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Fast and motion robust brain examination using simultaneous multi-slice turbo gradient spin echo BLADE Sequence1Siemens Shenzhen Magnetic Resonance Ltd., Shenzhen, China
Synopsis
Keywords: Motion Correction, Motion Correction
Motivation: PROPELLER/BLADE is robust to motion in brain imaging but comes at the cost of longer acquisition time.
Goal(s): Our goal was to reduce the acquisition time of commercial sequences (BLADE and EPI) based brain motion-insensitive workflow by a factor of 2.
Approach: The SMS-TGSE-BLADE sequence was developed with acceleration techniques, including in-plane GRAPPA, SMS, and EPI readout.
Results: Comparable image quality was obtained with the SMS-TGSE-BLADE sequence with more than 2-fold decrease in acquisition time.
Impact: The improvement in acquisition speed in the motion-insensitive brain examination (including T1-FLAIR, T2W, T2-FLAIR and DWI) through SMS-TGSE-BLADE sequence may increase patient comfort. It can also increase patient throughput and cost efficiency of healthcare providers.
Introduction
Head motion is a common issue in clinical MR examinations that can hinder their effectiveness. The Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER, or BLADE) sequence is an approach based on the Rapid Acquisition with Relaxation Enhancement (RARE, or TSE) sequence. In this technique, each readout train covers a central strip of k-space and is subsequently rotated to sample the entire k-space1. Each blade captures the central k-space, enabling correction for in-plane head motion and improving motion robustness.However, PROPELLER comes with the drawback of additional scanning time due to oversampling the central k-space region. Various methods can be employed to speed up the acquisition. In addition to the conventional in-plane acceleration using Generalized Auto-calibrating Partial Parallel Acquisition (GRAPPA), the simultaneous multi-slice (SMS) technique can further accelerate data acquisition with minimal impact on SNR2,3. The Turbo Gradient Spin Echo (TGSE, or GRASE) technique, which generates gradient echoes and spin echoes with EPI-like bipolar gradients, can accelerate the acquisition and reduce SAR4.
In this preliminary investigation, we assessed the feasibility of a fast and motion robust brain examination by implementing the SMS-TGSE-BLADE sequence, which incorporates these acceleration techniques. The examination includes the acquisition of T1-FLAIR, T2W, T2-FLAIR, and diffusion-weighted imaging (DWI) protocols.
Methods
To accelerate the conventional BLADE sequence, EPI-like readout gradients and blips are played out on both readout and phase encoding directions, producing radial-like k-space lines which are placed into separate blades, as shown in figure 1. The readout is repeated throughout the whole echo train, producing all k-space lines for each blade. These blades cover k-space completely.SMS and in-plane GRAPPA is implemented to speed up the acquisition. To reduce the g-factor penalty, blipped-controlled aliasing5 is used. The time to acquire the necessary SMS calibration data is kept to a minimum by applying a quick separate TSE-based calibration scan for the first blade. For other blades, the calibration data are interpolated from the acquired data. This calibration scan approach is also used for in-plane GRAPPA. In-plane GRAPPA makes it possible to acquire a wider blade, which can speed up the acquisition and increase the robustness against motion.
For T1- or T2-FLAIR contrast, an inversion recovery module executes at the beginning of each shot. For diffusion weighted contrast, diffusion encoding and phase insensitive modules are inserted between the excitation pulse and first refocusing pulse6,7.
The vendor-provided motion-insensitive workflow was taken as reference, which consists of product BLADE sequence based T1-FLAIR, T2-weighted and T2-FLAIR protocols, and EPI based DWI protocol. The proposed fast examination was based on the proposed SMS-TGSE-BLADE sequence, and imaging parameters (figure 2) were optimized to shorten the acquisition time while preserving similar contrast and SNR when compared to the reference. Both the proposed and reference examinations were tested with a healthy volunteer at a Siemens 3T MAGNETOM Lumina scanner (Siemens Healthcare, Erlangen, Germany) equipped with 20-channel head-neck coil.
Results
Images were successfully acquired and reconstructed using the SMS-TGSE-BLADE technique. The contrast of brain tissue and perceived SNR in these images were comparable to those in the standard BLADE or EPI images, despite a more than 2-fold reduction in total acquisition time. Moreover, the T1-FLAIR, T2W, and T2-FLAIR images acquired through the SMS-TGSE-BLADE method displayed a comparable level of sharpness when compared with the reference protocol.In contrast to EPI-based DWI, the SMS-TGSE-BLADE approach did not yield advantages in terms of scanning time. This is mainly because only half of the signal is sampled when compensating for the non-CPMG issue, resulting in lower SNR efficiency. Nevertheless, the TGSE-BLADE DWI displayed fewer geometric distortions and reduced signal overlap in regions of high susceptibility, e.g., in the frontal lobe shown in figure 3.
Discussion and conclusion
One limitation of this study is the absence of T2* contrast, which is commonly used in clinical practice. However, TGSE-based sequences can theoretically generate T2* contrast. Further exploration is needed to assess SMS-TGSE-BLADE's potential for fast and motion-resistant T2* imaging.Furthermore, it's worth noting that SMS-TGSE-BLADE-based images may exhibit some off-resonance effects. For instance, the T1-FLAIR image may appear slightly blurred in the neck region, and the fat signal in the T2W image can be reduced.
Despite a longer acquisition time and potential reduction in sharpness, SMS-TGSE-BLADE DWI can consistently preserve diagnostic image quality, in regions and situations challenged by significant B0 inhomogeneity, including cases involving metal implants8. It's also reported to be more motion-resistant compared to EPI DWI9.
In this preliminary study, whole-brain motion-robust MRI with SMS-TGSE-BLADE is achieved in under 6 minutes, while delivering image quality comparable to the 13-minute commercial BLADE-based reference examination.
Acknowledgements
No acknowledgement found.References
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8. Okuchi, S., Fushimi, Y., Yoshida, K., et al. Comparison of TGSE-BLADE DWI, RESOLVE DWI, and SS-EPI DWI in healthy volunteers and patients after cerebral aneurysm clipping. Scientific Reports, 2022 12(1), 17689.
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