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MRI Physics RV

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MRI physics

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MRI Physics RV

  1. 1. MRI PHYSICS-1 Roshan Valentine
  2. 2. Overview of steps • The patient is placed in a magnet, • Radio wave is sent in, • Radio wave is turned off, • Patient emits a signal, • Signal is received • Image is reconstructed
  3. 3. • But before that lets learn some BASIC PHYSICS
  4. 4. • Human body is ~53% water, and water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent. • Thus, most of the mass of the human body is oxygen, but most of the atoms in the human body are hydrogen atoms. • The average 70 kg adult human body • contains approximately 3 x 1027 atoms • of which 67% are hydrogen atoms!!!!
  5. 5. • But why we bothered about ONLY HYDROGEN
  6. 6. • Simplest element with atomic number of 1 and atomic weight of 1 • When in ionic state (H+), it is nothing but a proton. • Proton is not only positively charged, but also has magnetic spin (wobble)! • MRI utilizes this magnetic spin property of protons of hydrogen to elicit images!!
  7. 7. THUS
  8. 8. But why we can’t act like magnets? • The protons (i.e. Hydrogen ions) in body are spinning in a haphazard fashion, and cancel all the magnetism. That is our natural state!
  9. 9. Structure of an ATOM • Atom consistes of a centre nucleus which Contains a proton and neutron Around which an electron rotates around it and its own axis • Thus its analogous to our solar system • In the nucleus - besides other things - there are protons, little particles, that have a positive electrical charge • Any moving electrical charge is an electrical current • Any moving electric current induces a magnetic field
  10. 10. • Thus, the proton has its own magnetic field and it can be seen as a little bar magnet
  11. 11. SPIN • Proton in its natural state are arranged haphazardly with no net charge. • But when exposed to an external magnetic field(B0) they arrange parallel or antiparallel to the external field. • Naturally anti parallel needs more energy compared to parallel • In NMR it is the unpaired nuclear spins that produce a signal in a magnetic field
  12. 12. Precession • Protons as we all think doesnot just kay there Instead they undergo PRECESSION NOW WHAT IS PRECESSION
  13. 13. LARMOR EQUATION
  14. 14. PHASE • refers to the position of the magnetic moments on their circular precessional path • Out of phase or incoherent means that the magnetic moments of hydrogen are at different places on the precessional path. • In phase or coherent means that the magnetic moments of hydrogen are at the same place on the precessional path.
  15. 15. Measuring magnetization
  16. 16. TIME FOR SOME VECTORS
  17. 17. For easier understanding every proton can be considered as vectors. A vector can represent the direction of the force and the magnitude by its size.Here the force is referred to as the MAGNETIC FORCE Magnetization along an external magnetic field cannot be measured. For this a magnetization transverse to the external magnetic field is necessary.
  18. 18. • Human body can be considered as a large magnet with its vector along the Longitudinal axis called LONGITUDINAL MAGNETIZATION. • In a strong external magnetic field a new magnetic vector is induced in the patient, who becomes a magnet himself. This new magnetic vector is aligned with the external magnetic field. TRANSVERSE MAGNETIZATION LONGTIUDINAL MAGNETIZATION
  19. 19. • Magnetization along an external magnetic field cannot be measured. For this a magnetization transverse to the external magnetic field is necessary.
  20. 20. RESONANCE • Phenomenon that occurs when an object is exposed to a frequency close to its natural frequency of oscillation-LARMOR FREQUENCY
  21. 21. Lets take an example ENERGY RF ≠ LARMOR FREQUENCY
  22. 22. But when RF = ω
  23. 23. • For resonance of hydrogen to occur, an RF pulse of energy at exactly the Larmor frequency of hydrogen must be applied. • Excitation – application of RF pulse that causes resonance to occur.
  24. 24. • Flip angle • Magnitude of flip angle depends on the amplitude and duration of RF pulse. • The plane at 90 to B0 is termed transverse plane. • When resonance occurs, all the magnetic moments are in phase with each other.
  25. 25. MR Signal • As the NMV precesses at Larmor frequency in the transverse plane, a voltage is induced in the coil. This voltage constitutes the MR signal. • The magnitude of the signal depends on the amount of magnetisation present in the transverse plane.
  26. 26. Free Induction Decay • Recovery – gradual increase of magnetisation in longitudinal plane. • Decay – gradual decrease of magnetisation in transverse plane. • The magnitude of voltage induced in the receiver coil also decreases – FID signal • This is called free induction decay (FID): ‘free’ because of the absence of the RF pulse; ‘induction decay’ because of the decay of the induced signal in the receiver coil
  27. 27. CONTRAST MECHANISM • High Intensity ( white)- High Tm • Low intensity ( dark) – Low Tm
  28. 28. TR(Repitition time ) and TE(Echo time)
  29. 29. What determines the contrast • The inherent energy of the tissue • How closely packed the molecules are • How well the molecular tumbling rate matches the Larmor frequency of hydrogen
  30. 30. T1 Recovery • SPIN-LATTICE energy transfer • As they loose energy , they regain • their Lm • T1 time - defined as the time it takes for 63% of the Lm to recover. • The TR determines how much T1 recovery occurs in a particular tissue as it occurs during TR
  31. 31. T1 Recovery
  32. 32. T1 Recovery Short T1 (Fat) Long T1 (Water) contrast
  33. 33. T2 Decay • SPIN-SPIN Energy Transfer • Due to the intrinsic magnetic fields of the nuclei interacting with each other. • Time it takes for 63% of the transverse magnetization to be lost due to dephasing. • TE determines the T2 decay as dephasing occurs during then • Depends on how closely the molecular motion of the atoms • matches the Larmor frequency and the proximity of other spins.
  34. 34. T2 Decay • Fat has better energy exchange compared to water HENCE T2 is SHORT FOR FAT.
  35. 35. T2 Decay contrast
  36. 36. T1 Weighting T2 Weighting 300 600 10 30 ms ms msms >2000 >70
  37. 37. T1 T2
  38. 38. Proton Density(PD)Weighting • Contrast is predominantly due to differences in the proton density of the tissues • Low PD – dark • High PD – bright • Cortical bone and air are always dark • PDW – Decreasing T1 and T2 effects
  39. 39. T1 Weighting T2 Weighting 300 600 10 30 ms ms msms >2000 >70
  40. 40. Pulse Sequence Mechanisms • Magnetic field inhomogeneities cause the NMV to dephase before intrinsic magnetic fields of the nuclei can produce The main purposes of pulse sequences are: • to rephase spins and remove inhomogeneity effects and therefore produce a signal or echo ; • to enable manipulation of the TE and TR to produce different types of contrast.
  41. 41. Pulse Sequence Mechanisms To measure relaxation times and produce an image with good contrast we need to regenerate the signal .Hence pulse sequences
  42. 42. Types • Spin Echo Pulse Sequence • Gradient Echo Pulse Sequence
  43. 43. Spin Echo Pulse Sequence
  44. 44. A basic rephasing sequence.
  45. 45. • The time taken to rephase after the application of the 180 ° RF pulse equals the time to dephase when the 90 ° RF pulse was withdrawn. • This time is called the TAU time.
  46. 46. Spin Echo • Spin echo pulse sequences produce either T1, T2 or proton density weighting • • TR controls the T1 weighting • Short TR maximizes T1 weighting • • Long TR maximizes proton density weighting • • TE controls the T2 weighting • • Short TE minimizes T2 weighting • • Long TE maximizes T2 weighting
  47. 47. Spin Echo (SE) Using Single Echo The timing parameters used are selected to produce a T1 weighted image.
  48. 48. Spin Echo With Two Echoes Produce both a PD and a T2WI PD T2WI
  49. 49. FAST SPIN ECHO (TURBO SPIN ECHO) • Faster version of conventional spin echo. • More than one phase encoding is performed per TR, reducing the scan time. • FSE employs a train of 180° rephasing pulses, each one producing a spin echo. This train of spin echoes is called an echo train. The number of 180° RF pulses and resultant echoes is called the echo train length (ETL) or turbo factor.
  50. 50. FAST SPIN ECHO (TURBO SPIN ECHO) • The higher the turbo factor the shorter the scan time • In T2 weighted scans, water and fat are hyperintense (bright). This is because the succession of 180° RF pulses reduces the spin–spin interactions in fat thereby increasing its T2 decay time.
  51. 51. Spatial encoding in conventional spin echo.
  52. 52. The echo train.
  53. 53. Inversion Recovery Sequence
  54. 54. Inversion Recovery Sequence • Inversion recovery (IR) was developed in the early days of MRI to provide good T1 contrast on low field systems. • But became obsolete due to high scanning time • Now back when combined with fast spin sequences
  55. 55. Fast Inversion Recovery Sequence • 180 ° inverting pulse is followed after the TI time by the 90 °excitation pulse and the train of 180 ° RF pulses to fill out multiple lines of K space as in fast spin echo. • instead of being used to produce T1 weighted images, fast inversion recovery is usually used to suppress signal from certain ti ssues in conjuncti on with T2 weighti ng so that water and pathology return a high signal
  56. 56. Fast Inversion Recovery Sequence 2 Types • STIR( short tau inversion recovery) • FLAIR ( fluid attenuated inversion recovery)
  57. 57. STIR • the time it takes fat to recover from full inversion to the transverse plane so that there is no longitudinal magnetizati on corresponding to fat. This is called the null point • Uses STIR is an extremely important sequence in musculoskeletal imaging because normal bone, which • contains fatty marrow, is suppressed and lesions within bone such as bone bruising and tumors are seen more clearly (Figures 5.18 and 5.19 ). It is also a very useful sequence for suppressing fat in general MR imaging ( see Chapter 6).
  58. 58. FLAIR • FLAIR is used to suppress the high CSF signal in T2 weighted images • TI corresponding to the time of recovery of CSF from 180 ° to the transverse plane nulls the signal from CSF • To see periventricular and cord lesions more clearly
  59. 59. STIR FLAIR
  60. 60. Gradient Echo Pulse Sequence • Gradient is used to reduce magnetic homogeneity • Principle based on Larmor equation
  61. 61. Gradient Echo Pulse Sequence • After the RF pulse is withdrawn, FID signal is immediately produced due to magnetic field inhomogeneities and T2* dephasing occurs. • The gradient rephases the magnetic moments so that a signal can be received by the coil, which contains T1 and T2 information. • This signal is called a gradient echo
  62. 62. How gradients dephase
  63. 63. How gradients rephase
  64. 64. Advantages • Rephase faster. • Low flip angles (<90 ) • Shorter TR • Shorter scan times than spin echo sequences
  65. 65. Gradients GRADIENT SLICE SELCTION FREQUENCY ENCODING PHASE ENCODING
  66. 66. Slice Selection
  67. 67. Frequency encoding
  68. 68. Slice selection
  69. 69. Frequency encoding
  70. 70. Phase encoding
  71. 71. So in spatial encoding…..
  72. 72. Slice Selection
  73. 73. STEADY STATE • TR is shorter than both T1 and T2 of all tissues. • There is no time for the transverse magnetization to decay before the pulse pattern is repeated again • NMV remains steady during data acquisition
  74. 74. Steady state • Magnitude of tm keeps accumulating as it doesn’t have time to delay over successive TRs. • Tissues with long T2 times (mainly water) appear bright. • Most gradient echo sequences utilize the steady state because the TRs are so short that the fastest scan times are achievable.
  75. 75. Steady state
  76. 76. Gradient Recalled Acquisition in Steady State GRASS • Coherent Gradient Sequence • Uses steady state principle • Has a rewinder/rephasing gradient to keep residual TM coherent so that it is in phase at the beginning of next repetition. • By reversing the slope of phase encoding gradient after read out.
  77. 77. allows tissues with long T2 time to produce bright signal (blood,CSF,synovial fluid).
  78. 78. Spoiled Gradient Recalled Acquisition SPGR • Incoherent(Spoiled) Gradient Sequence • utilizes the steady state by using very short TRs. • Eliminates transverse magnetization so that tissues with long T2 times are not allowed to dominate image contrast but T1/proton density contrast prevails – spoiling • Gradient spoiling to dephase the residual transverse magnetization. • RF spoiling applies RF excitation pulses at different phases every TR so that residual transverse magnetization has different phase values • Primarily used for T1 weighting
  79. 79. Steady state free precession (ssfp) • steady state is maintained . • Every TR an excitation pulse is applied. When the RF is switched off, a FID is produced. • After the TR, another excitation pulse is applied . • Rephases the FID produced from the previous excitation.
  80. 80. ssfp
  81. 81. SSFP • a rewinder gradient is used to speed up the rephasing process after the RF rephasing has begun. • Rewinding moves the echo so that it occurs before the next excitation pulse, rather than during it. • Measures true T2 weighting long TE rephasing done by RF pulse and not gradient ( less T2* and more T2)
  82. 82. ULTRA FAST SEQUENCES very fast pulse sequences that can acquire several slices in a single breath-hold. Many ultrafast sequences use extra pulses applied before the pulse sequence begins
  83. 83. ULTRA FAST SEQUENCES How ? • Applying only a portion of the RF excitation pulse, so that it takes much less time to apply and switch off ; • Reading only a proportion of the echo (partial echo); • Using asymmetric gradients which are faster to apply than conventional balanced gradient; • Filling K space in a single shot or in segments (Echo Planar Imaging)
  84. 84. ECHO PLANAR IMAGING (EPI) • Collects all the data required to fill all the lines of K-space from a single echo train. • Multiple echoes are generated spin EPI Gradient EPI • Rapid data acquisition (axial images of brain in 2-3 secs, whole body imaging in 30 secs. )
  85. 85. EPI 18 seconds – mri pelvis

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