Image-guided tumour ablation - a new opportunity

Abstract

It is less than 20 years since the very first papers on ultrasound (US)-guided ablation of liver tumours were published. Initially ablation centred on US-guided percutaneous ethanol injection (PEI) in small, non-resectable hepatocellular carcinoma (HCC) and neodymium yttrium aluminium garnet (NdYAG) laser heating techniques in liver metastases. Since then radiofrequency ablation (RFA), microwave, percutaneous cryotherapy, percutaneous acetic acid injection, photodynamic therapy and high intensity focussed US have all been used. Technological development has seen increased power application, water-cooling and the development of an array of different applicators. Currently the majority of centres use radiofrequency, a heating technique whereby alternating current produces ionic agitation and frictional heating in the tissue immediately around the electrode. US guidance remains the optimal, quickest technique for applicator placement but monitoring has become more sophisticated with micro-bubble contrast-enhanced US, multi-planar, multi-slice or preferably co-registered 3D computed tomography or magnetic resonance (MR). Initial hopes for MR temperature monitoring have not yet been realised. Changes in T1 and proton resonance frequency can provide temperature information but require subtraction imaging and work well in areas where image co-registration is easy. This does not apply to the liver which remains the commonest site of tumour ablation.