chapter 9

Question 1

how many guass’s are in a 1.5T scanner?

Answer 1

15,000

Question 2

describe the right hand rule

Answer 2

right fingers wrapped around wire, thumb points to current, fingers point in the MF

Question 3

material for RF shielding

Answer 3

copper

Question 4

material to make permanent magnets

Answer 4

alnico

Question 5

material for passive shielding

Answer 5

steel

Question 6

material to make superconducting wire

Answer 6

niobium-titanium

Question 7

maintain low temperatures

Answer 7

cryostat

Question 8

how steep/strong the gradient is, measured mT/m or G/cm

Answer 8

gradient strength

Question 9

time to reach maximum amplitude, measured in microseconds, longer for stronger gradients

Answer 9

gradient rise time

Question 10

speed & strength, measured in mT/m/s

Answer 10

slew rate

Question 11

% of time the gradient is working

Answer 11

duty cycle

Question 12

converts signals to images

Answer 12

image processor

Question 13

where the FFT occurs

Answer 13

array processor

Question 14

the factors that change the strength of an electromagnet

Answer 14

• current passing through the windings
• number of windings
• diameter of the wire
• spacing between the windings

Question 15

what are the two “bi-poles” of a magnet

Answer 15

north & south

Question 16

Bo is vertical, heavy, low SNR

Answer 16

permanent magnets

Question 17

requires power supply, switched on & off, Bo horizontal or vertical, lightweight 0.2-0.3T

Answer 17

resistive magnets

Question 18

Bo is horizontal, higher SNR, Bo 1.5-14T
-269 C, -450 F, 4K

Answer 18

superconducting magnet

Question 19

heavy steel plates around the magnet

Answer 19

passive shielding

Question 20

uses solenoid electromagnets around the bore, equal but opposite effect on the magnet

Answer 20

active shielding

Question 21

at the time of installation, iron plates counteract inhomogeneities due to surrounding structures

Answer 21

passive shimming

Question 22

uses electromagnetic coils, between each patient, protocol or sequence

Answer 22

active shimming

Question 23

purpose of shimming

Answer 23

magnetic inhomogeneities

Question 24

purpose of shielding

Answer 24

reduce the fringe field

Question 25

the degree of resistance along a wire is determined by

Answer 25

ohms law

Question 26

V=IR

Answer 26

ohms law V is equal to the applied voltage I is the current R is the resistance within the wire

Question 27

the magnetic field strength in superconducting magnets is maintained with cryogens at a temperature of ___ K

Answer 27

4

Question 28

positioning is not as critical, increased wrap with small FOV

Answer 28

large coil

Question 29

high SNR, less likely for wrap

Answer 29

small coil

Question 30

which cryogen is commonly used to super cool wires in MRI

Answer 30

liquid helium

Question 31

apparently magnetization equation

Answer 31

Bo = H0(1+x)

Question 32

slew rate equation

Answer 32

mT/m/sec

Question 33

Tesla to Gauss conversion

Answer 33

1T = 10,000G

Question 34

paired electrons, no magnetic moments ex) diamond, graphite, wood, plastic

Answer 34

diamagnetism

Question 35

unpaired electrons, small attraction to the magnetic field. ex) gadolinium contrast, platinum alloys, gold alloys

Answer 35

paramagnetism

Question 36

half filled electron shells, strong attraction ex) iron & nickel

Answer 36

ferromagnetism

Question 37

units of MF homogeneity

Answer 37

parts per million (ppm)

Question 38

max field strength for clinical imaging in USA

Answer 38

8T

Question 39

given charge, motion, and magnetism, the interaction of 2 will induce the third

Answer 39

law of electromagnetic induction

Question 40

stray magnetic field outside the bore of the magnet

Answer 40

fringe field

Question 41

considered high field strength for clinical imaging

Answer 41

3T