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Digital X-ray Systems - Standard article

How Digital Radiography systems work?

A Digital Radiography (DR) X-ray system is a type of scanner in widespread use in medical domains such as dentistry, surgery and mammography. A DR scan of specific organs or parts of the body provides a 2D image of dense material such as bone and other tissues. 

A medical DR system consists of an X-ray tube and a Flat Panel Detector (FPD). The region of the body to be scanned is positioned between the tube and the detector. X-rays emitted by the tube pass through the body, and the FPD captures the attenuated X-ray photons.

The FPD is popular with radiographers because it offers high sensitivity, is light and compact, and produces accurate images of large areas.  High sensitivity means that high-quality, detailed images can be generated with relatively low doses of radiation, reducing the risk to both the patient and the radiographer. The size and weight of an FPD make it easy to accommodate the scanner in a radiography suite which has limited space, and to adjust the position and inclination of the scanner to the optimal position, both for the comfort of the patient and for image quality. The accurate and detailed images produced by an FPD enable the medical practitioner to diagnose conditions such as bone fractures, breast cancer or tooth decay with greater confidence. 

 

X-ray Flat Panel Detectors (FPD) inside Digital X-ray systems 

The FPD is composed of an array of sensing elements, called pixels, and its associated electronic circuitry, including readout integrated circuits (ROICs) and gate drivers. Soft tissue, bone and blood have different X-ray attenuation characteristics, so the composition of the area of the body in the scanner is represented by the relative strength of the signals captured by the pixel array. The electrical signals are converted by the ROICs into digital outputs supplied to a back-end computing system. The computer renders the digital signal output as an image which may be printed or displayed on a monitor.  An FPD may use either an indirect or direct method of converting the impinging X-rays to an electrical signal. An indirect detector contains a layer of scintillating material which converts the X-rays into visible photons (light). Behind the scintillator is an array of photodiodes which convert the light into an electrical signal. The array of photodiode pixels is similar in concept to a camera’s image sensor: a high density of pixels creates a high-resolution image in which small features are clearly and sharply rendered.  In an FPD which implements direct conversion, the photodiodes are made with a material which has the ability to convert incident X-ray photons directly into electric charge. Therefore, this type of FPD does not need a scintillator. Direct-conversion FPDs typically achieve higher spatial resolution with less blur than indirect detectors.  Both direct and indirect conversion FPDs feature an array of thin-film transistors (TFT), that connect each pixel to its corresponding ROIC channel, in order to perform the conversion from electrical charge into digital outputs. These TFTs are controlled by gate drivers that enable one row of pixels at a time, so that the captured X-ray image can be read out from the FPD in a rolling shutter fashion.

 

ams inside Flat Panel Detectors 

Advanced mixed-signal semiconductor technology underpins ams’ development of accurate, low-noise ROICs for FPDs. The ROIC amplifies the analog signal generated by an array of photodiodes, and converts it to a digital output that it transmits to the FPD’s computing back-end. 

The latest ams readout IC supplied as a standard product for FPDs is the AS5850B, a 16-bit, 256-channel charge-to-digital converter. It combines high speed and low noise to optimize image quality and minimize radiation dose while enhancing image quality in dynamic applications. Its ultra-low power consumption makes it suitable for use also in battery-powered portable FPD equipment.  ams also supplies gate drivers as application-specific ICs (ASICs) to the world’s leading manufacturers of X-ray equipment.


  > Find out more about the digital X-ray sensor AS5850B