The role of acoustic nonlinearity in tissue heating behind a rib cage using a high-intensity focused ultrasound phased array

Abstract

The goal of this study was to investigate theoretically the effects of nonlinear propagation in a high intensity focused ultrasound (HIFU) field produced by a therapeutic phased array and the resultant heating of tissue behind a rib cage. Three configurations of focusing were simulated: in water, in water with ribs in the beam path, and in water with ribs backed by a layer of soft tissue. The Westervelt equation was used to model the nonlinear HIFU field and a 1 MHz phased array consisting of 254 circular elements was used as a boundary condition to the model. The temperature rise in tissue was modelled using the bioheat equation, and thermally necrosed volumes were calculated using the thermal dose formulation. The shapes of lesions predicted by the modelling were compared with those previously obtained in in vitro experiments at low power sonications. Intensity levels at the face of the array elements that corresponded to formation of high amplitude shock fronts in the focal region were determined as 10 W·cm−2 in the free field in water and 40 W·cm−2 in the presence of ribs. It was shown that exposures with shocks provided a substantial increase in tissue heating, and its better spatial localization in the main focal region only. The relative effects of overheating ribs and splitting of the focus due to the periodic structure of the ribs were therefore reduced. These results suggest that utilizing nonlinear propagation and shock formation effects can be beneficial for inducing confined HIFU lesions when irradiating through obstructions such as ribs. Design of compact therapeutic arrays to provide maximum power outputs with lower intensity levels at the elements is necessary to achieve shock wave regimes for clinically relevant sonication depths in tissue.

Histotripsy methods in mechanical disintegration of tissue: Towards clinical applications

“…Furthermore, part of the transducer aperture can be obstructed by ribs which can further increase the power needed to create boiling in situ [51, 94]. Provided these conditions can be met, boiling histotripsy may provide a safe therapeutic modality for treatment of tumors in the liver.…”

A Nonparametric Temperature Controller With Nonlinear Negative Reaction for Multi-Point Rapid MR-Guided HIFU Ablation

“…The topology of the near-field thermal build-up depends on the sonication pattern, aperture number, and sparsity of the phased-array transducer and the field is almost linear in this region [26]. When shocks form within the region of the main focus, nonlinear waveform distortion is almost negligible in the pre-focal regions of lower amplitude [26].…”

“…When shocks form within the region of the main focus, nonlinear waveform distortion is almost negligible in the pre-focal regions of lower amplitude [26]. Therefore, scaling the emitted acoustic power does not change significantly the intrinsic topology of the acoustic intensity in the near field.…”

“…Reciprocally, for a given temperature elevation at the focus, near-field heating may be decreased. The potential impact of shock waves in the acoustic regime on the temperature controller's performance has not been investigated here because the initial intensity at the transducer surface was one order of magnitude less than that theoretically required [26] to enter that regime. At the interaction of the HIFU beam (which is a propagating wave) with the soft tissue/air interface (e.g., skin, air-filled bowel) or soft tissue/bone interface, various thermo-acoustical side effects can occur when using the volumetric sonication paradigm [27].…”

Spatio-temporal quantitative thermography of pre-focal interactions between high intensity focused ultrasound and the rib cage

“…A physical mask was inserted in the pre-focal beam pathway, external to the body, to block the energy normally targeted on the ribs. Non-linear propagation in a high-intensity focused ultrasound (HIFU) field produced by a therapeutic phased array [30] which may yield an increase in heating of tissue behind a rib cage. Rather than suggesting novel methods for rib sparing during intercostal HIFU sonication, the aim of this current study was to investigate in-depth the thermal effects generated by the HIFU beam's interaction with the rib cage.…”