MRI: Image Formation & Instrumentation

Question 1

How can 1-dimentional spatial information be encoded into the MR signal?

Answer 1

Altering the magnetic field using a magnetic field gradient alters the resonant frequency of the sample in that direction.
The magnitude of the frequency change depends on the sample’s position in the gradient.
This is the “read-out gradient”.

Question 2

On what does the amplitude of each frequency from a simple MR scan depend?

Answer 2

The total concentration of nuclei at that point in space.

Question 3

An ideal Fourier Transform samples from -inf to +inf, how is this dealt with in an MR sequence?

Answer 3

Negative gradient applied before sampling to simulate “negative time”.
Length of gradient should be 1/2 of sampling time.

Question 4

How is the second spatial dimension encoded into an MR signal?

Answer 4

“Phase Encoding gradient”.
Orthogonal to Read-Out.
Repeat sequence but reduce amplitude of PE gradient,
The phase of the signal is modified dependent on position orthogonal to the Read-Out gradient.
2D-Fourier Transform to convert modulation into an image.

Question 5

How is the third spatial dimension encoded into an MR image?

Answer 5

Slice Selection gradient applied during RF pulse only excites protons at a certain point along the z-axis. All signals come from that slice.

Question 6

What determines the slice thickness from a slice selection gradient?

Answer 6

Gradient steepness.

RF pulse bandwidth.

Question 7

What are the four main components of an MR scanner?

Answer 7

Gradient System.
Magnet/Shim System.
Radio-Frequency System.
Spectrometer-Computer System.

Question 8

What are the requirements of the magnet system?

Answer 8

High field strength
Highly uniform over a large volume
Temporally stable to 1 part in 10^8
Sensitive to frequency shifts of <1Hz

Question 9

What are some of the cryogen issues associated with a superconducting magnet?

Answer 9

As the magnet contains large volume of liquid helium so must be thermally isolated to minimise boil off.
The gas has a low thermal heat capacity, so there is a potential for a lot of gas to escape if heat is input - quench.
Quench pipe to vent gas to the atmosphere.

Question 10

How is passive shimming used to increase homogeneity in the magnet?

Answer 10

Structures in the local environment can distort the homogeneity in the magnet field. Small ferromagnetic plates are arranged inside the scanner bore to increase homogeneity
This process performed once during the installation.

Question 11

Why must the fringe field be managed using magnetic shielding?

Answer 11

Can cause interactions with the environment, which affect the image quality, and the people and equipment in the local environment.

Question 12

What are the three options for magnetic shielding?

Answer 12

Site magnet is a large open space with controlled access with the fringe field.
Place magnet in a steel shield to constrain the field.
Actively shielded scanner design.

Question 13

What are the advantages of a permanent magnet?

Answer 13

Open design - less claustrophobia.

Low running costs.

Question 14

What are the disadvantages of a permanent magnet?

Answer 14

Low field strength (<0.5T)
Very heavy
Unstable

Question 15

What are the advantages of a superconducting magnet?

Answer 15

Very stable

High field strength

Question 16

What are the disadvantages of a superconducting magnet?

Answer 16

Expensive to purchase

High running costs

Question 17

What are the components of the RF system?

Answer 17

RF Waveform Controller
RF Amp
Tx/Rx Switch
RF Coil
Pre-Amp
Demodulator (High field systems)
RF Reciever system
Digitiser

Question 18

What is the purpose of the RF Waveform Controller?

Answer 18

Generates the RF pulse shape at the correct frequency.

Question 19

What is the purpose of the RF amplifier?

Answer 19

Linear amplifier to amplify the basic low voltage RF pulse shape.
Tuned to work at the resonant frequency.

Question 20

What is the purpose of the Transmit Recieve (Tx/Rx) switch?

Answer 20

Controls whether the RF coil is being used to transmit or receive signal.
During transmission ensures no power goes into the receiver.

Question 21

What is the purpose of the RF coil?

Answer 21

Transmits the RF pulse into the patient and picks up the emitted RF signal.

Question 22

What is the purpose of the preamplifier?

Answer 22

High gain low noise amplifier to maximise SNR.

Question 23

What is the purpose of the RF receiver system?

Answer 23

Reduces the signal from around the Larmor frequency (10s MHz) to audio waveband (100s kHz bandwidth).

Question 24

What is the purpose of the Digitiser?

Answer 24

Converts the analogue signal to digital information for processing and storage.

Question 25

How does the sensitivity of an RF surface depend on size?

Answer 25

Small size - Higher sensitivity

Limited Field of View

Question 26

What are the requirements of gradient coils?

Answer 26

Variation in Bz with distance.
High gradient strength - high current density
Low inductance - fast switching

Question 27

How are eddy currents induced in the gradient coils?

Answer 27

Gradient coils created rapidly varying magnetic fields
Field extend inside and outside the coil structure
Coils are mounted in the scanner bore (a metal tube)
Interaction between external field and the bore induce Eddy currents.

Question 28

How are eddy currents in the coils controlled?

Answer 28

Self-Shielded gradient coils

Adding a secondary winding on a second larger coaxial cylinder, wound in opposite direction to cancel out field.

Question 29

What are the requirements for a gradient amplifier?

Answer 29

Must create high gradient strength.
Must establish gradient rapidly.
Must be temporally stable.

Question 30

What are the characteristics of a gradient amplifier?

Answer 30

Constant, High current (400-600A)
High voltage (800V)
High duty cycle (ideally 100%)
Water cooled.

Question 31

Why is active shimming needed?

Answer 31

Patients bring their own inhomogeneities and distort the original magnetic field.