  Background: Needle-based procedures such as fine needle aspiration (FNA) and
thermal ablation, are often applied for thyroid nodule diagnosis and
therapeutic purposes, respectively. With blood vessels and nerves nearby, these
procedures can pose risks in damaging surrounding critical structures. Purpose:
The development and validation of innovative strategies to manage these risks
require a test object with well-characterized physical properties. For this
work, we focus on the application of ultrasound-guided thermal radio-frequency
ablation (RFA). Methods: We have developed an anthropomorphic phantom mimicking
the thyroid and surrounding anatomical and physiological structures that are
relevant to ultrasound-guided thermal ablation. The phantom was composed of a
mixture of polyacrylamide, water, and egg white extract and was cast using
molds in multiple steps. The thermal, acoustical, and electrical
characteristics were experimentally validated. The ablation zones were analyzed
via non-destructive T2-weighted MRI scans utilizing the relaxometry changes of
coagulated egg albumen, and the temperature distribution was monitored using an
array of fiber Bragg sensors. Results: The physical properties of the phantom
were verified both on ultrasound as well as its response to thermal ablation.
The final temperature achieved (92{\deg}C), the median percentage of the nodule
ablated (82.1%), the median volume ablated outside the nodule (0.8 mL), and the
median number of critical structures affected (0) were quantified. Conclusion:
An anthropomorphic phantom that can provide a realistic model for development
and training in ultrasound-guided needle-based thermal interventions for
thyroid nodules has been presented. In the future, this model can also be
extended to novel needle-based diagnostic procedures.
