Volume of required pixels

I wanted to find the volume of the required pixels(impurities) in a grayscale image. I have a doubt regarding area and volume. It is given that the total number of pixels contributes to the area.Can't we say that the volume of these impurities is equal to the number of these impurities pixel values?

Answer 1

[Walter Roberson]

You cannot speak about volume in a grayscale image, not unless the white-level corresponds to a height.

[Image Analyst]

Or it's a 3D image volumetric image such as CT or MRI.

Answer 2

[Image Analyst]

See the code provided by Richard Brown here:

http://www.mathworks.com/matlabcentral/answers/35501-surface-area-from-a-z-matrix

Where did you upload your image to? And why do you think the intensity of the impurities is related to a physical volume? Are they fluorescent images? So the more impure the chunk of cell on the slide is, the more fluorescent dye it will have and thus glow brighter?