual functioning units that combine to make the detection of X-rays possible. These functional units are known as pixel arrays, and they are used for converting X-ray radiations to light energy that makeup the images. 

These pixel arrays are the sensitive parts of the flat panel detector which are usually square or rectangular shapes with varying dimensions depending on the size of the sample material under examination. However, depending on the desired spatial resolution this array may include thousands of pixels with each pixel having square shapes and micrometer-long sides. For every functional unit of the flat panel detector a very short radiation falls on the pixel array whenever an X-ray image is taken while the pixels collect and store this radiation until it is read out. 

Also, these pixels each include a photodiode that uses the impacting X-rays to generate an electrical charge. The pixels also include a switch which has a Thin-film Transistor (TFT) or Indium Gallium Zinc Oxide (IGZO) which are often utilized as a display technology. However, IGZO is more suitable since it is more recent and advanced than the TFT. Both the switch and the photodiode help to generate the image by direct or indirect conversion of the X-rays. 


Flat Panel Detector Working Principle


II. How Many Detectors are Used in Flat Panel Detectors?

Generally, detectors are classified based on their method of conversion of X-rays to light energy during the creation of the image. According to this definition there are two types of detectors used in flat panel detectors namely direct and indirect detectors both of which we will examine in this section of the post. 

  • Direct Detectors

The type of X-ray conversion used in direct detectors is the direct conversion method from which they derive their name. They establish their conversion using photoconductors like amorphous selenium or similar photoconductors to directly convert incident X-ray energies to an electric charge. Electron hole pairs are generated from the X-ray photons incident on the selenium layer using an internal photoelectric effect. By applying a bias voltage to the selenium layer depth, the holes and electrons can be drawn to corresponding electrodes to generate a proportionate electric current to the radiation’s intensity. To readout the signals a general TFT array like a readout electronic device is employed. 

  • Indirect Detectors 

Indirect detectors on the other hand employ a scintillating material layer such as gadolinium oxysulfide or cesium iodide to convert the X-ray energy to light with an amorphous silicon detector array embedded behind the scintillating layer. Similar to the image sensor chips found in a digital camera, photodiodes are included in every pixel that produces the electrical signals. These signals are comparable to the scintillator layer lights in front of the pixels which are used to generate a precise image of the X-ray image.


III. How does Flat Panel Detectors Work?

The workings of flat panel detectors are driven by an array of energy-sensing components called pixels which enables a readout scheme used to generate the image of a sample under X-ray examination. However, there are other electronic circuitry components along with the pixel that enable this readout system and they include readout integrated circuits (ROICs) and gate drivers integrated circuit. 

For the ROICs, they typically are multi-channel devices serving about 256 units, but they are often allocated along the pixel arrays’ side. To speed up the readout for large flat panel detectors, they are often placed on opposite sides. The gate driver ICs on the other hand are positioned perpendicular to the ROICs and by the side of the pixel array to activate or deactivate the switches that drive the controls for each row of pixels. 

To generate the images after an x-ray examination the X-rays are converted into light energies that are also converted into electrical signals for each pixel’s photodiodes. Furthermore, a thin-film transistor switch enables the readout of the electrical signals of each diode. Also, the photodiode and TFTs switch are connected using a signal wire with either an analog or a digital conversion which is used to generate an image after a low-noise amplification. 



Finally, flat panel detectors are very important for the modern X-ray imaging system as they are very effective and accurate at converting X-ray signals to light energies that enable it to produce an accurate image. The different types of detectors and the workings of flat panel detectors are very critical considerations that you should have if you intend to get the best result for your X-ray imaging system. Nonetheless, the benefits of flat panel detectors should not be overlooked including their cost savings, portability, higher image accuracy, time savings, and low radiation doses. The knowledge of flat panel detectors will help you choose a suitable one that fits your budget and X-ray inspection objectives. Now the ball is in your court, what are you going to do with it?


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