MR Image quality and artifacts

Image quality and artifacts

出版元 Denis Hoa

学習目的

After reading this chapter, you should be able:

  • To present the different parameters used to judge the quality of an image
  • Describe the factors influencing the signal-to-noise ratio and their interdependence
  • List the different MRI artifacts, their origin, effects on the image and ways of reducing them:
    • Movements, phantom images, flow
    • Magnetic susceptibility and metal artifacts
    • Truncation / Gibb’s
    • Folding / aliasing
    • Chemical shift
    • Cross-excitation
    • Magic angle
  • Present the basic criteria in MRI quality control

キーポイント

Signal to noise ratio

Principles

Compromise between:

  • Spatial resolution: limited by the voxel size which is determined by the matrix size, the field of view and slice thickness
  • Signal to noise ratio: depending on the voxel size, the number of averagings and the receiver bandwidth
  • Total scan time

Which also modify the available sequence parameters (TE) and the artifacts.

 

Remedies (and penalties)

 

The signal to noise ratio goes up Penalty
With the voxel size Limits the spatial resolution
When a local or surface coil is used instead of a large or body coil Reduced field of exploration
With the number of averagings (square root factor) Longer scan time
When the receiver bandwidth decreases (square root factor) Higher chemical shift artifacts

 

 

 

Motion and ghosting

Principles

Motion is responsible for a corruption in spatial localization in the phase-encoding direction, resulting in a blur and ghost images propagated in the phase-encode direction.

 

Remedies (and penalties)

 

Remedies Penalty
Reduce body movements (physical restraint, sedation) Patient's comfort
Breath-hold sequences Requires apnea and fast sequences
Gating and movement compensation Increase the scan time, restrict the available TRs,
a blur persists with some methods
Signal suppression of moving tissues (fat of abdominal wall...) Increased TR due to magnetization preparation or saturation bands
Swapping phase-encode and frequency-encode directions Only shifts the artifacts, consequences on sequence parameters and other artifacts

 

 

 

Magnetic susceptibility

Principles

At the interface between two tissues with different magnetic susceptibilities, there are local distortions in the magnetic field responsible for a signal loss (and sometimes an image distortion). These artifacts are much stronger in presence of metal.

 

Remedies (and penalties)

 

Remedies Penalty
SE rather than GE sequences  
Short TE Restricts available contrasts
Increased receiver bandwidth to reduce the minimal TE Decreased SNR

 

 

 

Usage

  • detection of hematomas (blood breakdown products)
  • quantification of liver iron content
  • detection of liver metastases
  • perfusion imaging

 

Truncation

Principles

The image is reconstructed from k-space by a 2D inverse Fourier transform. As k-space data are finite and defined by the matrix size, the reconstruction of high-contrast interfaces is imperfect and causes visible artifacts. These artifacts appear as multiple parallel lines adjacent to the interface or as false widening of the edges at this interface. Truncation artifacts can occur in both the frequency and phase-encode directions.

 

Remedies (and penalties)

 

Remedies Penalty
Incread matrix size Increased scan time
Decreased SNR

 

 

 

Aliasing

Principles

Aliasing or wrap-around results from a spatial mismapping caused by an undersampling in the phase-encode direction. Consequently, objects outside of the FOV overlap on the opposite side of the image.

 

Remedies (and penalties)

 

Remedies Penalty
Swapping the frequency-encode and phase-encode directions  
Increasing the FOV Decreased spatial resolution
Phase oversampling Increased scan time
No-phase wrap or Anti-aliasing Decreased SNR

 

 

Chemical shift

Principles

The chemical environment of protons can cause a shift in their precessional frequency. The shift in resonance frequency between protons in fat and water is roughly equal to 3.5 ppm, corresponding to a difference of about 225 Hz at 1.5 T.

This frequency shift is responsible for a spatial misregistration in the frequency-encode direction, resulting in a contour artifact with dark and white bands at the interfaces between fat and tissues in the frequency-encode direction : the chemical shift artifact of the first kind.
As the resonance frequency shift causes a phase shift which is not canceled in gradient echo sequences, there is a black line at all fat/tissue borders when fat and water are out of phase (TE = 2.2 ms at 1.5 T) : the chemical shift artifact of the second kind. This artifact occurs in both the frequency-encode and phase-encode directions.

 

Remedies (and penalties)

 

Remedies Penalty
Fat suppression Increased scan time
Swapping of the frequency-encode and the phase-encode directions Rotates the artifact without eliminating it
Increased receiver bandwidth Decreased SNR

 

 

Cross-talk

Principles

  • Imperfect slice profile or intersecting slice stacks
  • Contrast modification and/or signal loss
  • Mainly with multislice spin echo technique, fast spin echo and inversion-recovery sequences

 

Remedies

  • Gap between slices
  • Interlacing


Magic-angle

Principles

  • Dipolar interactions in fibrillar structures, varying according to the angle of the fibers in relation to the axis of field B0.
  • Minimum at 55° : T2 relaxation time lengthening and increased T2-weighted signal intensity

 

Remedies

  • T1-weighted sequences
  • Long TE
  • Modify angle between fibrillar structure and B0 axis

 

Usage

Tendons and ligaments imaging (T1-weighted, +/- gado / MTC)


Quality control

Principles

 

  • Signal: SNR, uniformity
  • Geometry: linearity and distortion, spatial resolution and encoding
  • NMR : T1 and T2 contrast, CNR, accuracy, precision
  • Artifacts
  • Spectroscopy : B0 homogeneity, SNR of main peaks

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