Gradient echo pulse sequences

Presented by:

Nisha karna

M.Sc. MIT 1^st year

Introduction

• Gradient-echo pulse sequences differ from spin-echo pulse sequences in two ways:
• They use variable RF excitation pulse flip angles as opposed to 90° RF excitation pulse flip angles that are common in spin-echo pulse sequences.
• They use gradients rather than RF pulses to rephase the magnetic moments of hydrogen nuclei to form an echo.
• The main purpose of these two mechanisms is to enable shorter TRs and therefore scan times than are common with spin-echo pulse sequences.

Variable flip angle

• A gradient-echo pulse sequence uses an RF excitation pulse that is variable and therefore flips the NMV through any angle (not just 90°).
• Typically, a flip angle of less than 90° is used.

Gradient rephasing

• After the RF excitation pulse is withdrawn, the FID immediately occurs due to inhomogeneities in the magnetic field and T2* decay.
• In gradient-echo pulse sequences, an RF pulse cannot rephase transverse magnetization to create an echo.
• The low flip angles used in gradient-echo pulse sequences result in a large component of magnetization remaining in the longitudinal plane after the RF excitation pulse is switched off.
• The 180° RF pulse would therefore largely invert this magnetization into the − z direction (the direction that is opposite to B0) rather than rephase the transverse magnetization .

How gradients dephase

How gradients rephase

GRE sequence

Fig: basic gradient echo pulse sequence

GRE sequence

• Gradients rephase the magnetic moments of hydrogen nuclei much faster than RF pulses, the TEs are therefore shorter
• When the TE is short, a given number of slices are acquired in a short TR, and, therefore, the scan time is shorter
• However, in gradient-echo sequences, there is no compensation for magnetic field inhomogeneities.
• They are also heavily reliant on T2* relaxation processes
• In gradient-echo pulse sequences, T2 weighting is termed T2* weighting, and T2 decay is termed T2* decay

T1 contrast in GE

T2* contrast

PD weighting

• To obtain a PD-weighted image, both T1 and T2* processes are minimized so that the differences in proton density of the tissues are demonstrated.
• TE short and TR long should be used.

GRE sequence

Table comparing extrinsic parameters between spin echo and gradient echo sequence

Types of gradient echo sequences

Pre-excitation refocused (only S− is sampled)

e.g: reverse echo GRE

Post-excitation refocused (only S+ is sampled)

e.g.: incoherent GRE

fully refocused (both S+ and S− are sampled)�e.g: coherent GE , balanced GRE

GRE sequences

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: preexcitation signal (S−) from echo reformation; and postexcitation signal (S+), which consists of free induction decay

Incoherent or Spoiled GRE

E.G. FLASH, MPGR

Coherent or steady state sequence

Steady state

• The steady state is generically defined as a stable condition that does not change over time.
•  Both LM and TM reach a nonzero steady state through the use of a repetition time that is shorter than the T2 relaxation time of tissue
•  Two types of signal are formed once steady state is reached:
• pre-excitation signal (S−) from echo reformation; and
• postexcitation signal (S+), which consists of free induction decay

Steady state

Steady state

• Therefore, in the steady state, image contrast is not due to differences in the T1 recovery and T2 decay times of tissues but rather due to the ratio of T1 recovery time to T2 decay time.
• In tissues where T1 recovery and T2 decay times are similar, signal intensity is high, and where they are dissimilar, signal intensity is low.

Steady state

Table: T1 and T2 relaxation times and signal intensity of brain tissues in the steady state at 1T

Ernst angle

• The Ernst angle is the flip angle that provides optimum signal intensity for a tissue with a given T1 recovery time scanned using a given TR.

Ernst angle

fig: illustrates typical Ernst angles for three tissues in the

brain using a TR of 30 ms.

Residual transverse magnetization

• In the steady state, there is coexistence of both longitudinal and transverse magnetization.
• The transverse component of magnetization does not have time to decay and builds up over successive TRs.
• It is produced because of previous RF excitation pulses but remains over several TR periods in the transverse plane
• Called residual transverse magnetization, and it affects image contrast, as it induces a voltage in the receiver coil.

Steady state

• In steady state , two signals are created at each RF pulse.:
• A FID
• An echo, these echoes are also termed Hahn or Stimulated echoes).

Steady state

Fig: echo formation in steady state I

Fig: echo formation in steady state II

Steady state sequences

Coherent GRE sequences

• Coherent or rewound gradient-echo pulse sequences use a variable flip angle RF excitation pulse followed by gradient rephasing to produce a gradient-echo.
• The steady state is maintained by selecting a TR shorter than the T1 and T2 relaxation times of tissues.
• These sequences maintain the coherency of this residual magnetization by rewinding.
• This is achieved by reversing the slope of the phase-encoding gradient after readout
• The resultant gradient-echo contains information from the FID and the stimulated echo.

Coherent GRE

Coherent GRE

Uses:

• Coherent or rewound gradient-echo pulse sequences are generally used to create T2*-weighted images in a very short scan time
• As water is hyperintense, they are often said to have an angiographic, myelographic, or arthrographic effect.
• They can be used to determine whether a vessel is patent or whether an area contains fluid.o have an angiographic, myelographic, or arthrographic effect.

Coherent GRE

Fig : sagittal coherent GRE of knee

Fig : axial coherent GRE seq of abdomen

Coherent GRE

Incoherent or spoiled gradient-echo

• These sequences dephase or spoil residual transverse magnetization so that its effect on image contrast is minimal.
• Only transverse magnetization from the previous excitation is used, i.e. the FID, enabling T1 and proton density contrast to dominate.
• There are two spoiling methods:
• RF spoiling
• Gradient spoiling

Incoherent GRE

Fig : RF spoiling in the incoherent gradient echo sequence

Incoherent GRE

Uses:

• As the stimulated echo contains mainly T2* and T2 information, and this is spoiled,
• These pulse sequences produce T1- or PD-weighted images.
• These sequences are used for 2D and volume acquisitions,
• Incoherent or spoiled gradient-echo sequences also demonstrate good T1 anatomy and pathology after gadolinium contrast enhancement

Incoherent GRE

Fig: sagittal incoherent GRE seq of ankle

Fig : coronal incoherent GRE after contrast enhancement

Incoherent GRE

Reverse-echo gradient-echo

• In gradient-echo sequences, the TE is not long enough to measure the T2 decay time of tissues,
• True T2 weighting is difficult to achieve.
• Reverse echo GRE obtains images that have a sufficiently long TE and less T2*
• In this sequence, only the stimulated echo is read.
• To do this, the stimulated echo must be repositioned so that it does not occur at the same time as the subsequent RF pulse .
• This is achieved by applying the rewinder gradient which speeds up the rephrasing so that the stimulated echo occurs sooner.
• Rewinding is achieved by applying the positive lobe of the frequency encoding gradient

Reverse echo GRE

Reverse echo GRE

• In this sequence, there are usually two TE’s:
• The actual TE is the time between the peak of the gradient-echo and the next RF excitation pulse
• This is the TE selected in the scan protocol but it is not the TEthat determines T2 contrast.
• The effective TE is the time from the peak of the gradient-echo to a previous RF excitation pulse (i.e. the RF pulse that created its FID).
• This is the TE that determines T2 contrast,
• TE eff= 2x TR- TE actual

Reverse echo GRE

Uses:

• Reverse-echo gradient-echo pulses sequences were used to acquire images that demonstrate true T2 weighting but FSE nowadays has replaced it .
• An example of this is in perfusion imaging

Reverse echo GRE

Fig: Perfusion imaging using an echo shifting sequence. Source: Westbrook 2015 [9]. Reproduced

with permission of John Wiley & Sons.

Fig : axial reverse echo GRE in brain

Reverse echo GRE

Fig : Echo formation

in coherent gradient echo.

Fig : Echo formation

in incoherent gradient echo.

Fig : echo formation in reverse echo GRE

Balanced GRE

• The balanced gradient-echo pulse sequence is a modification of the coherent gradient-echo sequence.
• It uses a balanced gradient scheme to correct for phase errors in flowing blood and CSF, and an alternating RF excitation scheme to enhance steady state effects.
• In this seq, both the FID and the stimulated echo are collected within a single readout
• This results in images where fat and water produce higher signal , greater SNR and fewer flow artefacts than coherent GRE in shorter scan times

Balanced GRE

Fig ; Balanced gradient system in balanced gradient echo.

Balanced GRE

• Higher flip angles and shorter TRs are used than in coherent gradient-echo producing a higher SNR and shorter scan times.
• This combination of flip angle and TR would result in saturation and therefore enhanced T1 contrast.
• Saturation is avoided by changing the phase of the RF excitation pulse every TR.

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Balanced GRE

Balanced GRE

• Axial balanced gradient-echo image of the lumbar spine.

Fig ; Axial balanced gradient-echo image of the lumbar spine.

Balanced GRE

Fast gradient-echo

• Acquire a volume in a single breathhold.
• Usually employ coherent or incoherent gradient-echo sequences, but the TE is significantly reduced.
• Only a portion of the RF excitation pulse is applied so that it takes much less time to apply and switch off.
• Only a proportion of the echo is read (partial echo) and the receive bandwidth is widened
• And a technique called ramped sampling is used.
• Fast GRE sequences permit multi slice acquisitions with TE’s as short as 0.7ms

CISS

• Constructive interference into steady state (CISS, Siemens)/FIESTA-C is a slow version of fully refocused steady-state sequences with a TR of approximately 15–20 msec
•  CISS combines two consecutive runs of three-dimensional (3D) balanced SSFP
•  The first run makes use of alternating +α and − α excitation pulses (where α = flip angle), and the second run is performed with constant α pulses
• The two image sets thus acquired show mutually shifted “banding artifacts.” 

Dual-Echo Steady-State Sequence�

• The dual-echo steady-state (DESS, Siemens) sequence is a variation of true FISP
•  In the DESS sequence, two echoes are produced: one coherent and one reverse echoes separately and are then combined to form a single image
•  The coherent gradient-echo provides resolution and the reverse-echo gradient-echo T2 contrast.

Echo planar imaging

• Echo planar imaging (EPI) is a rapid acquisition technique that begins with a sequence of one or more RF pulses and is followed by a series of gradient-echoes.
• The sequence begins either with a variable RF excitation pulse termed gradient-echo EPI (GE-EPI) or with 90° and 180° RF pulses termed spin-echo EPI (SE-EPI).
• GE-EPI begins with an RF excitation pulse of any flip angle and is followed by EPI readout of gradient-echoes
• In this, images are acquired in one TR period.

GE-EPI

SE-EPI

EPI

• EPI sequences are often run in conjunction with single-shot imaging
• In all single-shot techniques, all k-space is filled at once
• Hybrid sequences combine gradient- and spin-echoes, such as GRASE
• Series of gradient rephasing is followed by an RF rephasing pulse
• The hybrid sequence uses the benefits of both types of rephasing methods; i.e. the speed of gradient rephasing and the ability of the RF pulse to compensate for T2* effects.

EPI

Fig : GRASE EPI sequence

EPI

Clinical applications

• Cardiac imaging :
• It is used to assess myocardial viability, perfusion, pericardial diseases, and congenital heart diseases

FIG: a) cardiac imaging with true FISP two chamber and b) four chamber view, shows the normal cardiac anatomy. Dark myocardium and valve leaflets are well appreciated against a background of bright blood

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Clinical applications

• Abdominal imaging:
• The entire abdomen can be imaged with balanced SSFP during a single breath hold. Blood, bile, and fat are bright on true FISP images due to their high T2/T1 ratio

Fig : abdominal imaging with balanced SSFP sequence. On a coronal true FISP sequence of abdomen,

Vessels and biliary system are bright .  Note the calculus (arrow) in the neck of the distended

gallbladder (GB), with prominence of the common bile duct (arrowhead) lateral to the portal vein (PV)

Clinical applications

• Fetal imaging :
• The speed of balanced SSFP and its high SNR have made it useful in fetal MR imaging
•  This sequence is useful for whole-body fetal imaging and has significantly lower RF absorption than do fast spin-echo sequences such as half-Fourier single-shot turbo spin-echo

Fig : fetal imaging with a FIESTA sequence. Static image

shows a fetus in a sagittal orientation.

Clinical applications

Fig : Cranial nerve imaging with CISS. Axial CISS image of the posterior cranial fossa shows a normal facial nerve and eighth cranial nerve (VIII CN) and internal ear structures such as the cochlea and lateral semicircular canal (SCC).

Clinical applications

 fig ; Internal ear imaging with a 3D FIESTA-C sequence. Bilateral (a) and left-sided (b) maximum-intensity-projection images from 3D FIESTA-C data show the normal vestibule and cochlea. Post = posterior, SCC = semicircular canal.

Clinical applications

 Articular cartilage imaging with DESS in a 49-year-old woman with early signs of osteoarthritis. On this coronal DESS image of the left knee joint, articular cartilage is seen as an intermediate-signal-intensity line covering articular surfaces of bone in the lateral compartment (arrows). Note the loss of articular cartilage with reduction in the joint space in the medial compartment.

Refrences

• Mri in practice Catherine Westbrook, Carolyn Kaut Roth, John Talbot 4^th and 5^th edition
• Steady-State MR Imaging Sequences: Physics, Classification, and Clinical Applications RSNA Radiographics
• www.slideshare.net
• Various websites

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