5

• Magnetic Resonance Imaging (MRI)


  A noninvasive cross-sectional imaging modality that does not require any ionizing radiation
• MRI


  Uses the physical principle of magnetic resonance that was first described by Felix Bloch and Edward Purcell in 1946
• MR Imaging Principle


  1. Patient moved inside bore of magnet with strong static magnetic field (Bo)

  2. Oscillating electromagnetic radiofrequency fields and gradient fields used to acquire images

  3. Radiofrequency field (B1) produced by RF coil

  4. Fast-switching gradient fields (Gx, Gy, Gz) produced by coil systems
• Static Magnetic Field (Bo)


  • Used to prepare chemical compounds (mainly hydrogen protons in water and fat) for imaging
  • Strength measured in Tesla (T)
  • Stronger fields lead to higher signal-to-noise ratio and better image quality or faster scan time
  • Requires superconducting magnets cooled with liquid helium
• Radiofrequency Field (B1)


  • RF coils transmit and receive radiofrequency waves
  • Coils designed for different body parts and positioned close to anatomical structures
  • B1 field causes protons to alter alignment and enter higher energy state (excitation)
  • Protons return to lower energy state (relaxation) and reemit RF radiation at Larmor frequency
  • Returning RF waves picked up by RF coils, stored in k-space, and used to calculate MR image with FFT

  
• Gradient Magnetic Fields (Gx, Gy, Gz)


  • Used to localize MR signal from specific location in body
  • Responsible for acoustic noise in MRI scanner due to Lorentz forces
• MRI Pulse Sequences


  1. Programs containing timing and duration of RF pulses and magnetic gradients to produce image

  2. Two major groups: spin echo (SE) and gradient echo (GRE)

  3. SE has 90° excitation pulse and 180° refocusing pulses

  4. GRE has excitation pulse <90°, no 180° pulses, uses dephasing and rephasing gradients

  5. GRE sequences are faster and allow real-time imaging of moving organs
• Repetition time (TR)


  Time between two excitation pulses
• Echo time (TE)


  Time between excitation pulse and MR signal sampling when echo maximum occurs
• S = ρ(1-e^(-TR/T1))e^(-TE/T2)
• Gradient echo (GRE) sequence parameters


  • Flip angle of excitation pulse, TE, TR
• Magnetic Resonance Imaging (MRI)


  A noninvasive cross-sectional imaging modality that does not require any ionizing radiation
• MRI


  Uses the physical principle of magnetic resonance that was first described by Felix Bloch and Edward Purcell in 1946
• MR Imaging Principle


  1. Patient moved inside bore of magnet with strong static magnetic field (Bo)

  2. Oscillating electromagnetic radiofrequency fields and gradient fields used to acquire images

  3. Radiofrequency field (B1) produced by RF coil

  4. Fast-switching gradient fields (Gx, Gy, Gz) produced by coil systems
• Static Magnetic Field (Bo)


  • Used to prepare chemical compounds (mainly hydrogen protons in water and fat) for imaging
  • Strength measured in Tesla (T)
  • Stronger fields lead to higher signal-to-noise ratio and better image quality or faster scan time
  • Requires superconducting magnets cooled with liquid helium
• Radiofrequency Field (B1)


  • RF coils transmit and receive radiofrequency waves
  • Coils designed for different body parts and positioned close to anatomical structures
  • B1 field causes protons to alter alignment and enter higher energy state (excitation)
  • Protons return to lower energy state (relaxation) and reemit RF radiation at Larmor frequency
  • Returning RF waves picked up by RF coils, stored in k-space, and used to calculate MR image with FFT

  
• Gradient Magnetic Fields (Gx, Gy, Gz)


  • Used to localize MR signal from specific location in body
  • Responsible for acoustic noise in MRI scanner due to Lorentz forces
• MRI Pulse Sequences


  1. Programs containing timing and duration of RF pulses and magnetic gradients to produce image

  2. Two major groups: spin echo (SE) and gradient echo (GRE)

  3. SE has 90° excitation pulse and 180° refocusing pulses

  4. GRE has excitation pulse <90°, no 180° pulses, uses dephasing and rephasing gradients

  5. GRE sequences are faster and allow real-time imaging of moving organs

  
• Repetition time (TR)


  Time between two excitation pulses
• Echo time (TE)


  Time between excitation pulse and MR signal sampling when echo maximum occurs

  
• S = ρ.(1 - e^(-TR/T1)) e^(-TR/T2)
• Gradient echo (GRE) sequence parameters


  • Flip angle of excitation pulse, TE, TR