MR Image quality and artifacts

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.

Conséquences également sur les paramètres de séquences (TE) et les artéfacts

Remedies (and penalties)

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)

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)

Usage

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

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)

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)

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)

Usage

• 1st type : Spectroscopy, fat saturation
• 2nd type : Tissue characterization : fatty liver, adrenal adenomas

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

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)

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