1. MRI PHYSICS

2. PEHCHAN KAUN

3. NOBEL PRIZE 2003
PETERMANSFIELD
Paul lauterbur

4. MRI: What is it?
Magnetic Resonance Imaging
Magnet
Radio Frequency = Resonance
Imaging

5. • NMR measures the net magnetization of atomic nuclei in
the presence of magnetic fields
• Magnetization can be manipulated by changing the
magnetic field environment (static, gradient, and RF fields)
• Static magnetic fields don’t change (< 0.1 ppm / hr):
The main field is static and (nearly) homogeneous
• RF (radio frequency) fields are electromagnetic fields that
oscillate at radio frequencies (tens of millions of times per
second)
• Gradient magnetic fields change gradually over space and
can change quickly over time (thousands of times per
second)

6. WE ARE MAGNETS!
Really?

7. • The adult 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. Why the fuss about hydrogen?
• Nucleus needs to have 2 properties:
– Spin
– charge
• Nuclei are made of protons and neutrons
– Both have spin ½
– Protons have charge
• Pairs of spins tend to cancel, so only atoms with an
odd number of protons or neutrons have spin
– Good MR nuclei are 1H, 13C, 19F, 23Na, 31P

9. Y HYDROGEN?? Cont…
• 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!!
• We are magnets!

10. A Single Proton
There is electric charge
on the surface of the proton,
thus creating a small current
loop and generating
magnetic moment .
J
+
+
The proton also has
mass which generates
an
angular momentum
J when it is spinning.
+
Thus proton “magnet” differs from the magnetic bar in that it
also possesses angular momentum caused by spinning.

11. SPIN!!
• Protons and neutron spins are known as
nuclear spins.
• An unpaired component has a spin of ½
and two particles with opposite spins
cancel one another.
• In NMR it is the unpaired nuclear spins
that produce a signal in a magnetic field

13. 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!
• We need to discipline
them first, how?

14. • When placed in a large magnetic
field, hydrogen atoms have a strong
tendency to align in the direction of
the magnetic field
• Inside the bore of the scanner, the
magnetic field runs down the center
of the tube in which the patient is
placed, so the hydrogen protons will
line up in either the direction of the
feet or the head.
• The majority will cancel each
other, but the net number of protons
is sufficient to produce an image.
5-spin down 7spin up

15. Precession
• The static field causes the spinning proton
to ‘wobble’ in a regular manner called
‘PRECESSION’

17. LARMOR EQUATION

18. TIME TO WAKE UP…
REGARDING MRI SAY TRUE/FALSE
• The gyro magnetic ratio of a hydrogen atom is
42.57MHz/T
• The strength of magnetic field is measured in
gauss
• The Larmor frequency is defined as SUM of the
magnetic field and gyromagnetic ratio.
• An MR active nucleus spins at its own
precessional frequency.
• ONLY hydrogen nuclei are said to be MR active

19. SACH KA SAMNA….
REGARDING MRI SAY TRUE/FALSE
• The gyro magnetic ratio of a hydrogen atom is
42.57MHz/T T
• The strength of magnetic field is measured in
gauss T
• The Larmor frequency is defined as SUM of the
magnetic field and gyromagnetic ratio. F
• An MR active nucleus spins at its own precessional
frequency. T
• ONLY hydrogen nuclei are said to be MR active  F

20. IN A NUT SHELL

21. Now, its time to listen to radio in
RESONANCE.
►Pushing a swing in time with natural
interval of the swing will make the swing
higher and higher.
►Similarly, radio frequency pulses in
resonance push the aligned protons (H+) to
a higher energy level.

22. WATS RESONANCE????
Def:
Energy transition that occur when
object is subjected to frequency the
same as its own.
Here;
Radio Frequency "RF"
Same Frequency of H nuclei
At 90 degree to B0

23. 2 things happen at Resonance:
1- Energy Absorption
Increase number of High energy Spin Up
nuclei
2- Phase Coherence
NMV precesses in transverse plane at Larmor
Frequency

24. • Energy Absorption:
–The MRI machine applies
radio frequency (RF)
pulse that is specific to
hydrogen.
–The RF pulses are applied
through a coil that is
specific to the part of the
body being scanned.

26. MR Signal
• NMV rotates around transverse plane.
It passes across Receiver Coil inducing
voltage in it.
• RF Removed  Signal decreased  Amplitude
of MR Signal decreased
• Free Induction Decay "FID":
– Free (No RF Pulse)
– ID (because of Decay of Induced signal in Receiver
Coil)

27. SUMMARY TILL NOW…..ENJOYY…

28. Measuring the MR Signal:
– the moving proton vector
induces a signal in the RF
antenna
Measuring the MR Signal
z
– The signal is picked up by a
coil and sent to the computer
system.
the received signal is
sinusoidal in nature
– The
computer
receives
mathematical data, which is
converted through the use of a
Fourier transform into an
image.
RF signal from
precessing protons
y
x
RF antenna

29. Now, we re-transmit the energy for
image processing
►The emitted energy is too small (despite
2500 times the magnetic field with
resonance RF pulse) to convert them into
images.
►Hence, repeated “ON-OFF” of RF pulses are
required.
►The emitted energy is stored (Kspace), analysed and converted into images.

31. • Imaging:
– When the RF pulse is turned off the
hydrogen protons slowly return to their
natural alignment within the magnetic field
and release their excess stored energy. This
is known as relaxation.
• What happens to the released energy?
– Released as heat
OR
– Exchanged and absorbed by other protons
OR
– Released as Radio Waves.

32. The Image

33. Relaxation Process
1-NMV recovers and
realign to B0
this process called "T1
Recovery"
2-Nuclei loose
Precessional
coherence or dephase
and NMV decay in the
transverse plane
this process called "T2
Decay"

34. T1 time & T2 Decay are an intrinsic contrast
parameter that are inherent to tissue being
imaged.
T1WI

35. Definitions
Echo Time "TE"
Time between RF excitation pulse and
collection of signal
(it affects the length of relaxation period
after removal of RF excitation pulse
and the peak of signal received in receiver
coil)

36. • Repetition Time "TR"
Time from application of one RF pulse
To the application of the next
(it affects the length of relaxation period
after application of one RF excitation pulse
to the beginning of the next).

38. Flip Angle
Angle throw which the NMV moved as
result of a RF excitation pulse

39. Parameters
Image contrast controlled by:
1- Extrinsic Contrast parameters:
TR, TE & Flip Angle
2- Intrinsic Contrast parameters:
T1 Recovery, T2 Decay, Proton Density, Flow &
Apparent Diffusion Coefficient

40. T1 Recovery
T1 Recovery
Caused by exchange of energy from
nuclei to their surrounding environment or lattice
"Spin Lattice Energy Transfer"
and realign in B0
this occur in exponential process
at different rates in different tissue
NB: Molecules are constantly in motion; Rotational and
Transitional

41. T1 Recovery
T1 in Fat
T1 in Water
absorb energy quickly
T1 is very short
i.e. nuclei dispose
their energy to
surrounding fat tissue
and return to B0 in
very short time
inefficient at receiving
energy
T1 is longer
i.e. nuclei take a lot
longer to dispose
energy to surrounding
water tissue
FAT
WATER

44. T1 Recovery
Short TR T1 contrast
(T1
• TR 300-600 ms
• TE 10-30 ms
Weighted)

45. T2 Decay
Fat much better at energy exchange than Water
Because T2 depends on:
1-How closely molecular motion of atoms matches
Larmor Frequency
2-Proximity of other spins
So;
Fat's T2 time is very short compared to water
FAT
WATER

47. T2 Decay
Long TE T2 contrast
(T2 Weighted)
• TR 2000 ms
• TE 70 ms

49. T2* DECAY
• T2* relaxation – Disturbances in magnetic
field (magnetic susceptibility) increase the
rate of spin coherence T2 relaxation
• The total NMR signal is a combination of the
total
number
of
nuclei
(proton
density), reduced by the T1, T2, and T2*
relaxation components

50. Proton Density
• Proton Density
Long TR Proton density
• TR 2000 ms
• TE 10-30 ms

52. Gradient Magnets
• Used to vary magnetic field in known
manner
• Each point has slightly different rate of
precession & Larmor Frequency.
• Variety of signal released by Protons
returning to z-plane can used to determine
the composition of exact location of each
point.
• function of "Gradient Magnets”
– Slice selection
– Frequency encoding
– Phase encoding

53. BRAIN STIMULATOR
REGARDING MRI……
• T1 Recovery is due to interactions of
intrinsic magnetic fields of adjacent nuclei.
• T2 Decay is due to dephasing caused by
inhomogenities of external magnetic field.
• Fat has short T1 Recovery time.
• Water has a short T1 Recovery time.
• On T1 fat appears as high signal.

54. ANSWERS
• T1 Recovery is due to interactions of
intrinsic magnetic fields of adjacent
nuclei F
• T2*Decay is due to dephasing caused by
inhomogenities of external magnetic
fieldT
• Fat has short T1 Recovery time T
• Water has a short T1 Recovery time F
• On T1 fat appears as high signal T

55. IMAGE QUALITY
• Factors that determine image quality
Signal: noise ratio ,
Contrast ,
Spatial Resolution,
Scan time.

56. SIGNAL : NOISE RATIO
• Noise is a random variation in the MR
signal, occurring at all frequencies and all
the time, due to -1. PATIENT(white noise)
2.ENVIRONMENT.
3. SCANNER.
• All noise reduces and obscures contrast
bbetween tissues and is worst in areas of
low PD and low signal.

57. IMPROVE SNR…
• INCREASING THE
SIGNAL BY
• INCREASING THE
VOXEL SIZE
• DECREASING TE AND
INCREASING TR/FLIP
ANGLE.
• SE SEQUENCES > GRE
• HIGH TESLA MACHINE
• REDUCING NOISE
BY
• INCREASING THE
NUMBNER OF
EXCITATIONS
• REDUCING
BANDWIDTH
• REDUCING CROSS
TALK

59. The great headache—ARTIFACTS!!
• Image Artifact is something observed in a scientific
investigation that is not naturally present but occurs
as a result of the investigative procedure.
• A structure not normally present, but visible as a
result of malfunction in the hardware or software of
the device, or a consequence of environmental
influences as heat or humidity or can be caused by
the human body itself.

74. WHICH ARTIFACT????

75. Motion artifacts( ghost)

79. SPECIAL THANKS TO…….