Magnetic Resonance Imaging: Physical Principles - PowerPoint PPT Presentation

Magnetic Resonance Imaging: Physical Principles

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Magnetic Resonance Imaging: Physical Principles Richard Watts, D.Phil. Weill Medical College of Cornell University, New York, USA Physics of MRI, Lecture 1 Nuclear . – PowerPoint PPT presentation

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Title: Magnetic Resonance Imaging: Physical Principles

• Richard Watts, D.Phil.
• Weill Medical College of Cornell University,
• New York, USA

• Nuclear Magnetic Resonance
• Nuclear spins
• Spin precession and the Larmor equation
• Static B0
• RF excitation
• RF detection
• Spatial Encoding
• Slice selective excitation
• Frequency encoding
• Phase encoding
• Image reconstruction

• Fourier Transforms
• Continuous Fourier Transform
• Discrete Fourier Transform
• Fourier properties
• k-space representation in MRI

• 2D Pulse Sequences
• Spin echo
• Gradient echo
• Echo-Planar Imaging

• Medical Applications
• Contrast in MRI
• Bloch equation
• Tissue properties
• T1 weighted imaging
• T2 weighted imaging
• Spin density imaging
• Examples

• 3D Imaging
• Magnetization Preparation
• Chemical Shift and FatSat
• Water/Fat Separation
• Inversion Recovery and T1 Measurement
• Double Inversion Recovery

• Imaging blood flow
• Time-of-Flight
• Phase Contrast
• Fast Imaging
• Spoiling
• Spin Saturation
• Ernst angle, signal and contrast levels

• Spatial encoding
• Its all done with gradient fields
• Spatially varying Larmor precession frequency
• Slice select
• Frequency encode
• Phase encode

• Tissue contrast
• Different magnetic properties of tissues
• Different relaxation times
• T1 Longitidunal relaxation constant
• T2 Transverse relaxation constant
• Scan Parameters
• TE Echo time
• TR Repetition time

• Instead of exciting a thin slice, excite a thick
  slab and phase encode along both ky and kz
• Greater signal because more spins contribute to
  each acquisition
• Easier to excite a uniform, thick slab than very
  thin slices
• No gaps between slices
• Motion during acquisition can be a problem

• Contrast-enhanced MRA of the carotid arteries.
  Acquisition time 25s.
• 160x128x32 acquisition (kxkykz).
• 3D volume may be reformatted in
  post-processing. Volume-of-interest rendering
  allows a feature to be isolated.
• More on contrast-enhanced MRA later

• Precession frequency depends on the chemical
  environment e.g. Hydrogen in water and hydrogen
  in fat have a ?f fwater ffat 220 Hz
• Single voxel spectroscopy excites a small (cm3)
  volume and measures signal as f(t). Different
  frequencies (chemicals) can be separated using
  Fourier transform
• Concentrations of chemicals other than water and
  fat tend to be very low, so signal strength is a
  problem
• Creatine, lactate and NAA are useful indicators
  of tumor types

• Use a small bandwidth (long, 20ms) 90º pulse to
  excite only protons within fat
• Destroy the transverse magnetization by dephasing
  (spoiling)
• Not spatially selective no gradients
• Useful for reducing the fat signal in abdominal
  and breast imaging
• Cost extra acquisition time

• After 180 inversion pulse the longitudinal
  relaxation Mz grows back towards its equilibrium
  value, M0
• Mz 0 when e-t/T1 ½
• T T1.ln2
• Select the inversion time, TI to null out the
  required signal

• Image using different inversion times (TI) until
  the signal is minimized
• Signal ? Mz(TI)

• Measure signal as a function of echo time, TE
• Spin echo sequence gives T2 (irreversible)
• Gradient echo sequence gives T2 (reversible and
  irreversible)

• With two inversion pulses at appropriate times,
  two tissue types can be nulled out
• e.g. CSF and fat in the brain

• Fast imaging TRltT1, T2
• Spins do not fully recover after each repetition
• Magnetization producing MR signal decreases with
  the number of pulses until equilibrium reached
• Spins flowing into the slice have not seen
  previous excitations, so have greater signal

• With a given gradient field, stationary spins
  precess at a constant rate
• Moving spins experience a change in precession
  frequency, depending on their velocity v, and the
  gradient strength, G.
• This causes a velocity-dependent phase shift, ?f

• For constant flow velocity,
• With an x-Gradient Gx, the field strength is
• The phase shift is linear with velocity

• Add bipolar gradient in the flow direction after
  RF excitation
• Stationary spins are not dephased
• Repeat acquisition with reversed gradients
• Complex subtraction of images gives flow image

• Fast imaging TRltT1,T2
• Spins do not have time to relax between one RF
  pulse and the next
• Transverse magnetization Spin memory produces
  artifacts in images. Many RF pulses contribute
  to each acquisition

• Longitudinal magnetization Spins get beaten
  down so that after each pulse there is less
  magnetization available.
• Equilibrium between pulses and relaxation
• Optimum angle to maximize steady-state signal
  Ernst angle

• Destroy transverse magnetization so it doesnt
  contribute to later echoes
• Dephase the spins with large spoiling gradients
  after each acquisition
• Assume perfect spoiling

• At equilibrium full magnetization is available
• Each pulse and spoiling reduces the magnetization
• Some signal regrowth due to T1 decay