Effect of scanning direction on the statistical parameters of ultrasonic signals backscattered from the annular pulley and tendon

Previous studies have demonstrated that quantitative parameters estimated from backscattering signals are capable of characterizing pulley and tendon tissues for being applied to diagnose the trigger finger syndrome. Yet, the probability density functions (PDFs) of the envelope signals were found to vary significantly between those acquired from the transverse and sagittal views of samples. To extensively explore these effects, this study used a 30 MHz high-frequency ultrasound and theoretical considerations, to further comprehend the effect of the ultrasonic scanning direction on the estimated statistical parameters from the annular pulley and tendon. The ex vivo experiments began with the preparations of pulley and superficial digital flexor tendon (SDFT) excised from the first annular (A1) pulley region of cadavers. The tissue samples were then immersed in a saline solution tank. The ultrasound system was arranged to allow the scanning of tissues from the direction parallel to the fiber axis, at 0°, to that of perpendicular direction, at 90°, in which the increment of scanning angle is 5°. Statistical parameters, including Nakagami parameter (m) and scaling parameter, were estimated from regions of the acquired backscattering signals. Histological slices were also made for results verification. The scaling parameters associated with A1 pulley and SDFT of different scanning angles did not vary significantly; while those of the corresponding m was decreased significantly, as the scanning angle increased. The m of SDFT tended to decline with the increase of scanning angle faster than that of A1 pulley. Specifically, the m parameters of A1 pulley and SDFT decreased respectively from 1.06 ± 0.10 to 0.88 ± 0.07 and from 1.03 ± 0.06 to 0.78 ± 0.03. The empirical results are consistent with Nakagami statistical distribution, and which demonstrate that m could be applied to quantitatively assess to characterize the PDF of high-frequency ultrasonic envelopes from the A1 pulley and SDFT at various scanning angles.