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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.

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slide3

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

t2 artefacts
T2* artefacts

Good(ish)

shim

Phantom with

coin near it

Bad shim

resting cortex

Glucose and O2

Arteriole

Venule

Capillary Bed

Glucose and O2

Resting cortex

Blood cells containing deoxy- and oxy- haemoglobin

active cortex
Active cortex

Blood flow

Blood volume

Blood oxygenation

Glucose and O2

Arteriole

Venule

Capillary Bed

Glucose and O2

slide7

Assumed monoexponential:

Decay rate is R2*

M

xy

M

o

Active

Rest

0.5

x

Time after pulse

z

-0.5

1 march 2009 nature
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.

1 march 2009 nature1
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,”.

slide18

Central sulcus

departments.weber.edu/chfam/2570/Neurology.html

sensory areas of the brain
Sensory areas of the brain

From FMRIB, Oxford

slide20

Magnetic forces

Positive susceptibility:

object attracted

Ferromagnetic

Paramagnetic

Diamagnetic

-1

0

Susceptibility

Negative: repelled

Positive: attracted

slide21

Magnetic forces

Negative susceptibility:

object repelled or levitated

Ferromagnetic

Paramagnetic

Diamagnetic

-1

0

Susceptibility

Negative: repelled

Positive: attracted

slide22

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

-1

0

Susceptibility

Negative: repelled

Positive: attracted

special dissociation curves
Special dissociation curves

CO stop haemoglobin giving up oxygen

Fetal blood preferentially takes up oxygen in placenta

slide27

Assumed monoexponential:

Decay rate is R2*

M

xy

M

o

Low dHb

High dHb

0.5

x

Time after pulse

z

-0.5

slide28

a

b

resting cortex1

Glucose and O2

Arteriole

Venule

Capillary Bed

Glucose and O2

Resting cortex

Blood cells containing deoxy- and oxy- haemoglobin

active cortex1
Active cortex

Blood flow

Blood volume

Blood oxygenation

Glucose and O2

Arteriole

Venule

Capillary Bed

Glucose and O2

slide32

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

slide33

Lights

on

Lights

on

Lights

on

a

60

30

0

Bold

signal

b

Time (s)

slide34

Heamodynamic response function

Bold

signal

Stimulus

Time (s)

8 s

Initial dip

Post stimulus

undershoot

slide37

Effect of echo time

7 T

TE

18

25

34

43

slide38

B

C

stimulus

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.

image registration from welcome functional imaging lab

Squared Error

Rigid body transformations parameterised by:

Translations

Pitch

Roll

Yaw

Image registration (From Welcome Functional Imaging Lab)
image registration from welcome functional imaging lab1
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
slide50

Post Central Gyrus

Area 1

Dystonia

Normals

Centre of activation separation

Normals(6) 11  2 mm

Dystonics (5) 4.4  0.9 mm

p=0.00048

Little Finger

Index Finger

Both Fingers

slide51

Recovery from stroke

Motor task in relation to a small lesion

response to fat
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

slide58

fMRI & Cochlear Stimulation

250 Hz, biphasic right cochlear stimulation (9V)

R

L

Collaboration with C. Ludman (Radiology), S. Mason (Medical Physics), G. O’Donoghue (Otolaryngology)

possible labelling scheme

INVERSION PULSE

Possible labelling scheme
  • Could measure perfusion like this:

Blood flow

magnetization transfer

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

epistar

CONTROL

TAG

INVERSION PULSE

INVERSION PULSE

EPISTAR

Blood flow

Compare TAG and CONTROL conditions

TAG: tag arterial blood that will exchange with tissue

CONTROL: tag venous blood

perfusion
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
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

transit time
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

kinetic model
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

kinetic model1
Kinetic model

Transit time

Arterial input function

Depends on tagging scheme

Time

after tag

applied

Transit

time

kinetic model2
Kinetic model
  • Residue Function
  • Amount of contrast remaining after a time t

Input

function

r(t)

Time

kinetic model3
Kinetic model

r(t)

r(t)

Time

Time

slide70

Magnetization decay function

  • Describes T1 relaxation of tag
labelling schemes
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)
slide72

Kidney ASL

Dr Francis