T2* revision

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T2 revision
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Vector coherence
Schering
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Dephasing
Spins will precess at slightly different
frequencies due to variations in the local
magnetic field
Time
It is often easier to understand this dephasing
is a frame of reference that is rotating at the
average frequency of spins
Time
4
T2 artefacts
Good(ish) shim
Phantom with coin near it
Bad shim
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Resting cortex
Blood cells containing deoxy- and oxy-
haemoglobin
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Active cortex
Blood flow Blood volume Blood oxygenation
Glucose and O2
Arteriole
Venule
Capillary Bed
Glucose and O2
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Assumed monoexponential Decay rate is R2
M
xy
M
o
Active Rest
0.5
x
Time after pulse
z
-0.5
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BOLD effect due to T2 effects around blood
vessels
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1 March 2009 Nature
How do people maintain an active representation
of what they have just seen moments ago? The
visual areas of the cerebral cortex that are the
first to receive visual information are
exquisitely tuned to process incoming visual
signals, but not to store them. On the other hand
brain areas responsible for memory lack visual
sensitivity, but somehow people are able to
remember a visual pattern with remarkable
precision for many seconds, actually, for as long
as they keep thinking about that pattern.
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1 March 2009 Nature
Our question was, where is this precise
information being stored in the brain? "Using a
new technique to analyze fMRI data, we've found
that the fine-scale activity patterns in early
visual areas reveal a trace or something like an
echo of the stimulus that the person is actively
retaining, even though the overall activity in
these areas is really weak after the stimulus is
removed,.
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Phospholipid structures
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Blood brain barrier
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Central sulcus
departments.weber.edu/chfam/2570/Neurology.html
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Sensory areas of the brain
From FMRIB, Oxford
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Magnetic forces
Positive susceptibility object attracted
Ferromagnetic
Paramagnetic
Diamagnetic
Susceptibility
0
-1
?
Negative repelled
Positive attracted
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Magnetic forces
Negative susceptibility object repelled or
levitated
Ferromagnetic
Paramagnetic
Diamagnetic
Susceptibility
0
-1
?
Negative repelled
Positive attracted
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Magnetic forces
Superconductors -1
Permanent magnets 106
Deoxyg. Blood -6.52 10-6
Water -910-6
Air (oxygen) 0.36 10-6
Ferromagnetic
Paramagnetic
Diamagnetic
Susceptibility
0
-1
?
Negative repelled
Positive attracted
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Red blood cells
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Special dissociation curves
CO stop haemoglobin giving up oxygen
Fetal blood preferentially takes up oxygen in
placenta
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Effect of dHb on relaxation times
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Assumed monoexponential Decay rate is R2
M
xy
M
o
Low dHb High dHb
0.5
x
Time after pulse
z
-0.5
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a
b
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1/T2 against dHb for blood at 7T

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Resting cortex
Blood cells containing deoxy- and oxy-
haemoglobin
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Active cortex
Blood flow Blood volume Blood oxygenation
Glucose and O2
Arteriole
Venule
Capillary Bed
Glucose and O2
32
Spins will precess at slightly different
frequencies due to variations in the local
magnetic field
Time
It is often easier to understand this dephasing
is a frame of reference that is rotating at the
average frequency of spins
Time
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Lights on
Lights on
Lights on
a
60
0
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Bold signal
b
Time (s)
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Heamodynamic response function
Bold signal
Stimulus
Time (s)
8 s
Initial dip
Post stimulus undershoot
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Heamodynamic response function (effect of adding
CA)
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Effect of echo time
7 T
TE
18
25
34
43
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BOLD timecourses
Time course of signal change at optimum TE for
each field strength averaged over subjects
Cycle average for each field strength. Rising
edge of response intersects base-line earlier at
higher field.
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Minimize the sum of squared differences between
images
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Image registration (From Welcome Functional
Imaging Lab)
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Image registration (From Welcome Functional
Imaging Lab)

• Minimising mean-squared difference works for
  intra-modal registration (realignment)
• Simple relationship between intensities in one
  image, versus those in the other
• Assumes normally distributed differences

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Image registration (From Welcome Functional
Imaging Lab)
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Statistical analysis(From Welcome Functional
Imaging Lab)
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Convolution of paradigm with HRF
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Cross Correlation
From MNI
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Dont forget to Fill IN thE National Student survey
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Somatotopic mapping
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Post Central Gyrus Area 1
Dystonia
Normals
Centre of activation separation Normals(6) 11 ?
2 mm Dystonics (5) 4.4 ? 0.9 mm p0.00048
Little Finger
Index Finger
Both Fingers
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Recovery from stroke
Motor task in relation to a small lesion
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BBC In search of Perfection- Heston Blumentahl
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Response to fat
Correlation of BOLD response with all attributes
of oral fat delivery
Areas with a positive correlation of BOLD
response with fat concentration
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Different fat levels
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Supertaster effect
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Fetuses response to auditory stimulus(Motion
correction quite a challenge)
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Cochlear implant Cochlear Stimulation
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fMRI Cochlear Stimulation
250 Hz, biphasic right cochlear stimulation (9V)
L
R
Collaboration with C. Ludman (Radiology), S.
Mason (Medical Physics), G. ODonoghue
(Otolaryngology)
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Arterial Spin Labelling
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Possible labelling scheme

• Could measure perfusion like this

Blood flow
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Magnetization transfer

• Could measure perfusion like this
• The inversion pulse is off-resonance to slice
• Might expect it to have no effect on slice
• It does because of magnetization transfer
• Exchange between bound and free protons

Blood flow
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EPISTAR
Blood flow
Compare TAG and CONTROL conditions TAG tag
arterial blood that will exchange with
tissue CONTROL tag venous blood
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Perfusion

• Brain signal comes from mixture of tissue and
  blood
• Water assumed to be freely diffusible tracer
  exchanging between capillary and tissue
• Exchange time assumed to be zero
• Not quite true

IN
OUT
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Blood brain partition coefficient

• There are
• 80.5 g water /100g blood
• 84.0 g tissue /100g grey matter
• Blood flowing in has more magnetization per unit
  volume than tissue
• Blood brain partition coefficient l
• water content of brain 0.98

water content of blood
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Transit time

• It takes the labelled blood a finite time to
  reach the voxel
• And the even longer to reach the capillary
• This must be taken account of in models

Transit Time
Blood flow
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Kinetic model

• IF Mz is equal at start of tag and control
  conditions is same
• Then different signal is given convolution

Difference
Mz
Tag Control
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Kinetic model
Transit time
Arterial input function Depends on tagging scheme
Time after tag applied
Transit time
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Kinetic model
Residue Function Amount of contrast remaining
after a time t
Input function
r(t)
Time
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Kinetic model
r(t)
r(t)
Time
Time
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Magnetization decay function Describes T1
relaxation of tag
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Labelling schemes
FAIR (flow alternating inversion recovery)
Blood flow

• Blood in slice follows inversion recovery
• Blood outside slice alternates between
• following inversion recovery and
• being at equilibrium (Mo)

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Kidney ASL Dr Francis