Diffuse myocardial fibrosis occurs in various cardiomyopathies and can be indirectly assessed with blood and myocardial T1 mapping at baseline and after gadolinium administration. The widely used MOdified Look-Locker Inversion-recovery (MOLLI) [1] sequence is known to underestimate myocardial T1 at higher heart rates, but its dependence on T2 has not been explored. We investigate MOLLI’s T1 accuracy in phantoms and confirm with simulations and in-vivo studies. T1 values are further compared with a saturation-recovery T1 mapping sequence [2].
Methods
Phantoms
14 NiCl2 agarose phantoms with a broad range of T1 and T2 values were imaged with a gold-standard inversion-recovery spin-echo (IR-SE) sequence, MOLLI, and a new SAturation-recovery single-SHot Acquisition (SASHA) technique (Siemens Avanto 1.5T). IR-SE: 16 TIs 100-5000ms, TE=11ms, TR>5s, 90° flip. MOLLI: 2 inversion sets of 3 and 5 images, 75% partial Fourier, TImin=110ms with 80ms increment, 35° flip, TE/TR=1.3/2.9ms, simulated HR=60bpm. SASHA: single-shot SSFP images from 10 consecutive heartbeats with incremented TI spanning the RR interval in the last 9 images (no saturation in the first image), 70° flip, TE/TR=1.3/2.6ms, full k-space, simulated HR=60bpm. T2: spin-echo (SE) with 7 TEs 11-200ms. Simulations: Bloch equation simulations of MOLLI and SASHA were performed in MATLAB using actual acquisition and physiology parameters and SE measured T1 and T2 values.
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In-Vivo
For 10 healthy volunteers (5 male, 28.8±6.6yrs), blood and myocardial T1s were measured using MOLLI and SASHA (parameters as above) in a mid-ventricular short-axis slice at baseline and 20 minutes following 0.1mmol/kg Magnevist.
Results
In blood-like phantoms with long T2 (179-196ms), SASHA and MOLLI T1s agree well with IR-SE (0.7±0.5% and 2.2±1.8% absolute difference respectively), while shorter T2 (46-76ms) tissue-like phantoms have greater underestimation with MOLLI (8.4±3.5%) than SASHA (0.9±0.6%) (Fig. 1). MOLLI simulations predict underestimated T1s, with 1.3±0.9% absolute difference from observed values (vertical lines, Fig. 1). SASHA simulations also agree well with observations (0.8±0.5%, not shown). In healthy volunteers (63.3±8.4bpm), MOLLI T1s also show greater underestimation compared to SASHA in tissue than blood, although the difference is larger than observed in phantoms or predicted by simulations in all cases (Table 1).
Figure 1
Error in MOLLI and SASHA T1 values compared to gold standard inversion-recovery spin-echo (IR-SE) in "tissue-like" phantoms (T1s 339-1145ms) and "blood-like" phantoms (T1s 275-1452ms). A Bloch equation simulation of MOLLI using actual acquisition and physiology timing parameters is also shown, with the difference between simulated and actual results indicated with a vertical line for each phantom.
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Table 1
Comparison of MOLLI and SASHA T1 values in 10 healthy volunteers prior to and 20 minutes following 0.1 mmol/kg Magnevist. All comparisons between MOLLI and SASHA are significant (p<0.01, two-tailed, paired Student’s t-test).
T1 [ms]
Myocardium (mean±std)
Blood (mean±std)
Baseline
Post Gd (20 min)
Baseline
Post Gd (20 min)
MOLLI
935.5±24.9
614.4±33.8
1514.1±107.5
524.9±55.2
SASHA
1175.2±27.6
752.9±48.2
1687.4±85.8
542.6±56.3
Conclusions
MOLLI significantly underestimates T1s in shorter T2 tissue-like phantoms but less so in longer T2 blood-like phantoms, as predicted by simulations. Similar trends were observed in-vivo with MOLLI, although with greater T1 underestimation (compared to SASHA) than predicted. SASHA had good agreement with IR-SE T1 phantom measurements and simulations and can be acquired in less time than MOLLI.
Funding
N/A
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Acknowledgements
We acknowledge the work of Dr. Andreas Greiser in programming the MOLLI sequence used.
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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