Gradient Echoes,

1
Gradient Echoes, Diffusion, EPI

• Two recent MRI clinical research tools -
• Echo Planar Imaging ( EPI )
• Diffusion Weighted Imaging ( DWI )

2
Clinical EPI applications

• Very rapid imaging (fastest clinical).
• 128x128 image lt 100 ms.
• Multi-slice first pass contrast enhanced brain
  perfusion.
• Functional MRI ( BOLD fMRI ).
• Real-time cardiac imaging ?

3
DWI

• Measures diffusion of water.
• Diffusion -
• Random (Brownian) motion of water.
• RMS distance travelled in a fixed time would be
  a measure of diffusion.

Slow diffusion
Rapid diffusion
4
Clinical DWI applications

• Ischemia
• Ischemic tissue exhibits reduced diffusion.
• Intracellular water
• - low (restricted) diffusion.
• Extracellular water
• - higher diffusion.

ischemia ? cytotoxic edema ? reduction in
extracellular volume fraction ? reduction in
overall diffusion
5
Clinical DWI applications

• Ischemic tissue - reduced diffusion.
• Reduction observed within about half an hour of
  ischemia.
• T2 increase not seen for 6 - 12 hours
  (recruitment of excess tissue water).
• Assessment of acute stroke.

T2 weighted
diffusion weighted
6
Clinical DWI applications

• Tumours.
• Help distinguish tumour, cystic changes,
  vasogenic edema, normal white matter.
• White matter disease.
• Disruption of structure affects local diffusion
  of water.

7
Nuclear Magnetic Resonance Imaging

• Nucleus of atom is spinning.
• Causes it to behave like a tiny magnet.
• Nuclei align (almost) parallel to external
  magnetic field, like compass needle.

8
Magnetic Resonance Imaging

• Because nucleus is spinning, it can not align
  exactly parallel to the magnet field.
• This causes the N-S axis of the nucleus to rotate
  around the N-S axis of the main magnetic field.
• Precession.

North of main magnetic field
North of nucleuss magnetic field
9
Magnetic Resonance Imaging

• The rate of precession is important in MRI.
• The number of revolutions per second (frequency)
  of a precessing nucleus depends on the main
  magnetic field strength.
• Although individual nuclei are not aligned
  exactly parallel to the main magnetic field,
  their average alignment is parallel. Millions of
  nuclei involved in MRI.

10
Magnetic Resonance Imaging

• In the Earth magnetic field
• ( 000005 T ), hydrogen precesses at about 2100
  revolutions per second (Hertz).
• In the Vision MRI scanner ( 15 T ) hydrogen
  precesses at 63 million Hertz.
• Larmor frequency.

11
Magnetic Resonance Imaging

• wobble a nucleus at same rate as its
  precessing ? tips its alignment away from main
  magnetic field.
• flip angle.
• Use electromagnetic radiation to do the
  wobbling.
• 15 T ? 63 MHz ? radiowaves (FM)
• Interaction due to resonance between precessing
  nucleus and radiowaves.
• Radiowaves in ? RF pulse.
• Applied to whole sample.

12
Magnetic Resonance Imaging

• Switch off RF pulse ? nuclei realign with main
  magnetic field.
• As they realign, emit radiowaves at Larmor
  frequency ? NMR signal out.
• NMR signal detected by a RF coil (fancy FM
  aerial).
• Vast majority of NMR signal from hydrogen in
  water or fat only.

13
Magnetic Resonance Imaging

• MR images are primarily images of water and fat.
• To produce an image, apply smaller magnetic
  fields.
• Add / subtract with main magnetic field.
• Magnetic field gradients.

14
Magnetic Resonance Imaging

• Each point along x axis, different B.
• Nuclei precess at different rates.
• Emit r/w at different frequencies, depending on
  their x position.
• Spatial encoding of NMR signal.

15
Magnetic Resonance Imaging

• To acquire one line of image

• Apply gradient during acquisition to spatially
  encode NMR signal.

16
Magnetic Resonance Imaging

• Consider nuclei in left right eyes during FID
  and 2 ms readout gradient.

1501
15
15
1499
Rt.
Lt.
Magnetic field, B
Magnetic field, B
0
0
x
x
Freq. 63 63 62958 63042

• After 2 ms, Rt. eye 125916 rotations
• Lt. Eye 126084 rotations
• Out of step ? no gross NMR signal.

17
Magnetic Resonance Imaging

• Practically theoretically better to separate
  application of RF pulse and reception on NMR
  signal.
• Arrange peak (in phase) NMR signal in middle of
  readout gradient.

Gradient Echo
time
RF pulse in
NMR signal out
Readout gradient
time
18
Gradient Echo
B
C
D
A
Readout gradient
2
time
2
2
Rt.
Lt.
Rt.
Lt.
x
A A?B B?C C D Rt.
eye 0 126084 125916 252000
377916 Lt. Eye 0 125916 126084 252000
378084
in step again - echo
in step initially
de-phased by gradient
19
Gradient Echo - moving nucleus
B
C
D
A
Readout gradient
2
time
2
2
x
A A?B B?C C D Rt.
eye 0 125916 126084 252000
378084 Lt. Eye 0 126084 125916 252000
377916 moving 0 126084 126084 252168
378252 Same argument for nuclei only 1 µm apart.
20
Gradient Echo - moving nucleus

• On the scale of an image pixel within the object
  -
• Perfusion Coherent motion. Entire pixel has
  same phase shift.
• Diffusion Incoherent random motion. Signal in a
  pixel is sum of random phase shifts ? cancel one
  another out ? suppression of signal ? diffusion
  weighting.
• High diffusion ? large suppression of signal ?
  dark pixel in DWI.
• Low diffusion ? little suppression of signal ?
  bright in DWI.

21
Diffusion weighting

• Diffusion ? smaller effect than perfusion ? small
  amount of dephasing.
• Noticeable DW required very large magnetic
  gradients.
• Separate DW gradients from imaging gradients.

22
Diffusion weighting
RF
NMR signal
time
Readout gradient
time
Stationary dephased contributes to nucleus
rephased echo. Diffusing dephased reduced
contribution nucleus not fully to echo
rephased
Diffusion imaging gradients gradients
23
Diffusion weighting

• In practice, use spin echo with DW.

180º
NMR signal
90º
time
Readout gradient
time
Stationary dephased rephased
contributes to nucleus echo. Diffusing
dephased not fully reduced contribution nucleu
s rephased to echo
Diffusion imaging gradients gradients
24
Diffusion weighting

• Problem DWI sensitive to any random movements.
• Patient movements, e.g., cardiac pulsations,
  dominate over diffusion.
• One solution - use very fast MRI and freeze
  unwanted motions.
• Echo Planar Imaging ( EPI )

25
Gradient echo MRI

• Single gradient echo - one line of image.

RF
time
Readout gradient
time

• Build entire image with FLASH.

RF
.
time
Readout gradient
.
time

• About 1 second. Poor signal to noise.

26
EPI

• Multiple gradient echoes

RF
time
Readout gradient
time
Phase in out in out (stationary nucleus)
But, signal still potentially available. Re-phase
it with another gradient.
Readout gradient
time
Phase in out in out in out
27
EPI

• Multiple gradient echoes

RF
time
.
Sign - -
-
Readout gradient
.
time
.
Phase in out in out in out in out in
out in out in out
28
EPI

• NMR signal from one RF pulse lasts between 50 -
  200 ms ( T2 decay ). Have to acquire all echoes
  within this time.
• Limited to about 128 echoes, i.e., 128x128 image
  matrix.
• The faster gradients can be switched to - the
  more echoes in a fixed time.

? Entire image in less than 100 ms. ?
Physiological motion frozen. ? Relatively high
S/N (c.f. FLASH). ? More spatial distortion
artefacts. ? Not a high resolution technique. ?
High spec. hardware required.
29
DW-EPI
180º
90º
time
Readout gradient
time
Diffusion EP imaging gradients gradients
30
DW-EPI
T2 weighted EPI
DW-EPI

• High diffusion ? large suppression of signal ?
  dark pixel in DWI.
• Low diffusion ? little suppression of signal ?
  bright in DWI.

31
DW-EPI

• DW another MRI parameter
• (c.f., T1 and T2 weighting )
• DW-EPI also is heavily T2 weighted (need long TE
  to fit in extra diffusion gradients). EPI is
  inherently T2 weighted already.
• Bright signal in DWI could also be due to long T2
  and vice versa.
• T2 shine through
• To just measure diffusion, calculate the Apparent
  Diffusion Coefficient ( ADC ).
• Apparent because averaged over a pixel, and
  contains some perfusion.

32
Apparent Diffusion Coefficient
No diffusion (stationary)
Low diffusion
log( DWI )
High diffusion
b
0 1000

• b is strength of DW gradients.
• Larger b value ? more DW.
• slope of line is the ADC.
• To calculate ADC, need minimum of 2 points on
  line.
• We choose b0 (i.e., T2 weighted EPI) and
  b1000 (DW-EPI).
• ADC is a quantifiable parameter.

33
Apparent Diffusion Coefficient
b0 b1000 ADC map ( T2 weighted )

• In ADC map -
• Bright pixel ? large ADC.
• Dark pixel ? small ADC.
• infarct ? dark
• normal brain ? grey
• CSF ? bright
• No potential for T2 shine through in ADC map.

34
Apparent Diffusion Coefficient

• Amount of diffusion (ADC) also depends on
  direction.
• In free water, diffusion should be the same in
  all directions (isotropic)
• In structures (e.g., white matter tracts) get
  more diffusion along the tracts than
  perpendicular (anisotropic).
• Shown DW gradients along x axis. Acquire
  separate DWI with diffusion along y or z axes.
• Construct diffusion tensor.
• A tensor gives directional information.

35
Apparent Diffusion Coefficient
Normal brain.
ADC map. (amount of diffusion regardless of
direction)
Relative Anisotropy (RA) map (from tensor). (how
uni-directional diffusion is)

• White matter tracts bright -
• all diffusion in one direction along tracts.

36
Apparent Diffusion Coefficient
Post radiotherapy (AVM).
T2 weighted ADC map RA map
Stroke.
ADC map RA map
37
Summary

• Gradient echo always required to acquire a MR
  image.
• Sensitive to motion.
• Can use this to measure diffusion.
• Fast MRI sequence needed to freeze all other
  motions ? EPI.
• Quantify amount of diffusion using ADC maps.
• Quantify direction of diffusion using tensor
  maps.
• http//www.nottingham.ac.uk/radiology/