MR feature tracking

MR feature tracking refers to an MRI based post-processing method, used on normal cine SFFP sequences for the analysis of myocardial deformation and the determination of myocardial strain parameters.

Methodology

MRI feature tracking is a two-dimensional software algorithm applied on standard cine images. It uses a block matching approach to track those features. This is achieved by automatic identification of an image pattern and determination of the position of the best matching pattern or block within the target area ¹.

It relies on the identification of anatomic features of the myocardial contours, the endocardial and epicardial borders and due to the paucity of features of the mid-myocardial area midwall strain is not computed ^1,2.

The assessment of myocardial strain includes the following steps ^1,2:

• identification of end-diastole and end-systole
• detection of endocardial and epicardial borders either semiautomatic or by manual contouring (papillary muscles are typically excluded from the endocardial contour)
• definition of the segment to be tracked

Typically assessed strain parameters include the following ^1-3:

• longitudinal strain  (derived from 2-chamber, 3-chamber or 4-chamber cine images)
• circumferential strain (derived from short-axis cine images)
• radial strain (computed from short-axis cine images)
• associated diastolic and systolic strain rates typically calculated for peak systole and peak early diastole

Global strain values for longitudinal strain and circumferential strain can be also averaged from endocardial and epicardial values for better comparability.

Interpretation

As with other block matching algorithms e.g. speckle tracking echocardiography assessment of segmental strain is less reliable than global strain parameters ¹.

Reproducibility of radial strain was found to be lower than reproducibility of longitudinal and circumferential strain parameters, which might be done to the fact that the assessment involves simultaneous detection and tracking of the endocardial and epicardial borders, the latter being somewhat less distinctive than the former ².

There seems to be a slight gender difference in longitudinal strain values and age-related changes in circumferential strain. Systolic blood pressure constitutes another influence on circumferential strain ².

Normal reference ranges seem to vary also among different software versions and due to this issue, it is recommended that for the time being software specific cut off values are used ¹.

Approximated values for global longitudinal strain and circumferential strain are in the -20 ± 4% range and values for radial strain are roughly in the range of 40 ± 8%.  

Advantages/disadvantages

Advantages of MR feature tracking are ¹:

• post-processing software application
• no additional acquisition time
• easy to use

Disadvantages of MR feature tracking include ¹:

• two-dimensional approach
• contour-based tracking algorithm
• low spatial and temporal resolution
• inability to measure midwall strain

See also

• strain imaging