Detailed Study Notes on Gradient Echo Sequences

  • Learning Objectives and Essential Terminology

  • Spoiled (Incoherent) vs. Coherent: Note that these terms are used interchangeably in different contexts. "Spoiling" refers to the elimination of residual transverse magnetization, while "coherence" refers to its preservation

  • Coherent Gradient Echo Sequences

    • used to create T2*W images and are faster than conventional gradient echo pulse sequences

    • The speed and contrast is created by using an extra gradient pulse at the end of the pulse sequence ( _ is after the echo is collected) we then do an additional gradient pulse which alters contrast and reduced scan time

    • This extra gradient is called a REWINDER - they take protons out of phase make them become in phase

    • Please note: the rewinder gradient is done by the same physical gradient as the phase encoding gradient

    • Fast scan time , sensitive to flow which is useful for angiography, can be aquired in a volume acquistion

  • Spoiled (Incoherent) Gradient Echoes — T1W images

  • Spoiled gradient echoes are used to create T1W or PDW images (PD not used much)

  • Most of the time you will see T1W images due to the use of GRE sequences in abdominal imaging

  • Like the coherent, the spoiled GRE sequence is faster than a conventional GRE sequence

  • There are 3 ways to spoil— inchoernt (out of phase) — spoil means to dephase protons

  • 1. RF spoiling — by far the most common method

  • 2. Variable gradient spoilers —

  • 3. Lengthening TR

  • RF spoiling - The most common method, each RF excitation pulse has a slightly higher power than the last, spoiling transverse magnetization after the collection of the echo -

  • Variable gradient spoilers- Accomplished by introducing an additional gradient with variable strengths from cycle to cycle

  • Lengthening TR— if our goal is to make protons defase more we can wait and use a longer TR allowing protons to dephase more — but this means a longer scan time — this is comingly used in sequences when we are getting more than one slice or multi slice sequences at once (ex. MPGR)

Spoiled gradient echo Uses:

  • 1. 3D volume acquisitions

  • 2. 2D breath-hold T1W sequences

  • 3. Dynamic contrast enhanced images

( dynamic contrast enhanced = power injector)

  • shorter scan times , can be used after gadolinium injection, can be aqcuired in a volume acqusition , good snr and atomical detail in 3D

  • Steady State Free Precession (SSFP) — breaks all the rules of MRI *

  • The steady state free precession pulse sequence is a gradient echo that creates T2W images (NOT T2*W images), it has a TE that is longer than the TR, and it uses a rewinder gradient

  • This sequence works by using multiple variable flip angles to act as both excitation and rephasing pulses

  • Because rephasing is accomplished by an RF pulse, magnetic susceptibility, field inhomogeneities, and chemical shift are decreased

  • We still consider this pulse sequence to be a GRE because it does not use 90 or 180 degree RF pulses and the sequence also uses rewinder gradients

  • Yields heavily T2W images

  • Echo is recalled prior to the readout pulse by a gradient

  • This scan speed is increased with this technique withoutthe use of dedicated excitation and rephasing pulses

  • Fast Gradient Echo Sequences

  • Can cause motion artifacts and peripheral nerve stimulation

  • Uses ultra short TRs and TEs

Techinques used:

1. Ramped Sampling

2. Partial Echo

3. Increased Reciever Bandwidth

4. Use of inversion recovery pulses to alter contrast

  • Ramp sampling - we start collecting echo before the gradient has reached max amplitude!! - we reduce the sampling time while maintaining the TE

  • Partial Echo - uses conjugate symmetry in k space to reduce TE, which then allows the TR to be shortened

  • Reduction of sampling time by increasing receiver bandwidth

  • Using non selective 180 degree inversion pulses before the sequence begins to force protons into the transverse plane

  • Fast Gradient Echo Applications - temporal scanning of same slice after contrast injection (pituitary) — Disadvantages : decreased SNR and contrast due to ultra short TRs • increases chemical shift at very low TEs — To make the scan faster we increase bandwidth which decreases chemical shift

  • Echoplanar Imaging (EPI) — fastest pulse sequence

advantages : faster , less motion artifact , all 3 wieghting ,

disadvatages: chemical shift aftifact , periperal nerve stimulation due to the fast switching of gradients — most commonly used for DWI

  • 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

  • Generated by oscillation of the readout gradient

  • EPI’S can be gradient echo EPI sequences and spin echo EPI sequences

  • Gradient echo EPI – pulse sequence starts with a variable flip angle followed by a series of gradient pulses to create echoes

  • Spin Echo EPI – starts with a 90 degree RF pulse, then a 180 degree RF pulse followed by series of gradient pulses to generate echoes - The extra RF pulses help reduce chemical shift and field inhomogeneous

Single Shot EPI — Fastest pulse seq. we have

• Single shot EPI - all k-space lines filled by multiple gradient reversals which produce multiple gradient echoes in a single acquisition (1 TR time period)

• Readout gradient applied from positive to negative

•Variations in the resonating frequencies of protons (fat and water protons) lead to mismapping along the phase axis

•Magnetic susceptibility artifact (metal) particularly at air/tissue interfaces such as sinuses, are a problem

Multi-shot EPI

• A multi shot EPI uses more than one excitation pulse followed by collecting some, but not all, the echoes with gradient pulse

•Readout is divided into multiple “shots” or segments

• Advantages: places less stress on the gradients; phase errors have less time to build up, reducing diamagnetic susceptibility artifacts

• Disadvantages: takes longer to perform; therefore more susceptible to motion

  • increase shots = increase time

Ghost Artifacts in EPI

  • Phase artifacts may result from the multiple positive and negative passes through k-space (alternating polarity of the readout gradient )

  • Often called Nyquist Ghosting - only happens in echo planner imaging

  • Remedy: minimize eddy currents (shimming) ; proper tuning of gradients; not obliquing the slice position

Susceptibility Artifacts in EPI

  • Diamagnetic susceptibility effects may result in variations in frequencies and phase errors

  • Effect is reduced for multishot EPI because phase errors have less time to build up

  • Remedy: proper shimming, TE shortening, multishot echo

Chemical Shift Artifact in EPI

• Chemical shift artifact occurs along the phase encode axis

• Due to phase error, chemical shift can be seen in the phase encoding direction

• Chemical shift more pronounced than gradient echo or spin echo pulse sequences

only sequence where chemical shift occures in the phase encoding direction (bc of blipping) — in all other pulse sequences che mical shift only occures in the frequency encoding direction.

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