BASIC description of MRI physics

1. Basics of MRI
Physics
DR. YASNA KIBRIA
MD Resident , Phase-A
Department of RADIOLOGY AND IMAGING
BSMMU

2. NUCLEAR MAGNETIC RESONANCE ( NMR )
• NMR is a physical phenomenon
where a given nucleus absorbs and
re-emits electromagnetic radiation.
• It allows us to study the properties
of atoms & other substances.
• We use NMR to indicate the physics
of the nuclear magnetic resonance
phenomena .
• We use MRI to indicate the way
NMR is used to produce a medically
useful image.
Isidor Issaac Rabi

3. HISTORY OF MRI

4. • In 1972, at Stony Brook, New York,
PAUL LAUTERBUR , was able to
generate first two dimensional
NMR image of proton density and
spin lattice relaxation time .
• Lauterbur coined the term
ZEUGMATOGRAPHY (from the
Greek ZEUGMA ,meaning that
which joins together ) for his
technique.
• Joined together are a
radiofrequency magnetic field
and specially defined magnetic
field gradients that produce the
NMR image.

5. Very first image
using NMR
technique.

6. First MR body scanner.
Damadian was the first to perform a full
body scan of a human in 1977 to
diagnose tumors.

7. BASIC PRINCIPLES of MRI
Four basic steps are involved in getting an MRI image-
1. Placing the patient in the magnet
2. Sending radiofrequency (RF) pulse by coil
3. Receiving signals from the patient by coil
4. Transformation of signals into image by complex processing
in the computers.

10. WE are made up of ELEMENTS
• Human body is built of about 26 elements.
• Oxygen, hydrogen ,carbon ,nitrogen etc. constitute 96% of human
body mass.
• Most of the mass of the human body is oxygen and most of the
atoms in the human body are hydrogen atoms.
• An average 70 kg adult human body contains approximately 3x 10^27
atoms of which 67% are hydrogen atoms.

11. Why HYDROGEN? Why PROTON?
• Hydrogen is the Simplest element with
atomic number of 1 and atomic weight of
1.
• When in ionic state ( H+ ), it is nothing but
a proton.
• Hydrogen ions are present in abundance
in body water and H+ gives best and most
intense signal among all nuclei.
• 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.
• Essentially all MRI are hydrogen or proton
imaging.

12. MR ACTIVE NUCLEI
C 13
F 19
P 31
N 15
O 17
Na 23
 Due to unpaired proton nuclei of
these elements , they act as a tiny
magnet.

13. PROTONS and their magnetic fields
• Every proton has a single positive charge and proton spin around a
central axis ( much like the Earth ).
• This spinning charge is essentially a small current and ,therefore ,
induces a magnetic field ( Any moving charge generates current and
every current has a small magnetic field around it )
• So, every spinning proton has a small magnetic field around it ,also
called MAGNETIC DIPOLE MOMENT.

15. WE ARE MAGNETS !!
REALLY !!?
But why we can’t act like
magnets??
The protons ( Hydrogen
ions) in the body are
spinning in a haphazard
fashion and cancel all the
magnetism. That is our
natural state.

16. PRECESSION
• Normally , alignment of the proton magnets
is random.
• But when an external magnetic field is
applied ,these randomly moving protons align
( their magnetic moments align ) and spin in
the direction of external magnetic field ( as
the compass aligns in presence of earth’s
magnetic field ).
• Some of them align parallel and others anti-
parallel to external magnetic field.
• When a proton aligns along external magnetic
fields , not only it rotates around itself ( called
SPIN) ,but also its axis of rotation moves
forming a “cone”-this movement of axis of
rotation of a proton is called PRECESSION.

18. PRECESSION FREQUENCY
• The number of precession of a proton per second is called precession
frequency.it is measured in hertz.
• Precession frequency is directly proportional to strength of external
magnetic field.
• Stronger the external magnetic field , higher is the frequency.

20. So,
If Bo = 1 T then ,
Larmor frequency is 42.5
MHz .
Similarly , for 1.5 and 3
Tesla it is roughly 64 and
128 MHz respectively.

21. VECTORS
• A vector has both size and direction and is
represented graphically by an arrow.
• A vector’s size is represented by its length
in MRI physics , the size represents the
magnetic field of a proton.
• Each protons magnetic field can be
represented by a vector in a 3D co-ordinate
system where-
 Z=Direction of B =longitudinal
component
 X & Y =transverse component

22. LONGITUDINAL MAGNETIZATION
• External magnetic field is directed along Z axis which is the long axis of the
patient as well as bore of the magnet.
• Proton align parallel or anti-parallel to external magnetic field ,i.e. along
positive or negative sides of Z axis.Forces of protons on negative and
positive sides cancel each other out.
• However, there are always more protons spinning on positive side or parallel
to Z axis than negative side as it requires less energy to do so.
• After cancelling each others forces there are few protons on positive side
that retain their forces and these forces add up together to form a magnetic
vector along the Z axis.This is called net longitudinal magnetization.
• But this formed longitudinal magnetization we can’t measure directly as it is
along the external magnetic field.

27. TRANSVERSE MAGNETIZATION
• In order to measure the net magnetization ,we need to flip it towards
transverse plane by sending a radiofrequency pulse (RF pulse ).
• The precessing protons pick up some energy from the RF pulse and go to
higher energy level and start precessing antiparallel to Z axis.
• This imbalance results in tilting of magnetization into transverse (X-Y)
plane.
• This is called transverse magnetization.

29. NET MAGNETIC VECTOR ( NMV)
• Components of LM & TM can be
represented by a single vector.this
vector represents the sum of
these components and is called
NET MAGNETIZATION VECTOR.
• NMV lies somewhere in between
LM & TM.
• If there is no magnetization in
transverse plane , LM will be same
as NMV.
• Similarly , if there is no LM ,TM
will be equal to NMV.

30. RF PULSE and RESONANCE
• Radiofrequency pulse is the short burst of electromagnetic wave in the
radiofrequency range , used in combination with magnetic gradients to
generate a magnetic resonance imaging.
• For the exchange of energy , frequency of protons and RF pulse have to be
same . (Larmor frequency )
• When RF pulse and protons have same frequency ,protons of low energy state
can pick up some energy and can go to higher energy state-this phenomena is
known as RESONANCE –the R in MRI.
• RF pulse not only causes protons to go to higher energy level but also makes
them precess in step ,in phase or synchronously.

34. MR SIGNAL
• Transverse magnetization vector
constantly rotate at Larmur frequency
in transverse plane and induces a
electric current.
• The receiver coil receives this current
as MR signal.
• The strength of the signal is
proportional to the magnitude of the
transverse magnetization and this
signals are transformed into MR image
by computers using mathematical
methods.

35. RELAXATION : it means recovery of protons back towards
equilibrium after been disturbed by RF excitation.
WHAT happens when RF Pulse is switched off?
protons starts doing two things simultaneously –
Losing energy and returning to spin-up : longitudinal magnetization starts
increasing along Z axis.
Dephasing : transverse magnetization starts decreasing in transverse plane.

36. LONGITUDINAL RELAXATION
• When RF pule is switched off ,spinning
protons start losing their energy and start to
spin up along the positive side of Z axis.so
there is gradual increase in the magnitude (
recovery )of longitudinal magnetization.
• The energy released by protons is transferred
to surrounding (the crystalline lattice of
molecules)-hence the longitudinal is also
called as “spin-lattice” relaxation.
• The time taken by LM to recover its original
value after RF pulse is switched off is called
longitudinal relaxation time or T1.

37. TRANSVERSE RELAXATION
• The transverse magnetization represents composition of magnetic forces of protons
precessing at same frequency.These protons are constantly exposed to static or
slowly fluctuating local magnetic fields.
• So when RF pulse is switched off they start loosing phase and results in gradual
decrease in magnitude of transverse magnetization and is termed as Transversal
relaxation.
• Since dephasing is related to fluctuating local magnetic fields caused by adjacent
spins (protons ), transverse relaxation is also called ‘spin-spin’ relaxation.
• The time taken by TM to reduce its original value is transverse relaxation time or T2.

39. T1 CURVE
• T1 is the time taken by LM to
recover after RF pulse is
switched off.
• This is not an exact time , but it
is a constant.
• T1 is the time when LM reaches
back to 63% of its original value.
• The curve showing gradual
recovery of LM against time is
called T1 Curve.

40. T2 CURVE
• T2 is the time taken by TM to
disappear . Similar to T1 ,it is a
constant ,not an exact time.
• It is the time taken by TM to reduce
to 37 % of its maximum value.
• The curve showing decrease in
magnitude of TM against time is
called T2 Curve.

46. TR and TE
• TR : Time to REPEAT
is the time interval between
start of one RF pulse and start of
next RF pulse.
• TE : Time to ECHO
is the time interval between
start of RF pulse and reception of the
signal (echo).
**TR is always higher than TE.
• Short TR + short TE = T1 WI
• Long TR + long TE = T2 WI
• Long TR + Short TE = PD WI

47. Typical TR and TE values in milliseconds
SPIN-ECHO SEQUENCE GRADIENT-ECHO
SEQUENCE
Short TR 300-800 < 50
Long TR > 2000 > 100
Short TE 10-25 1-5
Long TE > 60 >10

49. T1 WEIGHTED IMAGE
• Here, the magnitude of the LM indirectly determines the strength of
MR signal.
• Tilting of stronger LM by 90 degree RF pulse greater magnitude
of TM stronger MR signal.
• Tissues with short T1 regain their maximum LM in short time
(after RF pulse is switched off) when next RF pulse is sent
TM will be stronger resultant signal will be stronger.
• Therefore, material with short T1 have BRIGHT signal on T1 weighted
image.

50. HOW does one make images T1 weighted?
This is done by keeping the TR SHORT.

51. T1 WEIGHTED IMAGE

52. T2 WEIGHTED IMAGE
• Immediately after its formation TM has greatest magnitude and
produces strong signal . When it starts decreasing gradually
reduction of the intensity of received signal.
• Different tissues ,depending on their T2 ,have variable time for which
TM will remain strong enough to induce signal.
• Tissues with longer T2 , such as water ,will retain signal for longer
time and tissues with shorter T2 , will lose their signal earlier after RF
pulse is turned off.

53. How does one make images T2 weighted?
This is done by keeping the TE longer.

55. PROTON DENSITY (PD ) IMAGE
• Contrast here is determined by the
density of protons in tissues.
• T1 effect is reduced by keeping
LONG TR.
• T2 effect is reduced by keeping TE
SHORT.
• The signal intensity difference
amongst tissues is functions of the
number protons they have.

56. MR SEQUENCES
• A pulse sequence is interplay of various parameters leading to a complex cascade of
events with RF pulses & gradients to form a MR image.so pulse sequence is a time
chart of interplay of-
1. Patient’s net longitudinal magnetization
2. Transmission of RF pulses
3. X ,Y & Z gradients activation for localization and acquisition of signal (echo).
4. K- space filling with acquired signals or echoes.
• Pulse sequences are broadly divided into two categories-
spin-echo & gradient-echo sequences
• Practically ,pulse sequences are- Spin-echo sequence (SE)
Gradient-echo sequence (GRE)
Inversion recovery sequence (IR)

57. SPIN-ECHO (SE) PULSE SEQUENCE
• It consists of 90 & 180 degree RF pulses.
• The excitatory 90 degree pulse flips net magnetization vector along Z axis
into transverse plane.
• TM precessing with Larmor frequency induces a small signal called FREE
INDUCTION DECAY in the receiver coil.This FID is weak and insufficient for
image formation.Also, TM starts to reduce as proton starts dephasing.
• Hence a rephrasing 180 degree pulse is sent to bring protons back into
phase.This rephrasing increases magnitude of TM & a stronger signal is
induced in receiver coil.this gives the sequence its name.
• SE sequence forms the basis for understanding all other sequences.

60. Modifications of spin-echo sequence
• In conventional spin-echo ,one line of K-space is filled per TR.it can be
modified to have more than one echo per TR by sending more than
one 180 degree pulses after excitatory 90 degree pulse.
• Three routine modifications are done-
1. Dual SE (two 180 degree pulse per TR)
2. Fast SE (multiple 180 degree pulses per TR)
3. Single-shot fast SE( fast SE only half of the k-space filled)

61. Advantages of spin-echo:
1)high SNR
2)True T2 weighting-sensitive to pathology.
Disadvantages of spin-echo:
1)Scan time relatively long
2)Use more RF pulse than in GRE

62. GRADIENT-ECHO SEQUENCE
• There are basic three difference between SE & GRE sequences :
1. There is no 180 degree pulse in GRE . Rephasing of TM is done by gradients.
2. The flip angle in GRE is smaller,usually less than 90 degree.As flip angle is
smaller,there will be early recovery of LM such that TR can be
reduced,hence reduced scanning time.
3. Transverse relaxation is caused by combination of two mechanism-
• dephasing of TM resulting from nuclear & molecular magnetic interaction
with protons.
• dephasing caused by magnetic field inhomogeneity.
** in GRE, dephasing effects by magnetic field inhomogeneity are not
compensated as there is no 180 degree pulse . T2 relaxation in GRE is called
T2* relaxation.

63. GRE
• Instead of 180 degree pulse,we use a MAGNETIC FIELD GRADIENT (MFG) to
refocus the protons.
• This is superimposed over the main magnetic field.
By switching on MFG for a very short period of time:
o causes lots of inhomogeneity in the imaged slice
oLeads to rapid dephasing of protons & quicker loss TM
By switching off MFG & switching it back on but in opposite direction:
oHas similar effect of using 180 degree pulse & leads to partial rephrasing.
we get a transient increase in signal intensity called GRADIENT ECHO.

65. GRE IS FASTER
• Here, smaller flip angle
is used (10-35
degree).this means
there is always a
substantial amount of
LM left that can be tilted
by next flip pulse.

66. T2* RELAXATION
• In addition to the magnetic field inhomogeneity intrinsic to the
tissues causing spin-spin relaxation ,inhomogeneity of external
magnetic field (Bo) also causes decay of the TM.
• Decay of the TM caused by combination of spin-spin relaxation &
inhomogeneity of external magnetic field is called T2* relaxation.

67. T2* CURVE
• Dephasing effect of the external
magnetic field inhomogeneity
are eliminated by 180 degree
pulse used in spin-echo
sequence.so, “true” T2
relaxation in spin-echo
sequence.
• T2* is shorter than T2.

68. INVERSION RECOVERY (IR)
• The inversion of the 180 degree pulse flips
LM along negative side of the Z axis.this
saturates fat & water completely at the
beginning.
• When excitatory 90 degree pulse is applied
after LM has relaxed through transverse
plane ,contrast of the image depends on
amount of longitudinal recovery of tissues
with different T1.
• An IR image is more heavily T1 weighted
with large contrast difference between fat
& water.IR sequence also used to suppress
particular tissue using different TI.
**TI: Time to invert : time between
inversion of 180 degree & excitatory 90
degree pulse is called TI.

69. Types of IR sequences
• IR sequence can be divided based on the value of TI used:
o1) Short TI IR sequence: TI in the range of 80 to 150 ms . Ex- STIR
o2) Medium TI IR sequence :TI in the range of 300-1200ms.Ex-MPRAGE
o3) Long TI IR sequence: TI in the range of 1500-2500ms.Ex-FLAIR

70. STIR( Short Tau Inversion Recovery)
• Used to suppress the signal from fat
• When 90 degree pulse is applied at short TI, LM for all tissues are still
on negative side. The tissues short T1,Ex- Fat have near zero
magnetization, so don’t have much signal.
• Most pathological tissues have increased T1 as well as T2.
• Moderately high TE used in STIR allows tissues with high T2 to retain
signal while tissues with short T2 will reduced signal.
• This results in increased contrast between tissues with short T1 –T2
and long T1-T2
** Most pathology appeared bright on STIR making them easier to pick
up

72. FLAIR (Fluid Attenuated Inversion Recovery)
• Used to suppress the signal from fluid.
• When 90 degree pulse is applied at long TI, LM of most tissues is
almost fully recovered.
• Since water has long T1 its LM recovery is at half way stage at long TI.
This results in no signal from fluid such as CSF.
• As in STIR most of the pathologies appeared bright on FLAIR
• In FLAIR long TE can be used to get heavily T2 weighted image
without problems from CSF partial volume effects

74. STIR VS FLAIR
Short TI of 80-150 ms is used 1 Long TI 1500-2500 ms is used
Combined T1 and T2 weighting is
obtained
2 Heavily T2 weighted image is
obtained
Fat and white matter is
suppressed
3 CSF and water is supressed
Mainly used in body imaging 4 Used in Neuro imaging
Can not be used in post contrast
imaging
5 Can be used in post contrast
imaging

75. STIR coronal image of the pelvis ,there is
suppression of subcutaneous fat.
FLAIR axial image of brain:CSF is
suppressed and is dark, scalp fat is not
suppressed and bright.

76. Frequency – selective fat
suppression
VS STIR
Only adipose tissue is suppressed. 1 Suppressed whole adipose tissue
including water and fat fraction
within it also suppresses mucoid ,
gadolinium,melanin and some
proteinaceous material.
Affected by magnetic field
inhomogeneity.
2 Insensitive to field inhomogeneity.
SNR to adipose tissue is reduced
but overall SNR is maintained.
3 Overall SNR is poor as compared to
other methods.
Generally good for post contrast T1
WI and T2 WI with short FOV.
4 Good for large FOV images, low
field strength. Can not be used in
post contrast imaging.

77. MAGNETIC RESONANCE INSTRUMENTATION
Basic four components make MR
system :
1. The magnet to produce external
magnetic field
2. Gradients to localize the signal
3. Transmitter and receiver coils for
RF pulse
4. Computer system

78. Magnetic field strength
• Magnetic field is expressed by
notation ’B’ ,the primary field as Bo
and secondary field as B1.
• The units of magnetic field strength
are GAUSS & TESLA.
1 TESLA =10 k G= 10,000 GAUSS
MR system for clinical purposes have
strength from o.2 to 3 Tesla.

79. MAGNETISM AND MAGNETS
• Fundamental property of a
matter.
• Depending on the magnetic
susceptibility, substances can be-
1. Paramagnetic
2. Diamagnetic
3. Ferromagnetic
• Three types of magnets are in
use for clinical MRI machine :
1. permanent magnet
2. Electromagnet
3. Superconducting magnet

81. PERMANENT MAGNET
• Usually made up of ferromagnetic substances,ALNICO which is a alloy of
alluminium,nickel and cobalt.
• Advantages:
 Do not require power supply
 Low cost
 Open MRI is possible in claustrophobic patient
• Disadvantages :
 thermal instability
 limited magnetic field strength( 0.2 to 0.5 T)
 heavy
Higher applications can not be performed

82. ELECTROMAGNETS/RESISTIVE MAGNETS
• An electric current is passed through a coil of wire to generate a
magnetic field.
• Advantage: can generate higher field strength than permanent
magnet.
• Disadvantage :1) are only magnetic when a current is flowing
therefore, require a lot of energy .
2) get very hot .

83. SUPERCONDUCTING MAGNET
• These are the most common magnets used in MR scanner today.
• Superconductor has zero electric resistant.
• Superconducting wires are made of Niobium- titanium alloy which
become superconducting at 10k.
• The coils need to be cooled to 4k by cryogens like liquid helium.
• Advantage : 1) high field strength 2) excellent magnetic field
homogeneity 3) continuous power supply is not required
• Disadvantage : 1) cryogens are expensive 2) costly

84. COILS
• Different coils are used in MR scanner:
 gradient coils: SSG, FEG, PEG
Shim coils: this coil permits fine adjustments to the main magnetic
field improves the homogeneity of the field.
Radiofrequency coils: these coils transmits the RF pulse and receives
the emitted signals. They should be as close to region of interest as
possible

85. RF COILS
• Based on design RF coils can be divided into:
Volume coils:1) surrounds either the whole body or a specific region.
2)Perpendicular to main magnetic field. 3) Ex- body and head coils.
Surface coils : 1)it is placed on the surface of a region of interest to
acquire images with very high SNR 2) receive only 3) either flexible or
rigid.
Phased- array ( PA) coils: 1) consists of two or more geometrically
aligned surface coils used in conjunction 2) high SNR, large FOV 3)
parallel imaging capability

86. K-SPACE
• K- space is an imaginary space of computer memory that stores the raw
data matrix. It represents stage between reception of signals and image
formation.
• It has two axes:
Horizontal axis represents the phase axis
Frequency axis is vertical and is perpendicular to phase axis.
• Signals are field in K- space as horizontal lines.
• Typically one row of K- space data is acquired per TR ( not always).
**Although K- space is a matrix, its coordinates system doesn’t start at the
top left. Instead, ( 0,0) is at the centre.

88. K-SPACE
• Arrays of signals in K- space doesn’t correspond with rows or columns of
pixel in the image.
• K- space Centre represents:
Higher signal intensity
Provides image contrast
• K- space periphery represents:
Lower signal intensity
Provides resolution and fine details.
** every point in K- space provides some form of information for every point
in the image.

89. FID SIGNAL
• When 90 degree pulse is switched
off,TM starts to decay and LM starts
to recover its original state.
• Since the net magnetic field is
changing, an electric charge is
induced in our receiving equipment.
This is our raw MR signal.
• The frequency of this raw signal
remains constant but its intensity
decreases in time.
• This raw signal is known as free
induction decay ( FID) signal.

90. LOCALIZATION OF THE SIGNAL
• Three magnetic fields are superimposed on the main magnetic field
along X, Y and Z axes to localize from where body signals are coming.
• This magnetic field has different strength in different location, hence
are called gradient fields or simply gradients.
• The gradient fields are produced by gradient coils.
• The three gradients are:
1. Slice selection gradient- Z axis
2. Phase encoding gradient- X axis
3. Frequency encoding gradient- Y axis

91. SLICE SELECTION GRADIENT:
• Slice selection gradient has gradually
increasing magnetic field strength
from one end to another as protons
precess at different frequencies
depending on where they are in SSG.
• It is possible to orientate the SSG in
any plane without moving the
patient.
• Slice thickness is determined by slope
of the gradient and bandwidth of RF
pulse.
• Steeper SSG = thinner slice
• Wider bandwidth = thicker slice.

92. FREQUENCY ENCODING GRADIENT
• The FEG causes proton to precess
at different frequencies where
along the gradient they lay.
• All protons in the same column of
the slice precess at the same
frequency and are in phase.
• Protons in different columns
precess at different frequencies
but are still in phase .

93. PHASE ENCODING GRADIENT
• To figure out from which row a
signal originates we need to apply a
third gradient PEG.
• When PEG is on, protons in a
particular column precess at
different speeds depending on
where along the gradient they lay.
• When PEG is switched off protons in
that column start to precess at
same frequency as before but they
are now out of phase.

94. • The gradients are applied perpendicular to each other.
• SSG is turned on at the time of RF pulse.
• PEG is turned on for a short time after SSG.
• FEG is turned on in the end at the time of signal reception.
** information from all 3 axes is sent to computers to get the point in
that particular slice from which the signal is coming.

95. FLOW-VOID
Flow voids refers to the signal loss with
blood or other fluids moving at
sufficient velocity relative to MRI
apparatus.
Protons in flowing fluid move out of
plane of imaging in time between giving
RF pulse and production of signal.

97. IMAGING/ACQUISITION TIME
Here,
a.t.=acquisition time
TR=Time to repeat
N=Number of pixels (matrix )
NEX=Number of excitations

98. Parameters of scanning
1. Matrix
2. FOV( field of view)
3. Number of excitations ( NEX)
4. Flip angle
5. Bandwidth

100. MR CONTRAST MEDIA
• May be oral and parenteral.
• Most common contrast agent is
Gadolinium
• Other agents – Iron oxide, Mn-
DPDP, Dysprosium chelates

101. • Gd ( contrast agent) causes both T1
and T2 relaxation of the tissues in
which it is accumulated.
• Increased T1 relaxation leads to
bright signal in T1WI images.
• T1 effects of Gd are used more
commonly in clinical practice.
• T2 effect of Gd leading to reduction
of signal on T2WI is generally
insignificant and clinically not
relevant

103. Adverse Reaction of GADOLINIUM
• Overall reaction rate is 3-5 percent includes nausea, headache, injection
site symptoms.
• Patient with history of allergy, asthma and previous drug reaction, Gd is
more prone to adverse reaction.
• Gd doesn’t pass through intact BBB, but will traverse if BBB is damaged.
• Nephrogenic systemic fibrosis (NSF): rare but fatal,when used in patients
with renal failure.
** Pregnancy and lactation: Gd is known to cross placenta and via breast
milk.

104. MR Diffusion
• Diffusion means random movement of water
protons. The process by which water proton
diffuse randomly in the space is called BROWNIAN
motion.
• The difference in the mobility of water molecules
between tissues give the contrast in DWI.
• In ISOTROPIC diffusion ,possibility of water
proton moving in any particular direction is equal
to the probability that it will in any other
direction( ISOTROPY = uniformity in all directions).
• In ANISOTROPIC diffusion, water diffusion has
preferred direction. Water proton moves more
easily in some direction than others.
** ISOTROPIC diffusion forms the basis of routine
DWI .

106. DWI
• We therefore get a high signal
from tissues with restricted
diffusion.
• DWIs are all T2WI of varying
degrees

107. b Value
• The b value: indicates the magnitude of diffusion weighting provided
by diffusion gradient. It also indicates sensitivity of the sequence to
the diffusion.
• The b value increases with diffusion gradient strength.
• As the b value increases the signal from water molecule reduces.
• At high b value ( b = 1000) only tissues with T2 relaxation time or
those with restricted motion of water molecule will have high signal.

108. ADC: Apparent Diffusion Coefficient
• ADC is a measure of diffusion which is calculated mathematically from b
value = zero and various higher b value images.
• Signal attenuation of a tissue with increasing b value is plotted on a graph
with relative signal intensity on Y axis and b values on X axis. The slope of
the line represent ADC.
• ADC removes the effect of T2.
• The area of REDUCED ADC ( restricted diffusion) bright area on DWI
while same area will turn DARK on ADC map.
• ADC is expressed mm2/sec.
• ADC map helps to differentiate T2 shine through from actual restricted
diffusion.

114. Parameters 1.5 T 3 T
Larmor frequency 63.9 MHz 127.8 MHz
Susceptibility Less. More.
T1 relaxation time Less as compared to 3T. T1 is increased by about
25% at 3T.
T2 relaxation time More as compared to 3T. T2 is reduced by about
10-15% at 3T.
Signal to noise ratio (SNR) Less as compared to 3T. SNR is double at 3T.

118. ABSOLUTE Contraindications of MRI
• Internal cardiac pacemaker
• Implantable cardiac defibrillator
• Cochlear implants
• Electrically programmed drug
infusion pumps , vascular access
port
• Intraocular foreign body
• Non-titanium aneurysmal clip