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MRI > Part IV > Flashcards

Part IV Flashcards

1
Q

heart of MRI system

A

primary magnets

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2
Q

function of Primary Magnets

A

provide a sustained
homogenous Bo during examination

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3
Q

Criteria for selection for primary magnets

A
  1. desired field strength - magnetic flux density (↑strength, ↑ price)
  2. siting limitation - location of installation
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4
Q

types of primary magnets

A
  1. permanent magnets
  2. electromagnets - resistive and superconductive
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5
Q

first permanent magnet

A

lodestone

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6
Q

-occurs naturally
-can be synthesized
-inexpensive & widely used

A

permanent magnet

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7
Q

ealiest commercial magnets

A

ferrite magnets made up of Fe

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8
Q

1930 magnets developed, higher mf intensity than ferrite magnets

A

AlNiCo

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9
Q

recently introduced magnets with higher mf intensity

A

rare earth magnets
ex. neodymium

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10
Q

pupose of open mri system

A

parents can stay with kid, claustro and anxious pt

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11
Q

produces mf in open mri system

A

individual brick size ferromagnetic ceramic material, oriented into an array, up to 1m on a side (2-5 layers)

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12
Q

part of OMS, positioned in each magnet for shimming - above magnet

A

pole face

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13
Q

[oms] refine the homogeneity of the mf after installation of MRI system

A

adjusting screws

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14
Q

iron yolk purposes

A
  1. mechanical frame for assemble and stability
  2. confines the fringe field
  3. intensifies the Bo
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15
Q

permanent magnet advantages (SLA)

A

signif. fringe field: 0.5mT
low electric power consumption
absence of cooling system

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16
Q

permanent magnet disadv.

A

limited Bo intensity
poor mf homogeneity
excessive wt. 90000 kg/ approx. 100 tons)

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17
Q

z-axis of permanent magnet

A

vertical instead of horizontal

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18
Q

pm is mostly used for [..]

A

low mf imaging - extremity imaging

0.3 T - whole body

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19
Q

[pm] mf Bo

A

up to 0.3T (usually less)

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20
Q

[pm] mf homogeneity

A

10-50 ppm (↑ppm, ↑ inhomegeneity)

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21
Q

[pm] weight

A

90 000 kg

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22
Q

[pm] cooling system

A

none

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23
Q

[pm] power consumption

A

20kW

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24
Q

[pm] dist to 0.5 mT fringe field

A

<1m

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25
Q

magnets used in older products, Bo produced by a classical electromagnet

A

Resistive EM

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26
Q

no. of coils of resistive em that balances Bo

A

4 separate coils- intensity, uniform Bo

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27
Q

Resistive EM produces [orientation] Bo

A

vertical coupled to permanent magnets

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28
Q

material that Solenoid resistive em uses

A

Aluminum Strips

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29
Q

adv and disadv of aluminum strips

A

widely available, light mat.
lower conductivity rate

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30
Q

aluminum weight

A

1/3 of that of Cu but 60% conductivity of Cu

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31
Q

Resistive EM field strength

A

0.3 T

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32
Q

Resistive EM Adv [LePTSEL]

A

-less expensive - lower f. strength
-r. less precision
-easily turned off - desired mf strength
-shimming less difficult
-easier siting
-lighter than perm magnet (4,000 kg)

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33
Q

Resistive EM mf homogeneity

A

10-50 ppm

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34
Q

Resistive EM Disadv

A

-consumes large current (0.2=60-80 kW)
-req for cooling: Water-cooler with secondary single pass system thru heat exchanger

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35
Q

used for analytical spectroscopy and high energy physics (reach up 14 T)

A

Superconducting EM

36
Q

requires broader rf bandwidth

A

Superconducting EM

37
Q

[Superconducting EM] Mf bo

A

0.5-4T

38
Q

[Superconducting EM] mf homogeneity

A

0.1-5 ppm

39
Q

[Superconducting EM] weight

A

10 000 kg

40
Q

[Superconducting EM] cooling system

A

cryogenic

41
Q

[Superconducting EM]power consumption

A

20kW

42
Q

[Superconducting EM] distance to 0.5 ml fringe field

A

10 m

43
Q

[Superconducting EM] Adv

A
  1. high mf intensity (↑MRI signal=↑SNR)
    2.↑ SNR = ↑SR and CR
  2. field could be ho0monized/shimmed
44
Q

[Superconducting EM] Disadv

A
  1. intense fringe field (compromises site selection) 10m
  2. usage of cryogen ( expensive, ↑work load)S
45
Q

superconducting em

A

nobium-titanium(NbTi) alloys

46
Q

critical temp:

A

9K - conductor becomes superconductor

47
Q

liquified gasses that keeps the conductor cold

A

cryogen

48
Q

2 liquid gasses

A

lHe vaporizes @ 4 K
lN vaporizes at 27 K

49
Q

container housing the superconducting wire and cryogen (DEWAR)

A

cryostat

50
Q

separation to maintain temp

A

helium vessel

51
Q

cause of heat up and vaporization of liquid Helium

A

thermal radiation

52
Q

solution for thermal rad :

A

surround helium compartment with concentric insulating compartments (lN)

53
Q

replenishing device that replaces nitrogen compartment

A

cryocoolers

54
Q

purpose of cryocoolers:

A

cool radiation shields
recondense the helium in main mag system

55
Q

recondenses helium vapors back into liquid helium bath

A

cool head

56
Q

the superconductor becomes resistive that it starts to heat up

A

quench

57
Q

cause of quench

A

lack of lHe or mechanical trauma to crystat

58
Q

location of GC

A

secondary cylinder

59
Q

GC materials

A

broad thick copper conducting bands (10mm wide and 4 mm thick)
placed in a strong epoxy resin

60
Q

GC casing purpose

A

prevents the coils that causes thump/noise

61
Q

GC material

A

broad thick Cu conducting bands (10mm wide & 4 mm thick) placed in a strong epoxy resin casing

62
Q

Z GC / Gz, Gss / Maxwell Coils
Location:

A

wound on cylinder at opposite ends

63
Q

Z GC / Gz, Gss / Maxwell Coils
Purpose

A

Bz: changes mf into the z-axis
Bss: transverse selection (top-down / AXIAL/ separates superior from inferior)

64
Q

X GC / Gx, GR
Location

A

2 positioned saddle shaped coil in set of 2 positioned on either side of the cylinder

65
Q

X GC / Gx, GR
Purpose

A
  1. produce mf lat; side to side
  2. slice selection (sagittal)
  3. Br - readout gradient mf
  4. phase &frequent encoding
66
Q

Y GC / Gy, Go
Location

A

2 positioned saddle shaped coil in set of 2 -vertical axis

67
Q

Y GC / Gy, Go
Purpose

A
  1. produce mf vertically; A-P
  2. slice selection (coronal)
  3. phase and frequency encoding
68
Q

gc energized simultaneously (3 gradient)

A

combined gradients

69
Q

Combined Gradients
Purpose

A
  1. produce a single selection
  2. used for slice selection (oblique images)
70
Q

made up of coils of wire that can produce a radiosignal at the larmour frequency
behaves as transmitter/receiver

A

RF probe

71
Q

source of RF, master of freq source

A

Frequency Synthesizer

72
Q

2 types RF Probe

A

Homogenous volume coils (T&R)
Inhomogenous coils (R coils only)

73
Q

Quadratus Coil
Purpose

A
  1. improves SNR by detecting MR signal from multi. detectors
  2. replaces saddle design for all homo application
  3. birdcage resonator
74
Q

widely used coil T & R
low signal sensitivity

A

saddle coils

75
Q

robust; homogenous volume coil

A

body coil

76
Q

body coil (location)

A

inside the gradient coil - close to gc

77
Q

body coil
purpose

A

can image the whole body; T & R

78
Q

head/extremity coil
pupose

A

cranial ana and LE (w/ high reso)
typical quadratus birdcage head coil-neck coil and thoracic/lumbar surface, spine coil -total spine

79
Q

surface coil
location:

A

placed nead the poi

80
Q

surface coil is encased with a [..]

A

rubberized/plastic matrix

81
Q

surface coil
purpose

A
  1. obtain high SNR images of the specific anatomy
  2. used as R but used in head/body coils to transmit RF
  3. better CR and SR
82
Q

surface coils disadv

A
  1. small fov
  2. positioning coil = more time
83
Q

phased array and matrix coil purposes

A

-to overcome limited FOV
-can image spine, br, and pelvis
-uniform img - large imging vol
-use eithe linear R/quaddrature R can be used
-allows parallel imaging

84
Q

contains 32,64 more indiv coils; used for gen imaging and parallel imaging

A

matrix coils

85
Q

Superconducting EMs operators @:

A

0.5T, 1.0T, 1.5T

86
Q

Superconducting EMs specialty system:

A

3T and 4T

MRI (7 decks)

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