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MRI physics

Basic concepts of MRI physics for doctors

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MRI physics

  1. 1. MRI Physics
  2. 2. Quantum Mechanics Davison, Germer and Born 1927 Heisenberg 1927 Schroedinger 1926 De Broglie 1925 Pauli 1924 Compton 1922 Bohr 1913 Einstein 1905 Planck 1901 Researcher Year
  3. 3. MRI <ul><li>M agnetic R esonance I maging </li></ul>
  4. 4. M is for Magnetic <ul><li>Atomic structure: </li></ul><ul><ul><li>Proton </li></ul></ul><ul><ul><li>Neutron </li></ul></ul><ul><ul><li>Electron </li></ul></ul>
  5. 5. M is for Magnetic <ul><li>Motion within the atom: </li></ul><ul><li>SPIN </li></ul>
  6. 7. M is for Magnetic <ul><li>MR active nuclei: </li></ul><ul><ul><li>H1 </li></ul></ul><ul><ul><li>C13 </li></ul></ul><ul><ul><li>F19 </li></ul></ul><ul><ul><li>P31 </li></ul></ul><ul><ul><li>N15 </li></ul></ul><ul><ul><li>O17 </li></ul></ul>
  7. 8. M is for Magnetic <ul><li>Why H1? </li></ul><ul><ul><li>1000 Billion Billion Atom in adult </li></ul></ul><ul><ul><li>Solitary Proton gives it a large magnetic moment </li></ul></ul>
  8. 9. M is for Magnetic <ul><li>What happen when placed in external magnetic field? ( Quantum Theory ) </li></ul>NNSS NSNS High Energy Low Energy Spin Down Spin Up
  9. 12. M is for Magnetic <ul><li>&quot; NMV &quot; </li></ul><ul><li>Net Magnetization Vector </li></ul>
  10. 13. M is for Magnetic <ul><li>Precession </li></ul>
  11. 15. Larmor Equation <ul><li>Frequency α Magnetic Field </li></ul><ul><li>ω α β </li></ul>
  12. 16. M is for Magnetic <ul><li>W0 = B0 * Gamma ( Larmor Equation ) </li></ul><ul><li>Gamma &quot; Gyro magnetic Ratio “ </li></ul><ul><li>Omega “Frequency” </li></ul><ul><li>Gamma = 42.6 in 1T </li></ul>
  13. 17. M is for Magnetic Incoherent magnetic moment of H is At different place in Precessional Path Coherent magnetic moment of H is Placed in the same place in Precessional Path
  14. 18. M <ul><li>Atomic structure </li></ul><ul><li>SPIN </li></ul><ul><li>MR active nuclei H1 </li></ul><ul><li>What happen when placed in external magnetic field ( Spin Up and Spin Down ) </li></ul><ul><li>NMV </li></ul><ul><li>Precession </li></ul><ul><li>Larmor Equation </li></ul><ul><li>Coherent </li></ul>
  15. 19. R is for Resonance <ul><li>Def: </li></ul><ul><li>Energy transition that occur when object is subjected to frequency the same as its own </li></ul>
  16. 20. R is for Resonance <ul><li>(Examples) </li></ul>
  17. 22. R is for Resonance <ul><li>Here; </li></ul><ul><li>Radio Frequency &quot; RF &quot; </li></ul><ul><li>Same Frequency of H nuclei </li></ul><ul><li>At 90 degree to B0 </li></ul>
  18. 23. R is for Resonance <ul><li>2 things happen at Resonance: </li></ul><ul><li>1- Energy Absorption </li></ul><ul><li>Increase number of High energy Spin Up nuclei </li></ul><ul><li>2- Phase Coherence </li></ul><ul><li>NMV precesses in transverse plane at Larmor Frequency </li></ul>
  19. 25. R is for Resonance <ul><li>MR Signal </li></ul><ul><li>NMV rotates around transverse plane </li></ul><ul><li>It passes across Receiver Coil </li></ul><ul><li>Inducing voltage in it </li></ul>
  20. 26. R is for Resonance <ul><li>RF Removed  Signal decreased  Amplitude of MR Signal decreased </li></ul><ul><li>Free Induction Decay &quot;FID&quot;: </li></ul><ul><ul><li>F ree (No RF Pulse) </li></ul></ul><ul><ul><li>ID (because of D ecay of I nduced signal in Receiver Coil) </li></ul></ul>
  21. 27. R <ul><li>Resonance </li></ul><ul><li>Radio Frequency &quot; RF ” </li></ul><ul><li>2 things happen at Resonance </li></ul><ul><ul><li>Energy Absorption </li></ul></ul><ul><ul><li>Phase Coherence </li></ul></ul><ul><li>MR Signal </li></ul><ul><li>Free Induction Decay &quot;FID&quot; </li></ul>
  22. 28. I is for Imaging <ul><li>Areas of High Signal </li></ul><ul><li>Areas of Low Signal </li></ul><ul><li>Areas of Intermediate Signal </li></ul>
  23. 29. I is for Imaging <ul><li>NMV can be separated in to </li></ul><ul><li>Individual Vectors of tissue present in the patient </li></ul><ul><li>Such as Fat , CSF & Muscle </li></ul>
  24. 30. I is for Imaging Small transverse component of magnetization Large transverse component of magnetization Grey Black White Intermediate Low Signal High Signal
  25. 31. I is for Imaging <ul><li>Gradient Magnets </li></ul><ul><li>Used to vary magnetic field in known manner </li></ul><ul><li>Each point has slightly different rate of precession & Larmor Frequency. </li></ul><ul><li>Variety of signal released by Protons returning to z-plane can used to determine the composition of exact location of each point. </li></ul>
  26. 32. I is for Imaging <ul><li>Conclusion of function of &quot; Gradient Function &quot; </li></ul><ul><ul><li>Slice selection </li></ul></ul><ul><ul><li>Frequency encoding </li></ul></ul><ul><ul><li>Phase encoding </li></ul></ul><ul><li>X, Y, F (Omega) & B </li></ul><ul><li>256*256 </li></ul><ul><li>512*512 </li></ul>
  27. 33. Contrast Mechanisms Small transverse component of magnetization Large transverse component of magnetization Grey Black White Intermediate Low Signal High Signal
  28. 34. Contrast Mechanisms ( Relaxation Process) <ul><li>Relaxation Process </li></ul><ul><li>after removal of RF pulse </li></ul><ul><li>Signal induced in Receiver Coil decreased </li></ul><ul><li>Why? </li></ul>
  29. 35. Contrast Mechanisms ( Relaxation Process) 2-Nuclei loss Precessional coherence or dephase and NMV decay in the transverse plane this process called &quot; T2 Decay &quot; 1-NMV recovers and realign to B0 this process called &quot; T1 Recovery &quot;
  30. 36. Contrast Mechanisms ( T1 Recovery) <ul><li>T1 time is </li></ul><ul><li>an intrinsic contrast parameter that inherent to tissue being imaged </li></ul>
  31. 37. Contrast Mechanisms ( T2 Decay) <ul><li>T2 Decay is </li></ul><ul><li>an intrinsic contrast parameter and is inherent to the tissue being imaged </li></ul>
  32. 38. Contrast Mechanisms ( Definitions ) <ul><li>Repetition Time &quot;TR&quot; </li></ul><ul><li>Time from application of one RF pulse </li></ul><ul><li>To the application of the next </li></ul><ul><li>(it affects the length of relaxation period </li></ul><ul><li>after application of one RF excitation pulse </li></ul><ul><li>to the beginning of the next) </li></ul>
  33. 39. Contrast Mechanisms ( Definitions ) <ul><li>Echo Time &quot;TE&quot; </li></ul><ul><li>Time between RF excitation pulse and </li></ul><ul><li>collection of signal </li></ul><ul><li>(it affects the length of relaxation period </li></ul><ul><li>after removal of RF excitation pulse </li></ul><ul><li>and the peak of signal received in receiver coil) </li></ul>
  34. 40. Contrast Mechanisms ( Definitions ) <ul><li>Flip Angle </li></ul><ul><li>Angle throw which the NMV moved as result of a RF excitation pulse </li></ul>
  35. 41. Contrast Mechanisms ( Parameters ) <ul><li>Image contrast controlled by: </li></ul><ul><li>1- Extrinsic Contrast parameters : </li></ul><ul><li>TR , TE & Flip Angle </li></ul><ul><li>2- Intrinsic Contrast parameters : </li></ul><ul><li>T1 Recovery , T2 Decay , Proton Density , Flow & Apparent Diffusion Coefficient </li></ul>
  36. 42. Contrast Mechanisms ( T1 Recovery) <ul><li>T1 Recovery </li></ul><ul><li>Caused by exchange of energy from </li></ul><ul><li>nuclei to their surrounding environment or lattice </li></ul><ul><li>&quot; Spin Lattice Energy Transfer &quot; </li></ul><ul><li>and realign in B0 </li></ul><ul><li>this occur in exponential process </li></ul><ul><li>at different rates in different tissue </li></ul><ul><li>NB: Molecules are constantly in motion; Rotational and Transitional </li></ul>
  37. 43. Contrast Mechanisms ( T1 Recovery) inefficient at receiving energy T1 is longer i.e. nuclei take allot longer to dispose energy to surrounding water tissue WATER absorb energy quickly T1 is very short i.e. nuclei dispose their energy to surrounding fat tissue and return to B0 in very short time FAT T1 in Water T1 in Fat
  38. 47. Contrast Mechanisms ( T1 Recovery) <ul><li>Short TR  T1 contrast </li></ul><ul><li>( T1 Weighted ) </li></ul><ul><li>TR 300-600 ms </li></ul><ul><li>TE 10-30 ms </li></ul>
  39. 48. Contrast Mechanisms ( T2 Decay) <ul><li>T2 Decay </li></ul><ul><li>Caused by exchange of energy from one nucleus to another </li></ul><ul><li>&quot; Spin-spin Energy Transfer “ </li></ul><ul><li>as result of intrinsic magnetic fields of nuclei interlacing with each other </li></ul><ul><li>this energy exchange  loss of coherence or dephasing </li></ul><ul><li>and as result NMV decay in transverse plane </li></ul><ul><li>T2 is exponential process </li></ul><ul><li>occur at different rates in different tissues </li></ul>
  40. 49. Contrast Mechanisms ( T2 Decay) <ul><li>Fat much better at energy exchange than Water </li></ul><ul><li>Because this T2 depend on: </li></ul><ul><li>1-How closely molecular motion of atoms matches Larmor Frequency </li></ul><ul><li>2-Proximity of other spins </li></ul><ul><li>So; </li></ul><ul><li>Fat's T2 time is very short compared to water </li></ul>FAT WATER
  41. 52. Contrast Mechanisms ( T2 Decay) <ul><li>Long TE  T2 contrast </li></ul><ul><li>(T2 Weighted) </li></ul><ul><li>TR 2000 ms </li></ul><ul><li>TE 70 ms </li></ul>
  42. 53. Contrast Mechanisms ( Proton Density) <ul><li>Proton Density </li></ul><ul><li>Long TR  Proton density </li></ul><ul><li>TR 2000 ms </li></ul><ul><li>TE 10-30 ms </li></ul>
  43. 54. Contrast Mechanisms ( Contrast Media) <ul><li>Contrast Media </li></ul><ul><li>as Gadolinium </li></ul><ul><li>local magnetic field fluctuation occur near Larmor frequency </li></ul><ul><li>T1 Relaxation times of nearby protons are reduced </li></ul><ul><li>So they appear brighter in T1 weighted Image </li></ul><ul><li>What else???!!! </li></ul>
  44. 55. References <ul><li>MRI at a Glance book </li></ul><ul><li>Armstrong Diagnostic Imaging book </li></ul><ul><li>Grainger & Allison's Diagnostic Radiology book </li></ul><ul><li>Imaging for idiots website </li></ul><ul><ul><li>http :// cal . man . ac . uk / student_projects / 2000 / mmmr7gjw / technique . htm </li></ul></ul><ul><li>Answers.com website </li></ul><ul><ul><li>www.answers.com </li></ul></ul><ul><li>Wikipedia website </li></ul><ul><ul><li>www.wikipedia.com </li></ul></ul><ul><li>Google search engine </li></ul><ul><ul><li>www.google.com </li></ul></ul>
  45. 56. me <ul><li>Dr. Mohamad Mostafa Alasmar </li></ul><ul><ul><li>House Officer, KasrAlainy Faculty of medicine. </li></ul></ul><ul><li>E-mails </li></ul><ul><ul><li>[email_address] </li></ul></ul><ul><li>Mobile </li></ul><ul><ul><li>0103502987 </li></ul></ul><ul><li>Website </li></ul><ul><ul><li>www.alasmar.info </li></ul></ul>

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