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 N_y)

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

See Figure 23-2.

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)

See Figure 23-3.

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

See Figure 23-4.

Mechanism

Rectangular FOV is used (FOV is typically reduced in phase direction because N_y is typically less than N_x).
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