2. Introduction
■ MRI is an imaging modality based on interaction
between hydrogen nuclei in human body under
the influence of strong magnetic field.
3. PRINCIPLE
■ It works on the principle of nuclear magnetic
resonance
■ Nuclear magnetic resonance (NMR)measures the
net magnetization of atomic nuclei in our body
in the presence of magnetic fields and utilise the
information to produce the image
4. MR nuclei
■ Nuclei is made of Protons and
neutrons
■ But pairs of spins tend to
cancel
■ so only atoms with an odd
number of Protons or neutrons
have spin
■ therefore good MR Nuclei are
1H, 13C ,19F, 23 Na, 31 P.
5. • A charged particle in motion will create a
magnetic field
• The postitively charged, spinning hydrogen
nucleus generates a magnetic field
6. WHY HYDROGEN
■ Human body is predominantly made of water
■ Water has hydrogen atom. Hydrogen atoms
exhibits spin and charge
■ They have relatively high magnetic resonance
sensitivity
7. SPIN
■ Protons and neutron spins
are known as nuclear spins
■ An unpaired component has
a spin of ½ and two
particles with opposite spin
cancel one another.
■ in NMR it is the unpaired
nuclear spins that produce a
signal in the magnetic field
8. COMPONENTS OF MRI
■ 1. MAIN COIL
■ 2. SHIMS
■ 3. GRADIENT COILS
■ 4. RADIOFEREQUENCY COIL
9. MAIN COIL
■ Outermost coil
■ When current passes through these
coils there is a formation of strong
main magnetic field denoted as B0.
■ Utilises the principle of
superconductivity.
■ Niobium titanium alloy is used
■ This main magnetic field aligns the
H+ Protons in one direction and
forms a Vector(M0)
10. ■ Continuous passage of current produces heat in the coil
■ To cool down the system we need to continuously
circulate the liquid helium to keep the temperature
below 4 kelvin
■ If the temperature is more than 4 Kelvin the liquid
helium converts to gas and is released -this process is
called as quenching
■ After quenching the MRI machine is non
superconducting and not act as a magnet.
12. GRADIENT COILS
■ They apply the
gradient along the
magnetic field
■ The change or
manipulate the
strength of magnetic
field along the
separate axis
13.
14. RADIOFREQUENCY COIL
■ It generates the
magnetic field (B1) in
the axis perpendicular
to the main magnetic
field (B0) that is in the
Y axis .
■ Makes the protons
inphase with one
another.
15. AXIS IN THE MRI
■ Z AXIS – longitudinal axis to the patient
■ Y AXIS – tranverse axis to the patient
■ X AXIS – oblique
■ Mo – Net magnetisation vector
■ Bo – main magnetic filed ( along the Z
axis )
■ B1 – Radiofrequency pulse ( along the Y
axis )
Y
Z
X
B0
B1
16. NET MAGNETISATION VECTOR
■ The Protons in our
body are spinning in a
haphazard fashion and
cancel all the
magnetism
■ So when this protons
are placed in a large
magnetic field they
align in the direction
of that magnetic field.
17. ■ When a body is placed into the bore of the scanner, the strong
magnetic field will cause the individual hydrogen nuclei to
either
A) ALIGN ANTI-PARALLEL TO THE MAIN MAGNETIC FIELD
(B0
OR
B) ALIGN PARALLEL TO THE MAIN MAGNETIC FIELD (B0)
.parallel or anti parallel depend to external magnetic field and
thermal energy level of the nuclei.
B0
18. ■ The majority will cancel each other but the net
number of Protons is sufficient to produce an
image.
■ This vector is called as net magnetization
vector(Mo)
■ Interaction of NMV with Bo is the basis of MRI
■ The unit of B0 is Tesla.
19. ■ We can’t measure the net magnetization vector
in the longitudinal axis as the main magnetic
field (Bo) is strong in this axis.
■ In order to measure the NMV we have to flip the
vector towards the transverse axis.
20. ■ In order to change the NMV from longitudinal to
transverse axis we have to apply an another
magnetic field called as radio frequency
pulse(B1).
■ After applying this radio frequency pulse the
NMV Changes from longitudinal to transverse
axis.
■ Now we’ll measure the NMV in the transverse
axis.
21. RADIOFRQUENCY PULSE
■ Follows the Law of Electromagnetism (charged
particles in motion will generate a magnetic field)
■ This magnetic field is known in RF pulse denoted as
B1
■ RF Applied as a “pulse” during MR sequences
■ The RF pulse is applied so that B1 is 90 to B0
22. ■ The application of an RF
pulse that causes
resonance to occur is
termed excitation . This
absorption of energy
causes an increase in the
number of spin - down
hydrogen nuclei
populations.
■ Now the NMV vector flips
from longitudinal to
transverse axis
23. FLIP ANGLE
■ The angle to which
the NMV move out of
alignment is called as
flip angle.
■ the NMV is given
enough energy by the
RF pulse to move
from 90 relative to
B0.
24. The nuclei gain the energy and resonate and align
in INPHASE if the energy delivered at :
Exactly its precessional frequency.
90 degrees to NMV and B0
25. PRECESSION
■ Due to the influence of
B0 the hydrogen nucleus
wobbles or precesses.
■ The axis of the nucleus
forms a path around the
B0 known as
precessional path.
26. ■ The speed at which the hydrogen precesss
depends on the strength of B0 and is called as
precessional frequency.
■ The precessional frequency of hydrogen in a 1.5
Tesla magnetic field(B0) is 63 .86 Mhz.
28. Larmar equation tells us two
important facts
■ All Active Nuclei have their own gyromagnetic
constant so that when they are exposed to the
same field strength they precess at different
frequencies.
■ This allows us to specifically image hydrogen and
ignore the other MR active nuclei in the body.
29. HOW TO MEASURE
■ When the nuclei precess in phase in the B-1
plane a changing magnetic field is created.
■ If you place a receiver coil / antenna in the path
of this changing magnetic field a current will be
induced .
■ This is called as Faraday’s law of induction.
30. ■ A signal or voltage is only induced in
the received coil if there is
magnetization in transverse plane, that
in phase.
31.
32. RELAXATION
■ When the RF pulse is switched
off , the NMV is again
influenced by B 0 and it tries
to realign with it.
■ The process by which
hydrogen loses this energy is
called relaxation.
33. ■ The amount of magnetization
in the longitudinal plane
gradually increases – this is
called recovery.
■ A t the same time, but
independently, the amount of
magnetization in the
transverse plane gradually
decreases – this is called
decay.
B1
B0
T1
RECOVERY
T2
DECAY
34. TR - REPETITION TIME
■ Time from the application of one RF pulse to
another RF pulse by ms .
■ TR determinates the amount of T1relaxation
that has occurred.
35. ■ TE - ECHO TIME
■ Time from the application of the RF pulse to
the peak of the signal induced in the coil. (ms).
■ TE determinates how much decay is occur before
the signal is read .
■ TE controls the amount of T2 relaxation
36.
37. T1 RECOVERY
■ Also called as longitudinal recovery or or spin lattice
relaxation.
■ Caused by nuclei giving up their energy to the
surrounding environment or lattice(spin lattice
relaxation ) .
■ The rate of recovery is an exponential process, with a
recovery time constant called T1.
■ T1 is the time it takes 63% of the longitudinal
magnetization to recover in the tissue.(time &signal
intensity)
39. T1 RECOVERY IN FAT
■ T1 recovery in fat: the
recovery of the fat
relatively rapid so the
magnetic moment of fat
are able to relax and
regain their longitudinal
magnetization quickly.
■ NMV of the fat realigns
rapidly with B0 and
therefore the T1time of
fat is short.
40. T1 RECOVERY IN WATER
■ In water molecular mobility is
high resulting in less efficient
T1recovery.
■ The magnetic moments of
water take longer to relax and
regain their longitudinal
magnetization .
■ The NMV of water take longer
to realign with B0 and so the
T1time of water is long
41. T2 DECAY
■ Caused by nuclei exchanging energy with neighboring nuclei
by magnetic field of each nucleus interacting with its
neighbour.
■ Termed spin spin relaxation .
■ The rate of decay is exponential process.
■ T2 relaxation time of tissue is its time constant of decay.
■ It’s the time takes 63%of the transverse magnetization to be
lost(37% remains).
42.
43. T2 STAR DECAY
■ T2*decay is decay of the FID following the RF excitation pulse.
■ This decay is faster than T2decay since it is a combination of two
effects:
– 1-T2 decay it self.
– 2-dephasing due to magnetic field inhomogeneities.
■ Dephasing caused by inhomogeneities can be compensated for by
a180 Rf pulse.
■ A pulse sequence that used a 180 RF pulse to compensate for
dephasing is called a spin pulse sequence.
44. T2 DECAY IN FAT
■ This process is efficient in hydrogen in fat as the
molecules are packed closely together and therefore
spin – spin interactions are more likely to occur.
■ As a result spins dephase quickly and the loss of
transverse magnetization is rapid
■ As energy exchange in more efficient in fat, the T2time
is short.
45.
46. T2 DECAY IN WATER
■ T2time of hydrogen of water is long(the molecules are
spaced apart and spin – spin
■ interactions are less likely to occur.
■ As a result, spins dephase slowly and the loss of
transverse magnetization is gradual
47.
48. IMAGE CONTRAST AND
WEIGHTING
■ The factors that affect image contrast in diagnostic
imaging are usually divided into two categories:
■ Intrinsic contrast parameters are those that cannot be
changed because they are inherent in to the body ’ s
tissues.
• Extrinsic contrast parameters those that can be
changed.
49. INTRINSIC FACTORS
■ T 1 recovery time
■ T 2 decay time
■ proton density
■ flow
■ apparent diffusion coefficient (ADC).
50. EXTRINSIC FACTORS
– T R
– T E
– flip angle
– T I
– turbo factor/echo train length
– b value.
51. ■ MR image has contrast if there are areas of high signal
(white on the image) and areas of low signal (dark on
the image).
■ Some areas have an intermediate signal (shades of gray
in between white and black)
■ Image obtain contrast mainly through the
mechanisms of T1recovery,T2decay and proton
density.
52. ■ T1 and T2 relaxation depend on three factors:
– The inherent energy of the tissue .
– How closely packed the molecules are.
– How well the molecular tumbling rate matches the
Larmor frequency of hydrogen
53. T1 WEIGHTING
■ A T1 weighted image is one where the contrast depends
predominantly on the differences in the T1 times
between fat and water.
■ TR control the amount of T1weighting.
■ For T1 weighting the TR must be short. nei ther fat nor
water has sufficient time to fully return to B0.
■ TR controls how far each vector can recover before it is
excited by the next RF pulse.
54. SHORT TR AND SHORT TE
■ With a short TR, the
difference in T1 shows up
■ With a short TE, difference in
tissue intensities is not well
appreciated. So, T2 doesn’t
influence the signal
■ Thus, the resulting image is
T1 weighted
55. T2 WEIGHTING
■ The TE controls the amount of T2 decay that is allowed
to occur before the signal is received.
■ TE must be long enough to give both fat and water to
decay (large contrast different between fat and water).
■ If the TE is too short, neither fat nor water has had time
to decay, and therefore the differences in their T2
times are not demonstrated
56. LONG TR AND LONG TE
■ With a long TR, T1 doesn’t
influence the signal
■ With a long TE, difference in
tissue intensities is becomes well
pronounced
■ Thus the resulting image is T2
weighted
57. PROTON DENSITY IMAGING
■ If image acquired at Short TE and long TR the effect of
T1and T2contrast must be diminished.
■ Along TR allows both fat and water to fully recover
their longitudinal magnetization and diminished
T1weighting.
■ A short TE dose not give fat or water time to decay and
therefore diminshed
58. LONG TR AND SHORT TE
■ With a long TR, T1 doesn’t
influence the signal
■ With a short TE, difference in
tissue intensities is not well
appreciated. So, T2 doesn’t
influence the signal
■ Thus, the signal here is influenced
by proton density
59.
60. FLOW VOIDS
■ When we send in our first 90° pulse, all the protons
■ in the cross section are influenced by the radio wave.
■ After we turn the RF pulse off, we "listen" into the section and
record a signal.
■ At this time, all the original blood in our vessel may have left
the examined slice. So there is no signal coming out of the vessel
-it appears black in the picture.
■ This phenomenon is called flow-void phenomenon.
61. EFFECT OF CONTRAST
■ Gadolinium, a paramagnetic substance, is used as an MR contrast
medium.
■ Chemically the substance is a rare earth.
■ As Gadolinium is toxic in its free state, it is bound to DTPA
■ Gadolinium does not go through the intact, but rather the
disrupted blood-brainbarrier.
62. ■ The effect of the contrast medium is a change of the signal
intensity by shortening T1 and T2 in its surroundings
■ With a short T1, the signal from the tissue will be stronger than
before
■ With a short T2, less signal comes from the tissues
■ As loss of signal often is more difficult to appreciate than a signal
enhancement, T1-weighted images are the predominant imaging
technique used after contrast medium injection.
63. SLICE SELECTION
■ we can select a slice to be examined by using a gradient field,
which is superimposed on the external magnetic field.
■ Protons along this gradient field are exposed to different
magnetic field strengths, and thus have different precession
frequencies.
■ As they have different precession frequencies, we can send in an
RF pulse that contains only those frequencies, which excite the
protons in the slice which we want to image.
64. SLICE THICKNESS
Slice thickness can be altered in two ways:
■ by changing the band width of the RF pulse,
■ by modifying the steepness of the gradient field.
65. SIGNAL PROCESSING
■ To determine the point in a slice from which a
certain signal is coming, we use two other
gradients, the frequency encoding and the phase
encoding gradient
66. FREQUENCY ENCODING
■ The frequency encoding gradient is sent in after the
slice selection gradient.
■ It is applied in the direction of the y-axis.
■ This results in different precession frequencies along
the y-axis, and thus different frequencies of the
corresponding signals
67. PHASE ENCODING
■ the phase encoding gradient is turned on for a short time
■ after the RF pulse along the x-axis.
■ During this short time, the protons along the x-axis precess with
different frequencies.
■ When this gradient is switched off, they go back to their former
precession frequency, which was the same for all of them.
■ Due to this phase encoding gradient, however, the protons and
their signals are now out of phase, which can be detected.
68. PROCESSING
■ 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.
