Digital Flat Panel (From the GE website)

GEs Digital Flat Panel Detector History
 

Result of extensive corporate R&D since 1985, the GE Revolution™ Digital Flat Panel (DFP) detector replaces the film in Mammography and Rad applications as well as the analog image intensifier, with its camera optics, pickup tube or CCD camera, and analog-to-digital converter, in Cardio-Vascular applications.

Using a common technology platform that requires only limited customization for each application, GE pioneered the deployment of DFP detectors in Mammography (1999), in Rad (1999) and in Cardiac (2000).

Characterized by a very high Detective Quantum Efficiency, the GE Revolution™ detector captures nearly all the information available at its entrance and transfers it with almost no degradation to the observer. For all applications, the result is outstanding image quality at reduced dose.

GEs Digital Flat Panel Technology
The principle of the flat-panel detector is illustrated in the drawing below.


 

Principle of the GE Revolution™ Digital Flat Panel Detector.

The cesium iodide (CsI) scintillator absorbs x-ray photons, converting their energy into light photons emission. This light is then channeled toward the amorphous silicon photodiode array where it causes the charge of each photodiode to be depleted in proportion to the light it receives. Each of these photodiodes is a picture element (pixel); the spatial sampling of the image, which is the first step in image digitization, is thus performed exactly where the image is formed, whereas it is realized almost at the end of the chain in an Image Intensifier (see more in part 2 of the education series). The electronic charge required to recharge each photodiode is then read by ultra-low-noise proprietary electronics and converted into digital data that are then sent to a real-time image processor. In the GE cardiac system, over 30 million pixels per second are read out, processed, and displayed in real time.

GEs Digital Flat Panel Structure
 

Mono-substrate Amorphous Silicon panel coated with CsI scintillator.

The heart of the flat panel digital detector consists of a two-dimensional array of amorphous silicon photodiodes and thin-film transistors (TFTs), all deposited on a single substrate.
Utilizing thin film technology similar to that used in the fabrication of integrated circuits, layers of amorphous silicon and various metals and insulators are deposited on a glass substrate to form the photodiodes and TFTs matrix, as well as the interconnections, and the contacts on the edges of the panel.


The CsI scintillator, which converts x-ray photons into visible light photons, is deposited directly on top of the amorphous silicon structure.
Using a proprietary process, it is grown in very thin needles (5µm width) that channel the light photons towards the photo-diode, like a fiber optics would do. This allows one to increase the thickness of the CsI, and thus to stop and detect more X-rays, without degrading spatial resolution because of wide-spreading light scatter as observed in typical radiographic phosphor screens.

Electron microscope views of CsI needles that constitute the scintillator layer.

The photo-diode comprising each pixel is used as a bucket for electrons and each TFT behaves as a switch to access the associated photo-diode. The TFT conductive state is controlled through the voltage applied by scan electronics modules to matrix rows.
When a TFT is conductive, the charge of the corresponding photo-diode can be measured through a matrix column by the readout electronics modules and converted to a digital value by the analog to digital converter attached to each colomn.

The second step of image digitization after spatial sampling: pixel quantification, is thus also performed next to image formation, and not at the end of a long transformation chain like in an Image Intensifier-based system. (for more details on Image Intensifier imaging chain, see part 2 in the Digital Flat Panel education series).

 

Flat Panel and Imaging
Scan modules and readout modules are GE proprietary designs and use state-of-the-art high density packaging technology to minimize sources of noise. Associated with the optimized design of the amorphous silicon flat panel, the electronic noise generated in the entire detection chain, from the photo-diode to the output of the analog-to-digital converter, is equivalent to the signal generated by a single X-ray photon. Thus, the read-out noise added by the panel is significantly less important then the quantum noise in X-ray imaging. The image quality is therefore limited only by the X-ray quantum noise, i.e. by the dose, and not by the detector performance. This low noise performance, which is particularly important in fluoro where very low dose is required. Combined with other advantages of the flat panel detector, such as large dynamic, response stability over dose variations and time, response uniformity over the entire image area, and absence of distortion, it provides a breakthrough in image quality. All this adds not only to intrinsic image quality but also and opens new opportunities for further image processing.

Processing Data with a Flat Panel
 

In cardio-vascular imaging, information is typically associated with small objects such as arteries, stents, guide wires, and catheters - objects that overlap each other and large organs with different contrasts such as lungs or diaphragm. Because the display has a finite number of gray levels, representing the organs at their acquired brightness levels may compromise the representation of smaller objects of clinical interest. At GE, we have developed state-of-the art computational methods to represent the information in an intelligent manner, so that features of interest are allocated optimal display values. This requires that the original image be captured with high fidelity over its entire dynamic range.

As a result, the detector gives images a unique look and feel. This allows diagnostic information to be presented with optimal utilization of display properties and human visual perception. This technology also provides the ability to selectively enhance the contrast of objects such as stents regardless of the anatomical background against which they are acquired, providing better visibility of object details across the entire image, regardless of the background anatomy.

Conclusion
The family of digital detectors manufactured by GE is based on a common technology platform whose heart is a two-dimensional amorphous silicon array of photo-diodes and thin-film transistors deposited on a single piece substrate and directly coated with needle grown Cesium Iodide. The technology platform strategy forced the design to be able to answer the most challenging needs of each application, such as large field of view for chest Rad, high resolution for Mammography, real time and low noise image acquisition for Cardiac. This strategy has several advantages:
 

The high performances of the amorphous silicon flat panel are complemented by the proprietary electronics for detector control and readout. Associating one Analog-to-Digital converter with each of the 1024 or 2048 pixels forming a single image row is a good example of a design without compromise to minimize noise sources in all conditions.

All that results in a final design which offers Image Quality performances as well as simplicity, with a single large sensitive area requiring neither tile stitching with the associated lost pixels, nor detector motion prohibiting fast acquisition, and thus a reliability demonstrated by the most large and diverse installed base of Digital Detectors.
 

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