How would I go about analyzing the formal complexity of this algorithm under Big Theta Θ notation?

I’m needing some help understanding algorithm analysis using Big Theta Θ notation:

Big O is the most commonly used asymptotic notation for comparing functions, although in many cases Big O may be replaced with Big Theta Θ for asymptotically tighter bounds.

How would I analyze this algorithm step by step? I know the final answer is ϴ(n²), but I’m having trouble seeing it. Any help would be appreciated.

You may assume for simplicity that n = 2k, for some positive integer k.

 int a = 0;
 int i = n * n;
 while (i > 0) {
      for (int j = 0; j < i; j++) {
          a++; 
      }
      i = i/2;
 }