New Scanning Features



New Scanning Features





Introduction

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.

Mechanism



  • 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.

Advantages



  • Increased speed (caused by reduced Ny)

Disadvantages



  • Decreased signal-to-noise ratio (SNR)


  • May increase artifacts

Applications



  • 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.

Mechanism



  • 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.)

Advantages



  • 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])

Disadvantages



  • 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.

Applications



  • 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


Mechanism



  • 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).

Advantages



  • 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.

Applications



  • 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)


Mechanism



  • 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


Mechanism



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


  • May get square or rectangular pixels.

Advantages



  • 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).

Disadvantages



  • 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.

Applications



  • Spine


  • Extremities


  • Abdomen

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