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T2* revision. Vector coherence. Schering. 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. - PowerPoint PPT Presentation
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T2* revision
Vector coherence
Schering
TimeTime
Spins will precess at slightly different frequencies due to variations in the local magnetic field
TimeTime
It is often easier to understand this dephasing is a frame of reference that is rotating at the average frequency of spins
Dephasing
T2* artefacts
Phantom with coin near it
Good(ish) shim
Bad shim
Resting cortex
Blood cells containing deoxy- and oxy- haemoglobin
Arteriole VenuleCapillary Bed
Glucose and O2
Glucose and O2
Active cortexBlood flow
Blood volume
Blood oxygenation
Arteriole VenuleCapillary Bed
Glucose and O2
Glucose and O2
Time after pulse
Mxy
Mo
ActiveRest
Assumed monoexponential:Decay rate is R2
*
z
x
-0.5
0.5
BOLD effect due to T2* effects around blood vessels
1 March 2009 NatureHow 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.
1 March 2009 NatureOur 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,”.
Phospholipid structures
Blood brain barrier
departments.weber.edu/chfam/2570/Neurology.html
Central sulcus
Sensory areas of the brain
From FMRIB, Oxford
Susceptibility0
Fer
rom
agne
tic
Par
amag
netic
Dia
mag
netic
-1
Positive susceptibility: object attracted
Magnetic forces
Negative: repelled Positive: attracted
dz
dBBF BmT
Susceptibility0
Fer
rom
agne
tic
Par
amag
netic
Dia
mag
netic
-1
Negative susceptibility: object repelled or levitated
Magnetic forces
Negative: repelled Positive: attracted
Susceptibility0Negative: repelled Positive: attracted
Fer
rom
agne
tic
Par
amag
netic
Dia
mag
netic
-1
Water-910-6
Air (oxygen)+0.36 10-6
Deoxyg. Blood-6.52 10-6
Permanent magnets106
Superconductors-1
Magnetic forces
Red blood cells
Special dissociation curves
CO stop haemoglobin giving up oxygen
Fetal blood preferentially takes up oxygen in placenta
Effect of [dHb] on relaxation times
Time after pulse
Mxy
Mo
Low dHbHigh dHb
Assumed monoexponential:Decay rate is R2
*
z
x
-0.5
0.5
a b
a
d
b c
e f
1/T2* against % dHb for blood at 7T
Resting cortex
Blood cells containing deoxy- and oxy- haemoglobin
Arteriole VenuleCapillary Bed
Glucose and O2
Glucose and O2
Active cortexBlood flow
Blood volume
Blood oxygenation
Arteriole VenuleCapillary Bed
Glucose and O2
Glucose and O2
TimeTime
Spins will precess at slightly different frequencies due to variations in the local magnetic field
TimeTime
It is often easier to understand this dephasing is a frame of reference that is rotating at the average frequency of spins
Lightson
Lightson
Lightsonaa
bb
Time (s)
00 3030 6060
Boldsignal
8 s
Time (s)
Stim
ulus
Initial dipPost stimulus undershoot
Boldsignal
Heamodynamic response function
Heamodynamic response function (effect of adding CA)
EPI pulse sequence
RF
Gslice
Gphase
Gread
Time
Repeat 128 times
A B C
A
B C
kx
ky
EPI k-space trajectory
18
25
34
43
7 TTE
Effect of echo time
0.98
1
1.02
1.04
1.06
1.08
0 50 100 150time (s)
norm
alise
d s
ign
al i
nte
nsi
ty
7 T
3 T
1.5 T
B
0.98
1
1.02
1.04
1.06
1.08
1.1
0 5 10 15 20 25 30
time (s)
norm
alis
ed s
ignal
inte
nsi
ty
7 T3 T1.5T
C
stimulus
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.
BOLD timecourses
Minimize the sum of squared differences between images
Image registration (From Welcome Functional Imaging Lab)
1 0 0 Xtrans
0 1 0 Ytrans
0 0 1 Ztrans
0 0 0 1
1 0 0 0
0 cos() sin() 0
0 sin() cos() 0
0 0 0 1
cos() 0 sin() 0
0 1 0 0
sin() 0 cos() 0
0 0 0 1
cos() sin() 0 0
sin() cos() 0 0
0 0 1 0
0 0 0 1
Translations Pitch Roll Yaw
Rigid body transformations parameterised by:
Squared Error
• Minimising mean-squared difference works Minimising mean-squared difference works for intra-modal registration (realignment)for intra-modal registration (realignment)
• Simple relationship between Simple relationship between intensitiesintensities in one in one image, versus those in the otherimage, versus those in the other– Assumes normally distributed differencesAssumes normally distributed differences
Image registration (From Welcome Functional Imaging Lab)
Image registration (From Welcome Functional Imaging Lab)
Statistical analysis(From Welcome Functional Imaging Lab)
Convolution of paradigm with HRF
From MNI
Cross Correlation
DONT FORGET TO FILL IN THE NATIONAL STUDENT SURVEY
Somatotopic mapping
Centre of activation separation
Normals(6) 11 2 mm
Dystonics (5) 4.4 0.9 mm
p=0.00048
Post Central Gyrus
Area 1Dystonia
Both Fingers
Little Finger
Index Finger
Normals
Recovery from stroke
Motor task in relation to a small lesion
BBC ‘In search of Perfection’- Heston Blumentahl
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
Different fat levels
Supertaster effect
Fetuses response to auditory stimulus(Motion correction quite a challenge)
Cochlear implant & Cochlear StimulationCochlear implant & Cochlear Stimulation
fMRI & Cochlear StimulationfMRI & Cochlear Stimulation
LLRR
Collaboration with C. Ludman (Radiology), S. Mason (Medical Physics), G. O’Donoghue (Otolaryngology)Collaboration with C. Ludman (Radiology), S. Mason (Medical Physics), G. O’Donoghue (Otolaryngology)
250 Hz, biphasic right cochlear stimulation (9V)250 Hz, biphasic right cochlear stimulation (9V)
ARTERIAL SPIN LABELLING
Possible labelling scheme
• Could measure perfusion like this:
Blood flow
INVERSION PULSE
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
INVERSION PULSE
INVERSION PULSETAG
EPISTAR
Blood flow
Compare TAG and CONTROL conditionsTAG: tag arterial blood that will exchange with tissueCONTROL: tag venous blood
INVERSION PULSECONTROL
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
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 = water content of brain = ~ 0.98
water content of blood
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
Blood flow
TransitTime
Kinetic model
• IF Mz is equal at start of tag and control conditions is same
• Then different signal is given convolution:
DifferenceMz
TagControl
Kinetic model
Arterial input functionDepends on tagging scheme
Timeafter tag applied
Transit time
Transit time
Kinetic model
Residue FunctionAmount of contrast remaining after a time t
r(t)
Inputfunction
Time
Kinetic model
r(t)
Time
Time
r(t)
Magnetization decay functionDescribes T1 relaxation of tag
Labelling schemesFAIR (flow alternating inversion recovery)
Blood in slice follows inversion recoveryBlood outside slice alternates between
• following inversion recovery and • being at equilibrium (Mo)
Blood flow
Kidney ASLDr Francis
