Abstract
High-intensity focused ultrasound (HIFU) is a well-accepted tool for noninvasive thermal therapy. To control the quality of HIFU treatment, the focal spot generated in tissues must be localized. Ultrasound imaging can monitor heated regions; in particular, the change in backscattered energy (CBE) allows parametric imaging to visualize thermal information in the tissue. Conventional CBE imaging constructed in the spatial domain may be easily affected by noises when the HIFU focal spot is visualized. This study proposes frequency-domain CBE imaging to improve noise tolerance and image contrast in HIFU focal spot monitoring. Phantom experiments were performed in a temperature-controlled environment. HIFU of 2.12 MHz was applied to the phantoms, during which a clinical scanner equipped with a 3-MHz convex array transducer was used to collect raw image data consisting of backscattered signals for B-mode, spatial-, and frequency-domain CBE imaging. Concurrently, temperature changes were measured at the focal spot using a thermocouple for comparison with CBE values by calculating the correlation coefficient r. To further analyze CBE image contrast levels, a contrast factor was introduced, and an independent t-test was performed to calculate the probability value p. Experimental results showed that frequency-domain CBE imaging performed well in thermal distribution visualization, enabling quantitative detection of temperature changes. The CBE value calculated in the frequency domain also correlated strongly with that obtained using the conventional spatial-domain approach (r = 0.97). In particular, compared with the image obtained through the conventional method, the contrast of the CBE image obtained using the method based on frequency-domain analysis increased by 2.5-fold (4 dB; p < 0.05). Frequency-domain computations may constitute a new strategy when ultrasound CBE imaging is used to localize the focal spot in HIFU treatment planning.
Highlights
High-intensity focused ultrasound (HIFU) is a therapeutic technique for inducing thermal lesions in tissues by focusing acoustic energy to generate a high temperature at the focal point
The results showed that, compared with conventional spatial-domain change in backscattered energy (CBE) imaging, frequency-domain CBE imaging provided improved image contrast for HIFU monitoring; the CBE value maintained a proportional relationship with temperature change
This study proposed a new method for ultrasound CBE imaging based on frequency-domain analysis
Summary
High-intensity focused ultrasound (HIFU) is a therapeutic technique for inducing thermal lesions in tissues by focusing acoustic energy to generate a high temperature at the focal point. The temperature increase alters the properties of ultrasound backscattered signals, including the echo time shift (caused by changes in the tissue thermal expansion and speed of sound)[7,8,9], acoustic attenuation[10], and change in backscattered energy (CBE)[11,12,13]. This implies that ultrasound parameters may serve as an alternative for localizing the HIFU-induced focal spot. Through a frequency-domain transformation, a frequency selection can be performed for data analysis to provide improved noise tolerance and image contrast for CBE imaging during HIFU focal spot monitoring
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