Introduction

Cortical parcellation for unambiguous in-vivo identification of distinct cortical areas has been performed using high resolution MRI that provides some details of myeloarchitecture [1-3]. Quantitative T₁ maps provide evidence of heavily myelinated layers [4] and can easily be obtained with high spatial resolution at 7T [5]. However, the low flip-angle 3D FLASH-type sequences which are commonly used for T₁ mapping have poorer SNR per unit time compared with techniques such as EPI and GRASE that use high flip angles [6]. Further, when inversion pulses are efficiently included in 3D sequences, the point spread function (PSF) becomes broad, because the acquisition window is wide compared with T₁. An alternative approach is to use 2D inversion-recovery EPI which has high SNR per unit time and a short echo train length compared with T₁ [7]. However, if each slice in a multislice brain volume is required to have the same inversion time, the scan time becomes long, especially if segmentation is added to improve spatial resolution, because the longitudinal magnetization must recover after each inversion pulse. Removing these requirements can greatly accelerate data acquisition, but entails the computation of T₁-maps for each slice using different inversion times. Here we develop an efficient 2D, multi-shot, inversion-recovery EPI (MS-IR-EPI) acquisition combining separately excited k-space segments after each inversion pulse and shifted slice ordering to provide high SNR per unit time.

Methods

A 2D MS-IR-EPI sequence 0.5x0.5mm² in-plane resolution (TE=20ms, TR=3.2s, EPI-factor=17, 12 shots, field-of-view (FOV)=180(AP)x154(RL), SENSE=1.5(RL), 38.4s acquisition time) was evaluated at 7T for T₁-mapping. Data sets consisting of 24 axial slices (1.5mm slice thickness) were acquired with varying slice ordering schemes, thus providing a number of equally spaced inversion times for each slice (Fig.1). Data was acquired at a number of slice offsets=[0, 4, 5, 6, 8, 10, 12, 15, 16, 18, 20] and used to reconstruct T₁-maps based on 2, 3, 4, 5 or 6 slice offsets and thus range of inversion times (TI). Data sets were acquired with and without fat suppression. In addition, a single slice dataset was acquired using standard IR-EPI at 10 TIs=[100, 200, 400, 600, 900, 1100, 1300, 1600, 2100, 3100], TR=5s. 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)]. For the MS-IR-EPI, Monte Carlo simulations were performed to evaluate the accuracy and variance of the fitted T₁s with different numbers of TIs and different level of noise (1024 repeats). High spatial resolution (0.5 mm isotropic) data were collected on two subjects using a MS-IR-EPI (48 slices, 4 TIs/slice offsets each with 3 averages, 461s total acquisition time) and MP2RAGE sequence (TR/TE=14/6.4ms, TI₁/TI₂=680/2080ms, TR_shot=3.5s, SENSE=2 (RL), 724s acquisition time) with matched FOV but 100 slices coverage. A PSIR-reconstruction [8] of the MP2RAGE data was used to generate images proportional to T₁.

Results and discussion

T₁-maps from the MS-IR-EPI sequence were homogeneous across slices, despite each slice being fit with different inversion times (Fig.2A), and were independent of the number of TIs when more than 2 inversion times were used in the fit (Fig.2B). Fitted T₁-values were shorter than those obtained with a single slice standard IR-EPI sequence (Fig.2B). This difference was reduced when collecting data with no fat suppression, suggesting this discrepancy is likely due to magnetization transfer effects [9]. Future work will explore the use of different fat suppression techniques while reducing the impact on fitted T₁-values. A Monte Carlo simulation showed that the accuracy of the fitted T₁ is independent of the number of TIs used in the fit whilst the standard deviation decreases as the number of TIs increases (Fig.3A), and as the SNR increases (data not shown). Although the fitted T₁ remains relative constant across slices (Fig.2B) there is variation across slices when using 2 and 3 TIs for WM (Fig.3B) and 2 TIs for GM (Fig.3C). Figures 4 and 5 compare the 0.5mm isotropic acquisition T₁-maps generated from the MS-IR-EPI (no fat suppression) with MP2RAGE processed images. Both methods yield similar SNR, but note that while the MS-IR-EPI is based on a true T1-fit, MP2RAGE is currently proportional to T1, and future work will generate a look-up table to estimate T₁ [5]. Figure 5 shows that the 2D MS-IR-EPI T₁-maps have minimal image distortion (B) and are sharper than 3D MP2RAGE maps (Fig.5C).

Conclusion

High quality T1-maps can be generated using a high resolution 2D MS-IR-EPI sequence. Future work will combine the MS-IR-EPI scheme with simultaneous multi-slice excitation (SMS) to achieve whole brain coverage in short acquisition times.

Acknowledgements

This work was funded by a MRC grant MR/M022722/1 and a Leverhulme fellowship.

References

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