# How is a 180° RF pulse in NMR?

A 180° pulse appears to be an RF pulse that is equivalent to rotating the direction of the magnetization vector by exactly 180° in MRI or NMR observations; That is, it may invert the direction of the macroscopic magnetization vector of nuclear spins placed in a certain uniform static magnetic field by 180° from the positive to the negative direction of the z-axis, or flips the transverse magnetization component in the x-y plane by 180°. What is 90 or 180 "degree" RF pulse in MRI?

My questions.

• What intensity, polarization, phase, pulse duration, ... would a 180° pulse be characterized by?
• So, a 90° RF pulse will generate a 90° flip angle. This is true right?

Here, according to the following lecture, let $$B_1$$ be RF pulse, and, it satisfies,

$$B_1\left(t\right)=B_1^e\left(t\right)\exp{\left(-j\left(\omega_0t-\theta\right)\right)}$$

https://www.coursera.org/learn/mri-fundamentals/home/week/6

Then, the flip angle (α) is represented by the following equation;

$$\alpha=\gamma\int_{0}^{\tau}{B_1^e\left(t\right)}$$

Where, τ represents the duration of the $$B_1(t)$$ according to Larmor's equation, the Larmor frequency ($$ω_0$$) of a nuclear spin placed at a point is proportional to the strength of the magnetic field at that point ($$B_0$$),

$$\omega_0 =\gamma B_0$$

where γ is the proportionality constant, γ = 42.58 MHz/T for hydrogen atoms

What intensity, polarization, phase, pulse duration, ... would a 180° pulse be characterized by?

A 180 deg RF pulse is $$12\mathrm{\ \mu T}$$ for a duration of $$1\mathrm{\ ms}$$, or $$24\mathrm{\ \mu T}$$ for a duration of $$0.5\mathrm{\ ms}$$, etc. Any polarization would be acceptable.

So, a 90° RF pulse will generate a 90° flip angle. This is true right?

Yes, it is true by definition.