Flat Panel Detector Array
Flat Panel Detector Array Theory
The flat panel detector array is made up of thin film transistor switch circuitry that controls the signal from a photo diode which gathers the light from a CsI conversion layer.
In the past image intensifiers (shown in figure 2) were the main device used to convert X-ray photons into light photons. The image intensifier (II) is a vacuum tube with an input window that can range in size from 3 inches to 18 inches. The output window is usually about an 25 mm or approximately one inch. There are grid voltages that are set up on the II for focusing and sizing. Depending on the quality of the II the manufacturer would select them for 1, 2 or 3 magnification settings by switching the high voltage levels on the tubes grids. The light gain compared to a phosphorus screen is many thousand times higher. The image intensifier can burn artifacts in the image at high radiation doses. Additionally, because it is a tube, is is fragile and the vacuum can be broken quite easily. The II also requires shielding from the earths magnetic field to eliminate small distortions in the image. There is a series of lenses that are usually set at infinity focus and light splitter mirrors that are used to filter and split the beam between CCTV cameras and film recording mediums so the resulting image is geometrically distorted.
The flat panel technology was developed by Xerox PARC in the 1970's and it was too expensive and slow to use in copiers however it was found to be resistant to X-rays. dpiX LLC was established in 1999 after a $50 million DoD grant led to the founding of dpiX Inc. The technology utilized is based upon the Xerox PARC program that was created in 1970 as the future in the “architecture of information.” dpiX’s move from Palo Alto, CA, to Colorado Springs was completed by 2011. dpix LLC developed the radiographic amorphous silicon (a-Si) panel. The a-Si panels were first used in the non-destructive testing applications before it was introduced into the medical market because the new technology had to be proven safe and effective by the FDA. The a-Si panels provided the operator with an image that was not geometrically distorted and a pixel size of around 127 microns. a-Si panels provided superior images when used in industry because the digital imaging enhancements, algorithms, filters could be used and the signal to noise was a huge improvement over image intensifiers and film.
The results of tests by aerospace and automotive QA labs determined that the cost savings of digital imaging over film provided in most cases enough revenue savings to change process control procedures. Additionally, the image quality of an a-Si panel compared to II image was far better in every respect. Later after premarket approval was awarded for the utilization of a-Si in medical devices, the product became mainstream for all new X-ray systems. The cost for a patient exam provided a business case to clinics and hospitals, especially large facilities to change to digital imaging.
Note: Direct conversion cassettes are used predominantly in medical radiology but are limited in there use for NDT CT for many reasons including energy limitations and speed.
Flat Panel Pixel Configuration
Flat panel pixel configuration is an important part of the reliability and the quality of the image. CT reconstruction requires the best possible results that are accurate and efficient. The photomicrograph of a 127 micron pixel shows the configuration of the photodiode being obstructed by the bias trace. The design of the pixel takes into account the usable surface of the pixel. The efficiency of the pixel has increased over the past twenty years by designing such that the area of the photo diode increase to allow more light photons to penetrate the photo diode. The term that is used for the efficiency of the pixel is the fill factor. Most a-Si panels photo diodes have a fill factor of approximately 60% with the rest of the architecture dedicated to gate lines, data lines, bias etc...
Why Flat Panel Size Important?
The area of your flat panel receptor is one of the determining factors in the resolution of your slice data. Ones ability to set up a CT scan requires the proper magnification factor to utilize as many pixels as possible to reach the required resolution for the image. Additionally, we must take into account the size of the pixel so that we can optimize the fill factor and receive as many light photons as possible in each pixel. If one has a very small pixel size the dose from the X-ray source will have to increase. If the pixel is too large the size of the manipulator will increase to get the higher resolution. So one can see that all of the factors in capturing the best image results are defined by the total configuration and physics of the system. When choosing a CT system you want the optimum size and manipulation to meet your imaging needs.
What is Cone Beam CT?
Cone beam CT utilizes the flat panel array to capture several lines and columns of pixel data information at a time. This reduces the scan times and provides the operator with geometrically precise images or slices. We will discuss this in the next chapters under the TECHNOLOGY heading.
Figure 2 Image Intensifier Schematic