| BACKGROUNDMalignant tumors have become the major diseases that threatenedhuman life seriously. The main direction of modern medicine aims attargeted killing tumors, which requires a minimally invasive or trulynoninvasive technique for tumor therapy. A series of fundamentalresearches and clinical trials have shown that high-intensity focusedultrasound (HIFU) is a noninvasive technique for tumor ablation. Theirreversible and instantaneous coagulation necrosis is caused by sufficientlyhigh ultrasound energy at the focal region using extracorporeal HIFUmodality.For the ablation of solid tumors using HIFU such as hepatocellularcarcinoma, prostate cancer, breast cancer, pancreatic cancer, malignantbone tumor and renal carcinoma, the results are greatly encouraging overthe last ten years. HIFU therapy has the advantages of conformal ablation,precise foci, deeper penetration, non-ionizing radiation. In addition, HIFUsonication can activate host antitumor immune response without increasing the risk of metastasis. For these reasons, the clinical application of HIFUtherapy has attracted great concern.Howerer, the focal region caused by HIFU ablation is usually severalmillimeters. To date, the predominant application of HIFU therapy is basedon ablation of the entire tumor to produce complete destruction andnecrosis by nesting lesions side-by-side. Therefore, for the treatment oflarge-sized tumors, deep-seated tumors or advanced malignancies, it isdifficult to achieve complete ablation of the entire tumor. Meanwhile, weshould not ignore the dilemma of the lengthy treatment time, high ablationdose, rising medical costs and the relatively low therapeutic efficacy.On the other hand, because of the biological characteristics and thenatural course of malignant tumors, their infiltrative growth margins oftenpossess hypervasculature that is intricately developed, highly oxygenatedand well nourished and therefore has a rapid heat transfer capacity.Correspondingly, the marginal tumor tissues are the fastest-growing partsof a tumor and the most prominent contributors to the malignant behaviors.By contrast, in the central region of a tumor, necrosis caused by poornutrition is common, especially in large tumors, which exhibit anoutgrowing blood supply, poor oxygenation, low metabolism andinsufficient perfusion. Based on this rationale, our research team presentsthe hypothesis that the formation of a completely necrotic barrier at theperiphery of a target tissue could induce secondary ischemic necrosis and hypoxic necrosis of the inner untreated tissue and effectively restrain tumorgrowth, ultimately leading to complete necrosis of the entire tumor.The goal of our study was to explore whether the use of a peripheralscanning mode with HIFU could produce a completely necrotic barrieralong the target tissue, to research the effective scanning pathway,optimized dose combinations and the strategy of HIFU energy deliveringand to investigate the influence of varying flow rates on the consumption ofultrasound energy using an isolated porcine liver machine perfusion model.Thus, the novel scanning mode with HIFU is focused on decreasing theablated volume, reducing total treatment times and the overall energyconsumption, and ultimately improving the therapeutic efficacy for thecontrol of large-sized tumors or deep-seated tumors.The subject was conducted mainly from the following three parts.Part ⅠTissue ablation using peripheral scanning mode with HIFU:A study on ex-vivo bovine liverObjective: To explore the effective scanning pathway, the feasibilityand the ultrasonic energy delivering strategies with HIFU peripheralscanning mode in ex-vivo bovine liver, and to elucidate the overalltreatment times and energy consumption with the novel scanning mode,compared with traditionally completed scanning mode using HIFU. Materials and Methods: Fresh bovine livers slaughtered within6hwere cut into cuboids with120mm×80mm×60mm in size, degassed inthe saline for30min using a vacuum pump. The tumor therapy system ofModel JC200(concave-focused,0.9MHz, focal length140mm) wasguided with ultrasonography monitoring. The focal region was an ellipsoidwith the maximum dimensions of8mm along the beam’s longitudinal axisand3mm along the transverse axis, which was measured by a PVDFneedle hydrophone. The appearance of a hyperechoic spot or a hyperechoicregion after HIFU exposure was the determination of the effective ablationwith the appropriate acoustic power output. HIFU sonication usingcontinuous wave with point-scanned exposure (i.e. sequential discretescanning mode) was conducted from the40-mm,32-mm,24-mm and16-mm focal depths, respectively. After HIFU ablation, the bovine liverswere sliced into2mm each along the long axis of the beam to find themaximal section of the region of coagulation necrosis, and to measure thesize of each single-point lesion for the sake of the correspondingly ablativeparameters. Then, we conducted the formation of completed linear-shapedlesion using multiple point-scanned mode with the40-mm track length,based on the above four focal depths. With the presupposed scanningcoordinate value, the HIFU beam was swept at each focal depth from thedeep to the shallow surface of the target tissue until the entire scanningprotocol was completed. In detail, the top surface of the target tissue was induced complete coagulation necrosis using point-by-point scanning. Forthe intermediate sections, the HIFU transducer was displaced only aroundthe perimeter of the target region and produced a quadrangular trajectory.The bottom surface was completely ablated just as the scanning pathway ofthe top surface. For the control group, the target regions were ablated usingthe complete scanning mode on each of the four focal depths, respectively.Each of the ablative range was measured after TTC staining, and thehistologic sections were observed under a light microscope after H&Estaining. The energy consumption in each sample was the acoustic powermultiplied by the exposure times at each focal depth, and the total energyconsumption was the sum of the energy consumption at each focal depth.The exposure time and energy consumption were performed statisticalanalysis.Results: The ellipsoidal coagulation necrosis was produced with2sexposure times and the acoustic power range from180W to330W at40-mm,32-mm,24-mm and16-mm focal depths, respectively. Thecompleted linear-shaped necrosis was formed with10s interval times,3-mm spacing distance between two adjacent exposure points, combinedwith the above parameters of single-point ablation. The completed necrosisbarrier along the periphery of the target regions was well defined andsharply demarcated while the inner non-ablated region was no obviousevidence of necrosis. The peripheral necrosis ranges between the two groups had no significant differences (P＞0.05). The overall exposuretime and overall energy consumption in the experimental group withperipheral scanning mode were significantly lower than those in the controlgroup with traditionally complete scanning mode (all P values＜0.05).Conclusions: According to the peripheral scanning mode, a completenecrosis barrier along the periphery of the target tissues can be shaped withcontinuous wave and point-scanned exposure of HIFU on ex-vivo bovineliver. By elaborately adjusting the acoustic power, exposure time of eachlesion, the interval times and spacing distance between two adjacentexposure points, and the interval times between two adjacent focal depths,the peripheral necrosis region was well defined and sharply demarcated. Inex-vivo tissue study, the novel scanning mode may contribute to lower theoverall energy consumption and reduce the overall exposure time thanthose with complete scanning mode, and therefore may achieve highertherapeutic efficacy using HIFU ablation.Part ⅡModeling an isolated porcine liver machine perfusionObjective: Throughout the procedure of HIFU ablation withperipheral scanning mode, several factors such as the ribs, gas in thegastrointestinal tract, abdominal wall tissues, respiratory movement andinsufficient coupling conditions of the skin may disturb the deposition of ultrasonic energy in vivo. Thus, the goal of this section was to explore thefeasibility of establishing an isolated porcine liver machine perfusionmodel and to investigate the related methods for monitoring and evaluation.Materials and Methods: Thirty-six isolated porcine liver obtainedfrom a commercial slaughterhouse (the warm ischemia time was less than30minutes) were perfused with iso-osmotic perfusate via the portal veinand hepatic artery, using an extracorporeal circulation machine. Within the3-h duration, the perfusion circuit was monitored with the liver color,texture, liver weight gain, fluid dynamic parameters using a multichannelbiological blood pressure sensor, color Doppler flow imaging, bile outputvia the common bile duct and the hepatic histopathology.Results: Thirty-three isolated porcine livers were successfullycannulated with the dual vessels and unobstructed drainage of the bile viacommon bile duct, and failure of the hepatic artery cannulation occurred inthree porcine livers. Within the3-h perfusion, thirty-one livers maintainedyellowish, soft texture, smooth liver capsules and sharp edges, and red andyellow in color occurred in two perfused livers. The liver weight gain rateswere (18.3±3.5)%after3-h perfusion. Within the physiological pressureranges of the portal vein (7～10mmHg) and the hepatic artery (70～80mmHg), the mean flux via the portal vein and hepatic artery were (37.1±1.3) ml/min/100g and (17.5±0.8) ml/min/100g, respectively. Using colorDoppler flow imaging, the liver parenchyma was shown homogeneous hypoecho, the intrahepatic vessels were similar to that of normal livers, andflows in the portal vein and hepatic artery were clear. The mean flowvelocities within the main portal vein and the main hepatic artery were(13.6±2.0) cm/s and (37.4±2.8) cm/s, respectively. The total bile outputwas (19.6±0.8) ml after3-h perfusion. H&E staining showed varyingdegrees of dilated sinusoids, partial mononuclear cell infiltration andhydropic degeneration of a small number of hepatocytes, but in general, theoverall appearance indicated an intact parenchymal morphology andarchitecture of the livers throughout the3-h perfusion.Conclusions: This isolated porcine liver perfusion model is stable,repeatable, less interference factors, better controllability and quantitativeassessment. The model can not only decrease large animals for ourexperiments, but also meet the experimental requirements of HIFU study.Part ⅢEffects of varying flow rates on peripheral scanning mode with HIFUObjective: Based on the isolated porcine liver perfusion model, ourgoal of this study was to explore the feasibility of using the peripheralscanning mode with linear-scanned HIFU to produce a completely necroticbarrier, to research the energy delivering strategies and the ablativeparameter combinations, and to investigate the effect of varying flow ratesvia major hepatic vessels on the energy consumption. Materials and Methods: Based on the isolated porcine liverperfusion model previously established, we conducted a scanning protocolalong the periphery of the target tissues using continuous linear-scannedHIFU exposure, and carefully adjusted the various ablation parameters.Forty-four porcine livers were divided into four ablation groups:experimental group1(n=12, with dual-vessel perfusion), experimentalgroup2(n=11, with portal vein perfusion alone), experimental group3(n=10, with hepatic artery perfusion alone) and a control group (n=11, withno-flow perfusion). The standard of sufficient ablation was the appearanceof complete linear-shaped hyperechoic region immediately after HIFUexposure. The presupposed ablation range was30×20×18mm3in eachgroup. All the samples were cut open consecutively at3-mm thickness, theactual ablation ranges were calculated along the periphery of the targettissues after TTC staining and were presented the subsequent histologicalobservations. The total energy consumption was calculated as the sum ofthe energy requirements at various focal depths in each group and then wasperformed statistical analysis.Results: A complete coagulation necrosis barrier was generated ineach group with carefully adjusting the varying focal depth (40-mm,34-mm,28-mm and22-mm focal depth, respectively), acoustic power(output range:180W～380W), scanning speed (4mm/s～6mm/s) andspacing distance between two adjacent exposure lines (4-mm). The histological results showed a sharp demarcation between the ablativetissues and the untreated tissues. Moreover, the volume of the peripheralnecrotic area among the four groups demonstrated no significantdifferences (P>0.05). The total energy consumption in each group wassignificantly reduced with the corresponding decrease of the flow rate viadifferent major hepatic vessels (P <0.01).Conclusions: Our preliminary study has demonstrated that a completeperipheral necrosis barrier along the target tissues can be generated withcontinuous linear-scanned exposure of HIFU. By dynamically adjusting theexposure parameters and their combinations on an isolated porcine liverperfusion model, we have found that the flow rate in the major hepaticvessels is a strong predictor of the total energy consumption following theperipheral scanning mode. This novel scanning strategy may contribute toreducing the ablation time and to improving the therapeutic efficacy andclinical safety in the treatment of large tumors with HIFU ablation. |
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