Experimental Comparison of Ultrasound -Derived Deformation Measurements for Soft Tissue Impact Biomechanics

Abstract

Background: Fall-related hip fractures represent a critical public health concern, with falls causing 95% of hip fractures in older adults and resulting in 25% mortality within one year. The global incidence of hip fractures is projected to nearly double by 2050, with particularly significant increases anticipated in Canada's aging population. Trochanteric soft tissue thickness plays a crucial role in impact dynamics during lateral falls by absorbing up to 70% of impact energy. From an epidemiology perspective, increased thickness is associated with reduced hip fracture risk. However, current understanding of soft tissue deformation during high-speed impact events remains limited, hindering the development of more effective fracture prevention interventions. Objective: This thesis aims to design and assess a dynamic ultrasound imaging protocol for quantifying soft tissue deformation during controlled, high-speed mechanical loading. The primary objectives are: (1) to design a 2D ultrasound motion tracking system using a linear actuator as a controlled mechanical loading standard, (2) to assess how ultrasound system sampling frequency affects measurement accuracy during rapid tissue deformation and (3) to evaluate the performance of the 2D ultrasound motion tracking system in a biologically representative porcine tissue specimen, assessing its accuracy under conditions that more closely mimic in vivo tissue complexity. Methods: A custom linear actuator system was used to deliver controlled compressive loading to a tissue-mimicking gel phantom at 0.5 m/s (low-speed) and 1.0 m/s (high-speed), while tissue deformation was simultaneously recorded using a Verasonics Vantage NXT 64LE ultrasound system. High-frame-rate B-mode ultrasound sequences were acquired using a 6 MHz transducer and processed via beamforming before motion tracking analysis. An Optotrak motion capture system served as the reference standard and provided synchronized displacement measurements by receiving a trigger from the ultrasound system. Ultrasound-derived displacement measurements were obtained using image-based motion tracking in ProAnalyst and compared with motion capture-derived deformation profiles. The protocol was further assessed using a biologically representative porcine tissue specimen. The effect of sampling frequency on measurement accuracy was assessed through direct ultrasound acquisition at multiple native frame rates and retrospective downsampling of 4000 fps acquisitions to evaluate the influence of temporal resolution independent of acquisition method. Statistical Analysis: Agreement between ultrasound-derived and motion capture-derived deformation measurements was assessed using Pearson’s correlation coefficient (r), root mean square error (RMSE), and peak deformation error. The effects of sampling frequency and loading velocity were evaluated using two-way and mixed-design analyses of variance. Independent-samples t-tests were used to compare direct acquisition and downsampled acquisitions at matched sampling frequencies. Statistical significance was defined as α = 0.05 with 95% confidence intervals. Results: Ultrasound-derived displacement measurements demonstrated strong agreement with motion capture measurements during phantom gel validation, with Pearson correlation coefficients of r = 0.970 at 0.5 m/s and r = 0.874 at 1.0 m/s. Sampling frequency significantly affected peak deformation accuracy, with the greatest errors occurring at sampling frequencies of 100 fps and below, particularly during faster loading conditions. At 1.0 m/s, peak deformation error increased to −78.40 ± 7.86% at 30 fps and −41.29 ± 35.55% at 100 fps, whereas sampling frequencies of 1000 fps and above maintained substantially lower errors. Downsampling analysis demonstrated that peak deformation accuracy was preserved from 4000 to 1000 fps, with significant degradation occurring only at 100 fps and 30 fps, indicating a threshold effect rather than a progressive decline with decreasing sampling frequency. Direct acquisition and downsampled acquisitions produced comparable peak deformation measurements between 100 and 3000 fps, although differences were observed at 30 fps. Porcine tissue results demonstrated the feasibility of using B-mode ultrasound motion tracking to quantify deformation and characterize layer-specific tissue responses during dynamic loading. Conclusion: High-frame-rate B-mode ultrasound combined with image-based motion tracking provides a reliable approach for quantifying soft tissue deformation during controlled dynamic loading. Accurate peak deformation estimation was maintained at sampling frequencies of approximately 1000 fps and above, whereas lower sampling frequencies resulted in substantial errors, particularly during rapid deformation events. These findings highlight the importance of sufficient temporal resolution for dynamic ultrasound deformation measurements and demonstrate the potential of high-frame-rate ultrasound for investigating tissue-specific mechanical responses. Significance: This research addresses critical gaps in understanding soft tissue deformation during high-speed impact events relevant to hip fracture biomechanics. By assessing dynamic ultrasound imaging protocols under controlled conditions, this thesis establishes a foundation for future in vivo studies of soft tissue behavior during falls and inform the development of more optimized hip protector designs and improved finite element models that account for subject-specific soft tissue properties.