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MRI Pulsesequences



1. SPIN ECHO (SE )

2. FAST SPIN ECHO (FSE)

3. INVERSION RECOVERY ( STIR, FLAIR)

4. GRADIENT READING ECH0 ( GRE )



The spin echo sequence

The spin echo sequence is a very basic and sequence

The two main parameters of a spin echo (SEare the TR and the TE.

TR is the time between two 90° excitation puregular SE sequences, TR is about 100 to 30

TR is the time allowed for longitudinal magnerecover (T1 relaxation). The longer the TR iscomplete the longitudinal magnetization recoIf the TR is short, not all tissues will have und

complete T1 relaxation and image contrast wmore dependent on the T1 relaxation proces



With a regular SE sequence, only one line of k-sduring a repetition (which lasts for time TR). Theacquisition (TA) of the full k-space is therefore pthe TR (TA = TR * number of lines).

TE is the time between the 90° excitation pulse signal acquisition. The echo concerns transversmagnetization and is the result of spin rephasin180° RF pulse applied at time TE/2. The 180° Rresponsible for a cancelation of spin dephasingfield inhomogeneities so that the transverse madecay is T2-dependent instead of T2*-dependen



Spin Echo

SE sequences serve as reference for tissue con

Pros:

Anatomic imaging

Reference for tissue signal and image contrast

Cons:

Very long acquisition time

Clinical use: Almost all the organs explored in MRI



T1-weighted

If the TR is very long, the longitudinal magnetization tissues will have recovered completely (complete T1

On the contrary, if the TR is short, tissue signal and

will depend on the T1 characteristics of tissues as nowill have completely recovered their longitudinal magIf the TE is also short, there is little time for T2 relaxacontrast will not be very dependent on T2.

When both the TR and the TE are short, the image isweighted.



Spin Echo

T1-weighted

Pros:

Anatomic imaging

Cons:


Clinical use:

Almost all the organs explored in MRI



T2-weighted

To obtain a T2-weighted image in SE, tmust be long so that the longitudinalmagnetization of the tissues has time tocompletely and the TE must be long to

relaxation happen.

A long TR is about 2000 ms or more anTE is about 80 to 140 ms.

With such parameters, the tissues withtime will have a stronger signal than thewith a short T2 time.



T2-weighted

Pros:

Imaging of water/fluids (CSF, edema, b

Cons:




PD-weighted

If we use a long TR (> 2000 ms) and a (10 - 20 ms), the tissue signal will not bdependent on T1 and T2 relaxation.

Therefore, image contrast is mainly duedensity.

Tissues with a rich Hydrogen content (will be bright whereas tissues with low

content will be dark.



PD-weighted

Cons:


Clinical use:

Osteoarticular

Pediatric Neuroradiology



2.Fast Spin Echo ( Turbo Spin A regular SE sequence requires as many repetitions

lines in the k-space to complete a slice acquisition.

Instead of acquiring k-space lines of other slices at d

positions during the wasted time, it is possible to acqspace lines for the same slice.

Such fast spin echo sequences use one 90° excitatiotwo or more 180° pulses in the same repetition time different phase-encoding gradient steps, to acquire m

that will fill the k-space.



The number of echoes acquired after a singexcitation is called the Turbo Factor or EchoLength.

As each echo undergoes more and more T2image contrast is modified.

The effective echo time, determined by whelines of k-space are acquired, indicates appwhat the image contrast obtained is like (ratweighted or T2-weighted).



Pros:

Reduction of scan time

Low sensitivity to magnetic susceptibili

Cons:

Modification of tissue contrast

Clinical use:

Almost all the organs explored in MRI



Single Shot Spin Echo

Single Shot Spin Echo, Rare

If there are as many echoes as the numb

space lines to fill, the entire k-space can

acquired after a unique 90° excitation puis called a single-shot sequence.

The resulting images are strongly T2-we

most of the k-space lines are acquired w

TE.



Rare Pros:

Excellent contrast between tissues and still fluids

Very fast : useful for moving organs

Cons:

Decreased signal to noise ratio

Low spatial resolution

Clinical use:

Biliary and urinary tract exploration ,myelograph



HASTE, SSFSE, SSTSE

Single Shot Spin Echo with Partial Filling of K-Space

Commercial name: HASTE (Siemens), SSFSE (GE), SST

Pros:Excellent contrast between tissues (gray or dark) and still fl

Very fast : less sensitive to motion, breath-hold sequences

Cons:

Decrease of signal to noise ratio (low signal amplitude of laeffective TE, partial filling of k-space)

Low spatial resolution

Clinical use: Hepato-biliary and urinary tract exploration

Cardiac MRI without gating



Inversion Recovery

One method for manipulating contrast is called Inversion-R

An Inversion-Recovery consists in applying a 180° inversiobeginning of the sequence.

The 180° inversion pulse changes the direction of the longmagnetization vector to its opposite. Then, the longitudinagoing to recover as defined by T1 relaxation.

At time TI (inversion time), a regular spin echo (or gradienecho planar) sequence is performed, starting with an excit



The TI is defined so that the longitudinal magnetizationtissue is null.

Consequently, this tissue will have a null transverse mafter the excitation pulse, resulting in a signal suppresstissue.

The optimal TI for eliminating the signal of a given tisson the tissue's characteristic T1 time.

The displayed image usually shows the magnitude of which corresponds to the absolute value of the signal isigned value

Tissues with no magnetization appear in dark and tissumagnetization (positive or negative) appear in gray or

Inversion-Recovery allows to eliminate the signal of tisaccording to their T1 time by choosing an appropriate T



Inversion Recovery

Fat Signal Suppression STIR

Fat : T1 time is short, And hence fat signal can be elimin

an inversion-recovery with a short

(about 140 milliseconds).



STIR Pros:

Fat signal suppression

Cons:

Increased scan time

Clinical use:

MSK SYSTEM



Water Signal Suppression

FLAIR In the same way, with a TI time ab

ms, water signal is eliminated (wa

long).



FLAIR Pros:

Elimination of CSF signal

Useful with T2-weighted images (edem

Cons:

Long scan time

Clinical use:

Neuroradiology



4. Gradient Echo

Two major differences distinguish the gradietechnique from the spin echo technique:

An excitation pulse with a flip angle lower th

No 180° rephasing pulse



A flip angle lower than 90° (partial flip anglethe amount of magnetization tipped into the plane.

The consequence of a low-flip angle excitat

faster recovery of longitudinal magnetizationshorter TR/TE and decreases scan time.

The advantages of low-flip angle excitationstechniques are faster acquisitions, new contbetween tissues and a stronger MR signal inshort TR.



In the following eg., there differences in longitudinal and trans

magnetization after a 90° excitation pulse and a 30° excitatio

partial flip angle excitation, the decrease in longitudinal magnvery low (only a 13 percent loss) but the transverse magnetiz

of its maximum value (sin 30°).



Gradient Echo

The flip angle determines the fraction of magnetin the transverse plane (which will produce the Nand the quantity of magnetization left on the lon

If the flip angle decreases, the residual longitud

magnetization will be higher and the recovery ofmagnetization for a given T1 and TR will be mo

On the other hand, the result of a lower flip angis a lower tipped magnetization.



Gradient Echo

The actual decay of the transverse magnetizatioseveral factors:

spin-spin tissue-specific relaxation (T2) which iB0 field inhomogeneities and magnetic suscwhich are static

As GE techniques use a single RF pulse and norephasing pulse, the relaxation due to fixed caureversed and the loss of signal results from T2* T2 + static field inhomogeneities).



Gradient Echo

Signal weighting in GE imaging lies on 3 parameters

TR

TE

Flip angle

The resulting contrast in basic GE sequences is a vaT1 and T2*:

the higher the flip angle chosen, the more T1-weightwill be

the shorter the TE obtained, the less T2*-weighted th



Gradient Echo

There is an optimal combination between th

the flip angle so that the NMR signal is max

The optimal flip angle is called the Ernst ang

It is calculated from the TR value and the T1

specific value to give the best flip angle to c

want to obtain the maximal signal for a give



Gradient Echo



If you compare it to an SE sequence dican note that a GE sequence is simplesingle RF pulse, which allows for shorteTE and faster acquisition.

Data is acquired during the gradient ecby the bipolar readout gradient (with a lobe).Note this is the same principle with datacquisition in SE sequences (but the spand the gradient reading echo match)



Gradient Echo

Pros:fast technique

Cons: T2*-weighted images instead of T2

More sensitive to magnetic susceptibili

Clinical use:

eg. Hemorrhage , calcification



5. Advanced Gradient Echo

Steady-state

The GE technique allows for very short TR and TE.

The TR can be so short that the spins on the slice plhave enough time to dephase completely the MR sigdecays completely. These sequences are called stea

They require that TR is less than T2*.

The steady-state technique produces T2*-weighted ifast (in less than 1 second with TRs < 10 msec).

The contrast between tissues and fluids is very high have a good signal to noise ratio.

As TEs are also very short, blood generally appears

does not have enough time to move out of the slice p



Advanced Gradient Echo

Steady-state In order to maintain the steady-state,

shifts from one excitation to the next mremain constant.

This is done by applying a phase-encrewinder gradient after the signal read

The rewinder gradient is of equal ampopposite sign of the phase-encoding g



It restores the original phase state before phencoding, which is needed for spatial localizsequences are called steady-state GRE.

Image contrast resulting from steady-state ismix of T1 and T2* so that steady-state can bwe want to obtain T1-weighted images.

In these cases, a spoiler gradient or RF is udestroy the transverse steady-state. These are called spoiled GRE sequences.



Advanced Gradient EchoSteady-state

Commercial name: FISP (Siemens), FFE (Philips),

Pros:

Accentuated signal form liquids

Very good contrast between liquids and soft tissues

Very fast (breath-hold sequences, dynamic cine imag

Cons:

Image contrast and sensitivity to motion very compleon TR, TE and flip angle

Clinical use:

urinary tract and biliary explorations



Advanced Gradient Echo

Enhanced Steady-state

Commercial name: True-FISP (Siemens), Balance(Philips), FIESTA (GE).

Using a fully-balanced gradient waveform, the statio

returns to the same phase it had before the gradienapplied.

This pulse sequence produces images with increasfluid.

True FISP is a reliable technique, very fast and relainsensitive, with very good contrast between liquidstissues.



Pros:

Increased signal from fluid

Very good contrast between liquids and

Very fast (motion imaging)

Clinical use:

urinary tract, pelvic and biliary exploratio

Pediatrics and antenatal imaging ++

Cardiac and vascular imaging ++



Advanced Gradient EchoSpoiled gradient echo

Commercial name :SPGR (GE), FLASH (Siemens), FFE

Pros:

T1 weighted anatomic imaging

Short scan time, volume acquisitions, breath-hold sequenc

MR angiography

Cons:

Magnetic susceptibility artifacts

Clinical use:

Cardio-vascular MRI



Image quality and artifact

The quality of an MR image depends on seve

1. Spatial resolution and image contrast

2. Signal to noise ratio (and contrast to noise rat3. Artifacts

An MR exploration is a compromise between image quality.

An MR exploration protocol and its sequence will have to be optimized in function of the orgpathology.



Spatial resolution Spatial resolution corresponds to t

of the smallest detectable detail.

The smaller the voxels are, the hig

potential spatial resolution will be.

Voxel volume is determined by the

size (256 x 256 or 512 x 512 etc..)

of view (10 cm, 20 cm, etc....), andthickness.



FOV 25



FOV 35



FOV 50



MATRIX SIZE (128 X 128



MATRIX SIZE (256 X 256



MATRIX SIZE (512 X512)



Signal and Noise

Noise is like interferences which present as a irregranular pattern.

This random variation in signal intensity degradeinformation.

The main source of noise in the image is the pati

(RF emission due to thermal motion). The whole measurement chain of the MR scanne

electronics...) also contributes to the noise.

This noise corrupts the signal coming from the tramagnetization variations of the intentionally excitthe selected slice plane).

The signal to noise ratio (SNR) is equal to the rataverage signal intensity over the standard deviatnoise.



INCREASING NOISE



Signal and Noise

The signal to noise ratio depends both on someare beyond the operator's control (the MR scanspecifications and pulse sequence design) and that the user can change:

Fixed factors : static field intensity, pulse seqtissue characteristics

Factors under the operator's control

RF coil to be used

Sequence parameters :

voxel size (limiting spatial resolution), numbeaveragings, receiver bandwidth



Surface coil The smaller the sensitive volume

the lower the noise from the adjac

structures of the selected slice pla

it can detect, and the better the signoise ratio will be.

A local coil, or better, a surface co

higher signal to noise ratio than a



Signal and Noise

The signal comes from the excited protons on thslice plane. The number of spins in parallel stateproportional to the static magnetic field intensity

the field intensity is, the higher the excess numbparallel state (available to make the MR signal) the signal intensity varies almost linearly with thintensity.

Assuming a uniform proton density, the numberspins is proportional to the voxel size and so is intensity. The signal goes up linearly with the vo

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