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PHYSICS of modalities > MRI > Flashcards

MRI Flashcards

(40 cards)

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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

Study These Flashcards
A

gradient reversal

18
Q

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

Study These Flashcards
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

Study These Flashcards
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

Study These Flashcards
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

Study These Flashcards
A

liver parenchyma
muscle
kidneys
spleen
fat
white matter
grey matter
CSF

22
Q

short or long TE maximises T2 effect

23
Q

what is the purpose of image pulse sequence

Study These Flashcards
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

Study These Flashcards
A
  • 180deg RF pulse (Spin Echo)
  • gradient reversal (GRE)
25
what is TR
- 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
what is TE
- 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
what is the maximum TR for a T1 weighted image
< 600 ms
28
tissues with short T1 values appear
hyperintense
29
what is the approximate TR to suppress T1 effect in a T2weighted image
2000msec
30
what is the minimum TE to enhance T2 effect
>30ms
31
on T2 weighted image, tissues with long T2 values appear
hyperintense
32
what is the minimum TR to minimise T1 effect in PD weighted image
>2000ms
33
what is the maximum TE to minimise any T2 effect in PD weighted image
>20 ms
34
slice thickness can be selected by
- adjusting RF bandwidth - keeping RF pulse bandwidth unchanged but using a *steeper gradient, exciting a thinner slice *shallower gradient, exciting a thicker slice
35
what are the steps to obtaining spatial information
1. slice selection 2. phase encoding 3. frequency encoding
36
what happens during the phase encoding step
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
what happens during the frequency encoding step
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
peripheral lines of k-space contributes mainly to
spatial resolution
39
what does low pass filter do
- 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
what does high pass filter do
- 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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