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Physics > Chapter 9 > Flashcards

Chapter 9 Flashcards

1
Q

Magnetic field strength can be expressed in units of Tesla (T) or gauss (G), where:

A

1 T = 10 000 G

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2
Q

Ohm’s law states:

A

V = I (R)

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3
Q

What rule determines the direction of current flow through a coil, and the direction of the magnetic field that the current produces?

A

Right hand thumb rule

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4
Q

To maintain the magnetic field of a resistive magnet, the following should be applied to the magnet coils:

A

Current

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5
Q

Fourier transform (FT) is a mathematical process that is used to convert MR signal from a FID into a spectrum. FT occurs in the:

A

Array processor

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6
Q

Passive magnetic shielding can be accomplished by lining the scan room walls with:

A

Steel

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7
Q

Which cryogen is commonly used to super cool wires in MRI?

A

Liquid helium

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8
Q

What is the maximum field strength allowed for clinical imaging of adults in the USA?

A

8 T

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9
Q

In a typical superconducting magnet, the direction of the magnetic field is:

A

Horizontal

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10
Q

The homogeneity of a magnetic field is expressed in units of:

A

ppm (parts per million)

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11
Q

Which material is most commonly used to make superconducting wires?

A

Niobium-titanium alloy

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12
Q

The SNR is enhanced when using local coils. (T or F)

A

true

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13
Q

What is the most common material used to make permanent magnets

A

Alnico

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14
Q

Why do we use shimming

A

To make the field even or homogenous

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15
Q

The magnetic field strength in superconducting magnets is maintained with cryogens at a temperature of:

A

4 K

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16
Q

The purpose of magnetic field shimming is to reduce the size of the fringe field. (T or F)

A

false

The purpose of magnetic field shimming is to correct for field inhomogeneity. The purpose of SHIELDING is to reduce the fringe field.

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17
Q

The law of electromagnetic induction indicates that whenever a charge particle such as an electron exhibits motion, a corresponding magnetic field is induced.(T or F)

A

true

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18
Q

Gradient strength is expressed in units of all of the following EXCEPT:

A

w/kg

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19
Q

Once a ferromagnetic material is exposed to an externally applied magnetic field, it becomes magnetized (a permanent magnet). Permanent magnets are said to be:

A

Bipolar

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20
Q

Select all of the following that determine the strength of an electromagnet:

A

The amount of current passed through the windings
The number of windings in the coil
The distance or spacing between the windings
The diameter of the wire used in the windings

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21
Q

Which correctly describes the right hand rule?

A

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

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22
Q

Which is true concerning superconducting magnets?

A

1.5-3.0T

Bo is horizontal

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23
Q

Which of the following are true concerning the temperature of superconductive magnets?

A

-269 C
-450 F
4 K

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24
Q

What are the factors that change the strength of an electromagnet?

A

current passing thru the windings
# of windings
diameter of the wire
spacing between the windings

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25
Q

TRUE or FALSE: The stronger the gradient, the longer the rise time.

A

True

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26
Q

passive shielding

A

steel plates around the magnet. heavy

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27
Q

active shielding

A

uses solenoid electromagnets around the bore. Equal but opposite effect of the magnet

28
Q

passive shimming

A

at time of install. iron plates counteract in homogeneities due to surrounding structures

29
Q

active shimming

A

uses electromagnetic coils. between each patient, protocol or sequence

30
Q

purpose of shimming

A

magnetic in homogeneities

31
Q

purpose of shielding

A

reduce the fringe field

32
Q

V

33
Q

I

34
Q

R

A

resistance

35
Q

equation for Ohm’s law

36
Q

not equation for Ohm’s law

37
Q

advantage of permanent magnets

A

do not require a power supply

38
Q

disadvantage of permanent magnets

A

very heavy

39
Q

Bo of permanent magnets

A

vertical, flux lines run south to north

40
Q

apparent magnetization

A

Bo = H0(1+x)

41
Q

Ohm’s Law

42
Q

Tesla to Gauss conversion

A

1 T = 10,000 G

43
Q

parts per million

A

units of MF homogeneity

44
Q

8T

A

max field strength for clinical

45
Q

law of electromagnetic induction

A

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

46
Q

fringe field

A

stray magnetic field outside the bore of the magnetic

47
Q

3T

A

considered high field strength for clinical imaging

48
Q

large coil

A

positioning not as critical, increased wrap w small FOV

49
Q

small coil

A

high SNR, less likely for wrap

50
Q

diamagnetism

A

paired electrons. no magnet moments. (diamond, graphite, mercury, and copper)

51
Q

paramagnetism

A

unpaired electrons. small attraction to magnetic field. (gad)

52
Q

ferromagnetism

A

half filled electron shells. strong attraction. (iron)

53
Q

gradient strength

A

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

54
Q

gradient rise time

A

time to reach maximum amplitude. measured in microseconds.

55
Q

slew rate

A

speed and strength. measured in mT/m/s

56
Q

duty cycle

A

% of time the gradient is working

57
Q

advantage of resistive magnets

A

light weight

58
Q

disadvantage of resistive magnets

A

must have power supply

59
Q

max field strengths

60
Q

max field strengths

A

1.5 T - 3.0 T

61
Q

magnet

A

nuclear alignment

62
Q

RF source

A

excitation

63
Q

gradients

A

spatial encoding

64
Q

image processor

A

converts signals to images

65
Q

array processor

A

where the FFT occurs

66
Q

NEED TO LOOK UP MATERIALS Q 16 STUDY GUIDE

Physics (15 decks)

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