Preliminary analysis of ultrasound elastography imaging-based thermometry on non-perfused ex vivo swine liver

Aims Real-time monitoring of tissue temperature during percutaneous tumor ablation improves treatment efficacy, leading clinicians in adjustment of treatment settings. This study aims at assessing feasibility of ultrasound thermometry during laser ablation of biological tissue using a specific ultrasound imaging techniques based on elastography acoustic radiation force impulse (ARFI). Methods ARFI uses high-intensity focused ultrasound pulses to generate 'radiation force' in tissue; this provokes tissue displacements trackable using correlation-based ultrasound methods: the sensitivity of shear waves velocity is able to detect temperature changes. Experiments were carried out using a Nd:YAG laser (power: 5 W) in three non-perfused ex vivo pig livers. In each organ, a thermocouple was placed close to the applicator tip (distance range 1.5-2.5 cm) used to record a reference temperature. Positioning of laser applicator and thermocouple was eco-guided. The organ was scanned by an echography system equipped with ARFI; propagation velocity was measured in a region of interest of 1 x 0.5 cm located close to thermocouple, to investigate influence of tissue temperature on shear waves velocity. Results Shear wave velocity has a very low sensitivity to temperature up to 55-60 degrees C, and in all cases, velocity is < 5 m s(-1); for temperature > 55-60 degrees C, velocity shows a steep increment. The system measures a value "over limit", meaning a velocity > 5 m s(-1). Conclusions Ultrasound thermometry during laser ablation of biological tissue based on elastography shows an abrupt output change at temperatures > 55-60 degrees C. This issue can have a relevant clinical impact, considering tumor necrosis when temperature crosses 55 degrees C to define the boundary of damaged volume.