Figuring out the bounds of an integral

I understand how to solve double integrals, but I am having trouble figuring out what the bounds are. This is a problem we did in class:

Evaluate the integral (assume S = integral)

SS (4 - x^2 - y^2) dxdy where R is the first quadrant sector of the circle x^2 + y^2 = 4 between the lines y = 0 and x = 0. For reference there is supposed to be an R under the double integral.

The teacher came up with these polar coordinates

0 <= r <= 2 and 0 <= theta <= pi/4

I understand how he got the bounds for r but what about for theta? Thanks in advance