Magnetic Resonance Imaging

1
Magnetic Resonance Imaging

• BEST Summer Course 2004
• Laurent Hermoye
• Radiology Department
• E-mail hermoye_at_rdgn.ucl.ac.be

2

3
Outline

• Reminder of electromagnetism
• Physical principles
• MR Security
• Advanced MR techniques

4
Reminder of electromagnetism

5
Magnetic field

Source 1
Magnetic field units Tesla (T)
6
Electromagnetic induction
Source 1
7
The electromagnetic spectrum
8
Fourier Transform
Source 2
9
Physical principles

10
Magnetic moment
The protons (1H) have a magnetic moment (spin)
11
Alignment

• In an external magnetic field, the spins align
  with the field direction

12
Larmor Precession

• The spins precess around the direction of the
• magnetic field at the Larmor frequency
• f0 g B0
• where g 42.57 MHz / T is the gyromagnetic ratio

13
RF pulse

• An RF pulse at the Larmor frequency rotate the
  spins by an angle a (flip angle)
• ? RESONANCE
• a depends on the amplitude and the length of the
  RF pulse
• (frequently a 90 or 180)

14
Signal reception

• A variable magnetic field in the transverse plane
  induces a signal in the coil

15
T1 and T2 relaxations
T2 Relaxation
T1 Relaxation
Spin dephasing Decay of the transverse
magnetization
Return of the longitudinal magnetization to its
equilibrium
16
Combination T1 T2

• In reality, the T1 and T2 effects occur
  simultaneously
• T1 gt T2

Source 2
17
Contrast

• The relaxation times T1, T2 and the protons
  density r depend on the tissue

18
Contrast

• If we can create a dependency between T1,T2 or r
  and the signal received by the coil, we are able
  to differentiate the tissues

19
Spin Echo
20
Spin echo sequence

• 2 parameters
• TE echo time
• TR repetition time

21
Signal Localization

• How can we localize the signal ?
• Spatial variation of the magnetic field strength
  (gradient)

22
Slice selection gradient
Source 2
23
Frequency / phase encoding

• Phase encoding gradient along the Y direction
• Frequency encoding gradient along the X direction

24
Spin Echo Sequence

25
Other sequences

• Gradient Echo
• Inversion - Recovery (FLAIR, STIR)
• Echo Planar Imaging (EPI) fast imaging

EPI sequence with T2 weighting
FLAIR Sequence
26
K- Space ? Image

k-Space
Fourier Transform
Image
Source 2
27
System architecture

Source 2
28
Lesion or artefact ?

29
MR Security

30
Magnetic forces
31
If we forget

32
Contraindications

• Pacemaker
• Aneurismal clip
• Implanted Electrodes
• Metallic Implants
• Intra-orbital metallic particle
• Pregnant women

33
Biological Effects

• 3 possibles causes
• The static magnetic field
• The gradients
• The RF pulses

34
Advanced MR techniques

35
fMRI

• Purpose detect the brain areas activated by a
  task
• Sequential acquisitions of the brain volume with
  an EPI sequence
• Alternate 2 conditions rest task
• BOLD effect a variation of the oxyhaemoglobin
  deoxyhaemoglobin ratio causes a variation of the
  MR signal

36
SPM
37
fMRI Clinical Application

• Detection of the language areas before surgery

38
fMRI Clinical Application

• Detection of the motor areas before surgery

39
Diffusion

• The molecules have a random translational motion
• Brownian motion
• diffusion
• Main molecule water
• The molecules probe the tissue structure at a
  microscopic level

40
Diffusion imaging (I)

• MRI can be made sensitive to these diffusion
  motions
• Pulsed gradient spin echo sequence
• Spin echo sequence (90 - 180)
• Additional diffusion gradients around the 180
  pulse
• The  b-factor  controls the diffusion weighting
• The  b-factor  depends on G, d et D

41
Diffusion Imaging (II)

• If free diffusion
• The spins acquire a random phase
• Phase opposition
• Signal decay
• If reduced diffusion (cellular membranes, axonal
  architecture, myelin shield )
• Lower signal decay

42
Diffusion Sequence

• Acquisition without diffusion gradient
• 3 acquisitions with the diffusion gradients (b
  1000) along the slice encoding, frequency
  encoding and phase encoding directions.
• We compute the apparent diffusion coefficient
  (ADC)

43
Diffusion Clinical Application (I)

• Hyperacute stroke

FLAIR Day 2
FLAIR Day 0
44
Diffusion Clinical Application (II)

Diffusion Day 0
ADC Day 0
45
Anisotropy

• In water
• No preferential diffusion direction
• Isotropic diffusion
• In white matter fibers
• The architecture of the axones and their myelin
  shield facilitate the diffusion along their axis
• Anisotropic diffusion

46
Diffusion Tensor Imaging

• We detect and track the white matter fibers
  (fiber tracking)

47
DTI sequence

• Identical to a diffusion sequence
• The diffusion gradients are applied along minimum
  6 directions (3 main directions 3 diagonales)
• For each pixel
• We compute the diffusion tensor (3 ? 3 matrix)
• We compute the preferential diffusion direction
  (eigen vectors)
• We color-code these directions (red for RL,
  green for AP, blue for FH)
• We can track the fibers (fiber tracking)

48
3 kinds of fibers

• Projection fibers

Corticospinal tract
49
3 kinds of fibers

• Association fibers

Superior longitudinal fasciculus
50
3 kinds of fibers

• Commissural fibers

Anterior Commissure
51
Limbic fibers

Cingulum
52
Pathological case

• Left hemiparesis

53
Perfusion Imaging

• Dynamic imaging after injection of a contrast
  agent bolus (Gd-DTPA)
• Application hepatic perfusion measurement

54
Angio-MRI
55
Spectroscopy

• Uses the chemical shift to measure the
  concentration of brain metabolites
• Main métabolites
• N-acetyl aspartate (NAA)
• Creatine (Cr)
• Choline (Cho)
• Myo-inositol (mI)

NAA
Cr
Cho
mI
56
Reference Book
57
Sources

• 1 Young and Freedman,  University Physics -
  10th Edition , Addison Wesley, 2000
• 2 McRobbie, Moore, Graves and Prince,  MRI
  From Picture to Proton , Cambridge University
  Press, 2003