Gelat, PN;
(2014)
Focusing the Field of a HIFU Array Transducer through Human Ribs.
Doctoral thesis (PhD), UCL (University College London).
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Abstract
High intensity focused ultrasound (HIFU) enables highly localised, non-invasive tissue ablation, and its efficacy in the treatment of a range of cancers, including those of the kidney, prostate and breast has been demonstrated. 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 remain, 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. In this thesis, an approach which predicts the acoustic field of a multi-element HIFU array scattered by human ribs, the topology of which was obtained from CT scan data, has been developed, implemented and validated. It is based on the boundary element method (BEM). Dissipative mechanisms were introduced into the propagating medium, along 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 solve the inverse problem of determining the complex surface normal velocities of a multi-element HIFU array 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 section multi-element array in the presence of human ribs. It was compared on six array-rib topologies against other methods of focusing through the ribs, including binarised apodisation based on geometric ray tracing, phase conjugation and the DORT method (décomposition de l’opérateur de retournement temporel). The constrained optimisation approach offers greater potential than the other focusing methods in terms of maximising the ratio of acoustic pressure magnitudes at the focus to those on the surface of the ribs whilst taking full advantage of the dynamic range of the phased array.
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