1. Magnetic Resonance Imaging - MRI
King Saud University
College of Dentistry
Magnetic Resonance Imaging - MRI
Asma’a Al-Ekrish
BDS
Demonstrator ( OMF Radiology )

2. Sources Source of illustrations: Understanding MRI-
An interactive guide to MRI principles and applications
Philips Medical Systems
Best, The Netherlands
Theory of MRI is based on the magnetic properties of an atom and its response to radiofrequency stimulation

3. Some MRI images reproduced from:
Sources
Some MRI images reproduced from:
Som, P.M. and H.D. Curtin (2003). Head and neck imaging. St. Louis, Mosby.
White, S. C. and M. J. Pharoah (2004). Oral radiology.Principles and interpretation. St. Louis, Mosby.
Theory of MRI is based on the magnetic properties of an atom and its response to radiofrequency stimulation

4. Introduction
RF pulse
Theory of MRI is based on the magnetic properties of an atom and its response to radiofrequency stimulation

5. Lecture Contents Magnetic Resonance Image Production
Diagnostic Applications

6. Magnetic Resonance

7. Electric charge + spinning
Magnetic Nuclei
Electric charge + spinning
Tiny magnetic field

8. Strength and direction of magnetic field represented by a vector:
Magnetic Nuclei
magnetic
moment
Strength and direction of magnetic field represented by a vector:
MAGNETIC MOMENT

9. Magnetic Nuclei
Uneven number of protons which causes them to have a net “spin”
Spinning nuclei are MR active

10. Abundant in the human body and have a large magnetic moment
Magnetic Nuclei 1H
Abundant in the human body and have a large magnetic moment

11. Magnetic Nuclei
Moments are constantly changing their alignments making this a dynamic system

12. Precession

13. Precession
Magnetic nucleus placed in a scanner’s magnetic field rotates in a cone around the main field’s direction

14. Precession Larmor Frequency: speed or frequency of precession
Proportional to the strength of the magnetic field

15. strength of magnetic field
Precession
LARMOR EQUATION
Larmor frequency
(MHz)
gyromagnetic ratio
(MHz/T)
strength of magnetic field
(T) = x
Every element has a specific gyromagnetic ratio

16. Magnetic Resonance
Magnetic resonance induced by exposing nuclei to a second magnetic field B1 and a radiofrequency pulse
Resonance only occurs if the radiofrequency applied matches the Larmor frequency of the nuclei

17. Magnetic Resonance MR signal is emitted
After removal of the RF signal, nuclei gradually return to their position relative to main magnetic field and the MR signal “decays”

18. Magnetic Resonance VOLUME COILS TMJ COILS Head coil Body coil
( fixed inside magnet )
Head coil
VOLUME COILS
TMJ COILS

19. Individual magnetic moments cannot be measured
Net Magnetization
Individual magnetic moments cannot be measured
Signal produced in MRI is produced by the sum of all the magnetic moments: the NET MAGNETIZATION (represented by the magnetic vector)

20. Net Magnetization
NET MAGNETIZATION points in the same direction as the main magnetic field

21. Net Magnetization LONGITUDINAL MAGNETIZATION
Net magnetization is situated on the z-axis of a 3-dimensional coordinate system  the “static frame”
The z-axis indicates the direction of the scanner’s main field

22. Net Magnetization LONGITUDINAL MAGNETIZATION
Net magnetization precesses at the larmour frequency
At equilibrium, precession is not detectable and MR signal cannot be measured

23. Net Magnetization TRANSVERSE MAGNETIZATION
To detect precession, the magnetization vector must be tipped into the horizontal plane
Basis of MR signals

24. Net Magnetization TRANSVERSE MAGNETIZATION Larger net magnetization
Larger transverse magnetization
Stronger MR signal

25. The MR Signal
Nuclei at equilibrium with main magnetic field  only longitudinal magnetization which cannot be measured
RF pulse applied  transverse magnetization
Net magnetization spirals down towards the horizontal plane

26. The MR Signal
Strength and duration of the RF pulse determine the degree of tilt or flip angle (a)
Pulses are named after the flip angle they induce

27. The MR Signal
Receiver coil
As net magnetization precesses in the transverse plane, an oscillating electrical signal is generated which is detected by a receiver coil

28. This current is the MR signal measured in MRI
Current in receiver coil s t
This current is the MR signal measured in MRI

29. The MR Signal
Free Induction Decay s t
The MR signal immediately after an RF signal is The Free Induction Decay

30. The MR Signal Free Induction Decay Properties:
= amplitude
Free Induction Decay s t
Properties:
The signal strength or amplitude of the signal is the largest value in one oscillation
Dictated by size of transverse magnetization vector

31. The MR Signal Free Induction Decay Properties: Measured in mVs
= amplitude
Free Induction Decay s t
Properties:
Measured in mVs
The amplitude gradually decays as net magnetization gradually returns to equilibrium

32. The MR Signal Free Induction Decay Frequency = cycles per second (MHz)

33. The MR Signal
Free Induction Decay
The signal’s phase is the position in the signal’s cycle of oscillation
Measured in degrees

34. RF Pulse Flip Angles 90o pulse 180o pulse
Tips net magnetization to the horizontal plane
180o pulse
Tips net magnetization to the negative z-axis

35. Time interval between individual RF pulses
Repetition Time ( TR )
Time interval between individual RF pulses

36. Time interval between the RF pulse and detection of the image
Echo Time
Time interval between the RF pulse and detection of the image

37. Image Production

38. Image Production
Tissues and lesions are differentiated from eachother when they have different signal intensities  tissue contrast
Strong signal  white areas
Weak signal  dark areas
Intermediate signal  gray areas

39. Image Production Signal intensity depends on:
Proton Density (PD) Contrast: Number of hydrogen nuclei in tissue
­ H  high signal intensity
¯ H  low signal intensity
Determines size of the equilibrium magnetization of a tissue

40. Image Production Signal intensity depends on:
Differences in T1 and T2 relaxation rates between tissues
Stronger source of contrast
Flow
Susceptibility
Diffusion
Perfusion

41. Relaxation T1
relaxation T2
relaxation
Is the gradual return of net magnetization to the longitudinal axis after excitation with an RF pulse
Two independent components: T1 and T2 relaxation rates

42. Relaxation
Different tissues have different T1 and T2 relaxation rates  their signal intensities appear different
Most MRI acquisition techniques are influenced by T1 or T2 contrast

43. T2- “Spin-spin” Relaxation
T2 Relaxation
T2- “Spin-spin” Relaxation
Decay of transverse magnetization after RF pulse
Fast

44. T2 Relaxation Occurs as magnetic moments interact with eachother
They have different precession frequencies so they de-phase  decay of transverse magnetization

45. T2 Relaxation
T2 relaxation time: the time needed for a 63% reduction of transverse magnetization

46. T2 Relaxation
Water and abnormal tissues: long T2 relaxation time  bright
Normal tissues: intermediate T2 relaxation time  gray
Normal
Abnormal
63% reduction

47. T2 Relaxation
T2 weighted images are favored when searching for pathological conditions

48. T2* Relaxation
Dephasing caused by spin-spin relaxation accelerated by de-phasing caused by imperfections in the main magnetic field
Cumulative effect leads to “effective T2 ” or “ T2* ” relaxation

49. T1- “Spin lattice” Relaxation
T1 Relaxation
T1- “Spin lattice” Relaxation
Recovery of longitudinal magnetization
Occurs due to interaction of hydrogen nuclei with their surroundings
Slow
Affected by flip angle (a) of RF pulse
Small (a)  faster net magnetization returns to z-axis

50. T1- “Spin lattice” Relaxation
T1 Relaxation
T1- “Spin lattice” Relaxation
Time required for recovery of 63% of longitudinal magnetization

51. T1- “Spin lattice” Relaxation
T1 Relaxation
T1- “Spin lattice” Relaxation
63% recovery
Fat: short T1 relaxation time  high signal intensity
Good image contrast High anatomic detail

52. T1 Relaxation
T1 weighted images are useful for demonstration of anatomy, especially of small regions where high spatial resolution is needed (eg TMJ)

53. Pulse Sequences
Pulse sequences are carefully coordinated and timed sequence of RF pulses, gradient applications, and intervening time periods which generate a particular type of image contrast

54. Pulse Sequences 2 main categories: Spin Echo sequences
Gradient Echo sequences

55. Pulse Sequences Spin Echo Contrast can be adjusted by variations in:
Repetition time ( TR )
Echo time ( TE )
Transverse magnetization characterized by T2 because extra relaxation due to field inhomogeneities is counteracted

56. Pulse Sequences Gradient Echo
Contrast may be adjusted by variations in:
Relaxation time (TR )
Echo time ( TE )
Flip angle ( a )
Decay of transverse magnetization characterized by T2* ( T1 and PD also ??)

57. Pulse Sequences

58. Obtained by manipulating the parameters of a sequence of RF pulses
Image Production T1
weighted
T2*
weighted T2
weighted PD
weighted
Obtained by manipulating the parameters of a sequence of RF pulses

59. Gradient Coils
3 sets of gradient coils set at right angles to eachother
Each coil produces a magnetic gradient in a particular direction ( x-y-z axes)
When all three used together, a gradient can be produced in any direction and images may be acquired from any plane.
Provide 15 mT/meter

60. Slice Selection
Strength of secondary magnetic field varies linearly along the length of the field to produce a magnetic gradient
Therefore, Larmour frequency of nuclei varies along length also
Select slice to be imaged by applying an RF pulse whose frequency matches the Larmour frequency of the nuclei in that area

61. Slice Selection
Orientation of the slice is perpendicular to the field gradient

62. Slice Thickness Steepness of field gradient Bandwidth of RF pulse
May be selected by manipulating:
Steepness of field gradient
Bandwidth of RF pulse

63. Spatial Encoding
Signals from individual voxels must be distinguished from eachother
Achieved by 2 gradient fields at right angles:
First, the Phase gradient
Second, the Frequency gradient

64. Spatial Encoding
Each voxel: unique phase and frequency

65. Multi-slice and Volumetric Imaging
Multi-slice scanning
Volume ( 3D) scanning

66. Multi-slice and Volumetric Imaging

67. Diagnostic Applications

68. Diagnostic Applications
Axial
Acquisition of images in any plane
Coronal
Sagittal

69. Diagnostic Applications
Obtaining corrected views of TMJ
Oblique sagittal views
Oblique coronal views

70. Diagnostic Applications
Evaluation of articular disc position and morphology
Detection of joint effusion

71. Diagnostic Applications
High Resolution MRI
Evaluation of articular disc integrity

72. Diagnostic ApplicationsCT CT
MRI
MRI
More accurate evaluation of internal structure and extent of soft tissue lesions

73. Diagnostic Applications
Separation of pathological and normal soft tissues
Evaluation of effect of lesions on adjacent soft tissues

74. Diagnostic Applications
MR sialography

75. Diagnostic Applications
Functional Imaging Techniques
Utilize ultra-fast imaging sequences in order to assess function and physiology ( eg of TMJ when opening and closing)

76. Diagnostic Applications
ADVANTAGES OF MRI
Superior anatomic and pathological details in soft tissues
No ionizing radiation
Non-invasive
Imaging possible in several planes without moving the patient
Fewer artifacts

77. Diagnostic Applications
DISADVANTAGES OF MRI
High cost
Special site planning and shielding
Patient claustrophobia
Inferior images of bone
Long scanning times

78. Diagnostic Applications
Contraindications to MR Imaging
Cerebral aneurysm clips
Cardiac pacemakers
Ferromagnetic implants
Metallic prosthetic heart valves
Claustrophobic or uncooperative patients
First trimester of pregnancy ( ?? )

79. Conclusion

80. Image characteristics in MRI are dependent on several factors
These factors may be manipulated to achieve the required quality and contrast according to the specific diagnostic need
Advances in MRI technology are allowing the use of this modality an increasingly versatile ways

81. Thank You
Questions?