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Chapter 2

  1. What factors determine the T1 and T2 relaxation times of a tissue?
    • inherent energy of the tissue
    • how well the molecular tumbling rate matches the Larmor Frequency
    • How closely spaced the molecules are
  2. Why does fat have short T1 and T2 relaxation times?
    • -fat has a low inherent energy
    • -slow molecular tumbling rate
    • -molecules are packed together
    • -this means energy exchange is efficient and therefore relaxation occurs quickly
  3. Weighting
    parameters are selected to make one contrast mechanism dominate over the others
  4. What values of TR and TE are needed for PD weighting in a spin echo sequence and why
    • the TR must be long so that neither fat nor water has had time to fully recover their longitudinal magnetization
    • the TE much also be short to minimize the T2 differences between tissues
  5. List the main factors that make gradient echo sequences different from Spin Echo
    • Variable Flip angles
    • gradient re phasing
    • shorter TRs and scan times
  6. What parameter controls T2 decay and why?
    The TE controls T2 decay as it determines how much dephasing is allowed to occur before the signal is read.
  7. What Types of contrast will these produce?
    TR 50 ms, TE 5 ms, flip 120
    T1 weighting
  8. What Types of contrast will these produce?
    TR 400 ms, TE 15 ms, flip 35
    T2* weighting
  9. Intrinsic Contrast Parameters
    cannot be changed because they are inherent to the body's tissues
  10. Extrinsic contrast parameters
    can be changed
  11. Intrinsic Contrast parameters include:
    • T1 recovery time
    • T2 decay time
    • Proton Density
    • Flow
    • Apparent diffusion Coefficient
  12. Extrinsic Contrast parameters include:
    • TR
    • TE
    • Flip Angle
    • TI
    • turbo factor/ echo train length
    • b value
  13. White on the images
    High signal
  14. Black on the image
    low signal
  15. ____ ____ has large transverse component of coherent magnetization at the time of TE
    High Signal
  16. Proton Density
    number of protons per unit volume of a tissue
  17. in Proton Density the higher density of a tissue the ___ signal available.
    A. More
    B. Less
    A. More
  18. What 3 factors do T1 and T2 relaxation depend on?
    • -inherent energy of the tissue
    • -How closely packed the molecules are
    • -How well molecular tumbling rate matches the Larmor frequency of Hydrogen
  19. How does inherent energy of a tissue affect the T1 and T2 relaxation?
    • -low energy = molecular lattice is more able to absorb energy from hydrogen
    • -High energy= cannot easily absorb energy from hydrogen nuclei
    • -these are especially true for T1
  20. How does molecular density affect T1 and T2 relaxation?
    • -closely spaced molecules are more efficient
    • -energy exchange
    • -very important for T2
  21. How does the tumbling rate matching the Larmor Frequency affect T1 and T2 relaxation?
    • -good match= efficient molecular lattice
    • -bad match = inefficient energy exchange
    • -is important for both T1 and T2
    • -is similar to resonance
  22. describe relaxation in Fat
    • -slow tumbling rate
    • -high molecular density
  23. describe relaxation in water
    • -fast tumbling rate
    • -oxygen steals the electron away from hydrogen, rendering it more available to the effect of the magnet
    • -molecules spaced far apart
  24. the Larmor frequency of hydrogen in ___ is higher than in ___
    • water (1)
    • fat (2)
  25. Fat recovers more rapidly along the longitudinal axis than water and loses transverse magnetization ___.
    faster
  26. T1 recovery in fat
    • short (fast)
    • -slow tumbling rate
    • -low inherent energy
  27. T1 recovery in water
    • Long (slow)
    • -high inherent energy
    • - tumbling rate doesn't match Larmor Frequency
  28. T2 decay in fat
    • T2 time is short (fast)
    • -tumbling rate matches Larmor Frequency
    • -spins dephase quickly
  29. T2 decay in water
    • T2 time is long (slow)
    • -spins dephase slowly
    • -loss of trans mag is gradual
  30. T1 contrast
    • next RF is applied at a TR shorter than total relaxation times of both tissues
    • -flips the longitudinal magnetization into trans (90 degree pulse)
  31. Signal depends on the amount of coherent ____ magnetization
    transverse
  32. T2 time of fat is ___ than water


    C. less
  33. Trans mag of fat decays ___ than water


    A. faster
  34. less ___ still has a readable signal for T2 contrast
    A. Fat
    B. Water
    A. Fat
  35. why is water bright on T2 weighted imaging?
    higher signal
  36. Proton Density contrast
    differences in signal intensity between tissues that are a consequence of their relative number of protons per unit volume
  37. ___ proton density has large trans mag (high signal)
    high (think brain tissue
  38. ___ proton density has small trans mag (low signal)
    low (think cortical bone)
  39. proton density contrast is always present on all images.
    A.True
    B. False
    A. True
  40. T1 and T2 relaxation times of tissue are ___ but dependent on the ___ strength.
    • inherent (1)
    • field (2)
  41. as field strength goes up, tissue relaxes quicker. 
    A. True
    B. False
    B. False
  42. What do fat and water look like on T1 weighted images
    • bright fat
    • dark water
  43. what do fat and water look like on T2 weighted images?
    • bright water
    • dark fat
  44. What looks bright and dark on PD weighted images?
    • bright- high density
    • dark- low density
  45. all intrinsic contrast parameters simultaneously affect contrast
    A. True
    B. False
    A. True
  46. what do you do to 'weight' an image?
    put the advantage of 1 intrinsic parameter in priority over the others
  47. T1 weighting depends on:
    differences of T1 times between fat and water
  48. explain the TR on T1 weighting
    must be short enough that neither fat or water can fully return to B₀
  49. What does TR control
    • -how much everything is allowed to relax
    • -the amount of T1 weighting
  50. What does TE control?
    the amount of T2 decay before the signal is read
  51. Explain TE on T2 weighting
    must be long enough to give both fat and water time to decay
  52. How do you get PD weighting
    by diminishing the effects of T1 and T2
  53. in PD weighting what does a long TR allow?
    • -fat and water to fully recover
    • -diminishes T1 weighting
  54. in PD weighting what does a short TE do?
    • -doesn't allow fat or water time to decay
    • - diminishes T2 weighting
  55. what is considered partially saturated?
    NMV beyond 90⁰
  56. What is fully saturated?
    NMV 180⁰ or more
  57. What happens if you have partial saturation of both fat and water?



    C. T1 weighting
  58. What happens if you have neither fat or water in partial saturation?



    C. PD weighting
  59. What is T2* decay?
    • -decay of FID following the RF excitation pulse
    • -the decay is faster than T2 since its a combination of 2 effects
  60. What 2 effects combine to make T2*?
    • -T2 decay
    • -dephasing due to inhomogeneitie
  61. Inhomogeneities
    areas within the magnetic field that dont exactly match the external magnetic field strength
  62. due to inhomogeneites the Larmor equation can cause___ of the NMV
    immediate dephasing
  63. What are Pulse Sequences used for?
    • -to compensate for T2* dephasing
    • - so that a signal regenerated at T1 or T2 can be measured
  64. What are the 2 ways to compensate for T2* dephasing?
    • Spin Echo Pulse Sequence
    • Gradient Echo Pulse Sequence
  65. Spin Echo Pulse Sequence
    uses a 180⁰ pulse to regenerate signal
  66. what uses a 90⁰ pulse to flip NMV into trans
    spin echo
  67. what occurs immediatly with the 90⁰ RF is removed
    • free induction decay (FID)
    • T2*
  68. what is used to compensate for dephasing in spin echo?
    180⁰ RF pulse after the initial 90⁰ pulse
  69. What information is contained in spin echo as T2* dephasing is reduced?
    T1 and T2
  70. what is TAU?
    time it takes to rephase after 180⁰ pulse
  71. What is the TE for spin echo?
    double the TAU
  72. Spin Echo with 1 echo
    produce T1 images if short TR and TE are used
  73. Spin Echo with multiple echos
    produce both proton density and T2 images
  74. What is the 1st image produced using spin echo with multiple echos?
    PD weighted
  75. What is the 2nd image produced using spin echo with multiple echos?
    T2 weighted
  76. how do you maximize T1?
    short TR
  77. how do you maximize PD?
    long TR
  78. what minimizes T2
    short TE
  79. What type of image would have bright water
    T2 with long TE
  80. what type of image is likely to have dark water?
    • T1 with short TR
    • (some PD images have dark water)
  81. What are the 3 main things to know about gradient echo pulse sequence?
    • -variable excitation pulse
    • - less trans mag than spin echo
    • -re phasing occurs by using a gradient
  82. Gradient
    generated by coils of wire (inside the gantry)
  83. magnetic isocenre
    • middle axis of the gradient
    • always remains at the field strength
  84. polarity of the gradient
    can add or subtract from the main magnetic field depending on the direction of the current passing through the coils
  85. when a gradient is switched on the magnetic field along the gradient is ___/___
    slopped/graded
  86. how do gradients dephase?
    • -gradient is applied
    • - alters the magnetic field
    • - some nuclei slow down and some speed up
    • -magnetic moments fan out
  87. spoilers
    gradients that dephase
  88. How do gradients rephase?
    • gradient is applied
    • -magnetic field is altered in a linear fashion along the axis of the gradient
  89. rewinders
    gradients that re phase
  90. what are the 4 advantages of gradient echo?
    • - gradients re phase faster than 180⁰ RF
    • - minimum TE is much shorter
    • - TR can be reduced
    • - Gradient Echo has overall shorter scan times
  91. What are the 3 disadvantages of gradient echo?
    • + no compensation for homogeneity
    • +contain magnetic susceptibility artifact
    • + T2* effects aren't eliminated
  92. flip angle is an ___ contrast parameter that affects image contrast
    extrinsic
  93. the flip angle combined with ___, determines weather T1 effects are maximized or minimized
    TR
  94. For more flexibility, flip angle is reduced to ___ ___ ___, it doesn't take NMV as long to recover.
    less than 90⁰
  95. describe T1 weighting in Gradient Echo
    • - large flip angle
    • - short TR (fat and water are still recovering at next RF)
    • - short TE (minimize T2*)
  96. Describe T2* weighting in Gradient Echo.
    • - large flip angle
    • - short TR (fat and water are still recovering at next RF)
    • - short TE (minimize T2*)
  97. Describe PD weighting in Gradient Echo
    • - short TE
    • -small flip angle
    • - long TR
  98. What type of flip angle is used in Gradient Echo?
    variable
  99. Typical Values in gradient Echo:
    • -long TR 100 ms+
    • -short TR less than 50 ms
    • - short TE 5-10 ms
    • -long TE 15-25 ms
    • - small flip angle 5⁰-20⁰
    • - large flip angle 70⁰-110⁰
  100. Typical Values in Spin Echo
    • -Long TR 2000 ms+
    • -Short TR 250-700 ms+
    • -Long TE 60 ms+
    • -Short TE 10-25 ms
    • -Flip angle always 90⁰
  101. What parameters would be used for each weighting factor in Gradient Echo?
    • T1- short TR, Short TE, Large flip angle
    • T2- long TR, long TE, small flip angle
    • PD- Long TR, short TE, small flip angle
Author
adavis
ID
351110
Card Set
Chapter 2
Description
MRI in practice Ch 2
Updated

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