Multi-shot inversion recovery EPI with SMS excitation for high spatial resolution T1-mapping

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Abstract Submission 


Robert Turner1,2,3, Rosa Sanchez Panchuelo1, Olivier Mougin1, Susan Francis4


1University of Nottingham, Nottingham, United Kingdom, 2University of Amsterdam, Amsterdam, Netherlands, 3MPI for Human Cognitive and Brain Sciences, Leipzig, Germany, 4Univeristy of Nottingham, Nottingham, United Kingdom


Quantitative T1-maps show myelinated cortical layers [1] and can be obtained with high spatial resolution MP2RAGE at 7T [2]. However, such low flip-angle 3D sequences have lower SNR per unit time than the high flip-angle techniques of 2D EPI and GRASE [3], and the point-spread-function (PSF) of T1-maps derived from MP2RAGE is broadened by the long duration of the acquisition window, compared with T1. 2D inversion-recovery EPI provides high SNR per unit time and a shorter echo train length [4]. To enable T1 mapping with higher spatial resolution, we recently proposed an efficient 2D, multi-shot, inversion-recovery EPI (MS-IR-EPI) [5] acquisition, combined with multiple acquisitions with shifted slice orderings [6]. This sequence gives excellent T1-maps which are homogenous across slices, although the required fat suppression pulses somewhat shorten the fitted T1-values due to magnetization transfer effects. Here, we combine MS-IR-EPI with simultaneous multi-slice (SMS) excitation to generate high spatial resolution T1-maps with whole brain coverage at 7 and 3 T, and explore the effect of different fat suppression schemes on the fitted T1-values.


A 2D MS-IR-EPI sequence was implemented at 7T and 3T (Philips Achieva and Ingenia scanner respectively) using a varying number of slice ordering offsets [5,6], each providing a set of equally spaced inversion times (TIs) for each slice, Figure 1A(i). Monte Carlo simulations determined the minimum number of slice offsets needed to obtain homogenous T1-maps for a given TR.
At 7T, 0.5 mm isotropic MS-IR-EPI (TE=20ms, TR=3.2s,12 shots, SENSE=1.5(RL), 48 slices, 4 offsets each with 3 averages, 461s total acquisition time) was compared to MP2RAGE (TR/TE=14/6.4ms, TI1/TI2=680/2080ms, TRshot=3.5s, SENSE=2 (RL), 100 slices coverage, 724s acquisition time). Data were acquired with different levels of fat suppression ('weak', 'medium', 'strong') and with fat suppression pulses applied prior to every other excitation pulse ('alternate') to assess the effect on fitted T1-values. A PSIR-reconstruction [7] of MP2RAGE data was used to generate images proportional to T1.
The SMS MS-IR-EPI sequence provided rapid whole brain coverage. At 7T, 0.7 mm isotropic resolution data were acquired using (i) SMS=2 (TR=6.4, 11 shots, 6 offsets, 423s acquisition time) and (ii) SMS=3 (TR=3.2, 11 shots, 5 offsets,179s acquisition time). Whole-head 0.7 mm isotropic MP2RAGE images were acquired using the UK-7T protocol (TR/TE=6.3/2.6ms, TI1/TI2=725/2150ms, TRshot=3.5s, 375 s acquisition time). At 3T, 0.8 mm isotropic MS-IR-EPI data were acquired using SMS=4 (TR=2.1s, 9 shots, 12 offsets, 360s acquisition).
To compute T1-maps, the polarity of the modulus data was corrected (based on phase) and fitted for T1 and S0: S(TI) = S0[1-2exp(−TI/T1)+exp(-TR/T1)].


To remove fat artefacts at 7T and 3T, the fat suppression pulses needed to be 'medium'/'weak' respectively, giving shorter T1 values than with no fat suppression. Using 'alternate' fat suppression pulses gave high quality maps at 7T and reduced this shortening effect (Fig.1A(iii)). Figure 1B shows that 2D MS-IR-EPI T1-maps (no fat suppression) have minimal image distortion and are sharper than 3D MP2RAGE data. Figure 2A compares the 0.7 mm isotropic MP2RAGE processed image with T1-maps generated from the SMS MS-IR-EPI at 7T, whilst Figure 2B shows a 0.8 mm isotropic 3T T1-map. T1-maps are homogeneous across slices even though each slice derives from different inversion times. The SNR per unit time of the MS-IR-EPI sequences with SMS=3 is superior (Fig.2C) to that of MP2RAGE.
Supporting Image: Figure1_wc.png
Supporting Image: Figure2_wc.png


IR-EPI can be combined with multi-shot and SMS excitation to achieve whole brain coverage with high spatial resolution in short acquisition times. T1-maps generated with the MS-IR-EPI sequence are homogenous across slices, have minimal image distortion and are sharper than the corresponding MP2RAGE images.

Imaging Methods:

Anatomical MRI 1


Cortical Anatomy and Brain Mapping 2


Other - T1-mapping, Quantitative MRI

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Structural MRI

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[1] Stüber C et al (2014). ‘Myelin and iron concentration in the human brain: A quantitative study of MRI contrast’. Neuroimage, 3(Pt 1):95-106
[2] Marques J et al. (2010) ‘MP2RAGE, a self bias-field corrected sequence for improved segmentation and T1- mapping at high field’. Neuroimage. 49(2):1271-81.
[3] Trampel R et al. (2014) ‘Anatomical brain imaging at 7T using two-dimensional GRASE’. Magn Reson Med,72(5):1291-301
[4] Mansfield P et al (1986). ‘Measurement of T1 by echo-planar imaging and the construction of computer-generated images’. Phys Med Biol. 31(2):113-24.
[5] Sanchez Panchuelo R et al (2018) ‘A 2D multi-shot inversion recovery EPI (MS-IR-EPI) sequence for high spatial resolution T1-mapping at 7T’. Proceedings ISMRM, 60.
[6] Ordidge R et al (1990) ‘High-speed multislice T1-mapping using inversion-recovery echo-planar imaging’. MRM,16: 238-256
[7] Mougin O et al (2016) ‘Imaging grey matter with concomitant null point imaging from the phase sensitive inversion recovery sequence’. MRM, 76:1512-1516.