Question

Explain why an indirect DR system would need a TFT flat panel array or a CCD but not both.

Answer 1

Indirect Conversion with a CCD

A CCD is a light-sensitive sensor for recording images that consists of an integrated circuit containing an array of linked or coupled capacitors. X-ray energy is converted into light by a scintillator such as Tl-doped cesium iodide. The amount of light emitted is then recorded by the CCD, and the light is converted into electrical charges.

CCDs can be used for radiography as part of either a lens-coupled CCD system or a slot-scan CCD system

Indirect Conversion with a Flat-Panel Detector

Indirect conversion DR systems are “sandwich” constructions consisting of a scintillator layer, an amorphous silicon photodiode circuitry layer, and a TFT array. When x-ray photons reach the scintillator, visible light proportional to the incident energy is emitted and then recorded by an array of photodiodes and converted to electrical charges. These charges are then read out by a TFT array similar to that of direct conversion DR systems

There are two main technologies available in Digital Radiography: Charge Coupled Device (CCD) based detectors and Flat Panel (either direct or indirect capture) detectors
Flat Panel systems - either direct or indirect - use a thin film transistor (TFT), layers that capture the X-ray generated electrons. Unlike CCD technology, the electronics in Flat Panel systems are in the direct radiation beam which is damaging and can cause the electronics to fail. To protect the electronics and reduce the risk of failure, Flat Panel manufacturers “harden” them, a process which reduces the efficiency. Even in this case, under the frequent, direct and unavoidable radiations of X-ray, some sensitive electronics in Flat Panel may easily be damaged over time. Every 2-3 years, Flat Panel must be serviced or replaced. Therefore, the maintenance cost of Flat Panel is very expensive. In contrast, NOVA detector protects its CCD sensor and sensitive electronics from any direct radiation of x-ray by design. Hence, it enjoys a much longer working life and easy maintenance.

Additionally, these complex electronic devices have layers of components that are surrounded by protective casings. In order to produce these Flat Panel systems, arrays of smaller devices are tiled together which prevents an entirely seamless image.

Another important difference between CCD and Flat Panel technology pertains to fill factor. NOVA has 100% fill factor which means 100% of the pixel area is used. Flat Panel systems have far lower fill factors because they have electronics running through each pixel reducing the X-ray capture surface area. The higher the fill factor, the higher the resolution of the image and the lower the dose needed. NOVA’s 100% fill factor provides maximum efficiency with lower radiation