The current use of CR image plate technology is expanding at a rate that makes it difficult for seasoned technologists to keep up with. Since radiography programs are now including more digital radiography curriculum, there seems to be a gap between the knowledge that new grads have about these systems, and the knowledge that experienced technologists have. Some have attended CEU training, or courses that have kept information current, but there is a large population that have not, and who need that gap bridged. I just wanted to write a quick post about three of the different types of exposure indicators that CR systems are currently using today... if there are other ones not posted here, I personally do not know about them, and would appreciate any info that any readers are willing to share!
Sensitivity Number (S#)
This is probably the most familiar one. A typical optimum range might be from 200 - 400, although, with all of these examples, every hospital will have a different acceptable range for different exams. Radiographic density and Sensitivity Number have an inversely proportional relationship. If I double my mAs, my S# will decrease by 1/2 its original value. Conversely, if I 1/2 my mAs, my S# will double.
Exposure Index (EI)
A typical optimum range for Exposure Index might be 1700 - 2000. There is a direct, but not proportional relationship between Exposure Index and radiographic density. For every double in mAs, the EI should increase about 300. Every time you 1/2 your mAs, it should reduce the EI by 300.
Log Mean Exposure (LgM or LME)
A typical optimum range for Log Mean Exposure is around 1.9 - 2.1. This one closely resembles the H&D curve representation of log relative exposure in the x-axis. There is a direct, but not proportional relationship between LME and radiographic density. For every double in mAs, the LME will increase by 0.3. Every time the mAs is cut in 1/2, the LME is reduced by 0.3.
Knowing how to correct for insufficiencies in technical factors is the first step to producing diagnostic images using CR/DR systems. Hopefully, we can all help each other to understand these principles and continue the efforts to improve quality and radiation protection standards.
Sensitivity Number (S#)
This is probably the most familiar one. A typical optimum range might be from 200 - 400, although, with all of these examples, every hospital will have a different acceptable range for different exams. Radiographic density and Sensitivity Number have an inversely proportional relationship. If I double my mAs, my S# will decrease by 1/2 its original value. Conversely, if I 1/2 my mAs, my S# will double.
Exposure Index (EI)
A typical optimum range for Exposure Index might be 1700 - 2000. There is a direct, but not proportional relationship between Exposure Index and radiographic density. For every double in mAs, the EI should increase about 300. Every time you 1/2 your mAs, it should reduce the EI by 300.
Log Mean Exposure (LgM or LME)
A typical optimum range for Log Mean Exposure is around 1.9 - 2.1. This one closely resembles the H&D curve representation of log relative exposure in the x-axis. There is a direct, but not proportional relationship between LME and radiographic density. For every double in mAs, the LME will increase by 0.3. Every time the mAs is cut in 1/2, the LME is reduced by 0.3.
Knowing how to correct for insufficiencies in technical factors is the first step to producing diagnostic images using CR/DR systems. Hopefully, we can all help each other to understand these principles and continue the efforts to improve quality and radiation protection standards.
