Misconceptions Regarding CCD Based Digital Radiography
A document has recently been forwarded to us in which a reseller of flat panel technology for digital x-ray makes many inaccurate and misleading statements about CCD based digital radiography. This paper will address those points in the hope of providing a more accurate technical view of the technology.
Assertion:
CCD really only exists with chiropractors, you would not find this technology in any human hospital.
Fact:
Imaging Dynamics’ Xplorer DR systems were approved by the US FDA in April of 2000 for use in human medical imaging. Today these systems are in use in hospitals in more than 25 countries with the largest installed base in the United States. Many prestigious universities have multiple systems installed. The University of California at Davis is mentioned by the author of the inaccurate paper. That institution presently has 5 Xplorers in use.
Assertion:
CCDs are too small to produce a high resolution image.
Fact:
Small sensors can produce very detailed images. ( Indeed every x-ray that has ever been produced has used a very small sensor for final image capture, namely the human retina. ) CCDs are the only technology that has been used in space borne imaging where expense is immaterial and performance is key. Every satellite from spy satellites to the Hubble space telescope use CCDs. They are simply the finest imaging devices ever made. There are no flat panel imagers in space.
Assertion:
Fill factor of 100% is a problem:
Fact:
This is a very odd thing to say. The fill factor is a measure of how much of the sensor is actually sensitive to incoming light. Flat panel manufacturers strive to achieve the highest possible fill factor but generally fall well short at about 70%. Fill factor contributes directly to quantum efficiency and nobody with any knowledge of imaging sensors would describe a low fill factor as a good thing. CCDs have improved their fill factor over many years of development and today have achieved 100% through the use of transparent gate structures. This means no light is wasted. Flat panel has fundamental technical limitations that will make a similar feat very difficult to achieve and will not be possible in the near future. It remains a goal for them however as it does with all sensors.
Assertion:
Overflowing pixels (“blooming”) represent damage.
Fact:
Blooming can occur when a CCD sensor is overexposed. The bloomed area contains no information and that portion of the image is uniformly black even after image processing. The CCD is not damaged however. A similar effect can be seen when x-ray film is over exposed. The area of over exposure is black and cannot be read, even with a hot light. It is true that without antiblooming technology a severely overexposed region can spill over into neighbouring areas of the image. Imaging Dynamics has recently introduced anti-blooming technology so this is no longer an issue at all. It is interesting to note however that some types of flat panel detector can be damaged beyond repair if severely over-exposed.
Assertion:
16 Megapixels are required to produce an acceptable image.
Fact:
A curious statement from a flat panel reseller. No flat panel has 16 megapixels. We would acknowledge however that they do produce acceptable images. The Imaging Dynamics Xplorer has the largest matrix of any commercial DR system with a full 16 megapixels (4K x 4K) IDC also has a 3K x 3K system giving 9 megapixels, still well above the resolution of most flat panels.
More important than pixel count is pixel size. The smaller the pixel the finer the resolution. The IDC Xplorer has a 108 micron pixel, the IDC Xaminer has a 144 micron pixel. By comparison a GE flat panel has a 200 micron pixel.
Assertion:
CCDs are too noisy to deliver a high quality image.
Fact:
As mentioned previously, the Hubble space telescope uses CCD for exactly the reason that CCDs have the finest noise and dynamic range of any sensor. IDC DR systems use them for the same reason. IDCs noise levels are close to the theoretical minimum possible for this technology.
Assertion:
Dark noise rises exponentially above 6 degrees.
Fact:
Dark noise can be an issue in a poorly designed CCD camera however the author got his math wrong. Dark noise drops by half for each 7°C reduction in temperature. There is nothing special about the 6°C point. IDC systems operate at -10°C +/- 0.5°C. At that temperature dark current is less than 1 electron/pixel/second. Cooling is achieved through a solid state Peltier cooler with no liquids or pumps. Dark noise is not an issue on these systems.
Assertion:
“While the possibilities of delivering lower cost CCDs over flat panel DR systems is theoretically present[sic], it is a moot point until the minimum acceptable standards of image quality, DICOM data handling, PACS, Distributed Imaging and other very important factors are met… at a competitive price to flat panel DR systems.”
Fact:
Finally we agree. IDC’s CCD system delivers higher resolution than flat panels at significantly lower costs with full DICOM compatibility and PACS integration.
Assertion:
No CCD company represents a viable alternative and market share position in the world of digital radiographic imaging.
Fact:
In IDC’s first year of sales in China, we outsold GE and Siemens combined, placing second overall to Phillips. Leading industry analysts Frost and Sullivan awarded the Xplorer 1600 their 2006 Medical Imaging Product Value of the Year Award. They stated that “IDC has emerged as one of the most innovative companies in the digital radiography industry…”
For more information about our technology, please visit our website at www.imagingdynamics.com
Robin Winsor
Chief Technical Officer
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