| Summary: | 博士 === 臺灣大學 === 機械工程學研究所 === 96 === One of the main problems encountered when using conventional B-mode ultrasound (US) for targeting and monitoring purposes during ablation therapies employing high-intensity focused US (HIFU) is the appearance of strong interference in the obtained diagnostic US images. In this study, instead of avoiding the interference noise, we demonstrate how we used it to locate the focus of the HIFU transducer in both in-vitro tissue-mimicking phantoms and an ex-vivo tissue block. We found that when the B-mode image plane coincided with the HIFU focal plane, the interference noise was maximally converged and enhanced compared with the off-focus situations. Stronger interference noise was recorded when the angle between the US image plane and the HIFU axis was less than or equal to 90. By intentionally creating a target (group of bubbles) at the 3.5-MHz HIFU focus (7.1 mm in length and 0.7 mm in diameter), the position of the maximal noise convergence coincided well with the target. The differenced between the predicted focus and the actual one (bubbles) on x and z axes (axes perpendicular to the HIFU central axis, Fig. 1) were both about 0.9 mm. For y axis (HIFU central axis), the precision was within 1.0 mm. For tissue block ablation, the interference noise concentrated at the position of maximal heating of the HIFU-induced lesions. The proposed method can also be used to predict the position of the HIFU focus by using a low intensity output scheme before permanent changes in the target tissue were made.
The utilization of magnetic resonance imaging (MRI) for HIFU not only real-time monitoring of HIFU ablation but also allows the evaluation of HIFU-induced lesions after treatment. Our study proposed an interleaved dual gradient-echo technique to simultaneously estimate temperature changes and magnetization transfer (MT) contrast, reflecting respectively heating conditions and degree of tissue damage during HIFU treatment.
If the target hepatocellular carcinoma (HCC) is close to the surface of the liver, HIFU may overheat intervening tissue such as the diaphragm, abdominal wall, and skin. To avoid this complication, we propose inducing artificial ascites in the abdominal cavity so as to separate the liver from the peritoneum, and to serve as a heat sink to cool overlying structures and thereby avoid inducing permanent damage. Target tissue that was 10 mm below the liver surface was ablated in 12 New Zealand White rabbits: 6 in the experimental group and 6 in the control group. Artificial ascites was established in the experimental group by injecting normal saline into the abdominal cavity until the pressure reached 150 mmH2O. Artificial ascites not only reduced the probability and extent of thermal damage to intervening structures (P<0.05), but also had no adverse affect on the efficacy of HIFU ablation (P>0.05).
One of the major disadvantages of HIFU ablation was the small lesion size and thus the long treatment duration. In this study, the effect of using ultrasound contrast agent (UCA) to enlarge the lesion size was studied both in vitro and in vivo. The mechanisms of lesion formation in the presence of UCA microbubbles were studied in vitro and in vivo.
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