A pulse sequence is a sequence of events, which we need to acquire MRI images. Basic pulse sequences 1. Spin echo sequence(SE) 2. Gradient echo sequence(GRE) 3. Inversion Recovery Sequence(IR) 4. Echo Planar Imaging(EPI)

1. BASIC PRINCIPLES OF MRI
PHYSICS: PART 2
PULSE SEQUENCES
AUTHOR
DR. GULSHAN KUMAR MADHPURIYA
Clinico-radiologist

2. Pulse Sequences
 A pulse sequence is a sequence of events, which we need to acquire MRI images.
 These events are: RF pulses, gradient switches and signal collection

3. Basic pulse sequence
 1) switching on the Slice Select gradient
(GSS). Simultaneously
 (2) a 90º RF-pulse was given to ‘flip’ the
net-magnetization into the X-Y plane.
 Then (3) the Phase Encoding gradient
(GPE) was switched on to do the phase
encoding.
 Then (4) the Frequency Encoding or Read
Out gradient (GRO) was switched on
during which
 (5) the signal, the Free Induction Decay
(FID), was sampled.

4. TR (Repetition Time): TR is the time between two 90º excitation pulses. TR can be in
the range of 100 to 3000 ms.
TE (Echo Time): This is the time between the 90º excitation pulse and the echo. TE
can be in the range of 5 to 250 ms.
FA (Flip Angle): Refers to the amount of degrees the net-magnetization is flipped
into the X-Y plane. It has nothing to do with the 180º rephasing pulse.

5. Basic pulse echo sequence
1. Spin echo sequence(SE)
2. Gradient echo sequence(GRE)
3. Inversion Recovery Sequence(IR)
4. Echo Planar Imaging(EPI)

6. Image Contrast
 Before we move on to other pulse sequence techniques, it is essential to discuss
image contrast.
 We have seen that there are two relaxation processes, T1 and T2, going on at the
same time.
 The image contrast is highly dependent on these relaxation processes.

7. T1 Contrast/ T1 WI
 Assume we scan with the following parameters: TR 600 and TE 10.
 We allow for T1 relaxation to take place for 600 milliseconds and,
more important, T2 relaxation only for 5 milliseconds (10÷2).

8.  FigureA, we see that after 5 ms. hardly any dephasing has taken place?. We receive
a lot of signal from all tissues. The image contrast is, therefore, very little influenced
by T2 relaxation.
 In FigureB we see that after 600 ms. not all tissues have undergone complete T1
relaxation. Fat is nearly there, but CSF has still a long time to go. So, for the next
excitation the net magnetization vector of the CSF spins, which can be flipped into
the X-Y plane is small.

9.  This means that the contribution from CSF to the overall signal will be
small too.
 In short, the image contrast becomes dependent on the T1 relaxation
process.
 In the final image CSF will be dark, fat will be bright and gray matter
will have an intensity somewhere in between.
 In this case we say that the image is “T1 weighted” because the
contrast is more dependent on the T1 relaxation process.

10. T2 Contrast/ T2WI
 We use the following parameters: LONG TR 3000 and LONG TE 120.
 Now we allow T2 relaxation to happen for 60 ms. (120÷2).

11.  Figure A most of the tissues have dephased and won’t produce that much signal.
Only CSF (water) has still some phase coherence left.
 Here the Figure B shows that practically all tissues have undergone complete T1
relaxation.
 The long TR of 3000 ms does not contribute much to the image contrast. The 3000
ms. are only needed to allow CSF to recover completely before the next excitation. TE
is the dominant factor for the image contrast.

12.  Figure A most of the tissues have dephased and won’t produce that
much signal. Only CSF (water) has still some phase coherence left.
 Here the Figure B shows that practically all tissues have undergone
complete T1 relaxation.
 The long TR of 3000 ms does not contribute much to the image
contrast. The 3000 ms. are only needed to allow CSF to recover
completely before the next excitation. TE is the dominant factor for
the image contrast.

13.  In THIS image we’ll see CSF bright,
while the other tissues show up in
various shades of gray. In this case we
say the image is “T2 weighted”
because we allowed for T2 to happen
for a “long” time.

14. Proton Density Contrast/ PDI
 There is one more type of image contrast called Proton Density.
 Now we choose the parameters: TR 2000 and TE 10.
 Again we allow T2 relaxation to happen for only 5 ms., which means that T2
relaxation contributes very little to the image contrast. With a TR of 2000 ms. the net
magnetization of most tissues will have recovered along the Z- axis. The image
contrast in PD images is neither dependent on T2 relaxation, nor T1 relaxation.
 The signal we receive is completely dependent on the amount of protons in the
tissue

15. Figure:- shows.
1. A short TR and short TE gives T1 weighted contrast.
2. A long TR and a short TE gives PD contrast.
3. A long TR and long TE gives T2 weighted contrast.

16. T1
TR = short
TE = short
CSF = dark
PD
TR = long
TE = short
CSF = gray
T2
TR = long
TE = long
CSF = bright

17. 1. Spin echo sequence(SE)
 It consists of 90 and 180 degree RF pulses. The excitatory 90 degree pulse flips net
magnetization vector along Z-axis into the transverse (X-Y) plane. The transverse
magnetization (TM) precessing at Larmor frequency induces a small signal called free
induction decay (FID) in the receiver coil. FID is weak and insufficient for image
formation. Also, the amount of TM magnetization reduces as protons start dephasing.
Hence a rephasing 180 degree pulse is sent to bring protons back into the phase. This
rephasing increases magnitude of TM and a stronger signal (spin echo) is induced in the
receiver coil. This gives the sequence its name.

19. Basic rephasing sequence

20.  Because it is “rebuilt” from the FID. Notice that the 180º rephasing pulse is
exactly in the middle of the 90º pulse and the echo.

21. 1. Spin echo sequence(SE)
 SE sequence forms the basis for understanding all other sequences.
 It is used in almost all examinations.
Advantages Disadvantages
Good quality images It takes time to do the rephasing step.
Very versatile Long scan time
The Signal is strong It increases the amount of RF(not that it’s
dangerous, but there are certain limits).
Available on all systems
Gold standard for image contrast and
weighting

22. Modifications of SE Sequence
 A) Dual SE sequence :
 Two 180 degree pulses are
applied to get PD -T2 double
echo sequence
 This sequence is run with long
TR.
 After the first 180 degree pulse,
since TE is short, image will be
proton density weighted (long
TR, short TE).
 After second 180 degree pulse,
TE will be long giving a T2-W
image (long TR, long TE).
 Both these echoes contribute
separate K-Space lines in two
different K-Spaces.

23. B) FAST (TURBO) SPIN-ECHO Sequence
 Also called as Multi spin-echo or
Turbo spin-echo sequence
 In fast SE sequence, multiple 180
degree rephasing pulses are sent
after each 90 degree pulse.
 In this sequence, multiple echoes
are obtained per TR, one echo with
each 180 degree pulse.

24. B) FAST (TURBO) SPIN-ECHO Sequence
 All echoes are used to fill a single
K-Space.
 Since K-Space is filled much faster
with multiple echoes in a single TR
the scanning speed increases
considerably.

25. Turbo factor
 Turbo factor is the number of 180
degree pulses sent after each 90
degree pulse. It is also called as echo
train length
TE effective
 The amplitude of signal (echo)
generated from the multiple
refocusing 180 degree pulses varies
since the TE goes on increasing. The
TE at which the center of the K-
Space is filled is called as ‘TE effective’

26. Turbo factor/ ETL vs T effective
Short turbo factor decreases effective TE and increases T1 weighting.
However, it increases scan time. Long turbo factor increases effective TE,
increases T2 weighting and reduces scan time.

27. Image blurring increases with turbo factor because more number of
echoes obtained at different TE form the same image.

28. Multi-echo sequence images at varying TR :
TE

29. Multi-SE with flip-back 90° pulse
 When a negative 90 degree pulse is sent at the end of the echo train, the
magnetization for tissues with high T2 flips quickly back in the longitudinal
plane at the end of each TR.
 This makes fast SE sequences even faster. The fast SE sequences using this
90 degree flip-back pulse at the end of each TR are called fast recovery
sequences.

30. SINGLE-SHOT FAST SPIN-ECHO Sequence
 This is a fast SE sequence in which all the echoes required to form an image are acquired
in a single TR. Hence it is called ‘single-shot’ sequence.
 ADVANTAGES:
1. Breathhold sequence-virtually freezes motion. Hence preferred in uncooperative,
claustrophobic patients & in infants.
2. Allows multiple heavily T2 weighted images in the same TR – esp. used for abdominal
imaging.
 DISADVANTAGES: Suffers from image blurring, esp. for tissues with short T2 relaxation.

32. HALF FOURIER SINGLE-SHOT TURBO SPIN ECHO
HASTE
 In this fast spin echo sequence, not only all K-Space lines are acquired in a single
excitation but also just over a half of the K-Space is filled.
 The other half of the K-Space is mathematically calculated with Half-Fourier
transformation.
 For example, to acquire an image with matrix of 128 x 128 it is sufficient to acquire
only 72 K-Space lines.
 Half-Fourier transformation reduces the scan time by half.

36. SPOTTERS
???? Pulse sequence
???? Nature of pathology

37. SPOTTERS
Axial T1-weighted (a) and T2 weighted (b) fast SE images show a low-grade
glioma. Because of hypercellularity, the tumor appears with hypointense signal
in a and hyperintense signal in b. The cystic components and edema are better
depicted in b than in a.

38. ???? Pulse sequence
???? Nature of pathology

39.  Axial T2-weighted fast SE image,
 SCAN TIME For fast SE imaging with
an echo train length of 16 was 34
seconds.
 Axial T2-weighted conventional SE
image,
 SCAN TIME For conventional SE
imaging was 7min 17 sec

40. Gradient Echo (GRE) Sequence
 They differ from the Spin Echo sequence in the way the echo is formed. Where a Spin
Echo sequence uses an 180º rephasing pulse to rephase the spins, the Gradient Echo
sequence uses a gradient polarity reversal .
1. RF excitation pulse.
2. Simultaneous Slice selecting with Gss
3. Phase encoding.
4. Switch on GRO. First negative polarity, and
then change polarity to positive.
5. Signal sampling during GRO.

41. ADVANTAGE
Changing the polarity of the GRO has the same effect as an 180º RF pulse. The
advantage is that it can be done much faster than the 180º pulse. That
makes this sequence useful when fast scans are needed.
DISADVANTAGE:-
The disadvantage is that it does not correct for local magnetic field
inhomogeneities, which translates into the presence of artifacts in the image.

42. Gradient Echo (GRE) Sequence
 There are basic three differences between SE and GRE sequences.
1. There is no 180 degree pulse in GRE. Rephasing of TM in GRE is done by
gradients so called as Gradient echo sequence.
2. The flip angle in GRE is smaller, usually less than 90 degree. Reduced
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.

43. Gradient Echo (GRE) Sequence
Are you remember T2*????
 In SE sequence, the dephasing caused by magnetic field inhomogeneity is
eliminated by 180 degree pulse.Hence there is ‘true’ transverse relaxation
in SE sequence.
 In GRE sequence, dephasing effects of magnetic field inhomogeneity are
not compensated, as there is no 180 degree pulse. T2 relaxation in GRE is
called as T2* (T2 star) relaxation.
 Usually T2*< T2.

44. GRE
Steady State GRE
Spoiled GRE
The residual TM is not destroyed. In
fact, it is refocused such that after a
few TRs steady magnitude of LM and
TM is reached, are called steady-state
or coherent 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..
GRE sequences can be divided into two types depending on what is
done with the residual transverse magnetization (TM) after
reception of the signal in each TR.

47. Flip angle
 Flip angle, also called tip angle, is the amount of rotation the net
magnetization (M) experiences during application of a radiofrequency
(RF) pulse.
 The flip angle is usually at or close to 90 degrees for a spin echo
sequence but commonly varies over a range of about 10 to 80
degrees with gradient echo sequences.
The larger Flip angles (in a GE sequence) has these effects:
• More T1 contrast.
• More signal.
Smaller flip angles give more T2 or actually T2* weighting to the images.

48. Here are two images where TR and TE are kept the same, while changing the FA.
TR=150
TE=10
FA=10
TR=150
TE=10
FA=70
A high FA has more T1
weighting (CSF dark).
A low FA has more T2
weighting (CSF bright)

50. SPOILED/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.
 Spoiled GRE sequences are 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. Examples include 3D FLASH

51. STEADY STATE (SS) Sequences
 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:
1. Preexcitation signal (S−) from echo reformation;
2. Postexcitation signal (S+), which consists of free induction decay.
 Depending on what signal is used to form the images, SS sequences are
divided into 3 types.
1. preexcitation refocused (only S− is sampled)
2. postexcitation refocused (only S+ is sampled)
3. fully refocused (both S+ and S− are sampled) sequences/

52. 3) Inversion Recovery (IR) Sequence
 IR sequence consists of an inverting 180 degree pulse before the usual
spin-echo or gradient echo sequence. In practice, it is commonly used
with SE sequences
 The time between the initial 180 degree pulse and the 90 degree pulse is
the inversion time (TI). A diagram of the sequence is shown below.

53. Advantages
It provides very strong contrast between tissues having different T1
relaxation times or to suppress tissues like fluid or fat.
Disadvantages
 Longer scan time due to the additional 180 inversion pulse

54. Types of IR Sequences
 IR sequences are divided based on the value of TI used.
 IR sequences can be of short, medium or long TI.
1. Short TI IR sequences use TI in the range of 80-150 ms and example is
STIR (SHORT TI INVERSION RECOVERY).
2. In Medium TI IR sequences, TI ranges from 300 to 1200 ms, and
example is MPRAGE.
3. Long TI ranges from 1500 to 2500 ms and example is FLAIR.

55. STIR (SHORT TI INVERSION RECOVERY)
STIR sequence can be used for fat suppression, where a relatively short
inversion time is used to null the fat signal while maintaining water and soft
tissue signal.
 USE: In cases where the high signal from fat may obscure pathology such
as Bone contusions.
 DRAWBACKS:
1. Partial loss of proton signal during the TI time.
2. TR time must be longer than that of a spin echo sequence for recovery of
longitudinal magnetization

57. Coronal STIR image shows
striking marrow edema in
the mid right tibia.

58.  The fat suppression possible by STIR is generally uniform and relatively
independent of magnetic field inhomogeneities. STIR may be superior to
other fat saturation methods (such as spectral "fat-sat")
a) Sagittal T2-weighted fast SE image obtained
with spectral fat suppression, which requires a
uniform magnetic field, shows incomplete fat
saturation in regions where there is field
inhomogeneity,
b) Sagittal image obtained with STIR, which is less
susceptible than fast SE sequences to mag-
netic field inhomogeneities, provides more uniform
and more complete fat saturation (arrow)

59.  ???? Pulse sequence
 ???? Nature of pathology

60.  Coronal T1-weighted fast SE image (a) and coronal STIR image (b) both
show pancarpal rheumatoid arthritis; however, the extent of bone
marrow edema throughout the carpal bones, distal radius, and ulna is
better depicted in b than in a.

61. FLAIR (FLUID ATTENUATED
INVERSION RECOVERY) SEQUENCE :
 FLAIR is a special inversion recovery sequence with long TI to remove the
effects of fluid from the resulting images. The TI time of the IR pulse
sequence is adjusted to the relaxation time of the component that should
be suppressed. For fluid suppression the inversion time (long TI) is set to
the zero crossing point of fluid, resulting in the signal being 'erased'.
 USE: Used to study demyelinating diseases, such as Multiple Sclerosis.
With this TI value, Multiple Sclerosis lights up like a light bulb.

63. ???? Sequence

64. FLAIR sequence T2 weighted image

65. B) Axial T1 with gadolinium contrast
demonstrating enhancement of the
lesion margins.
A) Axial FLAIR MRI demonstrating
large tumefactive areas of
demyelination.

66. ??? Sequence/WI
??? Pathology

67. a) Axial T2-weighted fast SE
image shows white matter
abnormalities in the left
temporal lobe.
b) Axial T2-weighted FLAIR image
obtained with nulling of the signal
from cerebrospinal fluid shows
the metastatic lesions more
clearly.

68. ??? Sequence/WI
??? Pathology

69. a) T1WI b) T2WI c) CONSTRAST T1WI
DIFFUSE ASTROCYTOMA WHO GRADE II

70. STIR{SHORT T1 (tau) IR SEQUENCE} Vs FLAIR{FLUID ATTENUATED INVERSION
RECOVERY SEQUENCE}
Short TI of 80-150 ms is used 1 Long TI of 1500-2500ms is used
Combined T1 & T2 weighting is
obtained
2 Heavily T2 weighted images are used
Fat and white matter are suppressed 3 CSF and water is suppressed
Mainly used in body imaging 4 Used in neuroimaging
Cannot be used in post contrast
imaging as short T1 tissue are
suppressed and contrast shortens T1 of
tissues that uptake the contrast
5 Can be used in post contrast imaging

71. Medium Ti Inversion Recovery Sequence
MPRAGE (Siemens)
Magnetization Prepared Rapid Acquisition Gradient Echo
 T1-w sequences that use inversion pre-pulse with medium inversion
time (TI) in the range of 600-1200 ms.
 ADVANTAGES:-
1. This increases contrast between gray and white matter.
2. Because of their good gray-white differentiation ability they are used
in the epilepsy protocol for the evaluation of mesial temporal sclerosis
and detection of cortical dysplasia.

72. ???? SEQUENCE
???? PATHOLOGY

73. A) MPRAGE sagittal image of the brain. Appreciate the gray- white
differentiation ability of the sequence. (B) MPRAGE oblique coronal
image of the brain shows atrophic left hippocampus (arrow) in a
patient with epilepsy

74. ?????

76. STIR coronal image of the knee, show Bright signals are seen in
the medial condyle of tibia and femur (arrows) suggestive of
marrow edema in this case of osteoarthritis

77. STIR sagittal image of the cervico-dorsal spine, Show Multiple bright
spots are seen in vertebral bodies as well as posterior elements
suggestive of metastases in this patient with known primary tumor.

78. STIR coronal image of the orbit show The left optic nerve is
atrophic with prominent CSF space around it (arrow)

79. STIR coronal image of sacroiliac joints in ankylosing spondylitis. Early inflammatory
changes in the form of edema are seen as bright signals bilaterally (arrows)

81. • A) Edema anterior to the lesion (arrow).
• (B) Multiple chronic infarcts in left
periventricular region indicated by arrows.
• (C) Multiple plaques are seen running
perpendicular to the callosal margin and in
the occipital lobe
• (D) Hippocampi in cross section. The left
hippocampus is atrophic and bright (arrow)
in this patient with epilepsy.

82. Echo planar imaging
 also known as Instascan.
 Echo planar imaging is an technique that collects all the data required to
fill all the lines of k space from a single echo train
 Filling of k space in echo planar imaging involves rapidly switching the
read out gradient from positive to negative. This rapid switching of
polarity of read out gradient is known as oscillation

83. ADVANTAGES:
 Echoplanar images may be acquired in less that 1/10th of a second and
therefore allows -- dynamic (rapidly changing) processes, like cardiac
imaging and intrauterine fetal imaging, diaphragmatic hernia,
 Motion free images.
 Can be used along with the diffusion weighted imaging

84. Comparison between FLAIR and EPI
 FLAIR-multiple periventricular
lesions with motion artifacts
 EPI-multiple periventricular
lesions without motion
artifacts
FLAIR
EPI

85. DISADVANTAGES:
1. Extremely sensitive to image artefacts like signal loss and distortions.
2. Extreme noise
3. Increased RF deposition
4. Peripheral nerve stimulation
5. relatively demanding on the scanner hardware, in particular on gradient
strengths, gradient switching times, and receiver bandwidth.

86. USES:
1. DIFFUSION WEIGHTED IMAGING -- Integral part of stroke imaging (since
diffusion changes can be detected within minutes of cerebral ischaemia)
differentiating epidermoids from cysts, & tumour necrosis from abscesses.
2. PERFUSION IMAGING – Calculates regional cerebral blood volume & total
blood flow, Studying tumour metabolism, Differentiating radiation necrosis vs
recurrent tumours, & prediction of stroke outcomes
3. NEURONAL ACTIVATION STUDIES – BOLD (Blood Oxygen Level Dependant
Contrast)
4. BREATHHOLD ABDOMINAL STUDIES
5. CARDIAC IMAGING – cine loops, flow quantifications, cardiac perfusion,
myocardial strain imaging

87. Depending on the data acquisition methods, EPI can be labeled as ---
1. SPIN ECHO EPI: (90⁰ - 180⁰ pulse)  T2 weighting
2. GRADIENT ECHO EPI: (90⁰ pulse without refocusing)  T2 weighting
3. IR-EPI: (180⁰ pre-pulse before spin echo pulse)  T1 weighting
4. DW-EPI: (using diffusion sensitized gradients)  Diffusion weighting

88. classification
SPIN ECHO SEQUENCES GRADIENT ECHO SEQUENCES
Single echo technique CSE
SSFP
PSIF
GRE
FLASH
FISP
GRASS
FAST
DESS
CISS
FADE
Single echo techniques with magnetisation
preparation
IR
STIR
FLAIR
Snapshot FLASH
Turbo FLASH
Snapshot GRASS
MP RAGE
Multi-echo techniques FSE
TSE
RARE
Segmented EPI
GRASE
TGSE
Multi-echo techniques with magnetisation
preparation
TIR
Turbo STIR
Turbo FLAIR
Fast FLAIR
IR FSE
Segmented IR EPI
Segmented DW EPI
Single shot techniques HASTE EPI
Single shot techniques with magnetisation
preparation
IR HASTE
HASTIRM
IR EPI
DW EPI
PULSE SEQUENCE CLASSIFICATION

89. CONCLUSION
The topic of MRI pulse sequences is
vast and ever evolving.. What was
covered in this seminar are the most
commonly used sequences and can
be described as just the tip of the
iceberg…
To just provide an idea of the
numerous sequences existing and
being modified to develop even
more… here is a composite list…

90. SPIN ECHO SEQUENCE
Dual Echo Sequence
Modified Spin Echo
Multi Echo Multiplanar
Partial Saturation Spin Echo
Variable Echo Multiplanar
Fast Spin Echo
Carr Purcell Sequence
Carr Purcell Meiboom Gill Sequence
Double Fast Spin Echo
Double Turbo Spin Echo
Dual Echo Fast Acquisition Interleaved Spin Echo
Half Fourier Acquisition Single Shot Turbo Spin Echo
Multiple Echo Single Shot
Rapid Acquisition with Refocused Echoes
Turbo Spin Echo
Ultrashort Turbo Spin Echo
Inversion Recovery Sequence
Flow Sensitive Alternating Inversion Recovery
Fluid Attenuation Inversion Recovery
Inversion Recovery Spin Echo
Short T1 Inversion Recovery
Turbo Inversion Recovery

91. GRADIENT ECHO SEQUENCE
Balanced Sequence
Balanced Fast Field Echo
Balanced SARGE
Balanced Turbo Field Echo
Fast Imaging with Steady Precession
Coherent Gradient Echo
Gradient Field Echo with Contrast
Inversion Recovery Fast Gradient Recalled
Acquisition in the Steady State
Fast Field Echo
Fast Imaging with Steady State Precession
Fourier Acquired Steady State
Reverse Fast Imaging with Steady State Precession
SHORT Repetition Technique Based on Free Induction Decay
Steady State Free Precession Sequence
Driven Equilibrium
Driven Equilibrium Fast Gradient Recalled Acquisition in the Steady State
Driven Equilibrium Fast Spin Echo
Driven Equilibrium Fourier Transformation
Driven Equilibrium Magnetization Preparation

92. Refocused Gradient Echo Sequence
Complex Rephasing Integrated with Surface Probes
Dual Fast Field Echo
Dual Echo Fast Gradient Echo
Fast Gradient Recalled Echo
Fast Field Echo
Fast Imaging with Steady State Precession
Fast Low Angle Recalled Echoes
Fourier Acquired Steady State
Gradient Field Echo with Contrast
Inversion Recovery Fast Gradient Recalled Acquisition in the Steady State
Resonant Offset Averaging in the Steady State
SHORT Repetition Technique Based on Free Induction Decay
Steady State Free Precession Sequence
Steady State Technique with Refocused FID
SPOILED GRADIENT ECHO SEQUENCE
Incoherent Gradient Echo (Gradient Spoiled)
Fast Low Angle Shot
Multiplanar Gradient Recalled Acquisition in the Steady State
Short Repetition Techniques
Small Tip Angle Gradient Echo
Incoherent Gradient Echo (RF Spoiled)
Gradient Field Echo
Radio Frequency Spoiled Steady State Acquisition Rewound Gradient Echo
RF Spoiled Fourier Acquired Steady State Technique
Small Tip Angle Gradient Echo T1 Weighted
Spoiled Gradient Recalled

93. Steady State Free Precession
Completely Balanced Steady State
Contrast Enhanced FAST
Contrast Enhanced Fast Field Echo with T2 Star Weighting
Fast Imaging with Steady Precession
Fourier Acquired Steady State
Driven Equilibrium Fast Gradient Recalled Acquisition in the Steady State
Reverse Fast Imaging with Steady State Precession
Steady State Gradient Echo with Spin Echo Sampling
Steady State Technique with Refocused FID
Ultrafast Gradient Echo Sequence
Echo Planar Imaging
Fast Spoiled Gradient Echo
Fourier Acquired Steady State
Gradient and Spin Echo
Magnetization Prepared Rapid Gradient Echo
Rapid Acquisition Matrix FAST
Rapid Scan
Short Minimum Angled Shot
Turbo Field Echo
Turbo Gradient Spin Echo
Turbo Gradient Recalled Acquisition in Steady State
Turbo Fast Low Angle Shot
Volumetric Interpolated Breath Hold Examination

94. NEWER MODALITIES . . .
Magnetic Resonance Angiography MRA
Black Blood MRA
Contrast Enhanced Magnetic Resonance Angiography
Phase Contrast Angiography
Time of Flight Angiography
Magnetic Resonance Spectroscopy
Binomial Pulses
Chemical Shift Imaging
Chemical Shift Selective Imaging Sequence
Depth Resolved Spectroscopy
Point Resolved Spectroscopy
Special Imaging
Arterial Spin Labeling
Blood Oxygenation Level Dependent Contrast
Diffusion Weighted Imaging
Diffusion Tensor Imaging
Diffusion Tensor Tractography
Functional Magnetic Resonance Imaging
Perfusion Imaging

95. Fat Suppression
There are 5 basic techniques of fat suppression
1. Frequency-selective fat suppression,
2. STIR,
3. Out-phase imaging,
4. Dixon method
5. Selective water excitation

96. Fat Suppression

97. Frequency-selective fat suppression
CHESS/Fat-Sat suppression

98. Typical "Fat Sat" pulse and spoiler gradient
used in a GRE sequence.

100. T1-weighted pelvis image without
fat-sat. Fat is the brightest
substance
T1-weighted image with fat-sat.
Note how muscle is now much
brighter than subcutaneous fat or
bone marrow

101. CAUTION POINT
 Spectral fat suppression can prove inadequate even at high fields
if magnetic homogeneity is poor.
1. This occurs typically around metal objects,
2. At tissue interfaces with different magnetic susceptibilities,
3. And where significant variations in tissue shape occur.

102.  The fat suppression possible by STIR is generally uniform and relatively
independent of magnetic field inhomogeneities. STIR may be superior to
other fat saturation methods (such as spectral "fat-sat")
a) Sagittal T2-weighted fast SE image obtained
with spectral fat suppression, which requires a
uniform magnetic field, shows incomplete fat
saturation in regions where there is field
inhomogeneity,
b) Sagittal image obtained with STIR, which is less
susceptible than fast SE sequences to mag-
netic field inhomogeneities, provides more uniform
and more complete fat saturation (arrow)

103. METHODS TO MINIMIZED INHOMOGENEITY
EFFECT
1. Performing shimming prior to imaging or
2. Packing saturation bags around irregularly shaped body parts.
 In spite of these measures, incomplete fat saturation regularly
occurs.

104. Spectral Presaturation with Inversion
Recovery (SPIR)

105. Spectral Attenuated Inversion
Recovery (SPAIR)

107. In-phase and out-of-phase sequences
 In-phase (IP) and out-of-phase (OOP) sequences correspond to paired
MRI gradient echo (GRE) sequence obtained with the same repetition time
(TR) but with two different echo time (TE) values.
 Because water and fat protons have slightly different resonance frequencies,
their spins go in- and out-of-phase with each other as a function of time. At
1.5T, the phase cycling period is 1/220 Hz or about 4.5 msec.
 In-phase and out-of-phase conditions occur twice per cycle, or approximately
every 2.2 msec at 1.5T. (At 3.0T the phase cycling is twice as fast, occurring
every 1.1 msec). GRE images obtained at 1.5T at TE's of 2.2, 6.6, 11.0 msec
are called out-of-phase (OOP); those obtained at 4.4, 8.8, etc. are called in-
phase (IP).

108. Phase-cycling between fat and water at 1.5T

109. Applications
 The main application of the IP-OOP sequences is to identify pathological
(microscopic) fat content of tissues in the abdomen by showing signal intensities
drop on the OOP images compared to the IP images.
 Examples where IP-OOP sequences are useful include:-
1. fatty liver and focal fatty sparing/infiltration
2. fat-rich adrenal lesions:
 adrenal adenoma (helping differentiate it from carcinomas and metastases)
 adrenal myelolipoma
3. lipid-poor angiomyolipoma
4. renal cell carcinoma (RCC)

111. Lipid-rich adrenal adenoma
(arrow). In-phase GRE
image at TE=4.4 msec
shows tumor of
intermediate signal
intensity.
Out-of-phase GRE image at
TE=2.2 msec. The adenoma
(arrow) falls in signal

112. Hepatic steatosis.

113. TAKE HOME MESSAGE

114. Dixon method
 This is based on in and out-of-phase imaging.
 In this method, Two sets of images are acquired, one at in-phase TE and
other at out-of-phase TE.
 Addition of these two sets of images gives ‘water-only’ image
 While subtraction gives ‘fat-only’ image
IP = W + F
OP = W − F
½ [IP + OP] = ½ [(W+F) + (W−F)] = ½ [2W] = W → Water only
image
½ [IP − OP] = ½ [(W+F) − (W−F)] = ½ [2F] = F → Fat only image

115. Coronal images of the abdomen show a fatty
lesion in the lower pole of the kidney (arrow) in
keeping with angiomyolipoma.
Four sets of images are available with Dixon
method in a single acquisition including
(A)In phase
(B) out-of-phase
(C) fat-only
(D) water-only images

116. Mathematically in two ways which result in a total of 4 sequences:
1. in-phase = (water + fat)
2. opposed-phase = (water - fat)
3. fat only = in-phase - opposed phase = (water + fat) - (water - fat)
4. water only = in-phase + opposed phase = (water + fat) + (water - fat)
The water only image can be used as a fat-suppressed image.

117. Advantage of Dixon method
 Suppression of fat signal is more uniform and less affected by artifacts
than many other techniques.
 Not only shows presence of microscopic fat but it can also quantify the
amount of fat.
 can be combined with a variety of sequence types (e.g. spin echo, gradient
echo, and steady state free procession sequences)
 can be combined with a variety of weightings (e.g. T1, T2 and proton
density)
 provides images with and without fat suppression from a single acquisition

118. Limitation of Dixon method
One limitation of this method is that of fat-
water swapping artifact which occurs in
cases of magnetic field inhomogeneity.

119. Case 1

120. Well defined lesion, which is hyperintense in T2WI relative to normal
hepatic parenchyma and suppressed in T2 fat sat. In T1 "in phase", the
lesion is isointense to hepatic parenchyma with a significant signal
drop in T1 "out of phase" denoting intracellular lipid. It elicits early
enhancement in the arterial phase with contrast washout in the portal
phase,, Possible differential includes fat-containing hepatic adenoma

121. CASE 2

123. The lesion in hepatic segment 7/6 and is slightly hyperintense to liver on T2 and
isointense on T1. Signal intensity does not decrease on out of phase T1 imaging
(unlike the rest of the background liver, due to mild steatosis). The lesion shows
arterial enhancement with washout through the portal venous and delayed
phases and a thin pseudocapsule. Thus, lesion in segments 7/6 is favored to
represent a lipid poor hepatic adenoma, on a background of hepatic steatosis.