What kind of testing did your surgeon do before deciding on the rib removal only?
Some do the rib and then go back for the scalenes if needed.
And was the whole rib removed? as opposed to leaving a stump?
If dr didn't clarify I believe you can find the info in the surgical report.

But the thing is - there are numerous places that can cause the compressions. It's not always the top rib alone- but I guess it's the most common thing to try first.

Could be pec minor, scalenes, large bones or ligaments, scar tissue, even over developed muscles or an anomaly in the structures..

The more expensive MRA is supposed to show more details and specifics - but not everyone uses those though.