Technology

The digital imaging market spaces are mostly serviced by charge-coupled device (CCD), complementary metal-oxide semiconductor (CMOS), and amorphous silicon (a-Si) flat panel image sensor technologies. Each technology serves specific markets with certain advantages. CCD technology, despite being expensive, dominated the visible imaging markets for two decades because of its high-quality, low-noise imaging performance. Large panel digital radiography markets have been well served by a-Si thin film transistor (TFT) technologies due to the manufacturability of large arrays of the relatively simple single transistor passive pixel circuits. CMOS Active Pixel Sensor (APS) technology has emerged recently to offer a cost and performance advantage over both CCD and TFT technologies in many radiography applications.

The uniqueness of X-ray imaging systems for nondestructive testing (NDT), medical, dental and other applications is due to the penetrating radiation that forms “shadow” images, with signal levels proportional to the density of material being penetrated. Penetrating radiation systems generally do not use focusing optics. Thus, the detection focal plane must be as large as the object under study. Clearly, coverage of large area objects such as the human chest or large circuit boards require large area detectors, which has been a challenge for CMOS and CCD technologies.

Standard foundry CMOS integrated circuit technology has been optimized for manufacture of hundreds of small devices on a round wafer, with reasonable tolerance for a few defective die. Filling a single wafer with one or two very large CMOS die can present significant challenges and special design features are required to minimize the effects of defects on the sensor circuits. Fortunately, strategies such as device tiling enable large focal planes to be constructed from smaller-format image sensors. Fundamental design considerations for CMOS x-ray image sensor technologies include the concept that image resolution is not a direct function of the pixel size. Furthermore, the signal value of each CMOS pixel is mostly independent of the pixel aperture. X-ray sensors require conversion of x-ray energy into visible light using an x-ray scintillator material such as gadolinium oxysulfide (GadOx), cesium iodide or other compounds.

The visible light output is proportional to the intensity of the X-ray beam that penetrates the object of interest and the CMOS detector converts the signal electrons generated in a silicon photodiode into a voltage using a voltage conversion amplifier called a source-follower. A CMOS pixel, consisting of three or more transistors that perform a voltage conversion proportional to the photogenerated signal, is known as an Active Pixel Sensor (APS).

For more information, please see our article, Digital Radiography Revealed by Michael G. Farrier, Gene P. Weckler, and Thorsten Achterkirchen (June 2008).