What Does An Ultrasound Of The Breast Reveal?

Hi



My mum had a stroke recently and has had a lot of scans and tests done and has been told she has a mass on her breast and she had a mamogram and the consultant has said he thinks there is no need for a biopsy as he couldnt feel a lump, but she has got to go for a ultra sound on her breast in stead. What that show that the mamogram didnt?

Tracey