What can't ultrasound do?
Ultrasound is excellent at showing us the structure of organs that in the past, would only be possible via an exploratory surgery but it does not give us information at a microscopic level. Whilst some diagnoses are possible through ultrasound alone, it is sometime still neccessary to get a sample of tissue to confirm the exact nature of a disease process.
Ultrasound also does not tell us how an organ is functioning. Combining ultrasound with blood tests or biopsy can answer these questions.
Finally ultrasound can miss small lumps or even areas that might be obscured by gas or another organ.
Just like all our diagnostic tools, ultrasound is not perfect.