change white into transparent

[marie louise]

Hi,

I would like to gather the grey scale distubution of an area using Analyze>Plot profile, However there are certain spots (which are white, 255,255,255 in the file) which i like to exclude from this analysis, so to not take them into account. Is there  a way to do this? Can i for example change all the white in my image to transparent?
Any hints are welcome!