Can Ultrasounds Detect Mets?

Or should I rely on CT and Pet scans? I've been having severe bloating for the past month and mild pain the upper-right side of my abdomen under my ribs, as well as loss of appetite. When my oncologist pressed on my abdomen, he said the pain was not coming from my gallbladder. Also, he said that he could not feel any lumps in my liver. 

I had an abdominal ultrasound last week and got the results back today. Apparently I have an 8-mm shadow on my liver. The radiologist said that it's most likely benign.

My question is, are ultrasounds reliable as far as determining what are mets and what are benign cysts? Especially on the liver? Or should I ask for further scans? I need to mention that my oncotype was 19, but I'm unable to take Tamoxifen because of a pre-existing medical condition, so I have a 20% chance of distant recurrence in spite of my grade 1, stage 1 status.

Thanks!