# An Automatic Approach for Individual HU-Based Characterization of Lungs in COVID-19 Patients
## 3 Results
### HI-Density Histogram Parameters--Extraction and Analysis of 166 COVID-19 Patients  
Through the script created in the Matlab environment, all the histograms were succesfully fitted; for each of them, the values of the HU thresholds between the aerated and intermediate components (th1) and between the intermediate and consolidated (th2) were calculated for each of the individual lungs using the methods described:  
1. Gaussian;
2. Maximum gradient on the fit;
3. Maximum gradient on the data. Examples of several histograms of representative lungs are shown in Figure 5 as results of the application of the three methods. Overall (see Table 3), the th1 threshold values using the Gaussian method were in the range of \(-870\) to \(-386\) HU, with a median and standard deviation of \(-768\) and \(73\) HU, respectively; values obtained with the th2 threshold were between\(-232\) and \(32\) HU with median and standard deviation of \(-114\) and \(41\) HU, respectively. Using the method of the maximum gradient on the fit, the HU range was \(-927\) to \(-322\) HU with median and standard deviation of \(-780\) and \(77\) HU for the threshold th1, and \(-706\) to \(10\) HU with median and standard deviation of \(-87\) and \(61\) HU, respectively, for the th2 threshold. Lastly, the values obtained with the maximum gradient method on the data were, for the threshold th1, a range of \(-900\) to \(-430\) HU and median and standard deviation of \(-798\) and \(71\) HU, and a range of \(-271\) to \(64\) HU with \(-34\)\(\pm\)\(41\) HU for the median and standard deviation of the th2 threshold. \begin{table}
\begin{tabular}{r|c c c c c c c} \hline \hline \multirow{2}{*}{**Insert**} & \multirow{2}{*}{**Attenuation Coefficient**} & \multicolumn{2}{c}{**Light Speed**} & \multicolumn{2}{c}{**Brilliance**} & \multicolumn{2}{c}{**Incisive**} \\ \cline{3-8}  & & **\#70 keV (1/cm)** & **N. CT ROI** & **SD** & **N. CT ROI** & **SD** & **N. CT ROI** & **SD** \\ \hline _Air_ & \(0\) & \(-981.23\) & \(38.35\) & \(-972.12\) & \(42.43\) & \(-972.57\) & \(49.64\) \\ _PMP_ & \(0.157\) & \(-181.55\) & \(48.03\) & \(-175.48\) & \(54.13\) & \(-181.07\) & \(66.68\) \\ _LDPE_ & \(0.174\) & \(-90.60\) & \(46.14\) & \(-85.43\) & \(53.53\) & \(-90.17\) & \(67.73\) \\ _Polystyrene_ & \(0.188\) & \(-35.30\) & \(46.56\) & \(-30.19\) & \(52.44\) & \(-34.59\) & \(69.33\) \\ _Acrylic_ & \(0.215\) & \(126.99\) & \(47.75\) & \(125.71\) & \(57.40\) & \(121.08\) & \(66.92\) \\ _Delrin_ & \(0.245\) & \(350.22\) & \(52.85\) & \(347.33\) & \(56.96\) & \(341.85\) & \(70.77\) \\ _Teflon_ & \(0.363\) & \(960.24\) & \(68.70\) & \(940.18\) & \(59.92\) & \(940.09\) & \(74.29\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average CT values for the three types of equipment. Figure 4: Plot of the number of CT inserts as a function of the attenuation coefficient for the equipment used. The trend line for each of the data sets is also represented. Figure 5: (**a–f**) Examples of graphical representation of the threshold values found by analyzing the HU-density histograms with the methods described. The Kruskal-Wallis non-parametric statistical significance test demonstrates that the distributions of the values of the thresholds _th1_ and _th2_ calculated with the three methods described were statistically different, with a significance level _p_-value <0.001 for both thresholds. While inter-patient variability was low for the th2 threshold, th1 threshold values were far more variable, as they concerned the fibrotic component of the consolidated lung. This is likely because the aerated component of the lung obtained from CT images, taken in conditions of maximum inspirium, is an extremely patient-dependent measure of lung function. As can be seen from the boxplots in Figure 6, the method with the least dispersion was the maximum gradient on the data. Also from the qualitative point of view, it was clear that the method that visually best identified the separation of the components of the lungs of COVID-19 patients was that of maximum gradient on the data.