Effect of frequency on the change in backscattered ultrasound energy as a function of temperature

Clinical trials have demonstrated that hyperthermia improves cancer treatments. Previous studies have developed temperature imaging methods based on the change in the backscattered energy (CBE) of ultrasound in order to monitor temperature variations during hyperthermia. The CBE with temperature is primarily due to the temperature dependency of the backscattering coefficient, which largely depends on the ultrasound frequency. This study explored the effect of frequency on the CBE as a function of temperature. Phantom experiments involving uniform heating from 37 to 45 °C were carried out in a temperature-regulated water tank, and the raw backscattered signals were acquired using ultrasound transducers with center frequencies of 5, 10, and 15MHz for CBE imaging. The compensation of echo shift due to temperature increase was not made for simplifying the algorithm. In the condition without motion compensation, the results showed that the absolute value of the CBE (ACBE) increases with increasing temperature. The dynamic range of the ACBE at the focus of the transducer between 37 and 45 °C increased from 5.6 to 10 dB when the ultrasound frequency increased from 5 to 15 MHz, demonstrating that the sensitivity of the CBE image in indicating the temperature variation may increase with the ultrasound frequency. However, the dynamic range of the ACBE over the same temperature range increased from only 5.5 to 6.2 dB when the CBE data were acquired from the far field of the transducer. This indicates that the beam diffraction of the transducer may reduce the dependence of the CBE on frequency, suggesting that the beam diffraction effect should be taken into account when CBE imaging employs a high-frequency transducer for sensitive temperature detection.