Synopsis

A thin-sliced (pseudo-3D) SMS accelerated PROPELLER with retrospective motion correction is demonstrated and compared to prospectively motion corrected 3D RARE using spiral navigators. The results show that our pseudo 3D PROPELLER sequence can produce higher image quality than 3D RARE, even in reformatted views, with and without the presence of head motion.

Purpose & Background

As high resolution 3D MRI data is reformattable in any plane, it is becoming increasingly used in the clinical routine. The sensitivity to motion of Cartesian 3D MRI can be mitigated by using prospective motion correction (1).

Gagoski et al. (2) showed that it is possible with SMS (simultaneous multi-slice) (3) to very rapidly obtain thin-sliced Cartesian 2D ('pseudo 3D') data that may be reformatted in other planes with similar resolution as a true 3D data sets. However, despite the short acquisition time, such a pseudo 3D volume would still be susceptible to motion.

In this work, we have used SMS to accelerate a PROPELLER (4) sequence and acquired pseudo 3D datasets of volunteers. These results were compared with the vendor's prospectively motion corrected 3D RARE (5) sequence.

Methods

A custom PROPELLER sequence with SMS acceleration capabilities (6) was used to acquire T2-weighted pseudo 3D data. Retrospective PROPELLER motion correction was performed by 3D rigid body registration between blade slice stacks ('bricks') in the image domain, to also account for out-of-plane head motion (7).

To demonstrate the image quality in native and reformatted views, as well as the robustness against head motion, the pseudo 3D PROPELLER was compared to a 3D T2-weighted RARE sequence that was prospectively motion corrected using spiral navigators (8) (vendor’s name: CUBE with PROMO).

The SMS PROPELLER sequence was scanned twice, with and without head motion . Both datasets were reconstructed with and without retrospective motion correction. The 3D RARE sequence was scanned three times; once without head motion and twice with motion. For all 3D RARE scans, the prospective motion correction was active, tracking the head movements. However, in order to see the magnitude of the motion artifacts, the first scan with motion was acquired without applying the estimates prospectively in the sequence.

The SMS PROPELLER data were acquired with a voxel size of 1.0×1.0×1.0 mm³, 16 blades and 153 1 mm axial slices, resulting in an acquisition time of 02:55 min. The 3D RARE was also acquired with a voxel size of 1.0×1.0×1.0 mm³, but with 160 1 mm sagittal partitions in the volume, resulting in an acquisition time of 03:28 min. Detailed acquisition parameters can be found in Figure 3.

For all the acquisitions with head motion, the subject was instructed to move their head in the same manner, drawing a circle with their nose and changing position every 20 seconds.

The experiments were performed on a 3T system (MR750w, GE Healthcare, Milwaukee, WI, USA) using a 32-channel RF receive coil (Nova Medical Inc., Wilmington, MA, USA).

Results

The resulting images are shown in Figure 1, with the PROPELLER data reformatted to sagittal and coronal views and the 3D RARE data reformatted to axial and coronal views. Compared to 3D RARE, the PROPELLER images have higher effective resolution due to lower T2-blurring thanks to the much shorter echo train (Figure 1a-b). Without motion correction, the PROPELLER is more prone to blurring, while the 3D RARE tends to get ghosting artifacts (Figure 1c-d). With motion correction, the PROPELLER produces a sharper image than the 3D RARE (Figure 1e-f) while also retaining a better gray/white matter contrast.

The motion estimates from the PROPELLER motion correction and PROMO are presented in Figure 2 and show that the amount of motion was similar for all scans.

Discussion

The 3D RARE’s sensitivity to motion depends significantly on when the motion occurs relative to the sequence components. If motion occurs after the navigators or during the readout, there will be a delayed correction. The PROPELLER, on the other hand, is sensitive to rapid motion during the acquisition of a brick, making it harder to align the bricks to each other. Therefore, the use of SMS is an advantageous acceleration technique for PROPELLER, as it can reduce the TR and the temporal footprint of a brick.

In this work, our SMS PROPELLER sequence with 3D retrospective motion correction has been compared with prospectively corrected 3D RARE. These preliminary results indicate that PROPELLER is better at handling patient motion. To make our pseudo 3D PROPELLER sequence yet more robust to motion, prospective motion correction will be added in the future (9).

Acknowledgements

References

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