1. Structure Similarity Index
Selection of Electrode Sites for Body Surface Potential Mapping: A Significance Ranking Study
Xin Li 1, Frederique J. Vanheusden 1, Tiago P. de Almeida 1,
Fernando S. Schlindwein 1,2 , G. André Ng 2
1 Department of Engineering, University of Leicester, Leicester, United Kingdom
2 University of Leicester Hospitals – Glenfield General Hospital, Leicester, United Kingdom Contact:
Introduction
Body surface potential mapping (BSPM) allows the display of detailed temporal and spatial distribution of cardiac activity1. However, the effort and cost of conventional leads BSPM makes reduced-lead BSPM systems desirable for clinical application. The aim of this study is to explore the importance of different electrodes for generating BSPMs, by comparing the maps produced using the full set of leads (N) with those using all but one leads (N-1). In this study, a human eye perception based index - structural similarity (SSIM) index was used for the comparison of the BSPMs.
Method
The body surface signal was acquired using a 128-electrode system (Active 2, Biosemi) from 10 healthy male volunteers for 60 s, sampled at Hz with band-limit between 0.16 Hz and 50Hz. At every sampling time point, 129 contour maps were generated, one using all 128 electrodes (original map Figure 1 (c) ) and 128 from reconstructed-maps of the remaining 127 electrodes (Figure 1 (d) ). For each electrode, the reduced set reconstructed-map was compared with the original map by calculating the SSIM. The above procedures were repeated for samples, and the average SSIM of each electrode was calculated. The significance ranking ( vector P) of electrodes was sorted by the reverse order of their mean SSIM.
Processing
The BSP contour maps are approximated using uniform electrodes spacing arrays with linear interpolation, in the format of 8-bit grayscale images, with a fixed colour map ranging between ± 2.5 mV.
𝑷={ 𝑷 𝟏 , 𝑷 𝟐 , 𝑷 𝟑 … 𝑷 𝟏𝟐𝟕 , 𝑷 𝟏𝟐𝟖 }
𝑴𝒆𝒂𝒏𝑺𝑺𝑰𝑴 𝒊 = 𝒏=𝟏 𝟐𝟎𝟒𝟖 𝑺𝑺𝑰𝑴(𝒊,𝒏)
Calculate SSIM between the map without electrode i and the original map at sample n.
Electrode number i with minimum MeanSSIM
Electrode number i with maximum MeanSSIM
Results
Figure 1 shows the significance ranking of the top 64 electrodes, with the highest significance (reds 1-8) being mostly on the left chest and with anterior electrodes providing more information than posterior ones.
Anterior Posterior
(a). 3D BSP contour map on a homogenous torso
(b). Layout of 128 electrodes original contour map
Anterior Posterior
Anterior Posterior mV
Anterior Posterior
Figure 2 Significant ranking of the main 64 electrodes
(c). 128 electrodes original contour map during QRS complex
(d). 127 electrodes reconstructed contour map without channel 29 (red dot) during QRS complex
Conclusions
This study provides a methodology and guidance for selecting the optimal electrode positioning for the purpose of constructing BSPM for clinical application. Research on more subjects and comparison with other studies using alternative approaches for the ranking are under way.
Figure 1 Details of the processing procedures
Structure Similarity Index
The contour line based body surface potential maps are highly structured: the pixels exhibit strong dependencies, especially when they are spatially proximate, and these dependencies carry important information about the structure of the objects in the visual scene2.
The SSIM measures the similarity considering the luminance, contrast and structure of the images.
References
Flowers NC, Horan LG: Body Surface Mapping Including Relationships with Endocardial and Epicardial Mapping. Annals of the New York Academy of Science 1990, 601:
Z. Wang, A. C. Bovik, H. R. Sheikh and E. P. Simoncelli, "Image quality assessment: From error visibility to structural similarity," IEEE Transactions on Image Processing, vol. 13, no. 4, pp , Apr
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