88. MRI: ELECTROMAGNETISM

Question 1

1. What are Magnetic Fields generated by?

Answer 1

• moving charges
• this is known as an electrical current
• it also determines the direction of the magnetic field

Question 2

2. In which directions can the Magnetic Field act in, with respect to the direction of the flow of current?

Answer 2

• it can act in a clockwise direction
• it can act in an anti-clockwise direction

Question 3

3. What is Ampere’s Law?

Answer 3

• this is also known as Fleming’s Right Hand Rule

THIS RULE:
- determines the magnitude
- determines the direction
- of the magnetic field
- due to the current

Question 4

4. How do we use Fleming’s Right Hand Rule
   (Ampere’s Law)?

Answer 4

THE CURRENT:
- is shown by the direction that the thumb is pointing in

THE MAGNETIC FIELD:
- is shown by the direction that the fingers are
pointing in

THE KNUCKLES OF YOUR HAND:
- act as the 00:00 point on the clock
- you look at the direction of the fingers relative to the
knuckles

Question 5

5. What do changing magnetic fields generate?

Answer 5

• they generate electric currents

Question 6

6. What motion happens in order to produce a current?

Answer 6

• a magnet is moved in and out of a closed circuit

THE CURRENT STOPS IMMEDIATELY:
- as soon as the magnet stops moving in and out of the
circuit

Question 7

7. Define: an Induced Electric Current.

Answer 7

• this is an electrical current that is caused by motion
• it is caused by the movement of a magnet
• this magnet continuously moves in and out of a closed
  circuit

THIS CURRENT CAN ALSO BE CAUSED:
- by a vector passing through a coil

Question 8

8. State Faraday’s Law.

Answer 8

• this law explains the phenomenon of an induced
  current

IT STATES THAT:
- the magnitude of the electromotive force (emf) or
of voltage
- is generated in a conductor
- due to electromagnetic induction
- this is the changing of magnetic fields

THIS ENDUCED EMF IN THE CONDUCTING CIRCUIT:
- is proportional to the rate of change of magnetic flux
linkage (Φ) within the circuit

Question 9

9. What machinery is Electromagnetic Induction a basic physical phenomenon of?

Answer 9

• an MRI

Question 10

10. How is Electromagnetic Induction used in an MRI scan?

Answer 10

1. THE SPINNING CHARGE OF THE HYDROGEN PROTON
   
   • causes a magnetic field to be induced around the MRI
     machine
2. THE NET MAGNETISATION VECTOR (NMW)
   - moves across the area of the receiver coil
   - this receiver coil induces an electrical charge in the
   coil

Question 11

11. What are the 3 types of magnets used in Clinical MRI Machines?

Answer 11

1. Permanent Magnets
2. Electromagnets
3. Superconducting Magnets

Question 12

12. What substances makes up Permanent Magnets?

Answer 12

• Ferromagnetic substances

THE MOST COMMON MATERIAL USED IS:
- Alnico

Question 13

13. What is Alnico?

Answer 13

• this is an alloy

IT IS MADE UP OF:
- Aluminium (Al)
- Nickel (Ni)
- Cobalt (Co)

Question 14

14. What are three advantageous characteristics of the Permanent magnets used in Clinical MRI Machines?

Answer 14

1. THEY HAVE AN OPEN DESIGN
2. THEY REQUIRE NO POWER SUPPLY
   
   • this means that it is low cost to operate them
3. THEY HAVE A VERTICAL MAGNETIC FIELD
   
   • this keeps the magnetic field virtually confined
   • it is confined within the boundaries of the scan room

Question 15

15. What are the 5 characteristics of Permanent Magnets that are disadvantageous?

Answer 15

1. THEY ARE EXCESSIVELY HEAVY
2. THEY HAVE LOW FIXED FIELD STRENGTHS
   
   • these are from 0.2 to 0.7 Tesla (T)
3. THEY HAVE A LOW SIGNAL TO NOISE RATIO (SNR)
4. THEY HAVE A LOW RESOLUTION
5. THEY TAKE A LONG TIME TO SCAN

Question 16

16. What Law do Electromagnets utilise?

Answer 16

• the Law of Electromagnetic Induction

Question 17

17. What does the Law of Electromagnetic Induction State?

Answer 17

• whenever a conductor is placed in a varying magnetic
  field
• an electromotive force is induced

NB:
- this is also known as Faraday’s First Law

Question 18

18. What is a simpler way of stating the Law of Electromagnetic Induction?

Answer 18

• an electric current will pass through a series of wires
• this produces a magnetic field

Question 19

19. What does the strength of the Magnetic Field depend on?

Answer 19

• it depends on the current that passes through the coils
  of the wire

Question 20

20. How do we determine the direction of the Main Magnetic Field?

Answer 20

• we use the Right-Hand Thumb Rule
• also known as Ampere’s Law

Question 21

21. What does the main magnetic field produce?

Answer 21

• it produces lines of flux
• these run horizontally
• from the head to the foot of the magnet

Question 22

22. What are the 4 advantages of Electromagnets?

Answer 22

1. THEY ARE LIGHTER IN WEIGHT
   
   • than permanent magnets
2. THEY HAVE A LOW CAPITAL COST
3. THEY ARE EASY TO INSTALL
4. THEY ARE EASY TO TURN ON AND OFF
   
   • this is also an inexpensive process
   • this helps with the safety of the machinery
   • it protects the patient in emergency situations

Question 23

23. What are the 5 disadvantages of Electromagnets?

Answer 23

1. THEY HAVE A HIGH OPERATIONAL COST
   - this is due to their enormous power requirements
2. THEY HAVE A MAXIMUM FIELD STRENGTH
   
   • that is less than 0.3 Teslas (T)
3. THEY HAVE SCAN TIMES THAT ARE LARGER AND
   LONGER
   
   • than that of permanent magnets
4. THEY HAVE A LOW SIGNAL TO NOISE RATIO
5. THEY HAVE A LOW RESOLUTION

Question 24

24. What does this image show?

Answer 24

• this is an electromagnet
• it is used in an MRI machine

IN THIS SPECIFIC CASE:
- it is being used to scan the hand and the wrist