MRI PULSE SEQUENCES.pptx/////////////////

MRI PULSE SEQUENCES
Presented by : Prativa Khanal
BSc.MIT 2nd yr
NMCTH

INTRODUCTION TO PULSE SEQUENCES
• The way in which the RF coil & gradient fields are turned
on & off is called pulse sequences
• Image quality is determined by the pulse sequence used
• PS is which kind of image contrast we want to see or even
which kind of pathology we want to detect
• A sequence of radiofrequency (RF) pulses applied
repeatedly during MR study to acquire MRI images

IMAGE CONTRAST
• The contrast characteristics of each image in MRI depends
on many variables,
• It is important to understand the mechanisms that affect
image contrast in MRI
• An image has contrast if it has high signal(white on the
image), as well as areas of low signal (dark on the image)
• Some areas have intermediate signal(shades of grey btn
white & black)
• NMV can be separated into the individual vectors of the
tissues present in the pt i.e.fat, CSF,& muscle

FAT AND WATER
• Fat is hydrogen linked to carbon & Water is hydrogen linked to
oxygen which tends to steal the electrons away from around the
hydrogen nucleus
• In fat, the carbon does not take the electrons from around the
hydrogen nucleus. They remain in an
electron cloud protecting the nucleus
from the effects of the main field
• Hydrogen in fat fecovers more rapidly
along the longitudinal axis than water
and loses transverse magnetization
faster than in water
• Subsequently, fat and water appear differently in MR image

CONTRAST MECHANISM
Images obtain contrast mainly through the mechanisms of :
• T1 recovery
• T2 decay &
• Proton or spin density- the proton density of a tissue is the
number of protons per unit volume of that tissue

T1 recovery in fat
• Occurs due to nuclei giving up their energy to the surrounding
environment
• The slow molecular tumbling in fat allows the recovery process to be
relatively rapid
• The NMV of fat realigns rapidly with B0 and therefore the T1 time of
fat is short

T1 RECOVERY IN WATER
• T1 recovery occurs due to nuclei giving up the energy acquired from
the RF excitation pulse to the surrounding lattice
• In water nuclear mobility is high
resulting in less efficient t1 recovery
• The magnetic moments of water
take longer to relax and regain their
longitudinal magnetization
• NMV of water takes longer to realign
with B0 and so the T1 time of water
is long

• T2-decay in fat :occurs as a rest of
the magnetic field of the nuclei
interacting with each other, therby
changing their energy to their
neighbours. As energy exchange is
more efficient in the hydrogen in fat
the T2 time is short. The T2 time of
fat is approx. 80ms
• T2 decay in water:As energy
exchange in water is less efficient
than in fat, the T2 time of hydrogen
in water is long. The T2 time of water
is approximately 200 ms

T1 CONTRAST
• T1 time of fat is shorter than water, the
fat vector realigns with B0 faster than
that of water
• There is less longitudinal magnetization in
water after the RF pulse. Water has low
signal & appears dark
• The longitudinal component of
magnetization in fat is larger than water
• A short TR & short TE will result in a T1
weighted image
• Excellent for demonstrating anatomy

T2 CONTRAST
• T2 time of fat is shorter than that of water,the
transverse component of magnetization of fat
decays faster
• Magnitude of transverse magnetization in
water is large
• Water has high signal and appears bright on T2
contrast image
• Magnitude of transverse magnetization in fat is
small so has low signal and appears dark

BASIC PARAMATERS
• TR (Repetition time) :- It is the time from the application of
one RF pulse to the application of next & is measured in
milliseconds (ms)

• TE (Echo Time) :- It is the time from the application of the
RF pulse to the peak of the signal induced in the coil & is
also measured in milliseconds (ms). It is usually the half of
the TR

• TI(Time from Inversion):It is the time from the application of
the 180° inverting pulse to the 90° excitation pulse

AT (ACQUISTION TIME)
• Time of acquisition is controlled by many factors :-
1. Signal to noise ratio ( SNR)
2. Contrast to noise ratio (CNR)
3. Spatial resolution
4. Scan time

FLIP ANGLE
• The first result of the resonance is
the NMV moves out of alignment
away from B0. The angle to which
the NMV moves out of alignment
is called flip angle. The magnitude
of the flip angle depends upon the
amplitude & duration of the RF
pulse. Usually the flip angle is 90°

• Phase : NMV move into phase with each other. The phase is
the position of each magnetic moment as the precessional
path around Bo. Two types 1) Out of phase 2 ) In phase

TYPES OF PULSE SEQUENCES
1.Spin Echo (SE)
a.Conventional spin echo ( CSE)
b. Fast spin echo (FSE)
c. Ultrafast spin echo (UFSE)
d. Turbo spin echo (TSE)
2. Inversion Recovery (STIR , FLAIR )
3. Gradient Echo (GRE)

SPIN ECHO
• It has at least two RF pulses,an excitation pulse and one or more
180 ° refocusing pulses that generate spin echo
• Utilizes 90° excitation pulse to flip the NMV into transverse
plane.NMV precesses in the T Plane induces voltage in the
receiver coil
• The paths of precession of the nuclei within the NMV are
translated into the T plane
• When the 90° RF pulse is removed an FID signal is produced, T2
dephasing occurs immediately & the signal decays
• A 180° RF pulse is then used to compensate this dephasing

• 180° pulse has sufficient energy to
move NMV through 180°
• The T2 dephasing causes the
magnetic moments to fan out in
the TP
• The magnetic moments becomes
out of phase with each other
• The magnetic moments that slow
down, form the trailing edge of
the fan (S) & that speed up, forms
the leading edge of the fan (F),

• The 180° RF pulse flips these individual magnetic moments through
180°
• They are still in the TP ,but now the magnetic moments that form
the trailing edge before the 180° pulse, form the leading edge
• Conversely, previously formed leading edge becomes trailing edge.
So the trailing edge begins to catch up with the leading edge after
specific time both edges superimposed
• At this instant –transverse magnetization
in phase –max. signal induced in the coil
which is called spin echo

TIMING PARAMETERS
T1 WEIGHTING
 short TE 10-20ms
 short TR 300-600ms
Typical scan time 4-6min
PROTON DENSITY/T2 WEIGHTING
Short TE 20 ms/long TE 80ms+
Long TR 2000 ms+
Typical scan time 7-15min

Advantages
• Good image quality
• Very versatile
• True T2 weighting sensitive to pathology
Disadvantages
• Scan times relatively long

• In most of the SE PS , more than one 180° RF pulse can be
applied after the 90° excitation pulse
• Each 180° pulse generates a separate SE . One two or four
180° RF pulses can be used to produce either one two or
four Images

Spin echo using single echo
• Used to produce T1 weighted images if short TR &TE r used
• One 180° RF is applied after 90° excitation
• Generates single spin echo

Spin echo using two echoes
• Used to produce both
PD & a T2 weighted
image in TR time
• The 1st spin echo
generated early by
selecting short TE
• A little T2 decay has
occurred & T2 diff, btn
the tissues are
minimized

• Two 180° pulses are sent after each 90° pulse to obtain dual
echoes per TR
• The first echo has a short TE (TE1) and a long TR and results in a set
of proton density weighted image
• The second echo has a long (TE2) and a long TR and results in a T2
weighted set of images. This echo has less amplitude than the first
echo because more T2 decay has occurred by this time

FAST SPIN ECHO (TURBO SPIN ECHO)
• A regular SE seq requires as many repetitions as there are
lines in the K-space to complete a slice acquisition
• Instead of acquiring K-space lines of other slices at diff.
positions during the wasted time,it is possible to acquire
several K-space lines for the same slice
• Such fast spin echo sequences use one 90 ° excitation pulse
& 2 or more 180 ° pulses in the same repetition time & with
different phase encoding gradient steps, to acquire multiple
echoes that will fill the K-space

• The NO. of echoes acquired after a single 90° excitation is
called the turbo factor or ECHO TRAIN LENGTH
• As each echo undergoes more & more T2 decay, the image
contrast is modified
• The effective echo time,detrmined by when the central lines
of K-space are acquired,indicates approxmately what the
image contrast obtained is like (rather T1-weighted or T2
weighted)

MRI PULSE SEQUENCES.pptx/////////////////

Advantage and disadvantage
Clinical use:- Billiary & urinary tract exploration, myelography
ADVANTAGE DISADVANTAGE
Short scan times Some flow artifacts increased
High resolution imaging Incompatible with some imaging
options
Increased T2 weighting Some contrast interpretation
problems
Magnetic suspectibilty decreases Image blurring possible

WEIGHTING
• To demonstrate either T1 proton density or T2 contrast,
specific values of TR & TE are selected for a given pulse
sequence
• The selection of appropriate TR& TE weights an image so
that one contrast mechanism predominates over the other
two

T1 weighting image
• TR controls how far each vector can recover before it is
excited by the next RF pulse, to achieve T1 weighting TR
must be short enough so that neither fat nor water return to
B0 . If the TR is too long both fat & water return to Bo &
recover their Longutidinal Magnetization fully
• TR controls the amount of T1 weighting
• For T1 weighting the TR must be short
• If the TE is also short,there is little time for T2 relaxation &
image contrast will not be very dependent on T2

T1 DIFFERENCES BETN FAT & WATER

T1 WTD IMAGE
• If the TR is very long, the longitudinal magnetization of all the tissues
will have recovered completely(complete T1 relaxation)
• If the TR is short,tissue signal & image contrast will depend on the T1
characteristics of tissues
as not all the tissues will have
completely recovered their
longitudinal magnetization
• When both the TR and the TE
are short, the image is said to
be T1 weighted

SE (T1 weighted image)
• Advantage - Anatomic Imaging
• Disadvantage- very long acquisition time
• Clinical use:- almost all the organs explored in MRI

T2 weighting image
• The contrast predominantly depends on the differences in the T2
times betweenn fat & water
• 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 & water time to decay
• A long TR is about 2000 ms or more & along TE is about 80 to
140ms
• With such parameters , the tissues with a long T2 time will have a stronger
signal than the tissues with a short T2 time

• T2 weighted-
• Advantage –imaging of water/fluids
( CSF,edema,biliary MRI)
• Disadvantage –very long acquisition
time

PROTON DENSITY CONTRAST
• Refers to differences in signal intensity between tissues
which are consequences of their relative no. of protons
/volume.
• The effect of T1&T2 contrast must be diminished
• If we use a long TR (>2000ms) & a short TE(10-20ms) , the
tissue signal will not be very dependent on T1 & T2
relaxation
• Tissues with rich hydrogen content(like water)will be bright
where as tissues with low hydrogen content will be dark

• Advantage
-Very long acquisition time
• Clinical use:
-osteoarticular
-paediatric Neuroradiology

SUMMARY OF SPIN ECHO
• Uses a 90° excitation pulse followed by one or more 180°
rephasing pulses to generate one or more spin echoes
• SE produces either T1, T2 or proton Density weighting
images
• TR controls the T1 weighting
Short TR maximizes T1 weighting
Long TR maximizes PD weighting
• TE controls the T2 weighting
Short TE minimizes T2 weighting
 Long TE maximizes T2 weighting

GRADIENT ECHO PULSE SEQUENCE
• Gradient Echo PS utilizes an RF excitation pulse that is
variable
• Therefore flips NMV through any angle (not just 90°)
• When a flip angle other than 90° is used, only part of the
longutidinal magnetisation is converted to transverse
magnetisation, which precesses in the transverse plane and
induces a signal in the receiver coil

• After the RF pulse is withdrawan,FID signal is immediately
produced due to inhomogenities in the magnetic field & T2
dephasing therefore occurs
• Magnetic moments within the transverse component of
magnetisition diphase, and are then rephased by a gradient
• The gradient rephases the magnetic moments so that signal
can be received by the coil which contains T1 and T2
information
• This signal is called gradient echo

There are basic three differences between SE and GRE sequences
1. There is no 180° pulse in GRE. Rephasing of TM in GRE is done
by gradients
2. The flip angle in GRE is smaller, usually less than 90° Since flip
angle is smaller there will be early recovery of longitudinal
magnetization (LM) such that TR can be reduced, hence the
scanning time
3. Transverse relaxation can be caused by combination of two
mechanisms:-
A. Irreversible dephasing of TM resulting from nuclear, molecular
and macromolecular magnetic interactions with proton
B. Dephasing caused by magnetic field inhomogeneity

Advantages & Disadvantages
• Advantages
• Rephase faster than 180° RF pulses
• TR can be shortened without producing saturation
• Shorter scan time than SE pulse sequence
• Disadvantages
• No compensation for magnetic field inhomogeneties
• Can also contain magnetic suspectively artifact

USES
• Can be used to acquire T2,T1 and proton density weighting
• GE allow for reduction in scan time so can be used for single
slice breath-hold acquistions in the abdomen and for
dynamic contrast enhacement
• May be used to produce angiographic type images

Timing parameters of gradient echo

SPOILED OR INCOHERENT GRE
SEQUENCES
• If the residual TM is destroyed by RF pulse or gradient such that
it will not interfere with next TR, the sequences are called
spoiled or incoherent GRE sequences.
• These sequences usually provide T1-weighted GRE images
These sequences can be acquired with echo times when water
and fat protons are in-phase and out-of-phase with each other.
• This ‘in- and out-of-phase imaging’ is used to detect fat in the
lesion or organs. It is modified to have time-of-flight MR
Angiographic sequences. The 3D versions of these sequences
can be used for dynamic multiphase post contrast T1-weighted
imaging

STEADY STATE (SS) Sequences
• The residual TM is not destroyed. When residual transverse
magnetization is refocused keeping TR shorter than T2 of the tissues, a
steady magnitude of LM and TM is established after a few TRs. Once
the steady state is reached, two signals are produced in each TR: FID
(S+) and spin-echo (S–). Depending on what signal is used to form the
images, SS sequences are divided into 3 types.
• 1. Post-excitation refocused steady-state sequences:- Only FID (S+)
component is sampled. Since S+ echo is formed after RF excitation
(pulse), it is called post-excitation refocused..
• 2. Pre-excitation refocused steady-state sequences. Only spin echo (S-)
component is used for image formation. S- echo is formed just before
next excitation hence the name pre-excitation refocused
• 3.Fully refocused steady-state sequences:- Both FID (S+) and Spin echo
(S-) components are used for the image formation. These are also
called ‘balanced-SSFP’ sequences as gradients in all three axes are
balanced making them motion insensitive

INVERSION RECOVERY(IR)
• One method for manipulating contrast is called inversion
recovery
• Is a pulse sequence which begins with a 180° inverting pulse
. It is used to produce heavily T1 weighted images to
demonstrate anatomy . Large contrast differences between
fat and water can be obtained
• The 180° IR pulse changes the direction of the longitudinal
Magnetisation vector to its opposite.Then it is going to
recover as defined by T1 relaxation
• At time T1(inversion time), a regular spin echo(or gradient
recalled echo or echo planar seq is performed,starting with
an excitation pulse

The inversion recovery pulse sequence

T1 weighting in inversion recovery

STIR (Short Tau Inversion Recovery )
• STIR is an IR pulse sequence that uses a TI that corresponds to the
time it takes the fat vector to recover from full inversion to the
transverse plane so that there is no longitudinal magnetization
corresponding to fat. This is called the null point.
• As there is no longitudinal component of fat when the 90° RF
excitation pulse is applied, there is no transverse component after
excitation, and signal from fat is nulled
• A TI of 100-175 ms usually achieves fat suppression, although this
value varies slightly at different field strengths
• 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
tumours are seen more clearly

Suggested parameters
• Short TI (tau) 150-175 ms (to suppress fat depending on field strength)
• Long TE 50 ms+ (to enhance signal from pathology)
• Long TR 4000 ms+ (to allow full longitudinal recovery)
• Long turbo factor 16-20 (to enhance signal from pathology)
• Scan tip:When not to use STIR
• STIR should not be used in conjunction with contrast
enhancement, which shortens the T1 recovery times of
enhancing tissues, making them relatively hyperintense. The T1
recovery times of these structures are shortened by the contrast
agent so that they approach the T1 recovery time of fat. In a STIR
sequence, therefore, enhancing tissue may also be nulled.

FLAIR ( fluid attenuated inversion
recovery )
FLAIR :- is the inversion recovery sequence where CSF is
nullified by selecting 180° inverting pulse to the inverse
plane . It is used to suppress the CSF signal in T2 & proton
density images. So pathology adjacent to the CSF is seen
more clearly
• In the same way, with a TI time about 2000ms, water signal
is eliminated (water T1 is long)

USES
• FLAIR is used in brain and spine imaging to see
periventricular and cord lesions more clearly because high
signal from CSF that lies adjacent is nulled
• lt is especially useful visualizing multiple sclerosis plaques,
acute sübarachnoid meningitis. This TI value nulls signal from
normal white matter so that lesions within it appear much
brighter by comparison
• This sequence (which requires a TI of about 300 ms) is very
useful for white matter lesions such as periventricular
leukomalacia and for congenital grey/white
matter abnormalities

Suggested parameters
• Long TI factor1700-2200 ms (to suppress CSF depending on field strength
• Long TE 70 ms+ (to enhance signal from pathology
• Long TR 6000 ms+ (to allow full longitudinal recovery)
• Long turbo factor 6-20 (to enhance signal from pathology)
• Learning tip: FLAIR and gadoliniumSometimes gadolinium is
given to enhance pathology in the FLAIR sequence. This oddity
(gadolinium enhancement in T2-weighted images) may be due to
the long echo trains used in FLAIR sequences that cause fat to
remain bright on T2-weighted images. As gadolinium reduces the
Ts recovery time of enhancing tissue so that it is similar to fat,
enhancing tissue may appear brighter than when
gadolinium is not given

THE STEADY STATE
• The condition where the TR is shorter than the T1&T2 times
of the tissue & thus no time for TM to decay before the pulse
sequence is repeated again

ECHO PLANAR IMAGING
• Ehoes can be generated by multiple 180° pulses termed spin
echo EPI (SE EPI) or by gradients termed gradient echo (GE-
EPI)
• It represents the fasest acquisition modes in MRI
• Real time ,dynamic &functional studies are possible using
this technique

REAL TIME IMAGING
• Very fast sequences such as EPI ,permit real time imaging of
moving structure
• It has proved to b very useful in interventional procedures
whereas a biopsy needle ,laser probe or other instruments
can be visualized in real time
• Biopsies thermal ablations of tumors angioplastics
endoscopies & limited field surgical operation are the most
promising applications of this technique

REFERENCES
• MRI Made Easy-schering
• MRI in practice catherine westbrook & carolyn kaut
• Internet sources

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