Relationship Among Temperature,Cavitation And Hyperecho In Ultrasound Images Induced By High Intensity Focused Ultrsound In Bovine Liver

BACKGROUDHigh-intensity focused ultrasound (HIFU) is a noninvasive treatment of certain primary solid tumors and metastatic disease, has been used to treat tumors on liver, kidney, pancreas, rectum, prostate, bone, uterus and other parts in clinical, and achieved excellent therapeutic effect.The physical mechanisms of HIFU include thermal effects,mechanical effects and cavitation .HIFU is a noninvasive treatment, so the real-time monitoring during treatment process is critical. At present,the available monitoring methods of HIFU treatment are Magnetic Resonance Imaging (MRI), Computed Tomography (CT) and type-B ultrasonography. Because of inexpensive, easy to operate, no radiation, good compatibility with HIFU equipment, type-B ultrasonography monitoring HIFU has been widely used in the medical domain, such as in clinic, especially in uterine fibroids, liver cancer, bone cancer and other solid tumors, and achieved a significant effect. The tissue coagulation necrosis was determined by the appearance of hyperechoic regions on the target of type-B ultrasound images after irradiating. But the reason for the appearance of hyperechoic regions is inconclusive currently. Generally believed that it is associated with boiling bubbles, cavitation bubbles and the formation of new interface caused by the generated of tissue coagulation necrosis. The reason for the appearance of hyperechoic regions on type-B ultrasound image is complex and unclear,lead to coagulation necrosis happened without hyperechoic on type-B ultrasonography monitoring HIFU.Therefore, the study of the relationship among temperature rising, cavitation and the appearance of hyperechoic regions during HIFU irradiation is useful for understanding type-B ultrasonography monitoring HIFU.In this paper, non-invasive MRI measured the temperatureat at the focus of HIFU irradiation process, boiling bubble was reflected by the temperature. Tissue coagulation necrosis was got through dissecting after irradiating. Cavitation signals collected by Passive Cavitation Detection (PCD), was used to reflect the participation of cavitation. The appearance of hyperechoic regions was obtained through the type-B ultrasonography. The relationship among temperature rise, cavitation and the appearance of hyperechoic regions was investigated in this paper. OBJECTIVE1. The relationship among the temperature rise of the HIFU focus, characteristics of the cavitation signal, and the appearance of hyperechoic regions under different HIFU irradiation power. Study the reason for the appearance of hyperechoic regions on type-B ultrasound image after HIFU irradiation.3. The cause of the appearance of hyperechoic regions on type-B ultrasound image under the low acoustic power.MATERIALS AND METHODS1. The study of the relationship among temperature rising, cavitation and the appearance of hyperechoic regions during HIFU irradiating boving liver tissue in vitro 10s with different power.1.1 Experiment Equipment: (1)MRIgHIFU treatment system, that was developed by Chongqing Haifu Technology Co., Ltd. and SIEMENS together.,including 1.5T MRI systems and HIFU treatment unit.The parameters of therapeutic transducer were: diameter 150 mm, focal length 180 mm, centre frequency 1.1 MHz. (2)Passive Cavitation Detection (PCD) system, it comprised two transducers, 0.5 MHz (panametrics: V 301, diameter 25 mm) and a 5 MHz (panametrics: V 309, diameter 13 mm) was used. NI High-Speed Data Acquisition Instrument with LabVIEW program was used to collect real-time data, the late data-processing was on LabVIEW platform. (3) Type-B ultrasonography monitoring HIFU system was composed of HIFU therapy system in MRIgHIFU and type-B ultrasonography device.1.2 Experiment Subjects: Taken freshly isolated boving liver within 6 h after slaughtering, the tissue where connective tissue and blood vessels less was selected, cut into 10 cm×8 cm×6 cm size of the tissue, immersed in saline, rewarming to 20℃, 30 min degassing for use.1.3 Experiment Method: HIFU point irradiated, powers elected 50 W, 100 W, 150 W, 200 W, times were 10_s, and set a fixed depth of 30 mm. Repeat 6 times each irradiation experiment. (1)MRI temperature measurement: First the liver sample was scanned with t2_trufi_sag_pat2 sequence , get a set of images, select the level with less connective tissue and blood vessels as the target, then move HIFU focus to the level and adjust the treatment depth. Magnetic resonance GRE_TMap_sag_phasediff sequences was used to get temperature of the voxel time-varying temperature during HIFU irradiation,the voxel with the maximum temperature was treat as the HIFU focus, last get the temperature rise of the HIFU focus during the process. (2) Cavitation signal acquisition through PCD. Set the PCD transducers face the HIFU focus straightly, acquire transducer voltage signal during HIFU irradiation, post signal processing to obtain the half harmonic emissions and broadband noise changes over time.(3) Type-B ultrasonography monitoring HIFU: adjusted the type-B ultrasonography probe to face the HIFU focus straightly, the focus was on the type-B ultrasound image, save type-B ultrasound images before and after HIFU irradiating .2. The study of the relationship among temperature rising, cavitation and the appearance of hyperechoic regions during HIFU irradiation under low-power.2.1 Experiment Equipment: the same as the first part2.2 Experiment Subjects: the same as the first part2.3 Experimental Method: HIFU point irradiated under 50W with a fixed depth of 30 mm.In order to save type-B ultrasound images, ater 10s irradiation stop 1s was taken.MRI temperature measurement saved temperature map during HIFU irradiation. During the 1s without HIFU irradiation, Type-B ultrasonography monitoring HIFU system saved the Type-B ultrasound image of tissue. PCD observed the characteristic signal of cavitation during HIFU irradiation.RESULT1. During HIFU irradiation process under different acoustic power , the temperature of the focus was increased with the irradiation time. With the same irradiation time and depth, the greater the acoustic power, the bigger the temperature of the focus and the area of coagulation necrosis. 2. When the acoustic power≥100 W, the half harmonic emissions and broadband noise were existing at the beginning of HIFU irradiation. The value of half harmonic emissions and broadband noise under 200W was greater than 100W and 150W. When the acoustic power was 50W, no half harmonic emissions and broadband noise generated.3. When the acoustic power≥100 W, with HIFU irradiation 10_s ,the tissue coagulation necrosis occured and cavitation happened, the hyperechoic region appeared on type-B ultrasound image.The greater the acoustic power, the greater the value of hyperechoic.4. When acoustic power was 50W, after HIFU irradiation 10_s , no coagulation necrosis generated. However, when acoustic power was greater than 100 W(included), with the increase of acoustic power, the larger of coagulation necrosis, with empty and fragmentation phenomena happened.5. During low-power (50 W) HIFU irradiation, the temperature of the focus was increased with the irradiation time,the hyperechoic region would appeare after long time irradiation.No cavitation happened,coagulation necrosis occurs was earlier than the hyperechoic region appeared.The coagulation necrosis areas were pale and uniform. During the entire irradiation the cavitation characteristics signal was not found, for long time HIFU irradiation,the hyperechoic region would appeare.CONCLUSION1. When acoustic power is higher (≥100 W), during 10_s HIFU irradiation,the half harmonic emissions and broadband noise are existing at the beginning of HIFU irradiation,this is the reason for appearing of hyperechoic on type-B ultrasound image.2. When 50 W HIFU irradiation, during long time HIFU radiation process, although no cavitation characteristics signal was found,appearing of hyperechoic on type-B ultrasound image associated with increasing of the focus temperature.3. During low-power HIFU irradiation,coagulation necrosis occurs is earlier than hyperechoic appearing,so exploring of this case specific and sensitive ultrasound monitoring is necessary.