Line Pairs Theoretical and Real

by Larry Emmott on June 13, 2013

in Radiography

lpmmguageBe careful when choosing a digital radiography sensor. Sensor resolution is an important factor. Spatial resolution is usually expressed in line pairs per millimeter or lp/mm. The bigger the number the better the resolution. That seems simple enough, however the problem develops when vendors use theoretical numbers when marketing products instead of actual resolution.

Theoretical resolution is determined by the number of pixels on the sensor. More pixels equals more data equals more resolution. Based on this some vendors say they have a resolution of greater than 25 lp/mm. That’s the theory the actual performance is sometimes only 12 to 18 lp/mm, not nearly so good. The reason the actual is so much less than the theoretical is that other factors affect the final resolution including the scintillator, the optical fiber or lack of, the internal software or firmware and the display software itself.

The actual scientific instrument (pictured to the right) that is used to measure line pairs only goes up to 20 lp/mm. If the vendor claims anything greater than 20 they are using theoretical numbers not actual measured resolution.

Higher resolution sensors are better and do provide more diagnostic data, especially when it is important to see very small details like the tip of a # 10 file. However what is real and what is hyped are sometimes quite different.

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