HIFU scattering by the ribs: constrained optimisation with a complex surface impedance boundary condition

12th Anglo-French Physical Acoustics Conference

Gélat, P.^1,2 ; Ter Haar, G.^3 ; Saffari, N.^2

National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom^1

Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom^2

Therapeutic Ultrasound Group, Joint Physics Department, Institute of Cancer Research, Sutton SM2 5NG, United Kingdom^3

关键词: Acoustic pressure field;    Clinical application;    Constrained optimisation problems;    Dissipative mechanisms;    Excitation frequency;    High intensity focused ultrasound;    Multielement array;    Treatment modality;   

Others  :  https://iopscience.iop.org/article/10.1088/1742-6596/498/1/012004/pdf
DOI  :  10.1088/1742-6596/498/1/012004

来源: IOP

【 摘 要 】

High intensity focused ultrasound (HIFU) enables highly localised, non-invasive tissue ablation and its efficacy has been demonstrated in the treatment of a range of cancers, including those of the kidney, prostate and breast. HIFU offers the ability to treat deep-seated tumours locally, and potentially bears fewer side effects than more established treatment modalities such as resection, chemotherapy and ionising radiation. There remains however a number of significant challenges which currently hinder its widespread clinical application. One of these challenges is the need to transmit sufficient energy through the ribcage to ablate tissue at the required foci whilst minimising the formation of side lobes and sparing healthy tissue. Ribs both absorb and reflect ultrasound strongly. This sometimes results in overheating of bone and overlying tissue during treatment, leading to skin burns. Successful treatment of a patient with tumours in the upper abdomen therefore requires a thorough understanding of the way acoustic and thermal energy is deposited. Previously, a boundary element (BE) approach based on a Generalised Minimal Residual (GMRES) implementation of the Burton-Miller formulation was developed to predict the field of a multi-element HIFU array scattered by human ribs, the topology of which was obtained from CT scan data [1]. Dissipative mechanisms inside the propagating medium have since been implemented, together with a complex surface impedance condition at the surface of the ribs. A reformulation of the boundary element equations as a constrained optimisation problem was carried out to determine the complex surface velocities of a multi-element HIFU array which generated the acoustic pressure field that best fitted a required acoustic pressure distribution in a least-squares sense. This was done whilst ensuring that an acoustic dose rate parameter at the surface of the ribs was kept below a specified threshold. The methodology was tested at an excitation frequency of 1 MHz on a spherical multi-element array in the presence of anatomical ribs.