This is a presentation describing in brief regarding the physics behind MRI and it's application from dental point of view. It contains few videos as well.

1. MRI
(MAGNETIC
RESONANCE
IMAGING)
Prepared by-
DIVYA PANDAY
Under the guidance of –
• Dr. Sunita
• Dr. Sujoy Ghosh
• Dr. Meera chaudhary
• Dr. khushboo

2. CONTENTS
 Introduction
 Protons
 Precession
 Resonance
 Magnetic resonance signal
 T1 and T2 relaxation
 Radiofrequency pulse sequence
 Tissue contrast
 Scanner gradient
 Advantages
 Disadvantages
 Applications
 references

4. PROTONS
 Proton – spin – angular momentum
 Spin – associated with charge – B generated in
nuclei – act as magnet– magnet dipole has
magnetic moment.

8.  Magnetic dipoles randomly oriented –Bext -
SPIN UP
SPIN
DOWN

9. PRECESSION
 Magnetic moment of H+ in a Bext – do not align
exactly with the direction of Bext
– TILT from a
position exactly parallel to Bext

11.  Bext – axis of spinning of H+ to wobble(precess) around
the lines of B ext.
 This frequency of precession
• PRECESSION FREQUENCY/
• RESONANCE FREQUENCY/
• LARMOR FREQUENCY
 Depends on
• Atom
• Strength of Bext- 0.1-4 Tesla

12. RESONANCE
 Energy to energy –
provided by radiofrequency(RF) spectrum of
EM spectrum.
 When RF = larmor frequency  resonance.

13. COIL PATIENT RESONANCE
Spin up to spin
down
BH+ longitudinal
ses
RF
Causes the
H+ to
precess in
phase
BH+
transverse

16.  Longer RF pulse –BH+ longitudinal
BH+ transverse
 90 degree RF pulse/flip angle of 90 degree
Significant intensity and duration –
BH+ longitudinal sed to 0

17. MAGENTIC RESONANCE
SIGNAL
Current in a
receiver coil
Precession of
H+ in phase
MR signal

19.  Frequency of this AC current = RF pulse =
larmor frequency.
 Magnitude of MR signal
1. Density of H+
2. Loosely bound H+

20. BH+ longitudinal
returned to
original
Dephase
RELAXATION
RF turned
off
 BH+
Transverse-
DECAY
MR signal

FREE
INDUCTION
DECAY SIGNAL

22. BH+ longitudinal
returned to
original
Dephase
RELAXATION
RF turned
off
 BH+
Transverse-
DECAY
T2 -
magnetic
moment of H+
interact with
each other
SPIN SPIN
RELAXATION
TIME
T1 – energy
absorbed by
lattice
SPIN –
LATTICE
REALAXATIO
N TIME
T1 and T2 RELAXATION

23. T1 RELAXATION TIME
 It is an exponential process and the time
required for 63% of the magnetization to
return to equilibrium (the time constant) by
this transfer of energy.

24.  Varies with tissue
 Ability to transfer their excess energy to lattice
fluid
content
TI TI
fat
content
 Inherent
energy of
water-
Inhibit
transfer of
energy
 Inherent
energy-
quickly
transfer
energy

25. T2 RELAXATION TIME
 Time constant that describes the exponential
rate of loss of transverse magnetization.

26.  Varies with tissue
 Depend on packing of tissues.
fluid
content
T2 T2
fat
content
Closely packed
- interaction
Loosely
packed -
interaction

27.  T2 occurs more rapidly then T1

28. RF PULSE SEQUENCE
Components of RF pulse sequence are set by
the operator.
Can be used to emphasize various features of
tissue being examined

30. TR and TE
 TR(REPETITION TIME) : duration between
repeat RF pulses.
 TE (ECHO TIME) : time after application of
the RF pulse when the MR signal is read.

31. BH+ longitudinal
returned to
original
Dephase
RELAXATION
 BH+
Transverse-
DECAY
Controlled
by TE
Controlled by
TR

32. IMAGE CONTRAST
INTRINSIC-
Features of
tissues.
EXTRINSIC-
Parameters of a
given pulse
• H+ density
• T1
• T2
• TR
• TE

33. T1 –Weighted Image
Emphasizes differences in T1 values of tissues
Accomplished by
1. short TR(300-700ms)
2. short TE(20ms)

34. Tissue with
short T1
Tissue with
long T1
BRIGHT
DARK
• anatomy
USE

35. T2 –Weighted Image
Emphasizes differences in T2 values of tissues
Accomplished by
1. Long TR(2000ms)
2. Long TE(≥60ms)

36. Tissue with
short T2
(eg-fat)
Tissue with
long T2
(eg-TMJ fluid
CSF fluid)
DARK
BRIGHT
USE
• Pathology- since pathologic tissue has more water.

37. OTHER METHODS
 STIR(short tau inversion recovery) – dec signal
from fat(FAT SATURATION) – better visualization
of adjacent structures.
 FLAIR(fluid attenuation inversion recovery) - dec
signal from fluid – better visualization of
pathology.
 CONTRAST AGENT – gadolinium.
It dec T1 of enhancing tissue – they appear
bright.

38. SCANNER GRADIENT
 3 gradient coils
1. X-axis : left to right
2. Y-axis : anterior to posterior
3. Z-axis : head to toe
 Intensity of Bext can be modified when gradient applied
 Z gradient turned on
 precessional frequency of H+ varies linearly along Bext
 RF pulse applied – nuclei with RF = PF – resonate
 Hence, desired slice selected.
 Z gradient coil turned off – X(phase encoding) and Y(frequency
encoding) gradients are turned on – location of signal within xand y
is determined.

40. ADVANTAGES
Time
Presence of ferromagnetic material
Claustrophobic patient
Best contrast resolution of soft tissues
No ionizing radiation
Multiplanar image without reorienting the patient
DISADVANTAGES

41. APPLICATIONS
 Soft tissue condition – position and integrity of disk of
TMJ
 Soft tissue disease –
 tongue, cheek, salivary gland, neck – neoplasia
 lymph nodes - malignant involvement
 Perineural invasion - by malignant neoplasia
 Extent of penetration of carcinoma- in cortex of mandible
– SWIFT
 Neoplasia - Gadolinium enhanced

42.  Osteomyelitis – edematous change in marrow
and surrounding tissue
 Location of mandibular nerve
 MR angiography

43. closed
NORMAL TMJ

44. Anterior disk displacement with reduction
closed
Lateral
displacement
Open-
Normal

45. T2
image -
effusion
Anterior disk displacement without reduction
T2 image-
effusion
T1 image-
anterior
displacement
T1 image-
remains
anterior to
condyle

46. a. T1 image-
bow tie
appearance
b. T2 image-
inflammatory
effusion
c. Anterior disk
displacement
TMJ

47. DEGENERATIVE JOINT
DISEASE

48. a. T1
image
b. T1 post
gadolinium
c. T2
image
RHABDOMYOSARCOMA- soft tissue of right face

49. MULTIPLE MYELOMA- right mandible and right carotid space
T1 post-
contrast(fat
saturation)
T1
image
T2 post-
saturation(fat
saturation)

50. T1 image post-
gadolinium,
Fat saturation
T2 image
T1 image
ADENOID CYSTIC CARCINOMA in submandibular gland

51. T2 image –
hyperintense to
muscle
T1 image –
isointense to
muscle
PLEOMORPHIC ADENOMA

52. CYST IN LEFT
PAROTID GLAND

53. SIALECTASES of parotid gland
T2
image

54. ANGIOGRAPHY

55. 1. RF pulse causes
a. B transverse
b. B longitudinal
c. Both
d. None
2. T1 weighted image is used for studying
a. Anatomy
b. pathology
3. T1 weighted image uses
a. Short TR, short TE
b. Long TR, long TE
c. Short TR, long TE
d. Long TR, short TE

56. 4. In T2 weighted image, tissue with long T2 (eg- TMJ fluid) appears
a. Bright
b. Dark
c. Both
d. None
5. Which diagnostic method would you prefer for diagnosing collection of effusion in
TMJ
a. T1
b. T2
c. Both
d. None
6. Which diagnostic method would you prefer for diagnosing disk displacement in
TMJ?
a. T1
b. T2
c. Both
d. none

57. 7. This is a case of lymphoepithelial cyst in right parotid
space, which is T2?
a. A
b. B

58. 8. MRI uses
a. Ionizing radiation
b. Non-ionizing radiation
c. Both
d. None
9. MRI is used for-
a. Neoplasia of soft tissue
b. Mallignant involvement of lymph nodes
c. Both
d. none
10. Contrast agent used is ________?

59. REFERENCES
 White and pharaoh – 1st South Asian edition

60. THANKYOU