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HSC3007 MRI > Week 4 > Flashcards

Week 4 Flashcards

1
Q

what is used to support superconductivity of MRI

A

Vacuum tank with 500 – 1500L of liquid helium at 4K as cryogen

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2
Q

___ of current increases slowly during ramping phase

A

500 - 1000A

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3
Q

passive shimming is done using __

A

Small pieces of iron placed at specific locations inside scanner to correct field distortion such that only body tissue disrupts it

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4
Q

active shimming is done using __

A

Special coils used to distort main field specifically for area to be scanned

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5
Q

magnetic field must be homogenous to __ variation

A

< 3.5 ppm

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6
Q

what are the use of gradients

A

Localizes signal & affects speed & resolution of scan; Combining gradients in directional pairs allows for oblique imaging

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7
Q

gradients uses large currents within magnetic fields to create __

A

lorentz forces

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8
Q

what are Lorentz forces

A

gradients vibrate and knock against each other causing loud acoustic noise in MRI

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9
Q

key parameters of gradients include

A
  • max amplitude
  • siew rate
  • gradient linearity
  • duty cycle
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10
Q

transmit coils are powered by __

A

RF power amplifier

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11
Q

transmit coils transmits ___

A

Radio waves transmitted at specific resonant frequency to excite nuclei, creating B1 field

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12
Q

transmit coils dissipate heat within patient measured at __

A

specific absorption rate (SAR)

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13
Q

receiver coils detects __

A

weak signals given off from nuclei > pre-amplified > digitised

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14
Q

the smaller the coil, the ___ signal, ___ depth penetration & coverage

A

higher; lower

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15
Q

receiver coils are placed ___ to B0

A

perpendicular

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16
Q

passive shielding uses __

A

Large amounts of ferromagnetic materials in the walls to contain fringe magnetic flux

restricts magnetic field from sensitive equipment

17
Q

active shielding uses __

A

Shielding coils outside windings of main magnetic field

18
Q

RF shielding AKA __

A

RF cage / Faraday’s Cage

19
Q

RF shielding is made of ___

A

copper / galvanized steel to enclose MRI room

20
Q

RF shielding blocks __

A

stray external RF signals from contaminating MR signal, causing artifacts

21
Q

Wires & tubes must pass through __ made of specific diameter & length to filter unwanted wavelengths

A

waveguide

22
Q

dynamic switching fields and gradients causes __

A

peripheral nerve stimulation & acoustic noise

23
Q

what should patients do to prevent dangerous current loop burns

A
  • Parts of the patient’s body should not touch each other
  • RF coils must not touch body
  • Skin does not make contact with tunnel lining
24
Q

how do dynamic switching fields cause peripheral nerve stimulation

A

rapid gradient switching causes currents in body = nerve depolarization; harmless but uncomfortable

25
Q

how to manage acoustic noises

A

hardware = absorptive materials, rigid mounting, force compensation

software = modulated gradients to reduce volume & zero TE sequences are nearly silent

26
Q

what patient care should be done for everyone

A

hearing protection

27
Q

time varying B0 field is created by __

A

low frequency subject movement in static/fringe field

28
Q

time varying B0 field induces __

A

low electrical currents in body causing sensory effects
- cranial nerves
- retina
- metallic taste

29
Q

light flashes experienced are due to __

A

stimulated magneto-phosphenes

30
Q

what causes metallic taste

A

electric current in tongue

31
Q

what should be considered when imaging patients with implanted leads

A
  • check implant compatibility
  • risk vs benefit
  • updated info
  • manufacturer’s guidelines
  • third party validation
32
Q

superconducting magnet can be switched off using __

A

helium

33
Q

what moves during scans

A

gradient coils

34
Q

what is the difference between active shimming & active shielding

A

Active shimming used to reduce B0 inhomogeneity / active shielding is to prevent electrical interference

35
Q

Peripheral nerve stimulation is dependent on __

A

scan orientation

HSC3007 MRI (10 decks)

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