julieannp,
It depends upon what they are done for. For some things, they are very accurate and reliable, such as for some of their uses in OB/GYN.
For other things, like in the musculoskeletal system, other studies may be better.
It is also a very operator dependent study. The more experienced the person who is doing the study is, the better the results.
However, all types of studies have flaws and also have to be correlated with the patient's history, symptoms, and exam.
You should speak with your physician, if you are concerned about the results of a study.
Good luck.