Coils

Question 1

What are the properties of the main magnet

Answer 1

Magnetic field strength, homogeneity, temporal stability, bore size

Question 2

Lamar Equation

Answer 2

W = y x B0

Question 3

Benefits for high field strength

Answer 3

Faster scan times, signal form patient increases and can probe physiologic processes

Question 4

Disadvantages for high field strength

Answer 4

bioeffects, heating, peripheral nerve stim, cost and increased susceptibility (image artefacts)

Question 5

Main magnet can be three types

Answer 5

permanent, resistive, and superconducting

Question 6

Permanent magnet adv

Answer 6

no power supply, low maintence and cost, large bore (good for claustro)

Question 7

disadvantages for permanent magnets

Answer 7

low SNR, long scan times, siting - heavy

Question 8

How does a superconducting magnet work

Answer 8

allows electrical current to run in coils without any resistance at temperatures below 250 deg (23K)

Question 9

Is superconducting ever turned off

Answer 9

only in emergencies

Question 10

What are the components of MRI system

Answer 10

Main magnet B0; gradient coils; radiofrequency source and image processor reconstruction system

Question 11

Advantages for resistive

Answer 11

good uniformity, light weight, low capital costs, can be turned off in emergency

Question 12

Disadv for resistive

Answer 12

requires cont electric power to produce magnetic field ; cost; sig amount of heat; low inherent SNR so scan times longer; stray magnetic field

Question 13

Adv of superconducting

Answer 13

achieve high field strengths, high field uniformity

Question 14

Disadv of superconducting

Answer 14

high initial capital and siting costs, difficulty in turning off main magnetic field in emergency and extensive fringe fields

Question 15

What is a fringe field

Answer 15

The stray peripheral magnetic field outside the bore of the magnet

Question 16

Shim coils

Answer 16

a device used to adjust the homogeneity of a magnetic field

Question 17

Passive shimming

Answer 17

when magnet is shimmed with metal

Question 18

Active shimming

Answer 18

when shimming is performed with lops of current carrying wire

Question 19

From inside to out, what are the different coils in magnet

Answer 19

RF coil, gradient coil, shim coil and then magnet

Question 20

Gradient coils enable differences

Answer 20

in the field to produce Larmor frequencies at different locations in the body - spatial encoding

Question 21

Selecting an appropriate gradient coil

Answer 21

will provide a specific slice location and orientation (axial, etc.)

Question 22

Gradient must have frequencies

Answer 22

matched

Question 23

What are three types of RF coils

Answer 23

Surface coil, phased array coil, volume coil or bird cage coil

Question 24

Volume coils are used for

Answer 24

transmitting RF signals over a large area

Question 25

Surface coils are used for

Answer 25

receiving signals from a small area with high SNR

Question 26

Phased array are used to

Answer 26

combine multiple surface coils to increase signal and improve image quality over a larger area

Question 27

Volume coils transmit

Answer 27

RF and receive the MR signal and encompass the entire anatomy - whole body

Question 28

Surface coils are

Answer 28

small and easily shaped to be placed near the anatomy and we can improve SNR when imaging structures near the surface

Question 29

Phased array coils have multiple coils and receivers that have

Answer 29

individ signals that combine to form one big signal with improved SNR and good coverage