Had Ultrasound BEFORE 6weeks?

Firsttimemama
Hi there!
I was wondering if anyone had an ultrasound before 6weeks?
I had back surgery so I saw a dr earlier than normal (I was 4 weeks 2 days) and he did an ultrasound but couldn't see anything. :( My hcg was at 487. 
I just wanted to know when people started seeing something and what their corresponding hcg level was?