Problem is that if you've ever had an ultrasound, it's not really something you can just tell someone how to do. They manipulate the transducer in real time, angling it just right, giving it just the right amount of pressure on the skin. Even if you did it in real time I can't imagine the frustration of a physician over the phone going "No move it to your right, now up, now back further, ok less pressure, now more pressure."