I'm trying to get quarter of the circle based on a certain direction (velocity)

The number that I want should be between -D3DX_PI and D3DX_PI, How do I determine it based on the velocity direction?

**Example:**

Lets say I have the velocity of: (X: 0, Y: 10, Z: 0), in this case the velocity is pointing at the upper direction, so the numbers between -D3DX_PI and D3DX_PI that I should get are based on the upper quarter of the circle.

So, I want to determine which quarter of the circle the velocity is pointing at.