Image quality and artifacts

  • Antoine Micheau, MD , Denis Hoa, MD
    • Antoine Micheau, MD : IMAIOS, 2 All Charles R. Darwin, Island Hall 2 34170 Castelnau Le Lez
    • Denis Hoa, MD : IMAIOS, 2 All Charles R. Darwin, Island Hall 2 34170 Castelnau Le Lez
  • Thursday, November 24, 2022
  • ISBN 978-1847537768

Learning objectives

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

Key points

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 upPenalties
With the voxel sizeLimits the spatial resolution
When a local or surface coil is used instead of a large or body coilReduced 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)

RemediesPenalties
Reduce body movements (physical restraint, sedation)Patient's comfort
Breath-hold sequencesRequires apnea and fast sequences
Gating and movement compensationIncrease 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 directionsOnly 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)

RemediesPenalties
SE rather than GE sequences 
Short TERestricts available contrasts
Increased receiver bandwidth to reduce the minimal TEDecreased 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)

RemediesPenalties
Incread matrix sizeIncreased 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)

RemediesPenalties
Swapping the frequency-encode and phase-encode directions 
Increasing the FOVDecreased spatial resolution
Phase oversamplingIncreased scan time
No-phase wrap or Anti-aliasingDecreased 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)

RemediesPenalties
Fat suppressionIncreased scan time
Swapping of the frequency-encode and the phase-encode directionsRotates the artifact without eliminating it
Increased receiver bandwidthDecreased 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 (and penalties)

• 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

References

  1. Elster. Questions and answers in magnetic resonance imaging. 1994:ix, 278 p.
  2. McRobbie. MRI from picture to proton. 2003:xi, 359 p.
  3. NessAiver. All you really need to know about MRI physics. 1997.
  4. Kastler. Comprendre l'IRM. 2006.
  5. Korin, Felmlee. Adaptive technique for three-dimensional MR imaging of moving structures. Radiology. 1990 Oct;177(1):217-21.
  6. Harris and White. Metal artifact reduction in musculoskeletal magnetic resonance imaging. The Orthopedic clinics of North America. 2006 Jul;37(3):349-59, vi.
  7. Hood, Ho. Chemical shift: the artifact and clinical tool revisited. Radiographics. 1999 Mar-Apr;19(2):357-71.
  8. Bydder, Rahal. The magic angle effect: a source of artifact, determinant of image contrast, and technique for imaging. J Magn Reson Imaging. 2007 Feb;25(2):290-300.
  9. de Certaines and Cathelineau. Safety aspects and quality assessment in MRI and MRS: a challenge for health care systems in Europe. J Magn Reson Imaging. 2001 Apr;13(4):632-8.
  10. Ihalainen, Sipila. MRI quality control: six imagers studied using eleven unified image quality parameters. European radiology. 2004 Oct;14(10):1859-65.
  11. Zhuo and Gullapalli. AAPM/RSNA physics tutorial for residents: MR artifacts, safety, and quality control. Radiographics. 2006 Jan-Feb;26(1):275-97.