New Scanning Features

New Scanning Features


In this chapter, we discuss some of the more recent techniques used by newer MR scanners with more advanced software. The following is a summary of these features and their functions:

  • To increase speed

    • Fractional number of excitations (NEX)

    • Fast spin echo (FSE)

    • Fast gradient-echo techniques

    • Parallel imaging (discussed in the next chapter)

  • To reduce TE

    • Fractional echo

    • Fractional radio frequency (RF)

  • To increase resolution (without time penalty)

    • Asymmetric field of view (FOV)

  • To reduce aliasing

    • No phase wrap

    • No frequency wrap

  • To increase coverage

    • Phase-offset RF pulses

  • To achieve contiguous slices

    • Contiguous slices

    • 3D acquisition

  • To achieve saturation

    • Spatial saturation

    • Spectral (chemical) saturation

  • To increase signal-to-noise ratio (SNR)

    • Low bandwidth

  • Reduce motion

    • Periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER)

Remember that FSE and fast gradient-echo techniques are separate pulse sequences, whereas the other features can be added to any pulse sequence.

Fractional NEX

See Figure 23-1, and also refer to Chapter 13.


  • Only a portion of k-space is used (e.g., ½ NEX, ¾ NEX [actually, the number of phase encodes Ny is reduced, not NEX]). Reconstruction is based on inherent symmetry of k-space along the phase axis.

  • Slightly more than half of k-space is used (called overscan) for phase correction.

  • The center of k-space is usually included because it contains the strongest signals.


  • Increased speed (caused by reduced Ny)


  • Decreased signal-to-noise ratio (SNR)

  • May increase artifacts


  • Used for localizer (scout) images

    Figure 23-1. Fractional NEX.

  • Used when speed is more important than SNR

  • Body imaging and magnetic resonance cholangiopancreatography (MRCP)

Fast Spin Echo

For more detailed discussion, refer to Chapter 19.


  • Use of multiple 180° refocusing pulses

  • Filling the k-space with multiple lines per TR (per “shot”)

  • Echo train length (ETL) denotes the number of 180° pulses (e.g., 2, 4, 8, 16, etc.)


  • Reduces scan time by a factor of ETL (2, 4, 8, 16, etc.)

  • Spin-echo contrast without reduction in SNR (SNR can actually be increased by using a very large TR)

  • Decreased magnetic susceptibility—useful in imaging near metal (particularly at higher bandwidth [BW])


  • Reduced coverage (caused by the presence of long TEs).

  • Cerebrospinal fluid may be bright on proton density images. This is a result of weighted averaging effect of later echoes. To reduce this undesirable effect, use a shorter ETL (e.g., ETL = 4), shorter TE (higher BW).

  • Fat is bright on T2-weighted images because of elimination of diffusion-mediated dephasing caused by the closely spaced 180° pulses as spins diffuse through regions of different magnetic field strength (e.g., fat and water).

  • Magnetic susceptibility effects (e.g., hemorrhage) are reduced because of decreased dephasing from closely spaced (refocusing) 180° pulses, which leave little time for spins to dephase as they diffuse through regions of magnetic nonuniformity.

  • May increase heating as a result of rapid train of RF pulses.


  • Fast scanning

  • High resolution (e.g., internal auditory canals)

  • Increase SNR with reasonable acquisition time

  • Single breath-hold technique

  • Isotropic T2-weighted data set (e.g., 3D FSE)

Fractional Echo


  • Only a fraction of the received echo is sampled (feasible because of the symmetry of the echo about TE and symmetry of k-space along frequency axis).


  • TE can be reduced

  • SNR is improved in early echoes (less T2 decay)

  • Improves T1 weighting (reduces T2 effect)

  • May decrease flow artifacts and susceptibility effects

Figure 23-2. Fractional echo. Note outer dashed box is the full echo while the inner dashed box represents the fractional echo.


  • T1-weighted images

  • To reduce flow artifacts and magnetic susceptibility effects

NOTE: Using a higher bandwidth can also result in a lower minimum TE, however, at a SNR loss.

Fractional RF (90°, 180°, or Partial Flip)


  • Same principles as in fractional echo (a fraction of the RF pulse is included in the pulse cycle because of the symmetry in the pulse).

  • TE can be reduced accordingly.

Features. The features are similar to those of fractional echo.

Figure 23-3. Fractional RF (90° or 180° or partial flip).

Asymmetric FOV


  • Rectangular FOV is used (FOV is typically reduced in phase direction because Ny is typically less than Nx).

  • May get square or rectangular pixels.


  • With rectangular FOV, we can obtain resolution of, say, 512 × 512 matrix in the time it takes to perform a 512 × 256 acquisition.

  • Increases speed while maintaining resolution.

  • Useful when the anatomy being imaged is asymmetric (smaller) in phase direction (e.g., the spine).


  • Reduced SNR compared with full FOV

  • May cause wraparound in phase direction

Figure 23-4. Asymmetric FOV. When a rectangular FOV is used, FOV typically is reduced in the phase direction. By acquiring more phase-encoding steps, a higher resolution can be achieved. In this example, you can acquire 256 phase-encoding steps and still get a 512 resolution.


  • Spine

  • Extremities

  • Abdomen

May 28, 2016 | Posted by in NUCLEAR MEDICINE | Comments Off on New Scanning Features
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