Synopsis

Keywords: Data Acquisition, Diffusion/other diffusion imaging techniques, Reduced FOV, Compressed SENSE

DWI is very important for MRI examination, but it has limited resolution due to distortion, blurring, and signal loss caused by B0 inhomogeneity. Reduced FOV imaging could decrease these impacts. However, due to the coil geometry penalty, it’s hard to combine it with parallel imaging to further improve the image quality, it will suffer from noise breakthrough issues and unfolding artifacts. We propose a framework that combines reduced FOV imaging, Compressed SENSE framework simultaneously to overcome these issues. This framework allows a new solution for reduced FOV based diffusion imaging with high resolution, low distortion, and without noise breakthrough issue.

Introduction

Diffusion weighted imaging (DWI) based on single shot EPI is a very helpful sequence for oncological applications and microstructure study. But it suffers severe geometry distortion, image blurring, and signal loss which are caused by the local B0 inhomogeneity¹. The distortion could be represented by the following equation:

△y=△f(x,y)⋅FOV_pe⋅ESP

where △y means the distortion in phase encoding direction, △f(x,y) means the local B0 inhomogeneity, FOV_pe means the FOV at phase encoding direction, ESP means the echo spacing for EPI based sequence.
Reduced FOV imaging (rFOV) based on 2D RF pulse_^2,3 or Out Volume Suppression⁴ could be used to improve resolution and reduce the distortion for DWI. However, rFOV is still suffered from the distortion and signal loss when the local B0 inhomogeneity is large.
Parallel imaging, such as SENSE could also be used to decrease the echo train length and improve the image quality⁵. However, it will increase noise-like artifacts on the center of the images due to the high geometry factor when large reduction factors are used due to the coil geometry penalty⁵, it will be more severe if it was used for rFOV. Compressed SENSE could be useful to achieve an optimal balance between noise reduction and data consistency⁶. Recent studies demonstrated Compressed SENSE reconstruction framework could improve image quality for single shot EPI based DWI^7,8,9,10, it’s called EPICS.
We combine reduced FOV imaging and EPICS simultaneously for the first time to achieve high acceleration factor to produce DWI images with high resolution, low distortion and relative high SNR.

Methods

EPICS is based on single shot EPI acquisition utilizing the Compressed SENSE framework, it could dramatically reduce noise-like artifacts, so it can be used for much higher accelerator than conventional parallel imaging methods. Conventional reduced FOV imaging base on 2D RF pulse without parallel imaging is called iZoom which is implemented as ref 2. iZoom applies a tilted 2D Echo-Planar RF excitation with only tilting the k-space along the phase-encoding direction to realize multi-slice imaging. We combined iZoom with EPICS simultaneously, we call it as rEPICS, the sequence diagram and reconstruction workflow for rEPICS were showed as Fig. 1.
To evaluate the performance of rEPICS, a pilot study was done for head and abdomen, conventional DWI based on ssEPI, iZoom without SENSE, iZoom with SENSE and our proposed rEPICS were acquired on a Philips 3.0T Elition system (Philips Healthcare, Suzhou, China) , a 16-ch head & spine coil for head and 32-ch torso & spine coil for abdomen. The study was approved by the local IRB. The characteristics of these DWI acquisition schemes were summarized on Table 1. For conventional DWI using ssEPI, SENSE was used, the SENSE factor is 2. For reduced FOV imaging, three different acceleration schemes were used, it means iZoom without SENSE, iZoom with SENSE and rEPICS (iZoom with EPICS). Detailed scan parameters were summarized in Table 2.

Results

Fig. 2 and Fig. 3 shows the comparison between conventional DWI based on single shot EPI, iZoom without SENSE, iZoom with SENSE and our proposed rEPICS for head and abdomen separately. Compared with other schemes, our proposal rEPICS has the higher resolution, lower distortion and better SNR than other techniques. The image quality was dramatically improved with rEPICS, as the red arrow showed in Fig. 2 and Fig.3. Both iZoom with SENSE 2 and SENSE 3 show noise breakthrough issue. rEPICS with acceleration factor 2 and 3 shows good SNR and better resolution than other acquisition schemes. rEPICS also shows less distortion than other acquisition techniques in Fig3. The results shows that our proposed rEPICS has better performance on distortion, SNR and resolution than other methods. Although further clinical investigation is needed, rEPICS might be clinically useful and promising for DWI in high resolution, low distortion and without noise breakthrough issue.

Discussion and conclusions

The proposed method rEPICS combines the reduced FOV imaging with Compressed SENSE framework for EPI (EPICS). rEPICS shows good performance with high resolution, lower distortion and also higher SNR than other schemes, such as conventional diffusion based on single shot EPI, and reduced FOV without or with parallel imaging. A pilot study was done for both head and abdomen. This strategy could enhance the applicability and offer a new solution of DWI in applications that is expected for high resolution and low distortion, such as brain, pancreas, prostate, kidney etc. Because this method doesn’t increase the scan time, it should also be promising for high resolution and low distortion fMRI.

Acknowledgements

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