Influence of ultrasound settings on carotid biomarker assessment by B-mode image processing

Introduction: This study aims to investigate how image settings affect carotid Intima-Media-Thickness (IMT) and diameter estimation performed with Carotid Studio (Quipu-Srl). Methods: Fourteen healthy volunteers (24.4 ± 4.4 years; eight men) were recruited. Longitudinal scans 3-cm-deep of common-carotid-arteries with the same region-of-interest were obtained by expert operator, starting from a "default-setup" (central Time Gain Compensation-TGC; gain: 50 dB; no persistence filter) and adjusting one parameter at a time (oblique TGC; gain: 10 dB increments from 30 to 70 dB; persistence filter). The acquisition was performed twice, and repeatability was expressed for each setup as Coefficient of Variation (CV). Significant changes (p <= 0.05) compared with the "default-setup" were analysed by Wilcoxon signed-rank-test. Results: Sixty and 70 dB gain resulted in overestimating IMT (from 0.460 ± 0.001 to 0.506 ± 0.030 mm and 0.538 ± 0.011 mm, p = 0.002 and 0.023 respectively vs. default-50 dB), underestimating the mean diameter (from 6.044 ± 0.040 mm to 5.763 ± 0.004 mm, p = 0.008 and to 5.698 ± 0.225 mm, p = 0.002), and for 70 dB, increasing IMT CV from 2.14 ± 1.92% to 6.85 ± 5.23% (p = 0.01) and diameter CV from 1.79 ± 1.22% to 4.77 ± 4.71% (p = 0.05). Distension CV increased from 5.71 ± 4.31% to 10.84 ± 5.91% (p = 0.041) and to 12.59 ± 7.97% (p = 0.021) for 60 and 70 dB respectively. Persistence and TGC did not affect repeatability, whereas diameter (5.955 ± 0.033 mm, p = 0.005) and distension (from 0.816 ± 0.019 mm to 0.655 ± 0.018 mm, p = 0.002) values were underestimated when persistence filter was active. Discussion: Precision and reproducibility of carotid parameters, automatically assessed by Carotid Studio, are affected by high gain levels, due to image grey-level saturation. Persistence filter activation leads to underestimate diameter and distension, possibly due to mathematical operator behaviour and image temporal filtering. Since ultrasound setup significantly affects carotid parameter assessment, it should be reported and replicated in follow-up scans.