T1 mapping is a magnetic resonance imaging technique used to calculate the T1 time of a certain tissue and display them voxel-vice on a parametric map. It has been used for myocardial tissue characterisation 1-6 and has been investigated for other tissues 5.
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Terminology
Native T1 is referred to as the T1 time measured in the absence of a contrast agent 1-3, whereas postcontrast T1 measured after the application of gadolinium is used to calculate extracellular volume (ECV), a surrogate parameter for the extracellular matrix 1-4. Both native T1 and ECV can be used as biomarkers 1,2.
Clinical applications
T1-mapping can detect a variety of myocardial pathologies, where it shows increased values 2-5:
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acute coronary syndrome/myocardial infarction 2-5
specifically, the differentiation of an acute coronary syndrome from the myocardial scar of an old infarct
assessing the area at risk 2
assessment of myocardial salvage after reperfusion 2
identification of the infarct core (lower values / pseudonormalisation) 5
detection of myocardial haemorrhage (low T1 values)
depiction of myocardial scar tissue
myocarditis 2-5
heart transplant rejection ref
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AL amyloidosis – higher values than in ATTR
In addition, it can be used in the following pathologies due to low values 2,5:
Anderson-Fabry disease 2,3,5 (septum)
fatty infiltration/metaplasia 2,3,5
The calculation of extracellular volume is considered reasonable in patients getting an extracellular contrast agent and gives additional information in some of the above-mentioned pathologies 2-5.
Technique
Physical principles
T1 is the spin-lattice or longitudinal relaxation time of tissue and a parametric T1 map depicts those values within its voxels 1-6. T1 reflects changes in intracellular and extracellular compartments and is affected by collagen, protein, water (oedema), lipids and iron content 1-5. The histopathological correlate of myocardial T1, however, has still not been completely elucidated 1.
Alterations in native T1 are not specific and in the myocardium, they reflect changes in tissue composition. Together with other imaging or clinical parameters, they can help in the diagnosis 1,2.
Image acquisition
T1 mapping can be conducted with several different acquisition methods (MOLLI, ShMOLLI, STONE, SASHA, SAPPHIRE) based on balanced SSFP sequences 2,5. Following either an inversion or saturation preparation a series of co-registered images is acquired at different T1 recovery times 2.
T1 values can then be computed pixel-wise from a signal intensity versus time curve fitting model 2,5. Motion between the images needs to be corrected otherwise this will have an adverse effect on the measured T1 values 2,3,5.
The voxels can then be quantified and evaluated either based on normal reference values in diffuse disease or compared to the healthy myocardium in focal disease 2.
If the second set of T1 images after the administration of a gadolinium-based contrast agent is acquired and co-registered to the first native set, an extracellular volume map can be generated 1-3.
Interpretation
Native T1 is related to the water, protein, lipid and iron content of the respective tissue and expresses the signal from the intracellular and extracellular compartments 1,5,7.
Postcontrast T1 is shorter than native T1, mainly reflects the extracellular compartments and is used to calculate extracellular volume after adjusting for haematocrit 1-5,7.
Extracellular volume reflects the space, which is not occupied by cells and also includes the intracapillary plasma volume 1. It correlates with the collagen volume fraction but also increases in the presence of amyloid or myocardial oedema 1. In the absence of the two latter and other forms of an infiltrative disease, it can be seen as a biomarker for interstitial disease or myocardial fibrosis 1-4.
Normal values
Normal values of T1 times differ depending on magnetic field strength (1.5 and 3 tesla) and acquisition sequence (MOLLI, shMOLLI, SASHA, SAPPHIRE). Because of variations between scanners, the primary use of a local reference range is recommended 1,2 and if a local reference range is not available quantitative results should not be clinically reported 1,2.
Limitations
Myocardial T1 depends on the pulse sequence, cardiac cycle as well as other factors and increases at higher magnetic field strength 6.
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