Doppler signals observed during high temperature thermal ablation are the result of boiling

Purpose: To elucidate the causation mechanism of Spectral Doppler ultrasound signals (DUS) observed during high temperature thermal ablation and evaluate their potential for image-guidance. Methods: Sixteen ex vivo ablations were performed in fresh turkey breast muscle, eight with radiofrequency ablation (RFA) devices, and eight with a conductive interstitial thermal therapy (CITT) device. Temperature changes in the ablation zone were measured with thermocouples located at 1 to 10 mm away from the ablation probes. Concomitantly, DUS were recorded using a standard diagnostic ultrasound scanner. Retrospectively, sustained observations of DUS were correlated with measured temperatures. Sustained DUS was arbitrarily defined as the Doppler signals lasting more than 10 s as observed in the diagnostic ultrasound videos captured from the scanner. Results: For RFA experiments, minimum average temperature (T1 ± SD) at which sustained DUS were observed was 97.2 ± 7.3°C, while the maximum average temperature (T2 ± SD) at which DUS were not seen was 74.3 ± 9.1°C. For CITT ablation, T1 and T2 were 95.7 ± 5.9°C and 91.6 ± 7.2°C, respectively. It was also observed, especially during CITT ablation, that temperatures remained relatively constant during Doppler activity. Conclusions: The value of T1 was near the standard boiling point of water (99.61°C) while T2 was below it. Together, T1 and T2 support the conclusion that DUS during high temperature thermal ablation are the result of boiling (phase change). This conclusion is also supported by the nearly constant temperature histories maintained at locations from which DUS emanated.