Influences Of Blood Flow Rate On Coagulation Necrosis Induced By Pulsed High-intensity Focused Ultrasound Ablation

BACKGROUNDHigh-intensity focused ultrasound (HIFU) therapy has been widelyapplied in clinical practice to treat solid tumors. It currently usescontinuous focused ultrasound waves with both high acoustic power toablate a targeted tissue through thermal effect. Pulsed high-intensityfocused ultrasound (PHIFU) differs from HIFU irradiation whereultrasound waves are applied continuously over the course of an exposure.The operation mode of PHIFU is that ultrasound waves are pulsed on andoff during one exposure, and each exposure is actually comprised of shortexposure time (pulse on) and break time (pulse off). Previous laboratorystudies have show that PHIFU could produce obvious cavitation，and theninduce tissue damage in a targeted region.As a pulsed irradiation，PHIFU may decrease HIFU-caused energydeposition on normal tissues along focused ultrasound pathways，thusreduce unexpected damage on them. However，PHIFU，as a noveltreatment mode，before clinical application it is essential to investigate both histological characteristics in PHIFU-treated tumor and subsequentchanges in real-time ultrasound imaging，as well as effects of blood flowrate on the efficiency of ablation. Using an isolated porcine liver perfusionmodel, this study is to employ a clinical HIFU therapeutic system toobserve the changes of both coagulation necrosis volume and ultrasoundimaging in treated liver tissues under different blood flow rates, and theinfluence of hepatic blood vessels on the efficiency of PHIFU ablation. Itwill also explore the effects of blood flow rate and blood vessel on theformation of PHIFU-induced coagulation necrosis and subsequent changesof real-time ultrasound imaging, and provide important data for clinicalapplication of this novel treatment.OBJECTIVE1. To explore the effects of blood flow rates on PHIFU-inducedcoagulation necrosis and changes in B-mode ultrasound imaging intargeted liver tissues.2. To explore the effects of blood vessel on both PHIFU-inducedcoagulation necrosis.MATERIALS AND METHODS1. Influence of different perfusion flow rates on characteristics ofPHIFU-ablated liver tissues and subsequent changes in B-mode ultrasoundimaging during treatment.Animals: Porcine liver was provided by slaughterhouse. It was obtained2~3min after pig was killed，and an isolated liver perfusionmodel liver was immediately established via the portal vein，hepatic arteryand inferior vena cava (IVC) for the study.Methods(1) HIFU and PHIFU were respectively classified as the controlgroup and experimental group in the study. The therapeutic parameterswere as follow: pulse repetition frequency=100Hz；acoustic power=80W；duty cycle=20%and exposure time=25s in the experimental group，dutycycle=100%，and exposure time=10s in the control group. The depth ofthe target was20mm from the liver surface. The portal vein flow rate ofthe liver perfusion model used for the study was442mL/min，588mL/min,746mL/min，and886mL/min，respectively.(2) With different portal vein flow rates，the targeted liver was treatedby PHIFU and HIFU exposure respectively. During irradiation，real-timeB-mode ultrasound was used to observe gray-scale changes in the targetedtissues. Each experiment was repeated for20times, and imaging datawere analyzed after treatment.(3) Immediately after HIFU exposure， a damaged tissue wasidentified in the perfusion liver， and its maximal dimensions weremeasured and recorded. Using mathematical equation，the volume ofcoagulation necrosis was calculated. Tissue samples were obtained in thetargeted tissue for histological examination. 2Effects of hepatic blood vessels on PHIFU-induced coagulation necrosisMethods(1) Both HIFU and PHIFU exposures were used as the control groupand experimental group. The following parameters used in both groups:acoustic power，80W in both groups；duty cycle，20%in PHIFU and100%in HIFU；exposure time，50s in PHIFU and10s in HIFU；pulse repletionfrequency，100Hz in both groups. The depth of the targeted tissue was40mm from the liver surface.(2) Using real-time ultrasound imaging device，3targeted regionswere separately selected in both groups. One target was the blood vessel，and the2ndone was liver tissue3mm before the blood vessel. The3rdtarget was liver tissue3mm beyond the blood vessel. Each target wasirradiated by one shot, and each experiment was repeated for15times.(3) Immediately after ultrasound exposure，gross examination wasperformed to identify both damaged liver tissue and blood vessels in theperfusion liver， and their maximal dimensions were measured andrecorded. Using mathematical equation， the volume of coagulationnecrosis was calculated. Tissue samples were obtained from both bloodvessel and ablated liver tissue， and histological examination wasperformed afterward.RESULTS1Characteristics of PHIFU-ablated liver tissue: In comparison with HIFU exposure, the necrosis induced by PHIFU presented typicalcharacteristics of coagulation necrosis with a clear margin between thetreated and untreated tissues. There was no obvious tissue hole in thetargeted tissue.2. Changes of B-mode ultrasound imaging in the PHIFU-ablated livertissue: During PHIFU exposure，B-mode ultrasound imaging could showreal-time gray-scale increase in the treated tissue. This change was adynamic process where the gray-scale was gradually increased，thendecreased，and increased again during one exposure. The time required forthe emergence of gray-scale change was various while the perfusion flowrates were different. An increase of perfusion flow rate could delay theemergence of gray-scale change during PHIFU exposure.3. Relationship between the volume of coagulation necrosis in ablatedtissue and perfusion flow rate：While perfusion flow rate was442mL/min，588mL/min，746mL/min and886mL/min，coagulation necrosis volumewas157.41±5.79mm3、118.70±5.96mm3、72.97±6.04mm3and44.45±3.49mm3in the experimental group，and156.41±6.23mm3、120.57±6.44mm3、74.39±7.62mm3and45.96±5.72mm3in the control group. There was anegative relationship between the necrosis volume and perfusion flow rate.With the increase of perfusion flow rate，the necrosis volume was graduallyreduced. Under the same perfusion flow rate，there was not a significantincrease of the necrosis volume in the PHIFU group while compared with the control group(P>0.05)..4. Relationship between the volume of coagulation necrosis inablated tissue and hepatic blood vessel: Both HIFU and PHIFU exposurescould induce typically coagulation necrosis in the targeted tissue close tothe blood vessel. While the targeted tissue was located3mm beyond orbefore the vessel，the necrosis volume was79.72±4.94mm3or85.6±10.23mm3in the PHIFU group, and25.53±2.08mm3or28.34±7.28mm3in theHIFU group，respectively. There was a significant difference between thePHIFU group and HIFU group (P <0.05).5. Destructive effect of PHIFU on blood vessel close to the targetedtissue: Both PHIFU and HIFU exposure was performed to ablate theselected three targets respectively. Immediately after exposures, grossexamination was carried out. There was no evidence of blood vesseldamage in the3targets, but coagulation necrosis was found in bothtargeted liver tissues. Both HE and Victoria’s blue and Ponceau’s stainingshowed that the structural integrity of the vascular wall was kept well, andthere was no damage on endothelial cells and fibrous tissues within thevascular wall in both PHIFU and HIFU groups.CONCLUSION1. During PHIFU exposure, B-mode ultrasound imaging canreal-time observe the gray-scale changes in the targeted tissue, whichpresented a dynamic process where the gray-scale was gradually increased, then decreased, and increased again.2. The necrosis induced by PHIFU has typical characteristics ofcoagulation necrosis. The margin is clear between the treated anduntreated regions， and there are no obvious tissue holes in the targetedtissue.3. Blood flow rate can influence the efficiency of PHIFU ablation.With the increase of blood flow rate, the necrosis volume is graduallyreduced. In addition, an increase of perfusion flow rate can delay theemergence of gray-scale change during PHIFU exposure.4. In order to achieve a similar volume of coagulation necrosis，theenergy used in PHIFU is much lower than that used in HIFU.5. In comparison with HIFU exposure, PHIFU can significantlyincrease the volume of coagulation necrosis in the targeted tissue while thetarget is close to the blood vessel.6. PHIFU has no directly destructive effect on both the blood vesseland the vessel close to damaged tissues.