Fabrication and Evaluation of Mixed Tissue Phantom for Medical Ultrasound Research

Abstract

An unmet need exists for a non-ionizing, economical, and effective intraoperative medical imaging modality to guide orthopedic surgeries. In the last few decades, ultrasound imaging has been recognized as a cost-effective and safe imaging modality for the diagnosis and intraoperative guidance of soft tissues like breasts and the liver. However, ultrasound (US) imaging around a bony structure is challenging because of the large acoustic impedance mismatch between bony tissue and soft tissue and the high acoustic attenuation properties of bone. In general, a US imaging phantom is an object specifically fabricated to replicate the acoustic characteristics of biological tissue. It is used in US imaging to evaluate, examine, and enhance the functionality of different US imaging equipment. For a US phantom to be useful in context of orthopedic surgeries such as spinal fusion surgery, it must have the appropriate acoustic and anatomical characteristics to mimic the bone and surrounding regions, such as the vertebra & soft tissue system. It broadly requires a Mixed Tissue Phantom (MTP) with three parts mimicking: soft-tissue, cancellous bone, and cortical bone. Previously, various groups have fabricated ultrasound bone phantoms using various materials and methods. Broadly they used only one single material such as acrylic, carbon fiber plastics, or ebonite to mimic the cancellous and cortical bones, including the fabrication of the phantom with 3D printable materials such as ABS, PLA, and resins. The acoustic properties of both bones are significantly different; hence, using a single tissue-mimicking material does not suffice to meet the requirement to mimic bone tissue. Some groups reported combining different materials or using various compositions, including mixtures of additives with resins, to create bone phantoms. However, these studies focused on matching acoustic properties of phantom material but did not address an aspect of anatomically correct shape. As part of this research work, a vertebra (bone) phantom and a MTP was fabricated. The soft tissue phantom was made with an agar-based mixture because of its ease of fabrication, cost effectiveness, and durability. The cancellous bone phantom was made of PLA by 3D printing an anatomically correct vertebra STL model. The cortical bone phantom was made with an epoxy-based material, and it was manually coated over the cancellous bone phantom (PLA) before solidification to make the vertebra phantom. The selection of the tissue mimicking materials for the phantoms is motivated by the review of scientific literature and their similarity in acoustic properties with real tissues. The vertebra phantom was scanned using Computational Tomography (CT) as well as with vernier callipers to assess its dimensional accuracy and match with anatomical structures as per the medical literature. The cortical layer thickness in the fabricated phantoms was found to be within the acceptable range. The process used for the fabrication of MTP in the current work is novel. This fabrication process provides an economical, readily available, and acoustically compliant bone phantom. This process can be generalized for any orthopedic structure. As part of the ultrasonic evaluation, US scans of the MTP were performed cross-sectionally using a portable ultrasound machine and curvilinear probe using two available imaging modes; standard and compounded. US scans were also conducted using a research 6 ultrasound system with three different imaging reconstruction algorithms: Conventional Bmode, Flash, and Synthetic Aperture. The resulting images were compared quantitatively and qualitatively for the US image quality. All the scanned image data using the best reconstruction algorithm among the above algorithms and imaging modes were imported into 3D slicer software to measure the anatomical landmarks and render the volumetric image. US images, rendered volume, and measurements on them helped measure and delineate a few anatomical landmarks of the vertebrae phantom, such as spinal processes, articular processes, laminas, upper end-plate depth, the thickness of the spinal process, transverse process width, height of the vertebra, skin to transverse process depth, insertion points for the pedicle screws, and distance between facet joints. These landmarks are considered essential for preoperative and intraoperative activities for spinal fusion surgeries. The research provides an invaluable tool to test various image reconstruction algorithms and customized ultrasound hardware for intraoperative 3D US usage for orthopedic surgeries in general and spinal fusion surgeries in particular, in a laboratory setting prior to clinical evaluations.