mri

1. MAGNETIC RESONANCE IMAGING
Seminar - 9
Presented by
N.Narmatha
II year pg

2. INTRODUCTION
• Paul Lauterbur described the first magnetic resonance image in
1973
• Peter Mansfield further developed use of the magnetic field and
the mathematical analysis of the signals for image reconstruction.
• MRI was developed for clinical use around 1980

3. INDICATIONS
• Assessment of intracranial lesions and the pituitary ,spinal cord.
• Tumor staging—evaluation of the site, size and extent of soft tissue
tumors and tumor like lesions, involving
– The salivary glands.
– The pharynx.
– The larynx.
• Investigations of the TMJ to show the marrow changes and soft
tissue components of the joint including the disk.

4. ABSOLUTE CONTRAINDICATION
• Patient with implanted metallic foreign objects or
medical devices that consist of or contain ferromagnetic metals
(e.g., cardiac pacemakers, some cerebral aneurysm clips or
ferrous foreign bodies in the eye).

6. Patient is placed inside large magnet
Magnetic field - nuclei of hydrogen atom to align
Scanner – radiofrequency pulse - Hydrogen nuclei
Resonate (absorb energy)
RF pulse is turned off
Stored energy is released from the body
Signal (coil)
MR image

7. • Nucleons – individual protons and neutrons in the nuclei of all
atoms
SPIN

8. • Hydrogen - most abundant
of these atoms in the body.
Nuclei that do not exhibit this
characteristic will not produce an
NMR signal.

9. MAGNETIC DIPOLES
• Magnetic resonance active nuclei, are hydrogen, carbon13,
nitrogen 15, oxygen 17, fluorine 19, sodium 23, and phosphorus
31.

12. proton rotating
about its own axis
and generates a
magnetic field
proton not only rotates about
its own axis but also
"wobbles" about the axis of
B0

13. Tilting of the spin axis - precession

14. • The rate or frequency of precession is called the precessional
resonance, or larmor frequency.
an external field depends on the strength of the field.
f Larmor= μ B0
• Magnetic resonance field strengths - 0.1 to 4 tesla (T)
• 1.5 T - most common.
• The larmor precession frequency of Hydrogen is 63.86 megahertz

15. RESONANCE
• Nuclei can be made to undergo transition from one energy state to
another by absorbing or releasing energy
• Electromagnetic energy in the RF portion of the electromagnetic
spectrum - Energy required for transition

16. Longitudinal Magnetic Vector

17. Transverse Magnetic Vector

18. 90-degree RF pulse or having a flip
angle of 90 degrees - will give the
maximum transverse magnetization

19. MAGNETIC RESONANCE SIGNAL

20. • Tightly bound hydrogen atoms - produce only a weak signal.
• Loosely bound or mobile hydrogen atoms - react to the RF pulse
- produce a detectable signal at the end of the RF pulse.
• The concentration of loosely bound hydrogen nuclei available
to create the signal is referred to as the proton density or spin
density of the tissue in question

21. RELAXATION
When the RF pulse is turned off, the nuclei begin to return to their
original lower-energy spin state, a condition called relaxation .
Reduction of the magnetization in the transverse plane - decay

22. Free induction decay (FID) signal - reduced voltage induced in the
receiving coil
Results from the
• loss of the transverse net
magnetization vector.
• return of the net magnetization vector to the longitudinal plane
and dephasing of the hydrogen nuclei.

23. T1 RELAXATION
Individual hydrogen nuclei (spin)
Energy transfer
Surrounding molecules (lattice)
Recovery of the longitudinal magnetization

25. • Time required for 63% of the net magnetization to return to
equilibrium by this transfer of energy – T1 relaxation time or
spin-lattice relaxation time
T2 RELAXATION TIME
The time constant that describes the exponential rate of loss of
transverse magnetization - T2 relaxation time or the spin-spin
relaxation time.

27. RADIOFREQUENCY PULSES SEQUENCES
TR time - duration between repeat RF pulses
• Determines the amount of T1 relaxation that has occurred at
the time the signal is collected
TE time - time after application of the RF pulse when the magnetic
resonance signal is read.
• It controls the amount of T2 relaxation that has occurred
when the signal is collected.

28. A BASIC PULSE SEQUENCE

29. TISSUE CONTRAST
Image contrast between tissues is governed by
• Intrinsic features of the tissues
• Proton density,
• T1 and T2 times,
• TR and TE times

30. T1 WEIGHTED IMAGE
• Differences in the T1times between fat and water
• TR must be short enough (300 to 700 ms), short TE times (20ms),
so that neither fat nor water has sufficient time to fully return to B0
and recover their longitudinal magnetism fully.

31. • Tissues with fast T1 times – fat - appear bright
• Long T1 times - cerebrospinal fluid (water) - appear dark.
• More commonly used to demonstrate anatomy.

32. T2 weighted image
• Differences in the T2 times between fat and water
• To achieve T2 weighting,
• TE must be long enough(60 ms or more) to give both fat
and water time to decay. Long TR times(2000 ms)
• If the TE is too short, neither fat nor water has time to
decay, and therefore the differences in their T2 times are
not demonstrated

33. • Tissues with long T2 times – CSF or temporomandibular (TMJ)
joint Fluid - appear bright
• Tissues with short T2 times – Fat - appear dark.
• Most commonly used for identifying inflammatory or other
pathologic changes.

35. Relationships between TR, TE and the resulting image contrast
Short TE Long TE
Short TR T1-weighted T1-and T2-weighted
– no practical
application
Long TR Not T1, not T2-weighted
T2 but density
weighted

37. HEMMORRHAGE

40. CONTRAST AGENTS
• Gadolinium - administered intravenously to improve tissue
contrast
• It shortens the T1 relaxation times of enhancing tissues - making
them appear brighter.

41. ENCODING
Gradients are alterations to the main magnetic field - generated by
coils of wire - within the bore of the magnet - current is passed -
induces a gradient (magnetic) field around it, which either subtracts
from, or adds to the main static magnetic field B0

43. Gradients - used to either dephase or rephrase the magnetic
movements of nuclei , can also perform
– Slice selection: locating a slice within the scan plane
Selected(Z)
– Frequency encoding: spatially locating (encoding) signal along
the long axis of the anatomy.(Y)
– Phase encoding: spatially locating (encoding) signal along the
short axis of the anatomy.(X)

44. • The data of each signal position is collected - information is
stored in the array processor of the system computer in K space -
then converted mathematically by a process called as Fast
Fourier Transform

45. Slice selection – exclusive excitation of spins in one slice performed
by the coincident combination of a gradient magnetic field and a
narrow bandwidth or slice selective RF pulse at a specific Larmor
frequency.

46. • Slice thickness – the thickness of an imaging slice.

47. Abnormalities on MRI is refered as

48. IMAGING PLANES

60. DIFFUSION WEIGHTED IMAGE:
• Asses the ease which the water molecues
move around within a tissue
• Fluid :no restriction to diffusion
• Known as shine through

65. Is it a CT OR MRI

66. Advantages :
• Best contrast resolution of soft tissues
• No ionizing radiation
• Direct multiplanar imaging is possible without reorienting the
patient

67. DISADVANTAGES
• Long imaging times
• Potential hazard imposed by ferromagnetic metals in the vicinity
of the imaging magnet.
• Claustrophobic
 Metals used in dentistry for restorations or orthodontics will not
move but may distort the image in their vicinity.
 Titanium implants cause only minor image degradation.

68. References:
1. Textbook of Dental and Maxillofacial Radiology,
Freny R Karjodkar,3rd edition
2. Principles and interpretion of oral radiology,white
and pharoah.
3. MRI – essentials in practice
4.Handbook of MRITechnique Catherine Westbrook

69. THANKYOU