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Magnetic Resonance Imaging Part 1 The Science Bit

Magnetic Resonance Imaging Part 1 The Science Bit. Lynn Graham DCR Msc Clinical Specialist in MRI. Particle Physics!. OUTLINE ( part 1). History + Local origins of MRI Fundamental Physics of MRI Tissue contrast + Versatility. History Lesson. Carl Fredrich Gauss. (1777-1855)

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Magnetic Resonance Imaging Part 1 The Science Bit

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  1. Magnetic Resonance ImagingPart 1The Science Bit Lynn Graham DCR Msc Clinical Specialist in MRI

  2. Particle Physics!

  3. OUTLINE ( part 1) • History + Local origins of MRI • Fundamental Physics of MRI • Tissue contrast + Versatility

  4. History Lesson • Carl Fredrich Gauss. (1777-1855) • German Physicist • Findings led to a knowlegdge of magnetism and its quantifiation • Gauss- unit of measurement of magnetism • Nikola Tesla (1856 –1943) • Serbian Electrical Engineer • Work in electromagnetic induction • Tesla –unit of measurement for Magnetic Field strength

  5. Born 11th July 1857 at Magheragall, Co Antrim Educated at RBAI, Belfast Graduated from Queens 1877 Appointed Professor @ St Johns College Cambridge 1903 Knighted 1909 Sir Joseph Larmor FRS.MA.DSCMathematician + Physicist 1857-1942

  6. The Larmor Equation Key to Nuclear Magnetic Resonance Clinical Magnetic Resonance Imaging

  7. NuclearMagnetic Resonance NUCLEAR ATOMIC SPIN : • +ve electric charge • Intrinsic spin/ Precession • nuclear magnetic moment

  8. PRECESSION

  9. + Nuclear Magnetic Resonance • MAGNETIC MOMENTS ALIGN WITH B0 No B0: random motion B0 : alignment

  10. Nuclear Magnetic Resonance Out of phase In phase

  11. “OUT OF” PHASE

  12. “IN” PHASE

  13. The Larmor Equation • L = Larmor frequency (MHz) • B0 = magnetic field (Tesla) • = gyromagnetic ratio Key to Nuclear Magnetic Resonance

  14. H H H H H H H H H H O C C C C Water Fatty acid chain NMR – CLINICAL MRI Fat + Water = 99% body tissue H+: ALIGNMENT + PRESCESSION

  15. Apply the Larmor equation NMR – CLINICAL MRI H1 @ 1 T :  = 42.58 MHz T-1 @ 1.5 T Larmor frequency = 63.87 MHz

  16. Electromagnetic Spectrum Precessional Frequency of H+

  17. Resonance + Excitation • Energy in the form of an RF pulse • Leads to misalignment with B0: antiparallel • Also leads to phase coherence. This is Excitation

  18. Resonance / Excitation B0 RF pulse

  19. Relaxation • Remove the RF and the spins will loose their energy. • Realign with B0 : “relax” • Loose phase coherance : “decay” Energy loss is variable

  20. Relaxation / Decay B0

  21. + Excitation + Relaxation = MR Signal Bo NMV 90 RF pulse Current induced in RF coil due to alternating B field = MRI signal

  22. Particle Physics : almost done!

  23. Image Formation X Z Y

  24. Spatial Localisation B0 + B0 Gradient B0 -

  25. Phase Slice Frequency Image Formation XX Y Z

  26. Pixel Mapping Each line of data is stored as the Image is built up gradually Fourier transform decodes data + forms the image Phase Frequency

  27. X Z Y

  28. Resolution Few pixels Short scan time Many pixels Long scan time

  29. The MR effect!

  30. T1 Fluid dark T2 Fluid bright Differing MR Images

  31. Relaxation : Free Induction Decay (FID) • The spins will loose their energy in two ways: Energy decays slowly Relaxing back to B0 T1 Recovery T2 decay Loose phase coherance

  32. T1 Recovery B0 Realign with

  33. T2 Decay In phase Out of phase Loose phase

  34. Brownian Motion • WATER : • Small , fast molecules • Fewer bumps • Slow energy loss • Long T1 + long T2 FAT : • Large , slow molecules • Lots of bumps • Fast energy loss • Short T1 + Short T2 Mr Blobby Vs Speedy Gonzalez!

  35. Typical T1 + T2 values for tissues (@1.5T)

  36. Pulse Sequences • Pre-set sequences of excitation, relaxation and signal organization that vary tissue contrast and image quality.

  37. Sag spine T1W Fluid dark Sag spine T2W Fluid bright T1 and T2 weightings

  38. Tissue differentiation • > 99% body tissues produce MR signal • Each tissue has unique properties - molecular structure - number of H+ ions - moving/stationary • Each tissue behaves differently in the MR environment  Unique MR signals from normal + abnormal tissues  Excellent disease diagnosis.

  39. Tissue contrast : Versatility of MRI T1 SE T2 SE T1 SE + gad GE brain

  40. Tissue contrast : Versatility of MRI Fat sat orbits FLAIR Black blood Angio

  41. Coming up Next !!! Clinical Applications of MRI • MRI Equipment • Safety issues of MRI • Advantages + Disadvantages of MR • MRI vs Other imaging modalities ( CT/ USS) • Clinical Images

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