Quantitative MRI of the brain in patients undergoing cardiac surgery: what imaging parameters relate to outcome?

Michael Adeshola Adebayo

Abstract


Background: Post-operative cognitive dysfunction (POCD) is an outcome of cardiac surgery which occurs as a result of microemboli formation and haemodynamic differences during the operation. We assess the cognition of patients who undergo endovascular aortic aneurysm repair (EVAR) prior to the operation and after the operation at 3 months and 1 year.  We also sought to find the structural changes made to the brain by EVAR on the brain by acquiring and analysing multimodal MRI images from before and after the operation. Whole brain DTI measures of fractional anisotropy (FA) and mean diffusivity were assessed pre- and post-operatively.

Methods: 12 patients underwent EVAR. Neurocognitive tests were performed preoperatively (at baseline), at 3 months and 1 year after the procedure. A composite z-score for each patient was calculated from the range of subtests. A z-score of -1.5 represented post-operative cognitive dysfunction. T1W, T2W, FLAIR, DWI, DTI MRI images were acquired. FLAIR images were assessed for white matter hyperintensities using Fazekas’ scale. T1W images were segmented to tissue probability maps of white matter (WM), grey matter (GM) and cerebrospinal fluid (CSF) and then coregistered to DTI images. The FA and MD maps were masked according to the native tissue types (GM, WM and CSF). Whole brain histograms were extracted of the FA of WM, MD of GM and MD of WM. The Wilcoxon signed-rank test was used to assess whether there was a significant difference between the z-scores at baseline and at 3 months, the z-scores at baseline and at 1 year and the z-scores at 3 months and at 1 year. We also used the Wilcoxon signed-rank test in order to assess for changes in the means of FA (of white matter), MD (of grey matter) and MD (of white matter) pre- and post- operation.

Results: We found a significant improvement between the z-score for global cognition at 3 months compared to baseline. There was no significant difference between the z-scores at baseline and at 1 year nor between the z-scores at 3 months and at 1 year. The 2 patients with extensive white matter hyperintensities (high-scoring on Fazekas’ scale) improved cognitively. No significant difference was found between the FA of white matter before and after the operation and there was no significant change in the MD (of grey matter) and the MD (of white matter) pre and post-operation.

Conclusions: These results suggest, with a small patient sample, that POCD is not a certain outcome after EVAR. The results we gained with whole brain FA and MD also suggest that the procedure makes little to no structural changes to the brain at an early post-operative time point. Further investigation of this topic with a larger patient sample and a longer time period of cognitive testing would increase the reliability of the results found in this study.


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