Try this Onde screen capture for Mac, it can capture circular region. Firstly, you need to press Command at the same time you should click "Ellipse", then you can grab a circular region very accurately.i think it can help you .Good luck
I would have thought it would be simpler to grab a regular screenshot and then do an elliptical crop in Preview, but if there's something out there that does it more quickly for you then it might be worth a shot.