Study Of Influence Of Attenuation And Non-linearity On Focused Sound Fields

High intensity focused ultrasound is used in many regions. HIFU surgery asa new technique in the 21th century gets hold of more and more attentionbecause of the successfully clinical application. Clinical observations indicatethere are many acoustic questions need to be solved and studied urgently. Oneimportant question is the influence of high intensity and heavy soundattenuation in medium on sound focal region.We provided an illustration of a concave spherical ultrasound transducer inthis paper. From the view of point of physical acoustics, the influence ofnon-linearity induced by high intensity and heavy medium attenuation on focalregion was analyzed. The numerical simulation was completed by Rayleighintegral superposition method to simulate the nonlinear sound fieldapproximately. And we compared the Rayleigh integral superposition method tothe KZK equation, then we found that our method could well describe thenon-linearity of focused sound field and it was easier than numerically solvingthe KZK equation in time-domain, and more clear in physical relations.We do some experiments to identify the numerical computation results. Inorder to identify the influence of non-linearity on sound focal region, axial soundpressure distributions were measured in water and castor oil in the cases of lowpressure amplitude and high pressure amplitude. Our numerical computationresults obtained by Rayleigh integral superposition were consistent with theexperimental results.Both theoretical analysis and numerical computation indicated that thedisplacement value of focal region towards the transducer had the order of one millimeter with the increase of attenuation and intensity under the condition ofthis paper. At the same time, the shape of focal region was gradually changedfrom a long ellipsoid to a short ellipsoid with a 'fatty head and thin tail' form.The results indicate that both high intensity and heavy attenuation haveimportant influence on the site and shape of sound focal region. It should betaken into full account in the accurate locating and dosage controlling, teststandard constituting, and the clinical application of HIFU equipment.