I am writing this letter about 'Getting Technical on CCD based Digital Radiography' in the February 2000 issue.

The article on optically coupled digital radiography caught my eye because optical coupling can be very inefficient when implemented without concern for the transfer of noise through the imaging system. Simply stated, when one tries to “look” at a large area with a single small detector using a lens system, the noise in the system can be dominated by sources other than the statistics of the X-ray photons impinging on the detector. An imaging system with this characteristic is usually described as having a “secondary quantum sink” which system designers try to avoid at all costs. The low value of optical coupling efficiency (less than 1 percent for large area/small detector) and the increase in noise resulting from “secondary quantum sinks” have been the subject of many scientific publications.

An increase in noise also has a severe impact on the value of the detective quantum efficiency (DQE) which is a fundamental measure of imaging performance. In general, the lower the value of this parameter, the less efficient is the imaging system in terms of imaging performance per unit radiation dose. Some of the optically coupled digital radiography systems described in your article have DQE values two to five times lower than a typical film/screen system doing the same imaging task. The DQE incorporates both resolution and noise in the measure of imaging performance.

The article focused on resolution and image processing concerns while neglecting noise considerations. It also ignored the more interesting tradeoffs facing designers of these digital radiography systems:

Single CCD Camera
• Inexpensive   
• Small amount of image processing
• High demagnification   
• Secondary quantum sink   
• Low to very low DQE

Multi-CCD Camera
• Expensive
• Difficult processing to stitch images
• Low demagnification
• Moderate to low DQE

Robert M. Gagne, Ph.D.
Senior Scientist
Medical Imaging and Computer Applications Branch
CDRH/OST/DECS, HFZ-142
Rockville, Md.