Lisa - I'm definitely not an expert on this - that's my disclaimer! But I am an RN and I work for Kodak in their health imaging group so have some knowledge.

So, basically my understanding is that there really isn't anything such as "false" positives or negative in US. That's more related to PET scans. Basically, an ultrasound can show the structures very well and a skilled radiologist can tell from the images what the characteristics of any abnormalities look like and make a more informed diagnosis on what the pathology might be. For example, when looking at my US the radiologist dictated "subcentimeter left level 1B lymph node which has benign characteristics. Under the area of palpable concern, there is a 10 mm level 2A lymph node with no suspicious features..."

And, as said before, much of this is determined by the quality of the equipment and the skill of both the sonographer and radiologist. And, in order to make any definitive dx, a biopsy is necessary... but that's the same with CT, MRI or PET...