Infrared thermography of the pig thorax: an assessment of selected regions of interest by computed tomographical and anatomical parameters

Abstract

Current methods of diagnosis of respiratory diseases in swine are invasive, time-consuming and expensive. Infrared thermography (IRT) of the thorax might provide a new method of high specificity to select swine affected with lung alterations for further diagnostics. In this study, layer thickness of different tissues was determined in frozen thorax slices (FTS) by computed tomography (CT) and then related to skin temperatures measured by IRT in healthy pigs. The aim was to determine appropriate regions of interest (ROI) for evaluation of IRT images. Organ layer thicknesses measured in CT images correspond to those measured in FTS. Temperature differences between lung ROIs and abdomen ROIs were positively correlated with lung layer thickness at certain localizations, and negatively correlated with the thickness of the thorax wall and of inner organ layers. Reference values of differences between skin temperatures were established for two ROIs on the thorax with potential practical use for lung health status determination. Respective ROIs were located on vertical lines crossing the 7th (right) and the 10th (left) thoracic vertebrae. The presence of ribs affected skin temperature significantly.

Figure 1

Frozen thorax slice, caudal view, transverse section at the 7th thoracic vertebra, db = dorsal lung border, vb = ventral lung border, blue ring = anatomical orientation point on the level of the 7th thoracic vertebras (OP7) on the left and right side of the thorax, white dots = position of the centres of the regions of interest (ROI) for IRT image evaluation, 7Hl‐7Hr = horizontal line dividing the ventral from the dorsal part of the thorax positioned at the ventral wall of the oesophagus. Auxiliary lines between the upper and middle thirds of the dorsal part (7dLl‐7dLr) and between the two halves of the ventral part (7vLl‐7vLr) of the lung.

Figure 2

CT image, Thorax, caudal view, transverse section at the 7th thoracic vertebra, blue rings = anatomical orientation point on the level of the 7th thoracic vertebras (OP7) on the left and right side of the thorax, white dots = position of the centres of the regions of interest (ROI) for IRT image evaluation, 7Hl‐7Hr = horizontal line dividing the ventral from the dorsal part of the thorax positioned at the ventral wall of the oesophagus. Auxiliary lines between the upper and middle thirds of the dorsal part (7dLl‐7dLr) and between the two halves of the ventral part (7vLl‐7vLr) of the lung. Red line = deep layer (DL), blue line = intermediate layer (IL), green line = superficial layer (SL), numbers in black boxes = total length of the three horizontal orange lines (7dLl‐7dLr, 7Hl‐7Hr and 7vLl‐7vLr) and the vertical line 7V.

Figure 3

IRT image of pig in sternal recumbency with ROIs and reference lines, left view, TA = 10.2°C, colour range: SW‐VarioCam, temperature range = 15–31°C, picture matrix: 384 × 288 pixel, emissivity of 0.96. 5.TV = vertical line of the 5th thoracic vertebra (TV), 7.TV = vertical line of the 7.TV (this line served as reference line to adapt the scale of the CT to the IRT images), 10.TV = vertical line of the 10.TV, 13.TV = vertical line of the 13.TV, Ref = ROI of the reference body, Abd1 and Abd2 = abdominal ROIs, R5 dL = ROI of line dL of the 5th thoracic vertebra (TV), R5H = ROI of line H of the 5.TV, R5vL = ROI of line vL of the 5.TV, R7 dL = ROI of line dL of the 7.TV, R7H = ROI of line H of the 7. TV, R7vL = ROI of line vL of the 7.TV, R10 dL = ROI of line dL of the 10.TV, R10H = ROI of line H of the 10. TV, R10vL = ROI of line vL of the 10.TV.

Figure 4

Influence of presence of ribs on temperature differences at location 5vL left. Boxplot showing the ranges of temperature differences between the lung ROI temperature at the left ventral thorax side at the level of the 5th vertebra (5vLl) and the maximal abdominal temperature. In group A (n = 15) pigs had ribs at that lung ROI location, while in group B (n = 5) pigs had no ribs at that location.