What is normal? A central question in the application of CMR mapping techniques
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New techniques in cardiovascular imaging are rapidly evolving and unfold new pathways in clinical practice. In cardiovascular magnetic resonance imaging (CMR) T1-mapping has revolutionized the understanding of the myocardial extracellular space and its importance in a variety of cardiac disorders.
Bearing in mind the basic principles of CMR, the T1 time represents the longitudinal relaxation time and is a measure of how quickly the net magnetization vector recovers to its ground state after an impulse. The technology of T1-mapping enables visualization and quantification of the specific T1 time of every voxel within a certain region of interest, which differs between normal myocardium and pathologies, such as fibrosis, edema and infiltrative disorders.
More recently, T2-mapping was introduced as a new method for detection of myocardial edema in addition to standard short-tau inversion recovery sequences, comparable to the value of T1-mapping on top of LGE imaging. Both T1-mapping and T2-mapping are currently being considered to replace the Lake Louise criteria for the diagnosis of myocarditis [7, 8].
Although they harbor a unique potential for noninvasive myocardial characterization, mapping techniques are limited by the lack of reference values. This is of particular interest since the possibility of CMR T1 and T2-mapping increases referrals for CMR scans, and referral indications are shifting. Non-academic outpatient radiology centers will also increasingly be asked to report T1 and T2-mapping results as well as extracellular volume estimates of the myocardium; however, the current consensus statement of the Society of Cardiac Magnetic Resonance Imaging underlines the importance of validation of reference values in every single center . Figure 1 furthermore illustrates another development of CMR: while cine and LGE imaging are mainly assessed qualitatively, T1-mapping only provides conclusive information if quantitative measurements are interpreted with respect to locally acquired normal values.
In this issue of the journal, Granitz et al.  report T1 and T2-mapping results of 60 healthy individuals undergoing CMR both on 1.5T and 3.0T systems. Consistent with previous findings, gender and heart rate dependency were reported for both T1 and T2 times. Reference values that are applicable in an older population are still a matter of debate. In this study, as in previous investigations, volunteers were young (mean age 41 years) and only 4 individuals were 60 years or older. Nevertheless, these data are of great importance not only for the center where they have been determined and thus enable the interpretation of local T1 maps. They will in the future, as part of a big puzzle, enhance our understanding of the variation of such measurements and the factors that determine local differences. The use of CMR mapping techniques has already revolutionized cardiovascular imaging and will continue to do so. The availability of reference values that do not need validation in every single CMR center will hence be crucial for the correct application of this innovative technique.
Conflict of interest
A.A. Kammerlander and J. Mascherbauer declare that they have no competing interests.
- 9.Messroghli DR, Moon JC, Ferreira VM, et al. Clinical recommendations for cardiovascular magnetic resonance mapping of T1, T2, T2* and extracellular volume: A consensus statement by the Society for Cardiovascular Magnetic Resonance (SCMR) endorsed by the European Association for Cardiovascular Imaging (EACVI). J Cardiovasc Magn Reson. 2017;19:75.CrossRefGoogle Scholar