88. MRI: ELECTROMAGNETISM Flashcards
- What are Magnetic Fields generated by?
- moving charges
- this is known as an electrical current
- it also determines the direction of the magnetic field
- In which directions can the Magnetic Field act in, with respect to the direction of the flow of current?
- it can act in a clockwise direction
- it can act in an anti-clockwise direction
- What is Ampere’s Law?
- 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
- How do we use Fleming’s Right Hand Rule
(Ampere’s Law)?
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
- What do changing magnetic fields generate?
- they generate electric currents
- What motion happens in order to produce a current?
- 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
- Define: an Induced Electric Current.
- 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
- State Faraday’s Law.
- 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
- What machinery is Electromagnetic Induction a basic physical phenomenon of?
- an MRI
- How is Electromagnetic Induction used in an MRI scan?
- THE SPINNING CHARGE OF THE HYDROGEN PROTON
- causes a magnetic field to be induced around the MRI
machine
- causes a magnetic field to be induced around the MRI
- THE NET MAGNETISATION VECTOR (NMW)
- moves across the area of the receiver coil
- this receiver coil induces an electrical charge in the
coil
- What are the 3 types of magnets used in Clinical MRI Machines?
- Permanent Magnets
- Electromagnets
- Superconducting Magnets
- What substances makes up Permanent Magnets?
- Ferromagnetic substances
THE MOST COMMON MATERIAL USED IS:
- Alnico
- What is Alnico?
- this is an alloy
IT IS MADE UP OF:
- Aluminium (Al)
- Nickel (Ni)
- Cobalt (Co)
- What are three advantageous characteristics of the Permanent magnets used in Clinical MRI Machines?
- THEY HAVE AN OPEN DESIGN
- THEY REQUIRE NO POWER SUPPLY
- this means that it is low cost to operate them
- THEY HAVE A VERTICAL MAGNETIC FIELD
- this keeps the magnetic field virtually confined
- it is confined within the boundaries of the scan room
- What are the 5 characteristics of Permanent Magnets that are disadvantageous?
- THEY ARE EXCESSIVELY HEAVY
- THEY HAVE LOW FIXED FIELD STRENGTHS
- these are from 0.2 to 0.7 Tesla (T)
- THEY HAVE A LOW SIGNAL TO NOISE RATIO (SNR)
- THEY HAVE A LOW RESOLUTION
- THEY TAKE A LONG TIME TO SCAN
- What Law do Electromagnets utilise?
- the Law of Electromagnetic Induction
- What does the Law of Electromagnetic Induction State?
- 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
- What is a simpler way of stating the Law of Electromagnetic Induction?
- an electric current will pass through a series of wires
- this produces a magnetic field
- What does the strength of the Magnetic Field depend on?
- it depends on the current that passes through the coils
of the wire
- How do we determine the direction of the Main Magnetic Field?
- we use the Right-Hand Thumb Rule
- also known as Ampere’s Law
- What does the main magnetic field produce?
- it produces lines of flux
- these run horizontally
- from the head to the foot of the magnet
- What are the 4 advantages of Electromagnets?
- THEY ARE LIGHTER IN WEIGHT
- than permanent magnets
- THEY HAVE A LOW CAPITAL COST
- THEY ARE EASY TO INSTALL
- 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
- What are the 5 disadvantages of Electromagnets?
- THEY HAVE A HIGH OPERATIONAL COST
- this is due to their enormous power requirements - THEY HAVE A MAXIMUM FIELD STRENGTH
- that is less than 0.3 Teslas (T)
- THEY HAVE SCAN TIMES THAT ARE LARGER AND
LONGER- than that of permanent magnets
- THEY HAVE A LOW SIGNAL TO NOISE RATIO
- THEY HAVE A LOW RESOLUTION
- What does this image show?
- 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
