# What is 90 or 180 "degree" RF pulse in MRI?

I am studying MRI, specifically spin echoes.

Now I kind of understand how magnetic resonance imaging works, but what I'm stuck is the definition of "degree" in MRI. What I thought was that if I say "90-degree RF pulse," I fire an RF pulse perpendicular to $$\mathbf{B}_{0}$$, so the magnetic field and RF coils inside an MRI machine fire the 90 degree pulse. But then what is a "180-degree" RF pulse? Structurally there, are no RF coils positioning at the feet of the apparatus, which is 180 degree away from the upward (toward the head) magnetic field.

So then, what does "degree" mean in MRI physics?

Thanks in advance.

## 2 Answers

Both a 90 degree and a 180 degree RF pulse are applied along an axis in the transverse plane in the rotating reference frame. The difference is how much they rotate the magnetization. The 90 degree pulse rotates the magnetization 90 degrees and the 180 degree pulse is applied twice as strong so that it rotates the magnetization 180 degrees.

• Thanks for your answer! Can I understand "degree" as a unit of pulse strength? Mar 27, 2023 at 8:02
• @AndyJunghyunKim yes, it is a measure of pulse strength, but it is also an angle
– Dale
Mar 27, 2023 at 11:29
• Thank you, Dale! Mar 28, 2023 at 23:19

It's a measurement of both the duration and amplitude of the RF signal depending on the tissue of interest. For example, for the same amplitude, 180 is just twice the duration of 90. Or, with the same duration, you double the amplitude to switch from 90 to 180.