 MRI sequence is order to combination of RF and
gradient pulse designed to acquire the data to form the
image.
 Timing,order,polarity & repetition frequency of
RF pulse and applied magnetic field gradients

BASIC SEQUENCES

Spin echo sequence(SE)
Inversion recovery sequence (IR)
Gradient echo sequence(GRE)
Echo planar imaging(EPI)

FREE INDUCTION DECAY

Free induction decay (FID) refers a short-lived sinusoidal
electromagnetic signal which appears immediately
following the 90° pulse

It is induced in the receiver coil by the rotating component of the
magnetization vector in the x-y plane which crosses the coil loops
perpendicularly

This sinusoidal signal decays exponentially with a time
constant of T2*.

It does not contribute to form MR image.

FREE INDUCTION DECAY
 Free induction decay (FID) refers a short-lived
sinusoidal electromagnetic signal which appears
immediately following the 90° pulse.
 It is induced in the receiver coil by the rotating
component of the magnetization vector in the x-y
plane which crosses the coil loops perpendicularly.
 This sinusoidal signal decays exponentially with a time
constant of T2*.
 It does not contribute to form MR image.

SPIN ECHO SEQUENCE
 It consists of 90 and 180degree RF pulse
 The excitatory 90 degree Rf pulse flips net
magnetization vector along z axis and due to non
homogeneity strat dephasing
 By giving 180 degree pulse they strat rephasing to
bring protons back into the phase
 This rephasing increase magnitude of TM and stronger
signal is induces in the receiver coil

It consists of 90degree and 180 degree
RF pulse

Initially 90 degree pulse is given

After 90 degree pulse TM vector
starts dephasing

Next we are going to give 180
degree pulse

After giving 180 degree pulse TM vector
direction is changed

Now TM vector started rephasing

Getting good signal to form the
image

PARAMETERS
• Short TR(300-800ms)
• Short TE(15-45ms)
T1-
anatomy
• Long TR(1000-2000ms)
• Long TE(90-140ms)
T2-
pathology
• Long TR(1000-3000ms)
• Short TE(15ms)PD

spin echo
sequence
Dual SE Fast SE
Single shot
fast SE
modification

DUAL SPIN ECHO SEQUENCE
 Two 180 degree pulse are sent after each 90 degree
pulse to obtain dual echoes per TR.
 PD+T2 double echo sequence is an example of this
modified SE sequence.
 It is run with longTR.
 After the first 180 degree pulse, since TE is short,
image will be proton density weighted (Long TR,
shortTE).
 Next 180degree pulse, TE will be long giving T2
weighted image (LongTR, LongTE).

DUAL SPIN ECHO SEQUENCE

FAST (TURBO) SPIN ECHO
 In fast SE sequence we are giving multiple 180 degree
rephasing pulses are sent after each 90 degree pulse.
 In these sequence multiple echos are obtained per TR
once echo with each 180 degree pulse.
 All echos are used to fill a single k-space.
 Since k-space is filled much faster with multiple echos
in a single TR the scanning speed is increases
considerablly.
 Reduce phase encoding and spatial resolution.

Generally speaking contrast in fast spin echo is similar
to spin echo and used in
Muscoloskeletal regions
Central nervous system
pelvis

SINGLE SHOT FAST SPIN ECHO
 Scan time is much reduced in SS-FSE than fast spin
echo
 All lines of k-space is filled in one TR
 SS-FSE combines a partial fourier technique
 Half of lines acquired in one TR and other half are
transposed

Uses
Cranial nerves
Inner ear
MR urogram
MRCP
Spinal imaging

INVERSION RECOVERY SEQUENCE
 In this sequence we are giving first 180 degree pulse.
 Second we are giving 90 degree pulse followed by
180degrees.
 After that are we are taking signal.
 TI- time of inversion is the time between first 180
degree pulse and 90 degree pulse.

INVERSION RECOVERY –MAJOR SEQUENCES
FLAIRSTIR

STIR FLAIR

STIR
S • SHORT
T • TIME
I • INVERSION
R • RECOVERY

STIR
 Fat has a short TI i.e. 80-150ms.
 So fat can be suppressed at the STIR sequence.
 This sequence can be used with T1 or T2W images.
 Mainly used in body imaging.

FLAIR
FL- fluid
A-
attenuation
I-inversionR-recovery

FLAIR
 The aim of FLAIR sequence is to suppress liquid
signals, mainly in the brain .
 Water has a long TI.
 Nulling of water signal is seen at TI of 2000-2400ms.
 Heavily T2W image is obtained.
USES
Neuro
imaging
Post
contrast
image

COMPARISION OF SES AND IRS

GRADIENT ECHO SEQUENCE
 There is no 180 degree pulse in gradient echo
sequence.
 Rephasing of TM in GRE is done by gradients;
particularly by reversal of the frequency encoding
gradient.
 Since rephasing by gradient gives good signal.

Initially 90 degree pulse is given

TM vector started dephasing due
to inhomogeneity

Gradient polarity is reversed

TM vector started rephasing

Completely rephasing TM vectors
and good signal is obtained

TYPES OF GRES
Gradient
ES
Steady
state
spoiled

ADVANTAGE OF GRES
 Faster than spin echo because of short TRs and Tes
 Shows flowing blood as bright good for MRI
 Good for functional MRI
 Good for subtle hemorrhage (micro hemorrhage
technique or magnetic susceptibility technique)
 Micro calcification present in the brain also image well
by using susceptibility technique
 Can be combined with spin echo to produce very fast
image

ECHO PLANAR IMAGING
 Scanning time can be reduced by filling multiple line
of k-space in a single TR.
 All the lines of k=-space required to form an image are
filled in a single TR.
 Since entire 2D raw data ser (a plane of data or echo)
could be filled in single echo decay.
 Slice acquired in 100ms thus minimizing the effect of
patient motion.
 It is a dynamic image.

Advances in gradient and digital data acquisition technology
have made it possible to obtain individual MR slices in the time
frame of 50-100 ms, thus minimizing the effects of patient
motion. It gives good dynamic imaging protocol .
Echo-planar imaging (EPI).

THANK
YOU