Experimental Study On A Novel Radiofrequency Ablation Imaging System Developed Based On The Determination Of Young's Modulus Threshold By Indentation Method And Its Validation

Chart 1Stiffness distribution and Young’s modulus testing for the radiofrequency ablated zone for liver:An ex-vivo study with a tissue elastometerObjective To investigate the stiffness distribution in the ablated zone after radiofrequency ablation(RFA),we used a device called tissue elastometer based on gross liver samples.Methods Twelve freshly excised porcine livers were subject to RFA under a same setup to form elliptic ablated samples.Each sample was cut open for gross examination,and then the surface of the section plane was sliced into one piece for Young’s modulus test using the tissue elastometer.Five test points along the longand short-axis on each piece were selected to evaluate stiffness distribution respectively.Among them,four points distributed equidistantly from center to boundary in the ablated zone and one was in the unablated zone.Results In the ablated zone,we found the Young’s moduli were significantly different among the four test points both in long-(F=99.04,p<0.001)and short-axis(F=79.47,p<0.001)directions.The Young’s modulus showed a downtrend in each direction,and was linearly related to the distance from the center to the test point(for long axis,R2=0.968;for short axis,R2=0.984,both p<0.001).A more significant downtrend was observed in short-axis direction.The Young’s moduli gained from the inner edge of ablated zone were comparable and significantly higher than those from the outer edge for both directions.The maximum value of 24.71kPa for Young’s modulus was the appropriate threshold to ensure the tissues were necrotic completely.Conclusions The stiffness inside the ablated zone represented a radial distribution with downtrend,following a linear law.The stiffness at the inner edge of ablated zone is stable and significantly higher than that at the outer edge.The maximum value of 24.71 kPa close to the inner edge of Wz may be used as the standard of complete ablation.Chart 2Comparison of the ablation range defined by Young’s modulus threshold with real ablation range based on the ultrasound bioengineering experimental platform,an in vitro experimentPurpose There are many brands of ultrasound equipment.Although the basic principles of elastography are the same,however,the algorithms and engineering manufacturing processes they are based on are quite different.Especially in the visualization process,the elastograms of ablation lesions presented are very different due to the different coding methods.Thus,we disigned this study to obtain an elastic map through an open source engineering platform and a controllable excitation device,and a new type of radiofrequency ablation imaging method will be developed through the strategy of "actively seeking boundaries".By comparing with the actual ablation lesion(Wz),the accuracy of the feedback of the real necrotic area in this area is verified.Methods Ten fresh liver tissues were selected and subjected to 5W power radiofrequency ablation for 30 minutes.Before ablation,an oscillator was installed on the needle shaft of the ablation electrode and 100 Hz vibration was applied to produce a shear wave of lateral motion in the target area.Then use the open source ultrasound bioengineering experimental platform to edit the background program,enter 24.71 ±2.5kPa as the focused recognized elasticity range,the range of 24.71 ±2.5kPa is assigned as yellow,the upper limit of the range is assigned as red,and the lower range is assigned as light green.Take the longitudinal section where the needle path is located as the observation surface,and automatically outline the outer edge of the yellow part as the edge of the ablation focus.After the DICOM image is exported,the artifacts are removed,and then the Gaussian filter is convolved with the image to smooth the outer edge of the yellow area to obtain the optimized shape of Ez,and the Ez is further extracted and the Wz seen in the general pathology is registered and compared step by step.Double coverage area is defined as true positive,Ez coverage and Wz uncovered are defined as false positive area,otherwise it is defined as false negative area.Results All the ablation lesions could not be clearly displayed in the ultra-gray scale sound,showing a roughly elliptical shape.However,the edges of Ez were not smooth before the post-processing optimization.After post-processing optimization,the false negative and false positive areas of all samples were randomly distributed on the edge of Ez,and the total areas of Ez and Wz were 10.22±0.30cm2 and 10.43±0.32cm2(P=0.561).Ez sensitivity and positive predictive value reached 96.4%and 98.5%,respectively.There was no significant difference between the true positive area(10.06±0.32cm2)and Wz area(P=0.554),but it was significantly larger than the false negative area(0.37±0.07cm2)and false positive area(0.16±0.07cm2)(both P<0.001).Through frame-by-frame comparison,the false negative area of Ez was significantly larger than the false positive area(P=0.034).Conclusion According to the results of the Chart 1,connecting the points where the Young’s modulus reaches 24.71kPa on the elastic diagram can delineate a relatively accurate necrotic area.After registration verification,the area is highly overlapped with the real ablation zone,and the outline is roughly the same.However,the boundary details is slightly different,that is,the false negative is significantly higher than the false positive,indicating that the Ez and Wz obtained by this method roughly coincide.Only the edge details are slightly "shrinked".