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Gradient Echoes, Diffusion, & EPI. Two recent MRI clinical research tools - Echo Planar Imaging ( EPI ) Diffusion Weighted Imaging ( DWI ). Clinical EPI applications. Very rapid imaging (fastest clinical). 128x128 image < 100 ms.

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slide1

Gradient Echoes,

Diffusion, & EPI

  • Two recent MRI clinical research tools -
  • Echo Planar Imaging ( EPI )
  • Diffusion Weighted Imaging ( DWI )
slide2

Clinical EPI applications

  • Very rapid imaging (fastest clinical).
  • 128x128 image < 100 ms.
  • Multi-slice first pass contrast enhanced brain perfusion.
  • Functional MRI ( BOLD fMRI ).
  • Real-time cardiac imaging ?
slide3

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

slide4

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

slide5

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

slide6

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.
slide7

S

N

S

N

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.
slide8

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 nucleus’s

magnetic field

slide9

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.
slide10

Magnetic Resonance Imaging

  • In the Earth magnetic field
  • ( 0·00005 T ), hydrogen precesses at about 2100 revolutions per second (Hertz).
  • In the Vision MRI scanner ( 1·5 T ) hydrogen precesses at 63 million Hertz.
  • Larmor frequency.
slide11

Magnetic Resonance Imaging

  • “wobble” a nucleus at same rate as it’s precessing  tips its alignment away from main magnetic field.
  • “flip angle”.
  • Use electromagnetic radiation to do the “wobbling”.
  • 1·5 T  63 MHz  radiowaves (FM)
  • Interaction due to resonance between precessing nucleus and radiowaves.
  • Radiowaves in  RF pulse.
  • Applied to whole sample.
slide12

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.
slide13

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.
slide14

1·5 T

Magnetic field, B

Magnetic field, B

0

x

x

With gradient

Without gradient

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.
slide15

Free Induction Decay (FID)

time

NMR signal out

RF pulse in

Readout gradient

time

Magnetic Resonance Imaging

  • To acquire one line of image
  • Apply gradient during acquisition to spatially encode NMR signal.
slide16

Readout

gradient

time

Magnetic Resonance Imaging

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

1·501

1·5

1·5

1·499

Rt.

Lt.

Magnetic field, B

Magnetic field, B

0

0

x

x

Freq. 63 63 62·958 63·042

  • After 2 ms, Rt. eye: 125916 rotations
  • Lt. Eye: 126084 rotations
  • Out of step  no gross NMR signal.
slide17

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

slide18

1·501

1·501

1·499

1·499

Magnetic field, B

Magnetic field, B

0

0

x

Gradient Echo

A

B

C

D

Readout

gradient

2

2

2

time

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

initially

de-phased

by gradient

in step again -

echo

slide19

1·501

1·501

1·499

1·499

Magnetic field, B

Magnetic field, B

0

0

x

Gradient Echo - moving nucleus

A

B

C

D

Readout

gradient

2

2

2

time

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.

slide20

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.
slide21

Diffusion weighting

  • Diffusion  smaller effect than perfusion  small amount of dephasing.
  • Noticeable DW required very large magnetic gradients.
  • Separate DW gradients from imaging gradients.
slide22

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

slide23

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

nucleus rephased to echo

Diffusion imaging

gradients gradients

slide24

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 )
slide25

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.
slide26

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

slide27

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

slide28

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.

slide29

DW-EPI

180º

90º

time

Readout

gradient

time

Diffusion EP imaging

gradients gradients

slide30

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.
slide31

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.
slide32

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 b=0 (i.e., T2 weighted EPI) and b=1000 (DW-EPI).
  • ADC is a quantifiable parameter.
slide33

Apparent Diffusion Coefficient

b=0 b=1000 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.
slide34

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.
slide35

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.
slide36

Apparent Diffusion Coefficient

Post radiotherapy (AVM).

T2 weighted ADC map RA map

Stroke.

ADC map RA map

slide37

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/