MRI sequence is order to combination of RF and
gradient pulse designed to acquire the data to form the
Timing,order,polarity & repetition frequency of
RF pulse and applied magnetic field gradients
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
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,
Next 180degree pulse, TE will be long giving T2
weighted image (LongTR, LongTE).
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
Reduce phase encoding and spatial resolution.
SINGLE SHOT FAST SPIN ECHO
Scan time is much reduced in SS-FSE than fast spin
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
INVERSION RECOVERY SEQUENCE
In this sequence we are giving first 180 degree pulse.
Second we are giving 90 degree pulse followed by
After that are we are taking signal.
TI- time of inversion is the time between first 180
degree pulse and 90 degree pulse.
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.
GRADIENT ECHO SEQUENCE
There is no 180 degree pulse in gradient echo
Rephasing of TM in GRE is done by gradients;
particularly by reversal of the frequency encoding
Since rephasing by gradient gives good signal.
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
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
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).