How much can they see in a breast ultrasound?

I can’t get mammograms or mri’s bc I cannot get my arms over my head, nor lay on my stomach. I have an area of concern but I also need a screening of both breasts. Can ultrasound be used well to make sure nothing weird is happening? I’ve heard women say that a mammogram found their early breast cancer but I’ve never heard that about an ultrasound. I’m not uber concerned but I have a huge cyst along with a mass in front of it. It has caused dimpling and the area around my nipple feels harder and more dense when arms are down. So they just wanna make sure. PLUS I wanna get the huge cyst drained!