Introduction

DWI provides image contrast that reflects the limiting water diffusion at the cellular level. Using DWI, various lesions, like ischemic disease and tumors in the body can be visualized as hyperintensity depending on the degree of diffusion limitation. Some studies reported that DWI of the optic nerve was useful for detecting optic neuritis and optic nerve tumors. ^1,2,3,4 However, DWI of the optic nerve has some problems about artifacts due to differences in magnetic susceptibility between tissue and air. For this reason, single shot (SSh) echo planar imaging (EPI)-DWI, which is a commonly used sequence has high risks of image distortion, signal loss, and spatial blurring caused by susceptibility artifacts to demonstrate the optic nerve. SSh TSE-DWI is known to be useful for reducing susceptibility artifacts. ^5,6 However, SSh TSE-DWI suffers from low signal noise ratio (SNR), and severe image blurring. On the other hand, multi shot (MSh) TSE-DWI combined with Multi Vane radial acquisition (MSh-MV-TSE DWI) has an advantage of reducing image blurring. However, MSh-MV-TSE DWI still suffers from low SNR. IRIS (Image Reconstruction using Image-space Sampling Function) is based on MSh EPI-DWI, which is combined with navigator-echo to correct the phase shift caused by the movement of each shot [Fig 1]. IRIS for the optic nerve is expected to create higher quality imaging with less ghost artifacts. In this study, we evaluated the usefulness of IRIS for depiction of the optic nerves by comparing image quality among SSh EPI-DWI, SSh TSE-DWI, MSh MV-TSE-DWI, and IRIS. ^7,8

Methods

Five healthy volunteers (4 males, 1 female, age 25-45 years old) were examined by 3.0T MRI (Ingenia, Philips Healthcare).　The study was approved by the local IRB (Clinical Trial Review Board) and all subjects gave written informed consent. The image parameters were shown in Table 1. SPLICE (Split-echo acquisition of TSE signals) was used as SSh TSE-DWI because SPLICE improves SNR by collecting Stimulated echo (STE). MSh TSE-DWI is combined with Multi Vane acquisition. A visual evaluation of the optic nerve (artifact, image contrast, and overall image quality) was performed by three radiologists and radiological technologists using a 5-point scale: 0, non-evaluative; 4, excellent quality. The signal profile analysis of the optic nerve was performed on SSh EPI-DWI, SSh TSE-DWI, MSh TSE-DWI, IRIS, and T1WI that was used as a standard image.

Results and Discussion

The susceptibility artifacts were significantly noted on SSh EPI-DWI, while IRIS showed the least artifacts. The image contrast of SSh TSE-DWI was the lowest, while SSh EPI-DWI and IRIS showed higher image contrast than both SSh and Msh TSE-DWI. IRIS was the best of overall image quality. The results of signal profile analysis were shown in Figure 3. The signal profiles of T1WI and IRIS were almost the same. The signal intensities of T1WI were almost the same as those of T1WI, while the image distortion was noted on SSh EPI-DWI. The signal intensities of SSh TSE-DWI and MSh TSE-DWI were lower, and SSh TSE-DWI and MSh TSE-DWI showed more prominent image distortion, compared to T1WI. SSh EPI-DWI is known to be sensitive to magnetic field inhomogeneity. In this study, SSh EPI-DWI was strongly affected by susceptibility artifacts which caused image distortion of the optic nerve even though SSh EPI-DWI had relatively high image contrast. Both SSh and MSh TSE-DWI succeeded to reduced blurring, however, lower SNR of TSE-DWI leaded to poorer image quality than SSh EPI-DWI and IRIS. IRIS is one of MSh EPI-DWI using phase correction to reduce ghost artifacts. In this study, IRIS was the best sequence to demonstrate the optic nerve. It was suggested that combination of high SNR on EPI-DWI and artifact suppression by Msh acquisition and phase correction of IRIS created good synergistic effects.

Conclusion

IRIS can suppress the susceptibility artifacts while maintaining the higher nerve conspicuity with sufficient spatial resolution, image contrast and SNR compared to other DWI techniques in a clinically feasible acquisition time, and it has a potential to be a new commonly used DWI for the evaluation of the optic nerve.

Acknowledgements

No acknowledgement found.

References

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