Table 2.

First-order texture features

Feature	Definition	Description
SUVmax, SUVavg, MTV, TLG	–	Conventional PET parameters
CV	Standard deviation of SUVs/SUVavg	A global heterogeneity marker. Higher values indicate more heterogeneity
AUC-CSH	Percent of total tumor volume above percent threshold of SUVmax, which threshold varies from 0 to 100%.	A global heterogeneity marker. Lower values indicate more heterogeneity
Skewness	$\frac{\frac{1}{E}{\sum }_i{\left(\mathit{\text{HISTO}}\left(i\right)-\overline{\mathit{\text{HISTO}}}\right)}^3}{{\left(\sqrt{\frac{1}{E}{\sum }_i{\left(\mathit{\text{HISTO}}\left(i\right)-\overline{\mathit{\text{HISTO}}}\right)}^2}\right)}^3}$ (with E = the total number of voxels in the VOI, $\overline{\mathit{\text{HISTO}}}$= the average of gray levels in the histogram)	The asymmetry of the gray-level distribution in the intensity frequency histogram
Kurtosis	$\frac{\frac{1}{E}{\sum }_i{\left(\mathit{\text{HISTO}}\left(i\right)-\overline{\mathit{\text{HISTO}}}\right)}^4}{{\left(\frac{1}{E}{\sum }_i{\left(\mathit{\text{HISTO}}\left(i\right)-\overline{\mathit{\text{HISTO}}}\right)}^2\right)}^2}$ (with E = the total number of voxels in the VOI, $\overline{\mathit{\text{HISTO}}}$ = the average of gray levels in the histogram)	The sharpness of the peak of the gray-level distribution in the intensity frequency histogram
Entropyhisto	$\sum _{i}p\left(i\right)\bullet log\left(p\left(i\right)+\epsilon \right)$	The randomness of the gray-level distribution in the intensity frequency histogram

AUC-CSH, area under curve of cumulative SUV-volume histogram; CV, coefficient of variation; MTV, metabolic tumor volume; SUV, standardized uptake values; SUV_avg, average of SUVs; SUV_max; maximum of SUVs; TLG, total legion glycolysis; VOI, volume of interest