Heating produced by therapeutic ultrasound in the presence of a metal plate in the femur of canine cadavers

Thermal Modeling of Ultrasound Diathermy in Tissues with a Circular Inclusion near a Curved Interface

The influences of implants on the temperature field in tissues during ultrasound diathermy is controversial. In addition, most previous studies have focused on plate implants, and the effects of irregular implants and bones are seldom discussed. In this study, a hybrid computational framework per se is proposed to investigate the effects of double circular inclusions on the temperature distribution during ultrasound diathermy. The tissue–inclusion–bone structure is simplified as a two-dimensional bilayer composite model consisting of soft tissue and bone with a circular inclusion imbedded in the soft tissue. The interface between the bone layer and the soft-tissue layer is assumed as a convex surface for the incident ultrasonic waves. Multiply scattered waves originate between the two acoustic scatterers, i.e., the circular inclusion and the convex bone. The proposed computational framework consists of two kernels tackling ultrasound propagation and heat conduction problems, respectively. Making use of theoretical solutions of pressure fields, the transformed heat sources are efficiently obtained in the first kernel without sacrificing much computational burden. Temperature distributions in the composite media under ultrasound diathermy are evaluated via finite element numerical simulations in the second kernel. Numerical results indicate that the temperature distributions in the composite system obviously change when the bone layer changes from flat to convex. In addition, the inclusion size, location, material, and ultrasound operation frequency will also affect the temperature distribution and peak temperature during ultrasound diathermy. Pertinent findings could serve as a guide for clinical innovations in therapeutic ultrasounds.

Simplified Theoretical Model for Temperature Evaluation in Tissue–Implant–Bone Systems during Ultrasound Diathermy

Deep heating procedures are helpful in treating joint contractures that frequently occur with fractures and joint diseases involving surgical implants and artificial joint prostheses. This study uses a one-dimensional composite medium model consisting of parallel slabs as a simplified approach to shed light on the influences of implants during ultrasound diathermy. Analytical solutions for the one-dimensional transient heat generation and conduction problem were derived using the orthogonal expansion technique and a Green’s function approach. The analytical solutions provided deep insight into the temperature profile by therapeutic ultrasound heating in the composite system. The effects of the implant material type, tissue thickness, and ultrasound operation frequency on temperature distribution were studied for clinical application. In addition, sensitivity analyses were carried out to investigate the influences of material properties on the temperature distribution during ultrasound diathermy. Based on the derived analytical solutions, the numerical simulations indicate that materials with high density, high specific heat, and low thermal conductivity may be optimal implant materials. Among available implant materials, a tantalum implant, which can achieve a lower temperature rise within the tissue (hydrogel) and bone layers during ultrasound diathermy, is a better choice thanks to its thermodynamics.