1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: SNR in X-ray imaging

  1. Feb 20, 2016 #1
    1. The problem statement, all variables and given/known data
    (1) An ideal digital detector only suffers from quantum noise. If, after being exposed to 5 µGy the mean pixel value in the image is 100 and the standard deviation of the pixel values in the image is 5, calculate the SNR?

    The relationship between pixel value and detector dose is linear.

    (2) What is the effect on SNR of applying a linear gain of factor 4 to increase all pixel values

    2. Relevant equations

    3. The attempt at a solution

    As I understand SNR = N/sqrt(N) so I would have said SNR = 10
    But I don't understand why they gave the S.D value of 5...

    Re part (2) I thought SNR was unaffected by gain as both noise and signal would increase by the same amount

    But I am not sure about either answer.
    Thanks for the help
  2. jcsd
  3. Feb 25, 2016 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
  4. Feb 25, 2016 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    SNR is defined in different ways for different purposes. I can't think of a purpose for which it would take the form N/√N. (Why doesn't that collapse to √N? Are the two Ns different?)
    The form μ/σ given at https://en.m.wikipedia.org/wiki/Signal-to-noise_ratio#Alternative_definition looks appropriate.
    Of course, for a given arrangement, if you just vary the sample size N then you will get something proportional to √N, but not simply equal to it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted