Its value is always independent of the main field strength and equals 3.5 ppm for fat and water however, the precessional frequency is proportional to the main magnetic field strength B o. The amount of chemical shift is often expressed in arbitrary units known as parts per million (ppm) of the main magnetic field strength. The slice thickness is larger than the shift of the water and fat images making it difficult to detect the effect on routine imaging 2. Since the slice position depends on the frequency of the spins, the "fat image" is shifted compared to the "water image". The Larmor frequency which determines the frequency at which a particular nucleus resonates is established at the nucleus, and therefore different tissues will have slightly different Larmor frequencies depending on their chemical composition.Ī chemical shift artifact can occur in the slice select direction for an analogous reason to the frequency encoded artifact. So precession frequency is higher in hydrogen atoms in water compared to hydrogen in fat. The lesser the electron cloud, the higher the effect of static magnetic field, so higher the precession frequency. Oxygen atoms in water molecules attract electron clouds and reduce electron clouds around hydrogen atoms while no significant such effect occurs in hydrogen atom of fat. Essentially it is due to the effect of the electron cloud to a greater or lesser degree shielding the nucleus from the external static magnetic field (B o). It occurs in the frequency-encode direction where a shift in the detected anatomy occurs because fat resonates at a slightly lower frequency than water. It is a common finding on some MRI sequences and used in magnetic resonance spectroscopy ( MRS). This artifact occurs in the frequency-encoding direction and is due to spatial misregistration of fat and water molecules.Ĭhemical shift is due to the differences between resonance frequencies of fat and water. Chemical shift artifact or misregistration is a type of MRI artifact.
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