MRI: Basics

Question 1

How are the different energy levels needed to make a signal created in an MRI scanner?

Answer 1

(Conventional) MRI looks at H nuclei (protons).
When the protons are placed in a magnetic field the interaction between that applied field and the nuclear dipole moment creates the different energy levels.

Question 2

What is the gyromagnetic ratio for a proton?

Answer 2

g = 2.6752×10^8 rad/s/T

Question 3

What is the equation for the energy difference between the two quantum levels in a proton in a magnetic field?

Answer 3

dE = (g*h*B)/2*pi
g = gyromagnetic ratio
h = Plancks' Constant
B = applied magnetic field

Question 4

Briefly explain how a signal is created in an MRI scanner in terms of the quantum energy levels.

Answer 4

Patient placed in Magnetic Field.
Protons in the patient either align with (stable, low energy state) or against (unstable high energy state) the magnetic field and occupy different energy levels.
RF wave, with energy equal to energy difference in levels, emitted by the scanner into the patient.
Lower energy protons absorb RF wave and transition to higher energy level.
Protons in higher energy level eventually decay back to lower energy level, emitting same frequency RF wave that was absorbed.

Question 5

Briefly explain how a signal is created in an MRI scanner in terms of the net magnetisation vector.

Answer 5

B0 filed aligns protons to the z-axis.
B1 field (supplied by magnetic component of RF wave at Larmor frequency) causes net magnetisation to precess about z-axis, and tilts it into x-y plane.
Rotating magnetisation induces a current in a suitably placed coil.
Generates MR signal.

Question 6

What is free induction decay?

Answer 6

The loss of signal due to relaxation of the net magnetisation vector in the x-y plane.

Question 7

Spin-Spin relaxation is characterised by which time constraint?

Answer 7

T2

Question 8

Spin-Lattice relaxation is characterised by which time constraint?

Answer 8

T1

Question 9

What is the cause of T2 (spin-spin) decay?

Answer 9

T2 decay is caused by the incoherent exchange of energy between nuclei.

Question 10

What is the equation for spin-spin decay?

Answer 10

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

There M is the component of the mangetisation in the x-y plane.

Question 11

What is the cause of T1 (spin-lattice decay)?

Answer 11

T1 decay is caused by loss of energy from the nuclei to their surroundings.

Question 12

For a 90deg pulse, what is the equation for T1 decay?

Answer 12

M(t) = M(0)[1-exp(-t/T1)]

Where M is the component of the Magnetiasation in the z-axis.

Question 13

For a 180deg pulse, what is the equation for T1 decay?

Answer 13

M(t) = M(0) [1-2exp(-t/T1)]

Where M is the component of the Magnetiasation in the z-axis.

Question 14

Which time constant is shortest, T1 or T2?

Answer 14

T2 is shorter than T1.