How to add a noise in Shepp Logan Phantom?

Hello.

I wish to know how to add for example SNR=(-5)DB or SNR=(-3)DB to a Shepp Logan Phantom. I already know how to deploy the original image but I don’t know how to add these kind of noises. I be have the formula SNR=10*log_10(Var S/sigma^2) Var S is the array of noiseless projections. Sigma ^2 is the noise power fixed variance Help is appreciated!