Background
The recent introduction of rapid parameter mapping into cardiovascular magnetic resonance (CMR) imaging provides the invaluable ability for noninvasive quantitative myocardial tissue characterization. The quantification of the native longitudinal magnetization recovery time as a spatially resolved map (native T
1-mapping) shows promising prognostic and diagnostic value in various cardiomyopathies [
1]. The combination with post-contrast T
1-time measurements allows for the estimation of the extracellular volume fraction (ECV), which reflects fibrotic remodeling [
2], a common endpoint of many pathological cardiac conditions [
3].
A number of cardiac T
1-mapping methods have been proposed, each offering a distinct profile of advantages. The modified Look-Locker inversion recovery (MOLLI) sequence [
4] and variations thereof, like the shortened MOLLI (ShMOLLI) [
5], are commonly used for myocardial T
1-mapping. However, confounding factors to the method’s quantification accuracy including heart rate [
6], T
2 relaxation time [
7], and magnetization transfer [
8] lead to underestimation of the T
1-time of the healthy myocardium by ~20 % at 1.5T [
9,
10].
Alternatively, saturation-recovery (SR) based myocardial T
1-mapping methods have been proposed [
11] and were recently revisited by the SAturation-recovery single-SHot Acquisition (SASHA) sequence [
12]. To increase the low dynamic range in SR T
1-mapping, the hybrid sequence for Saturation Pulse Prepared Heart-rate independent Inversion-REcovery (SAPPHIRE) T
1-mapping was introduced, using a combination of saturation and inversion pulses for magnetization preparation [
6]. While SASHA and SAPPHIRE result in excellent accuracy, the sequences still suffer from reduced precision in assessing T
1-times compared with MOLLI, as previously shown at 1.5T [
9].
The application of inversion-recovery T
1-mapping at 3T has recently received increasing interest. Multiple studies have shown promising T
1-map quality and improved quantification precision, due to the increased imaging Signal-to-Noise ratio (SNR) at 3T [
5,
13,
14]. Thus, in this work we sought to study the visual quality and precision of SR T
1-mapping and to establish accurate reference values for native T
1-times and ECV-values of the healthy myocardium at 3T.
Discussion
In this study, we assessed reference values and in-vivo precision of SR T1-mapping at 3T in comparison with MOLLI. SR T1-mapping provided robust image quality throughout the study. MOLLI T1-maps were shown to consistently provide the highest image quality rating and lowest artifact incidence. However, significantly better ex-vivo accuracy was confirmed for SR methods for the trade-off against a slight reduction of in-vivo precision. No significant difference was found in the inter-subject variability and the inter-and intra-observer variability among the three methods.
Native T
1-times of the human myocardium using SR T
1-mapping were found to be around 1550 ms. This reveals a field strength dispersion of approximately 30 % compared with 1.5T (1210–1220 ms [
9]), which is in good agreement with reported literature values for cardiac tissue of animals [
23,
24]. MOLLI T
1-times from our own findings and previous reports at 3T (1166 ms [
14]) demonstrate a significant underestimation of about 20–30 % compared with the present results of SR T
1-mapping. This underestimation, as confirmed by the phantom study, indicates decreased in-vivo accuracy of MOLLI. SASHA T
1-mapping was previously reported to have about 150 % higher in-vivo variability than MOLLI at 1.5T [
10]. Our results demonstrate that the loss in precision when using SR over MOLLI is drastically reduced compared with 1.5T. The present results indicate that at 3T, MOLLI remains to provide higher visual image quality than SR methods. However, the high ex-vivo accuracy, the low level of precision-loss, and the good inter-subject variability, indicate only a small gap to SR T
1-mapping. Hence, SR methods at 3T provide a valuable option for trading-off increased quantification accuracy against a reduction of overall image-quality.
Alternative T
1-map reconstructions have been proposed for SR T
1-mapping, to improve precision albeit at the cost of reduced accuracy and increased sensitivity of the T
1-time to the choice of scan parameters. A two-parameter fit for SASHA T
1-mapping was recently proposed [
10] and initial results on an extension using a variable flip-angle scheme for the bSSFP imaging readout to minimize the loss in accuracy were presented [
25]. Two parameter fitting has also been used for SAPPHIRE post-contrast T
1-mapping [
6]. However, imperfect inversion efficiency might impair the accuracy of SAPPHIRE, when using a two-parameter fit for native T
1-mapping. The use of a predetermined correction factor for incomplete inversion, as previously proposed [
17], might be warranted for this application.
The reported reference ECV values for MOLLI (~0.26) and SR T
1-mapping (~0.21) obtained in this study are in good agreement with previous literature. For MOLLI, ECV-values between 0.25 and 0.27 have been reported at 1.5T [
9,
15,
26] and between 0.26 and 0.28 at 3T [
26‐
29]. Furthermore, the slight increase in ECV-values between the two post-contrast time points has been previously observed with MOLLI at 3T [
29], and the ECV deviation between the two time points is in agreement with previous reports (0.258–0.272 for times between 10 and 25 min [
28]. Close agreement of SAPPHIRE ECV-values are obtained with a previous study at 1.5T (ECV: 0.20 [
9]). SASHA ECV-values were reported as 0.18 [
9], 0.21 [
30] and 0.22 [
31,
32] in healthy subjects at 1.5T. The close agreement of these values with our results, as well as with ECV-values obtained with SR T
1-mapping in an animal study at 3T (AIR: 0.20–0.21 [
33]) proves high cross field-strength consistency for SR based ECV-measures.
Despite the higher precision of MOLLI compared to SR T
1-mapping, previous studies did not report significant differences in the scan-rescan reproducibility [
9,
26]. To add on this, our results show no significant difference in the inter- or intra-observer variability between the methods either. All three methods showed consistency with ICCs > 0.90, which is considered excellent for diagnostic tools [
34]. The values of observer variability characteristics obtained in this study are well in line with previous reports [
27,
35‐
38]. However, some studies from specialized centers achieved consistently higher inter- and intra-observer variability, with ICCs > 0.99 [
27,
29,
39]. This difference might be explained by the limited clinical experience of our readers. Therefore, extensive observer training and an extensive common learning phase for both readers seems to be required to achieve optimal reproducibility results in T
1-mapping.
Imaging at 3T using bSSFP has considerable challenges compared with 1.5T. Off-resonance artifacts are commonly induced by magnetic susceptibilities at tissue interfaces, e.g. epicardium-lung interface. In this study, frequency scouts were used to minimize off-resonance artifacts. However, careful volumetric shimming is still essential at 3T to ensure robust image quality. Also, the rapid imaging readout reaches specific absorption rate (SAR) limitations at 3T. As SR T
1-mapping methods were shown to be independent of the imaging flip-angle [
12], improved imaging SNR could potentially be achieved using optimized excitation pulses with higher flip-angles and low SAR, for the trade-off against suboptimal slice profiles.
Non-rigid motion correction algorithms, as used in this study, are dependent on strong contrast within the area of interest [
40]. Hence, MoCo was more effective for MOLLI than for the SR methods. Tailored motion correction algorithms might be required if a further reduction of residual motion in the SR imaging series is necessary.
This study has several limitations. Due to the lack of feasible methods for the assessment of “true” T
1-times in the myocardium, no direct evidence of the in-vivo accuracy of SR methods can be given. Instead, phantom accuracy was used as an indicator of in-vivo accuracy. Evaluation of the sequence characteristics was restricted to accuracy and precision, specifically no inter- or intra-session reproducibility was considered in this study. A tightly controlled cohort of young healthy volunteers was recruited for the study, in order to obtain reproducible reference values of the healthy myocardium that are not affected by potential age-related fibrosis in the muscle. As the T
1-time of the myocardium is known to be age and sex dependent [
41], cohorts that are age/sex matched to the particular patient population are to be assessed if more specific T
1-reference values with reduced intra-cohort variability are required.
Acknowledgements
We thank Mehmet Akçakaya for editorial comments and Svetlana Hetjens for statistical consulting.