MRI Basics

Question 1

Explain the NMR experiment? (3 marks)

Answer 1

Place protons (spin-1/2) in a magnetic field.
Send in RF pulse to change energy levels to a less stable state.
We only get a signal if the the Bohr condition is met where the energy put in matches the energy level gap.
Wait a time for the energy to be released.
Receive the signal and create an image.

Question 2

What is the Larmor equation and where does it come from? (3 marks)

Answer 2

w = gamma B

From E=hw and the splitting of the energy levels, E = h gamma*B /2.

Gamma is the gyromagnetic ratio.

Question 3

Give the equation for, sketch and explain T2 relaxation? (3 marks)

Answer 3

T2 is equal to the transverse magnetisation component.

At t=0 it is 1 and decays away exponentially.

M_xy(t) = M(0)exp(-TE/T2)

Relaxation occurs due to incoherent energy exchange, spin-spin interactions, These can be caused by spatial inhomogenieties, or temporal.

Question 4

Give the equation for, sketch and explain T1 relaxation? (3 marks)

Answer 4

T1 is the longitudinal component of the magnetisation, M_z.

M_z(t)=M(0) [1-2exp(TR/T1)] for 180 degree pulse

T1 is generated from interactions with the lattice (loss of energy).

Question 5

What is faster out of T2, T2*, T1. Why? (3 marks)

Answer 5

T2* is the shortest as it if affected by spatial inhomogenities in the magnetic field, whereas in T2 because of the 180 pulse, the dephasing of the spins due to the spatial inhomogenities is refocussed. T1 is always the longest. There cannot be any T2 is the T1 signal has decayed.

Question 6

Sketch and explain the pulse sequence diagram for a gradient echo? Explain how a signal is generated, in terms of k-space etc (12 marks)

Answer 6

First apply the 90 deg RF pulse with the slice selection gradient to excite a particular slice.

Then apply a phase encoding gradient for encoding in the y-direction.

Then apply a read-out gradient in the negative then positive direction, and acquire once it goes positive.

This provides encoding in three directions.

One pulse sequence gives one line in k_x space, changing the G_phase will shift a line in k_y. This is repeated for the whole of k_space. A fourier transform is then applied to generate an image. The outer edges of the k-space image provide detail in the image, where the inner sections provide contrast.

Question 7

Sketch and explain the pulse sequence diagram for a spin echo? Explain why more signal can obtained compared to a GRE (3 marks)

Answer 7

First a 90 degree RF pulse is applied with a G_ss.

Then phase encoding is applied.

Then another 180 degree RF pulse is applied with G_ss to flip the magnetisation and re-focus the spins.

Then the G_read and signal is acquired.

This generates more signal than a GRE due to the compensation of the spatial inhomos.