Partial Flip Angle
INCORRECT. The T2* time is not changed by RF pulses or magnetic field gradients.
INCORRECT. Smaller flip angles can be quicker to apply, but this does not significantly affect the repetition time (TR) of the pulse sequence. How does the flip angle affect the TR?
Partial flip angle. The rotation of net magnetisation into the xy-plane need not be 90°. Less time is required for the recovery of Mz.
CORRECT. We can reduce the amount of time it takes to get an image in a large number of ways in MRI. However, the most simple is to reduce the amount of magnetisation which we rotate into the xy-plane.
Partial rotation of the net magnetisation vector produces a smaller signal; less magnetisation is precessing in the xy plane to start with. But there is usually enough for a good signal measurement. Since Mz has a headstart on it's way back to thermal equilibrium (M0), we don't need to wait for as long before we can rotate the net magnetisation again and make another signal measurement (with a different phase encoding, of course).
Partial rotation of the net magnetisation is described by a lower angle of rotation (e.g. a 15° RF pulse).