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RAD413 > MRI physics and principles > Flashcards

MRI physics and principles Flashcards

1
Q

general overview on how MRI works

A
  1. strong static magnetic field to align protons which create net magnetisation
  2. radio frequency magnetic filed to tip the magnetisation when both have the same frequency
  3. time-varying gradient magnetic filed to spatially localise the MR signals and reconstruct the image
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2
Q

what percentage of the human body is made of water

A

60-75%

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3
Q

what type of nuclei have a non-zero net spin

A

all nuclei with odd numbers of protons and or neutrons

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4
Q

what characteristic of an electron or proton creates a magnetic field

A

spinning charged particlel

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5
Q

Ines of magnetic fields ALWAYS form a closed loop

A
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6
Q

what do you call the magnet formed from electron or proton spin

A

magnetic dipole moment

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7
Q

explain the spin of a proton in and without B0

A

absence of b0= proton spins about its own axis

presence of b0= proton spins about its own axis AND precesses about axis of b0

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8
Q

what is larmor frequency

A

spins rotate (precess) at fixed frequencies

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9
Q

wroe 0 = 2 pi f = y B0
wroe 0 - larmor frequency
2 pi f - larmor frequency
y - gyromagnetic ratio
b0 - external magnetic field

A
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10
Q

what is the order of the electromagnetic spectrum from lowest to highest frequency

A

radio
micro
infrared
visible
ultraviolet
x-ray
gamma

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11
Q

when protons are randomly orientated, what net magnetisation do they hole

A

0, no net magnetisation

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12
Q

compare the energy of protons which have spins parallel/antiparallel to B0

A
  • spins parallel to b0 (+z) have lower energy than those antiparallel (-z)
  • more of spins are in parallel state than antiparallel
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13
Q

explain how the net magnetic moment (M) correlates with B0

A
  • total parallel/antiparallel spins has M
  • this behaves as a magnet pointing in the direction of B0
  • M increases with B0
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14
Q

define flip angle

A

MRI phenomenon by which the axis of the hydrogen proton shifts from its longitudinal plane (static magnetic field B0) Z axis to its transverse plane XY axis by excitation with the help of radiofrequency (RF) pulses.

  • the flip angle determines the amount of RF energy that is absorbed by the body during each pulse
  • the amount of rotation the net magnetization (M) experiences during application of a radiofrequency (RF) pulse.
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15
Q

what 2 things do flip angle depend on

A
  • pulse energy
  • pulse duration
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16
Q

how is the angle (flip angle) controlled

A
  • by varying time or amplitude of pulse
  • a 90 degree excitation pulse produces a flip angle of 90 degrees
17
Q

why do we need to flip the net magnetisation

A
18
Q

what is net magnetisation vector

A

The net magnetisation vector in MRI is the summation of all the magnetic moments of the individual hydrogen nuclei.

19
Q

how is a signal detected in mri

A
  • current induced in conductor is exposed to changing magnetic field
  • in transverse (xy) plane theres Mxy, the rotating component of M
  • a coil whose axis is in the xy plane will detect Mxy
20
Q

define FID

A

free induction decay

  • short-lived sinusoidal electromagnetic signal which appears immediately following the 90° pulse.
21
Q

what is FID

A
  • after RF pulse, short duration decay
  • RF pulse removed, M returns to original equilibrium position M0, pointing along Bo
  • spins precess out of phase = Mxy decays
  • spins release energy = Mz recovers to Mo

Free= spins precess freely after RF pulse switched off
Induction = spins induce current in RF receive coil
Decay= signal decays with time due to longitudinal recovery and dephasing

22
Q

what is the pathway needed to generate an MR signal/ conditions to satisfy

A
  • Mxy precessing in TRANSVERSE plane
  • Net magnetisation Mz
  • Strong magnet (Bo) to make Mz
  • RF pulse to flip Mz to xy plane and get Mxy
  • RF transmit coil to generate pulse
  • Magnetic moments spin IN PHASE
  • Re phase the spin back after dephasing
  • Receive coil in path of precessing Mxy
23
Q

what 3 things happen when the RF transmitter turns off

A
  1. absorbed RF energy retransmitted (at resonance frequency)
  2. magnetisation returns exponentially to equilibrium
  3. excited protons begin to dephase
24
Q

what type of recovery/decay and relaxation is T1

A

longitudinal recovery
- spin lattice relaxation

25
Q

what type of recovery/decay and relaxation is T2

A

transverse decay
- spin spin relaxation

26
Q

define spin-lattice relaxation

A

energy is given to surrounding lattice

27
Q

define spin-spin relaxation

A

dephasing caused by interactions with other nuclei

28
Q

why is the dephasing occurring during t2 relaxation

A

due to inhomogeneities within the magnetic field B0

29
Q

what 2 things is dephasing of t2 dependant on

A
  • scanner (intrinsic inhomogeneities of B0)
  • local environment (susceptibility-induced field distortions produced by tissue or other materials placed within the field)
30
Q

what is spin echo

A

the refocusing of spin magnetisation by a pulse of resonant electromagnetic radiation

  • A spin echo sequence aims to remove the effects of the static field (T2*) but leave the tissue characteristic T2 effect.
  • A single RF pulse generates a free induction decay (FID), but two successive RF pulses produce a spin echo
31
Q

what are the 3 classifications of materials based on their interactions with magnetic fields

A
  1. diamagnetic
  2. ferromagnetic
  3. paramagnetic
32
Q

define diamagnetic

A
  • substance not magnetic under normal conditions but when exposed to external field, reduced the strength of the field locally
  • e.g wood, glass, water
33
Q

define ferromagnetic

A
  • always exhibits magnetic properties
    -e.g iron, cobalt, nickel
34
Q

define paramagnetic

A
  • substance enot magnetic under normal conditions but when exposed to external field, increases the strength of field locally
  • e.g gadolinium (contrast)
35
Q

define magnetic susceptibility

A

degree to which various materials can be magnetised

36
Q

compare diamagnetic, ferromagnetico and paramagnetic in terms of their magnetic susceptibility

A

diamagnetic= very low magnetic susceptibility (doesnt increase field strength)

paramagnetic= high magnetic susceptability (increases field strength)

ferramagnetic= very high magnetic susceptibility (greatly increases field strength)

37
Q
A

RAD413 (24 decks)

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