MRI PHYSICS RADIOLOGY

1. Dr Girish Gunari
Moderator – Dr Ram Prakash HV

2. Atomic structure
All things are made of atoms , including the human body
Atomic number is
the sum of the
protons
Mass number is
the sum of the
protons and
neutrons in the
nucleus
Electrons Protons Neutrons

3. Nuclei with an odd mass number
i.e different number of protons to neutrons ----
are important in MRI
Number of neutrons = protons [ mass number is an
even number]

4. Three types of motion are present within the atom
• electrons spinning on their own axis
• electrons orbiting the nucleus
• the nucleus itself spinning about its own axis. [ spine of
proton –spine of neutron]

5. Net spine of nucleus = spine of proton –spine of
neutron

6. Law of electromagnetic induction
Michael Faraday

7. MR active nuclei
Charged particle ------- spinning [in motion]-----
act as magnet – align their axis of rotation to an applied
magnetic field

8. Important examples of MR active nuclei,
together with their mass numbers[isotopes]
Hydrogen -1
Carbon -13
Nitrogen -15
Oxygen -17
Fluorine- 19
Sodium -23
Phosphorus- 31

9. Hydrogen is very abundant in the
human body

10. The hydrogen nucleus [protium - isotope]
hydrogen nucleus - one positively
charged-proton
spin [moves]—magnetic field induced
Acts as a small magnet
[MR active nucleus ]--used
in clinical MRI.
Electron
Proton

11. Magnetic moment of each nucleus has vector
properties,
Direction of the vector
designates the direction
of the magnetic momentum
Length of the vector
designates the size of
the magnetic moment

12. Alignment

13. Max Planck in 1900-quntum theory

14. spin - up nuclei
spin- down nuclei

15. Net magnetic moment= larger number aligned parallel -
small number in anti parallel
Magnitude of the NMV is larger at ----high field
strengths than low field strengths, resulting in
improved signal

17. Precession
Each hydrogen nucleus is spinning on its axis as in The influence of B0
produces an
additional spin or wobble of the magnetic moments of hydrogen around B 0 .
This secondary spinis called precession
It causes the magnetic moments to follow a circular path
around B 0 . This
path is called the precessional path and
the speed at which they wobble around B 0 is called the
precessional frequency/ Larmor frequency .

18. The Larmor equation
ω0 = B0 × λ
ω0 is the precessional frequency
B 0 is the magnetic field strength of the magnet
λ is the gyromagnetic ratio. It is constant and specific to the atom
Different MR active nuclei have different gyromagnetic ratios, so have
different precessional frequencies at the same field strength. In
This allows us to specifically image hydrogen and ignore the other MR active
nuclei in the body. The gyromagnetic ratio of hydrogen is 42.57 MHz/T.

19. RESONANCE
Energy transition that occur when object is subjected
to frequency the same as its own.

20. If energy is delivered at a frequency to that of
the Larmor frequency of the nucleus, -nucleus
gains energy

21. If energy is delivered at a different frequency to that of
the Larmor frequency of the nucleus, resonance does
not occur.

22. 1] Energy
Absorption
The magnitude
of
the flip angle
depends on the
amplitude and
duration of the
RF pulse.
Resonance
results in

23. Usually the flip angle is 90 ° , i.e. the NMV is
given enough energy by the RF pulse to move
through 90 ° relative to B 0 .
B 0 is now termed the longitudinal plane.
The plane at 90° to B 0 is termed the transverse plane.

25. 2] Other result of resonance is that the
magnetic moments of hydrogen nuclei
move into phase with each other.

26. Out of phase (or incoherent ) are not in
the same place on the precessional
path

27. result of resonance, in phase or coherent
magnetization precesses at the Larmor
frequencyin the transverse plane

28. The MR signal
NMV rotates around transverse plane. It passes across
Receiver Coil inducing voltage in it.
[Faraday ’ s law of electromagnetic induction]

29. When RF removed
Recovery – gradual increase of magnetisation in
longitudinal plane.
During relaxation hydrogen nuclei give up absorbed
RF energy to surrounding lattice and the NMV
returns to B 0 [Spin lattice relaxation ]
Recovery of longitudinal magnetization is
caused by a process termed T1 recovery

31. The free induction decay signal ( FID)[T2 decay ]
Decay – gradual decrease of magnetisation in
transverse plane.
Magnetic moments of
hydrogen lose coherency
due to dephasing.
decay of magnetization in
the transverse
plane.[spin-spin
relaxation]
The decay of transverse
magnetization is caused
by a process termed T2
decay.

34. T1 recovery, T2 decay depend on three
factors:
1] The inherent energy of the tissue .
2] How closely packed the molecules are.
3] How well the molecular tumbling rate
matches the Larmor frequency of hydrogen.

35. 1] The inherent energy of the tissue
Inherent energy is low, then the molecular lattice is more able to absorb energy
from hydrogen nuclei during relaxation. –T1 recovery occurs fast
In tissues with a high inherent energy that cannot easily absorb energy from
hydrogen nuclei. during relaxation.
T 1 recovery occurs slow
This is especially important in T1 relaxation processes, which depend on
energy exchange between the hydrogen nuclei and the molecular lattice (spin
lattice).

36. 2 ] How closely packed the molecules are.
How closely packed the molecules are.
In tissues [FAT} where molecules are closely spaced, there is
more efficient interaction between the magnetic fields of
neighboring hydrogen nuclei[spin –spin interaction] –
dephasing will be fast –T2 decay will be fast
In tissues where molecules are spaced apart [WATER}- less
interaction between the magnetic field of neighboring
hydrogen nuclei- dephasing will be slow – T2 decay slow

37. 3] How well the molecular tumbling rate matches the
Larmor frequency of hydrogen.
1] I f there is a good match between the two, energy
exchange between hydrogen nuclei and the molecular
lattice is efficient. (resonance,)
2] When there is a bad match, energy exchange is not as
efficient.

38. 1] Molecules are closely packed
together [spin-spin]
-dephase fast-T2 decay fast
2] low inherent energy- give energy to
lattice easily –T1 recovery fast
3] Molecular tumbling rate matches the
Larmor frequency and allows efficient
energy exchange from hydrogen
nuclei to the surrounding molecular
lattice
-T1 recovery fast
T1 RECOVERY FAST [FAT IS FAST]
T2 DECAY FAST
Fat water
1]Molecules spaced apart-
[spin-spin]dephase slow -T2 Decay
slow
2] High inherent energy-
Lattice will not take energy
easily –T1 recovery slow
3] Molecular tumbling rate does
not match the Larmorfrequency
and does not allow efficient
energy exchange –
T1 recovery slow
T1 RECOVERY SLOW
T2 DECAY SLOW
[WATER WAITS ]
Relaxation in different tissues

42. Contrast mechanisms
MR image has contrast
If there are areas of high signal --- white on the image
large transverse component of coherent magnetization at
time TE
Areas of low signal ----------------- dark on the image
Tissue returns a low signal if it has a small transverse
component of coherent magnetization at time TE
Areas with intermediate signal ---- gray in between white
and black

43. T 1 contrast [FAT –WHITE WATER-DARK]

44. T 2 contrast FAT- DARK WATER-WHITE

45. Proton density contrast
proton density of a tissue is the number of mobile hydrogen
protons per unit volume of that tissue.
The higher the proton density of a tissue [brain tissue], the
more signal available from that tissue
The Lower the proton density of the tissue [cortical bone], the
less is the signal available from that tissue

47. Weighting
All the contrast parameters [T1, T2 and Proton
density] simultaneously affect image contrast and
would therefore produce images of mixed contrast.
So we need to weight image contrast towards one
of the parameters [T1] and away from the others
[T2] [to get T1 weighted image ].

48. T 1 weighting – short TR

49. T 1 to increase TE
T 2 to suppress

50. T 2 weighting- longer TE

51. T 1 to suppress TE T 2 to increase

52. Proton density weighting
To achieve proton density weighting, the effects of T1
and T2 contrast must be diminished
Small TR- T1
Long TE –T2
Long TR and Small TE- proton density
T 1 to suppress T 2 to suppress