What role does the 90° pulse play at the TI time during an IR sequence?

The role of the 90° pulse during an Inversion Recovery (IR) sequence is to flip the longitudinal magnetization of the tissues into the transverse (X-Y) plane. This process is crucial in MRI as it allows for the manipulation of the magnetization vectors of different tissues, which can help in differentiating between them based on their relaxation properties.

When a 90° pulse is applied after the inversion time (TI), it takes the longitudinal magnetization that has been growing back from a negative value towards equilibrium and tilts it into the X-Y plane. In this plane, the transverse magnetization can then be measured to create an image. By flipping the magnetization to this plane, the MRI system is able to sample the signal effectively, which is essential to form the images we see.

This maneuver also plays a significant part in selecting specific tissues to enhance or suppress in the final image, thus improving contrast. Other options provided do not accurately describe the mechanics that occur during this part of the IR sequence, highlighting why the correct choice focuses specifically on the flipping function of the 90° pulse.