Evaluating tissue changes with ultrasound during radiofrequency ablation.

ABSTRACT

The purpose of this study was to estimate tissue changes during radiofrequency (RF) ablation by correlating echo frequency shifts and temperature elevations. Experiments were performed on phantoms (tissue mimicking gel) and in-vitro turkey breast. Heating was performed with a modified RF-ablation system. Intermittent RF was applied and the temperature at the electrode tip was continually measured by an embedded thermocouple. Various voltages (10-30V) were applied to achieve a wide range of temperature elevations between 10 and 80 degrees C and ablation sizes between 5 and 27 mm in width. B-mode images and raw data were acquired every 5 s by a modified ultrasound imaging system. The raw data from each line and frame was processed using an algorithm to measure spectral shifts of the echo signals in the power spectrum. The phantom experiments showed positive frequency shifts as the temperature rose, with dependency on the heating rate. A linear relationship (R(2) > 0.96) was found between the RF-applied voltage and the width of the heated area, defined by frequency changes larger than 0.05 MHz. In-vitro experiments showed a correlation (R(2) = 0.84) between the width of the coagulated area and the maximal width of the region with more than 0.12 MHz frequency shifts, but a lower correlation (R(2) = 0.4) between the width of the coagulated area and the temperature elevation. In conclusion, correlation was found between echo frequency shifts and temperature elevations and between echo frequency shifts and the width of the ablated area during intermittent RF ablation. Our results suggest that, with further refinement and validation, ultrasound could be used to measure RF heating and its induced coagulation.