It depends on what exactly is meant by "questionable." Is it that the doctor/ radiologist is unhappy with the quality or clarity yielded by the ultrasound? Or, is that there's something on the ultrasound that has the doctor/ radiologist concerned? It's possible that another form of diagnostic imagery (i.e. mammogram or MRI) would be tried before doctors decide if a biopsy is even necessary. Each diagnostic approach renders a different type of image... a mammogram is similar to an x-ray; the ultrasound utilizes sound waves to capture an image and the MRI provides image information by subjecting the subject to a strong magnetic field.
I had cancer and it was invisible on the mammogram but a tiny piece of it was found with an ultrasound. He made it quite clear to me that what was there in my breast did NOT belong there and needed a biopsy immediately if not sooner. He was that emphatic. So I left there knowing I had cancer before they said the C word to me.
A biopsy was done and then an MRI.
If your DR is saying biopsy or MRI, do it. If they're not, relax :)
Best wishes to you!