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Xray goes digital February 26, 2008

After a long break I returned to one of my beloved hobbies, photography. I was very happy when my brand new DSLR (digital single-lens reflex) camera arrived. Coming back wasn’t that easy, though I had years of practice with SLR and lately used more digital compacts too. The development, we went through is remarkable, let it be hardware, software, quality, ease of use or techniques. One dealing with digital photography really has to know more than the basis and should be up to date, to create the best pictures. So I ran over some wikis…

One really fundamental thing is the image sensor, as there is no film. Sensors works as film. This is a digital light sensitive flan – a photoelectric sensor, which perceives the quantity of light coming through the lens, and then forwards this essential information as pixels to the processor. So it’s obvious why companies emphasize developing better and better sensors.

Maybe you heard of these, like CCD (charge-coupled device) or CMOS (complementary metal-oxide-semiconductor). CCD is an analog shift register, enabling analog signals (electric charges) to be transported through successive stages (capacitors) controlled by a clock signal.

Basicaly there are two groups of image sensors (IS). CCD-CMOS and CCD-NMOS (n-channel metal-oxide-semiconductor). In weekdays we call them -not so accurately- CCD and CMOS. Each has unique strengths and weaknesses giving advantages in different applications. Neither is categorically superior to the other, although vendors selling only one technology have usually claimed otherwise. The difference between these two is in the manufacturing process. Both types of imagers convert light into electric charge and process it into electronic signals. In a CCD sensor, every pixel’s charge is transferred through a very limited number of output nodes (often just one) to be converted to voltage, buffered, and sent off-chip as an analog signal. All of the pixel can be devoted to light capture, and the output’s uniformity (a key factor in image quality) is high. In a CMOS sensor, each pixel has its own charge-to-voltage conversion, and the sensor often also includes amplifiers, noise-correction, and digitization circuits, so that the chip outputs digital bits. With each pixel doing its own conversion, uniformity is lower. But the chip can be built to require less off-chip circuitry for basic operation. Both CCD and CMOS imagers can offer excellent imaging performance when designed properly. CCD and CMOS will remain complementary. The choice continues to depend on the application and the vendor more than the technology.

The reason I wrote about ISs was the creation of The University of Sheffield, namely large and sensitive CMOS sensors for the next generation of X-ray based imaging systems.

Easier to use and faster than the imagers used in current body scanners, and with very large active pixel sensors with an imaging area of approximately 6cm square, the technology has been specifically developed to meet demanding clinical applications such as x-ray imaging and mammography. This silicon imager is about 15 times larger in area than the latest Intel processors. The next step of the project is to produce wafer-scale imagers which can produce images that approach the width of the human torso. This will eliminate the need for expensive and inefficient lenses and so enable lower-cost, more sensitive and faster medical imaging systems.

These sensors were developed by the CMOS Sensor Design Group at STFC´s Rutherford Appleton Laboratory in association with the University of Sheffield and University College London.