| BACKGROUNDHIFU provides a non-invasive method which can be precisely focused without the skin incision. After the past decades development, HIFU has been successfully used in clinical uterine fibroids, prostate cancer, liver cancer, pancreatic cancer, breast cancer and other benign and malignant tumors treatment. However, in the course of treatment for uterine fibroids, often there will be some of the side effects of treatment should not happen. Despite thermal effects, studies had shown that the presence of cavitation bubbles would significantly promote tissue temperature and the occurrence of additional tissue damage. Therefore, the understanding of multibubble distribution as a function of acoustic power would make huge contribute to the lesion as a result of the presence of air bubbles. OBJECTIVEWith the help of sonoluminescence, passive cavitation detector(PCD) et al, we make a detail study on the multi-bubble activity level and spatial distribution as a function of different ultrasound power, different oxygen capacity of the working water based on a concave spherical and an open ends spherical focused ultrasound transducer, which would make a significance on the understanding of side effect at pre-focus and post-focus. METHOD(1) Study on multibubble distribute and sonoluminescence in concave spherical fieldThe Transducer used in our research was a 0.4 MHz shell self-focused transducer with a focal length of 150 mm and the rang of output ultrasonic power is 50 W- 410 W. With the aid of high speed and PCD acoustic powers dependence of multibubble distribution and multibubble dynamics respectively. Build a light-proof box, by using a digital camera and photomultiplier tube system to record spatial distribution and luminous intensity of SL under different acoustic power.(2) Study on multibubble distribute and sonoluminescence in spherically focused fieldThe transducer used in this research was a 0.4 MHz shell spherical focused transducer with a diametera of 650 mm, a height of 433 mm. By the using of oxygen measuring instrument to adjust the oxygen content of working water(3.0 mg/L, 4.0 mg/L and 4.5 mg/L, respectively). With a aid of a digital camera, we record the multibubble spatial distribution and luminous intensity of SL under different output voltage. Applying MATLAB image processing functions to preprocess the picture(remove the background noise and gray image binarization processing). Select the entire range of the sound field as a interest region, calculate the number of pixels of lighted and consider the the number of pixels as the SL intensity as a function of output voltage.(3) Place biological tissue in front of the focusUse fresh porcine abdominal tissue(include skin, fat and muscle) as experimental material. Apply oxygen measuring instrument to adjust the oxygen content of working water(3.0 mg/L) and place biological tissue in front of the focus(3 cm, 4 cm, 5 cm). With a aid of a digital camera, we record the SL spatial distribution at different power and observe the lesion. RESULTS(1) Study on multibubble distribute and sonoluminescence in concave spherical fieldFirst of all, we fixed oxygen capacity as 3.0 mg/L, and observed the bubble spatial distribution at different sound power(50 W-410 W, spacing 30 W) of focused sound field. When the low input power was given, we could see a typical standing wave pattern produced by the acoustic reflect in the liquid- gas interface and a cluster of bubbles was trapped in the antinodes of the standing wave field, while the PCD results showed insignificant broadband noise. The SL region mainly located in pre-focus firstly. When the input power was increased, the free-floating nucleated bubbles were subjected to the radiation forces and pushed to the focus which disturbed standing wave pattern field. The results in high speed camera showed that bubbles were firstly gathering at the focus, then growing toward the post-focus region and disappearing about 6 ms later. A number of higher harmonics apperaed and the amplitude of broadband noise increased monotonically, the SL region start growing toward transducer and the SL near the focus increased gradually. When the input power was sufficient, the bubbles gathered at the focus, collaped persistently, meanwhile, a large hissing noise was heard with an oval-shaped bright spot would emerge in field which weaken the SL intensity sharply.Secondly, we fixed the acoustic power as 140 W, and observed the bubble spatial distribution under different oxygen capacity(2.5 mg/L, 3.0 mg/L, 3.5 mg/L). It was found that when oxygen capacity of the working water is low(such as 2.5 mg/L), the MBSL spatial distribution seems like a conical region and concentrated in pre-focus in focused ultrasound field; the spectrum analysis of the scattering signal by bubble cloud at focus reminded that cavitation signal concentrated harmonic and second harmonic with nonsignificant broadband noise. After increasing the oxygen capacity(such as 3.0 mg/L),MBSL growth toward the transducer surface;the spectrum analysis of the scattering signal by bubble cloud at focus reminded that harmonic breeding,but there was no significant broadband noise appeared in the beginning of acoustic irradiation until a period of time increased suddenly. When the oxygen capacity increased to 3.5 mg/L, the MBSL beyond the pre-focus and there was a disc-shaped region at post-focus;the spectrum analysis of the scattering signal by bubble cloud at focus reminded that harmonic breeding and the time required for the broadband noise appeared significantly increase has been shortened.(2) Study on multibubble distribute and sonoluminescence in spherically focused fieldFirst of all, we fixed oxygen capacity as 3.0 mg/L. When the low output voltage(20 V-90 V) was given, we can see that the free diffusion of bubbles before irradiation was trapped in the antinodes of the standing wave field. With the increase of output voltage(such as 90 V), bubbles in focused field under the power of radiation force gathered in the center of transducer and the light emitting region was divided into two parts(a bunch of light-emitting point group at Y direction and a radial circular aperture at a certain distance from the center of the sphere). As the drive voltage increased, the outer radial circular aperture growed toward the inner wall of the transducer. When the drive voltage increased to 310 V, a bubble cluster appeared at the focus, collaped persistently, meanwhile, a large hissing noise was heard. Only an oval-shaped bright spot would emerge at the center of transducer. Further increasing the output voltage, there was isignificant change.Oxygen capacity were 3.0 mg/L, 4.0 mg/L and 4.5 mg/L respectively, we observed the MBSL spatial distribution as a function of driving voltage. The results showed that when the driving sound pressure(20 V-110 V) is low, the SL intensity emitted by larger oxygen capacity was stronger than the others. After increasing the driving voltage, the SL intensity emitted by smaller oxygen capacity was stronger than the others.(3) Place biological tissue in front of the focusWhen biological tissue is placed in front of the focus, since the barrier effect of the bubble tissue interface makes light-emitting region seems like conical-shaped distribution. When the biological tissue was placed 3 cm in front of the focus and the acoustic output power were 320 W and 410 W, there was a round like lesion on the skin. CONCLUSIONS(1) When the low input power was given in a self-focused field and the oxygen capacity of the media water was 3.0 mg/L, we could see a typical standing wave pattern produced by the acoustic reflect in the liquid- gas interface. A cluster of bubbles was trapped in the antinodes of the standing wave field and the MBSL region mainly located in pre-focus firstly. When the input power was increased, MBSL growed toward transducer. When the input power was was sufficient, the bubbles gathered at the focus, collaped persistently, meanwhile, a large hissing noise was heard with an oval-shaped bright spot would emerge in field which weaken the SL intensity sharply;(2) When the input power was 140 W in a self-focused field, improving the oxygen capacity of the media water can reduce cavitation threshold promoting cavitation activity, thus improving MBSL intensity;(3) When the low output voltage was given in spherically focused field and the oxygen capacity of the media water was 3.0 mg/L, we could see a typical standing wave pattern produced.With the increase of voltage,the MBSL region was divided into two parts(a bunch of light-emitting point group at Y direction and a radial circular aperture at a certain distance from the center of the sphere). As the drive voltage increased further, the outer radial circular aperture growed toward the inner wall of the transducer. When the drive voltage was sufficient, a bubble cluster appeared at the focus, collaped persistently, meanwhile, a large hissing noise was heard, the appearance of cluster weaken the SL intensity sharply;(4) When the low output voltage was given in spherically focused field Low power was given, the increase of the oxygen capacity of the media water can reduce cavitation threshold promoting cavitation activity, thus improving MBSL intensity. While high output voltage was given, the bubbles acoustic attenuation was more obvious. The medium of lower oxygen capacity emitting greater luminous intensity;(5) When biological tissue was placed at pre-focus in a self-focused field and the oxygen capacity of the media water was 3.0 mg/L. As for the attenuation of acoustic energy by biological tissue, there was no obvious luminous near the focus. When the power was increased, the MBSL was mainly concentrated in the the surface of tissue skin. When the biological tissue was placed 3 cm in front of the focus and the acoustic output power were 320 W and 410 W, there was a round like lesion on the skin. |
PDF Full Text Request