The Localized Elasticity Measurement for Coagulation Detection during HIFU Therapy

Abstract

High Intensity Focused Ultrasound (HIFU) treatment is one of minimally invasive treatment methods for cancer. To control a beam position and dose amount of HIFU, visualizing inside of the body during treatment is essentially required. Our objective is to develop an ultrasound monitoring system for thermally induced coagulation. Localized Motion Imaging (LMI) is a monitoring method to detect a localized mechanical response depending on stiffness change caused by thermal coagulation. In LMI, amplitude modulated HIFU causes oscillation of tissues in the HIFU focal area. Because an elastic modulus at coagulated area will increase, coagulated area will be detected as a decreased area of oscillation amplitude. To increase detection sensitivity for small coagulated area, a localization control of oscillation by changing modulation frequency were tested in experiments of porcine liver. A 2MHz transducer for HIFU and 7.5MHz transducer for imaging were used. The amplitude modulation frequency was ranging from 50 to 200Hz. Acoustic intensity of HIFU was 2000 w/cm2 at the focus and HIFU exposure time was 45 s. Amplitude decrease of tissues oscillation at focal point was detected in 5-10 s HIFU exposure at the highest modulation frequency. Detected amplitude was decrease to 0.2. This suggested that in LMI, high modulation frequency is suited to detection for small coagulation area or begging of coagulation.