6/13/2018
`
`T2* vs T2 relaxation time - Questions and Answers in MRI
`
`QUESTIONS
`AND
`ANSWERS
`IN MRI (/)
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`T2 vs T2*
`
`What is the difference between
`T2 and T2*?
`
`
`(/uploads/3/4/5/7/34572113/9782955_orig.png)
`In the preceding Q&A (/what-is-t2.html), T2 was defined as a time constant for the decay of transverse magnetization
`arising from natural interactions at the atomic or molecular levels. used as a measurement of those processes
`contributing to the transverse decay of the MR signal that arise from natural interactions at the atomic and molecular
`levels within the tissue or substance of interest.
`
`In any real NMR experiment, however, the transverse magnetization decays much faster than would be predicted by
`T2* ("T2-star")
`natural atomic and molecular mechanisms; this rate is denoted
`. T2* can be considered an "observed"
`or "effective" T2, whereas the first T2 can be considered the "natural" or "true" T2 of the tissue being imaged. T2* is
`always less than or equal to T2.
`
`T2* results principally from inhomogeneities in the main magnetic field. These
`inhomogeneities may be the result of intrinsic defects in the magnet itself or from
`susceptibility-induced field distortions produced by the tissue or other materials
`placed within the field.
`
`Certain MR sequences using gradient echoes and relatively long TE values are
`called T2*-weighted. They are used to accentuate local magnetic homogeneity
`effects to aid in the detection of hemorrhage or calcifications. T2*-sensitive
`
`sequences also form the basis for functional MRI (fMRI) using the BOLD
`
`
` technique. (Blood Oxygen Level Dependent)
`
`
`(/uploads/3/4/5/7/34572113/6173896_orig.jpg)
`T2* image showing low signal area due to
`old blood products.
`If one makes a certain assumption about the line shapes contributed by these processes we may write
`
`where 1/T2i = ΔBi is the relaxation rate contribution attributable to field inhomogeneities (ΔBi) across a voxel. Note
` γ
`
`that the equation is a sum of relaxation rates
`
` (1/T2's) rather than relaxation times
` (T2's).
`
`
` If you are still confused, you might enjoy this little song
`by Greg Crowther from his on-line album,
`Muscles and
` (2004).
`Magnets
`
`00:35
`
`02:40
`
`Twinkle, Twinkle, T2*
`
`Advanced Discussion (show/hide)»
`
`Twinkle, twinkle, T2*; | How I wonder what you are! |
`XY signal soon decays; | Why do the spins go out of
`phase? | Twinkle, twinkle, T2*; | Something pulls those
`spins apart. | Spin-spin crosstalk sets T2, | But by then
`T2* is through. | A brief duration here is sealed | By an
`inhomogeneous field. | Twinkle, twinkle, T2*; | Now I
`know just what you are!
`
`References
` Chavhan GB, Babyn PS, Thomas B et al. Principles, techniques, and applications of T2*-based MR imaging and its
`special applications (/uploads/3/4/5/7/34572113/t2_review.pdf). Radiographics 2009;29:1433-1449.
`
`Related Questions
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