MRI Pulse Sequence in radiology physics.pptx

COLLEGE OF MEDICINE AND HEALTH SCIENCES
RADIOLOGY DEPARTMENT
Seminar On :- MRI pulse sequences
Prepared By:
Dr. Atalay Mola
Dr.Belayneh Bishaw
Dr.Endeg Lake
Dr.Tsegaye
Bahir dar UNIVERSITY

By solomon RR1, BDU 2
Course Outline
• Introduction of MRI pulse sequence
• Timing parameters of pulse sequence
• TR( time of repetition )
• TE( echo time)
• Flip angle
• Types of basic pulse sequence
• Spin echo
• Gradient eco
• Inversion recovery
• Others
8/22/2019

Learning Objectives
On completing this session, you should able to:-
• Understand the basic principles of MRI sequence
• Identify commonly used MRI sequence
• Differentiate between T1-weighted and T2 weighted image based on tissue
contrast
• Explain the clinical use of different MRI sequence
• Recognize the appearance of normal and pathologic finding on various MRI
sequence
• Select appropriate sequence for specific clinical indications
8/22/2019

06/15/2025 4
Introduction
• MRI pulse sequences are programmed combinations of radio frequency pulses
and magnetic field gradients used to create, manipulate, and encode signals in
MRI.
• These sequences determine the appearance of the resulting image, influencing
contrast, resolution, and overall quality.
• They are carefully designed to control the way protons in the body respond to the
magnetic field, allowing for different image contrasts and the visualization of
various anatomical structures.

06/15/2025 5
Cont…
• MRI pulse sequences involve a specific combination of radio frequency pulses
(RF pulses) and magnetic field gradients.
• RF pulses are used to excite the protons, and gradients are used to encode spatial
information, which allows the scanner to determine the location of the signals.
• Each pulse sequence is characterized by a set of parameters, such as
• TR (Repetition Time)
• TE (Echo Time)
• flip angle, and gradient strength.
• These parameters influence the image contrast, resolution, and acquisition time.

06/15/2025 6
Cont…
Repetition time
• Is the time to repeat the sequence
• Repetition is necessary for a number of reasons:
• to acquire signals with different phase encoding gradients, or
• to increase SNR by averaging signals together.
• In a GRE sequence ,TR is the time between two α° pulses; in a spin echo sequence it
is the time between two 90° pulses.
Echo time
• Is the time from the excitation pulse to the centre of the echo.
• For spin echo sequences, the 180° pulse must be exactly halfway between the 90°
excitation and the desired TE.

06/15/2025 8
Cont…
• These steps are repeated many times, depending on the desired image quality.
• some time elapse before an MR signal form after the hydrogen protons have been
excited.
• Before an MR signal can be collected, the phase-encoding gradient must be
switched on for spatial encoding of the signal.
• Some time is also needed to switch off the slice-selection gradient and switch on
the frequency-encoding gradient.
• Finally, formation of the echo itself also takes time, which varies with the pulse
sequence used.

06/15/2025 9
Pulse sequences can be broadly grouped as follows:
Three major pulse sequences
• Spin echo (SE) Pulse sequences
• Inversion recovery (IR) Pulse sequences
• Gradient echo (GE Pulse sequences
Others include
• Diffussion weighted Pulse sequences
• Saturation recovery Pulse sequences
• Echo planer Pulse sequences
• Spiral Pulse sequences
• Steady State Free Precession Imaging (SSFP)

06/15/2025 10
Spin-echo pulse sequences
• one of the earliest developed sequence and still widely.
• The pulse sequence timing can be adjusted to give T1W,T2W & PDW images.
• Dual echo and multi echo sequences can be used to obtain both proton density and
T2-weighted images simultaneously.
• The two variables of interest in spin echo sequences are the repetition time (TR)
and the echo time (TE).

06/15/2025 11
cont…
• All spin echo sequences include a slice selective 90-degree pulse followed by one
or more 180 degree refocusing pulses as shown in the next diagrams.
• describes the excitation of the magnetized protons in a sample with a 90 degree RF
pulse
• followed by a refocusing 180-degree RF pulse to produce an echo.
• The 90-degree pulse converts Mz into Mxy,&
• creates the largest phase coherent transverse magnetization that immediately begins to decay at
a rate described by T2* relaxation.

cont…
• The 180-degree RF pulse, applied at TE/2, inverts the spin system and induces
phase coherence at TE.
• Inversion of the spin system causes the protons to experience external magnetic
field variations opposite of that prior to TE/2.
• resulting in the cancellation of the extrinsic inhomogeneities & associated
dephasing effects.
8/22/2019

06/15/2025 13
Cont…
• In the rotating frame of reference, the echo magnetization vector reforms in the
opposite direction from the initial transverse magnetization vector.
• Subsequent 180-degree RF pulses during the TR interval produce corresponding
echoes with peak amplitudes
• that are reduced by intrinsic T2 decay of the tissues, and are immune from extrinsic
inhomogeneities.
• Digital sampling and acquisition of the signal occurs in a time window symmetric
about TE, during the evolution and decay of each echo.
• These steps are repeated many times.

06/15/2025 17
Spin Echo Image Contrast
• At short TRs, there is not enough time for full T1 relaxation,
• so that Mz is reduced when the next excitation pulse is applied.
• At these short TRs, there is more contrast—signal difference—between tissues,
than at long TRs.
• However, the signal is also reduced. We call this effect ‘saturation’.
• At long TRs there is time for complete relaxation of T1.

…
• If TE is short, there is little time for T2 relaxation & Mxy will be close to its
starting value
• at longer TEs, T2 decay reduces height of the echo, w/c means reduced SNR.
• However, there is also most contrast b/n different tissues, with fluids (long T2)
staying bright.
• There are four possible combinations of TR and TE, but only three are useful .
8/22/2019

06/15/2025 19
T1 Weighting
• A “T1W” SE sequence is designed to produce contrast chiefly based on the T1
characteristics of tissues,
• with de-emphasis of T2 and proton density contributions to the signal.
• Achieved by using a relatively short TR to maximize the d/c in longitudinal
magnetization recovery during the return to equilibrium,&
• a short TE to minimize T2 decay during signal acquisition
• T1W SE contrast requires a short TR and a short TE (spin-lattice relaxation time)
• Fat is the most intense signal, followed by white matter, gray matter, and CSF.

06/15/2025 22
Proton Density Weighting(PDW)
• Relies mainly on differences in the number of magnetized protons/unit volume of
tissue.
• At equilibrium, tissues with a large PD, such as lipids, fats, and CSF, have a
corresponding large Mz compared to other soft tissues.
• Contrast based on PD differences is achieved by using a long TR & a short TE
• Contrast is generated from variations in proton (water content) .
• Fat & CSF display as a relatively bright signal, & a slight contrast inversion
between white and gray matter occurs

06/15/2025 25
T2 Weighting(T2W)
• T2 contrast weighting follows directly from the PD-weighting sequence
• reduce T1 differences in tissues with a long TR, and emphasize T2 differences
with a long TE.
• The T2-weighted signal is generated from the second echo produced by a second
180-degree pulse of a long TR spin echo pulse sequence,
• where the first echo is proton density weighted, with short TE.

cont…
• T2 contrast differences are manifested by allowing M xy signal decay .
• Compared with a T1-weighted image, CSF is bright, and gray and white matter
are reversed in intensity.
• As TE is increased, more T2-weighted contrast is achieved.
• but at the expense of less M xy signal and greater image noise.
8/22/2019

06/15/2025 30
Multislice Imaging
• Remember that we excite only one slice at a time.
• After we excite slice no.1 & sample its signal at TE , we must wait until TR before delivering RF
that excites that slice again.
• b/c the spins within slice no.1 are recovering longitudinal magnetization after the RF pulse.
• However, this slice-selective RF pulse did not excite the rest of the patient.
• All tissue outside slice no.1 is fully relaxed, including tissue in the location of adjacent slices.

06/15/2025 31
Cont…
• If we apply a second RF pulse immediately after TE, with the slice select gradient
and RF tuned to excite a different slice.
• the relaxation of slice no. 1 will continue unaffected while slice no. 2 is excited .
• Then sample signal after a time TE after the second RF.
• This signal is recorded in a new data file,forming the first line of k-space for slice
no.2.

cont…
• Then we can immediately excite slice no.3, sample the signal after an additional
time TE &
• Record the signal in another data file forming the first line of k-space slice no.3.
• It will be possible to excite additional slices until the time TR after the first RF has been reached.
• At that point, we must revert to exciting slice no.1 in order to retain the image
contrast the TR dictates.
8/22/2019

06/15/2025 34
Multiecho Imaging
• In this variant use of the dead time after TE, but before TR
• Instead of immediately exciting another slice,
• we will generate several images of the same slice with different contrasts .
• The signal recorded at TE we will call it TEl at this point-will of course be entered
into k-space.
• Next, before applying any additional RF pulses
• we will turn on the frequency-encoding gradient again and sample the signal.

cont…
• Notice that no additional phase encoding or slice selection is performed.
• The time at which we sample the signal this second time will be termed TE z, &
• it will be recorded in a new k-space.
• Each time repeat the RF pulse at TR, we will again record signal at TEl & TE z
• recording each in its respective k-space.
• Notice that within a given TR period, signal from each TE has identical spatial
localization (slice, phase, and frequency encoding),
8/22/2019

06/15/2025 36
Cont…
• but it is recorded at a different time after the RF and in a separate k-space.
• Each k-space will yield a unique image:
• The contrast of the two images will differ in the degree to which they show contrast based on
T2.
• the multiecho technique is used with spin echo pulse sequences , a relatively long
TR & one short and one rather long TE (perhaps 20 &120ms).
• The result is two images: one with contrast mostly based on proton density (long
TR and short TE) and one based on T2 (long TR and TE).

06/15/2025 37
Inversion recovery pulse sequences(IR)
• used to selectively null the signal for certain tissues (e.g. fat or fluid).
• Can generate heavily T1W images & was originally developed for this purpose.
• Basically, IR pulse sequence is a SE preceded by a 180° RF pulse.
• The preparatory pulse inverts longitudinal magnetization (Mz) , it flips Mz to its
negative value, -Mz.
• Tissues regain Mz at different longitudinal (T1) relaxation rates determined by their
T1 relaxation times.

06/15/2025 38
Cont…
• The spin echo 90° readout pulse is applied at the exact time when longitudinal
magnetization reaches the null point for the tissue we wish to suppress.
• The time elapsed b/n the preparatory 180° pulse and the 90° excitation (slice
selection) RF pulse is termed time to inversion.
• By choosing the appropriate TI, suppression of different tissues is possible:
• Short Tau Inversion Recovery, or STIR: fat is nulled
• Fluid Attenuated Inversion Recovery - fluid is nulled

06/15/2025 40
Inversion Recovery Sequences (STIR and FLAIR)
• Short TI inversion recovery & fluid attenuated inversion recovery are widely used.
• Both are based on a spin echo sequence, but with an additional 180° pre-pulse.
• Acting on equilibrium M0 this pulse inverts the magnetization (Mz = –|M0|).
• After a delay time called TI the 90° pulse of the spin echo sequence is applied.
• the tissues partially recover their longitudinal magnetization with T 1 relaxation.
• If the inversion delay is such that one of the tissues is at the null point (i.e.Mz =0),
its signal after the 90° pulse will be zero.

06/15/2025 41
Short Tau Inversion Recovery, or STIR
• The aim is to create a fat-suppressed T2w image.
• Fat has a short T1 and so a short TI is needed (120 ms at 1.5 T, 150 ms at 3.0 T).
• At these short TIs, most other tissues still have a negative Mz.
• When the image is formed, however, we ignore the sign of the echo & only use its
magnitude.
• Fluids, with the longest T1s, have the most negative signal &
• have the highest signal on the final images, giving the required T2w contrast.
• A short TE is needed, to maximize the signal from fluids.

06/15/2025 42
Fluid Attenuated Inversion Recovery
• used exclusively in brain and spine imaging, to null CSF signals in a T2w image.
• Helpful to distinguish periventricular lesions from the high signals in ventricles.
• Since CSF has a very long T1, a long TI is needed (1700–2500 ms).
• There is a wide range of TIs because the signal is changing only slowly;
• a few tens of ms difference can still give adequate CSF suppression.
• At such long TIs, all other tissues already have positive Mz & in fact may be close
to |M0|.
• A long TE is used to create T2w contrast.

06/15/2025 49
Gradient echo sequences (GRE)
• Are an alternative technique to SE, differing from it in two principal points:
• Utilization of gradient fields to generate transverse magnetization
• Flip angles of less than 90°
• Compared to the SE, IR & gradient echo sequences are more versatile.
• Not only is the basic sequence varied by adding dephasing or rephasing gradients
at the end of the sequence

06/15/2025 50
Cont…
• but there is a significant extra variable to specify in addition to the usual TR &
TE.
• This variable is the flip or tip angle of the spins.
• The gradient echo is generated by the frequency-encode gradient, except that it is
used twice in succession, and in opposite directions:
• It is used in reverse at first to enforce transverse dephasing of spinning protons
and then right after.

06/15/2025 51
Cont…
• Another important feature of GRE is that the dephasing of spinning protons
occurs as a result of T2* decay.
• which is more rapid than the T2 decay process underlying a spin-echo sequence (leading to
shorter TE) and is susceptible to static field inhomogeneitie.
• The ability to use a short TE is also important because signal decays much more
rapidly in GRE, with T2*.
• used as a readout gradient to re-align the dephased protons and acquire signal.

cont…
• B/c low flip angles are used, there is some retention of the original longitudinal
magnetization as opposed to the 90° pulse used in spin echo,
• which completely eliminates the longitudinal magnetization.
• As a result, the build-up time for longitudinal magnetization is significantly
reduced for the subsequent pulses, allowing faster image acquisition in GE.
• A gradient echo is essentially a means for minimizing the signal loss incurred
during signal sampling under the frequency-encoding gradient.
8/22/2019

06/15/2025 53
Cont…
• To form a gradient echo, first turn on frequency-encoding gradient magnetic field.
• This leads to signal loss.
• After gradient is switched off, spins are out of phase &
• signal has been lost in proportion to the strength of the gradient magnetic field.
• If we now turn on the same gradient magnetic field with the same strength but
opposite polarity,
• the spins will undergo further "dephasing," but enough dephasing actually leads to rephasing
• This is a gradient echo.

06/15/2025 56
Flip angle
• Is usually at /close to 90 degrees for a spin echo but is less on GRE sequences
• commonly varying over a range of 10 to 80 degrees (usually denoted by α).
• For the basic GRE sequence FLASH, the larger flip angles give more T1W to the
image &
• the smaller flip angles give more T2, or actually T2* weighting, to the images.
• So a small flip angle can be used to avoid saturation, even when TR is very short.
• Flip angle becomes the parameter w/c controls M z, while TE still controls Mxy.

06/15/2025 57
Cont…
• Combinations of flip angle and TE, but only three are useful .
• Small α°, short TE:
• contrast is dominated by the proton density (water content) of the tissues
• Small α°, long TE:
• contrast depends on T2 (spin-spin relaxation time, or to be more correct T2* the effective spin-spin relaxation time)
• Large α°, short TE:
• contrast is dictated by T1 (spin-lattice relaxation time)
• The fourth combination, large α° with long TE, has no value.

06/15/2025 61
Coherent Gradient Echo
• The timing of the RF pulse with the dephasing & rephasing implemented by reversal of
gradient polarity to generate an echo at a TE for the frequency encode gradient (FEG).
• Also involved phase encode gradient (PEG), which is applied & incrementally changed
for each TR
• for identify proton position in the direction perpendicular to the FEG based upon phase changes of
the protons after the PEG is turned off.
• The combined FID and simulated echo signals are generated during the GE, and produce
tissue contrast dependent on flip angle, TR, and TE.
• With moderate flip angles of 30 -60 degrees, the d/c in tissue contrast are primarily based upon
T2/T1 ratios.

06/15/2025 62
Incoherent, “Spoiled” Gradient Echo
• With short TR steady-state acquisitions,T1W can,t be achieved to any great extent.
• owing to either a small d/c in longitudinal magnetization with small flip angles or
• dominance of the T2* effects for larger flip angles.
• T2* influence can be reduced by using a long TR or
• by “spoiling” the steady-state transverse magnetization by introducing incoherent phase
differences from pulse to pulse.
• The latter is achieved by adding a phase shift to successive RF pulses during the excitation
of protons.
• Both the RF transmitter and RF receiver are phase locked, so that the receiver discriminates
the phase of the GE from the SE generated by the previous RF pulse,

06/15/2025 63
Others….
Diffusion-weighted imaging (DWI)
• is a form of MR imaging based upon measuring the random Brownian motion of
water molecules within a voxel of tissue.
• highly cellular tissues or those with cellular swelling exhibit lower diffusion
coefficients.
• Diffusion is particularly useful in tumor characterization and cerebral ischemia

06/15/2025 64
Saturation recovery (SR) sequences
• Their primary use is to measure T1 times more quickly than an IR pulse sequence.
• consist of multiple 90 degree RF pulses at relatively short repetition times.
• Longitudinal magnetization develops during the TR period after the dephasing
gradient is rotated into the transverse plane by another 90 degree pulse.
• A gradient echo is acquired immediately after this.
• The signal will reflect T1 differences in tissues because of different amounts of
longitudinal recovery during the TR period.

06/15/2025 65
Cont…
Echo planar imaging (EPI)
• is an MRI acquisition methodology with an excellent temporal resolution that is
required in specific clinical settings e.g. cardiac imaging.
• There are single-shot and multi-shot echo-planar sequences.
• Is performed using a pulse sequence in which multiple echoes of different phase
steps are acquired using re phasing gradients
• instead of repeated 180-degree RF pulses following the 90°/180° in a SE sequences.

06/15/2025 66
Cont…
• Accomplished by rapidly reversing the readout or frequency-encoding gradient.
• This switching or reversal may also be done in a sinusoidal fashion.
• Echo planar sequences may use entirely gradient echoes or may combine a spin-
echo with the train of gradient echoes.
• In a single-shot echo-planar sequence, the entire range of phase encoding steps,
• In multi-shot echo-planar imaging, the range of phase steps is equally divided into
several "shots" or TR periods.
• Each subsequent echo results in a progressively T2-weighted signal.

06/15/2025 67
Cont…
Spiral scanning on MRI:
• the word "spiral" refers to the pattern of sampling k-space.
• On conventional imaging sequences including SE ,GRE &on fast imaging
sequences,
• a line or multiple lines of k-space in the frequency direction are acquired consecutively.
• In spiral scanning, k-space is acquired in a spiral trajectory.
• The entire k-space can be acquired during a single acquisition/interleaved using
more than one acquisition.
• Spiral scanning tends to have fewer artifacts than echo-planar imaging.

06/15/2025 68
Cont…
Steady State Free Precession Imaging (SSFP):
• Is a MRI sequence which uses steady state of magnetization.
• when the TR is shorter than both the T1 and T2 relaxation times of all the tissues.
• Therefore there is no time for the transverse magnetization to decay before the
pulse pattern is repeated again.
• The only process that has time to occur is T2*.
• In general,SSFP MRI sequence are based on gradient echo a short repetition time,
• it is also called FLASH MRI Technique.

06/15/2025 69
Reference
• Diagnostic radiology physics
• Totally accessible mri,a user's guide to principles, technology, and applications,
• Physics for diagnostic radiology,3rd
edition
• The essential physics of medical imaging
• The physics of diagnosting imaging 2nd
edition
• Radiopedia

Thank you!!
8/22/2019

MRI Pulse Sequence in radiology physics.pptx

More Related Content

Similar to MRI Pulse Sequence in radiology physics.pptx

Recently uploaded

MRI Pulse Sequence in radiology physics.pptx