Approaches for modelling interstitial ultrasound ablation of tumours within or adjacent to bone: Theoretical and experimental evaluations

Abstract

Purpose The objectives of this study were to develop numerical models of interstitial ultrasound ablation of tumors within or adjacent to bone, to evaluate model performance through theoretical analysis, and to validate the models and approximations used through comparison to experiments. Methods 3D transient biothermal and acoustic finite element models were developed, employing four approximations of 7 MHz ultrasound propagation at bone/soft tissue interfaces. The various approximations considered or excluded reflection, refraction, angle-dependence of transmission coefficients, shear mode conversion, and volumetric heat deposition. Simulations were performed for parametric and comparative studies. Experiments within ex vivo tissues and phantoms were performed to validate the models by comparison to simulations. Temperature measurements were conducted using needle thermocouples or MR temperature imaging (MRTI). Finite element models representing heterogeneous tissue geometries were created based on segmented MR images. Results High ultrasound absorption at bone/soft tissue interfaces increased the volumes of target tissue that could be ablated. Models using simplified approximations produced temperature profiles closely matching both more comprehensive models and experimental results, with good agreement between 3D calculations and MRTI. The correlation coefficients between simulated and measured temperature profiles in phantoms ranged from 0.852 to 0.967 (p-value < 0.01) for the four models. Conclusions Models using approximations of interstitial ultrasound energy deposition around bone/soft tissue interfaces produced temperature distributions in close agreement with comprehensive simulations and experimental measurements. These models may be applied to accurately predict temperatures produced by interstitial ultrasound ablation of tumors near and within bone, with applications toward treatment planning.

Interstitial ultrasound ablation of vertebral and paraspinal tumours: Parametric and patient-specific simulations

“…This model was experimentally validated by our group in bone/soft tissue phantoms and in ex vivo bones and soft tissues, through comparison to temperature distributions derived both invasively and using MRTI [38, 47, 48]. $$Q=2\alpha I=2\mathcal{T}\alpha I_s\frac{r_t}{r}{e}^{-2{\int }_{r_t}^r\mu d{r}^{\prime }}$$ where α is the ultrasound absorption coefficient (Np/m), I is acoustic intensity (W/m 2 ), $\mathrm{scriptT}$ is the transmission coefficient (unitless), I s is the acoustic intensity on the transducer surface (W/m 2 ), r t is the transducer radius (m), r is the radial distance from the transducer's central axis (m), and μ is the ultrasound attenuation coefficient (Np/m).…”

“…Furthermore, the high acoustic absorption coefficient of bone, one to two orders of magnitude above that of soft tissue [35], limits energy penetration and produces significant heating at the bone surfaces [36]. Thus, bone adjacent to or encapsulating a tumour may effectively isolate the acoustic energy and effectively produce a secondary heat source away from the ultrasound applicator, enhancing heating [37, 38]. Bone adjacent to a tumour can also serve to protect nearby sensitive structures, such as the spinal cord, through acoustic and thermal insulation [18].…”

Catheter-based ultrasound technology for image-guided thermal therapy: Current technology and applications

“…Tumor destruction was identified through histological staining. Simulation and ex vivo tissue studies were reported recently where performance of such interstitial ultrasound devices was characterized across a range of design parameters to explore treatments options for tumors in different locations and sizes [23]. Patient specific modeling studies were also conducted to investigate treatment delivery strategies [24], demonstrating that the interaction of ultrasound energy with surrounding bone can be used to enhance localization of therapy.…”

Investigation of interstitial ultrasound ablation of spinal and paraspinal tumors: A patient-specific and parametric simulation study

“…14 3 MHz transducers (3.2 mm OD, 5 or 15 mm L) within a 4 mm ID, 4.5 mm OD catheter were also considered. It has been previously shown that acoustic heat deposition in soft tissue and bone from a cylindrical interstitial ultrasound source may be modeled as a radially divergent intensity pattern well-collimated to the length of the transducer 15,16 : (2) where is the ultrasound absorption coefficient (Np/m), is the transmission coefficient (unitless) calculated assuming normal incidence, 21 is the acoustic intensity on the transducer surface (W/m 2 ), is the transducer radius (m), is the radial distance from the transducer's axis (m), and is the ultrasound attenuation coefficient (Np/m). The absorption coefficient is assumed to be equal to the attenuation coefficient.…”