T1 mapping - myocardium

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 characterization ^1-6 and has been investigated for other tissues ⁵.

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 (edema), lipids and iron content ^1-5. The histopathological correlate of myocardial T1, however, has still not been completely elucidated ¹.

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.

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.

Methodology

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 on the basis of 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 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 used to calculate extracellular volume after adjusting for hematocrit ^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 edema 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.

Influencing factors

Myocardial T1 depends on the pulse sequence, cardiac cycle as well as other factors and increases at higher magnetic field strength ⁶.

Clinical applications

T1-mapping can detect a variety of myocardial pathologies, where it shows increased values ^2-5:

• acute coronary syndrome/myocardial infarction ^2-5
  • specifically 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 / pseudonormalization) ⁵
  • detection of myocardial hemorrhage (low T1 values)
  • depiction of myocardial scar tissue
• myocarditis ^2-5
• cardiac sarcoidosis ^ref
• Takotsubo cardiomyopathy ^2-5
• heart transplant rejection ^ref
• cardiac amyloidosis
  • ATTR-amyloidosis
  • AL-amyloidosis – higher values than in ATTR
• myocardial fibrosis

In addition, it can be used in the following pathologies due to low values ^2,5:

• Anderson-Fabry disease ^2,3,5 (septum)
• iron overload cardiomyopathy ^1-3
• myocardial hemorrhage ^1-3
• fatty infiltration / metaplasia ^2,3,5
• athlete's heart

The calculation of extracellular volume is considered as reasonable in patients getting an extracellular contrast agent and give additional information in some of the above-mentioned pathologies ^2-5.

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.

See also

• myocardial mapping
• extracellular volume (ECV)
• T2 mapping
• T2* mapping