The impact of laser ablation on optical soft tissue differentiation for tissue specific laser surgery-an experimental ex vivo study

Abstract

Background: Optical diffuse reflectance can remotely differentiate various bio tissues. To implement this technique in an optical feedback system to guide laser surgery in a tissue-specific way, the alteration of optical tissue properties by laser ablation has to be taken into account. It was the aim of this study to evaluate the general feasibility of optical soft tissue differentiation by diffuse reflectance spectroscopy under the influence of laser ablation, comparing the tissue differentiation results before and after laser intervention.

Methods: A total of 70 ex vivo tissue samples (5 tissue types) were taken from 14 bisected pig heads. Diffuse reflectance spectra were recorded before and after Er:YAG-laser ablation. The spectra were analyzed and differentiated using principal component analysis (PCA), followed by linear discriminant analysis (LDA). To assess the potential of tissue differentiation, area under the curve (AUC), sensitivity and specificity was computed for each pair of tissue types before and after laser ablation, and compared to each other.

Results: Optical tissue differentiation showed good results before laser exposure (total classification error 13.51%). However, the tissue pair nerve and fat yielded lower AUC results of only 0.75. After laser ablation slightly reduced differentiation results were found with a total classification error of 16.83%. The tissue pair nerve and fat showed enhanced differentiation (AUC: 0.85). Laser ablation reduced the sensitivity in 50% and specificity in 80% of the cases of tissue pair comparison. The sensitivity of nerve-fat differentiation was enhanced by 35%.

Conclusions: The observed results show the general feasibility of tissue differentiation by diffuse reflectance spectroscopy even under conditions of tissue alteration by laser ablation. The contrast enhancement for the differentiation between nerve and fat tissue after ablation is assumed to be due to laser removal of the surrounding lipid-rich nerve sheath. The results create the basis for a guidance system to control laser ablation in a tissue-specific way.

Figure 1

Experimental set-up for optical measurements: 1) Diffuse Reflectance Spectroscopy before laser ablation, 2) Tissue ablation with an Er:YAG laser (spot size ø 4 mm), 3) Diffuse Reflectance Spectroscopy in the area of ablation (a. Spectrometer, b. Pulsed xenon lamp, c. Reflection/backscattering probe, d. Tissue sample (pig, ex vivo), e. Er:YAG laser).

Figure 2

Histological slice of ablated skin: The ablation crater had range of depth of 350 to 500 μm on all soft tissue types used in this study. The superficial epithelium of the skin was removed by the laser ablation uncovering sub-epithelial tissue layers. A very small darkened margin is detectable on the surface of the ablated area, indicating minimal carbonization (Staining: H.E., magnification: 2,5x).

Figure 3

Diffuse reflectance spectra before ablation; Mean Standard Deviation of Measurements within the same tissue type: 7.238.

Figure 4

Diffuse reflectance spectra after ablation; Mean Standard Deviation of Measurements within the same tissue type: 7.216.

Figure 5

Centered and scaled diffuse reflectance spectra before ablation.

Figure 6

Centered and scaled diffuse reflectance spectra after ablation.