The Relationship Between Cavitation And Temperature Rise、Coagulation Necrosis During High Intensity Focused Ultrsound Induced Thermal Ablation Process

BACKGROUNDHigh intensity focused ultrasound tumor therapy，a precise focus onnon-invasive tumors treatment with no skin incision, has a good efficacyand safety and shows good prospects. The HIFU resulting in tissue thermalcoagulation necrosis is a very complex transient process. The principle is tomake use of the ultrasound beam which has convergence characteristics,and penetrating in the organization and the inherent sound absorptioncharacteristics. The ultrasound will be launched in vitro ultrasonic focusingto promote target tissue to a small region of body. With the thermal effects,cavitation and mechanical effects, it produce transient high temperatureabove60℃, and thus the formation of irreversible coagulation necrosis inorder to achieve the purpose of treatment.Based on the results of thermal dose research, it has to reach a certaintemperature and last for some time in HIFU treatment process to producecoagulation necrosis. Therefore, real-time temperature monitoring in target region has great significance on efficacy evaluation and therapy dosedetermination. HIFU destruction mechanism in target tissue is mainlythermal effect and cavitation effect. Heat is converted from ultrasoundabsorption in target tissue. When ultrasound irradiates with the soundpressure exceeding a certain threshold, tiny air core or cavities expands andforms cavitation bubbles. Cavitation is the series of dynamics of cavitationbubbles under sound pressure, such as oscillation, growth, contraction andcollapse. Studies have shown that strong cavitation activity will lead tohigh temperature, high pressure, shock wave, high-speed jet and suchphysical phenomena, all of which are likely to impact the thermal effectand coagulation necrosis formation.Thermocouple temperature measurement and passive cavitationdetection is used to monitor target region’s temperature elevation andcavitation changes over time in the HIFU irradiation in vitro bovine liver,to explore the relationship between cavitation and temperature rise.Records of coagulation necrosis formation in bovine eye lens taken byHigh-speed photography, combined with cavitation signal analysis, areused to analyze the relationship between cavitation and coagulationnecrosis.OBJECTIVE1. Temperature and cavitation’s variation with time in target region,and the relationship between cavitation occurrence and temperature elevation, under different irradiation time and acoustic power.2. The coagulation necrosis formation process under the same doseconditions, and how cavitation impacts it.MATERIALS AND METHODS1. The realationship between cavitation and temperature rise duringhigh intensity focused ultrasound induced thermal ablation process.Experimental materials: Fresh vitro bovine tissue washed by normalsaline (concentration0.9%), was cutted into pieces with the size of8cm×8cm×6cm, to avoid the large blood vessels and connective tissue.These bovine liver tissue was placed in the degassing system added to thenormal saline, degassed for35minutes. After that restore the temperature to20℃.Experimental installation:①Ultrasound-guided high intensityfocused ultrasound therapy system (USgHIFU)，developed by ChongqingHaifu (HIFU) Technology Co., Ltd. The parameters of the HIFU transduceris that: frequency0.94MHz, focal length145mm, diameter220mm.②Passive cavitation detection (PCD) system: cavitation probe (panametricsV301-SU, diameter13mm, frequency5MHz, focal length40mm), NIhigh-speed data acquisition card (PXie-5122,20MHz sampling rate),Labview software platform(v10.0.1).③Temperature acquisition system:Thermocouple probe, made up by T-type thermocouple wire; Temperaturedata logging devices (5Hz sampling rate); PC and serial port debugger. Experimental methods: Before the experiment, radiation pressuremethod was used to measure the acoustic output of the HIFU transducer ofand ensure there is a stable sound output in different parameter conditions.Set HIFU irradiation time of5s, the irradiation power, respectively50W,100W,150W,200W,250W, irradiation depth of30mm, each parameterexperiment was repeated five times. Under the guided of B-modeultrasound, thermocouple probe was inserted into the bovine tissues andadjust the PCD probe. Temperature data logging devices was used to recordthe temperatue changes over time. Cavitation signals collected by the PCDsystem was to do Fast Fourier transform, select non-harmonic broadbandnoise in the frequency range3-7MHz as the characteristic signal oftransient cavitation, Calculate the RMS value of the signal in order to drawthe curve of the RMS signal amplitude changes over time.2. Based on high-speed photography of the realationship between theformation process of HIFU induced coagulation necrosis and cavitation.Experimental materials: in vitro bovine lens.Experimental installation:①Ultrasound-guided high intensityfocused ultrasound therapy system (USgHIFU).②Passive cavitationdetection (PCD) system.③High-speed photography system：High-speedcamera (Fastcam SA4), LED light source, camera lens，PVF controlsoftware.Experimental methods: The focus is positioned in the bull’s-eye lens center,under the guided of B-mode ultrasound. The same does wasdelivered into the target tissue. The specific parameters are100W-6s,200W-3s,300W-2s,600W-1s. The cavitation singal was collected by PCDsystem, and the formation process of HIFU induced coagulation necrosiswas photographed by high-speed photography system.RESULT1. As the acoustic power rises, the cavitation intensity andaccumulative transient cavitation dose increases. But transientcavitation intensity declines in the exposure under the same acousticpower.2. Temperature increasing is linear under low acoustic power (50W,100W,150W). The temperature elevation rate rise is due to theacoustic power increase. Temperature increasing is nonlinear underhigh acoustic power(200W,250W). Transient cavitation occurswhen the temperature rapidly elevated.3. Transient cavitation occurs before the coagulation necrosis at300Wand600W acoustic power, and after the coagulation necrosis at200W.4. Coagulation necrosis gradually expanded from a small point at100W and200W acoustic power. When acoustic power are300Wand600W, Coagulation necrosis grew from several independentvertical aligned points to a line, and expanded to the surrounded tissue.CONCLUSION1. The RMS and temperature curves show different trends in differentHIFU acoustic power exposure, nonlinear temperature elevation intarget region involves of transient cavitation.2. As the acoustic power increases, the required time for coagulationnecrosis and cavitation becomes shorter.3. For different acoustic power, transient cavitation and coagulationnecrosis are not the same. So is the coagulation necrosis formationprocess.