This study examined limits of agreement of two rapid methods for quantifying myocardial T1: Look-Locker (LL) and MOLLI (modified Look-Locker). Over wide ranges in myocardial T1, both LL and MOLLI showed good agreement in intrasequence comparisons but MOLLI shows substantially tighter limits of agreement than LL. Short and long axis cardiac planes showed similar results for myocardial T1 values for both sequences.
T1 mapping via the LL method is one of the fastest approaches for T1 quantification(11
). In addition, the LL sequence has been widely applied to delayed gadolinium CMR of the myocardium principally to determine the null point of the myocardium. In the LL technique, multiple phases are acquired after an inversion pulse. After a sufficient pause to allow magnetization to return to equilibrium, the experiment is repeated, until k-space is filled. There are some limitations in the LL method for quantifying myocardial T1. For post-contrast myocardial T1 time around 500 ms, two RR intervals enable almost full magnetization recovery and accurate T1 quantification. For non-contrast myocardium T1 time around 1100ms at 1.5T, two RR intervals are not sufficient for magnetization to return to equilibrium; thus, the estimated T1 will be in error and this error is increased with faster heart rate or at higher magnet field strength (when the T1 time is longer). Heart rate correction cannot entirely resolve this inaccuracy. In addition, LL data is prospective ECG triggered and acquired as a cine sequence. Thus, T1 mapping requires careful contouring of epicardial and endocardial boundaries on all cine phases, as described by Gai et al(11
The MOLLI sequence is a variant of inversion recovery LL, which introduces two changes to the standard LL sequence: 1) selective data acquisition at a fixed delay time of the cardiac cycle over successive heartbeats, and 2) the possibility to combine multiple LL image sets into one data set to facilitate the final analysis. However, the accuracy of pixel-by-pixel T1 estimation is compromised by the myocardial motion between frames, mainly due to beat-to-beat variation and respiration. Xue et al(14
), proposed a novel registration algorithm based on estimating motion-free synthetic images presenting similar contrast to original data. Robust motion correction is achieved by registering synthetic images to corresponding MOLLI frames(14
). MOLLI allows a rapid and highly reproducible T1 map of the heart with high levels of intra and inter-observer agreement (8
Overall, the MOLLI and LL sequences showed similar trends and consistency for post contrast images of the myocardium. However, the T1 values derived from the LL measurement are about 60 ms longer than those of LL. MOLLI images are easier to quantitatively analyze since artifacts from motion are more readily eliminated. This is likely to result in more reproducible data and thus smaller sample sizes for scientific studies. Nevertheless, it appears that either method, used consistently, may be applicable to post gadolinium MRI data acquisition.
In this study, the mean myocardial time was measured in both the long and short axis. No systematic difference was found between LL and MOLLI techniques. Since different segments of myocardial tissue are measured in the two imaging planes, these results suggest no global differences can be detected in myocardial T1 using either method. We did however observe greater standard deviation for LL short axis images compared to other acquisitions, including MOLLI. The reason for this is not clear, but could be related to small sample size or motion artifacts that are more apparent using LL sequence. A current limitation of both LL and MOLLI sequences are their two-dimensional nature. Ideally a 3D (15
) acquisition of the entire myocardium with high resolution and accuracy will eventually be feasible, similar to applications in other areas of the body(17
LL methods demonstrate increased measurement variability in precontrast images compared with MOLLI methods. This poses a particular challenge for methods for interpreting T1 values that rely on both pre- and post-contrast T1 data – such as gadolinium volume of distribution (Vd) and extracellular volume calculations (ECV) (18
). Our results suggest that MOLLI should be preferred for Vd and ECV calculations. When only LL is available, corrected post-contrast T1 measurements should be utilized(11
While the use of MOLLI has advantages in prospectively acquired studies, there is a wealth of previously acquired CMR data in large population studies that included a Look-Locker sequence for optimization of delayed enhancement images (21
). Our results provide validation for use of these previously acquired LL sequences to retrospectively measure myocardial T1 time in such study populations. While myocardial T1 measurement is a promising tool for the noninvasive assessment of fibrosis, there is as yet little data tying these measurements to clinical outcomes. Such retrospective analyses may help illuminate the most promising applications for this technique.
The study was limited to normal subjects, so that motion artifacts were probably less than for typical patient studies. Since we collected T1 values over a long period after contrast agent injection (3.5 to 25 minutes), a large range of T1 values (from about 350 to 1000 ms) was evaluated. This range likely covers the situation of low T1 values proposed to be associated with diffuse fibrosis in disease states. The overall T1 values we derived are in general agreement with previous studies (1
). Only 1 MRI scanner manufacturer was included. In order to determine the applicability of T1 mapping, it will eventually be necessary to evaluate T1 values on different MRI scanners.
In conclusion, this study examined limits of agreement of two rapid methods for quantifying myocardial T1: Look-Locker (LL) and MOLLI (modified Look-Locker). Over wide ranges in myocardial T1, both LL and MOLLI showed good agreement in intrasequence comparisons but MOLLI shows substantially tighter limits of agreement than LL. Short and long axis cardiac planes showed similar results for myocardial T1 values for both sequences.