Table 3 Phantom experiment results. Data are expressed as the median and the interquartile range (^† normality test passed). The correlation coefficients r_s and p values obtained from the Spearman correlation analysis for amplitude and entropy as a function of the PCF are also provided.

1 MHz nonfocused

5.5

0.58 (0.52–0.64)

5.35 (5.33–5.36)

5.27 (5.24–5.31)

5.18 (5.10–5.23)

7.5

1.09 (0.81–1.38)

5.32 (5.28–5.35)

5.32 (5.17–5.39)

5.21 (5.06–5.30)

15

1.03 (0.89–1.14)

5.33 (5.31–5.35)

5.23 (5.16–5.30)

5.12 (4.99–5.25)

20

1.36 (1.18–1.58)

5.32 (5.28–5.36)

5.25 (5.19–5.32)

5.17 (5.01–5.39)

30

1.19 (1.11–1.38)

5.32 (5.29–5.34)

5.25 (5.20–5.32)

5.17 (5.10–5.32)

r_s: 0.75 (p < 0.05)^†

r_s: − 0.29 (p > 0.05)^†

r_s: − 0.15 (p > 0.05)^†

r_s: 0.15 (p > 0.05)^†

3.5 MHz nonfocused

5.5

0.33 (0.28–0.34)

4.95 (4.88–4.99)

5.11 (5.07–5.16)

5.14 (5.10–5.17)

7.5

0.37 (0.33–0.40)

4.98 (4.95–4.99)

5.14 (5.11–5.17)

5.14 (5.10–5.20)

15

0.41 (0.35–0.57)

5.03 (4.98–5.06)

5.19 (5.16–5.22)

5.15 (5.10–5.19)

20

0.53 (0.43–0.60)

5.05 (5.02–5.07)

5.15 (5.10–5.18)

5.12 (5.08–5.14)

30

0.46 (0.40–0.59)

4.96 (4.91–5.02)

5.12 (5.09–5.15)

5.10 (5.05–5.13)

r_s: 0.77 (p < 0.05)^†

r_s: 0.38 (p > 0.05)^†

r_s: 0.2 (p > 0.05)^†

r_s: − 0.47 (p < 0.05)^†

3.5 MHz focused

5.5

0.56 (0.51–0.63)

5.06 (4.95–5.13)

5.07 (4.93–5.16)

4.96 (4.84–5.06)

7.5

0.61 (0.54–0.69)

5.01 (4.99–5.02)

5.01 (4.97–5.05)

4.94 (4.78–5.00)

15

0.65 (0.58–0.71)

4.88 (4.65–4.96)

4.91 (4.70–4.98)

4.82 (4.72–4.91)

20

0.59 (0.54–0.62)

4.77 (4.53–4.92)

4.84 (4.62–4.98)

4.83 (4.59–4.93)

30

0.47 (0.41–0.57)

4.66 (4.39–4.79)

4.69 (4.35–4.89)

4.67 (4.47–4.80)

r_s: − 0.41 (p > 0.05)

r_s: − 0.83 (p < 0.05)

r_s: − 0.77 (p < 0.05)

r_s: − 0.73 (p < 0.05)