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A Rapid, Whole-brain Quantification of T1 Relaxation Time Using Submillimetre Inversion-recovery EPI at 7T
Seong Dae Yun1 and N. Jon Shah1,2,3,4
1Institute of Neuroscience and Medicine 4, INM-4, Forschungszentrum Juelich, Juelich, Germany, 2Institute of Neuroscience and Medicine 11, INM-11, JARA, Forschungszentrum Juelich, Juelich, Germany, 3JARA - BRAIN - Translational Medicine, Aachen, Germany, 4Department of Neurology, RWTH Aachen University, Aachen, Germany

Synopsis

Keywords: Contrast Mechanisms, Quantitative Imaging, 7T, Inversion-recovery EPI, Rapid T1 mapping, Submillimetre and Whole-brain

Knowledge of T1 relaxation time is of great interest for clinical diagnosis or MRI sequence optimisation. For the quantitative measurement of T1, the inversion-recovery method is widely used due to its relatively good accuracy or tolerance to B1 inhomogeneity. However, 2D multi-slice- or 3D segmentation-based readout methods often preclude the effect of different signal recovery modulation depending on slice locations. Therefore, this work presents a single-slice-based inversion-recovery 2D EPI method, combined with TR-external EPI phase correction at 7T, to provide rapid, whole-brain T1 mapping with a voxel size of 0.73 × 0.73 mm2.

Introduction

The quantitative measurement of T1 relaxation time is of great interest in the MR community as it can provide valuable information on physiological state for clinical diagnosis or physical tissue parameters to optimise imaging sequences.1-4 Previously, various methods have been presented for T1 mapping, and of these, the inversion-recovery method is considered a gold standard method, as it can provide an enhanced accuracy or tolerance to B1 inhomogeneity when compared to other types of methods (e.g. variable flip angle).1,5 However, the approach of tracking the dynamic recovery of longitudinal magnetisation after an inversion inherently requires a relatively long acquisition time, which can significantly hinder the mapping of whole-brain T1 with high spatial resolution. To effectively shorten its acquisition time, an inversion-recovery method, the readout of which is implemented with echo-planar-imaging, namely IR-EPI, was demonstrated using the 2D multi-slice-based or 3D segmented EPI regime.1,5,6 However, the acquisition of multiple slices in successive excitation loops within the same T1 recovery period can impose a different signal recovery modulation for each slice. That is, the earlier time point during the recovery may not be effectively captured in the slice acquired at a later time point. Specifically, this problem becomes more severe as the number of slices increases for larger brain coverage. Therefore, this work presents a single-slice-based IR-EPI method (i.e. one inversion-recovery per each slice) where the number of required inversion-recovery loops is reduced by means of the multiband acceleration, resulting in a much shorter acquisition time.7,8 Moreover, the EPI readout was further configured with the TR-external EPI phase correction scheme (TRx) to substantially reduce the minimum TE required for submillimetre resolution EPI.9,10

Methods

Figure 1 shows a diagram of the proposed imaging sequence. The inversion-recovery loop is repeated as many times as the number of slices, which can be reduced by the multiband acceleration. Each inversion-recovery kernel starts with combined saturation and inversion preparation pulses and subsequently performs dynamic sampling only for a single slice or a group of slices in case of multiband acceleration. This strategy ensures data at each slice location have identical inversion time points. Here, each temporal point is acquired with 2D EPI combined with the TRx scheme. The feasibility of using the multiband and the TRx techniques for IR-EPI was verified by comparing the signals from the following four sequences: 1) IR-EPI, 2) IR-Multiband EPI (IR-MEPI), 3) TRx-IR-EPI and 4) TRx-IR-MEPI. For this comparison, low-resolution data sets from a uniform spherical phantom were acquired without any acceleration condition (see Fig. 2a). Next, submillimetre-resolution protocols were configured using IR-MEPI and TRx-IR-MEPI, each of which was optimised to offer the highest possible spatial resolutions (0.97 × 0.97 mm2 and 0.73 × 0.73 mm2, respectively) under the imaging condition shown in Fig. 2b. Data sets from a healthy volunteer screened with a standard safety procedure were acquired using the two submillimetre protocols. All experiments in this work were performed at a Siemens Magnetom Terra 7T scanner with a 1-Tx/32-Rx head coil.

Results

Figures 3a-c depict the temporal signal evolution of the phantom data obtained from the four different sequences. This result suggests that the use of the multiband or TRx techniques has no significant impact on the behaviour of the signal recovery. The very small difference observed in the above plots was also numerically verified by computing the mean signal differences (see Fig. 3d). Figure 4 shows T1 maps at a representative slice location obtained from IR-MEPI (0.97 × 0.97 mm2) and TRx-IR-MEPI (0.73 × 0.73 mm2). This result demonstrates the impact of the superior spatial resolution achieved in TRx-IR-MEPI, resulting in a clearer delineation of the T1 values around the cortical ribbon. Figure 5 shows the T1 maps for all the slices (75) from TRx-IR-MEPI, and depicts reliable T1 estimation for all slice locations. The mean T1 values for whole-brain WM/GM were computed as 1200.72/1640.30 ms for IR-MEPI and 1140.20/1617.19 ms for TRx-IR-MEPI, both of which are in good agreement with those reported in previous submillimetre T1 mapping studies obtained with MP2RAGE (1140/1653)5, IR-Turbo-spin-echo (1017/1654)11 or IR-EPI (1170/1783)5 methods at 7T.

Discussion and conclusions

This work demonstrates rapid, whole-brain T1 mapping using TRx-IR-MEPI at 7T. The method was able to achieve a relatively large matrix size, consequently resulting in a substantial improvement in the spatial resolution of T1 maps when compared to IR-MEPI. The achieved spatial resolution suggests its potential use for cortical depth-dependent T1 quantification. The TE employed here was 17.6 ms, which was also similar to the TEs (16~20 ms) employed in the previous submillimetre IR-EPI study.5 However, in contrast, our method is based on single-shot EPI, which can mitigate increased aliasing artefacts, physiological noise or subject motion stemming from the multiple shots typically seen in 2D multi-shot or 3D segmented EPI. For the submillimetre-protocols tested here, the required time for the entire T1 recovery data (0.73 × 0.73 mm2 × 75 slices × 60 time-points) was only 3.5 minutes, which was mainly the result of the use of the multiband acceleration. In future works, a study with more subjects including the inversion efficiency measurement12,13 is planned for more quantitative analysis on TRx-IR-MEPI with improved T1 mapping accuracy.

Acknowledgements

No acknowledgement found.

References

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  7. Feinberg DA, Moeller S, Smith SM, Auerbach E, Ramanna S, Gunther M, et al. Multiplexed echo planar imaging for sub-second whole brain FMRI and fast diffusion imaging. PLoS One. 2010;5(12):e15710.
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  12. Raman, F.S., Kawel-Boehm, N., Gai, N. et al. Modified look-locker inversion recovery T1 mapping indices: assessment of accuracy and reproducibility between magnetic resonance scanners. J Cardiovasc Magn Reson 15, 64 (2013). https://doi.org/10.1186/1532-429X-15-64.
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Figures

Figure 1. Sequence diagram of proposed IR-EPI with TR-external EPI phase correction (TRx-IR-EPI). The inversion-recovery loop is repeated as many times as the number of slices (i.e. the bracket marked in red) and can be reduced by a factor of the multiband acceleration rate. The saturation and inversion preparation pulses are the adiabatic, non-selective 90°-τ-180° magnetisation preparation (tau = 2.4 s). Each temporal point is sampled with TRx-EPI where the navigator echoes are acquired in a separate loop to avoid the increase of the minimum TE required (αPC = 9°).

Figure 2. (a) A low-resolution (voxel: 3.28 × 3.28 mm2) protocol to check the feasibility of using the multiband technique and the TRx scheme for IR-EPI. (b) Submillimetre-resolution protocols configured with IR-MEPI (voxel: 0.97 × 0.97 mm2) and TRx-IR-MEPI (voxel: 0.73 × 0.73 mm2) where each protocol was optimised to offer its highest possible spatial resolution under the imaging conditions shown in the table.

Figure 3. Comparison of the temporal signal evolution from the four different inversion-recovery sequences for the following pairs: (a) IR-EPI and IR-MEPI, (b) TRx-IR-EPI and TRx-IR-MEPI and (c) IR-EPI and TRx-IR-EPI. The signal curve was obtained by taking the mean value of the phantom data at all slice locations. (d) The mean signal difference of the signal curves is plotted in the image panels from a to c. The results shown here suggest the feasibility of the multiband technique and TRx scheme in the inversion-recovery EPI method.

Figure 4. Submillimetre T1 maps at a representative slice location obtained from IR-MEPI and TRx-IR-MEPI. The enlarged depiction of the selected ROI (see below each slice) demonstrates the use of a superior spatial resolution in TRx-IR-MEPI (0.73 × 0.73 mm2) in comparison to IR-MEPI (0.97 × 0.97 mm2). A clearer delineation of T1 values around the cortical ribbon can be observed in TRx-IR-MEPI.

Figure 5. Whole-brain T1 maps (0.73 × 0.73 mm2 × 75 slices) obtained from TRx-IR-MEPI. The acquisition time required for the T1 recovery data with 60 time points was 3.5 minutes.

Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)
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DOI: https://doi.org/10.58530/2023/1361