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PHYSICS of modalities > MRI > Flashcards

MRI Flashcards

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1
Q

advantages of clinical MRI

A
  • excellent soft tissue contrast with high resolution
  • display of several images and oblique cuts
  • no ionising radiation
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2
Q

what is the most challenging problem in abdominal MRI

A

breathing motion and movement

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3
Q

what happens to spin when external magnetic field is introduced

A

spins will align either parallel or anti-parallel state

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4
Q

more spins in the parallel or anti-parallel state and why

A

parallel - because lower energy

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5
Q

what is larmor frequency

A

precessing frequency of magnetic moment of proton

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6
Q

which state spins are more unstable

A

anti-parallel - higher energy state

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7
Q

what happens when a 90 deg RF pulse is injected

A
  • “in excess” spins will absorb the external energy and jump to the unstable high energy state (from parallel to anti-parallel)
  • all nuclei become “in phase” in the x-y plane
  • amount of energy causing the NMV flip down onto x-y plane
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8
Q

signal is detected only when spins are precessing in which plane

A

xy plane

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9
Q

what happens when RF pulse is removed

A
  1. generation of free induction decay (FID) signal
  2. amplitude of NMV is exponentially decaying
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10
Q

without 90 deg RF pulse, spin dephase occurs due to

A

inhomogeneity of local magnetic field

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11
Q

what happens during T1 relaxation

A

MNV flips back towards its equilibrium position after removal of RF pulse, excessive energy is released and dissipated as heat in the tissue lattices

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12
Q

factors affecting T1 relaxation

A
  • T1 relaxation increases with the complexity of the lattice: very easily give away energy to surrounding lattice structure, T1 relaxation time decreases
  • T1 relaxation decreases as the lattice becomes increasingly spare
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13
Q

rank tissue from the fastest T1 relaxation time to the longest T1 relaxation time

A

fat
liver
kidney
spleen and white matter
muscle
gray matter
csf

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14
Q

short or long TE maximises T1 relaxation

A

short TE

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15
Q

what happens during T2 relaxation

A

magnetic moments at different locations in space are subjected to a slightly different magnetic field strength, thus precess with slightly different frequencies, causing loss of phase coherence and spin dephasing

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16
Q

how to counter spin dephasing (T2) due to magnetic field

A

applying a 180deg rephasing pulse

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17
Q

how to recover spin dephasing due to activation of magnetic gradients

A

gradient reversal

18
Q

how to recover spin dephasing due to internal local field of tissue

A
  • cannot be recovered because it is fluctuating
  • however, internal local field of tissue is responsible for generation of tissue contrast on MR images
19
Q

what can be determined from T2 relaxation time

A
  • tells us about the internal local field strength of a particular type of hydrogen-containing tissue
  • stronger internal field = higher spin dephasing speed = shorter T2 relaxation time
20
Q

what type of tissues have shorter T2 relaxation time

A

solid tissue
- rigid molecular structure have strong IF
- rapid loss of phase coherence of magnetic moments

21
Q

rank the tissues of shortest to longest T2 relaxation time

A

liver parenchyma
muscle
kidneys
spleen
fat
white matter
grey matter
CSF

22
Q

short or long TE maximises T2 effect

A

long TE

23
Q

what is the purpose of image pulse sequence

A
  • rephase the dephased spins
  • remove magnetic inhomogeneity effects by 180deg RF pulse
  • produce signal or echo that contains decay characteristics of different tissue
  • enable manipulation of different TE and TR setting to produce different types of contrast weighting images
  • spatial encoding
24
Q

how are signals being rephased

A
  • 180deg RF pulse (Spin Echo)
  • gradient reversal (GRE)
25
Q

what is TR

A
  • repetition time
  • time from application of one RF pulse to the next
  • measured is ms
  • affects length of relaxation period allowed after application of one RF pulse to the beginning of the next
26
Q

what is TE

A
  • echo time
  • time between RF excitation pulse and the collection of signal
  • affects length of the T2 relaxation period allowed after the removal of an RF excitation pulse and the peak of the signal receiver coil
  • measured in ms
27
Q

what is the maximum TR for a T1 weighted image

A

< 600 ms

28
Q

tissues with short T1 values appear

A

hyperintense

29
Q

what is the approximate TR to suppress T1 effect in a T2weighted image

A

2000msec

30
Q

what is the minimum TE to enhance T2 effect

A

> 30ms

31
Q

on T2 weighted image, tissues with long T2 values appear

A

hyperintense

32
Q

what is the minimum TR to minimise T1 effect in PD weighted image

A

> 2000ms

33
Q

what is the maximum TE to minimise any T2 effect in PD weighted image

A

> 20 ms

34
Q

slice thickness can be selected by

A
  • adjusting RF bandwidth
  • keeping RF pulse bandwidth unchanged but using a
    *steeper gradient, exciting a thinner slice
    *shallower gradient, exciting a thicker slice
35
Q

what are the steps to obtaining spatial information

A
  1. slice selection
  2. phase encoding
  3. frequency encoding
36
Q

what happens during the phase encoding step

A

y-axis gradient turns on very briefly, causing a transient difference in precessing speed in different rows

results:
- spins along the same column will have same frequency but varies in phase between rows
- phase value of spins in the same column contain “pseudo frequency” - which can be decided later by Fourier transformation to generate back Y coordinate information

37
Q

what happens during the frequency encoding step

A

assuming no phase encoding was done,
spins in the different column varies -> which can be decoded later by Fourier transform to generate x-coordinate information

38
Q

peripheral lines of k-space contributes mainly to

A

spatial resolution

39
Q

what does low pass filter do

A
  • cuts our all high spatial frequency data from k-space
  • results in a lack of details on the image, although coarse contrast of image remains
40
Q

what does high pass filter do

A
  • removes low spatial frequencies data from k-space
  • resultant image shows little contrast, yet edge definition remains
  • fine details of the image are contained in the high spatial frequency data

PHYSICS of modalities (4 decks)

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