The analysis and compensation of cortical thickness effect on ultrasonic backscatter signals in cancellous bone

Abstract

To determine the influence of the overlying cortical shell on ultrasonic backscatter signal in cancellous bone, ultrasonic backscatter simulations were carried out by a three dimensional finite difference time domain method. The simulated signals were obtained for various cortical thickness (CTh) and three central frequencies (1, 2.25, and 3.5 MHz). The integrated reflection coefficient (IRC), integrated transmission coefficient, and apparent integrated backscatter (AIB) were calculated from the signals. The results showed that the IRC oscillated with amplitudes decreasing with increasing CTh and the amplitudes diminished gradually when CTh was over 2.18 mm. The AIB fluctuated and decreased as the CTh increased (R = −0.72 ∼ −0.90, p < 0.05), and the amplitudes were much smaller when the CTh was over 2.18 mm. After removing the multiple reflections in the cortical shell, the corresponding AIB (referred as AIB−c) decreased and the fluctuations were smaller (R = −0.80 ∼ −0.96, p < 0.05). An explicit compensation method for the cortical effect was proposed. No significant correlations were observed between the CTh and the compensated AIB (CAIB: R = −0.19 ∼ 0.26, p > 0.05, and CAIB−c: R = −0.09 ∼ −0.00, p > 0.05, respectively), and the fluctuations in CAIB−c were also reduced. The results demonstrated that the effect of cortical thickness on backscatter signals was removed by the compensation method proposed in this study.