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

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Presentation on theme: "Magnetic Resonance Imaging - MRI Asma’a Al-Ekrish BDS Demonstrator ( OMF Radiology ) King Saud University College of Dentistry."— Presentation transcript:

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

2 Sources Source of illustrations: Understanding MRI - An interactive guide to MRI principles and applications Philips Medical Systems Best, The Netherlands

3 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.

4 Introduction RF pulse

5 Lecture Contents Magnetic Resonance Image Production Diagnostic Applications

6 Magnetic Resonance

7 Magnetic Nuclei Electric charge + spinningTiny magnetic field

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

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

10 Magnetic Nuclei Abundant in the human body and have a large magnetic moment 1H1H

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

12 Precession

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

16 Magnetic Resonance Magnetic resonance induced by exposing nuclei to a second magnetic field B 1 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 Body coil ( fixed inside magnet ) VOLUME COILS TMJ COILS Head coil

19 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 points in the same direction as the main magnetic field Net Magnetization

21 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 LONGITUDINAL MAGNETIZATION Net Magnetization

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

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

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

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 (  Pulses are named after the flip angle they induce

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

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

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

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

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

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

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

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

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

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

37 Image Production

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

39 Image Production Signal intensity depends on: 1. 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: 2. Differences in T1 and T2 relaxation rates between tissues Stronger source of contrast 3. Flow 4. Susceptibility 5. Diffusion 6. Perfusion

41 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 T1 relaxation T2 relaxation

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

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 63% reduction Normal Abnormal

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

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

51 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: 1.Spin Echo sequences 2.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 (  ) Decay of transverse magnetization characterized by T2* ( T1 and PD also ??)

57 Pulse Sequences

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

59 Gradient Coils 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 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)

60 Slice Selection 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 Strength of secondary magnetic field varies linearly along the length of the field to produce a magnetic gradient

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

62 Slice Thickness May be selected by manipulating: 1. Steepness of field gradient 2. 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 VolumetricMulti-slice

67 Diagnostic Applications

68 Acquisition of images in any plane Sagittal Axial Coronal

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

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

71 Diagnostic Applications Evaluation of articular disc integrity High Resolution MRI

72 Diagnostic Applications CT MRI CT 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 Questions? Thank You


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