Research On Key Problems Of Dose Control For High Intensity Focused Ultrasound Therapy

HIFU therapy has been regarded as one of the most potential new method about non-invasive tumor treatment. Its basic principle is through using the acoustic focusing function to focus the low energy ultrasound on the treatment area in biological medium, the temperature in the treatment area is up to65℃in a short time, which makes the tissue protein coagulation necrosis in target area. At the same time, the thermal effects scarcely hurt the surrounding normal tissue. The purpose in the future is minimally invasive even non-invasive treatment.Ultrasound dose control is a premise to ensure the efficiency and safety of HIFU therapy, which is also the key problem to be solved urgently and worthy of in-depth study in the HIFU therapy at prsent. The foundation of dose control is acoustic focusing function and temperature control. Evaluation of the therapeutic effect is an important basis for dose optimization.In this paper, the key problems of HIFU dose control were researched, and the multi-layer biological tissue model was established. A lot of experiments based on the multi-layer tissue model were simulated; the influences of various factors on the sound field and temperature field were analyzed, and the acoustic focusing and temperature control method was established. The research results showed that it is important significance to promote the application of HIFU technology and improve the performance of HIFU equipment. We did some work as follows:First, influences of the correlation characteristics of the biological media and the characteristics of the sound source on ultrasonic field distribution were studied in detail, and the essence of the thermal effects by the interaction of ultrasound and biological medium was discovered.Second, a multi-layer tissue model was established, sound field and temperature field induced by self-focusing concave spherical transducer in multi-layer media were simulated through solving the Pennes equation with finite difference method, influences of the correlation parameters of the transducer and characteristics of biological medium on sound field and temperature field were analyzed.Third, analyzing ultrasonic propagation characteristics in the multi-layer biological media, the problem about the refraction of ultrasound focus shift and deformation of the focal region by HIFU therapy was solved through introducing the phase compensation factors. The position control function of the Pseudo-inverse algorithm was simplified, thus, which could relatively simplify the control of the ultrasonic phased array, the calculation time was shorten greatly, and real-time processing was facilitated. At the same time, the sound field induced by the ultrasonic phased array in the multi-layer biological media was optimized with genetic algorithm, and the sound intensity gain was improved obviously.Fourth, the temperature field distribution based on the convective heat transfer model was analyzed with various parameters of vessel location, size, and blood velocity. The simulation results showed that the temperature of the tissue around vascular would decrease while increasing the vessel diameter, the effective therapeutic region in the focusing region would be reduced, when the vessel diameter was increased to a certain degree, even the therapeutic region in the surrounding vascular tissues may not be formed, as the temperature was not up to65℃at the same irradiation time.Fifth, according to speckle noise characteristics of B-mode image and the existing problems of B-mode image de-noising methods, a new image de-noising method based on adaptive mask filtering-improved wavelet transform was proposed, B-mode image generated by HIFU therapy was de-noised by this method, the results showed that this method could effectively remove the noise in the B-mode image, and could retain detail in the original image. The therapeutic region by HIFU was divided by level set method, which could detect more accurately the therapeutic region. According to image feature exacted from B-mode subtraction image, grade of lesion area by HIFU treatment was classed, this classification was verified to rationality according to the injury area obtained by experimental anatomy. In addition, the improved pseudo-color conversion method was also proposed in this paper, which could convert the treatment region, and help medical staff intuitively judge the degree of the injury.Sixth, the effect of HIFU therapy was greatly relation to the ultrasound dose in the clinic treatment, the role of the size of ultrasound dose was reflected by the temperature in the focus area, the characteristics of the existing temperature controller was analysized, an adaptive reference model of the temperature control suitable for HIFU therapy was established, which could regulate the temperature in the therapeutic area by adjusting the ultrasound transducer sound intensity and irradiation time. The simulation results showed that the effect of the temperature control obtained by this method was very good; there was no overshoot and oscillation phenomenon during the entire processing of HIFU therapy; the temperature in the focus area could reach the desired temperature threshold.