Spins with gyromagnetic ratio y are uniformly distributed with uniform spin density po along the z-axis from -20 to zo in a 1D imaging experiment. Suppose they are excited at t = 0 by an rf pulse such that the signal at that instant would be given by s(t = 0) = dz po = 2z0p0 (9.40) A negative constant gradient field -G is immediately applied at t = flipped to the positive constant gradient field +G at time t = T. Find an expres- sion for the signal for t > T and show that it exhibits a gradient echo at time t = 2T. 0t and %3D