T2 relaxation, also known as the transverse or spin-spin relaxation, is characterized by the dephasing of spins, leading to a reduction of the total magnetization in the x-y plane. In practical terms, the T2 value represents the upper limit of the signal decay time under ideal imaging conditions. In practice, magnetic field inhomogeneities cause the transverse magnetization to decay faster than what is captured by T2 relaxation. These inhomogeneities can be macroscopic, caused by factors such as metallic implants or air-tissue interfaces, or microscopic, resulting from differences in magnetic susceptibility between tissues Chavhan et al., 2009Cohen-Adad, 2014 . When considering this phenomenon, we refer to the transverse relaxation as T2*.
T2 mapping, a quantitative magnetic resonance imaging method, offers images of T2 relaxation times (called T2 maps) providing valuable insights into tissue composition. Given that T2 relaxation is sensitive to specific microstructural changes associated with diseases, such as iron accumulation, myelination and inflammation Dortch, 2020, it is considered a promising modality for clinical and research applications. Commonly used T2-mapping techniques are split into two categories: the mono-exponential methods, which consider that the voxel’s signal arises from a single tissue compartment, and multi-exponential methods, where various tissues that contribute to a single voxel’s signal are considered. Recently, novel techniques for T2 mapping have emerged beyond the conventional techniques that were developed for NMR, such as MR fingerprinting (Ma et al., 2013), a fast relaxation mapping technique that uses a single image acquisition to quantify multiple parameters Hamilton et al., 2017.
In the following sections, we will introduce the current state-of-the-art signal modeling and data fitting theory for both mono-exponential and multi-exponential T2 mapping, as well as the benefits and pitfalls of both approaches. An overview of some common applications of T2 mapping (e.g., myelin water fraction (MWF) imaging) will also be presented. We will also cover the fundamentals of T2* mapping and explore how its signal decay curve can differ from that of T2 relaxation.
- Chavhan, G. B., Babyn, P. S., Thomas, B., Shroff, M. M., & Haacke, E. M. (2009). Principles, techniques, and applications of T2*-based MR imaging and its special applications. Radiographics, 29(5), 1433–1449.
- Cohen-Adad, J. (2014). What can we learn from T2* maps of the cortex? Neuroimage, 93 Pt 2, 189–200.
- Dortch, R. D. (2020). Quantitative T2 and T2* Mapping. In Advances in Magnetic Resonance Technology and Applications (pp. 47–64). Elsevier.
- Hamilton, J. I., Jiang, Y., Chen, Y., Ma, D., Lo, W.-C., Griswold, M., & Seiberlich, N. (2017). MR fingerprinting for rapid quantification of myocardial T1, T2, and proton spin density. Magn. Reson. Med., 77(4), 1446–1458.