Credible articles can be found in a variety of places: medical journals, reliable websites, expert interviews, and medical databases, to name a few.
Your search is driven by what kind of articles you need.
You will also find many useful articles in the following journals:
Journal for Vascular Ultrasound (JVU)
Journal of Diagnostic Medical Sonography (JDMS)
Journal of the American Society of Echocardiography (JASE)