MRI: Image Formation and Instrumentation

Question 1

How can 1-dimensional 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 the 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 excited 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 a large volume of liquid helium; this 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 - this is 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 is 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 affects 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 within 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 receiver systems
Digitiser

Question 18

What is the purpose of the RF wave controller?

Answer 18

Generate 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 Receive (Tx/Rx) switch?

Answer 20

Controls whether the RF coil is being used to transmit or receive signal.
During transmission, it 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

Reduced 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 current induced in the gradient coils?

Answer 27

Gradient coils created rapidly varying magnetic fields.
Fields extend inside and outside the coil structure.
Coils are mounted n the scanner bore.
Interaction between external field and the bore induce Eddy currents.

Question 28

How are eddy current in the coil controlled?

Answer 28

Self-shielding 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.

Question 32

(i) How many litres of liquid helium might typically be found in an MRI scanner?
(ii) What volume of helium gas would this turn into if it boiled off to atmospheric pressure?
(iii) What is the boiling point of liquid helium at atmospheric pressure?

Answer 32

(i) 1500 litres
(ii) each litre of liquid becomes approximately 750 litres of gas, so a complete boil off would produce 1.1 million litres of gas
(iii) 4.2K.

Question 33

(i) What is a shim system, and why must the magnet be shimmed for each patient?
(ii) How are the room temperature shims adjusted?
(iii) How often must this be done during a scan?

Answer 33

(i) Putting an object, such as a patient, into the scanner distorts the magnetic field. The purpose of the room temperature shim gradients is to adjust the magnetic field to be as uniform as possible.
(ii) During the preparation phases of the scan, the B0 distribution will be measured within the prescribed volume of interest, decomposed into spherical harmonics and currents applied to the various shim coils to compensate the distortion of the field.
(iii) It must be done at least once per individual and more than once if the scan volume changes during the scan session.

Question 34

When considering the radiofrequency coils as a tuned circuit:

(i) Does the coil itself principally provide capacitance or inductance?
(ii) What other two components need to be added?

Answer 34

(i) Inductance
(ii) At least two further capacitances must be added, to allow tuning of the circuit to the resonant frequency (a capacitor in parallel with the inductance) and to have an overall impedance of 50 ohms (a matching capacitor, in series with the inductance).

Question 35

With respect to a transmit/receive rf coil, what other equipment is required before transmission and after reception.

Answer 35

There must be a waveform controller, an rf amp and a transmit receive/switch to transmit signal. For reception there must again by a transmit/receive switch, followed by a pre-amplifier.

Question 36

In comparing permanent magnet types to superconducting magnets, what are the

(i) advantages and
(ii) disadvantages of the permanent magnet?

Answer 36

(i) The advantages are the open design (reduces patient claustrophobia and allows bariatric patients to be scanned) and the low running costs (since no cryogens or cryogen refrigeration equipment is needed).
(ii) The disadvantages are that permanent magnet types are only available at lower field strength, they are very heavy and they are less stable than superconducting magnets.

Question 37

If the gyromagnetic ratio of the proton is 42.57 MHz/T, what field strengths corresponds to the processional frequency of hydrogen being:

(i) 127.7 MHz
(ii) 298.0 MHz.
(iii) What would the processional frequency of phosphorus be at 1.5 T (gyromagnetic ratio of phosphorus is 17.24 MHz/T)?

Answer 37

(i) 3T
(ii) 7T
(iii) 25.9 MHz

Question 38

How do magnetic field gradients help to encode position?

Answer 38

Adding a magnetic field gradient to the static magnetic field, means that the magnetic field strength (and hence the resonant frequency) of the protons varies along the position of nuclei in the gradient. For a linear gradient we can describe the Larmor condition as ω = γ(Bo + Gr x1).

Question 39

(i) Describe how slice selection works in MRI
(ii) What determines the width of the slice selected?
(iii) What determines the slice position in the object?

Answer 39

(i) A slice selection gradient is applied at the same time as the rf pulse. The slice selection gradient modifies the resonant frequency of the hydrogen nuclei in that direction. The rf pulse will only excite the regions of the sample whose resonant frequency is within the bandwidth of the r.f. pulse.
Therefore (ii) the width of the slice is determined by the strength of the gradient and the bandwidth of the rf pulse and
(iii) the slice position is determined by the frequency of the r.f. pulse.

Question 40

How would you increase the slice thickness?

• Increase the transmitted RF bandwidth and the slice selection gradient strength
• Any of these answers.
• Decrease the transmitted RF bandwidth, or increase the slice selection gradient strength
• Increase the transmitted RF bandwidth, or decrease the slice selection gradient strength

Answer 40

Increase the transmitted RF bandwidth, or decrease the slice selection gradient strength