0517
Evaluation of Four T1 Mapping Sequences for Obtaining the Extracellular Volume Fraction in Abdominal Imaging1Radiology and Imaging Sciences, Indiana University School of Medicine, Indianapolis, IN, United States, 2Indiana University School of Medicine, Indianapolis, IN, United States, 3Radiology and Clinical Sciences, Indiana University School of Medicine, Indianapolis, IN, United States, 4MR Application Predevelopment, Siemens Healthcare, Erlangen, Germany, 5MR R&D Collaborations, Siemens Medical Solutions USA, Inc, Chicago, IL, United States, 6Siemens Medical Solutions, USA Inc, Chicago, IL, United States
Synopsis
Many different T1 mapping sequences and extracellular volume (ECV) fraction have proven to be useful tools for evaluation of tissue fibrosis, however their potential has not been explored in abdominal imaging. We evaluated 4 different T1 mapping techniques; Dual-flip angle VIBE (DFA VIBE), MOLLI, SASHA and IR-SNAPSHOT and obtained similar ECV fractions of the liver and pancreas. DFA VIBE has the highest spatial coverage in 1 breath hold but suffers from inhomogeneous T1 in the aortic blood. IR-SNAPSHOT has advantage of not requiring cardiac gating and provides the most homogenous T1 of the blood within the aorta.
INTRODUCTION
Extracellular volume (ECV) fraction, which requires obtaining T1 relaxation times in pre- and post-contrast phases in both the tissue and inflowing arterial blood has been successfully used as a biomarker for tissue fibrosis, but mostly for myocardial imaging. There are several T1 mapping sequences, however there is relatively limited data in the literature assessing their feasibility and potential use in abdominal imaging. Measurement of T1 in aortic blood is challenging due to pulsatile flow, but is important for computation of ECV in abdominal organs. The purpose of this study was to evaluate different sequences for obtaining T1 maps and ECV fraction.METHODS
This prospective study was approved by our institutional review board and informed consent was obtained from the patients. T1 maps were acquired in 22 patients using Dual flip angle VIBE DIXON in-phase (DFA VIBE) (1), MOLLI (2), prototype SASHA (3) and a prototype IR-SNAPSHOT (4) in pre- and 5-minute delayed post-contrast phases on a 1.5T MR scanner (MAGNETOM Avantofit, Siemens Healthcare GmbH, Erlangen, Germany) (Figure 1). Prior to the DFA VIBE, B1+ mapping was performed and its results were used to correct the subsequent T1 maps. For IR-SNAPSHOT, MOLLI and SASHA, a series of T1-weighted images were acquired at various time points following the initial non-selective inversion or saturation pulse. Cardiac gating was used for SASHA and MOLLI to synchronize image acquisition with slow phase of pulsatile flow. Gadobenate dimeglumine was administered in all patients using the standard dose of 0.1 mmol/kg. Regions of interest (ROIs) were drawn on the T1 maps over the aortic lumen, pancreas and liver and used to generate ECV maps using the formula at each pixel:
ECV= [1–hematocrit] x ΔR1organ / ΔR1blood
where ΔR1organ, and ΔR1blood are defined as the change in organ and blood pool relaxivity before and after contrast administration. ECV maps were generated offline using prototype software (MR Arithmetics; Siemens Healthcare, Erlangen, Germany) that also performed non-rigid registration between the pre- and post-contrast T1 maps. Statistical analysis was performed using one-way analysis of variance (ANOVA) and Tukey-Kramer pairwise comparison test. The standard deviation of T1 in aortic blood was used as a measure of robustness of the technique against the effects of pulsatile flow, with higher values indicating poorer performance.
RESULTS
Average age of the patient was 58 and 40% of them were male. In pre-contrast T1 maps, there was statistically significant difference in the standard deviations (SD) of the T1 of the blood obtained with 4 different sequences (p<0.001) (Figure 2a). Pairwise comparisons showed that SD of blood T1 from IR-SNAPSHOT, MOLLI and SASHA were not significantly different from each other, but were significantly lower (p>0.05) than DFA VIBE (Figure 2b). In the post-contrast phase, standard deviations of the blood T1 were similar in all sequences (p=0.13).
There was no statistically significant difference among all sequences for mean ECV fractions of the liver (p=0.08) and pancreas (p=0.43) (Figure 3a). T1 times of the pre-contrast liver were different (p=0.004) however, those for pancreas were similar (p=0.13) (Figure 3b).
DISCUSSION
All imaging techniques were feasible for obtaining ECV maps in the abdomen (Figure 4). DFA VIBE sequence has advantage of scanning of the entire abdomen (64 image slices) in 1 breath hold, as compared to 3 slices obtained with the IR-SNAPSHOT and 1 slice from the SASHA and MOLLI. Disadvantage of the DFA VIBE is inherent sensitivity to pulsatile flow in aortic blood that causes significant variability in T1 measurements. One would expect that variability of blood T1 would lead to unreliable ECV. However, our study showed no statistically significant difference among all sequences for ECV fraction of the liver and pancreas. This can be explained by ECV formula which takes into account of the difference between the post-contrast and pre-contrast relaxivity in the aorta, not the absolute value of the measured T1 (Figure 5). IR-SNAPSHOT has benefit of providing 3 image slices in 1 breath hold, lowest SD of the aortic blood and achieving this without requiring cardiac gating. MOLLI and SASHA techniques were originally developed for myocardial imaging, provide 1 image per breath hold and requires gating.
CONCLUSION
All of the T1 mapping sequences were feasible for obtaining ECV maps in the abdomen. IR-SNAPSHOT, MOLLI and SASHA provided more homogenous T1 in aortic blood compared to the DFA VIBE. While DFA VIBE offers most spatial coverage, its sensitivity to blood-flow can be problematic, although this was not the case in our small cohort. IR-SNAPSHOT provide higher spatial coverage per breath-hold than MOLLI and SASHA and does not require external gating.Acknowledgements
References
1. Cheng HM, Wright GA. Rapid High-Resolution T1 Mapping by Variable Flip Angles: Accurate and Precise Measurements in the Presence of Radiofrequency Field Inhomogeneity. Magn Reson Med 2006;55:566-574
2. R. Deichmann and A. Haase, “Quantification of T1 Values by SNAPSHOT-FLASH NMR Imaging” Journal of Magnetic Resonance 96, 608-612 (1992)
3. Messroghli DR, Radjenovic A, Kozerke S, Higgins DM, Sivananthan MU, Ridgway JP. Modified Look-Locker inversion recovery (MOLLI) for high-resolution T1 mapping of the heart. Magn Reson Med. 2004; 52(1):141–146
4. Chow K, Flewitt JA, Green JD, Pagano JJ, Friedrich MG, Thompson RB. Saturation recovery single-shot acquisition (SASHA) for myocardial T1 mapping. Magn Reson Med. 2013
Figures
Figure 2. Comparison of pre- and post-contrast standard deviations (SD) of the measured T1 of the aortic blood using 4 different sequences.
a) This bar chart compares the pre- and post-contrast SD of the blood T1 within aorta. Significant variability of the DFA VIBE is seen in pre-contrast phase (p<0.001). This variation of the DFA VIBE equalizes in the post-contrast phase (p=0.13).
b) Table lists the mean SD of the 4 techniques in pre- and post-contrast phase. Pair-wise comparison showed that during the pre-contrast phase, DFA VIBE showed significantly (p<0.05) higher SD of the blood T1.
Figure 3. Comparison of the ECV fractions computed in the liver and pancreas using 4 different T1 mapping sequences.
a) There was no statistically significant difference between the mean ECV fractions of the liver (p=0.08) and pancreas (p=0.43). Bar graphics show mean ECV and lines indicate 95% confidence intervals.
b) Mean ECV fractions of the liver and pancreas were not statistically significant among all sequences.
