T2*-weighted imaging
T2*-weighted imaging is an
Physics
T2*-weighted imaging is built from the basic physics of magnetic resonance imaging where there is spin–spin relaxation, that is, the transverse component of the magnetization vector exponentially decays towards its equilibrium value. It is characterized by the spin–spin relaxation time, known as T2. In an idealized system, all nuclei in a given chemical environment, in a magnetic field, relax with the same frequency. However, in real systems, there are minor differences in chemical environment which can lead to a distribution of resonance frequencies around the ideal. Over time, this distribution can lead to a dispersion of the tight distribution of magnetic spin vectors, and loss of signal (free induction decay). In fact, for most magnetic resonance experiments, this "relaxation" dominates. This results in dephasing.
However, decoherence because of magnetic field inhomogeneity is not a true "relaxation" process; it is not random, but dependent on the location of the molecule in the magnet. For molecules that aren't moving, the deviation from ideal relaxation is consistent over time, and the signal can be recovered by performing a spin echo experiment.
The corresponding transverse relaxation time constant is thus T2*, which is usually much smaller than T2. The relation between them is:
where γ represents gyromagnetic ratio, and ΔB0 the difference in strength of the locally varying field.[3][4]
Unlike T2, T2* is influenced by magnetic field gradient irregularities. The T2* relaxation time is always shorter than the T2 relaxation time and is typically milliseconds for water samples in imaging magnets.
T2*-weighted imaging can be created as a postexcitation refocused gradient echo (GRE) sequence with small flip angle. The sequence of gradient echo T2*-weighted imaging (GRE T2*WI) requires a high uniformity of the magnetic field.[2]
Clinical applications
T2*-weighted sequences are used to detect
The T2*-weighted GRE sequence can detect a "middle cerebral artery susceptibility sign", which is a dark linear filling defect that is wider than the corresponding artery on the contralateral side. This sign is 83% sensitive and 100% specific for thrombotic occlusion of the internal carotid artery.[2]
It can detect hemosiderin deposition in joints as seen in arthropathy by
T2*-weighted sequences can be used with
See also
- MRI sequence
References
- PMID 25647424. CC-BY 4.0 license
- ^ PMID 19755604.
- PMID 19755604.
- ^ "T2* vs T2 relaxation time". Questions and Answers in MRI. Retrieved 2018-08-13.