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Neuroimaging 2 > MRI Scanning (Physics) > Flashcards

MRI Scanning (Physics) Flashcards

1
Q

Give an overview of what is happening/being measured in MRI

A

Water present in tissue contains hydrogen, hydrogen in a magnetic field with radio-frequency applied acquires energy frequency, we can measure frequencies coming back from H (relaxed) are measured , put into matrix and have an algorithm applied to give MRI

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2
Q

outline MR scanner hardware

A

patient inside machine with magnetic field, radio frequency coils and magnetic field gradient coils.
Radio frequency coils generate spatially varying magnetic fields to determine where water is in an object
by transmitting radio frequency we can manipulate H signal- detected by RF coils and sent back through amplifiers, matrix, algorithm to appear on a screen

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3
Q

outline nuclear spin

A

an atom’s nucleus is composed of protons and neurons with a spin property. When we have an odd number of either they possess spin e.g. H = 1 proton

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4
Q

What is magnetic moment

A

where spin combines with nuclear charge

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5
Q

in an atom spin + charge =

A

a tiny magnet

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6
Q

how do H atoms align in a magnetic field?

A

because they are acting like tiny bar magnets, they will line themselves up N-S to orientate with the magnetic field (not all)

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7
Q

when spins/H are aligned with magnetic field, are they in high or low energy

A

low

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8
Q

where spins/H are not aligned with magnetic field, are they in high or low energy state?

A

high

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9
Q

overview what the larmor equation is

A

in addition to high and low energy states (dependent on orientation), the nucleus also has spin, so precessed about the main magnetic field- this rate of spin obeys larmor’s equation

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10
Q

describe the difference between quantum and classical magnetisation

A

quantum takes into consideration all magnetisation- e.g. the energy levels of each H, however Classical just looks at the net using the difference between high a low (excess) to measure

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11
Q

nuclei with an odd number of protons/neutrons possess…

A

nuclear spin

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12
Q

what defines the spin frequency?

A

larmor equation

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13
Q

the larger the magnetic field,

A

the higher the magnetisation (signal)

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14
Q

why does magnetisation (MR signal) have to be excited from equilibrium state?

A

magnetisation is not detectable at equilibrium

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15
Q

what is used to excite signal?

A

radio frequency

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16
Q

how is the rf applied?

A

short pulse, orthological (perpendicular) to main magnetic field (B0)

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17
Q

what does the orthological rf pulse do to the magnetisation?

A

rotates it from z axis (B0) to xy axis

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18
Q

how can magnetisation direction be controlled

A

through the duration/number of rf pulses

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19
Q

after rf is applied and magnetisation is tipped into xy plane, how is it detected?

A

it induces a current in a loop of wire placed in XY plane, changing voltage can be recorded as oscillations- analogue signal

20
Q

how is the analogue signal digitised?

A

signal is recorded and stored then sampled at discrete times according to nyquist condition in order to digitise the correct frequency of oscillations

21
Q

in which plane can magnetisation only be detected?

A

XY plane

22
Q

what does fourier transform algorithm do?

A

converts time domain signals to the frequency domain

23
Q

once the radio frequency field is switched off, the nuclei undergo 2 processes:

A

T2
T1

24
Q

after rf pulse, the magnetisation is in a

A

non-equilibrium state - signal then decays back to equilibrium via relaxation mechanisms

25
Q

what is T1 relaxation?

A

time to relax back to equilibrium state

26
Q

what is T2 relaxation?

A

time to relax back to zero

27
Q

what is a pulse sequence diagram?

A

a sequence of events that control the scanner to control magnetisation with 2 events- 1 RF pulse, 2 signal

28
Q

overview magnetic field gradients

A

you can superimposed an additional magnetic field (field gradient), where one end is lower and the other end higher frequency, varying linearly across distance- this frequency and distance is added to lamor

29
Q

what are the 3 dimensions in which an object is defined in MR scanner?

A

X axis- read direction
y axis- phase direction
z axis- slice direction

30
Q

define slice selection/z axis

A

a selection of spins in the plane through an object where a 1D linear magnetic field gradient is applied during RF pulse application

31
Q

define read encoding/x-axis

A

signal acquired simultaneous with gradient with differening frequnecy representing spatial projection along x-axis

32
Q

define phase encoding/y-axis

A

a magnetic field gradient changes the signal’s phase depending on its location

33
Q

what is k-space

A

matrix where signal is stored

34
Q

what does each column in K-space represent?

A

deigitised data point

35
Q

what does each row in k-space represent?

A

phase encoding step

36
Q

how do we transform k-space into image?

A

apply fourier transform (FT) algorithm to convert complex oscillating signals to discrete frequencies

37
Q

all of k-space spatial frequencies contribute to…

A

the image

38
Q

is there a 1:1 ratio between k-space and image?

A

no

39
Q

centre of k-space:

A

maxinum signal, contributing most to intensity

40
Q

periphery of k-space:

A

minimum signal, contributes to detail

41
Q

spatial frequency is the inverse of

A

wavelength

42
Q

how can resolution of MRI be increased?

A

acquiring more data points

43
Q

what are the 4 methods to sample k-space?

A
  • cartesian
  • radial
  • spiral
  • zig-zag
44
Q

outline cartesian k-space sampling

A

most common and is sequentially acquired in any order

45
Q

outline radial k-space sampling

A

covers central cortion more than periphery- ives intense image with low resolution (unless we acquire more radial direction to fill in peripheral gaps)

46
Q

outline spiral k-space sampling

A

start in centre and move outwards in a spiral. gives more even coverage of spatial information in k-space but image quality depends on whether we can cover k-space correctly

47
Q

outline zig-zag k-space sampling

A

phase encoding gradient applied in different direction whilst acquiring signal. a fast strategy (used in fMRI),, howcer, suseptible to artefacts to to rapid gradient flips

Neuroimaging 2 (7 decks)

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