Myocardial T1- and ECV- Mapping at 3 Tesla using the Saturation-Recovery Techniques SASHA and SAPPHIRE
Nadja M Meßner1,2, Sebastian Weingärtner1,3,4, Johannes Budjan5, Dirk Loßnitzer6, Uwe Mattler5, Theano Papavassiliu2,6, Lothar R Schad1, and Frank G Zöllner1

1Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany, 2DZHK (German Centre for Cardiovascular Research), partner site Heidelberg/Mannheim, Germany, 3Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, United States, 4Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, 5Institute of Clinical Radiology and Nuclear Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany, 61st Department of Medicine Cardiology, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany

Synopsis

Myocardial T1- and ECV-mapping for the detection of fibrosis is commonly performed at 1.5T with inversion-recovery (IR) techniques such as MOLLI.

As an alternative, we studied the robustness and precision of the saturation-recovery (SR) T1-mapping techniques SAPPHIRE and SASHA at 3T in 20 healthy volunteers. The resulting T1- and ECV- reference values for SR T1-mapping were 1578±42ms/0.30±0.03 (SAPPHIRE) and 1523±46ms/0.31±0.03 (SASHA), revealing the underestimation of T1-times by MOLLI to be approximately 20-29%.

Therefore, we suggest SR T1-mapping with its high accuracy, low precision-loss, and good inter-subject variability as a valuable alternative to IR T1-mapping at 3T.

Background

Quantitative mapping of myocardial T1 and the extracellular volume fraction (ECV) shows promising diagnostic value in cardiomyopathies such as diffuse fibrosis [1]. The most commonly used method, the modified Look-Locker Inversion-recovery (MOLLI) sequence [2], is known to underestimate myocardial T1 by ~20% at 1.5T [3] and is prone to several factors confounding quantification accuracy.

To overcome these problems, saturation-recovery (SR) based myocardial T1-mapping methods like the SAturation-recovery single-SHot Acquisition (SASHA) [4] and a combined SR and IR method, the Saturation Pulse Prepared Heart-rate independent Inversion-REcovery (SAPPHIRE) magnetization preparation, have been introduced [5].

Recently, several studies took advantage of increased imaging SNR at 3T to achieve increased T1-map quality and quantification precision compared to 1.5T.

Purpose

In this study we sought to investigate the feasibility and robustness of saturation-recovery (SR) myocardial T1-mapping at 3T and to establish accurate reference values for native T1-times and ECV of the healthy myocardium.

Methods

SAPPHIRE and SASHA T1-mapping were compared with the MOLLI sequence on a 3T MRI scanner (Magnetom Skyra; Siemens Healthcare, Erlangen, Germany) with a 30 channel receiver coil array. In all three sequences, imaging readout was performed with an ECG-triggered single-shot bSSFP readout and the following parameters: TR/TE/α=2.6ms/1.0ms/35°, in-plane resolution=1.7×1.7mm2, slice-thickness=6mm, field-of-view=440×375mm2, bandwidth=1085Hz/px, #k-space lines=139, linear profile ordering, startup-pulses=5 Kaiser-Bessel, GRAPPA-factor=2. Magnetization saturation was achieved using a composite “Water suppression Enhanced through T1-effects” (WET) [6] saturation module, magnetization inversion with an adiabatic full passage tan/tanh pulse [7]. The 5(3)3 MOLLI scheme was employed for native and the 4(1)3(1)2(1) scheme for post-contrast T1-mapping. Pulse-efficacy, accuracy and precision of the methods were studied in phantom.

20 healthy subjects (27±5 years,10 M) were scanned for native and post-contrast T1-times, and regional ECV values were calculated by involving blood hematocrit. The in-vivo scan protocol is depicted in Fig.1.

T1-times and ECV values were compared between the sequences using ANOVA. Subjective image quality and susceptibility artifact rating, as assessed by two blinded readers, and in-vivo precision were statistically analyzed with Kruskal-Wallis- and Mann-Whitney U-tests. Inter-subject variability was compared using Bartlett- and F-tests.

Results

WET saturation tests in phantom resulted in an average saturation efficacy >99% across a broad T1-range. The SR methods showed excellent accuracy in phantom (<3.9% deviation from the spin-echo reference). However, intra-compartment variability of T1-time was 29% and 50% lower using MOLLI compared with SAPPHIRE and SASHA, respectively.

Example in-vivo T1-maps of two subjects are shown in Fig. 2. Fig. 3 shows the segment-wise analysis of T1 and ECV. Averaged over all volunteers and segments, SAPPHIRE and SASHA yielded significantly higher T1-times (SAPPHIRE: 1578±42 ms, SASHA: 1523±46 ms), ECV values (SAPPHIRE: 0.30±0.03, SASHA: 0.31±0.03) and T1-time variation (SAPPHIRE: 60.1±8.7 ms, SASHA: 70.0±9.3 ms) compared with MOLLI (T1: 1181±47 ms, ECV: 0.27±0.03, precision: 53.7±8.1 ms). No significant difference was found in the inter-subject variability of T1-times or ECV values between the three methods (T1: p=0.90, ECV: p=0.78). The average quality and artifact scores of the T1-mapping methods were: MOLLI: 3.4/3.6, SAPPHIRE: 3.1/3.4, SASHA: 2.9/3.2; 1: poor - 4: excellent, as displayed in Fig. 4.

Discussion

For all methods, native T1-maps revealed a robust image quality throughout the study. The SR image quality rating was slightly lower and artifact incidence higher compared with MOLLI. However, SR showed significantly better quantification accuracy. SR results in 15-30% lower in-vivo precision compared with MOLLI. Previous studies report differences up to 150% at 1.5T [3], rendering SR at 3T a more competitive alternative to IR. Accordingly, no significant difference was found in the inter-subject variability among the three methods.

ECV-values were increased compared with reported values at 1.5T [3], potentially due to inaccurate assessment of blood pool T1-values, as caused by inflowing blood and imperfect saturation. Imaging at 3T using bSSFP yielded high SNR, but led to off-resonance artifacts and SAR limitations. Frequency scouts were successfully used to minimize off-resonance effects. For further improved image quality, SAR reduction is warranted by excitation pulse optimization.

Conclusion

Given the high accuracy, the low level of precision loss, and the good inter-subject variability, SR T1-mapping can be a considered a valuable alternative to MOLLI T1-mapping at 3T.

Acknowledgements

No acknowledgement found.

References

[1] Burt et al., Radiographics 2014 [2] Messroghli et al., MRM 2004 [3] Roujol et al., Radiology 2014 [4] Chow et al., MRM 2013 [5] Weingärtner et al., MRM 2014 [6] Ogg et al., JMRB 1994 [7] Kellman et al., MRM 2014

Figures

Figure 1: Imaging schedule including T1-mapping before (Pre) and after Gd-contrast (15min=Post1; 25min=Post2). MOLLI, SAPPHIRE and SASHA were performed in three short-axis slices. For post-contrast acquisition, same slices were grouped to minimize effects of contrast-washout on inter-sequence comparison. The sequence order within a group and amongst slice groups was randomized.


Figure 2: T1-maps of two healthy subjects (#1: 27y, m; #2: 24y, f), acquired with all three sequences before contrast injection and 25min afterwards in a short-axis mid-ventricular slice. Homogeneous T1-values, the absence of artifacts, and a sharp delineation of the myocardium yield visually high T1-map quality for all methods.

Figure 3: Native T1-times (top), precision (middle) and ECV-values (bottom) for all methods, depicted in bullseye plots (AHA-16-segment model) in three slices (A=apical, M=mid-ventricular, B=basal), averaged over 20 subjects. The average across all myocardial segments is shown in the center of each bullseye, the slice averages in the boxes below.

Figure 4: Pie-charts illustrating the distribution and mean of quality/artifact scorings across all T1-maps by two blinded readers. 81% of images were scored with at least “good” quality, with MOLLI having the highest average and lowest artifact scoring. SAPPHIRE yielded higher average quality and similar artifact scores compared with SASHA.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3152