3. Father of MRI
First to perform a full
body scan of a human
being in 1977 to
diagnose cancer.
Dr. Shubhankar
4. HISTORY
• Phenomenon of Nuclear Induction later termed as NMR (Nuclear
Magnetic Resonance) first described by Bloch and Purcell in 1946.
• They won Nobel Prize for Physics in year 1952.
• In 1971 Damadian (father of MRI) noted that tumors have elevated
MR Relaxation Times and filed a patent ‘Apparatus and methods for
detection of cancer in tissues.’
• Human in vivo images first published in 1977 by Damadian et al.
Dr. Shubhankar
5. Nuclear Magnetic Resonance (NMR)
• Nuclear Magnetic Resonance (NMR)measures the net
magnetization of atomic nucleiin the presence of
magneticfields.
• 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):
Themain 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 changegradually over spaceand
canchangequickly over time (thousandsof times per second)
Dr. Shubhankar
6. Production of a Magnetic Field
• When an electron travels
along a wire, a magnetic
field is produced around
the electron.
• When an electric current
flows in a wire that is
formed into a loop, a
large magnetic field will
be formed perpendicular
to the loop.
Electrons flowing along a wire. An
electric current in a loop of wire will
produce a magnetic field (black arrow)
perpendicular to the loop of wire.
Dr. Shubhankar
7. Main Magnetic Field
• The main magnetic field of an MR
system comes from a large
electric current flowing through
wires that are formed into a loop
in the magnet of the imaging
system.
• The wires are immersed in liquid
helium (at superconducting
temperatures) so that very large
currents can be used to produce
the strong magnetic field.
A large electric current in loops of wire
at superconducting temperatures will
produce a very large magnetic field.
Dr. Shubhankar
8. MR Nuclei
• Nucleusneeds to have2 properties:
– Spin
– charge
• Nuclei are made of protons andneutrons
– Both have spin½
– Protons havecharge
• Pairsof spinstend to cancel, soonly atomswith an odd number of protons
or neutrons have spin
– Good MRnuclei are 1H, 13C, 19F,23Na, 31P
Dr. Shubhankar
9. Why we use H+
• The adult human body is ~ 53% water, and water is
~11% hydrogen by mass but ~67% hydrogen by atomic
percent.
• Present in virtually all biological material.
• Exhibits relatively high MR sensitivity.
• The proton can be regarded as a small, freely
suspended bar magnet spinning rapidly about its
magnetic axis.
Dr. Shubhankar
10. Alignment of protons with the B0 field. With no
external magnetic field, hydrogen protons are
oriented randomly. When the protons are placed in
a strong magnetic field (B0 ), a net magnetization
will be produced parallel to the main magnetic
field.
Dr. Shubhankar
11. SPIN
• Protons and neutron spins are
known as nuclear spins.
• In NMR it is the unpaired
nuclear spins that produce a
signal in a magnetic field.
• Due to combination of charge
and nuclear spins these
nuclei behave as Magnetic
dipole.
The positively
charged hydrogen
proton spins about
its axis and acts like
a tiny magnet.
Dr. Shubhankar
13. PRECESSION
Precession of a spinning top
and nuclear precession are
similar in that an external
force combined with the
spinning motion causes
precession.
Dr. Shubhankar
14. Larmor Equation
• The proton precessional
frequency is determined
from the Larmor equation.
• Precession frequency is
dependent on strength of
external magnet field
• Stronger the external
magnetic field higher the
precession frequency
- f is precession
frequency in MHz
- Bo in magnetic field
strength in Tesla
- g is gyro-magnetic ratio,
for proton it is 42.6
MHz/Tesla
Dr. Shubhankar
19. Measuring the MR Signal
• The moving proton vector induces
a signal in the RF antenna.
• 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.
Z
Y X
Dr. Shubhankar
20. Now, we re-transmit the energyfor image processing
• The emitted energy is too small
(despite 2500 times the magnetic
field with resonance RFpulse) to
convert them into images.
• Hence, repeated “ON-OFF” of RFpulses
are required.
• The emitted energy is stored (K-space),
analysed and converted into images.
Dr. Shubhankar
22. Relaxation Process
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.
Dr. Shubhankar
23. 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"
Dr. Shubhankar
24. Longitudnal Magnetization/T1 Relaxation
• After a 90°RF pulse, the
longitudinal magnetization is
zero. The magnetization
then begins to grow back in
the longitudinal direction.
This is called longitudinal
relaxation or T1 relaxation.
Application of a 90° RF pulse causes
longitudinal magnetization to become
zero. Over time, the longitudinal
magnetization will grow back in a
direction parallel to the main magnetic
field.
Dr. Shubhankar
25. T1 Relaxation
• The rate at which this
longitudinal magnetization
grows back is different for
protons associated with
different tissues and is the
fundamental source of
contrast in T1-weighted
images.
T1 is a characteristic of tissue and
is defined as the time that it takes
the longitudinal magnetization to
grow back to 63% of its final value.
Dr. Shubhankar
26. T1 Relaxation
• T1 is a parameter that is characteristic
of specific tissue and is related to the
rate of regrowth of longitudinal
magnetization.
• The magnetization of tissues with
different values of T1 will grow back in
the longitudinal direction at different
rates.
• White matter has a very short T1
time and relaxes rapidly.
• CSF has a long T1 and relaxes slowly.
• Gray matter has an intermediate T1
and relaxes at an intermediate.
Different tissues have different rates of T1
relaxation. If an image is obtained at a time
when the relaxation curves are widely
separated, T1-weighted contrast will be
maximized.
Dr. Shubhankar
27. Transverse Magnetization.
• After a 90°RF pulse rotates the
longitudinal magnetization into
the transverse plane, this
magnetization called transverse
magnetization.
• Immediately after application of
a 90°RF pulse, transverse
magnetization is maximized; it
then begins to dephase due to
several processes.
1. Spin-spin interactions
2. Magnetic field inhomogeneities
3. Magnetic susceptibility
4. Chemical shift effects
Transverse (T2*) relaxation
Dr. Shubhankar
28. T2 Relaxation
• Different tissues have
different values of T2
and dephase at different
rates.
• White matter has a short T2
and dephases rapidly.
• CSF has a long T2 and dephases
slowly.
• Gray matter has an
intermediate T2 and dephases
intermediately.
T2 is a characteristic
of tissue and is
defined as the time
that it takes the
transverse
magnetization to
decrease to 37% of
its starting value.Dr. Shubhankar
29. Flip Angle
• Angle which the NMV
moved as result of a RF
excitation pulse.
• The amount of rotation
depends on the strength
and duration of the RF
pulse.
Dr. Shubhankar
30. Echo Time "TE"
• Time from application of RF pulse to the measurement
of signal in receiver coil.
• TE determines how much decay of transverse
magnetization is allowed to occur.
• Thus TE controls amount of T2 relaxation that has
occurred when signal is read.
Dr. Shubhankar
31. Time of Repetition 'TR'
• Time from application of one RF pulse to application of
Next RF pulse.
• TR determines the amount of longitudinal relaxation
that is allowed to occur before application of next RF
pulse.
• TR determines amount of T1 relaxation that have
occurred when the signal is read.
Dr. Shubhankar
37. Slice selection
• Rf excitation pulse is applied with simultaneous linear magnetic
field gradient in direction of desired slice.
• Now we can predict the Bnet (B+G or delta G) at any desired
point.
• And thus we can apply a specific radiofrequency pulse to excite
the protons of desired point or place.
• The thickness of slice is the frequency band width of RF Pulse.
Dr. Shubhankar
39. Phase Encoding
• Phase encoding is performed following excitation and prior to the
formation of the signal echo.
• A gradient is applied along one direction within the plane of the
excited slice.
• Gradient causes a linear change in spin precession frequency along its
direction.
• It introduces an incremental phase shift along its direction, the size
of which depends upon its magnitude. This corresponds to one sampled
spatial frequency along the phase-encode direction.
Dr. Shubhankar
41. Frequency encoding
• Performed during data sampling of the signal echo.
• A linear field gradient is applied in the imaging plane,
perpendicular to the slice-selection direction.
• At different positions along the frequency-encoding direction,
spins will be resonating at different resonant frequencies as the
signal is sampled.
• As the echo evolves, successive spatial frequencies of the object
are encoded along this dimension.
Dr. Shubhankar
43. Spin Echo
• The spin echo is the “trick” that can be used to recover
dephasing due to all effects except spin-spin interactions.
• After a 90°RF pulse, protons that were in phase begin to
dephase in the transverse plane due to some spins going
faster than the average and some spins going slower than
the average.
• After a certain amount of time, if a 180°RF pulse is
applied, the spins will rotate over to the opposite axis.
• Now, rather than the spins continuing to dephase, the
spins will begin to rephase.
Dr. Shubhankar
45. • The spin-echo pulse sequence can produce proton
density weighting, T1 weighting, and T2 weighting.
• TE and TR are set to achieve these weightings.
• Typical values of TE and TR (at 1.5 T) :
Flip angle:90° TE (msec) TR (msec)
T1WI 20 (20) 500 (580)
T2WI 80 (85) 2000 (4500)
PDWI <50 (39) >2000 (2800)
Dr. Shubhankar
46. • Short TE (producing minimal T2
weighting) and intermediate TR
(producing maximal T1 weighting)
will result in a T1-weighted image.
• Long TE (producing maximal T2
weighting) and long TR (producing
minimal T1 weighting) will result in
a T2-weighted image.
• Short TE (producing minimal T2
weighting) and long TR (producing
minimal T1 weighting) will result in
a proton density–weighted image.
Dr. Shubhankar
48. Multiecho Spin Echo
• This sequence uses a 90°
RF pulse with multiple
180°RF pulses to form
multiple echoes.
• Each echo can be used to
create a separate image data
set with different contrast
weighting.
Dr. Shubhankar
49. Turbo Spin Echo
• This sequence uses a 90°
RF pulse with multiple 180°
RF pulses.
• Multiple echoes are formed,
and the data are used to
create a single data set.
• Multiple rows of raw data
are filled during one TR
period; this feature allows
the pulse sequence to be run
fewer times, thus saving
imaging time.
Dr. Shubhankar
50. Inversion Recovery
• This sequence is similar to
the basic spin-echo sequence
with the addition of an initial
180°inversion pulse.
• This sequence can be used to
suppress the appearance of
unwanted signals (eg, those
due to fat or fluid).TI inversion time.
Dr. Shubhankar
51. • The 180°RF pulse causes an initial inversion of the longitudinal magnetization (so that
it is aligned in the -z direction)
• The magnetization then begins to grow back in the direction of the main magnetic
field(+z).
• The magnetization of different tissues will grow back at different rates.
• When the signal from the tissue to be suppressed crosses the zero axis, application of
a 90°RF pulse will rotate all other signals into the transverse plane.
• Since the signal from the tissue at the zero point is zero, there is nothing to rotate
into the transverse plane.
• Thus, this tissue will not contribute any brightness to the resulting image.
• Fat relaxes relatively quickly, and a short TI (inversion time) of approximately 170msec
is used to suppress signal from fat at a field strength of 1.5 T.
Dr. Shubhankar
52. • 180° RF pulse prior to the
regular spin-echo pulse sequence
given.
• After initial inversion of the
longitudinal magnetization, T1
relaxation occurs and the signals
from different tissues cross the
zero axis at different times.
• When the signal to be suppressed
crosses the zero axis, a 90°RF
pulse will rotate all other signals
into the transverse plane for
image formation.
Dr. Shubhankar
53. Gradient Recalled Echo
• This sequence is similar to the
spin-echo sequence except that
the initial RF pulse is less than
90° and there is no 180°RF
pulse.
• Signal dephasing and rephasing by
means of gradient pulses results
in formation of a gradient echo,
which is used to produce T1- or
T2*- weighted images.
Dr. Shubhankar
54. Take Home Message
Dark(Hypo) on T1
• Edema, tumor, infection,
inflammation.
• Hemorrhage (hyperacute,chronic)
• Low proton density, calcification
• Flow void
Bright(Hyper) on T1
• Fat, melanin, protein rich fluid.
• Hemorrhage (subacute)
• Slowly flowing blood
• Paramagnetic substances
(gadolinium,copper,manganese)
Dr. Shubhankar
55. Take Home Message
Dark(Hypo) on T2
• Low proton density, calcification,
fibrous tissue
• Paramagnetic substances
(deoxy hemoglobin, intracellular
methemoglobin,ferritin,hemosiderin,
melanin.)
• Protein rich fluid
• Flow void
Bright(Hyper) on T2
• Edema, Tumor, Infection,
Inflammation
• Subdural collection
• Extracellular methemoglobin (in
late subacute hemorrhage)
Dr. Shubhankar
