Synopsis

In this work, we propose a novel motion corrected Phase Sensitive Inversion Recovery (PSIR) method with integrated T₁ mapping derived from MP2RAGE acquisition. Motion correction is achieved using PROMO (PROspective MOtion correction), as well as Optimal Weighted Average (OWA) combination of multichannel data. This proposed method will be useful in obtaining high quality T₁ images for children and other subjects who are prone to move during scans.

INTRODUCTION

T₁ is an essential component of a volumetric brain exam, and motion remains a major cause of image quality failures. The Magnetization-Prepared 2 Rapid Acquisition Gradient Echoes (MP2RAGE) sequence has been introduced to obtain bias-field corrected T₁-weighted images with T₁ maps at high field(1,2). Recently, a new method was proposed to obtain PSIR images from MP2RAGE sequences(3):

$$$PSIR=\frac{\pm\left|SI_{1}\right|}{\left|SI_{1}\right|+\left|SI_{2}\right|}$$$ [1]

The polarity of $$$\left|SI_{1}\right|$$$ was defined as being negative if the phase change between signal at TI₁ ($$$SI_{1}$$$) and signal at TI₂ ($$$SI_{2}$$$) lay in the range $$$\pi/2$$$ and $$$3\pi/2$$$.

PROMO is a method to use navigator data to estimate and correct for subject motion in every TR, with little scan time penalty. PROMO significantly reduces motion artifacts, which improves brain image quality for morphometry and cortical surface reconstruction (4,5). OWA has been used to combine multi-channel image to further reduce noise and image degradation due to motion(6)(7). In the proposed work, both PROMO and OWA were used to optimize the image for subjects with motion.

METHODS

All experiments were performed on a 3T GE Discovery MR750 scanner on two healthy volunteers after obtaining informed consent. Two different receive coils were used to evaluate the robustness of the channel combination method.

The first volunteer was scanned using a GE HNS 12-Channel head coil under three conditions: 1) Head motion without PROMO; 2) Head motion with PROMO; 3) No intentional motion. Prior to scan, the subject was trained in a MRI simulator to achieve consistent head-nodding motion upon auditory cue: nod backwards and then return to original position; cues every 1 minute. Approximately 20° of “pitch” rotation was consistently achieved after training. The MP2RAGE pulse sequence (as shown in Figure 1(a)) acquisition parameters were: TR_MP2RAGE=5s, TR=5ms, TI₁/TI₂= 0.7s/2s, $$$\alpha_1=7$$$°, $$$\alpha_2=5$$$°, parallel imaging (ARC) acceleration factor = 2. The acquisition matrix size is 256x128 with 172 sagittal slices, scan duration=6.5 minutes.

The second volunteer was scanned using a GE 32-channel head coil using the same sequence settings except: TR=7.5ms. The acquisition matrix size is 512x266 with 64 axial slices, final reconstructed voxel size = 0.45x0.45x3mm³, scan duration=9 minutes.

The complex images for each coil $$$({SI_1})_{coil_i}$$$ and $$$({SI_2})_{coil_i}$$$ were obtained during the data acquisition. The $$$({PSIR})_{coil_i}$$$ was obtained from Equation [1]. The coil combined PSIR was calculated with OWA. The ‘PSIR versus T₁’ plot was calculated based on the equation for signal $$$SI_{1}$$$ and $$$SI_{2}$$$ (1), which is used as a lookup table to obtain T₁ maps. Figure 1(b) shows the flowchart of this processing.

RESULTS

Figure 2 compared the $$$SI_{2}$$$ images, the corresponding PSIR and T₁ map for subject with and without motion. As shown in Figure 2(2), with PROMO, motion artifacts can be substantially reduced. However, there is still some residual motion artifact. With OWA, the motion artifact in PSIR images (figure 2(5)) was further reduced. Since the T₁ map (Figure 2 (8)) is obtained from the PSIR image, the motion artifact can hardly be detected.

Figure 3 shows a representative high-resolution MP2RAGE (a) and PSIR (d) image. The T₁ maps, (b) and (e), were shown correspondingly. The image intensity versus T₁ values for MP2RAGE(c) and PSIR(f) were plotted too. As shown in figure3(f), the PSIR signal versus T₁ is linear. However, the MP2RAGE signal from approximately -0.5 to -0.25 corresponds to two T₁ values (figure3(c)), which causes difficulties in determining some values in the T₁ map. The T₁ map in figure3 (b) demonstrates areas of some of these intensity discontinuities.

DISCUSSION

CONCLUSION

Acknowledgements

References

1. Marques JP, Kober T, Krueger G, van der Zwaag W, Van de Moortele P-F, Gruetter R. MP2RAGE, a self bias-field corrected sequence for improved segmentation and T1-mapping at high field. Neuroimage 2010;49:1271–1281.

2. O’Brien KR, Kober T, Hagmann P, Maeder P, Marques J, Lazeyras F, Krueger G, Roche A. Robust T1-Weighted Structural Brain Imaging and Morphometry at 7T Using MP2RAGE. PLoS One 2014;9:e99676.

3. Mougin O, Abdel-Fahim R, Dineen R, Pitiot A, Evangelou N, Gowland P. Imaging gray matter with concomitant null point imaging from the phase sensitive inversion recovery sequence. Magn Reson Med 2016;76:1512–1516.

4. White N, Roddey C, Shankaranarayanan A, Han E, Rettmann D, Santos J, Kuperman J, Dale A. PROMO: Real-time prospective motion correction in MRI using image-based tracking. Magn Reson Med 2010;63:91–105.

5. Alexandru V. Avram, Joelle E. Sarlls, Cibu P. Thomas, Vinai Roopchansingh, Dan Rettmann, Ajit Shankaranarayanan, Peter J. Basser. Prospective Motion Correction (PROMO) enabled MP2RAGE for multi-contrast high-resolution brain imaging. In: Toronto, Canada; 2015.

6. Taylor JR. Introduction to Error Analysis, Second Edition, John R. Taylor. 2nd ed. University Science Books; 1997.

7. Zhang J, Bjornson B, Xiang Q-S. Combining Multi-channel MP2RAGE Images with Minimized Noise. In: ISMRM 24th Annual Meeting. Singapore; p. #1917.