Introduction/Purpose: CT provides the ability to quantitatively measure attenuation values of tissues using Hounsfield Units (HU). The goals of this study were to compare the CT attenuation values of different types of pleural effusion in canine and feline patients and to assess the association between specific cytologic parameters and HU. We hypothesized that CT attenuation values would significantly differ between types of effusion and that increased cell counts or total protein would be associated with higher attenuation.
Methods: Dogs and cats with pleural effusion that underwent thoracic CT and diagnostic thoracocentesis within 16 days of each other, between January 2000 and July 2015, were included in the study. Effusions were classified based on protein and cell counts into 5 different categories: exudates, transudates, modified transudates, hemorrhage, or chylous effusions. HU of the effusion was measured on three different CT images with the greatest quantity of fluid. HU means were compared between groups, linear correlation was performed between HU and cytologic parameters, and ROC curve analysis was performed.
Results: 111 studies were included, 37 cats and 74 dogs. 44 pleural effusions were exudates, 5 were transudates, 19 were modified transudates, 11 were hemorrhagic, and 32 were chylous. Significant differences were observed between groups (ANOVA, P<0.0001) with chylous effusion (8.5 +/- 5.7 HU (mean +/- SD)) and transudate (6.6 +/- 2.8) significantly different from exudate (19.1 +/- 9.0) and hemorrhage (22.5 +/- 8.8). No significant differences where found between modified transudate (14.5 +/- 9.6) and any other groups. A significant but weak linear correlation was identified between HU and each of total protein (P=0.031, R2=0.048), red blood cells (P=0.009, R2=0.065) and total nucleated cells (P=0.007, R2=0.069). Using a cut-off value of <14 HU identified transudate or chylous effusion with a sensitivity of 92% (95% CI, 78-98%) and a specificity of 64% (52-74%) (ROC area=0.82, P<0.0001). A cut-off value >20 HU had a specificity of 97% (86-100%) and a sensitivity of 41% (31-54%) for identifying exudate and hemorrhage.
Discussion/Conclusions: A significant difference in HU values exists for different pleural effusion categories, however overlap exists especially in the HU range from 14- 20 HU. Although HU measurements could not replace cytology, HU values <14 increase the suspicion for chylous or transudate and HU values > 20 likely indicate hemorrhage, exudate, or modified transudate. HU of pleural effusion weakly correlates with the protein concentration and number of cells in the fluid. This suggests that other factors, such as lipid content in chylous effusion, likely have an effect on HU values. Measurement of HU should be performed when pleural effusion is identified as it brings valuable information.