Study Of The Effect Of Ultrasound Frequency On The Lesion Induced By High-Intensity Focused Ultrasound In Layered Tissue

High Intensity Focused Ultrasound (HIFU) is a novel non-invasive surgical technique, which has been used in various tumor or other diseases treatment. HIFU treatment relies on absorption of ultrasound in tissue. However, more ultrasound absorption of tissue means quicker decrease of ultrasound and shorter penetrated distance in tissue. With the aim to achieving the largest tissue thermal lesion in the clinical treatment of deep tumors, we need make a balance between thermal absorption efficiency of tissue and penetrated distance of ultrasound.In this paper, the nonlinear propagation of ultrasound in layered tissue is calculated with the KZK equation and layered medium model, which is confirmed by acoustic field scanning experiment. The calculation of temperature distribution in tissue is conducted with Pennes bio-heat transfer equation. Simultaneously, the change of sound velocity and attenuation coefficient of tissue with temperature rise should been taken into consideration. Thermal dose in tissue is calculated with the equivalent thermal dose model, and threshold of thermal lesion is set at 30 EM. Coagulation necrosis experiment with HIFU irradiation to bovine liver tissue in vitro is conducted to validate the method of numerical calculation of thermal lesion range.Spherical shell ultrasonic transducers whose external diameter 22cm, internal diameter 8 cm and focal length 16cm is used in this study. Under the condition that the total output power of HIFU is 400 W, the ultrasound is focused on target region in liver tissue, which crossing through three layers tissue that is kin, fat and muscle. With different kinds of target depth which means the different distances from focus to skin face (5,7,10,15 cm) and different ultrasound frequency (0.4,0.6,0.8,1.0,1.2,1.4,1.6MHz), the influence of different frequency on focal domain size, the largest sound intensity and thermal absorption ratio is calculated and the reason that those variables change with frequency is analyzed. The change of the harmonic breeding coefficient and mechanical index with frequency is also calculated to analyze the effect of the mechanical effects to the tissues lesion.The experimental results of acoustic field scanning are in good agreement with the results of simulation. In the experiment of bovine liver tissue in vitro, when the acoustic intensity in tissue is low, the experimental results agree well with the simulation, or the result is greater than the simulation, which can been reasoned that contribution of cavitation and microbubbles to mechanical and thermal lesion of tissue has not been taken into consideration in the simulated model.Size of the focal region (-6dB size) has a sharp decline with the ultrasonic frequency increasing in a same target depth, while in the same ultrasonic frequencies, the size of the focal region (-6dB size) almost constant with the increasing target depth. During ultrasonic frequency increases, the maximum intensity, the largest heat absorption ratio and lesion volume in the focal region increased first and then decreased, but in different target depths(5,7,10,15cm), the optimal maximum frequency to obtain the largest heat absorption are not the same, which is 1.5MHz、 1.3MHz、1.1MHz and 0.9MHz in different target depths, and the optimal maximum frequency to reach the largest lesion is 0.8MHz、0.8MHz、 0.6MHz and 0.6MHz. With the ultrasonic frequency increases, harmonics breeding coefficient firstly increases, then decreases and achieves to a maximum level at a certain frequency, while mechanical index linearly decreases, and the same to the probability of the occurrence of cavitation.Considering the heat absorption efficiency, tissue thermal lesion volume,mechanical index and other factors of target tissue, we should select a lower ultrasonic frequency in a larger target depth, to obtain the greatest thermal lesion and mechanical effect.