INTRODUCTION

Pediatric patients are challenging to perform magnetic resonance imaging (MRI) scans as they may not stay still during the scan. It is therefore often required to scan with protocols which are resistant to motion artifacts or which complete in a short scan time. Three-dimensional Magnetization Prepared Rapid Acquisition Gradient Echo (MPRAGE) provides excellent T1 contrast and anatomic cerebral structure to identify pathologic changes. However, MPRAGE sequence requires elongated scan time compared with two-dimensional scans. This study was aimed to investigate the extent of head motion artifact suppression in two different MPRAGE sequences which both thought to contribute for the motion artifact suppression by different strategy. The first type of MPRAGE sequence is a modified multi-echo Magnetization Prepared Rapid Acquisition Gradient Echo (MEMPRAGE) which performs prospective motion correction using three-dimensional EPI volumetric navigators (vNav Moco)1. The second type is a wave-controlled aliasing in parallel imaging (wave-CAIPI) MPRAGE which reduces its scan time with only a small g-factor noise penalty2 and which our previous study showed the suppression of the head motion artifacts in pediatric brain by shortening the scan time in about 3 minutes compared with the conventional MPRAGE sequence3. Our current study investigated the extent of head motion artifact suppression by visual assessment among vNav Moco MPRAGE, wave-CAIPI MPRAGE, and conventional MRPAGE. To understand the T1 contrast differences among these three MPRAGE sequences, the study assessed contrast-to-noise ratio (CNR) of the gray and white matter in five different area of the cortical brain.

METHODS

Six patients (A to F) aged 2 to 16 years were enrolled. All underwent sequential MRI scans with conventional MPRAGE, prototype wave-CAIPI MPRAGE2, and prototype vNav Moco MEMPRAGE1 sequences on a 3T MRI scanner (MAGNETOM Prisma, Siemens Healthcare, Erlangen, Germany) with a 64-channel head-and-neck coil. Three patients received contrast. The wave-CAIPI MPRAGE sequence modified conventional gradient echo readouts by playing sinusoidal gradients in phase-encoding directions during the sampling period, leading to a “cork-screw” trajectory. The vNav Moco MEMPRAGE sequence modified conventional MPRAGE by inserting 3D-encoded EPI acquisition into each TR with a multi-echo readout. The extent of the artifacts was visually assessed with scores 1 to 5 (1, brain structure unrecognizable; 5, no artifact). White and gray matter contrast-to-noise ratio (CNR) differences were analyzed in the frontal, temporal, occipital, anterior cingulate, and posterior cingulate cortices. One-way ANOVA between subjects was conducted.

RESULTS

Figure 1 shows the parameters of the study. Figure 2(A) shows representative images from the three MPRAGE sequences. CNR showed no significant differences among the sequences (Figure 2(B)) either with or without contrast injection. The six patients showed different extent of head motion during in this study (Figure 3). The average visual scorings affected by the motion artifacts were 2.8, 3.7, and 4.2 for conventional MPRAGE, wave-CAIPI MPRAGE, and vNav Moco MEMPRAGE, respectively (p=0.028) (Figure 4). A ringing artifact was observed in five patients and was assumed to originate from vessel flow. The ringing artifacts became milder in four patients, either in wave-CAIPI or vNav Moco MEMPRAGE, compared with conventional MPRAGE (Figure 5).

DISCUSSION

The extent of the head motion was presented in translations and rotations for each subject depicted from the vNav Moco MEMPRAGE scan (Figure 3). It presented varieties of the head movement. In our previously study, wave-CAIPI MPRAGE showed significantly reduced motion artifacts in pediatrics’ cerebral brain3. In this study, we have investigated the extent of the motion artifact suppression by performing vNav Moco MEMPRAGE scan and then by comparing with wave-CAIPI MPRAGE and conventional MPRAGE images. The average visual scorings affected by the motion artifacts was highest in vNav Moco MEMPRAGE images, which may imply that prospective motion correction is much effective to suppress motion artifacts rather than using a fast scan technique. On the other hand, the scan time became twice longer when using vNav Moco MEMPRAGE compared with wave-CAIPI MPRAGE. Additional finding was the suppression of the vessel flow artifact using either vNav Moco MEMPRAGE or wave-CAIPI MPRAGE. However, this needs to be examined further since our study could not conclude the reason that some showed better vessel flow artifact suppression in vNav Moco MEMPRAGE and some showed better in wave-CAIPI MPRAGE. CNR had no significant differences among the three sequences, which indicated that vNav Moco MPRAGE and wave-CAIPI MPRAGE can be similar to conventional MPRAGE to provide visual diagnosis in pediatric cerebral brain with or without contrast injection.

CONCLUSION

Wave-CAIPI MPRAGE and vNav Moco MEMPRAGE had better suppression against head motion artifact than conventional MPRAGE, but vNav Moco MEMPRAGE effectively reduced motion artifact compared to wave-CAIPI MPRAGE. Both wave-CAIPI MPRAGE and vNav Moco MEMPRAGE sequences reduced flow artifact from vessel flow.

Acknowledgements

No acknowledgement found.

References

1. Tisdall MD, Hess AT, Reuter M et.al. Volumetric Navigators (vNavs) for Prospective Motion Correction and Selective Reacquisition in Neuroanatomical MRI. Magn. Reson. Med. 2012 68(2):389-399. 2. Bilgic B, Gagoski BA, Cauley SF et.al. Wave-CAIPI for highly accelerated 3D imaging. Magn Reson Med 2015;73:2152-2162. 3. Niisato E, Chen YC, Cheng S et.al. Fast scan with a wave-CAIPI MPRAGE sequence to minimize motion artifacts in pediatric T1-weighted imaging. ISMRM 2021 #0090.