(Fig 5-A.04) Chemical Shift Artifact

https://youtu.be/gP1GMIH6ajQ

https://youtu.be/YyQWPfrVkxI

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📌 Understanding T2 Anisotropy, Magic Angle Effect, and Chemical Shift Artifacts

✅ T2 Anisotropy

• T2 anisotropy arises from the highly organized collagen network in cartilage. 
• This organization leads to variations in signal intensity depending on the orientation of the collagen fibrils relative to the magnetic induction field (B0). 
• The anisotropic nature of the collagen fibers causes a gradation of signal intensity at MRI, primarily observed in intermediate-weighted or T2-weighted sequences. 
• In these images, the deep layer of cartilage often exhibits low signal intensity due to the vertical orientation of the collagen fibers.

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✅ Magic Angle Effect

• The magic angle effect significantly influences the appearance of cartilage on MRI. 
• When collagen fibrils are oriented at approximately 55° to the main magnetic field (B0), signal intensity increases. 
• This effect is more pronounced in the deep zones of cartilage where the fibrils are parallel to B0, leading to rapid T2 decay and consequently lower signal intensity. 
• This phenomenon can cause regional variations in cartilage signal intensity, which can vary by more than 200% depending on the orientation to B0.

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✅ Chemical Shift Artifacts

• Chemical shift artifacts occur at the interface between tissues with different chemical environments, such as the fat-containing medullary cavity and the bone plate. 
• These artifacts can lead to an overestimation of the thickness of the subchondral bone plate and, therefore, the depth of cartilage lesions. 
• The appearance of the bone plate in MRI is influenced by the receiver bandwidth; a decreased bandwidth can exacerbate chemical shift artifacts, which is often done to improve signal-to-noise ratios in T2-weighted sequences. 
• Fat-suppression techniques can mitigate these artifacts by reducing or eliminating the fat signal.

References

1. RadioGraphics 2018; 38:1478–1495.
2. RadioGraphics 2022; 42:1457–1473.

 

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#MRI #Radiology #MusculoskeletalImaging #T2Anisotropy #MagicAngleEffect #ChemicalShiftArtifacts #CartilageImaging #BonePlate #FatSuppression #MedicalImaging