Introducing a structural similarity index and map for quality control in tractography performed using multi-band EPI
Yuichi Suzuki1, Akira Kunimatsu1, Kouhei Kamiya1, Masaki Katsura1, Harushi Mori1, Katsuya Maruyama2, Thorsten Feiweier3, Kenji Ino1, Yasushi Watanabe1, Jiro Sato1, Keiichi Yano1, and Kuni Ohtomo1

1The Department of Radiology, The University of Tokyo Hospital, Bunkyo-ku, Japan, 2Siemens Japan K.K., Shinagawa-ku, Japan, 3Siemens AG, Erlangen, Germany

Synopsis

We quantitatively evaluated the quality of tractography images captured using the multi-band EPI (MBEPI) compared with those obtained without using MBEPI. We also demonstrated the potential weakness of classic Dice similarity coefficients (DSCs) and introduced a structural similarity (SSIM) index and map as a new method for evaluating the quality of tractography images. A numerical evaluation was enabled by the SSIM index and that the SSIM map was advantageous in that it allows visual confirmation of the structural similarity ratio; in contrast, the DSCs only offered a numerical evaluation.

Purpose

The multi-band echo planar imaging (MBEPI) technique 1,2 can reduce scanning time; however, the quality of the images produced is suboptimal because of the effects of incomplete slice separation 3. In this study, we quantitatively evaluated the quality of tractography images captured using MBEPI compared with those obtained without using MBEPI. We also demonstrated the potential weakness of classic Dice similarity coefficients (DSCs) and introduced a structural similarity (SSIM) index and map 4 as a new method for evaluating the quality of tractography images.

Materials and Methods

Twelve men participated in this study (average age, 28.25 ± 3.62 years). Diffusion MRI (dMRI) data were acquired with a SIEMENS MAGNETOM Avanto 1.5T B17 using a single-shot spin echo EPI sequence and following parameters: b-value, 3000 s/mm2; field of view, 24.5 × 24.5 cm2; acquisition matrix, 98 × 98; slice thickness, 2.8 mm; axial sections, 50; GRAPPA factor, 2; and average, 1. The following three methods were compared.

Non-MBEPI method (gold standard): MPG, 64 axes and acquisition time, 518 s.

MBEPI method 1 (shortened scanning time): MPG, 64 axes; MB factor (MBf), 2; and acquisition time, 327 s.

MBEPI method 2 (an increased number of MPG directions): MPG, 104 axes; MBf, 2; and acquisition time, 507 s.

The MBEPI sequence version used was Release R011a for VB17A 5. After the distortion correction of dMRI data using eddy_correct in FSL, and Q-ball imaging (QBI) analysis and tractography were performed using Diffusion Toolkit 0.6 and TrackVis 0.5.1. The tractography of the bilateral pyramidal tract was reconstructed using the standard method; by placing the seed and target in the cerebral peduncle and motor cortex, respectively 6. The quality of the tractography images obtained using MBEPI methods 1 and 2 was evaluated using the non-MB method as the gold standard. The DSCs and SSIM index were used as quantitative measures of coincidence with the non-MBEPI method. A visual assessment (VA), a method similar to the gold standard was performed by a neuroradiologist and radiologic technologist with 10 years’ experience in neurosurgery MRI. The SSIM index is generally used to measure the similarity between two images or videos and has been widely applied in the evaluation of image compression 7, 8. The SSIM index can be considered a measure of the quality of one of the images under comparison, provided that the other image is of perfect quality. We performed McNemar's test using the VA and the DSCs or the SSIM index and calculated the coincidence rate (accuracy) between the VA and the DSCs or the SSIM index with SPSS v.20. Both the DSCs and SSIM index were used to generate an SSIM map by voxel-wise calculation using MATLAB codes.

Results

The DSCs for coincidence with the non-MBEPI method of the right/left pyramidal tracts obtained using MBEPI methods 1 and 2 were 0.614/0.618 and 0.609/0.647, respectively, of the DSC obtained using the non-MB method. The SSIM indices of MBEPI methods 1 and 2 were 0.936/0.942 and 0.935/0.947, respectively (Fig.1). McNemar’s test identified no significant differences between the VA and DSC (p = 0.143) or the VA and SSIM index (p = 0.267). The accuracies were 0.646 and 0.729 for the VA and DSC, and the VA and SSIM index, respectively (Fig.2). It was easy to visualize the similarity ratio using the SSIM map (Fig.3).

Discussion and Conclusion

The results of McNemar's test and coincidence rates showed that the SSIM index is more similar to the VA than the DSC. This result supports Wang’s suggestion 1 that the SSIM calculates a value almost equivalent to that selected by subjective judgment. We believe that a numerical evaluation is enabled by the SSIM index and that the SSIM map is advantageous in that it allows visual confirmation of the structural similarity ratio; in contrast, the DSC only offers a numerical evaluation. Therefore, this initial study demonstrates the utility of the SSIM index and map for the evaluation of the quality of tractography images; moreover, the success of the SSIM index and map may preface the development of new methods for the evaluation of tractography. Regarding MBEPI methods 1 and 2, there were no great differences between these methods of evaluation. However, it’s difficult to declare that the result with 64 axes MPG set is the gold standard for QBI tractography. Therefore, examinations with other conditions are required in future works.

Acknowledgements

I am deeply grateful to Yuji Komaki of Central Institute for Experimental Animals.

This study was supported by Grants-in-Aid for Scientific Research (26460721).

References

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4. Wang Z, Bovik AC, Sheikh HR, et al. Image quality assessment: from error visibility to structural similarity. IEEE Transactions on Image Processing, 2004 Apr; 13(4):600-612.

5. https://www.cmrr.umn.edu/multiband/

6. Wakana S, Caprihan A, Panzenboeck MM, et al. Reproducibility of quantitative tractography methods applied to cerebral white matter. Neuroimage. 2007 July 1; 36(3):630–644.

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8. Voruganti AK, Mayoral R, Vazquez A, et al. A modular video streaming method for surgical assistance in operating room networks. Int J Comput Assist Radiol Surg. 2010 Sep; 5(5):489-99.

Figures

Fig.1. the DSCs and the SSIM index of example images

Fig.2. the results of McNemar's test

Fig. 3. the SSIM map comparison

The brighter area indicates better quality. Green arrows particularly indicate differences in the tractography structure of the right pyramidal tracts detected by MBEPI methods 1 and 2.




Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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