Final answer:
An inversion recovery sequence uses an initial 180 RF pulse followed by a 90 pulse to measure the recovery of magnetization vectors. It is commonly used in MRI to create tissue contrast based on recovery rates.
Step-by-step explanation:
The pulse sequence that employs an initial 180 RF pulse before a 90 pulse is called an inversion recovery sequence.
In an inversion recovery sequence, a 180 RF pulse is applied first, which flips the magnetization vectors in the opposite direction. This creates an inverted state. Then, after a certain time delay, a 90 RF pulse is applied to measure the recovery of the magnetization back to its original state.
This sequence is commonly used in Magnetic Resonance Imaging (MRI) to create contrast between different tissues based on their recovery rates.