MR Physics 1

Question 1

What do MRI scanners consist of?

Answer 1

Main magnet- use superconducting magnets which generate much larger fields

Ferromagnetic blocks & shim coils- both inside the bore- even out the magnetic field, keep it homogenous

Gradient coils (amplifiers)- spatially modulate the main magnetic field in a predictable way causing the Larmor frequency of spins to vary as a function of position

Radiofrequency transmitter and receiver- excite and detect the MRI scanner

Computer- generates the image using information from the scanner

Question 2

What is the typical magnetic field strength in MRI?

Answer 2

Usually between 0.5 and 1.5 tesla

Question 3

Is a higher magnetic field strength better than low magnetic field strength?

Answer 3

Yes

Means the signal being read by the coils and transmitted to the computer is increased
= better image due to less obstruction from noise
= higher spatial and temporal resolution

Question 4

What are gradient coils in MRI?

Answer 4

Loops of wire or thin conductive sheets on a cylindrical shell inside the bore

When an electrical current passes through these coils, the result is a secondary magnetic field

This gradient field distorts the main magnetic field in a slight but predictable pattern

Question 5

How many gradients are there?

Answer 5

3 - one for each axis (X, Y & Z)

Question 6

What is the fundamnetal basis of MRI scanning?

Answer 6

MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field

When a radiofrequency current is then pulsed through the patient, the protons are stimulated, and spin out of equilibrium, straining against the pull of the magnetic field

When the radiofrequency field is turned off, MRI sensors detect the energy released as the protons realign with the magnetic field

Question 7

What is nuclear spin?

Answer 7

Nuclei with unpaired neutrons and/or protons (odd numbers) have nuclear spin

Spin combines with nuclear charge to produce a magnetic moment

Spin + charge = tiny magnet

Question 8

Protons align in what two orientations?

Answer 8

‘Spin up’ and ‘spin down’ states
If the electron spins clockwise on its axis, it is described as spin-up, if it spins anti-clockwise it is spin-down

These two states have a difference in energy that is proportional to the magnetic field strength

Question 9

What is a magnetic moment?

Answer 9

The microscopic magnetic field originating from nuclear spin

Question 10

What is the larmour equation?

Answer 10

The precession frequency of the nuclear spins, or the resonance frequency for nuclear magnetic transitions

The precession frequency obeys the equation:
f0 = y . B0 (Larmor equation)

Question 11

How many spins give rise to a signal?

Answer 11

Only a small amount of the spins give rise to a signal: few per million (ppm)

Question 12

What is the magnetisation vector?

Answer 12

When the body is placed in a strong magnetic field (the MRI scanner), the protons’ axes all line up

This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner

The net magnetisation vector in MRI is the summation of all the magnetic moments of the individual hydrogen nuclei

We can treat excess spins in the lower energy level as a single magnetisation vector aligned to the main magnetic field

Question 13

The predominant signal from the body is due to what?

Answer 13

Water and fat

Question 14

What is chemical shift in MRI?

Answer 14

The chemical shift refers to signal intensity alterations that result from inherent differences in the resonant frequencies of precessing protons

Chemical shift is due to the differences between resonance frequencies of fat and water.

It occurs in the frequency-encode direction where a shift in the detected anatomy occurs because fat resonates at a slightly lower frequency than water

Question 15

How do we detect the MRI signal?

Answer 15

To produce ‘signal’, MRI scanner interacts with protons in the body

Randomly orientated protons become aligned with the powerful magnetic field in the bore of the scanner

A rapidly repeating sequence of radiofrequency pulses produced by the scanner causes ‘excitation’ and ‘resonance’ of protons

As each radiofrequency pulse is removed, the protons ‘relax’ to realign with the magnetic field, and as they do so they give off radiofrequency ‘signal’ which is detected by the scanner and transformed into an image.

Question 16

How can RF pulses control magnetisation?

Answer 16

By changing their strength and/or duration

Question 17

How do we detect the MRI signal?

Answer 17

As each radiofrequency pulse is removed, the protons ‘relax’ to realign with the magnetic field, and as they do so they give off radiofrequency ‘signal’ which is detected by the scanner and transformed into an image

Question 18

How do we record the signal (induction)?

Answer 18

Nuclear spins precess like a spinning magnets

Changing magnetisation induces voltage in detector coil

Measure rate of oscillation as a voltage change

Question 19

How do we record the signal (free induction decay)?

Answer 19

Changing magnetisation induces voltage in detector coil

Detect analogue oscillating voltage

Analogue signal digitised at discreet points – analogue to digital converter (ADC)

Sampling rate > 2 highest frequency- Nyquist condition

Question 20

What is the MRI pulse sequence?

Answer 20

An MRI pulse sequence is a programmed set of changing magnetic gradients

Each sequence will have a number of parameters e.g. TR, TE, 2D vs 3D, diffusion weighted etc

Question 21

Summarise MR Physics

Answer 21

The ‘bulk’ magnetic moment of the water protons aligns and ‘precesses’ about the direction of the field at a frequency dependent on the Larmor equation

A radiofrequency field B1 applied perpendicular to the main magnetic field can supply the correct energy pulse to tip the water magnetic moment away from alignment with the field direction

Different pulses can be used to control the magnetisation

The precessing magnetic moment generates a field which can be detected by a tuned receiver coil

The analogue signal is discreetly sampled as a Free Induction Decay (FID)

Question 22

What changes image contrast?

Answer 22

Relaxation

When the pulse is switched off spin are not at equilibrium - more spins in the upper energy level

Once the radiofrequency field is switched off the nuclei undergo two processes:
T2 (sometimes called spin-spin or transverse relaxation)
T1 (sometimes called spin-lattice or longitudinal relaxation)
-due to microscopic motion of water and are collectively known as relaxation.

Question 23

What is T2 relaxtion?

Answer 23

Signal decay

Spins experience tiny difference in magnetic field strength due to neighbouring spins

Each spin has a slightly different frequency from other spins

As spins precess at different frequencies to magnetisation, bulk vector spreads out, dephases in the XY plane losing signal

T2: loss of signal due to each spin signal cancelling each out
Decay rate is exponential with a time constant T2

Question 24

What is T2* relaxation?

Answer 24

T2 effects transverse magnetisation in the xy plane

Signal decay rate exponential with a time constant T2 (ms).

T2* = T2 loss + extra loss due to variation in applied magnetic field (e.g imperfect magnet)

T2* always shorter than T2, i.e. signal disappears faster

Influences the appearance of the image

Question 25

What is T1 relaxation?

Answer 25

Requires the correct energy (frequency) for exchange to occur

Over a longer period the magnetic moments realign with the magnetic field with an exponential recovery time constant T1

Bulk magnetisation vector returns to the z-axis

Recovery rate exponential with a time constant T1

99.5% of the signal is recovered in 5*T1

Question 26

What happens after the RF pulse?

Answer 26

T1 and T2 relaxation occur simultaneously and return to equilibrium

Question 27

Sumarise T1, T2 and T2* relaxation

Answer 27

After an rf pulse the magnetisation is in a non-equilibrium state

Signal decays back to equilibrium via relaxation mechanisms

T1: relaxation along the z-axis. Spins return to lower energy

T2: relaxation in the xy plane. Variation in magnetic field produced by the spins and is an intrinsic property of the sample

T2*: as T2 but affected by locally distortions in the applied magnetic field

T1/T2/T2* offer the ability to change image contrast to highlight anatomical features