MRI

Question 1

The four main components of an MRI scanner:

Answer 1

1. The magnet
2. a radiofrequency system
3. a gradient system
4. the host computer

Question 2

3 main types of magnet:

Answer 2

1. Superconducting magnets
2. Permanent magnets (0.06-3.0T)
3. Resistive magnet (0.1 - 0.5T)

Question 3

What is the rang of magnetic field strengths clinically?

Answer 3

0.2 to 3.0T

Question 4

What is the main type of magnet used in clinical practice?

Answer 4

Superconducting magnets

Question 5

Why is superconducting magnets the most used magnet in clinical practice?

Answer 5

This is due to the high field strength (better SNR)
and and very good homogeneity.

Question 6

What is the superconducting magnet made of?

Answer 6

solenoid (coil of wire) made of Niobium and Tin, which carries a large electric current

Question 7

The wire is cooled at a very cool temperature of…

Answer 7

Below 9K (-264 degrees Celsius)

Question 8

Why is the wire cooled?

Answer 8

The wire loses all electrical resistance at this
temperature. It becomes superconducting. As long as the
solenoid is kept at this temperature it will run indefinitely

Question 9

How is the wire cooled?

Answer 9

using liquid helium, which is surrounded by liquid nitrogen

Question 10

What is the main disadvantage of the superconducting magnets?

Answer 10

the long narrow bore which can cause claustrophobia in
some patients

Question 11

Why are the superconducting magnets always on?

Answer 11

to maintain this magnetic field homogeneity

Question 12

What are permanent MRI magnets made of?

Answer 12

Ferrite

Question 13

What is an advantage of ferrite?

Answer 13

• It is very heavy and has a low magnetic flux density
• The magnetic field can be positioned horizontally or vertically
• Can be used to make open magnets

Question 14

However the magnetic field can become weak if…

Answer 14

the ferrite is not kept at a constant temeprature

Question 15

What were the first magnets used in the early MRI scanners?

Answer 15

Resistive MRI magnets, which use electrical coils wrapped around an iron core to increase magnetic susceptibility

Question 16

Requires _______ amount of electricity to produce a relatively ____
strength magnetic field.

Answer 16

Large, Low

Question 17

Advantage of Resistive Magnets

Answer 17

they can be designed
as an open magnet and they can be switched off in the evening

Question 18

What role does the radiofrequency system play in MRI?

Answer 18

The RF system is required to transmit RF into the patient
at an appropriate Larmour frequency and to detect and
gather the very tiny RF echo, which the patient emits sub
sequentially

The host computer calculates the required RF pulses for
the sequence and these pulses are amplified along the RF
transmitting chain

Question 19

True or False: The coil that detects the echo can be the same coil that
was used to transmit the initial RF pulse

Answer 19

True

Question 20

True or False: Separate receive coils cannot be used

Answer 20

False

Question 21

What size is the amplitude of the echo?

Answer 21

Very small (microvolts)

It needs to be amplified before being processed!

Question 22

What is the gradient used for in MRI?

Answer 22

The gradients are used to alter the main magnetic field of
the scanner in order to get spatial information about the
patient being scanned

Question 23

How many gradient coils are used?

Answer 23

Three

Question 24

What do the coils do in gradient?

Answer 24

These can alter the main B0 field strength in the X,Y and Z
directions

They are resistive coils driven by 3 separate gradient
amplifiers.

It is the switching of the gradient coils that causes the
acoustic noise in an MRI scanner

Question 25

What is B0?

Answer 25

When the external magnetic field is applied

Question 26

What is MRI based on ?

Answer 26

Nuclear resonance of Hydrogen atoms

Question 27

What are 2 important nuclear properties for MRI?

Answer 27

Spin and Charge

Question 28

There are 2 models for explaining the phenomenon of magnetic resonance:

Answer 28

The Classical Theory
Quantum Mechanics

Question 29

what does the precessional frequency depend on?

Answer 29

the magnetic flux density (B)

Question 30

What happens to H+ protons once B0 is applied?

Answer 30

spins align themselves with and against the magnetic field.

This creates two energy levels, the spins that are antiparallel to the magnetic field having the higher energy

Question 31

When the radiofrequency is emitted what happens?

Answer 31

To transition from one axis (the higher energy) to another (the lower one) the proton must emit a photon of energy.

When proton drops to lower energy level it must give up
energy in the form of an RF signal at the resonant
frequency

Question 32

Water & biological tissues are…

Answer 32

Diamagnetic

Question 33

The Larmor frequency is….

Answer 33

the resonant frequency of the tissue for a particular magnetic field strength

Question 34

The strength of the MR signal depends on:

Answer 34

1. Low temperature (cannot lower in vivo)
2. Large magnetic dipole moment (m) and gyromagnetic ratio
3. Large N: use abundant element 1H
4. B0