Coagulation depth estimation using a line scanner for depth-resolved laser speckle contrast imaging

Abstract

Partial-thickness burn wounds extend partially through the dermis, leaving many pain receptors intact and making the injuries very painful. Due to the painfulness, quick assessment of the burn depth is important to not delay surgery of the wound if needed. Laser speckle imaging (LSI) of skin blood flow can be helpful in finding severe coagulation zones with impaired blood flow. However, LSI measurements are typically too superficial to properly reach the full depth of the adult dermis and cannot resolve the flow in depth. Diffuse correlation spectroscopy (DCS) uses varying source-detector separations to allow differentiation of flow depths but requires time-consuming 2D scanning to form an image of the burn area. We here present a prototype for a hybrid DCS and LSI technique called speckle contrast diffuse correlation spectroscopy (scDCS) with the novel approach of using a laser line as a source and using the speckle contrast of averaged images to obtain an estimate of static scattering in the tissue. This will allow for fast non-contact 1D scanning to perform 3D tomographic imaging, making quantitative estimates of the depth and area of the coagulation zone from burn wounds. Simulations and experimental results from a volumetric flow phantom and a gelatin wedge phantom show promise to determine coagulation depth. The aim is to develop a method that, in the future, could provide more quantitative estimates of coagulation depth in partial thickness burn wounds to better estimate when surgery is needed.

Fig. 1.

System set-up and flow phantom. (a) Laser light is shined on a line and speckle contrast is measured at different distances from it, corresponding to increasing measurement depth with distance. The system was tested on (b) an optical phantom with flow tubes at different depths from the surface and (c) a liquid phantom with a gelatin wedge on top to simulate coagulation from a burn wound with varying thickness. The red line marks the laser line orientation.

Fig. 2.

(a) Measurement and (b) simulation of the perfusion estimate. The rightmost tube is the most superficial and can be seen in level with the laser line source at y = 0 mm while the deeper tubes appear further away from the laser line. The speckle contrast has been normalized to be 1 in the center of the images (around x = 12.5, y = 0 mm) where only static scattering is expected. Dashed blue lines mark approximate tube locations (c) Measurement and (d) simulation of the static scattering estimate K_S. Dashed red lines mark the laser line illumination.

Fig. 3.

(a) The angle of the laser light is displaced by varying phantom height. (b) Average intensity over 50 pixels in the x direction over the pure liquid phantom part and over the gelatin wedge at 4 mm thickness. (c) Diffuse reflectance light intensity from the laser line at y = 0 mm. (d) Displacement of the peak position of the laser line due to the height of the gelatin wedge. This displacement was the used to deskew the image so that the laser line is centered on y = 0 in Fig. 4.

Fig. 4.

(a) Estimated perfusion from a gelatin wedge phantom over a liquid phantom with 1% fat intralipid as scattering and moving media thresholded at a perfusion level of 5. The perfusion isolevels can be seen to be further away from the laser line at y = 0 mm with increasing wedge thickness. (b) Comparisons of perfusion estimates at different thicknesses of the wedge compared with reduced Brownian motion from added glycerol to the liquid intralipid phantom. The perfusion estimates from 30% and 50% glycerol are similar to the estimates for 1 and 2 mm layer thickness respectively but the slope increases more slowly with distance from the laser line in the glycerol cases without gelatin layer on top compared to the varying gelatin thickness. (c-d) Estimated static scattering, K_S, in the same measurement. The traditional perfusion estimate is not only affected by the thickness of the coagulation layer but also greatly affected by the underlying perfusion, here modelled with varying viscosity form varying glycerol concentrations. The static scattering estimate, however, is mostly affected by the thickness of the coagulation layer. Note that in the ideal case, the perfusion estimates should increase with distance from the laser line while the static scattering estimate should decrease. However, opposite trends appear as artefacts once the distance is so large that the signal-to-noise becomes poor.

Fig. 5.

Isolevel distances, L, from the laser line. (b) Piecewise linear regression between the coagulation depth modelled with the wedge thickness, h, and static isolevels squared, L². When estimating h, The 30% isolevel is used as long as ${\text{L}}_{30\mathrm{\%}}^2$ ≥ 3 mm², then 20% isolevel as long as ${\text{L}}_{20\mathrm{\%}}^2$ ≥ 0.8 mm², and finally the 15% when ${\text{L}}_{20\mathrm{\%}}^2$ < 0.8 mm². The curves end when the wedge is so thick that the static scattering estimate never becomes as low as the corresponding isolevel at any distance. (c) Least squares fit between h and the static scattering S at the center of the laser line using a logarithmic function $\hat{\mathrm{h}}=-\text{a}\ln (\text{b}-{\text{K}}_{\text{s}})+\text{c}=-1.04\ln (0.87-{\text{K}}_{\text{s}})+0.96$ .

Fig. 6.

(a) Estimation of coagulation thickness through piecewise linear regression against static scattering isolevels, L, utilizing the increasing depth penetration of the light with increasing distance from the laser line. (b) Estimation of coagulation thickness from least-squares fitting to a logarithmic function of the static scattering, S, at the center of the laser line, corresponding to a traditional LSI measurement without depth resolution. The fitting works well for a wedge thickness up to about 1.5 mm (at x around 25 mm), but when the wedge gets thicker, the fitting becomes much less accurate than when using the longer distances from the laser line where the signal comes from light that has travelled deeper.

Fig. 7.

Scanning in steps of 2 mm over a 1.8 mm solid wedge phantom over a liquid phantom of 1% intralipid with perfusion estimates at 0–4 mm from the laser line normalized with the perfusion estimates from the liquid phantom only (red area).

Fig. 8.

(a) Thickness estimate from static scattering isolevels L (b) Error in Thickness estimate, limited to the range 0 to 1 mm.. (c) Thickness estimate from static scattering S at the center of the laser line., (d) Error in Thickness estimate from center line, limited to the range 0 to 1 mm..