MR tagging

Cardiac MR tagging or myocardial tagging refers to an MRI-based acquisition method designed for the analysis of myocardial deformation.

Methodology

The method exploits tissue magnetization as a tissue property. A local magnetic saturation grid of dark-lined tissue markers known as tags are induced onto the myocardium with radiofrequency pulses applied in orthogonal planes. The tag lines deform with the myocardial tissue during cardiac contraction and consequently, the ‘tagging’ grid can be used to track and assess the displacement of the myocardial tissue as a basis for calculating cardiac strain ^1-3.

Cardiac MR tagging involves the following steps ^2,3:

• image preparation

• detection of endocardial and epicardial borders

• definition of the segments to be tracked

• tag tracking

  • tracking of dark lines or intersections

  • harmonic phase analysis (HARP)

  • other optical flow techniques

• motion reconstruction

These principles of tagged MRI have also found novel research applications such as in the analysis of upper airway motion e.g., for the study of obstructive sleep apnea^4,5.

Advantages/disadvantages

MR tagging is considered and widely accepted as a reference standard in the assessment of regional myocardial strain imaging and has been used in large clinical trials e.g. the MESA trial ^1,2.

However, due to the requirement of a dedicated acquisition sequence, additional acquisition time and time-consuming post-processing algorithms MR tagging has been less widely used than other recent methods.

The advantages of MR tagging are ¹:

• direct measurement by physical tissue properties

• extensive in vitro and in vivo validation

Disadvantages of MR tagging include ^1,2:

• low temporal resolution (20-30 frames/heartbeat)

• delay of tag deposition at the beginning of systole potentially leading to an underestimation

• low accuracy at the endocardial border and in thin-walled regions

• tags fade through the cycle

History and etymology

Cardiac MR tagging was developed in the late 1980s ^6,7.

See also

• strain imaging