## 2 Materials and Methods
### Radionic Analysis  
#### 2.2.1 Feature Extraction  
ROIs were drawn on each US image in a polygonal shape to encompass the largest area (Figure 3a). Image normalization was performed according to the software manufacturer's recommendation, with a gray level of 256 and bin number of 10. A total of 104 radiomic features were extracted, including 53 first-order features of intensity-based and intensity histogram features and 51 high-order features, including gray-level co-occurrence matrices (GLCM), gray-level run length matrices (GLRLM), and gray-level size zone matrices (GLSZM) as follows (features in intensity histogram, GLCM, GLRLM and GLSZM were calculated after image normalization):  
* First-order features
* Intensity-based features: mean intensity, intensity variance, intensity skewness, intensity kurtosis, median intensity, minimum intensity, maximum intensity, 10 th intensity percentile, 25 th intensity percentile, 50 th intensity percentile, 75 th intensity percentile, 90 th intensity percentile, standard deviation, intensity interquartile range, intensity range, intensity based mean absolute deviation, intensity based robust mean absolute deviation, intensity based median absolute deviation, intensity based coefficient of variation, intensity based quartile coefficient of dispersion, area under curve of cumulative intensity volume histogram, intensity based energy, root mean square intensity
* Intensity histogram features: mean discretized intensity, discretized intensity variance, discretized intensity skewness, discretized intensity kurtosis, median discretized intensity, discretized minimum gray level, 10 th discretized intensity percentile, 25 th discretized intensity percentile, 50 th discretized intensity percentile, 75 th discretized intensity percentile, 90 th discretized intensity percentile, intensity histogram standard deviation, discretized maximum gray level, discretized intensity range, discretized intensity range, discretized intensity mean absolute deviation, discretized intensity median absolute deviation, discretized intensity coefficient of variation, discretized intensity quartile coefficient of dispersion, discretized intensity entropy (based log10), discretized intensity entropy (based log2), area under curve of cumulative intensity volume histogram, discretized intensity uniformity, discretized intensity root mean square, maximum histogram gradient, maximum histogram gradient gray level, minimum histogram gradient gray level
* High-order features
* GLCM: joint maximum, joint average, joint variance, joint entropy (based log2), joint entropy (based log10), difference average, difference variance, difference entropy, sum average, sum variance, sum entropy, angular second moment, contrast, dissimilarity, inverse difference, normalized inverse difference, inverse difference moment, normalized inverse difference moment, inverse variance, correlation, autocorrelation, cluster tendency, cluster shade, cluster prominence
* GLRLM: short runs emphasis, long runs 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, run length non-uniformity, run percentage
* GLSZM: small zone emphasis, large zone emphasis, low gray level zone emphasis, high gray level zone emphasis, small zone high gray level emphasis, large zone low gray level emphasis, large zone high gray level emphasis, gray level non-uniformity, normalized gray level non-uniformity, zone size non-uniformity, normalized zone size non-uniformity, zone size non  
For ICC calculation, ROIs were redrawn on 50 US images in a square shape containing the greatest area of parotid gland parenchyma (Figure 3b). The shape was changed to reduce the variations in results due to different ROI shapes.