To accurately produce an image based on differences in T1, the time interval between 90° excitation pulses should be:

To produce an image based on differences in T1 relaxation times, the time interval between consecutive 90° excitation pulses should be short.

In T1-weighted imaging, the goal is to emphasize the differences in the longitudinal relaxation time of various tissues. A short time interval allows for the complete recovery of the longitudinal magnetization of the tissues before the next 90° pulse is applied. This ensures that the contrast in the image reflects the inherent T1 characteristics of the tissues. When tissues with different T1 values are excited in quick succession, it highlights their varying degrees of relaxation, resulting in a clearer distinction in the final image.

Using long intervals would not effectively optimize the T1 contrast, as tissues with longer T1 relaxation times would have more time to recover and could affect the image quality and contrast negatively. Similarly, refocusing or inverting pulses are techniques used in other imaging protocols or contexts but are not specifically relevant to maximizing T1 contrast in this scenario.