Table 2 Regression analysis results (Prob > F, R²) and regression equation for each model (rater, body position, MRI-based calculation method)

Novice (ultrasound)

Seated

p = 0.02; R² = 0.29; RMSE = 1.30

\({\text{MRI}}=4.86+0.37xUS\)

p = 0.05; R² = 0.21; RMSE = 1.30

\({\text{MRI}}=6.83+0.30xUS\)

Supine

p = 0.02; R² = 0.29; RMSE = 1.30

\({\text{MRI}}=4.63+0.39xUS\)

p = 0.02; R² = 0.28; RMSE = 1.25

\({\text{MRI}}=6.04+0.37xUS\)

Expert (ultrasound)

Seated

p = 0.01; R² = 0.32; RMSE = 1.28

\({\text{MRI}}=4.68+0.43xUS\)

p = 0.03; R² = 0.26; RMSE = 1.27

\({\text{MRI}}=6.48+0.37xUS\)

Supine

p = 0.002; R² = 0.48; RMSE = 1.12

\({\text{MRI}}=3.23+0.57xUS\)

p = 0.04; R² = 0.23; RMSE = 1.29

\({\text{MRI}}=6.45+0.38xUS\)