Table 1 Texture features calculated from the [¹⁸F]FDG-PET/CT images in Lifex to characterize thyroid incidentalomas

Homogeneity 1

Energy_GLCM

Contrast_GLCM

Correlation_GLCM

Entropy_GLCM

Dissimilarity

Correlation measures the linear dependency of the gray levels in the GLCM matrix and is defined as the average over the 13 directions of: \( \left(\sum_{i,j}\frac{\left(i-\mu \right)x\left(j-\mu \right) xC\left(i,j\right)}{\sigma_i{\sigma}_j}\right) \) where μ_i,j correspond to the average on row i or column j and σ_i,j correspond to the variance on row i or column j.

Contrast_NGTDM

Coarseness

Short-Run Emphasis

Long-Run Emphasis

Low Gray-level Run Emphasis

High Gray-level Run Emphasis

Short-Run Low Gray-level Emphasis

Short-Run High Gray-level Emphasis

Long-Run Low Gray-level Emphasis

Long-Run High Gray-level Emphasis

Gray-Level Non-Uniformity for run

Run Length Non-Uniformity

Run Percentage

Short-Zone Emphasis

Long-Zone Emphasis

Low Gray-level Zone Emphasis

High Gray-level Zone Emphasis

Short-Zone Low Gray-level Emphasis

Short-Zone High Gray-level Emphasis

Long-Zone Low Gray-level Emphasis

Long-Zone High Gray-level Emphasis

Gray-Level Non-Uniformity for zone

Zone Length Non-Uniformity Zone

Percentage

Sphericity

Compacity

Compacity (Shape and size): \( \frac{A^{3/2}}{V} \) where A and V correspond to the area and the volume of the volume of interest from the Delaunay triangulation.

Skewness

Kurtosis

Entropy_Hist

Energy_Hist

Skewness \( \frac{a\sum_i{\left(H(i)- Mean\right)}^3}{{\left(\sqrt{a\sum_i{\left(H(i)- Mean\right)}^2}\right)}^3} \) where a is the inverse of the total number of voxels in the volume of interest and mean is the average of the intensity values in the histogram. The sum is extended to all voxels in the volume of interest.

Kurtosis \( \frac{a\sum_i{\left(H(i)- Mean\right)}^4}{{\left(a\sum_i{\left(H(i)- Mean\right)}^2\right)}^2} \)

SUV minimum, SUV maximum, SUVmean and SUVstandard deviation, SUVpeak within a sphere of 0.5 and 1 ml volume (mL) Total lesion glycolysis (TLG)

SUV_max maximum of the standardized uptake value in the volume of interest

SUV_std standard deviation of the SUV distribution in the volume of interest

MTV the metabolic tumor volume was determined as the total number of voxels with SUV > 40%

TLG is defined as the product of SUV_mean times the volume