Until now, we have assumed that the transverse signal (Mxy) decays exponentially with the echo time divided by the T2 constant (see Eq. 3.3). However, in practice, other factors such as B0 inhomogeneities can cause a more rapid loss of the transverse signal; this results in a faster transverse decay, which is referred to as T2* relaxation (see Figure 3.1).
The relation between T2 and T2* is described as follows Brown et al., 2014:
Where T2′ quantifies the portion of relaxation which is due to magnetic field inhomogeneities. Some studies suggest that T2′ mapping, which can be performed by removing the T2 relaxation effect from T2*, could offer valuable insights for brain disease diagnosis, notably by quantifying blood oxygenation levels Lee et al., 2014 and to predict infarct growth in acute stroke patients Siemonsen et al., 2008.
The T2* decay can then be calculated using the same methods as T2, where Eq. 3.3 can be rewritten as follows :
Unlike T2 mapping, which uses spin echo type sequences, T2* mapping is performed using gradient echo sequences (GRE), as they are sensitive to magnetic field inhomogeneities Cohen-Adad, 2014Markl & Leupold, 2012. Nonetheless, there are specialized sequences such as the spin and gradient echo (SAGE) sequence Stokes et al., 2014 that enable the simultaneous acquisition of both T2 and T2*.
- Brown, R. W., C. Norman Cheng, Y., Haacke, E. M., Thompson, M. R., & Venkatesan, R. (2014). Magnetic resonance imaging: Physical principles and sequence design (R. W. Brown, Y.-C. N. Cheng, E. M. Haacke, M. R. Thompson, & R. Venkatesan, Eds.; 2nd ed.). Wiley-Blackwell.
- Lee, D., Kim, E. Y., Yoon, H., Park, S., & Ye, J. C. (2014, April). T2 prime mapping from highly undersampled data using compressed sensing with patch based low rank penalty. 2014 IEEE 11th International Symposium on Biomedical Imaging (ISBI).
- Siemonsen, S., Fitting, T., Thomalla, G., Horn, P., Finsterbusch, J., Summers, P., Saager, C., Kucinski, T., & Fiehler, J. (2008). T2’ imaging predicts infarct growth beyond the acute diffusion-weighted imaging lesion in acute stroke. Radiology, 248(3), 979–986.
- Cohen-Adad, J. (2014). What can we learn from T2* maps of the cortex? Neuroimage, 93 Pt 2, 189–200.
- Markl, M., & Leupold, J. (2012). Gradient echo imaging. J. Magn. Reson. Imaging, 35(6), 1274–1289.