MRI Instrumentation

Question 1

What are the main parts of an MRI scanner?

Answer 1

a powerful magnet, cooling system, gradient coils, RF transmit coils, RF receive coils

Question 2

What are the three main types of magnets?

Answer 2

permanent, resistive magnet, & semiconducting

Question 3

What is a permanent magnet?

Answer 3

large ferromagnetic blocks with the magnetic field permanently on; no cryogens needed; mainly for low fields (0.2 T)

Question 4

What is a resistive magnet?

Answer 4

magnet that requires power (can be turned off) and generates a lot of heat; depends on stability of power supply

Question 5

What is a semiconductive magnet?

Answer 5

these magnets are always on, requires no power (just for ramping up the field), need cryogens (liquid helium for cooling); electromagnet with a few km of superconducting wires

Question 6

What are gradient coils used for in an MRI scanner?

Answer 6

selecting the slices

Question 7

What do we expect from Gradient coils?

Answer 7

1. High spatial resolution in all directions
2. High speed for fast imaging
3. Clean profile

Question 8

What are the parameters of gradient coils?

Answer 8

amplitude, slew rate, and rise time

Question 9

Amplitude

Answer 9

The maximum strength of a gradient coils (mT/m)

Question 10

Slew rate

Answer 10

The gradient slope (vs time) = how fast it rises or falls (T/m/s)

Question 11

Rise time

Answer 11

time it takes to reach the desired amplitude (ms)

Question 12

Shimming (in the gantry)

Answer 12

making the magnet homogenous (smoother)

Question 13

Passive shim

Answer 13

attching iron plates to the magnet to reduce distortions in magnetic field (make homogeneous)

Question 14

Active shim

Answer 14

Use of shim coils which produce small static currents to smooth out the field distortions

Question 15

What is magnetic shielding?

Answer 15

containing the magnetic field

Question 16

Fringe field

Answer 16

The field outside the bore of the magnet

Question 17

Passive shielding

Answer 17

The soft iron shields around the magnet to reduce the fringe field

Question 18

Active shielding

Answer 18

Additional coils outside the magnet which counteract the fringe field

Question 19

5 Gauss Line

Answer 19

The border beyond which the magnetic fringe field falls below 5 G

Magnetic field is good if it’s equal to or less than 5 gauss

Question 20

What is a Faraday Cage?

Answer 20

An enclosure designed to exclude electromagnetic fields (lined with copper plates so no external RF can get into the scanner room)