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Sequence Diagrams  [Home] [Archive] [Sequence Diagrams]

Gradient Echo (GE)

  • simpliest sequence, TR, TE, and flip angle (FA) variable.
  • Pulse Sequence Timing Diagram [BW]
 Features:
  • T2* decay occurs between the dephasing and rephasing gradients, and generates image contrast
  • Longer TE means greater signal loss
  • Signal loss from B0 inhomogeneity, magnetic susceptibility, and water-fat incoherence unrecoverable
Advantages:
  • Fast imaging
    --can use shorter TR and TEs than SE
  • Low flip angle deposits less energy
    --more slices per TR than SE --decreases SAR
  • Compatible with 3D acquisitions
  • Sensitive for flow / blood.
Disadvantages:
  • Difficult to generate good T2 weighted images
  • B0 Magnetic field inhomogeneity causes signal loss
    --worse with increasing TE
    --susceptibility sensitive
    --chemical shift dephasing
Acquisition Time per 256x256 single slice image : 15-30 seconds.
by using low flip angle, 0.5-10 seconds.

Sequencies based on Gradient Echo:
  • Conventional: GRASS, FAST
    --T1 weighting: larger flip angle
    --T2* weighting: longer TE
  • Spoiled GR: SPGR, RF-FAST
    --spoiling destroys accumulated transverse coherence
    --maximizes T1 contrast
  • Contrast enhanced GR: SSFP, CE-FAST
    --poor SNR, rarely used
    --heavily T2* weighted images
  • Other:
    --MTC (T2 - like weighting)
    --IR prepped (180 preparatory pulse)
    --DE (driven equilibrium) prepped (90-180-90 preparatory pulses and T2 contrast)

Magnetization-Prepared Gradient Echo (MPGE)

  • A preparation pulse is excuted before a GE sequence, TR, TE, and flip angle (FA) variable.
 Features:
  • non-steady-state
  • Image contrast is determined by the time between the preparation pulse and the measurement of the low-amplitude phase-encoding steps
  • 180 degree preparation pulse for T1-weighted imaging
  • 90-180-90 pulse train preparation for T2-type weighting
  • Very sensitive to choices of measurement parameters and the order of raw data collection

Spin Echo (SE)

  • widely used sequence, TR and TE variable.
  • Pulse Sequence Timing Diagram [BW]
 Features:
  • Signal decay is caused by T2 relaxation only
  • Signal losses from B0 inhomogeneity, susceptibility, and water/fat dephasing are recovered
  • Generates T1, T2, and PD (proton density) weighted images
Advantages:
  • High SNR
  • High spatial resolution
  • Least artifact prone sequence
  • Clear contrast mechanism:
    --Short TR (450-850) + Short TE (10-30) = T1 weighted
    --Long TR (2000+) + Long TE (>60) = T2 weighted
    --Long TR + Short TE = PD weighted
Disadvantages:
  • Higher SAR than GE, caused by 90 and 180 RF pulses
  • Incompatible with 3D acquisitions due to long TR
  • Delay to return magnetization to equilibrium.
Acquisition Time per 256x256 single slice image : 100-1000 seconds

Special sequences based on SE for faster T2 weighted imaging:
  • Half-NEX, half-Fourier imaging (SNR or spatial resolution penalty)
  • rectangular FOV (reduced FOV)
  • PRISE, THRIFT (altered flip angle SE imaging)

Multi Echo Spin Echo

  • designed to obtain simultaneously multiple echos, at a given TR,
    number of slices depends on TE
  • Pulse Sequence Timing Diagram [BW]
 Features:
  • Simultaneously generates PD and T2 weighted images
  • No time penalty for acquisition of PD weighted image
  • No mis-registration between echos
  • Can do multiple echos (usually 4) to calculate T2 relaxation values

Fast Spin Echo

  • for faster T2 weighted imaging. TR, TE, ETL varible
  • Pulse Sequence Timing Diagram [BW]
 Features:
  • Collect multiple echos per TR
  • Most time are used for data acquisition, no waiting penalty
  • At a given imag resolution, the number of phase encodings determines the imaging time
  • Each phase encoding is obtained as a unique echo following a single excitation with a 90 degree RF pulse
  • Fast for T2 or proton denstity-weighted acquisitions.
Scan Time:
  • TR = time between 90 degrees
  • phase_encodings = total number of unique echos
  • NEX, NSA = number of averages
  • ETL = echo train length
Scan_timeminutes = TRmsec*phase_encodings*NEX/ (60,000 * ETL)

Advantages:
  • Acquisition time reduced proportinal to echo train length (ETL)
  • Can trade-off some of the time savings to improve images
    --increased NEX
    --increased resolution
  • Image contrast and SNR similar to SE
Disadvantages:
  • Much louder gradient noise
  • Higher SAR (many 180 degree flips closely placed)
  • Motion sensitive
  • Reduced number of slices for equivalent TR SE scan
  • MT effects alter image contrast
  • TE time imprecise
  • T2 effects along phase encode direction
  • Image blurring may occur
  • Fat remains relatively bright on long TR/long TE scans
  • "J-coupling"
  • Each each "belongs" to a different TE image
  • Combining the echos to form a single image creates artifacts
    --worse with shorter effective TE times
  • Insensitive to susceptibility differences
Acquisition Time per 256x256 single slice image: 30-60 seconds.

Typical sequence based on TSE : RARE.

Inversion Recovery

  • for heavily T1 weighted imaging. TI, TR, and TE varible
  • [BW]
 Features:
  • "Fat Nulling" - TI about 180ms for 3T
  • Exploits the zero crossing effect of IR imaging
Advantages:
  • Robust technique
    --works better than fat saturation over a large FOV (>300mm)
    --better at low field strengths (open magnets)
  • High visibility for fluid
    --long T1 bright on STIR
    --long T2 bright on STIR, given long enough TE
Disadvantages:
  • Poor SNR
    --improved with multiple averages
    --improved with shorter TE times
  • Imcompatible with gadolinium
    --shorter T1 relaxation post-contrast
  • Red marrow signal can obscure subtle edema
    --use TE=48 to knock signal down from marrow

Hybrid Gradient Echo Spin Echo

  • A series of spin echoes lead gradient echo trains
 Features:
  • Gradient Echoes symmetrically placed around each spin echo, with two or four gradient echoes acquired per spin echo
  • Primarily used for T2 weighted imaging
  • T2* effects more pronounced
  • Magnetic susceptibility differences
Advantages:
  • Rapid acquisition
  • RF power deposition is reduced compared to a comparable echo train spin echo
  • Less sensitive to chemical shift and magnetic field inhomogeneities
Disadvantages:
  • Delay to return magnetization to equilibrium
Acquisition Time per 256x256 single slice image: 1-10 seconds.

Typical hybrid sequence: GRASE

Echo Planar Imaging (EPI)

  • Characterized by a series of rapid gradient reversals by the readout gradient, each of which produced a gradient echo
 Features:
  • SE-EPI: Off-center 180 degree pulse gives T2* weighting
  • SE-EPI: Diffusion weighting possible using extra gradient pulses
Advantages:
  • Rapid acquisition
Disadvantages:
  • Low SNR
  • SE-EPI: Poor spatial resolution
  • SE-EPI: special gradient coils required
  • GE-EPI: Chemical shift artifact
  • GE-EPI: Sensitive to magnetic field inhomogeneties
Acquisition Time per 256x256 single slice image: 0.01-1 seconds.

BURST

  • This sequence figures a family of fast imaging sequences capable of image acquisition in less than 100 ms. Basically a train of low flip angle pulses generates a long train of echoes. This complete sequence is performed with the application of a constant read gradient, and phase encoding may be implemented using short phase encoding gradients between echoes.
 Features:
  • Rapid Acquisition
Advantages:
  • Less demanding on gradient speed than other fast techniques (e.g. EPI)
  • The images are substantially free of susceptibility artifacts
  • No special gradient hardware required
Disadvantages:
  • Less sensitive than competing methods
  • Low SNR
Acquisition Time per 256x256 single slice image: 0.01-1 seconds.
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