T1 Mapping for Myocardial Fibrosis by Cardiac Magnetic Resonance Relaxometry-A Comprehensive Technical Review

Abstract

Cardiac magnetic resonance (CMR) imaging has been widely used to assess myocardial perfusion and scar and is the non-invasive gold standard for identification of focal myocardial fibrosis. However, the late gadolinium enhancement technique is limited in its accuracy for absolute quantification and assessment of diffuse myocardial fibrosis by technical and pathophysiological features. CMR relaxometry, incorporating T₁ mapping, has emerged as an accurate, reproducible, highly sensitive, and quantitative technique for the assessment of diffuse myocardial fibrosis in a number of disease states. We comprehensively review the physics behind CMR relaxometry, the evidence base, and the clinical applications of this emerging technique.

Keywords: MRI; T1 mapping; cardiac magnetic resonance; cardiovascular imaging; relaxometry.

Figure 1

T₂ and $T_2^{*}$ curve. The $T_2^{*}$ has shorter time than T₂ time. $T_2^{*}$ exponential decay (≈30–100 ms, and shorter for higher B₀).

Figure 2

Diagram of a conventional 2D Look–Locker pulse sequence. The inversion-pulse/α-pulse train is repeated for every ky phase encode step. For N α-pulses, a series of N images are formed corresponding to times TI_n = t_d + (n − 1)τ(n + 1, 2, …, N) after the inversion pulse, where t_d is the time between the inversion pulse and the first α-pulse (26).

Figure 3

Modified Look–Locker inversion recovery pulse sequence scheme. There are three Look–Locker (LL) experiments, each prepared by a separate 180°inversion pulse (“inv”). The first is defined as TI_minimum, and then TI of the second and third LL experiments is determined by TI_minimum − TI_increment and TI_minimum − 2TI_increment. After inversion pulses, readout is in a non-segmented fashion with a single flip angle (α). A defined pause of a certain number of R–R intervals allows for signal recovery (40).

Figure 4

T₁ map of a healthy volunteer: using 17 heartbeats to reconstruct 11 images with different inversion times (TIs) at end of diastole phase. By merging these images into one data set, T₁ values are computed for every pixel with three parameter curve fitting (39, 41). A reconstructed T₁ map with parametric color scale is produced for these pixel values, and the segmental and global T₁ times can be estimated.

Figure 5

Graph shows recovery of absolute T₁ (mean ± SD in milliseconds) at 1.5 T in mid-cavity short axis slices at pre- and post-contrast (0–20 min) after administration of 0.15 mmol/kg of gadopentetate dimeglumine (22).