Magnetic Resonance Imaging MRI

1
Magnetic Resonance Imaging MRI

• Woo Ka Ming
• Yip Kai Hou

2
Magnetic Resonance Imaging MRI
3
Outline

• Medical Imaging Techniques
• MRI Principles
• 2.1 Fundamental information
• 2.2 Manipulating magnetization
• 2.3 Relaxation times
• 2.4 MR signal detection
• 2.5 Structure of MRI machine
• About MRI
• 3.1 History of MRI developments
• 3.2 Applications
• 3.3 Future

4
1. Medical Imaging Techniques

• X-ray imaging
• Digital subtraction angiography
• Computed tomography CT
• Positron computed tomography
• Diagnostic ultrasound
• Nuclear Magnetic Resonance Imaging
• NMR or MRI

5
Advantages of MRI over others

• Non-invasive
• High resolution
• Cross-sectional images available
• Yielding images that depend on physiology or
  functional properties
• Allow direct information about metabolic
  processes in vivo
• Providing real-time images

6
MRI images
7
2. MRI Principles

• 2.1 Fundamental information
• 2.2 Manipulating magnetization
• 2.3 Relaxation times
• 2.4 MR signal detection
• 2.5 Structure of MRI machine

8
2.1 Fundamental information Spin angular momentum

• Classically, any circulating charged particle
  possesses a magnetic moment
• In QM, all quantum particles have intrinsic spin
  property and so have the spin angular momentum
  
• In case of proton of the H atom ?p 2.675 108
  rad T-1 s-1
• which is termed the gyromagnetic ratio

9
2.1 Fundamental informationBasic concepts

• Main character Protons
• Hydrogen nuclei in water molecules
• Hydrogen nuclei in organic substances
• Proton is a fermion. Its a spin ½ particle.
• When placed inside a magnetic field, a proton
  would experience a torque, as a result, it has an
  additional magnetic potential energy.
• Splitting of energy level by magnetic field
  Zeeman effect
• Magnetization Magnetic dipole moment per unit
  volume

10
2.2 Manipulating magnetizationApplication of
magnetic field B0

• In the first stage of MRI
• a uniform B0 is applied along z-axis (0.07 T to
  1.5 T)
• From classical EM, the spin magnetic moment will
  experience a torque
• Subsequent motion
• In 2-dimension similar to SHM
• In 3-dimension precess about z-axis with Larmor
  frequency
• At the same time, splitting of energy levels of
  protons occurs
• with energy difference
• Similar to Zeeman effect for the electron in a H
  atom.

11
2.2 Manipulating magnetizationApplication of
magnetic field B0
Precessing proton in B0?
?
Splitting of energy states?
12
2.2 Manipulating magnetizationArise of
Magnetization Mz

• The precession will not last too long as some of
  the magnetic energy transfers to its surrounding
  and finally reaches the equilibrium state.
• The population of protons is described by
  Boltzmann energy distribution
• On average, at room temp and B0 0.1 T, there
  are 7 protons lined up with the B0 among 107
  protons.
• As a result, a net magnetization Mz occurs
  parallel B0

13
2.2 Manipulating magnetizationArise of
Magnetization Mxy

• In the second stage of MRI
• An oscillating pulse B1 is applied in x-direction
  for a while
• Thus the magnetization flips onto xy-plane and
  produce a rotating net magnetization Mxy.
• B1 also known as 90 pulse.

14
2.2 Manipulating magnetizationPictures of
magnetization
?Spin distribution
Magnetization?
Fluctuating magnetization vector ?
15
2.2 Manipulating magnetizationMagnetic Resonance

• B1 has to give the right amount of energy to the
  protons
• So, the excitation will take place only for a
  very definite frequency f of B1
• This is called magnetic resonance.

16
2.2 Manipulating magnetizationHow to map?

• Main idea
• To map these spins
• which depend on the physical and chemical
  properties of its surrounding
• Methods
• Applying different combinations of magnetic field
• Monitoring the respective characteristics resulted

17
2.3 Relaxation times T1 and T2

• Just after B1 pulse
• Mz regrows slightly and finally attains its
  original eqm state.
• T1
• Spin-Lattice Relaxation
• Mxy dephases slightly and finally no more
  magnetization on x-y plane.
• T2
• Spin-Spin Relaxation

18
2.3 Relaxation times T1 and T2

• Bloch equations

19
2.3 Relaxation timesFree Induction Decay

• Due to presence of intrinsic inhomogeneities
• - arise from magnetic field generator itself
• - and varied from person to person chemical
  shift
• - these also contributes to the dephase of Mxy
• Actual decay of T2 is even faster

20
2.4 MR signal detectionTR and TE

• Only the rotating Mxy produce emf signal by means
  of Faraday EM induction.
• Repetition Time TR
• time interval between two successive excitations
  of the same slice.
• Echo Time TE
• time interval between application of the
  excitation pulse and collection of the MR signal.

21
2.4 MR signal detectionContrast of image

• T1 weighted image
• Short TR, strong T1 weighting
• Long TR, low T1 weighting
• Tissue with short T1 appears bright
• Tissue with long T1 appears dark
• T2 weighted image
• Short TE, low T2 weighting
• Long TE, strong T2 weighting
• Tissue with short T2 appears dark
• Tissue with long T2 appears bright

22
2.4 MR signal detectionSpatial Encoding
Originally unknown


Result
_______________________
So what is A?Ans Fourier transform of
Detected MR signal
22
23
2.4 MR signal detectionSpatial Encoding

• 1. Phase Encoding
• Switch on a magnetic field gradient of amplitude
  Gy in y-direction just after the spins have been
  excited and precess on xy-plane
• - Phase Shift of spins relative to each others
• - phase angle

24
2.4 MR signal detectionSpatial Encoding

• 2. Frequency Encoding
• At time ty, the gradient Gy is turned off and
  then an orthogonal gradient Gx is applied for a
  time tx
• Thus the spins precess at frequency
• 3. What do we know now?
• The exact values of both phase and frequency of
  precession at each point (x, y) on the tissue
• In Spin-Wrap imaging, the amplitude of gradient
  is raised incrementally and thus forms N1 N2
  picture elements.

25
2.4 MR signal detectionSpatial Encoding

• 4. Fourier Transform
• - Through Fourier Transform of the MR signal
• - we can know the amplitudes of different
  frequencies and phases in the k-space, which in
  turns proportional to the brightness of the
  picture elements.

26
2.4 MR signal detectionSpatial Encoding
27
2.5 Structure of MRI machine
28
2.5 Structure of MRI machine
Radio Frequency Coils
Patient
Patient table
Gradient Coils
Magnet
Scanner
29
3. About MRI

• 3.1 History of MRI developments
• 3.2 Applications
• 3.3 Future

30
3.1 History of MRI developments

• 1938 Nuclear magnetic resonance by I.I. Rabi
• Mid-1940s First detection of NMR in bulk matter
• 1950s Discovery of chemical shift and spin-spin
  coupling
• 1960s Development of pulse Fourier-transform
  NMR
• 1973 First NMR image by Paul Lauterbur, who
  shared the Nobel Prize in medicine in 2003
• 1975 2D NMR by Ernst, which earned him the 1991
  Nobel Prize in chemistry
• 1977 First study performed on human
• 1980s k-space formalism

31
3.1 History of MRI developmentsPaul Lauterburs
images
? Oil in peanuts
Cross-section of a mouse ? (shadows are lungs)
32
3.2 Applications

• Clinical diagnosis
• Physiological research
• Petrophysical analysis
• Ceramic manufacturing
• Food processing
• And so on

33
3.3 Future

• More powerful computer
• Electron-nuclear double resonance
• Electron-nuclear Overhauser effect
• Emergence of new superconducting materials
• Up to 10 Tesla
• Increase signal-to-noise ratio
• Improve spatial resolution

34
Summary

• Nuclear magnetic resonance
• Flipping of spinning nuclei by B1 of suitable
  frequencies
• Apply combinations of fields detect the
  resultant signals
• How Fourier?
• Distribution funct. from a signal
  funct.
• Method
• Gy? phase
• Gx? frequency

Phase knows y Freq. knows x
Signal
Lock a position (x, y)
Amplitude knows
35
Thanks!
36

• Medical imaging techniques
• P.R. Moran, R.J. Nickles and J.A. Zagzebski, The
  Physics of Medical Imaging, Physics Today (July
  1983) pp.36-42.
• Electron-nuclear Overhauser effect
• Overhauser, Albert W. (1953-10-15). "Polarization
  of Nuclei in Metals" Phys. Rev. 92 (2) 411-5.

37
Is it dangerous?

• Remain mysterious.
• Radio frequencies is used.
• Its not energetic enough to harm any molecule in
  human body.
• Typically, x-ray are 1011 times more energetic
  can break up molecules.
• RF waves of high intensities may cause brief
  local heating, but less than 0.5K.
• For B fields below 2T, no biological hazards
  appear to present.
• Cell physiology, such as enzyme reactivity, may
  not be affected.

38
Other methods?

• Yes
• Irradiate the nuclei with rf energy of const.
  energy, scan it with a constant B.
• Given up
• Pulsed rf excitation followed by detection of
  resultant free-precession signal.
• Still in use
• Etc

39
Who are banned from using MRI?

• Patients with metal instruments inside his body
• E.g. heart pacemakers, ferromagnetic prostheses,
  clips, etc.
• Someone who is allergy to magnetic field may feel
  dizzy.
• Mobile phones, watches should be banned from MRI.