Ultrasound-based assessment of skin and bone thickness in the temporal bone region

Abstract

Hearing loss is a major global health concern, and bone-anchored hearing systems (BAHS) such as the Ponto™ system provide an important solution for patients who cannot benefit from conventional air conduction (AC) hearing aids. Successful implantation, however, depends strongly on the anatomical conditions at the implant site, where both cortical bone thickness and soft tissue thickness play critical roles in implant stability and soft tissue healing. In current clinical practice, bone thickness is typically assessed intraoperatively using a multi-step drilling procedure, while soft tissue thickness is measured manually with a needle and ruler during surgery. Preoperative computed tomography (CT) is therefore reserved mainly for patients with suspected abnormal anatomy, due to the associated ionizing radiation exposure and clinical cost. Ultrasound presents an attractive alternative because it is non-invasive, radiation-free, and widely accessible, and can be performed in the office prior ordering implants and performing the surgery. While previous studies have explored ultrasound-based estimation of either soft tissue thickness or cranial bone thickness individually, limited research has addressed the combined assessment required for preoperative assessment in BAHS implantation. This thesis therefore investigated the feasibility of using ultrasound to simultaneously and site-specifically determine soft tissue and cortical bone thickness in the temporal bone region relevant to implantation of the Ponto™ system. The study was conducted using analytical modelling and numerical simulations of ultrasound wave propagation in a multilayer structure. Different excitation waveforms, operating frequencies, and anatomical configurations were evaluated for their effects on signal amplitude, spatial pulse length (SPL), echo separability, and timeof- flight (ToF) accuracy. The results demonstrated that ultrasound-based thickness determination is technically feasible under the investigated conditions. Across all simulated configurations, consistent interface reflections enabled reliable identification of relevant tissue boundaries. The findings showed that accurate determination of ToF is governed not by signal amplitude or SPL alone, but by the balance between signal strength, temporal compactness, and echo separability. Chirp excitation achieved a favourable combination of high voltage peak and low SPL, while pulse excitation demonstrated comparable overall performance. Stable ToF errors below 2–3 % were achieved in several configurations, with cortical bone consistently exhibiting lower error than soft tissue. The simulations further demonstrated robustness in the presence of blood vessels and secondary reflections, with a selected intermediate-frequency range providing the best compromise between penetration depth and temporal resolution. Overall, the findings indicate that ultrasound has strong potential as a complementary tool for preoperative assessment in Ponto™ system implantation. Although further experimental validation is required, this work establishes a theoretical and numerical foundation for future development of ultrasound-based assessment methods for BAHS.