How to find area of target in an image?

I need to find out signal to noise or clutter ratio like this.

SNR=double summation(A(i,j)^2/B(m,n)^2) where B[M × N] is B-san data, A[I × J ] is the area of target, A(i, j) and B(m,n) are normalized gray

value (I could not insert a formulae that's why I have written snr equation very badly). my query is how to find area of target in image as I have uploaded

[darova]

Where is the target?

[Image Analyst]

Can you outline the target in red or something so we know where in the image it is? Is the target a rectangle or an irregular shape? Is it defined by thresholding? Like it's anything more than a certain intensity away from the background intensity?

And what is B? Is B the entire image? Or the entire image except for the target pixels?

[santhosh kumar buddepu]

The target is hyperbola shape. where B is entire image. It is B-scan image which is formed by concatenating all A-scans. for plotting the image I have used imagesc, it is not like normal image which can read and shown by imread and imshow.