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Mri basics

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Mri basics

  1. 1. MRI Made Simple HH Schild
  2. 2. History <ul><li>Felix Bloch & Edward Purcell </li></ul><ul><li>Discovered magnetic resonance phenomenon in 1946 </li></ul><ul><li>Received Nobel Prize in 1952 </li></ul><ul><li>Used for chemical and physical molecule analysis </li></ul><ul><li>Raymond Damadian in 1971 showed magnetic relaxation of tumor and normal tissue differed </li></ul>
  3. 3. MRI <ul><li>Patient is put in a magnet </li></ul><ul><li>RF signal is sent </li></ul><ul><li>RF signal is switched off </li></ul><ul><li>Patient emits a signal which is used for </li></ul><ul><li>Reconstruction of image </li></ul>
  4. 4. Proton are little magnet Proton spin around its axis like a planet, has positive changer and electrical field around it, and moving electrical field has a magnetic field like a bar magnet
  5. 5. Proton alignment <ul><li>Protons align to external magnetic field parallel or anti-parallel. </li></ul><ul><li>Both have different energy level </li></ul><ul><li>Like walking on feet require less energy than on hand, parallel state is preferred to anti-parallel </li></ul><ul><li>Difference is 10,000,000/10,000,007 </li></ul>
  6. 6. Precession Proton moves in like spinning top In two axis wobbling motion called Precession, depends on magnetic field strength
  7. 7. Precession frequency <ul><li>Precession frequency is dependent on strength of external magnet field </li></ul><ul><li>It is determined by Larmor Equation </li></ul><ul><li>Wo = yBo </li></ul><ul><ul><li>Wo is precession frequency in Hz or MHz </li></ul></ul><ul><ul><li>Bo in magnetic field strength in Tesla </li></ul></ul><ul><ul><li>y is gyro-magnetic ratio, for proton is 42.5MHz/Tesla </li></ul></ul><ul><ul><li>Stronger the external magnetic field higher the precession frequency </li></ul></ul>
  8. 8. Coordinate system Representation of magnetic force in Z axis, Proton vector as red arrow
  9. 9. Net magnetic force Proton pointing in opposite direction cancels each others magnetic effect in respective direction. 9 proton align up and 5 down, resulting in 4 proton up force
  10. 10. Net magnetic force <ul><li>As there are more protons aligned parallel to the external magnetic filed, there is a net magnetic movement aligned with or longitudinal to the external magnetic field </li></ul>
  11. 11. Human magnetic vector <ul><li>In a strong external magnetic field a new magnetic vector is induced in the patient, who becomes a magnet himself. </li></ul><ul><li>This new magnetic vector is aligned with the external magnetic field </li></ul>
  12. 12. Measuring magnetization Magnetization along an external magnetic field cannot be measured For this a magnetization transverse to the external magnetic field is necessary
  13. 13. RF pulse frequency After protons aligned along the external magnetic force, RF pulse is send for energy exchange. Which is only possible if RF pulse has same frequency as protons precession frequency
  14. 14. Effect of RF pulse <ul><li>The RF pulse exchange energy with the protons (a), and some of them are lifted to a higher level of energy, pointing down (b), in effect the magnetization along the z-axis decreases, as the protons which point down neutralize the same number of proton pointing up </li></ul>
  15. 15. Whip like action of RF pulse
  16. 16. Effect of RF pulse <ul><li>It lifts some protons to higher energy ( they point down), and it also causes the protons to precess in step, in phase. </li></ul><ul><li>The former result in decreasing the magnetization along the Z-axis, the longitudinal magnetization </li></ul><ul><li>The later establishes a new magnetization in the x-y-phase (->), a new transversal magnetization, which moves around with the precessing protons </li></ul><ul><li>A ship, passengers being distributed randomly all over the deck, the ship in a normal position </li></ul><ul><li>When all passengers walk in equal step around the railing </li></ul><ul><li>The ship is leaning towards the side where the people are, a new force is established, and become visible </li></ul>
  17. 17. RF pulse on proton <ul><li>In a strong external magnetic field a new magnetic vector along the external field is established </li></ul><ul><li>Sending in an RF pulse causes a new transveral magnetization while longitudinal magnetization decreases </li></ul><ul><li>Depending on the RF pulse , longitudinal magnetization may even totally disappear </li></ul>
  18. 18. Result of transverse magnetization <ul><li>The new transversal magnetization moves around with the precessing protons </li></ul><ul><li>Thus for an external observer, transversal magnetization constantly changes its direction, and can induce a signal in an antenna </li></ul>
  19. 19. Longitudinal relaxation <ul><li>After the RF pulse is switched off, protons go back from their higher to the lower state of energy. </li></ul><ul><li>Longitudinal magnetization increases and grows back to its original value </li></ul><ul><li>Energy of RF pulse is handed over to the surrounding lattice </li></ul><ul><li>This process is call longitudinal relaxation or spin lattice relaxation </li></ul>
  20. 20. T1 Curve Recovery of longitudinal relaxation vs. time curve
  21. 21. Transverse relaxation After the RF pulse is switched off, protons lose phase coherence, they get out of step.
  22. 22. T2 Curve Transverse magnetization vs. time after RF pulse is switched off
  23. 23. T1 > T2 <ul><li>Coupling of a T1 and T2- curve resembles a mountain with a slope </li></ul><ul><li>T1- 300-2000 msec. </li></ul><ul><li>T2- 30-150 msec. </li></ul><ul><li>It takes longer to climb a mountain than to slide or jump down. </li></ul>
  24. 24. Relaxation time <ul><li>It is difficult to pinpoint the end of the longitudinal and transverse relaxation exactly. Thus T1 and T2 were not defined as the time when relaxation is completed. </li></ul><ul><li>Instead T1 was defined as the time when about 63% of the original longitudinal magnetization is reached. </li></ul><ul><li>T2 is the time when transverse magnetization decreases to 37% of the original value. </li></ul><ul><li>These percentages are derived equation 1-1/3 = 63%, 1/e = 37% </li></ul><ul><li>1/T1 is also called longitudinal relaxation rate </li></ul><ul><li>1/T2 transverse relaxation rate </li></ul>
  25. 25. Liquids have long T1 & T2
  26. 26. Fat has short T1 & T2
  27. 27. What influences T1/T2? <ul><li>T1 is longer than T2 </li></ul><ul><li>T1 varies with the magnetic field strength </li></ul><ul><li>It is longer in strong magnetic field </li></ul><ul><li>Water has a long T1 and fat has a short T1 </li></ul><ul><li>T2 of water is longer than the T2 of impure liquids containing larger molecules. </li></ul>
  28. 28. 90 degree RF pulse <ul><li>If after the RF pulse, the number of protons on the higher energy level equals the number of protons on the lower energy level, longitudinal magnetization has disappeared, and there is only transverse magnetization due to phase coherence. </li></ul><ul><li>The magnetic vector seems to have been tilted 90degree to the side </li></ul><ul><li>The corresponding RF pulse is thus also called a 90degree pulse </li></ul>
  29. 29. 90degree RF pulse <ul><li>Before RF pulse </li></ul><ul><li>After 90d RF pulse, longitudinal magnetization decreases, proton also precess in phase, causes new transversal magnetization </li></ul><ul><li>After the RF pulse is switched off longitudinal magnetization increases, and transversal magnetization disappears, </li></ul>
  30. 30. Magnetizing vector <ul><li>Before RF pulse there is only longitudinal magnetization </li></ul><ul><li>After the 90dgr RF pulse there is only transversal magnetization and this is spinning around </li></ul><ul><li>With time after the removal of RF pulse the transversal magnetization decreases and longitudinal magnetization increases in spiral motion </li></ul>
  31. 31. Free induction decay <ul><li>The sum magnetic vector constantly, its direction and magnitude, while it performs its spiraling motion </li></ul><ul><li>The sum vector induces an electrical current in an antenna, the MR signal. </li></ul><ul><li>This is greatest immediately after RF signal is switched off and then decreases </li></ul><ul><li>The signal intensity reduces but frequency is constant </li></ul><ul><li>This is called free induction decay (FID) signal </li></ul>
  32. 32. Long and Short TR <ul><li>A and B are two tissue with different relaxation times. Frame 0 shows the situation before, frame 1 immediately after a 90degree pulse. When we wait for a long time (TR long) the longitudinal magnetization of both tissue will have totally recovered (frame 5). A second 90d pulse after this time results in the same amount of transversal magnetization (frame 6) for both tissue. </li></ul><ul><li>When we do not wait as long as in a, but send a second RF pulse after a shorter time (TR short), longitudinal magnetization of tissue B which has the longer T1, has not recovered as much as that of tissue A with shorter T1. The transversal magnetization of the two tissue after the second RF pulse will then be different (frame 5). Thus by changing the time between successive RF pulse we can influence and modify magnetization and the signal intensity of tissue. </li></ul>
  33. 33. Tissue signal with short TR <ul><li>Brain has a shorter longitudinal relaxation time than CSF. With a short TR the signal intensities of brain CSF differ more than after a long TR </li></ul>
  34. 34. T2 - Weighted image <ul><li>After the RF pulse is switched off, the protons dephase (a-c). </li></ul><ul><li>The 180deg pulse causes them to precess in the opposite direction and so they rephase again (d -f) </li></ul>
  35. 35. 180 turn <ul><li>When a rabbit and a turtle run in one direction for a certain time, then turn around and run in the opposite direction with the same speed for the same time they will arrive at the starting point at the same time. </li></ul>
  36. 36. T2 effect <ul><li>The 180 0 pulse refocuses the dephasing protons which results in a stronger signal, the spin echo after the time TE. The protons then dephase again and can be refocussed another time by a 180 0 pulse and so on. Thus it is possible to obtain more than one signal, more than one spin echo. The spin echo, however differ in intensity due to so-called T 2 – effect. </li></ul><ul><li>A curve connecting the spin echo intensities is the T 2 curve. If we did not use the 180 0 pulse, the signal intensity would decay much faster. A curve describing the signal intensity in that case the T * 2 (T 2 star) curve. </li></ul>
  37. 37. T2-effect <ul><li>If we record the signal of two buses as the buses drives away. The signal vanish due to extrinsic (bus speed) and intrinsic (exhaustion of the passengers) </li></ul><ul><li>Without having the bus come back (i.e. a 180 0 pulse), it is impossible to say whether a decrease in signal intensity is due to inherent tissue properties , or due to external influence, i.e. different bus speed. </li></ul>
  38. 38. T 2 - weighted sequence <ul><li>T2-curve of the two tissue with different transversal relaxation times </li></ul><ul><li>Tissue A has a shorter T2 than tissue B, thus looses transversal magnetization faster. </li></ul><ul><li>With short TE the difference in signal intensity is less pronounced than after a long TE </li></ul>
  39. 39. Spin Echo sequence <ul><li>The spin echo sequence consists of a 90 0 and a 180 0 pulse </li></ul><ul><li>After the 90 0 pulse protons are dephasing due to external and internal magnetic fields inhomogeneities </li></ul><ul><li>The 180 0 pulse rephase the dephasing protons, and stronger signal , the spin echo results. </li></ul><ul><li>The 180 0 pulse serve to “neutralize” the external magnetic field inhomogeneities </li></ul><ul><li>Signal decrease from one echo to the next, when using multiple 180 0 pulse is due to internal T2-effects </li></ul><ul><li>By choosing different TEs this signal can be T2 weighted in varying degrees with very short TEs, T2-effects have not yet had time to really show up </li></ul><ul><li>With longer TEs, the signal intensity difference between tissue will be depending very much on their T2s , their transversal relaxation times </li></ul><ul><li>With very long TEs, there should be even more T2-weighing, however , signal intensity as such would be so small, that at best it can just barely be distinguished from the background noise </li></ul>
  40. 40. Spin Echo sequence <ul><li>(90 0 – TE/2 – 180 0 – TE/2 -> record signal at TE) after TR (time from the beginning of one 90 0 pulse) follows another pulse cycle and signal measurement. </li></ul><ul><li>(90 0 – TE/2 – 180 0 – TE/2 -> record signal at TE) </li></ul>

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