EXAM #9 — PHYSICS UNIT 02

Question 1

static magnetic field.

Answer 1

the main magnetic field- doesn’t change over space or time

Question 2

fringe magnetic field.

Answer 2

the part of the static magnetic field that extends beyond the magnet bore

Question 3

gradient magnetic field.

Answer 3

a magnetic field that changes linearly over a distance (slopes), used for spatial localization

Question 4

magnetic field homogeneity.

Answer 4

the uniformity of the magnetic field

Question 5

active magnet shielding.

Answer 5

confining and concentrating the main magnetic field with smaller; opposing magnetic fields

Question 6

passive magnet shielding.

Answer 6

confining and concentrating the main magnetic field by placing large pieces of metal on the magnet, or lining the magnet room with steel

Question 7

active magnet shimming.

Answer 7

improving the homogeneity of the main magnetic field by superimposing smaller magnetic fields

Question 8

passive magnet shimming.

Answer 8

improving the homogeneity of the main magnetic field by placing pieces of iron around the magnet

Question 9

body coil.

Answer 9

the RF transmitter/receiver that is built into the main magnet housing

Question 10

RF coil.

Answer 10

a transmitter and/or receiver of RF

Question 11

transceiver.

Answer 11

an RF coil that can transmit AND receiver RF

Question 12

cryogen.

Answer 12

a naturally occurring gas that is compressed into liquid form

Question 13

array processor.

Answer 13

the part of the MRI computer that reconstructs the MR images

Question 14

slew rate.

Answer 14

the speed at which the gradients arrive at the peak amplitude

Question 15

Faraday cage.

Answer 15

a complete box of copper and steel that surrounds the magnet room, shielding the room from outside RF

Question 16

eddy currents.

Answer 16

electric currents that are induced in metal components of the MRI magnet housing

Question 17

identify the 2 most important characteristics of an MRI computer.

Answer 17

speed and storage capacity

Question 18

identify the notation used to represent the primary magnetic field.

Answer 18

Bo

Question 19

list the 3 types of primary magnets used in MRI.

Answer 19

permanent, resistive, superconducting

Question 20

identify the field strengths in Tesla that are usually associated with the:

a. permanent MRI magnet. ?
b. resistive MRI magnet. ?
c. whole-body superconducting MRI magnet. ?

Answer 20

identify the field strengths in Tesla that are usually associated with the:

a. permanent MRI magnet. less than . 3 T
b. resistive MRI magnet. less than .3 T
c. whole-body superconducting MRI magnet. .5- 3T or above

Question 21

identify the maximum static magnetic field strength allowed by the FDA for clinical imaging of patients over 1 month old.

Answer 21

8T

Question 22

explain the function of the yolk in a permanent MRI magnet.

Answer 22

to confine and concentrate the static field

Question 23

state 3 advantages and 2 disadvantages of permanent MRI magnets.

Answer 23

advantages: low initial cost, low operating cost, small fringe field
disadvantages: low static field, very heavy

Question 24

state 3 advantages and 3 disadvantages of resistive MRI magnets.

Answer 24

advantages: low initial cost, light weight, static field can be switched off
disadvantages: high power consumption, large fringe field, low static field

Question 25

state 3 advantages and 3 disadvantages of superconducting MRI magnets.

Answer 25

advantages: high field strength, good field homogeneity, low power consumption disadvantages: high initial cost, cryogen replacement, acoustic noise)

Question 26

2 advantages of imaging with high static magnetic field strength.

Answer 26

faster scan times, better image quality

Question 27

identify the superconducting metals most often used in MRI magnets.

Answer 27

niobium and titanium

Question 28

identify the cryogen most often used in superconducting MRI magnets.

Answer 28

liquid helium

Question 29

identify the 2 possible directions of the static magnetic field MRI magnets.

Answer 29

vertical or horizontal

Question 30

identify the unit measurement of homogeneity of a magnetic field.

Answer 30

parts per million (PPM)

Question 31

state the term used to identify the conductive bands that create a gradient magnetic field.

Answer 31

physical gradients

Question 32

list the 3 letters used to identify the physical gradients.

Answer 32

X, Y, and Z

Question 33

18. for a superconducting MRI electromagnet, identify the location of the: a. X gradient coil pair. ? b. Y gradient coil pair. ? c. Z gradient coil pair. ?

Answer 33

18. for a superconducting MRI electromagnet, identify the location of the: a. X gradient coil pair. sides of the bore b. Y gradient coil pair. top and bottom of the bore c. Z gradient coil pair. circular bands at the head and foot of the bore

Question 34

19. for a superconducting MRI electromagnet, identify the direction of the gradient magnetic field induced by the: a. X gradient coil pair. ? b. Y gradient coil pair. ? c. Z gradient coil pair. ?

Answer 34

19. for a superconducting MRI electromagnet, identify the direction of the gradient magnetic field induced by the: a. X gradient coil pair. from one side of the bore to the other b. Y gradient coil pair. from the top of the bore to the bottom or vice versa c. Z gradient coil pair. from one end of the bore to the other

Question 35

describe the direction of current flow in opposite loops of a gradient coil pair.

Answer 35

The current flows in opposite directions in each coil pair

Question 36

identify the 2 properties used to compare gradient coil systems.

Answer 36

peak strength, slew rate

Question 37

identify the unit of measurement of the slope or peak amplitude of a gradient magnetic field.

Answer 37

milliTesla per meter (mT/m)

Question 38

identify the piece of system hardware that provides power to the gradient coils.

Answer 38

gradient amplifiers

Question 39

identify the type of RF coil that can be used to receive and transmit RF.

Answer 39

volume RF coil

Question 40

list the 3 categories of RF coils.

Answer 40

linear polarized (LP); circular polarized (CP) or quadrature; phased array

Question 41

give an example of a common surface coil.

Answer 41

spine coil

Question 42

list 3 examples of common volume coils.

Answer 42

body coil, head coil, extremity coil

Question 43

describe the quadrature or circular polarized (CP) volume RF coil.

Answer 43

two loops of wire are arranged opposite each other, then another 2 loops are arranged opposite each other, and 90° offset from the first pair

Question 44

describe the quadrature or circular polarized (CP) surface RF coil.

Answer 44

a simple loop of wire is superimposed over a loop of wire that is in a figure-eight

Question 45

describe the phased array RF coil.

Answer 45

4 or more simple loop coils that are slightly overlapped, providing increased signal and increased coverage

Question 46

discuss the advantage and disadvantage of using smaller RF coils for MRI.

Answer 46

advantage: better signal disadvantage: smaller area of coverage

Question 47

32. identify the type of electric power used to create magnetic fields · in the: a. active shim coils. ? b. active shielding coils. ? c. primary magnet coils. ? d. gradient coils. ? e. RF transmitter coils. ?

Answer 47

32. identify the type of electric power used to create magnetic fields · in the: a. active shim coils. DC b. active shielding coils. DC c. primary magnet coils. DC d. gradient coils. DC e. RF transmitter coils. AC