In a spin echo sequence, the first RF pulse is known as?

In a spin echo sequence, the first radiofrequency (RF) pulse is referred to as the 90-degree RF pulse. This pulse is crucial because it tips the net magnetization vector from its equilibrium position (along the longitudinal axis) into the transverse plane. The application of this 90-degree pulse causes hydrogen nuclei within the tissue to resonate and create a transverse magnetization signal, which is then detected during the readout of the sequence.

The 90-degree pulse is essential as it prepares the spin system for echo formation. Following the 90-degree pulse, a later 180-degree pulse is typically applied to refocus the spins and produce the echo signal, but the first pulse specifically issues the initial excitation to achieve proper imaging.

Understanding the role of the 90-degree RF pulse helps in grasping the concept of how spin echo sequences work in MRI imaging, emphasizing its function in signal generation and the dynamic process of T1 and T2 relaxation that ultimately characterizes tissue contrast in the images produced.

In contrast, other options like the 180-degree RF pulse are involved later in the echo sequence, while terms like "Quick RF pulse" and "Phase RF pulse" do not accurately represent the terminology or functions within the context of standard spin echo sequences