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
Best Answer
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.
Yes, it is true by definition.