MR Instrumentation

Question 1

Sketch the main components of the MR system. (8 marks)

Answer 1

RF controller -> RF amp -> TR switch -> Pre-amp -> Demodulator -> Rx digitiser -> Spectrometer -> PC/PACS

Grad/shim controllers and amps

Magnet system and coils.

Question 2

What are the basic steps in the MR signal generation process? (6 marks)

Answer 2

1 - Polarise
2 - Excite
3 - Encode
4 - Receive
5 - Transform
6 - View/Save

Question 3

What are the magnet requirements? (3 marks)

Answer 3

• Large B-field volume which is homogenous and spatially stable
• High B field

Question 4

What are the two main magnet designs and describe them? (7 marks) Give pros and cons of each. (6 marks)

Answer 4

Permanent = operates at room temp, cannot be switched off, only generates low B-fields, does not require cooling, horseshoe design

Pros = cheap to buy and run, does not require cooling, better for claustrophobic patients.
Cons = temperature sensitive, low B fields, lower image quality, very heavy, cannot be switched off.

Superconducting = uses liquid He as well as possibly nitrogen to cool superconducting wires. Titanium/niobium material. Requires a compressor to minimise boil off in modern systems.

Pros = high B field, better IQ, more homogeneous B-field.
Cons = expensive to purchase and run, issues with quenching, high electricity usage, can be turned-off but expensive to do so.

Question 5

How does the RF coil work? (4 marks)

Answer 5

It is a tuned circuit which operates at the Larmor frequency.

Uses a circuit which consists of a matching capacitor to get the overall impedance correct and a tuning capacitor to ensure the coil transmits at the larmor frequency.

Question 6

Discuss two types of coil and their designs. (8 marks)

Answer 6

Basic surface coil:
- Just a simple loop, the larger the loop the larger the FOV but at a cost of increased noise and reduced sensitivity. Used for body coil.

Birdcage coils:
- Shape of a birdcage, with complex field distribution, sinusoidal. Standing wave at the centre. Can be transmit and receive like in a head coil, or just transmit, which are usually built into the bore.

Question 7

Essentially what are gradient coils? What types are there? And what requirements are there for them? (4 marks)

Answer 7

Current driven resistive coils which produce gradient fields as a result of the current flow.

There are two types: Golay and Maxwell (for gradients in z and x/y direction.

They need to produce high gradient field strengths and be able to switch rapidly.

Question 8

What are the requirements for gradient amplifiers? (4 marks)

Answer 8

High current and voltage, fast switching, generally water cooled, temporally stable.

Question 9

What does the shim system do? (3 marks)

Answer 9

Shimming corrects for B0 field inhomogenities. There are different types. Passive shimming which is carried out during installation -> pieces of ferromagentic material placed in the bore to create a more even field.

Active shimming which is carried out when a patient is in the scanner which will distort the B-field. Some systems just apply a linear correction with the gradient coils. But others use specific shim coils to apply higher order corrections.

Question 10

What does the spectrometer do? And what does it consist of? (4 marks)

Answer 10

It processes the signal and performs fourier transforms to convert the k-space image into a real space image.

It consists of controller and a reconstruction system, as well as a connection to PACS and the console.