T2 mapping is a magnetic resonance imaging technique used to calculate the T2 times of a certain tissue and display them voxel-vice on a parametric map. It has been used for tissue characterization of the myocardium 1-5 and has been investigated for cartilage 6,7 and other tissues 4.
T2 mapping offers the potential for more objective detection and quantification of myocardial edema than standard black-blood T2W and STIR images, which are often of limited value due to susceptibility or slow-motion artefacts and have limited value for quantitative evaluation 1,2,8.
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Clinical applications
T2-mapping can detect and assess myocardial edema in a variety of cardiac pathologies 1-5,9,10,15-17:
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acute coronary syndrome/myocardial infarction
specific differentiation of an acute coronary syndrome from a myocardial scar of an old infarct
assessing the area at risk
assessment of myocardial salvage after reperfusion
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assessing disease activity
In addition, it is of some use in the following pathologies due to low values 1,3:
Technique
Physical principles
The T2 time, also referred to as the spin-spin or transverse relaxation, is a time constant for the decay of transverse magnetization 1-3 and is tissue-specific regarding its ability to differentiate normal from abnormal 5,6.
Alterations in the T2 time are not specific for a single disease but reflect changes in tissue composition and can be used to receive further valuable information about certain disease processes and together with other parameters or in the context of a certain clinical scenario 1. This can help in the diagnosis of a disease or the assessment of disease activity 1 or its repair 6.
T2 values reflect water content in the respective tissue4 and within the myocardium, T2 maps are mainly used for the evaluation of myocardial edema in the context of myocardial inflammation or myocardial infarction, but also in other pathologies 1,4.
Image acquisition
T2 mapping has been conducted with T2 turbo spin multi-echo (T2-TSE) 2,5, T2 prepared steady-state free precession (T2p-SSFP) 2,5,8,9, as well as T2 gradient spin echo mapping sequences (T2-GraSE) 10,11. No matter which acquisition technique is used a series of co-registered images is acquired with different T2 echo times 1-4,6-9.
T2 values can then be computed pixel-wise from a signal intensity versus echo time curve fitting model 1-3,12. The variation of other weight factors, e.g. T1 off-resonance, needs to be corrected if not negligibly small, and displacement between the images of the series should be avoided to get accurate values 1,12.
The respective voxels can then be quantified and evaluated based on normal reference values in diffuse disease. In focal disease, the voxels can be compared to the spared healthy myocardium.
Interpretation
T2 time is related to the water content of the respective tissue, hence the myocardium and thus prolonged T2 reflects myocardial edema 1-4,8.
Normal values
Normal values of T2 times differ depending on magnetic field strength (1,5 and 3 Tesla), and acquisition sequence (T2-SSFP, GraSE). Because of variations between scanners, the primary use of a local reference range is recommended 1,4 and if a local reference range is not available quantitative results should not be clinically reported 1,4.
Limitations
Myocardial T2 tends to decrease at higher magnetic field strength 13. It is susceptible to artifacts.
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