MRI utilizes the magnetic spin property of protons in hydrogen atoms. During an MRI scan, protons in the body are exposed to strong external magnetic fields which cause the protons to align. Radio waves are then used to excite the protons, causing them to emit radio signals as they relax back to their original alignment. These signals are detected by receivers in the MRI machine and used to construct images. Different pulse sequences such as spin echo and gradient echo are used to manipulate proton relaxation times and produce T1, T2, or proton density weighted images with varying contrasts. Ultra fast sequences like echo planar imaging allow for very rapid full brain or body imaging.

1. MRI
PHYSICS-1
Roshan Valentine

3. Overview of steps
• The patient is placed in a magnet,
• Radio wave is sent in,
• Radio wave is turned off,
• Patient emits a signal,
• Signal is received
• Image is reconstructed

5. • But before that lets learn some
BASIC PHYSICS

7. • Human body is ~53% water, and
water is ~11% hydrogen by mass
but ~67% hydrogen by atomic percent.
• Thus, most of the mass of the human body
is oxygen, but most of the atoms in the
human body are hydrogen atoms.
• The average 70 kg adult human body
• contains approximately 3 x 1027 atoms
• of which 67% are hydrogen atoms!!!!

8. • But why we bothered about
ONLY HYDROGEN

9. • Simplest element with
atomic number of 1 and
atomic weight of 1
• When in ionic state (H+),
it is nothing but a
proton.
• Proton is not only
positively charged, but
also has magnetic spin
(wobble)!
• MRI utilizes this
magnetic spin property
of protons of hydrogen
to elicit images!!

10. THUS

11. But why we can’t act like magnets?
• The protons (i.e. Hydrogen ions) in body are
spinning in a haphazard fashion,
and cancel all the magnetism.
That is our natural state!

12. Structure of an ATOM
• Atom consistes of a centre nucleus which
Contains a proton and neutron
Around which an electron rotates around it and its own axis
• Thus its analogous to our solar system
• In the nucleus - besides other things - there are protons, little
particles, that have a positive electrical charge
• Any moving electrical charge is an electrical current
• Any moving electric current induces a magnetic field

13. • Thus, the proton has its own magnetic field
and it can be seen as a little bar magnet

14. SPIN
• Proton in its natural state are arranged haphazardly with no net
charge.
• But when exposed to an external magnetic field(B0) they arrange
parallel or antiparallel to the external field.
• Naturally anti parallel needs more energy compared to parallel
• In NMR it is the unpaired nuclear spins that produce a signal in a
magnetic field

15. Precession
• Protons as we all think doesnot just kay there
Instead they undergo PRECESSION
NOW WHAT IS PRECESSION

18. LARMOR EQUATION

19. PHASE
• refers to the position of the magnetic moments on their circular
precessional path
• Out of phase or incoherent means that the magnetic moments of
hydrogen are at different places on the precessional path.
• In phase or coherent means that the magnetic moments of hydrogen
are at the same place on the precessional path.

20. Measuring magnetization

21. TIME FOR SOME VECTORS

22. For easier understanding every proton can be considered as
vectors.
A vector can represent the direction of the force and the
magnitude by its size.Here the force is referred to as the
MAGNETIC FORCE
Magnetization along an external magnetic field cannot be
measured. For this a magnetization transverse to the
external magnetic field is necessary.

23. • Human body can be considered as a large
magnet with its vector along the
Longitudinal axis called LONGITUDINAL
MAGNETIZATION.
• In a strong external magnetic field a new
magnetic vector is induced in the patient,
who becomes a magnet himself. This
new magnetic vector is aligned with the
external magnetic field.
TRANSVERSE
MAGNETIZATION
LONGTIUDINAL
MAGNETIZATION

24. • Magnetization along an external magnetic field cannot be measured.
For this a magnetization transverse to the external magnetic field is
necessary.

25. RESONANCE
• Phenomenon that occurs when an object is exposed to a frequency
close to its natural frequency of oscillation-LARMOR FREQUENCY

26. Lets take an example
ENERGY
RF ≠ LARMOR FREQUENCY

27. But when RF = ω

28. • For resonance of hydrogen to occur, an RF pulse of energy at exactly
the Larmor frequency of hydrogen must be applied.
• Excitation – application of RF pulse that causes resonance to occur.

31. • Flip angle
• Magnitude of flip angle depends on the amplitude and duration of RF
pulse.
• The plane at 90 to B0 is termed transverse plane.
• When resonance occurs, all the magnetic moments are in phase with
each other.

32. MR Signal
• As the NMV precesses at Larmor frequency in the transverse plane, a
voltage is induced in the coil. This voltage constitutes the MR signal.
• The magnitude of the signal depends on the amount of magnetisation
present in the transverse plane.

34. Free Induction Decay
• Recovery – gradual increase of magnetisation in longitudinal plane.
• Decay – gradual decrease of magnetisation in transverse plane.
• The magnitude of voltage induced in the receiver coil also decreases –
FID signal
• This is called free induction decay (FID): ‘free’ because of the absence
of the RF pulse; ‘induction decay’ because of the decay of the induced
signal in the receiver coil

35. CONTRAST MECHANISM
• High Intensity ( white)- High Tm
• Low intensity ( dark) – Low Tm

36. TR(Repitition time )
and TE(Echo time)

37. What determines the contrast
• The inherent energy of the tissue
• How closely packed the molecules are
• How well the molecular tumbling rate matches the Larmor frequency
of hydrogen

38. T1 Recovery
• SPIN-LATTICE energy transfer
• As they loose energy , they regain
• their Lm
• T1 time - defined as the time it
takes for 63% of the Lm to recover.
• The TR determines how much
T1 recovery occurs in a particular tissue
as it occurs during TR

39. T1 Recovery

40. T1 Recovery
Short T1 (Fat)
Long T1 (Water)
contrast

42. T2 Decay
• SPIN-SPIN Energy Transfer
• Due to the intrinsic magnetic fields of the nuclei interacting with each
other.
• Time it takes for 63% of the transverse magnetization to be lost due to
dephasing.
• TE determines the T2 decay as dephasing occurs during then
• Depends on how closely the molecular motion of the atoms
• matches the Larmor frequency and the proximity of other spins.

43. T2 Decay
• Fat has better energy exchange compared to water
HENCE T2 is SHORT FOR FAT.

44. T2 Decay
contrast

45. T1 Weighting T2 Weighting
300
600
10
30
ms ms msms
>2000 >70

46. T1
T2

47. Proton Density(PD)Weighting
• Contrast is predominantly due to differences
in the proton density of the tissues
• Low PD – dark
• High PD – bright
• Cortical bone and air are always dark
• PDW – Decreasing T1 and T2 effects

48. T1 Weighting T2 Weighting
300
600
10
30
ms ms msms
>2000 >70

50. Pulse Sequence Mechanisms
• Magnetic field inhomogeneities cause
the NMV to dephase before intrinsic
magnetic fields of the nuclei can
produce
The main purposes of pulse sequences are:
• to rephase spins and remove inhomogeneity effects and therefore
produce a signal or echo ;
• to enable manipulation of the TE and TR to produce different types of
contrast.

51. Pulse Sequence Mechanisms
To measure relaxation times and produce an image with good contrast
we need to regenerate the signal .Hence pulse sequences

52. Types
• Spin Echo Pulse Sequence
• Gradient Echo Pulse Sequence

53. Spin Echo Pulse Sequence

55. A basic rephasing sequence.

57. • The time taken to rephase after the application of the 180 ° RF pulse
equals the time to dephase when the 90 ° RF pulse was withdrawn.
• This time is called the TAU time.

58. Spin Echo
• Spin echo pulse sequences produce either T1, T2 or proton density
weighting
• • TR controls the T1 weighting • Short TR maximizes T1 weighting
• • Long TR maximizes proton density weighting
• • TE controls the T2 weighting
• • Short TE minimizes T2 weighting
• • Long TE maximizes T2 weighting

59. Spin Echo (SE) Using Single Echo
The timing parameters used are selected to produce a T1 weighted image.

60. Spin Echo With Two Echoes
Produce both a PD and a T2WI
PD
T2WI

61. FAST SPIN ECHO (TURBO SPIN ECHO)
• Faster version of conventional spin echo.
• More than one phase encoding is performed per TR, reducing the
scan time.
• FSE employs a train of 180° rephasing pulses, each one producing a
spin echo. This train of spin echoes is called an echo train. The
number of 180° RF pulses and resultant echoes is called the echo
train length (ETL) or turbo factor.

62. FAST SPIN ECHO (TURBO SPIN ECHO)
• The higher the turbo factor the shorter the scan time
• In T2 weighted scans, water and fat are hyperintense (bright). This is
because the succession of 180° RF pulses reduces the spin–spin
interactions in fat thereby increasing its T2 decay time.

63. Spatial encoding in conventional spin echo.

64. The echo train.

65. Inversion Recovery Sequence

66. Inversion Recovery Sequence
• Inversion recovery (IR) was developed in the early days of MRI to
provide good T1 contrast on low field systems.
• But became obsolete due to high scanning time
• Now back when combined with fast spin sequences

67. Fast Inversion Recovery Sequence
• 180 ° inverting pulse is followed after the TI time by the 90 °excitation
pulse and the train of 180 ° RF pulses to fill out multiple lines of K
space as in fast spin echo.
• instead of being used to produce T1 weighted images, fast inversion
recovery is usually used to suppress signal from certain ti ssues in
conjuncti on with T2 weighti ng so that water and pathology return a
high signal

68. Fast Inversion Recovery Sequence
2 Types
• STIR( short tau inversion recovery)
• FLAIR ( fluid attenuated inversion recovery)

69. STIR
• the time it takes fat to recover from full inversion to the transverse
plane so that there is no longitudinal magnetizati on corresponding to
fat. This is called the null point
• Uses
STIR is an extremely important sequence in musculoskeletal imaging
because normal bone, which
• contains fatty marrow, is suppressed and lesions within bone such as
bone bruising and tumors are seen more clearly (Figures 5.18 and 5.19
). It is also a very useful sequence for suppressing fat in general MR
imaging ( see Chapter 6).

70. FLAIR
• FLAIR is used to suppress the high CSF signal in T2 weighted images
• TI corresponding to the time of recovery of CSF from 180 ° to the
transverse plane nulls the signal from CSF
• To see periventricular and cord lesions more clearly

71. STIR FLAIR

72. Gradient Echo Pulse Sequence
• Gradient is used to reduce
magnetic homogeneity
• Principle based on Larmor equation

73. Gradient Echo Pulse Sequence
• After the RF pulse is withdrawn, FID signal is immediately produced
due to magnetic field inhomogeneities and T2* dephasing occurs.
• The gradient rephases the magnetic moments so that a signal can be
received by the coil, which contains T1 and T2 information.
• This signal is called a gradient echo

74. How gradients dephase

75. How gradients rephase

76. Advantages
• Rephase faster.
• Low flip angles (<90 )
• Shorter TR
• Shorter scan times than spin echo sequences

77. Gradients
GRADIENT SLICE SELCTION
FREQUENCY
ENCODING
PHASE ENCODING

78. Slice Selection

80. Frequency encoding

81. Slice selection

82. Frequency encoding

83. Phase encoding

84. So in spatial encoding…..

85. Slice Selection

86. STEADY STATE
• TR is shorter than both T1 and T2 of all tissues.
• There is no time for the transverse magnetization to decay before the
pulse pattern is repeated again
• NMV remains steady during data acquisition

87. Steady state
• Magnitude of tm keeps accumulating as it doesn’t have time to delay
over successive TRs.
• Tissues with long T2 times (mainly water) appear bright.
• Most gradient echo sequences utilize the steady state because the
TRs are so short that the fastest scan times are achievable.

88. Steady state

89. Gradient Recalled Acquisition in Steady State
GRASS
• Coherent Gradient Sequence
• Uses steady state principle
• Has a rewinder/rephasing gradient to keep residual TM coherent so
that it is in phase at the beginning of next repetition.
• By reversing the slope of phase encoding gradient after read out.

90. allows tissues with long T2 time to produce bright signal
(blood,CSF,synovial fluid).

91. Spoiled Gradient Recalled Acquisition
SPGR
• Incoherent(Spoiled) Gradient Sequence
• utilizes the steady state by using very short TRs.
• Eliminates transverse magnetization so that tissues with long T2
times are not allowed to dominate image contrast but T1/proton
density contrast prevails – spoiling
• Gradient spoiling to dephase the residual transverse magnetization.
• RF spoiling applies RF excitation pulses at different phases every TR so
that residual transverse magnetization has different phase values
• Primarily used for T1 weighting

92. Steady state free precession (ssfp)
• steady state is maintained .
• Every TR an excitation pulse is applied. When the RF is switched off, a
FID is produced.
• After the TR, another excitation pulse is applied .
• Rephases the FID produced from the previous excitation.

93. ssfp

94. SSFP
• a rewinder gradient is used to speed up the rephasing process after the RF
rephasing has begun.
• Rewinding moves the echo so that it occurs before the next excitation pulse,
rather than during it.
• Measures true T2 weighting long TE
rephasing done by RF pulse and not gradient (
less T2* and more T2)

95. ULTRA FAST SEQUENCES
very fast pulse sequences that can acquire several slices in a single
breath-hold.
Many ultrafast sequences use extra pulses applied before the pulse
sequence begins

96. ULTRA FAST SEQUENCES
How ?
• Applying only a portion of the RF excitation pulse, so that it takes
much less time to apply and switch off ;
• Reading only a proportion of the echo (partial echo);
• Using asymmetric gradients which are faster to apply than
conventional balanced gradient;
• Filling K space in a single shot or in segments (Echo Planar Imaging)

97. ECHO PLANAR IMAGING (EPI)
• Collects all the data required to fill all the lines of K-space from a
single echo train.
• Multiple echoes are generated spin EPI
Gradient EPI
• Rapid data acquisition (axial images of brain in 2-3 secs, whole body
imaging in 30 secs. )

98. EPI
18 seconds – mri pelvis