Magnetic susceptibility artifact
This type of distortion (susceptibility artefact) occurs due to the interaction of various materials with the main magnetic field of the tomograph. The presence of superparamagnetic and ferromagnetic objects, such as metal alloys, iron, or dental materials, leads to pronounced distortion of the local magnetic field. On the image, this manifests as signal drop-out.
This effect is especially pronounced in patients with orthopedic structures (for example, in the presence of the femoral component of a hip arthroplasty). If a small matrix and a narrow receiver bandwidth (low receiver bandwidth) are used during scanning, severe geometric distortions, complete signal loss, and signal pile-up occur (signal pile-up).
Methods of elimination and minimization:
- Increasing the matrix size.
- Broadening the receiver bandwidth (sampling bandwidth).
- Using spin-echo (SE) sequences instead of gradient ones.
- Setting a short echo time (TE).
- Using specialized sequences for diffusion-weighted imaging (DWI).
Gradient sequences are significantly more susceptible to this artifact since they lack a 180-degree refocusing pulse.
Note: The susceptibility artifact can also bring diagnostic benefits. It is purposefully used in SWI (susceptibility-weighted imaging) to detect paramagnetic effects arising from gadolinium contrast or blood breakdown products.
Chemical shift artifact
The chemical shift artifact (chemical-shift artefact) has a completely different physical nature. It is caused by a small difference in the resonant frequency between water protons and fat protons. The tomograph interprets this frequency difference as a difference in spatial location, leading to spatial signal registration errors.
Visual manifestation: Characteristic fine bright and dark bands are formed at the interfaces between adipose tissue and water-containing structures.
Elimination: Various fat suppression techniques are used to suppress the chemical shift artifact.
Truncation artifact (Gibbs ringing)
The truncation artifact, also known as the Gibbs ringing artifact (truncation or Gibbs ringing artefact), occurs due to insufficient sampling (undersampling) of high-frequency data during image formation.
How to recognize: It manifests as alternating bright and dark bands located parallel to sharp boundaries between tissues with contrasting signals. These bands can be mistaken for a pathological formation (e.g., a meniscus tear) or, conversely, conceal a small real pathology.
How to eliminate:
- Using fat suppression methods (fat-suppression), which helps reduce overall contrast at the tissue interface.
- Directly increasing the matrix size during scanning.
To easily remember the methods for dealing with major MRI artifacts: "Increase M and B" — Matrix (helps with Gibbs ringing and metal) and Bandwidth (saves from metal). And fat suppression universally cures the chemical shift.
Class and exam questions
Why are gradient sequences more susceptible to susceptibility artifacts?
This occurs because gradient sequences lack the 180-degree refocusing radiofrequency pulse that compensates for local magnetic field inhomogeneities in standard spin-echo sequences.
Can clinical benefit be derived from the magnetic susceptibility artifact?
Yes, it forms the basis of SWI sequences. This effect allows for highly sensitive detection of paramagnetic blood breakdown products (hemorrhages) or the distribution of a gadolinium contrast agent.
What is the main cause of Gibbs ringing?
The main cause is insufficient sampling of high-frequency data during signal acquisition, which leads to the appearance of false bands at the boundaries of high-contrast tissues.
What else to study on this topic
- Principles of diffusion-weighted imaging (DWI) and ADC maps
- Whole-body MRI in myeloma staging and CRMO
- Cartilage mapping: T2 relaxation and dGEMRIC
- Physics of MR elastography and tissue stiffness assessment
- Single-photon emission computed tomography (SPECT)
- Positron emission tomography (PET) in neuro-oncology
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