High field MRI: clinical applications and safety

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(No Transcript)
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Quantitative methods and investigating BOLD
mechanisms

• Penny Gowland

Susan Francis
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Quantitative Imaging

• Relaxation times, MT, diffusion, perfusion/ blood
  volume
• Neuroapplications
• BOLD mechanisms
• Cortical layering
• MS etc
• Cancer

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Quantitative imaging

• Rapid imaging sequences
• EPI
• HASTEFISP
• Appropriate fitting

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What would we like to measure?

• CBV changes- venous and arterial
• CBF changes
• Oxygenation changes

REST
Glucose and O2
Venule
Arteriole
Capillary Bed
Glucose and O2
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What do we want to measure?

• CBV changes- venous and arterial
• CBF changes
• Oxygenation changes

ACTIVE
Glucose and O2
Arteriole
Venule
Capillary Bed
Glucose and O2
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What do we want to measure?

• CBV changes- venous and arterial
• CBF changes
• Oxygenation changes

• With a temporal resolution sufficient to test
  models of HDR

Stimulus
0
30
60
Time (s)
8 s
Time (s)
Post stimulus undershoot
Initial dip
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Measuring total DCBV by T2 effect of contrast
agents

• (and DY)

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Typical R2 timecourse
30
29
28
27
-1
26
R2 / s
25
24
Infusion
23
22
-2
0
2
4
6
8
10
12
14
16
18
Time / min
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Understanding the HDR

• Now repeating the study at higher temporal
  resolution, and shorter paradigm

1st Block
1100
2nd Block
3rd Block
4th Block
600
5th Block
EPI Signal (a.u.)
-5
15
35
55
5
Time (s)
Stimulus
Button press
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Measuring Venous Blood Volume using multiecho T2

• And oxygen extraction

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Blood oxygenation effect on T2
p
p
p
p
p
p/2
tcp
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Measuring DCBV by T1 effect
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Measuring DCBV by T1 effect

• Assuming fast exchange, in the presence of CA,
  the T1 of a voxel is given by

Schwarzbauer et al., Magn. Res. Med., 1993
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ME-LL-EPI sequence
TR
a0
ME-EPI
ME-EPI
ME-EPI
ME-EPI
TA
TI
TA
TA
Broadening Gradient
Rewind gradient
Signal
T2 decay
TE1
TE2
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T1 during an infusion of CA
1.14
1.13
1.12
1.11
Tl (s)
1.1
1.09
Stimulus
1.08
1.07
1.06
0
120
240
360
480
600
720
840
960
time (s)
Washout
CA given
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Results

• DCBVtot 27
• Rise times similar, DCBV falls slower than T2
• Faster changes than observed by Mandeville in
  anaesthetized rats (T2 method)

50
10
CBV
40
8
T2
30
6
CBV change ()
T2 change ()
20
4
10
2
0
0
80
120
0
20
40
60
100
Time (s)
-10
-2
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Combining CBV and CBF measurements
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LLEPI-FAIR

• LL-EPI with alternating selective and
  non-selective (S/NS) inversion pulses
• Better sensitivity than FAIR
• Previous models (ITS-FAIR, Turbo-TILT) can be
  improved
• Did not take account of transit times
• Starting magnetization different in NS and S
  cases
• cannot model difference signal

a0
EPI
EPI
EPI
EPI
TI
TA
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Results

• CBF increases by 85 on activation

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Measuring arterial CBVa
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EPI star with diffusion weighting
With and without diffusion weighting which will
suppress signal from arteries
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EPI star with diffusion weighting
With and without diffusion weighting which will
suppress signal from arteries
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CBVa using EPIstar
0.9
0.8
0.7
0.6
Change
0.5
0.4
0.3
0.2
0.1
0
0
1000
2000
3000
4000
5000
1
0.8
0.6
Change
0.4
0.2
0
0
1000
2000
3000
4000
5000
Inversion Time (ms)