Chemical shift artifact
Chemical shift is due to the differences between resonance frequencies of fat and water. 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. 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 (Bo). 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.
A chemical shift artifact can occur in the slice select direction for an analogous reason to the frequency encoded artifact. Since the slice position depends on the frequency of the spins, the "fat image" is shifted compared to the "water image". 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.
The amount of chemical shift is often expressed in arbitrary units known as parts per million (ppm) of the main magnetic field strength. 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 B0, for example, at 1.5 T the difference in precessional frequency is 224 Hz. That is, fat precesses 224 Hz less than water. At 1.0 T this difference is 147Hz at lower field strengths (0.5 T or less), it is usually insignificant
In MRS the shift in Lamor frequency allows separation of different chemical peaks. The actual amount of chemical shift as an absolute value is difficult to measure, so instead it is represented relative to a reference, and expressed in parts per million (ppm).
In MRI, both spin echo sequences (SE) and gradient echo sequences (GE) may demonstrate chemical shift misregistration or mismapping. The mismapping will occur in the frequency encoding direction, and show up as a bright band on one side and a dark band on the other side of a fat-soft tissue interface.
In addition to mismapping, GE sequences can show another type of chemical shift induced artifact known as the black boundary or india ink artifact. In the artifact a black line is seen in all directions at fat-water interfaces. In pixels with roughly equal amounts of fat and water, the fat and water spins are 180o out of phase at certain echo times because of their chemical shift or frequency difference causing cancellation of signal.
These effects can be used to confirm, for example, the presence of fat in a lesion.
- chemical shift increases with magnetic field strength.
- chemical shift increases with decreasing gradient strength.
- chemical shift depends upon the bandwidth; narrower the bandwidth higher is the chemical shift. Increasing the bandwidth will decrease the artifact.
- fat suppressed imaging can be used to eliminate the chemical shift misregistration and the black boundary artifact.
- use of a spin echo sequence instead of a gradient echo can eliminate the black boundary artifact but not chemical shift misregistration.
- 1. Reiser MF. Magnetic Resonance Tomography. Springer Verlag. (2007) ISBN:354029354X. Read it at Google Books - Find it at Amazon
- 2. Smith RC, Lange RC, McCarthy SM. Chemical shift artifact: dependence on shape and orientation of the lipid-water interface. Radiology. 1991;181 (1): 225-9. Radiology (abstract) - Pubmed citation
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MRI pulse sequences (basics | abbreviations | parameters)
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MR spectroscopy (MRS)
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- lactate peak: resonates at 1.3 ppm
- lipids peak: resonate at 1.3 ppm
- alanine peak: resonates at 1.48 ppm
- N-acetylaspartate (NAA) peak: resonates at 2.0
- glutamine-glutamate peak: resonate at 2.2-2.4 ppm
- gamma-aminobutyric acid (GABA) peak: resonate at 2.2-2.4 ppm
- 2-hydroxyglutarate peak: resonates at 2.25 ppm
- citrate peak: resonates 2.6 ppm
- creatine peak: resonates at 3.0 ppm
- choline peak: resonates at 3.2 ppm
- myo-inositol peak: resonates at 3.5 ppm
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