Chapter 72 - MRI

Question 1

1. Signals generated in MRI come from _____

Answer 1

• Hydrogen protons

Question 2

3. What is relaxation?

Answer 2

• When the excited protons lose energy and return to their base resting state

Question 2

2. In an MRI exam, what occurs to the hydrogen protons?

Answer 2

• They are placed under the influence of a strong magnetic field. The protons are then excited by an applied radiofrequency

Question 3

4. After termination of the radiofrequency pulse, what are the three different processes?

Answer 3

• T1, T2, T2*

Question 4

5. What is T1 relaxation (T1 recovery)?

Answer 4

• Refers to the release of energy from a proton into its surrounding molecular environment

Question 5

6. What is T2 relaxation (T2 decay)?

Answer 5

• Results from the energy transfer between the protons themselves

Question 5

7. What is T2* decay?

Answer 5

• Energy loss owing to inhomogeneities within the magnetic field, which are inherent to all magnets but increased by some types of material (especially hemorrhage or metal)

Question 6

8. What is pulse sequence?

Answer 6

• A programmed set of RF stimulation sequences that result in images with a particular appearance

Question 7

9. What is hyperintense?

Answer 7

• Areas of high signal (appear very white)

Question 8

10. What is hypointense?

Answer 8

• Areas of low signal (appear very black)

Question 9

11. T/F: Different pulse sequences can lead to different tissues having different relaxation characteristics

Answer 9

• True

Question 10

12. What is benefit of T1 images?

Answer 10

• T1 images typically have very good anatomic detail but less inherent contrast between tissues than on other sequences

Question 11

13. On T1 weighted pulse images: fluid, edema, and inflammation are _____. Cortical bone, tendons, and ligaments are ______.

Answer 11

• Hypointense/dark
• Hypointense

Question 12

18. What are proton density sequences?

Answer 12

• Designed to produce images that minimize both T1 and T2 weighting, contrast between tissues is solely related to the density of mobile hydrogen protons in the tissue

Question 12

14. On T1 imaging, describe the intensity of the following tissues: 1) fluid/edema, 2) muscle, 3) fat, and 4)cortical bone and tendon/ligament

Answer 12

• Hypointense
• Intermediate
• Hyperintense
• Hypointense

Question 12

15. What is benefit of T2 weighted sequences?

Answer 12

• Greater signal intensity differences between adjacent tissues compared to T1 images

Question 13

16. On T2 weighted sequences, fat is ______, tendons, cortical bone, and ligaments are _____. Fluid and edema and inflammation are _______

Answer 13

• Hyperintense (bright)
• Hypointense (dark)
• Hyperintense (bright)

Question 14

17. What is the primary image difference between a T1 and a T2 weighted sequence?

Answer 14

• Fluid and areas of edema or inflammation are hyperintense, often resulting in good contrast with adjacent hypointense cortical bone or tendons/ligaments (results in good visualization of areas of pathology.

Question 15

19. On proton density sequences, what is the signal intensity and contrast between tissues related to?

Answer 15

• Contrast between tissues is solely related to the density of mobile hydrogen protons in the tissue.

Question 16

20. On proton density images, fluid and inflammation are _______, fat are ____ intensity, cortical bone, ligaments and tendons are _____

Answer 16

• Hyperintense
• Intermediate
• Hypointense

Question 17

21. Inflammatory, neoplastic, and degenerative pathology in bone and soft tissue look _____ on T2 and PD imaging due to increased interstitial or intracellular fluid and cellular infiltrates increased

Answer 17

• Hyperintense

Question 18

22. Fat is ___ on T2 and PD images?

Answer 18

• Hyperintense
• Intermediate intensity

Question 19

23. Why could high fat signal intensity be a problem when looking for inflammatory, neoplastic, and degenerative pathology?

Answer 19

• In structures that contain fat, such as bone marrow, or subcutaneous tissue the increase pathologic signal may be obscured by the already high signal from the regional fat

Question 20

24. Pulse sequences have ____ technology that eliminate the signal from fat, while retaining the high signal with pathology

Answer 20

• Fat suppression

Question 21

28. What are short tau (or time) inversion recovery pulse sequences?

Answer 21

- Inversion recovery sequences that null the signal from fat

Question 21

30. What is the resulting STIR image?

Answer 21

- Image contrast that is generally similar to T2 weighting (fluid and pathology tends to be bright) - But with fat displaying very low signal and appearing dark gray or black on the image

Question 21

26. What are inversion recovery pulse sequences?

Answer 21

- A technique that employ an additional RF pulse that reverses or “inverts” the signal from fat protons so that when the tissue signal is collected, fat contains no signal to give

Question 21

25. What are the 2 most common methods of fat suppression?

Answer 21

- Inversion recovery - Fat saturation

Question 21

27. What is another term for ‘time’ when referring to MRI protocols?

Answer 21

- Tau

Question 22

29. What is STIR?

Answer 22

- Short Tau Inversion Recovery

Question 23

31. STIR images are generally (higher/lower) in signal compared to other sequences and have what appearance? What is the benefit?

Answer 23

- Lower - Grainy - Supreme contrast between structures and good sensitivity to pathology

Question 24

32. What is fat saturation?

Answer 24

- Reduces the signal from protons within fat, resulting in a hypointense appearance to fatty areas on the final image

Question 25

33. Fat saturation can be applied to what types of weighted images?

Answer 25

- T1, T2, and PD

Question 25

34. What is the advantage of fat saturation?

Answer 25

- It can be applied to one of the common pulse sequences, unlike STIR, which is a pulse sequence in itself

Question 26

35. Which fat suppression technique works on both high-field and low-field magnets and tends to result in a very homogenous suppression of fat across the entire image?

Answer 26

- STIR

Question 27

37. What are the two fundamental classes of pulse sequences?

Answer 27

- Spin echo - Gradient recalled echo

Question 28

36. Which fat suppression technique is considered unrelable in low-field systems and can struggle to have uniform fat suppression throughout the image (with less suppression near the margins of the image and a loss of suppression in areas of nonuniform magnetic field)

Answer 28

- Fat saturation

Question 29

38. What are the major advantages of GRE sequences?

Answer 29

- Shorter acquisition time (less motion artifact, less time under GA) - Potential for volumetric acquisition (3 dimensional)

Question 30

39. What are disadvantages of Gradient Recalled Echo (GRE) over spin echo (SE)? Why?

Answer 30

- Poorer contrast resolution - Due to increased susceptibility to field inhomogeneities

Question 31

40. GRE sequences are called ____ weighted instead of T2 weighted, though they appear similar to T2 weighted images

Answer 31

- T2*

Question 32

41. T2* weighted (GRE) images are more prone to ____ compared to T2 weighted images.

Answer 32

- Artifacts

Question 33

42. What are the standard pulse sequences taken for equine limb MRI? (3)

Answer 33

- Fast SE (T2, PD, or less commonly T1) - STIR - GRE (T2* weighted, PD or T1)

Question 34

45. Diseased tissues tend to have (higher/lower) water content

Answer 34

- Higher

Question 35

43. Which sequences are of benefit for anatomic detail, especially tendon and ligament damage?

Answer 35

- PD and T1

Question 36

44. On T2 images, fluid is ____

Answer 36

- Hyperintense

Question 37

48. To easily ID lesions on T2 and PD sequences on high field magnets use ____ to eliminate the fat signal and STIR sequences can be used on high and low field magnets

Answer 37

- Fat-saturation pulses

Question 38

46. T2 images and disease tissues have higher/lower water content and are hyperintense

Answer 38

- Higher

Question 39

47. Lesions in bone, like edema or osseous cysts, result in (high/low) signal intensity on T2/PD/STIR sequences

Answer 39

- High

Question 40

49. What is the transverse series useful for?

Answer 40

- Identification of pathology

Question 41

50. What are the transverse planes in relation to the axis of the limb?

Answer 41

- Perpendicular to the long axis of the limb

Question 42

51. How are sagittal plane images aligned in relation to the axis of the limb?

Answer 42

- Parallel to the long axis of the limb

Question 43

52. What are sagittal plane images useful for?

Answer 43

- Anatomic localization

Question 44

53. What are dorsal images useful for?

Answer 44

- Evaluation of joint space width and cartilage and anatomic localization

Question 45

54. Low field magnets have (longer/shorter) acquisition times

Answer 45

- Longer

Question 46

55. Standing magnets are (low/high) field magnets

Answer 46

- Low

Question 47

56. A 1 T magnet has a strength ____ times that of earth’s magnetic field

Answer 47

- 20,000 times

Question 48

57. High field means ____T; low field means ____ T

Answer 48

- >1T - <1T (generally between 0.2 and 0.4T)

Question 49

58. What is the major determinant of image quality?

Answer 49

- Field strength

Question 50

59. All horses must have ___ removed before MRI studies

Answer 50

- Shoes

Question 51

60. Support tables for equine MRI are custom build with _____ material like ___ and ____ with padding

Answer 51

- Nonferrous - Thick plywood - Synthetic material

Question 51

62. The region of interest should be positioned in ___ of magnet.

Answer 51

- The exact middle (isocenter)

Question 52

61. Low-field systems have (lower/higher) signal-to-noise ratio compared with high-field systems, which translates into (lower/higher) resolution images.

Answer 52

- Lower - Lower

Question 53

63. Why should the region of interest be at the isocenter?

Answer 53

- Image quality deteriorates near the edge of the magnet where the magnetic field is less homogeneous

Question 53

64. What are receiver coils that are required for MRI exams?

Answer 53

- They are the device that receives the signal created by the hydrogen atom excitation

Question 54

65. What is taped to the lateral aspect of the hoof to indicate left versus right?

Answer 54

- Vitamin E capsules

Question 55

67. What are the most common forms of MRI contrast medium?

Answer 55

- Chelates of Gadolinium

Question 56

66. What do vitamin E capsules show on MRI?

Answer 56

- Rounded, homogenous high signal intensities

Question 56

70. By what routes can MRI contrast agents be administered?

Answer 56

- IV or intrasynovially

Question 57

68. What does gadolinium do in tissue? What is resultant image?

Answer 57

- Enhances T1 relaxation time - More hyperintense on T1-weighted images

Question 58

71. What is a ballistic danger in MRI?

Answer 58

- Ferromagnetic metallic objects

Question 59

69. T/F: GD MRI contrast agents are useful on imaging sequences other than T1

Answer 59

- False, they are little use on sequences other than T1

Question 59

74. T/F: Only objects labeled MR safe can be assumed to compatible and safe

Answer 59

- True

Question 60

72. What does ferromagnetic refer to?

Answer 60

- an object comprised of a metal with similar magnetic properties to iron (notably cobalt and nickel)

Question 61

73. What are 3 nonferromagnetic metals?

Answer 61

- Aluminum - Titanium - Copper

Question 62

77. What human implants should avoid the MRI?

Answer 62

- Pacemakers or metallic ferrous implants

Question 62

75. Why can non ferromagnetic stainless steel implants not be used?

Answer 62

- They will still produce distortions if included in the imaging field of view

Question 62

76. How can objects be tested before entering magnet?

Answer 62

- With a handheld bar magnet

Question 63

78. What is the upper limit where a magnetic field of strength is of no potential concern for public, including those with implanted electronic devices?

Answer 63

- 0.0005T

Question 64

79. How do burns occur in an MRI? Specifically in equine?

Answer 64

- MRI can induce an electronic current and secondary heating of any conductive material within or contacting the body - ECG cables or other lines which may loop on themselves

Question 65

80. What changes can be seen on navicular bone MRI that are often not visible on radiographs? (3)

Answer 65

- Medullary cavity changes - Flexor cortex erosions - Fragmentation of the distal margin of the DSB at the origin of the impar ligament

Question 66

81. On histopathology, what do the areas of high signal intensity in navicular bones correspond to? (6)

Answer 66

- Degeneration - Necrosis - Osteonecrosis - Fibrosis - Hemorrhage - Edema

Question 67

82. How is navicular bursitis characterized?

Answer 67

- Fluid distension within the bursa with or without proliferation of the soft tissues

Question 68

83. Fluid in the navicular bursa has a high signal intensity on what sequences?

Answer 68

- PD and T2

Question 69

86. What are the MRI artifacts? (5)

Answer 69

- Magnetic Susceptibility artifact - Magic angle effect - Zipper artifact - Motion artifact - Volume averaging artifact

Question 70

84. What are the types of DDFT lesions? (3)

Answer 70

- Core lesions - Parasagittal splits - Dorsal irregularities or fibrillations

Question 71

88. A solar margin radiograph of the hoof to identify small nail fragments has been shown to have 100% sensitivity in detecting metallic particles ___mm or greater in size

Answer 71

- 1mm or greater

Question 71

85. How are adhesions of the DDFT most easily seen on MRI?

Answer 71

- When saline is injected into the podotrochlear bursa, causing distension that should separate the DDFT from the DSB, with lack of distension attributable to adhesions (requires 4-6mL of volume)

Question 72

87. What is magnetic susceptibility artifact?

Answer 72

- When 2 tissues of very different susceptibility are in close proximity, the distortion of the magnetic field is more pronounced, characterized by an area of hypointense signal void, surrounded by an area of high signal caused by overlap of signals from other tissues and anatomic distortion

Question 73

91. ___ sequences have the smallest susceptibility artifact on low field magnets

Answer 73

- SE STIR

Question 74

89. What color play doh should not be used for horses undergoing MRI? Why?

Answer 74

- Blue play doh - It is produced using a ferrous blue dye

Question 75

90. What sequences are more subject to susceptibility artifacts?

Answer 75

- GRE are more susceptible than SE sequences

Question 75

92. T/F: Field inhomogeneities are inherent to all magnets, being greater in magnets with closed bores and in magnets with narrow bores.

Answer 75

- False, greater in magnets with open bores and wider bores

Question 76

94. What is the “Magic Angle Effect”

Answer 76

- Because protons in dense, relatively acellular tissues greatly restrict the movement of hydrogen protons, they lose energy rapidly (resulting in low signal), except when the fibers are aligned at 55° ± 10° relative to the direction of the magnet’s main field, interproton reactions become minimal, resulting in greatly increased signal intensity in these areas

Question 76

97. when the collagen fibers are aligned at ____ degrees ± ___ degrees relative to the direction of the magnet’s main magnetic field, the interactions between protons become minimal

Answer 76

- 55° - 10°

Question 76

93. IN addition to metal, what else can cause a magnetic susceptibility artifact?

Answer 76

- Air or hemorrhage/hemosiderin

Question 77

95. What is effect of restricted proton motion on excitation?

Answer 77

- Very strong interactions between protons, causing them to very rapidly lose any energy imparted by the excitation RF pulse (low signal)

Question 78

96. What is the result of magic angle effect on MRI?

Answer 78

- When collagen fibers are aligned at 55 degrees ± 10 the proton interactions are minimal and this effect causes a greater increased signal intensity

Question 79

98. On T1, PD, and T2* sequences , what type of signal is seen in tendons and ligaments where they are aligned at the 55 degree angle relative to magnet’s main magnetic field? What is it called?

Answer 79

- Magic angle effect

Question 80

99. T/F: High-field, closed-bore magnets have magnetic fields that run perpendicular to the bore, essentially vertical to the ground

Answer 80

- False, parallel to the bore

Question 81

100. Magic angle artifact can mimic ____ in tendons and ligaments

Answer 81

- Lesions

Question 82

101. Where are the most recognized areas of magic angle artifact? How do they appear?

Answer 82

- Insertion of the DDFT - Hyperintense

Question 83

105. What is zipper artifact caused by?

Answer 83

- Leakage of electromagnetic energy into the magnet room

Question 84

103. What sequences show the magic angle effect most prominently? (3)

Answer 84

- T1, PD, and T2*

Question 85

106. What is leakage of electromagnetic energy caused by?

Answer 85

- Non MRI-compliant electronic devices - Opening of the room door while the scan is in progress

Question 85

102. As little as __ degrees off axis, can result in asymmetry in signal pattern of collateral ligaments

Answer 85

- 4°

Question 86

104. What represents a true pathologic lesion vs magic angle?

Answer 86

- Compare it to T2 and other sequences (equivalent to orthogonal views)

Question 87

107. What does zipper artifact look like on MRI?

Answer 87

- A region of increased noise with a width of 1 or 2 pixels that extends as lines of white and black dots along one axis of the image

Question 88

108. What does motion artifact look like on MRI?

Answer 88

- Blurring of the image and ghosting

Question 89

110. T/F: Flowing blood also results in motion artifact as the blood vessel diameter changes with time, and has been referred to as “flow artifact”

Answer 89

- True 111. How is patient motion reduced on sequences?

Answer 89

- Proper patient sedation or anesthesia - Respiration-related motion on recumbent upper limb exams can be minimized by placing sandbags on top of the distal limbs

Question 90

109. What is ghosting?

Answer 90

- The same region repeated multiple times

Question 91

112. Thicker slices result in (better/worse) signal from tissue with (faster/slower) scans

Answer 91

- Better - Faster

Question 91

3. Volume averaging is more likely to occur with thicker/thinner slices and high/low magnets

Answer 91

- Thicker - Low-field

Question 92

1. What is the disadvantage of thicker slices?

Answer 92

- Loss of resolution

Question 93

2. What is volume averaging? What is the concern?

Answer 93

- If the structure adjacent to the one being evaluated is markedly different, the high signal of the fluid may average with the edge of the tendon, ligament, or bone, making it appear that a lesion is present within that structure