Near-Infrared Spectral Similarity between Ex Vivo Porcine and In Vivo Human Tissue

Abstract

Background: In vivo diffuse reflectance spectroscopy provides additional contrast in discriminating nerves embedded in adipose tissue during surgery. However, large datasets are required to achieve clinically acceptable classification levels. This study assesses the spectral similarity between ex vivo porcine and in vivo human spectral data of nerve and adipose tissue, as porcine tissue could contribute to generate large datasets.

Methods: Porcine diffuse reflectance spectra were measured at 124 nerve and 151 adipose locations. A previously recorded dataset of 32 in vivo human nerve and 23 adipose tissue locations was used for comparison. In total, 36 features were extracted from the raw porcine to generate binary logistic regression models for all combinations of two, three, four and five features. Feature selection was performed by assessing similar means between normalized features of nerve and of adipose tissue (Kruskal-Wallis test, p < 0.05) and for models performing best on the porcine cross validation set. The human test set was used to assess classification performance.

Results: The binary logistic regression models with selected features showed an accuracy of 60% on the test set.

Conclusions: Spectral similarity between ex vivo porcine and in vivo human adipose and nerve tissue was present, but further research is required.

Keywords: adipose tissue; classification; diffuse reflectance spectroscopy; nerve tissue; spectra.

Figure 1

Pictorial assay for obtaining porcine samples. (A) Identification of the brachial plexus nerve in prescapular region (porcine shoulder) and sample planning. (B) Dissection of one sample. (C) Orient sample and digital mapping nerve locations. (D) Digital documentation of the 5 to 10 distinct locations measured on the sample.

Figure 2

Mean calibrated spectra (solid lines) flanked by the standard deviation (striped lines). (a) Ex vivo adipose porcine (blue) vs. nerve porcine tissue (red). (b) In vivo adipose human (cyan) vs. human nerve tissue (magenta).

Figure 3

Mean calibrated spectra (solid lines) flanked by the standard deviation (striped lines). (a) Porcine (blue) vs. human adipose tissue (cyan). (b) Porcine (red) vs. human nerve tissue (magenta). (c) Difference between porcine and human adipose tissue. (d) Difference between porcine and human nerve tissue.

Figure 4

Performance of the BLRMs generated with combinations of 2, 3, 4 and 5 features, respectively for the 24 features showing similar means with human data and showing the highest performance on cross validation set. Green: accuracy of the training set. Blue: accuracy of the cross validation set. Yellow: accuracy of the human test set. Note that each dot, square and triangle can represent multiple BLRMs which give the same classification performance.

Figure 5

Two reproductions of Figure 3, but now zoomed in to wavelength range 350–650 (top 2 graphs) and zoomed in to wavelength range 1440–1830 (bottom 2 graphs). Mean calibrated spectra (solid lines) flanked by the standard deviation (striped lines). Porcine (blue) vs. human adipose tissue (cyan), and porcine (red) vs. human nerve tissue (magenta). The gradient features Ft1 (in top 2 graphs), and Ft22 and Ft23 (in bottom 2 graphs) were found to be most accurately predicting the human dataset.