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3D Flow Compensated Interleaved EPI for a Fast High-Resolution Susceptibility-Weighted Imaging at 1.5T
Wei Liu1, Kun Zhou1, Shi Cheng1, and Kawin Setsompop2,3,4

1Siemens Shenzhen Magnetic Resonance Ltd, Shenzhen, China, 2Department of Radiology, A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 3Harvard Medical School, Boston, MA, United States, 4Harvard-MIT Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States

Synopsis

We implemented a first order gradient nulling (GMN) based partial flow compensation in 3D interleaved EPI and assessed its feasibility for a fast high-resolution SWI application at 1.5T. Specifically, we used GMN to zero the velocity-induced phase error at the center echo for each shot in both phase and frequency encoding directions. The slice direction implementation was identical to that for 3D GRE, with flow compensation for both slice selective gradient and partition encoding gradient. In addition, each shot was sequentially acquired twice with the swapped readout polarity in the second to further reduce the phase oscillates between the even and odd echoes in each shot. Flow phantom and in-vivo experiments were performed to validate that flow effect is effectively reduced even not all echoes have been fully flow compensated.

INTRODUCTION

Susceptibility-weighted imaging (SWI) has been widely used in a variety of clinical setting for evaluating iron-laden tissues, venous blood vessels and other sources of susceptibility effects1. The conventional SWI is acquired using a T2* weighted 3D GRE with three-dimensional full flow compensation, which is usually with a long scan time. A 3D interleaved EPI (3D-iEPI) has been proposed as a fast alternative to 3D GRE2-4. By using a short EPI train length, the typical EPI related artifacts (distortion and blurring) are limited, and images will gain in both SNR and efficiency whilst keeping the similar contrast for both magnitude and phase images in comparison with conventional 3D GRE5. However, the design of flow compensation in EPI is more complicated compared with that in GRE6. A most common way to alleviate the flow effect is first order gradient nulling (GMN), which cannot be achieved for all echoes for both phase and frequency encoding directions without a cost of scan efficiency in EPI (e.g. “flyback” approach used for compensating all echoes in frequency encoding direction)7. In this work, we implemented a GMN based partial flow compensation in 3D-iEPI and assessed its feasibility for a fast SWI application. In addition, each shot was sequentially acquired twice with the swapped readout polarity in the second to further reduce the phase oscillates between the even and odd echoes in each shot.

METHODS

The sequence diagram with flow compensation is shown in Fig.1.When applying GMN to EPI sequence, to take advantage of its scan efficiency, only the center echo of each shot can be compensated for both phase and frequency encoding directions. The slice direction implementation was identical to that for 3D GRE. All measurements were performed on a commercial 1.5T scanner (MAGNETOM AERA, Siemens Healthcare, Erlangen, Germany) equipped with a 20-channel head/neck coil. Both the prototype 3D-iEPI and conventional GRE were scanned. First a flow phantom was tested composed of a tube filled with flowing water driven by a pump and a commercial phantom as a reference (shown in Fig.2). To evaluate the flow compensation effect, the data were acquired using the 3D-iEPI when the pump was turned on and off. In vivo images were acquired from a healthy volunteer with imaging parameters shown in Table 1. After data collection, both 3D GRE and 3D-iEPI data were SWI-processed in the standard way.

RESULTS

The magnitude and phase images acquired using 3D-iEPI with and without the flow in phase and frequency encoding directions are shown in Fig.3 and Fig.4, respectively. As expected, the flowing water signal is visible with flow compensation (Fig.3b and Fig.4b) while is diminished without flow compensation (Fig.3c and Fig.4c). Compared with the images acquired with same protocol but the pump is powered off (Fig.3a and Fig.4a), the tube signal is lower when the pump is powered on (flow compensation on) because of the partial flow compensation. The minimum intensity projection (mIP) over the processed SWI images for a healthy volunteer brain are shown in Fig.5. After the application of flow compensation, both 3D GRE (Fig.5a) and 3D-iEPI (Fig.5c) show a remarkable reduction of arterial contamination. Note that the 3D-iEPI enjoys a higher SNR, even with much less acquisition time.

DISCUSSION

As far as susceptibility is concerned, the confounding phase information introduced by rapid arterial flow will lead to unwanted signal in SWI images. In order to keep the same scan efficiency of 3D-iEPI, we use GMN to alleviate the flow effect for the center echo of each shot, which means more lines in k-space center are fully flow compensated in three dimensions when more shots are acquired. In this work, we choose to acquire 11-15 echoes per shot to achieve an appropriate TE for a high-resolution SWI at 1.5T, with a balance of scan efficiency and flow compensation. In addition, as indicated by our flow phantom and in-vivo experiments, there is no obvious distortion and blurring in the images acquired with 3D-iEPI because of a short echo train length used.

CONCLUSION

We have demonstrated that 3D-iEPI with partial flow compensation can be used for SWI with significant reduction in acquisition time, while maintaining high resolution and gaining SNR by increasing average and avoiding the iPAT acceleration. Although the flow effect is not fully compensated for all echoes in 3D-iEPI, the brain vasculature is similar to that obtained from conventional GRE with full flow compensation. In addition, the increased in distortion in 3D-iEPI compared to 3D-GRE is minimal and should allow its usage for SWI applications in the clinic.

Acknowledgements

References

1. Haacke EM. et al, Susceptibility weighted imaging (SWI), Magn Reson Med. 2004;52:612–618

2. Patel MR. Detection of hyperacute primary intraparenchymal hemorrhage by magnetic resonance imaging. Stroke.1996; 27:2321–2324

3. P. Sati. et al, Ultra-Fast Acquisition of High-Resolution Susceptibility-Weighted-Imaging at 3T. Proc Intl Soc Mag Reson Med. 2011; 19:2364

4. Poser BA. et al, Three dimensional echo-plannar imaging at 7 Tesla. Neuroimage. 2010; 51:261-266

5. Zwanenburg JJ. et al, Fast high resolution whole brain T2* weighted imaging using echo planar imaging at 7T. Neuroimage. 2011;56:1902–1907

6. K. Butts. et al, Analysis of flow effects in echo-planar imaging. J. Magn. Ikon. Imaging 1992; 2:285-293

7. Glenn S. et al, Gradient Moment Smoothing: A New Flow Compensation Technique for Multi-Shot Echo-Planar Imaging. Magn Reson Med. 1997;38:368–377

Figures

Table 1. Imaging parameters for Axial SWI protocols

Figure 1: Sequence diagram of 3D-iEPI with nine echoes acquired per shot. The bipolar gradient waveforms shown in red are designed flow compensated gradients for three directions, in which the GMN calculation based on the phase and readout gradients is only for the center echo highlighted in black.

Figure 2: A schematic representation of experimental setup with a flow phantom.

Figure 3: Flow phantom images acquired by 3D-iEPI. The phase encoding is in the vertical direction. Magnitude image and phase images (a) without flow, (b) flow on with flow compensation, (c) flow on without flow compensation. Fifteen echoes were acquired after each RF pulses. The arrows indicated the conspicuous flowing water when flow compensation is used, which is consistent with the images acquired without flow.

Figure 4: Flow phantom images acquired by 3D-iEPI. Equivalent measurements have been done as Fig.3, but with the phase and frequency encoding directions switched. (a) without flow, (b) flow on with flow compensation, (c) flow on without flow compensation. Fifteen echoes were acquired after each RF pulses. The arrows indicated the conspicuous flowing water when flow compensation is used, which is consistent with the images acquired without flow.

Figure 5: Comparison of SWI processed mIP images with and without flow compensation applied to 3D GRE and 3D-iEPI respectively, acquired at 0.7x0.7x1.6 mm3 resolution. (a, b) 3D GRE with and without flow compensation respectively, acquisition time 5 min 49 sec, (c,d) 3D-iEPI with and without flow compensation respectively, acquisition time 2 min 57 sec. Thirteen echoes were acquired after each RF pulses. Same vessels are seen in the mIPs from both sequences. Note that the flow effect can be alleviated after the application of flow compensation in both sequences, and better in 3D GRE because all echoes have been flow compensated in three directions.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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